TttE INFLUENCE Of MOISTURE CHANGES
IN WOOD ON THE SHEARING STRENGTH
Of GLUED-JOINT ASSIMIALIES
January 1945
This Report is One of a Series
Issued In Cooperation with the
ARMY-NAVY-CIVII. COMMITTEE
on
AIRCRAFT DESIGN CRITERIA
Under the Supervision of the
AERONAUTICAL BOARD
No, \,1524
STA TE
AGRICULTURE
ItFOREST
SERVICE
OF
LOREST LTRODUCTS LABORATORY
Madis•n„ Wisconsin
In Cooperation with the University of Wisconsin
THE INFLUENCE OF MOISTURE CHANGES IN WOOD
ON THE SEWING STRENGTH OF GLUED-JOINT ASSEMBLIESBy
W. A. SANBORN, Engineer
Summary
When varying atmospheric conditions produce moisture changes in
glued-wood structural elements, the several wood components tend to shrink
or swell independently, and internal stresses are induced. Although
existing information does not provide a basis for the precise evaluation
of the effect of such internal stresses, the influence on the shearing
strength of glued structural elements cannot be overlooked in the applica. tion of design data developed under constant moisture conditions.
This report presents the results of several series of tests to
determine the effect of moisture changes in the wood on the shearing strength
of glued joints in structural combinations deemed most likely to cause trouble
when exposed to changes in relative humidity. A cold-setting urea formaldehyde glue commonly used in aircraft joints was used throughout the tests.
This glue. developed sufficient strength to cause wood failures in nearly all
the tests.
One hundred tests on two types of rib-to-spar fastenings using Sitka
spruce for the principal members were made to provide comparison of the performance of joints of structural size and type, supplemented by approximately
2,750 block-shear tests made in accordance with Army-Navy Specification AN-G-8.
Sitka spruce and hickory lumber and yellow birch plywood were used as components in the block-shear joints. Provision was thus made to include the
effects of joining members differing in species, in grain direction, and in
density, and of joining wood to plywood. Varying moisture conditions, such
as might be encountered by aircraft in service, were simulated by storing
specimens under relative humidities calculated to produce moisture content
values both more and less than that of the wood at time of gluing and also
by imposing re p eated cycles of different moisture conditions after gluing,
The methods emp:Pyed furnish a means of determining the extent to
which the shearing strength of a specific type of glued assembly has been
influenced by internal stress induced by moisture changes. They do not, however, provide measures of the magnitude or distribution of such internal
stresses.
1
This mimeograph is one of a series of progress reports prepared by the Forest
Products Laboratory to further the Nation's war effort. Results here
reported are preliminary and may be revised as additional data become
available.
Mimeo. No. 1524
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The results show that the effects of moisture changes, as determined by standard block-shear specimens, cannot be safely projected to
glued joints of larger size. There is evidence that the effects are more
pronounced in larger assemblies.
The results show that important internal stresses, not necessarily in proportion to the potential dimensional changes of the solid components, occur in the joint when the moisture content of the wood is
changed. The influence of these stresses is more pronounced when the moisture content is decreased below that of gluing than when it is increased.
The results also demonstrate the weakening effect of moisture
changes on joints in which pieces are glued with the grain of the joined
faces at right angles. The need of following good practice in design and
fabrication of glued structural elements as provided in ANC-1 g , "Design
of Wood Aircraft Structures" and ANC-19, "Wood Aircraft Inspection and
Fabrication", is emphasized by this study.
Introduction
The efficient use of material in wood aircraft construction frequently requires that members having different dimension-change potentials
be combined by gluing. Glued joints are commonly employed between members
made of species of different density, between components in which the radial face of one is joined to the tangential face of the other, between
components in which the joined gurfaces. have face grain of different directions, and between wood and certain dimensionally stable materials.
When such members are combined by glued joints, they cannot
shrink or swell independently when the moisture content is changed, and
stresses are induced by the restraint thus imposed. Such stresses, caused
by factors other than external loading, are properly called "internal
stresses." These stresses may change the shape of the structure or they
may combine with loading stresses to reduce the safe external load. If the
stresses are of sufficient magnitude, local failures may be produced that
permanently weaken the structure. Under conditions of constant relative
humidity, internal readjustment over a period of time may, to some extent,
relieve such internal stresses.
Aircraft in service encounter a considerable range of atmospheric
conditions, which when further amplified by extremes of temperature occurring within he structure, produce a wide variation in the moisture content
of the wood.- The influence of internal stresses thereby induced is an
essential consideration in the application of design criteria that have
been developed under constant moisture conditions. Such influence is of
particular importance in glued fastenings, the strength of which is usually
determined by the shearing strength of the glued joints.
..2-Study of Temperature and Moisture Content in Wood Aircraft Wings in
Different Climates. Forest Products Laboratory Mimeograph 1597.
Mimeo, No. 1524
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The purpose of this investigation was to determine the influence of
changes in moisture content on the shearing strength of glued joints in those
structural combinations deemed most likely to cause trouble when exposed to
changes in relative humidity.
Plan of Investigation
Tests of shearing strength were made on glued joints between various
structural combinations. The shearing strengths of specimens Subjected to
moisture changes were compared with those of matched specimens kept at constant moisture content.
Standard shear blocks for 4e block shear strength test, as described
in Army-Navy Specification AN-G-8,- were used for the major portion of this
study (fig. 1). In addition, two types of typical rib-to-spar fastenings were
investigated to provide a comparison of performance of actual structures with
that of standard shear blocks (figs. 2 and 3). To check the established
moisture-strength relationships for the solid wood used, standard shear tests
were made on representative pieces according to procedures established by the
American Society for Testing Materials (fig. 4).
The program was divided into eight series of tests, identified by Roman
numerals. The specimens for each series wore subjected to one of three conditioning methods as follows:
Series I.
Series II,
Series III.
) Glued at 12 percent moisture conJoints between same species.
Joints between unlike species. ) tent and subjected for various
Joints between solid wood and ) periods of time to a relative
) humidity that changed the moisplywood.
) ture content.
Series IV,
Joints between solid wood and comp reg (this series was dropped
from the program).
Series V.
Glued at 12 percent moisture content and subjected to cycles of
moisture change.
Series VI.
Glued with members at unlike moisture content values and then
conditioned to 12 percent moisture content.
Series VII.
Solid wood tested at various moisture content values.
Series VIII,
Typical rib-to-spar fastenings, Glued at 12 percent moisture
content and subjected for various periods of time to a relative humidity that changed the moisture content.
Each group of test specimens of one structural combination within a
series was further identified by a capital letter.
3
A more detailed description of the block shear joint test is given in ANC--19,
Wood Aircraft Inspection and Fabrication, pp. 164-165; U. S. Department of
Agriculture Technical Bulletin No. 205, Gluing Wood in Aircraft Manufacture,
pp, 11-12; and in U. S. Department of Agriculture Department Bulletin No. 1500,
The Gluing of Wood, p, 69.
Mimeo. No, 1524
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Materials and structural combinations for the glued block-shear
specimens were selected to provide various degrees of difference between
the dimensional change properties of the members and to orient this difference both parallel and perpendicular to the direction of loading.
The following combinations were investigated: (The angle given
is that between face grain and direction of loading.)
First Member and Second Member
Used in series
and group Nos.
No difference in dimensional change properties
Flat-sawed Sitka spruce 0° Flat-sawed Sitka spruce
0° I A,VI A
Flat-sawed hickory 0°
Flat-sawed hickory 0°
I AH, VI B
Difference in dimensional change properties placed
perpendicular to loading
Flat-sawed Sitka spruce 0°
Plat sawed Sitka spruce 0°
Quarter-sawed hickory 0°
Quarter-sawed Sitka spruce
0°
Flat-sawed Sitka spruce 0°
Flat-sawed Sitka spruce 0°
Flat-sawed Sitka spruce 0°
Quarter-sawed Sitka
spruce 0°
Flat-sawed hickory 0°
Flat-sawed hickory 0°
Flat-sawed hickory 0°
I 0
II A, VI C
I CH
II B, v A
Yellow birch plywood 0°
Yellow birch plywood 45°
Yellow birch plywood 90°
III A
III B
III C
Difference in dimensional change properties placed
parallel to loading
Flat-sawed Sitka spruce 90° Yellow birch plywood 90° III D
Difference in dimensional change properties placed both
and perpendicular to loading
Flat-sawed hickory . 0°
Flat-sawed hickory 90°
Flat-sawed Sitka spruce 0° Flat-sawed Sitka
spruce 90°
arallel
I BH
I B, V B
The typical rib-to-spar fastenings investigated were as
follows:
Series VIII, Group A.--The specimen used for this group was a
typical rib-to-spar fastening employing both a gusset plate and corner
block joints (fig. 3). Prom exploratory tests, the specimen was designed
to induce failure by shearing a gusset 2 inches wide from a spar 2 inches
thick under cantilever loading.,
Mimeo. No. 1524
-4-
Series VIII, Group B.--A symmetrical rib-to-spar joint employing corner blocks only was used for this group (fig. 2). The ribs were
made 15/16 inch thick to prevent their failure in compression. The triangular corner blocks for both groups had 1/2- by 6-inch gluing faces.
A.S.T.M. standard shear tests were made at each of three moisture contents of the following solid woods:
Series VII, Group A -- Sitka spruce
Series VII, Group B -- Hickory
Materials
Material for Block Shear Strength Tests and
A.S.T.M. Standard Shear Tests of Wood
Sitka spruce was selected as a low specific gravity species
used for aircraft. For contrasting density, hickory was chosen as the
aircraft wood of highest specific gravity and greatest shrinkage. Nineply yellow birch plywood, 0.695 inch thick, was chosen as a standard
aircraft plywood construction suitably adapted to block-shear specimens,
Sitka spruce of average density was selected from kiln-dried
laboratory stock. The flat-sawed material used was selected from the
outer part of one log where the annual rings had little curvature. The
quarter-sawed material and the solid wood shear blocks were taken from a
single spruce plank.
Hickory was obtained from a supply of air-dried 1-inch flatsawed boards. Only clear, straight grained pieces were used. The elimination of defects, such as wane, knots, and cross grain, generally prevented consecutive longitudinal matching, although one clear piece was
used for specimens for the A.S.T.M. standard shear tests. The material
selected was mostly sapwood with occasional heartwood inclusions.
Curvature of the rings was pronounced. Quarter-sawed hickory 2 inches
wide was obtained by gluing two flat-sawed laminations.
The 0.695-inch plywood was manufactured at the Laboratory from
0.080-inch yellow birch veneer cut from one log and selected for aircraft
grade. After conditioning to 9 percent moisture content, the veneers
were assembled' with phenol-formaldehyde film glue and pressed for 30
minutes at 250 pounds per square inch at a temperature of 320° F.
Materials for Typical Aircraft Fastenings
Sitka spruce from kiln-dried laboratory stock was used for the
solid wood parts. Consecutive pairs of 4-foot, flat-sawed planks were
glued with annual rings parallel to obtain quarter-sawed material 6 inches
Mimeo. No. 1524
wide for the spar parts of group A. The gusset plates for this group
were of 0.125-inch yellow birch plywood of aircraft grade purchased from
a commercial producer. The parts for group B were made from quartersawed planks cut side by side from one log.
Matching and Preparation of Specimens
Preparation of Glued Shear Blocks
The material for glued shear blocks was first prepared in
rectangular panels 3/4 inch thick ((%695
inch for plywood) and approximately 5
inches wide and 14 inches long. From each was obtained a panel
about 5 by 12 inches and an end-matched piece
1-3/4 by 2 inches for moisture and specific gravity determinations. The various structural combinations were assembled and glued in this
5- by 12-inch size and 10
shear blocks were cut
from each assembly. Before finishing, hickory
boards were planed over-size and conditioned at 70° F. and 64
percent relative humidity for about 1 week to minimize warping.
The plywood sheets were cut into 5- by 14-inch panels with the
direction of the face grain varied as
required.
With the exception of pieces for perpendicular construction,
the grain of the solid wood panels was parallel to the length. Of these,
all but qua r ter-sawed hickory were
cut and numbered in longitudinal order
from 16-foot boards. The quarter-sawed hickory was laminated from a
single flat-sawed board, and cut into pieces 2-1/4 inches wide and 2 g
inches
long. Two end-matched, 14-inch pieces were placed side by side to make one
panel. Panels for per
pendicular construction were made by cutting 5-inch
lengths from boards 7
inches wide, placing end-matched pairs side by side,
and numbering in pairs through the length of each board.
In assigning panels for assembly in test groups, consecutively
numbered pieces were used. Sitka spruce boards for each group were face-,
matched insofar as possible. Those of hickory were selected for similar
density.
Twenty-two panels of each constituent material were required for
each group of tests of series I, II, and III.
To minimize differences in
shrinkage between parallel, flat-sawed laminations of the same species
(groups A and AH, series I) 22 end-matched pairs provided both members of
these groups. Each set of 22 panels was further divided into
two matched
sub-groups by assigning alternate panels (or pairs of panels) to each
sub-group.
Two groups of tests of series V consisted of structural combinations identical wit ,1
groupe occurring in series I and II. Panels of the
weaker member .
in wIalek failule was anticipated, were face•,matched with
those of series I and II, mile
the stronger members were selected for
similar density. The 11 panels of each material for each group were taken
Mimeo. No. 1524
in order from a single board. For joints between the same species of
series VI (groups A and B), end-matched pairs made up each group.
After conditioning to the moisture content desired, the panels
for each group of tests were assembled and glued. Parallel laminations
of the same flat-, sewed species were glued with rings parallel. In all
other combinations, the convex or bark sides of flat-sawed members were
used for the glued surfaces. No choice was made between surfaces of
quarter-sawed or plywood members.
With the exception of series VI, all of the glued panels for
block shear specimens were in groups or sub-groups of 11 each. The individual specimens were marked out on the glued panels, each being identified by series, group, and specimen number. The numbering was so arranged
that, when cut and rearranged in numerical order, each consecutive set of
10 included one specimen each from 10 of the 11 panels. Thus 11 matched
sets of 10 specimens each were obtained from each group or subgroup of 11
glued panels.
The glued panels of series VI were similarly sub divided into
specimens but the 10 specimens cut from each were numbered consecutively
and comprised one set.
Preparation of Specimens for A.S.T.M.
Standard Shear Tests
Sixty Sitka spruce specimens were cut consecutively from a clear,
2- by 2-inch piece. Alternate specimens were cut with the annual rings
parallel and perpendicular to the shear plane. Three sets of 20 specimens
were matched by placing every third specimen of each type of cutting in a
set.
For hickory specimens, a clear, flat-sawed board 3/4 inch thick
and 5 inches wide was cut into 14-inch lengths. From each length a 12-inch
panel with an end-matched specific gravity specimen was made. Similar
hickory was glued to each face of these panels to make 2-inch material.
From the glued panels, two side••matched tiers of 30 end-matched specimens
were cut. Three sets of 20 specimens were matched by placing every third
specimen in a separate set.
Preparation rof Typical Rib-to-Spar Specimens
The parts for typical rib-to-spar fastenings were cut to size and
conditioned to approximately 12 percent moisture content. Parts for not
more than six specimens at a time were taken to the carpenter shop for final
trimming and assembly. These operations were completed in less than two
hours, and finished specimens were returned to the humidity room.
For the cantilever-type specimens of group A, series VIII, 11 sets
of five specimens each were manufactured. Each set contained three spar
Mimeo. No. 1524 -7-
pieces from one plank and two from another. The following parts were
selected at random: gusset plates from two plywood panels; rib parts
from one plank; and corner blocks from a 1/2- by 3-inch board. Fovr
each of spar, rib, and gusset parts were left unglued for moisture
determinations.
The symmetrical corner-block shear specimens of group B,
series VIII were assembled in groups of ten. Consecutive, end-matched
spar parts and consecutive pairs of end-matched rib parts were used in
each group. From five such assembly groups, 10 matched sets of five
specimens were obtained by taking one from each group to make a set.
Four each of spar and rib parts were left unglued for moisture determinations.
Assembly Gluing
The glue used for the assembly of glued shear blocks and typical aircraft joints was a cold-setting, urea-formaldehyde glue commonly
used for glued fastenings in aircraft. This glue was mixed in the proportion of 65 parts of water to 100 parts of dry glue and applied when
not less than 30 minutes nor more than 2 hours old. Maximum assembly
time was 15 minutes.
For glued shear blocks, glue was applied to the 5- by 12-inch
panels, representing the various structural combinations, by means of a
spreader at a coverage rate of 25 grams per square foot. The assemblies
that included Sitka spruce as a component were then cold-pressed for a
minimum of 6 hours at 150 pounds per square inch. Those consisting entirely of hickory were cold-pressed for at least 16 hours at 225 pounds per.
square inch.
The typical rib-to-spar fastenings were assembled in the carpenter shop, using a jig to hold the parts in position while gluing, Glue
was spread by hand brushing, and pressure was applied by means of removable
nailing strips.
Exposure Conditions
Three general methods of conditioning test specimens were
investigated. These were chosen to simulate conditions which might reduce
internal stresses in aircraft structures in service. The desired conditions for each method were attained by the use of conditioning rooms in
which the temperature and relative humidity were automatically controlled
as follows:
Mimeo. No. 1524
-s-
Approximate
:moisture content
attained
Room
•
A(70 0 F., 64 percent relative humidity):
B(80° F., 90 percent relative humidity):
C(80 0 F., 30 percent relative humidity):
Percent
12
18
7
With the exception of series VI, wherein members of unlike
moisture content were glued, all materials were brought to approximately
12 percent moisture content by conditioning in room A prior to assembly.
Immediately after pressing, glued panels were returned to room A where
they were marked for cutting. Specimens were than cut in the carpenter
shop, after which conditioning in room A was continued for a minimum of
weeks. The typical rib-to-spar specimens of series VIII were reconditioned in room A for periods of 14 and 7 days for groups A and B respectively.
First Conditioning Method
All of the structural combinations investigated were subjected
to the first method of conditioning, wherein materials glued at approximately 12 percent moisture content were subjected for various periods of
time to a relative humidity which produced a change in moisture content.
Ths following exposures were used prior to testing for all groups
of series I, II, III, and VIII:
Condition A. (12 percent moisture content).--The reconditioning
period in room A was designated as condition A. One set of specimens from
each group or subgroup was tested after this condition, which was also the
starting point of condition B or C.
Condition B. (12 to 18 percent moisture content).--After condition A, specimens were placed in room B and tested after various time
intervals.
Condition C. (12 to 7 percent moisture content).--After
tion A, specimens were placed in room C and tested after various time
intervals.
Condition BC. (12 to 18 to 7 percent moisture content).--After
condition A, specimens were stored in room B to approximately constant
weight, then placed in room C and tested after various time intervals.
Mimeo. No. 1524
-9-
is
Condition CB. (12 to 7 to
percent moisture content).--After
condition A, specimens were stored in room C to approximately constant
weight, then placed in room B and tested after various time intervals,
To expedite the work, the intermediate conditioning used for
BC and CB was terminated when daily weighing indicated no change in
weight. The duration was varied a few days within groups as required
to maintain a uniform testing schedule. The intermediate exposure for
typical fastenings (series VIII) was 25 days.
With the exception of controls at condition A, one test set of
10 specimens for each group of glued block-shear tests (series I, II, III)
and'for each exposure condition was tested after final conditioning for
the following periods of time:
With hickory as a component All others 2, 4, 7, 14, and 25 days.
1, 2, 7, 14, and 25 days.
One test set of five specimens for each group Of typical ribto-spar fastenings (series VIII) was tested after final conditioning for
the following periods of time:
Cantilever specimens (group A)
Condition B 25 days
Condition C....2, 4, 7, 14, and 25 days
Condition BC. ..... 3, 7, 14, and 28 days
Corner-block shear specimens (group B)
Condition B 28 days
Condition C....3, 7, 14, and 25 days
Condition BC...4, 7, 15, and 28 days
For series VIII, the 25-day tests of condition B also represented 0 days
exposure of condition BC.
Second Conditioning Method (Series V.
Cycles of Moisture Change)
Two structural combinations, flat-saved Sitka spruce glued at
right angles (group B), and quarter-sawed Sitka spruce glued to flat-sawed
hickory (group A) were subjected to cycles of moisture change. For controls,
a set of 10 specimens was tested at condition A. After condition A, the
remaining specimens were exposed alternately to room B and room C. Exposure in each room was for 7 days, since most of the moisture change takes
place within that time and exposures sufficient for equilibrium would prolong the investigation. Exposure in room B and then in room C constituted
one cycle. A set of 10 specimens was tested at the conclusion of each of
9 consecutive cycles. After 10 cycles, the final set of 10 specimens was
placed in room A and tested when constant weight was reached. This was
done to permit comparison with the controls without adjustment for moisture
content.
Mimeo. No. 1524
Third Conditioning Method (Series VI, Gluing
Yembers of Unlike Moisture Content)
Three structural combinations were investigated wherein joints
whose interfaces were at different moisture content at the time of gluing
were conditioned to a uniform moisture content before testing * The influence of other factors was minimized in flat...sawed Sitka spruce (group A)
and in flat..sawed hickory (group B) by parallel laminating end-matched
panels. Panels of the two species were combined for group C.
The members were conditioned in panel size by storing for 2g
days in rooms A, B, or C. Moisture differences of about 5 and 11 percent,
respectively, were obtained for each group by assembling members from
rooms A and C and from room B and C. For control specimens of each group,
both members were taken from room A .• Panels were glued immediately after
removal from the humidity rooms. Upon removal from the press, specimens
were cut from the glued panels and stored in room A for 2 g days before
testing.
Conditioning Specimens for A.S.T.M.
Standard Shear Tests of Wood
All solid wood specimens were conditioned to approximately 12
percent moisture content by storing in room A for 2 g days. The shearing
area of each specimen was measured, and 20 specimens of each group were
tested. Twenty specimens of each- group and for each exposure condition
were then further conditioned by storing in room B or room C for 2 g days
before testing.
Conditioning. Records
Special forms were used to record conditioning and testing dates
of the various test groups. Such a form is shown in figure 5.
Methods of Tests
For the block shear tests of glued joints, the method of testing
designated as Block Shear Strength Test in specification AM-G- g was used.
Specimens were placed in a testing machine eauiDped with a shearing tool
having shearing edges in the same vertical plane, and sheared by compressive loading at 0.015 inch of head movement per minute. The shearing tool,
for this test is shown in figure 1.
The A.S.T.M. standard shear test for wood was used for the solid
wood specimens. In the shearing tool used for this test, the shearing
edges are offset horizontally li g inch, permitting selection of the weakest
plane within the zone thus created, Figure 4 shows the tool for this test.
Mimeo. No. 15214
.11-
The typical rib-to-spar fastenings designed to fail by shearing the gusset from the spar were tested as cantilevers with the spar ,
fixeciandthe load applied to the top edge of the rib at a point 9 inches
from the spar. To prevent crushing, the load was applied through a 2inch steel block, and to prevent buckling, oak stiffeners were bolted
to the ena of the rib. The rate of head movement was 0.05 inch per minute. This method of test is illustrated in figure 6.
The typical rib-to-spar fastenings designed to fail at the
corner-block attachment were tested by shearing the corner blocks from
the spar or rib members under compressive loading. The load was applied
through a spherical head to the top of the spar piece while the specimen
wad supported by the bottom edges of the ribs. An adjustable hardwood
frame prevented the ribs from spreading, thus minimizing the bending
moments at the joints. Since the shape of the assembly was altered by
moisture changes, the loading surfaces of the specimens had to be planed
before testing to obtain uniform bearing. The rate of head movement used
for this test was 0.015 inch per minute. Figure 7 illustrates this method
of test.
Moisture Determinations
For the block-shear glue-joint test, moisture determinations
were made from the matched pieces cut for this purpose, These pieces were not glued but were conditioned with the groups of shear specimens
they represented, They were measured and weighed at the conclusion of
each humidity condition to which the group was subjected. When a set of
10 shear blocks was tested, the moisture content, dimensional change, and
specific gravity were determined for a free sample of each component.
Specimens for moisture and specific gravity determinations were
cut from the parts of the typical rib-to-spar fastenings at the time of
test. For those designed to fail by shearing the gusset from the spar
(group A) the piece was cut near the joint from the top 1/2 inch of the
spar piece, while for those designed to fail at the corner block attachment (group B), the piece wps cut from the member in which failure occurred.
In addition, records were kept of the dimensions and moisture content of
full-sized unglued parts to determine the rate of change during each conditioning,
nxplanation of Tables and Charts
The relationship between shearing strength and time of exposure
is shown on a separate graph for each of the four humidity conditions used.
For each structural combination investigated, the four graphs are included
on each of figures g through 19. On these graphs, the shearing strengths
are expressed as percentages of the average value of the controls (0-day
exposure). The individual test values have been plotted to show the effect
Mimeo. No. 1524
of moisture changes on their distribution, while a solid line connects
the average shearing strength of each set of 10. On the graphs of con-
ditions BC and CB, where an intermediate conditioning room was used, the
applicable points are duplicated from the results of tests made in that
room.
On each graph, a dotted curve shows the computed strength of
the solid wood in which failure occurred as determined from the moisture
changes of unglued specimens during exposure intervals. For this computation, the percentage change in strength for each 1 percent change in
moisture content was taken from table 2-.2 of ANC Bulletin 18, "Design of
Wood Aircraft Structures" 4 Since the relationship between moisture content and shearing strength at angles other than parallel to the grain is
not included in this table, it was assumed to be the same. The average
shearing strengths of sets of 10 specimens are recorded in table 1 for
each of the 12 structural combinations. The percentage of wood failure
is also shown in this table for those groups wherein glue failures occurred. The distribution of failures between yellow birch plywood and
Sitka spruce is shown graphically in figure 20 for those groups of specimens in which these materials were combined.
The average change in width and in moisture content of free
specimens conditioned with the shear blocks of table 1 is shown in table
2. These data are shown in graphic form (figs. 21 through 24) for each
of the materials used. Each figure is divided into four parts, showing
the rate of moisture and dimensional change for each of the four conditioning methods used. The dimensional change of each solid wood specimen was adjusted to the average specific gravity of its group. Each
plotted point represents the data for a component of one test group, and
the curve best representing the rate of change is shown on each graph.
The dimensional change of the yellow birch plywood was too small for
accurate measurement by the methods used, but the average of the four
groups, including various grain directions, is shown for comparison in
figure 24.
The effect of cycles of moisture change on the shearing strength
of the two structural combinations so investigated is shown in figures 25
and 26, On these graphs, the shearing strengths are expressed as percentages of the average of the controls (0 cycles). Each point represents one
test value and a solid line connects the average shearing strength at each
cycle. Beneath these, the moisture content Of the free specimens at each
point of change is shown, The average shearing strengths, together with
the moisture and dimensional change data of unglued specimens, are shown
in table 3.
Table 4 presents the average shearing strengths for the three
structural combinations investigated whose members were at unlike moisture
content when glued, together with the moisture and dimensional change data
of the free specimens at the time of gluing and at test.
Mimeo. No. 1524
-13-
The results of the A.S.T.M. standard shear tests of solid Sitka
spruce and hickory specimens at various moisture content values are plotted on semilogarithmic graph paper in figure 27. For each species, a
straight line shows the strength-moisture relationship applied to the
shearing strength values of U. S. Dept. Agr. Bulletin No. 479, corrected
to the average specific gravity of the test specimens.
Figures 25 and 30 show the results of tests for the two types
of typical rib-to-spar fastenings. On each figure the ultimate loads,
expressed as percentages of the average value of the controls, are plotted
against the time of exposure for each of two conditioning methods. The
moisture and dimensional change record of free rib and spar parts for the
two types of specimen are shown in figures 29 and 31. The average ultimate
strength of test specimens at each time of exposure are recorded in table 5.
The specific gravity values of materials used are shown in table 6.
`Discussion of Results
Moisture and Dimensional Change Data
Unavoidable variations from constant relative humidity in the
conditioning rooms are reflected in the moisture change data for each
group. These variations overshadow the variations in moisture-change
properties of the material. The dispersion of points at each test interval is about the same whether the points represent one specimen or the
'average of several. When several groups were conditioned nonconcurrently,
however, the average moisture content, plotted against time of exposure,
produced a smooth curve.
The moisture-time curves (figs. 21 through 24), show that only
quarter-sawed Sitka spruce came to approximate equilibrium within the 25day maximum exposure used at SO 0 F., 90 percent relative humidity. The
rate of moisture change is more rapid when decreasing, and all materials
came to approximately constant weight after 25 days at 50° F., 30 percent
relative humidity. Moisture change takes place more rapidly in quartersawed Sitka spruce than in flat-sawed, but in hickory no difference was
found between types of cutting.
Yellow birch plywood reared an equilibrium moisture content of
5.5 percent under relative humidity conditions producing 12 percent moisture
content in solid rood. The range in moisture content then relative humidity
conditions are changed, however, is about the same as for solid wood.
The ratio of dimensional change (across grain) to moisture change
remains nearly constant throughout the 25-day exposure periods for both
Sitka spruce and hickory.
Tests of Solid Wood
The results of tests of solid Sitka spruce at various moisture
contents in the A.S.T.M. standard shear test for wood agree closely with
Mimeo. No. 1524
the established strength value and moisture-strength relationship for
this species. The test results for hickory are in fair agreement.
Control Specimens
Shearing strength values for wood have not been established
for the block-shear strength test of specification AN-G- g . Specimens
not subjected to moisture change were tested to provide control values
for each structural combination. This method of test results in higher
values than the A.S.T.M. standard shear test for wood, and the difference between flat and quarter-sawed material is more pronounced. When
unlike materials are joined, failure occurs in the weaker component, but
its shearing strength thus determined appears to vary with the material
to which it is glued.
The average shearing strengths of control sets of 10 specimens
each are shown in table 7.
In table 7, the average values of the two matched sets of controls of the groups of series I, II, and III afford an estimate of the
variation which might be expected between the test sets in any group.
The greatest difference between matched sets was 6.9 percent and the
average difference was 2.S percent.
Since moisture was added to the wood at gluing, specimens reconditioned to the 12 percent moisture content at which they were glued
cannot be assumed to be stress-free. In flat-sawed hickory glued at
right angles (group I B H), the low control strength obtained and its
failure to decrease with increase in moisture content to 17 percent indicate that the shearing strength of the controls has been reduced by stresses
thus induced. The effect of such stresses is not apparent, however, for any
other structural combination studies.
For structural combinations where shearing takes place parallel
to the grain, the individual values of control specimens fell within a
range of 30 percent, with the lowest values 15 percent below the average.
For shear perpendicular, individual values were much more variable, with
the lowest about 25 percent below average.
It was not the intention of this investigation to test glues,
and the gluing technique used was intended to develop the full strength of
the wood. In all of the materials used for glued shear blocks, excepting
flat-sawed hickory, the failures were virtually 100 percent in the wood
under all conditions. From three panels of flat-sawed hickory, all of the
specimens were rejected because of the high percentage of glue failures
under all conditions, leaving S or 9 specimens to a set for group I A H.
While partial glue failures occurred in this group, the average shearing
strength of the controls appeared to be approximately that of the wood.
Mimeo ! No. 1524
-15..
Condition B. Block-shear specimens glued at
12 percent moisture content, subjected to
90 percent relative humidity at g O° F.
With the exception of flat-sawed hickory glued at right angles
(group I B H), for which the control value was lowered by induced stresses
in gluing, the shearing strength of each of the structural combinations
studied, when exposed to 90 percent relative humidity at $00 F., decreased
approximately the same as would be expected for solid wood under the same
moisture change. All combinations of Sitka spruce and yellow birch plywood followed the moisture-strength curve of yellow birch. When the face
grain of both was parallel to the direction of loading, the strength of
the cross plies was a controlling factor.
The average shearing strength, when plotted against time of
exposure (to g0° F., 90 percent relative humidity) produces a smooth curve
with little variation from computed solid rood values for the following
groups:
I A
Flat-sawed
I A H Flat--,sawed
I C H Flat-sawed
III A Flat-sawed
III B Flat-sawed
III C Flat-sawed
Sitka spruce, 00.hickory 0°
hickory 0° Sitka spruce 0° Sitka spruce 0 0 Sitka spruce 0°
Flat-sawed Sitka spruce 0°
Flat-sawed hickory 0°
Quarter-sawed hickory 0°
Yellow birch plywood 0°
Yellow birch plywood 45°
Yellow birch plywood 90°
The remaining combinations agreed fairly well at 2 g days, but
varied from the computed curve during earlier exposure, These variations
were, for the most part, within the deviation that might be expected between sets of specimens. The greatest variations were in flat-sawed glued
to quarter-sawed Sitka spruce 0 0 (group I C).
There is no apparent correlation between these variations and the
magnitude or direction of the difference in dimensional change properties
between members of the various groups. During exposure to 90 percent relative humidity, none of the test values fell appreciably below g 5 percent of
the computed strength of solid wood at equilibrium moisture content.
For all of the combinations tested, the dispersion of individual
test values decreased to an optimum of about one-half that of the controls
at an exposure of 2 to 7 days. After this point was reached, the dispersion increased somewhat but did not exceed that of the controls.
No visible damage to the glued joints was observed during conditioning of specimens to increasing moisture. Warping occurred in unlike
solid wood combinations and corresponded roughly in extent with the difference in dimensional change properties of the members (fig. 32). Parallellaminated specimens of the same species and those combining wood with ply.rood did not warp appreciably.
Mimeo, No. 1524
-16-
Condition CB. Block shear specimens glued at
12 percent moisture content, conditioned to
7 percent and subjected to 90 percent relative humidity at SO° F.
The influence of this exposure on the shearing strength was
essentially the same as for condition B. Again variations occurred
during exposure, but they did not parallel those of condition B, and
similarities between groups did not recur. An optimum range of test
values ras at 4 to 14 days.
In two, groups where the difference in dimensional change
'properties was parallel to the direction of loading [flat–sawed Sitka
spruce glued at right angles (group I B), and flat–sawed Sitka spruce
90° glued to yellow birch plywood 90° (group III D)] the shearing
strengths hero 10 percent lorrer than for condition B at 2S days.
Because of the relatively lor shearing strength and wider dispersion
of control values for shear perpendicular to the grain, however, this
difference may not be significant.
No visible damage to the joints was observed during conditioning, and warping of the specimens r y as the same as for condition B.
The recovery in shearing strength of flat–sawed hickory glued
at right angles (group I B H) indicates that the greater part of the
reduction in strength that occurred in condition C was due to internal
stress rather than to permanent damage.
Condition C. Block shear specimens glued at
12 percent moisture content, subjected to
30 percent relative humidity at SO° F.
The dispersion of test values was increased by low relative
humidity, and became greater with duration of exposure. For those
combinations that involved shear parallel to the grain, the dispersion
of test values about doubled that of the controls, and the lowest values
were often about the same as the lowest values of condition B.
The test values of flat.-sawed hickory glued at right angles
(group I B H) were greatly dispersed at all time intervals. For combinations in which Sitka spruce vas sheered perpendicular to the grain, the
dispersion of test values under this exposure wasabout the same as the
relatively ride distribution of the control values. Because of this dispersion, variations between test sets under this condition become less
significant.
Of the 12 combinations investigated, on1; ,, those two including
quarter–sawed Sitka spruce; glued to flat–sawed Sitka spruce (group I C),
and glued to flat–sawed hickory, (group II B), increased in shearing
strength as would be expected for solid wood for the same change in moisture content.
Mimeo. No. 1524
The shearing strength remained approximately at the control.
value while the moisture content . decreased from 12 to 7 percent for
eight of the groups, including those having no difference in shrinkage
between components, as follows:
II A Flat-sawed Sitka spruce 0° - Flat-sawed hickory 00
I A Flat-sawed
I. A H Flat-sawed
I C H Flat-sawed
III A Flat-sawed
III B Flat-sawed
III,C Flat-sawed
I B Flat-sawed
Sitka spruce
hickory 0°
hickory 0°
Sitka spruce
Sitka spruce
Sitka spruce
Sitka spruce
0° - Flat-sawed Sitka spruce 0°
- Flat-sawed hickory 0°
- Quarter-sawed hickory 0°
0° - Yellow birch plywood 0°
0° - Yellow birch plywood 45°
0° - Yellow birch plywood 90°
0° - Flat-sawed Sitka spruce 90°
The shearing strength of flat-sawed Sitka spruce glued to
yellow birch plywood with face grains of both perpendicular to the
direction of loading (group III D), was decreased by 20 percent.
Under parallel loading (group III A), the reduction in numerical value
of unit strength of this combination was about the same (90 pounds)
but this represented only 6 percent of the controls. The greatest
reduction in average strength under this condition was about 30 percent
for flat-sawed hickory glued at right angles (group I B H).
The greatest effect on shearing strength was usually found
at the first test period after exposure, when the reduction in strength
equalled or exceeded that of the 2 g-day period. This immediate reduction, or failure to increase, was pronounced in several groups as I C H
or I B H, and appeared in all except quarter- to flat-sawed Sitka spruce
(group I C).
There is no apparent correlation between the effects of exposure to low relative humidity on the shearing strengths of various structural combinations and the difference between the shrinkage properties
of their components.
Warping occurred in all specimens combining solid woods of
different shrinkage properties, but to a lesser extent than under condition B. No visible damage to the joints during exposure was observed in
specimens having Sitka spruce as a component. In many hickory specimens
combining flat '-sawed to flat-sawed or quarter-sawed to flat-sawed, a
small crack appeared in the ena of the joint within 1 day but disappeared
upon continued exposure. After testing, small crescent-shaped areas
resembling glue failure could sometimes be distinguished on the sheared
faces. Specimens of flat-sawed hickory glued at right angles (group I
B H) cracked at the edges shortly after exposure, and these cracks increased in size and number with time of exposure. The stress was partly relieved by end checking in some of the specimens. Figure 33 shows
cracking and checking of this group after 4 days at condition C.
Mimeo. No. 1.52.
Condition BC. Block shear specimens glued at
12 percent and conditioned to -I g percent
moisture content, subjected to 30 percent
relative humidiV at goo F.
For most of the groups, the effect of this exposure was
essentially the same as that of condition C, and the test values
are similarly dispersed. The immediate decrease, or failure to
gain, in shearing strength was less pronounced and does.not appear
at all in half of the groups. Quarter-sawed Sitka spruce, when
glued to flat-sawed Sitka spruce (group I C), again followed the
moisture-strength relationship of solid wood, but, when glued to
flat-sawed hickory (group II B), it tended to follow this relationship only until the control strength was reached, after which the
shearing strength declined to g 0 percent of the controls at 2g days.
Quarter-sawed hickory glued to flat-sawed hickory (group
I C H) was improved by the intermediate conditioning, increasing
in strength by 10 percent. In flat-sawed hickory glued at right
angles (group I B H), glue failures were reduced, and the extremely
low values of condition C did not recur. Thus the average shearing
strength remained substantially the same as that of the controls.
Under this exposure, the specimens recovered or nearly
recovered from the warping caused by the intermediate conditions.
Cracking of the joints in hickory specimens was similar to condition C, but was less extensive for those glued at right angles
(group I B H).
Block Shear Specimens, whose Members were at
Different Moisture Content when Glued,
Tested after Conditioning to Uniform 12
percent Moisture Content
While results of the shear tests vary somewhat from the
controls, as many test sets showed improvement as decreased in
strength. No relationship exists between changes in shearing
strength and the differences in potential dimensional changes induced by moisture inequality. Since sets of specimens could not
be matched as in' the previous series, differences were probably
due to variability in tha material.
All specimens combining members of unlike moisture content
warped when brought to uniform moisture content. The extent of such
warping corresponded roughly to the difference in moisture content
and the dimensional change properties of the components.
Mimeo. No. 1524
Block Shear Specimens Subjected to Cycles of
Moisture Change
For specimens tested at the lower moisture content (7 percent)
at the conclusion of the first cycle, the shearing strength of quarter—
sawed Sitka spruce glued to flat—sawed hickory (series V A) increased
about 5 percent, while those of flat—sawed Sitka spruce glued at right
angles (series V B) decreased about 10 percent. For both groups, the
shearing strength (at the lower moisture content) after each cycle gradually declined during nine cycles to about 80 percent of that of the
controls. The average shearing strength of specimens restored to 12
percent moisture content after being subjected to 10 cycles was also
about 80 percent of the control value.
Typical Fastenings Subjected to Moisture Change
Specimens of group VIII A (fig. 3) using both gusset plate
and corner block attachments were designed 'to fail by shearing' the
gusset fromAhe spar when tested as cantilevers (fig. 6). When specimens glued at 12 percent moisture content were subjected to 30 percent
relative humidity at 80° F. (condition 0), the average ultimate strength
declined to 60 percent of the control value in 28 days, when a moisture
content of about 7 percent was reached.
For specimens conditioned to 18 percent moisture content, the
ultimate strength decreased to 70 percent of the control value. When
corresponding s p ecimens were subsequently subjected to 30 percent rela.
tive humidity at 80° F. (condition BC), the ultimate strength increased
as the 12 percent moisture content (condition at gluing) was approached
and then declined to 65 percent of the control strength at 28 days and 7
percent moisture content.
Three of the control specimens failed by shearing the gusset
from the spar while the other two failed at approximately the same loaa
by shearing the gusset from the rib: The strength of the gusset—to—spar
joint decreased during the earlier test periods. As the moisture content
was further lowered, the differential shrinkage of the assembly produced
visible cracking of the corner—block joints causing most of the specimens
to fail in shear at the corner blocks and in tension perpendicular at the
gusset plate glue joints. These partial failures that occurred in the hu
midity room, and which were entirely in the ribs and spars, were greatest
at the ends and decreased toward the centers of the corner blocks. Differences in shrinkage of the portions of members thus left unrestrained caused
distortion and sometimes cracking of the gusset joints. A few corner—block
joints cracked throughout their length. One of these is shown in figure 34.
The symmetrical rib—to—spar specimens of group VIII B (fig. 2)
were tested in such a manner as to cause shearing at the corner block joints
(fig. 7). Under condition C, the average shearing strength declined to 4o
percent of the control value in 28 days of exposure, while the moisture content changed from 12 to 7 percent.
Mimeo. No. 1524 —20—
For specimens conditioned to 1S percent moisture content, the
average shearing strength after 2S days of exposure was 70 percent of that
of the controls. When these were subsequently subjected to 30 percent
relative humidity at 80° F. (condition BC), the shearing strength increased
as the moisture content approached 12 percent and then decreased to 50 percent of the control value at 2S days and 7 percent moisture content. Upon
reaching this moisture content, all the glued joints showed visible creel&
ing, and three of the five specimens had at least one joint completely open.
One of these is shown in figure 35.
Conclusions
OnAx one type of glue, cold-setting urea–formaldehyde, was used
throughout the tests. This glue developed sufficient strength to cause
wood failures in practically all tests. It is to be presumed that, so far
as the effects of internal shearing stresses are concerned, similar results
would have been obtained with any efficient glue, the use of which would
not modify the properties of the wood.
Influence of Moisture Changes
on Internal Stresses
The methods of investigation adopted for this study furnish a
means of determining the extent to which the load required to produce
shearing failure at a glued joint has been influenced by internal stresses
induced by moisture changes. They do not provide quantitative measures of
the magnitude or distribution of such stresses.
The difference between the shrinkage properties of the joined
members was used as a basis to provide varying degrees of internal stress
for a given change in moisture content, and hence, varying effects on
shearing strength. The results of tests on standard–sized shear blocks,
however, indicate that while internal stresses were created, their influence on the shearing strength was not related directly to the difference
in shrinkage.
When the moisture content of a glued assembly was changed, the
presence of internal stresses, not directly related to the potential difference in dimensional change, was indicated by the results of tests on shear
blocks in the following ways:
(a) When the moisture content was increased
above that at which the specimens were
glued, those combining unlike materials
were visibly warped, but the shearing
strengths of all combinations decreased
only in accordance with the moisture–
strength relationship for solid wood and
with no apparent reduction due to internal
stresses.
Mimeo. No. 1524
–21–
(b When the moisture content was decreased
below that at the time of gluing, warping was less pronounced, and the effect
of internal stresses offset the gain in
strength of the wood which would normally
accompany a reduction, in moisture content.
Combinations that had no difference in
potential dimensional changes between
members were affected in like manner. For
combinations in which a difference in potential dimensional changes existed, the
effect was similar whether the difference
was parallel or perpendicular to the direction of loading. Where the data indicate
the existence of internal stresses of considerable magnitude, there is no evidence
that the influence of these stresses was
reduced appreciably during the 2 g-day period
of exposure.
(c) For a short period of exposure to a relative
humidity that lowered the moisture content
of the wood, the effect of internal stresses
associated with a maximum moisture gradient
equaled or exceeded the corresponding effect
at a longer period of exposure, when moisture
equilibrium was attained but when dimensional
changes were greatest.
The effect of moisture changes as determined on standard-sized
shear test specimens cannot be safely projected to glued joints of larger
size. There is evidence that the effects are more pronounced in larger
joints. The greatest effect.of internal stresses in the larger joints
tested occurred when, the moisture content was decreased below that of
gluing. This effect was only partially offset by the increase in the
shearing strength of wood due to the decrease in moisture content.
The effect of internal stresses on the shearing strength of a
glued joint can be expected to be reduced by gluing at a relatively low
moisture content. This, however, entails the risk of distortion of the
assembly, since the potential upward range in moisture content is thereby
increased.
Effect of Gluing Members of
Different Mo3.9t1..r3 Ciol..tents
The glued shear blocks composed of members originally at different moisture content values warped when brought to uniform 12 percent moisture content, but the shearing strength apparently was not affected by in
ternal stresses.
Mimeo. No. 1524
-22-
Influence of Alternations in
Moisture Content
The shearing strength of glued joints between unlike materials
declined slowly with cycles of alternating high and low moisture content.
For the rather extreme combinations investigated, the shearing strengths
of standard shear blocks were reduced by about 20 percent after 10 such
cycles.
Failures Due to Moisture
Changes
Internal stresses at a glued joint resulting from changes in
moisture content that occur in service can become of sufficient magnitude
to cause partial or complete shearing failure in the wood, permanently
reducing the strength of the joints. Although the block shear specimens
of some combinations probably approached this condition, no actual failures of this kind were observed. In both types of rib-to-spar fastenings,
however, corner blocks 6 inches long were partially sheared from quartersawed Sitka spruce when the moisture content was reduced 5 percent below
that of gluing. A 5 percent increase in moisture content caused no permanent damage.
Applicability of Results of the Block
Shear Strength Test of Glued Joints
The shearing strengths of glued joints aa determined by the
block-shear strength test of specification AN-G .4 are not readily comparable with the shearing strenghts of wood established by the A.S.T.M.
standard shear test. The results obtained in the AN-G- g test were consistently higher. Furthermore, although shearing failure in a joint between
unlike materials occurred consistently in the weaker wood, the indicated
shearing strength was apparently influenced by the elastic properties of
the stronger wood. The shearing strengths of glued block shear specimens,
presented in this report, therefore, should be used for comparative purposes only and should not be regarded as design data.
Mimeo. No. 1524
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specimens tested by the block-shear strength
test of specification AN-G-8
Flat-sawed Sitka spruce, parallel to grain
Series
and group
Glued to
Yellow birch plywood, 90°
Yellow birch plywood, 45°
Yellow birch plywood, 0°
Flat-sawed Sitka spruce, 0°
Flat-sawed Sitka spruce, 0°
Flat-sawed hickory, 0°
Flat-sawed hickory, 0°
III
III
III
I
VI
II
VI
C
B
A
A
A
A
C
Shearing strength
Av.
Set 2
Set 1
Pounds Per Square Inch
1,137
1,151
1,33g
1,550
1,559
1,679
1,712
1,94g
1,430
1,144
1,384
1,52g
1,623
1,539
1,7 52
1,732
1, 94 g
1,591
1,679
Quarter-sawed Sitka spruce, Eprallel to grain
Flat-sawed Sitka spruce, 0° Flat-sawed hickory, 0 0
Flat-sawed hickory, 0 0
I C
V A
II B
1,299
1,41g
1,54g
1,294
1,540
1,296
1,41g
1,544
Flat-sawed Sitka spruce, perpendicular to grain
Yellow birch plywood, 90°
Flat-sawed Sitka spruce, 0°
Flat-sawed Sitka spruce, 0°
III D
V B
I B
463
495
480
459
...
512
461
495
496
Flat-sawed hickory, parallel to grain
Flat-sawed hickory, 0°
Flat-sawed hickory, 0° I A H
VI B
Quarter.-sawed hickory,
Flat-sawed hickory, 0°
ICH
2,74g
2,481
2,632
2,690
2,4g1
arallel to grain
2,264
2,215
2,240
Flat-sawed hickory, perpendicular to grain
Flat-sawed hickory, 90°
Mimeo. No. 1524
I B H
911
933
922
GLUE JOINT
SOL/0 WOOD GLUED TO SOLID WOOD
PLYWOOD GLUED TO SOLID WOOD
DETAILS gE TEST SPECIMENS
OIL HOLE
SELF-ADJUSTING
BEARING
TEST SPECIMEN
5£CT7ON AA
Figure 1.--Shearing tool and test specimens used for the block-shear strength
test of glued joints.
Z M 58446 F
Figure 2.--Specimen used tor the determination of shearing strength of the
corner-block joints of rib-to-spar fastenings.
Figure 3.--Cantilever-type specimen for the determination of strength of
typical rib-to-spar fastenings.
2 M 58447 F
TEST PIECE
TANGENT/AL
RAD/AL
S/TKA SPRUCE
HICKORY
DETAILS OF TEST SPECIMENS
Figure 4.--Shearing tool and test specimens used for the A.S.T.M.
standard shear test for wood.
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specimens glued at 12 percent moisture content.
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specimens glued at 12 percent moisture content.
Series I, Group B-- FLAT-SAWED. SITU SPRUCE glued to FLAT-SAWED SITS/1 SPRUCE
With grain of faces at right angles.
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Figure 11:I.—Effect of exposure to high or low relative humidity on the shearing strength of block
shear specimens glued at 12 percent moisture content.
Series I, Group C-- QUARTER-SAWED SITKA SPRUCE glued to EL'IVEDSITKII. SPRUCE
with grain of faces parallel to the direction of loading.
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40
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STRENGTH OF CONTROL SPECIMENS ADJUSTED FOR
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<>MOISTURE CONTENT
LESS THAN 80 PERCENT WOOD FAILURE
n
0 VALUES FROM PRECEDING GRAPH
nu
nIN
: 4101p. 11171.11.Mr
,
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1
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60
•
7
et
tkiQ0
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k "
c, cc
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0
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/5
90 PERCENT RELATIVE HUMIDITY
25
20
TIME (DAYS)
30
35
"4://aM
Figure 11.--Effect of exposure to high or low relative humidity on the shearing strength of block
shear specimens glued at 12 percent moisture content.
Series T, Group AH-- FIAT-SAWED HICKORY glued to FLAT-SAWED HICKORY with
grain of faces parallel to the direction of loading.
Z PI 58454 P
40
45
/VU
/40
,
•
12
E .011L'Elli
. /00 n
— ..i.--mmm
. s3, A
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o
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60
4
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ION
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n
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0
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n
SPE
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CONTENT
--
0
0
0
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0
0
0
c507:2 30 n4
0
80 F
RELATIVE 6 vor.
5
10
/WENS ADJUSTED FOR
--STRENGTH OF CONTROL
MOISTURE CONTENT
CORRESPONDING CHANGE
—
<>MOISTURE
o VALUES FROM PRECEDING GRAPH
I
I
I
I
I
1
I
1
►
90 PERCENT RELATIVE HUMIDITY
15
r
20
Z5
30
i1.111WArlid
35
TIME (DAYS)
Figure 12.--Effect of exposure to high or low relative humidity on the shearing strength of block
shear specimens glued at 12 percent moisture content.
Series I', Group BD-- FLAT-SAWED HICIDART glued to FIAT-SAWED HiCKDRA with
grain of faces at right angles.
.2 14 58455 F
45
sti
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100
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t,4
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in
1n1=111nn =1•101iLW'''-
1
111.111111111011 —1
Il
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II
110
8
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.
11
ill
• PERCENT REG4TIvi∎
HUMIDITY
eo F 90
70
{.,J ,vv
11
Ull8
n
.
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•
1......___
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Erma
90
n
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n
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n III
c44t crl 80 r illkiliVall.
ni
CONDITION BC
4. rt
•
CI
1, 70
80'F,
90
PERCENT
Z cl,
In 80•F. 30 PERCENT RELATIVE HCIMICITY
. . RELATIVE 1-16404/0/7- y
g
41
4
1"
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pi
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III."- 0 — 11111111111111
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1 z 100
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(LI lr)
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n
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"i
1
1
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1011
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to
b
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CONDITION CB
80.F 90 PERCENT RELATIVE HUM/D/TY MEW— .
AthimermirammursrlialM
/5
25
5
20
BOZYt,
RH
30
55
TIME (04V5)
Figure 1.1.--Effect of exposure to high or low relative humidity on the shearing strength of block
shear s pecimens glued al 12 percent moisture content.
Series I, Group CII-- (TPA/TIM-SAWED PTCHMRY glued to FTAT-SAWED PTCKORT with
grain of faces parallel to the direction of loading.
Z M 58456 F
n
n
40
45
r
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0
40.
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MEM
IMMI!
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n••
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LEGEND:0
AVERAGE TEST VALUE
ul
0
o
- --STRENGTH OF CONTROL SPECIMENS ADJUSTED FOR
0
/20
CORRESPONDING CHANGE IN MO/STORE CONTENT
<> MOISTURE CONTENT OF SITHA SPRUCE •
0
8
cl, 110
VALUES FROM PRECEDING GRAPH
0
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n
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30
PERCEN
RELATIVE 1-101,11DI
<---lj> I
0
.
S
1
6'`..."
l0
15
25
20
TIME (04V.5)
J
30
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35
Figure 11.--Effect of exposure to high or low relative humidity on the shearing strength of block
shear specimens glued at 12 percent moisture content.
Series IT, Group A-- FLAT-SAWED SITRA SPRITE glued to FLAT-SAWED VICWORY with
grain of faces parallel to the direction of loading.
Z 14 58457 F
n
n
40
45
MEE
III
nI
III
il
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ME nIII
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tic,
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441111
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CONDITION B C
60
`ct
00 •P, 90 PERCENT
ELATivE NUMPITY
et
Z kJ
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MIlliaillM=
=
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II nII iii
i
1.1
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GONDITION G
60*F. 30 PERCENT RELATIVE HUMIDITY
mil
LEGEND:-
•
El
111
PM
what
M MN
AVERAGE TEST VALUE
.
OF CONTROL SPECIMENS ADJUSTED FOR
--- — STRENGTH
a
<>.
0
CORRESPONDING CHANGE IN MOISTURE CONTENT
M0/5TURE CONTENT OF SITKA SPRUCE
VALUE,5 FROM PRECEDING GRAPH
1111M1111
MEIN
El 1111111gEMELPINEE-1111 •
III ME
EMI Mil
CONDITION CB n
111
1111
=MEM
MN
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IN err 30 PERCENT RELATIVE
81)
ie
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806 F., 30 PERCENT
LAT1vt m/Pii0IT
Z.4 il t
0
BO • ff. 90PERCENT RELATIVE HUM/D/TY
10
15
25
20
TIME (DAYS)
30
55
Figure 15.--Effect of exposure to high or low relative humidity on the shearing strength of block
shear specimens glued at 12 percent moisture content.
Series II, Group B-- qUARTER-SAWED SUPRA :WHITE glued to FLAT-SAWED HICKORY
with grain of faces parallel to the direction of loading.
'Z N 58458 F
40
45
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•
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iii.
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ME
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ril
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ram
Lk!
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Q Bo
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o
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n
n
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Rpm.
CONDITION
4
e
60 P. 90 PERCENT RELATIVE HUM/D/TY
-41;'''
44I-tb• n •nn==nniin iimon=wm...
111 111111111111113
• t
110
111
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posmonmi-imi
CONDIT/ON BC
8
•.
pp., 96 P£ LI r
RELATIVE HOW/Ob rY
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dr", 30 PERCENT RELATIVE HUMIDITY
'',:ltimminimmiim•nlimiLi142
. n••nnn=1101•1nnnn•••.-M
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ct
e
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n
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441'5
CV
II
0
70
ill n
0
.
AVERAGE TEST VALUE
OF CONTROL SPECIMENS ADJUSTED FOR
CORRESPONDING CHANGES //V MOISTURE CONTENT
<> MOISTURE CONTENT OF YELLOW BIRCH PLYWOOD
VALUES FROM PRECEDING GRAPH
NI i 0
mosigui
=
iMMIIMMn01.1
111.111LE
..
/0
=
__STRENGTH
•
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11111•EililltEEN
5
=
LEGEND:-
80'F, 30 PERCENT
RE LATIvE hi cimlofTV
5?.. (x.
'''
III n
III
CONDITION 4:
1
8 0 r. ..3c/ PERCENT RELATIVE h'UM/DITY
7944-.111.
1,1n1•1=1=.
414M
doillelMIN
MINI
MIMn
/30 =BM
MM iniiiME
/20
El
•
lii
ER
im
—1.10-1
E
nn
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IS
Ell
CONDITION CB
3
Im
•
80.f, 90 PERCENT RELATIVE HUMIDITY
..—
UMINIMEMENNIIMMIMMUIL
d.N.
20
25
30
35
40
T/ ME (DAYS)
Figure 16.--Effect of exposore to high or low relative humidity on the shearing strength of block shear
specimens glued after attaining equilibrium moisture content at 7n° F. and f4 percent
relative humidity.
Series III, Group A-- FLAT-SAWFO °TM sPftat glued to 0.flun-Wn SWIJTM
PLYWOOD with grain of faces parallel to the direction of loading.
Z M 58459 F
110
45
e4
11
.n•
100
al
•
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LIIn
;.'El
V
•
11111111
1
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, III
1!
90
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k8
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all..1
a
==MIIIIIIMMINMEMI 1= i
III
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MI
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Mr 11M111
n
11 n
WPM III
ENE
n111111
1111151
U
1IiInlin I
,z , , NMI
r:. k <EP
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vc:, Ell
CONDITION 8
60' F 90 PERCENT RELATIVE HUMIDITY
//0 m
;
,kl. ) Q 8 0
0
t
eo•
CONOIT/ON BC
MIn 11
490 .E SO PERCENT ;EL_ATIVE HUMIDITV
/000 -113
9 0 gillEZ
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11221..
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o ,44
et, 64ciiM
0
80 F SO PERCENT RELATIVE HUMIDITY MOM
III
n
n
ME=
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1111111
0
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F
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90
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LLI "c
c)
80
70
n
11
67
0
•
VALUE
--STRENGTH OF CONTROL SPECIMENS ADJUSTED FOR
CORRESPONDING CHANGES .IN M0/.5TURE CONTENT
<>MO/Sri/RE CONTENT OF YELLOW BIRCH PLYWOOD
VALUES FROM PRECEDING GRAPH
D\ 0
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11110
CONDITION CB
ME:kill".
n111=1111MINIRE — — III
an
0.
et
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,.'. 4-
n
LEGEND ,ni
immenin
AVERAGE TEST
120
vl
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ri
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MN
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110
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o
F , 90 PERCENT
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bo
1 20
,
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L v 7 0 EIMIA
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1
11111116mimitium.....m.n
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t 0
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nn
10 0 tli.
till
erFA)prfect-NrRELAT/yE HUMIDITY 04
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20
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25
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30
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40
Figure 17.--Effect of exposure to high or low relative humidity on the shearing strength of block shear
specimens glued after attaining equilibrium moisture content at 70° F. and rvF percent
relative.humidity.
series III, Group
7I11,Mr Pliwym
FLAT-QARIM SIM% Si q iTA glued to
with grain of faces at 45' and grain of Sitka spruce parallel to the
direction of loading.
M 58460 F
45
100
Ell
110.......1111151.1
91)
80 IN
k ' 70
cc
R''cJ 6
'kJ 0-
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$
q
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' k
5., , RELATIVE HUMIDITY
n
CONDITION BC
=
n
80.
ME
,: q , iimp
_ --- III -ENE
LEGEND
ak
100 MI
. .mENIIMAIMOMMEM
cs,
0
q
cg i
s
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LkJ k
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L,J,c: to
ii
0
n
II COND/T/ON C
.....
S.P
AVERAGE TEST VALUE
III
STRENGTH OF CONTROL SPECIMENS ADJUSTED FOR
/N M0/57- 1/RE CONTENT
<>M0/5TURE CONTENT OF YELLOW BIRCH PLYWOOD
CORRESPOND/NC CHANGES
Ilmilliquimgmiliali
0
VALUES FROM PRECEDING GRAPH
lb
I n
CONDITION C3
.111
80°F, .30 PERCENT
EtArive HUMIDITY
II
IIII
Milnd
•n •C''-4 SO
IIIIII
1111
cN.
110
II
•
IFIIIINIMINIIIIIIIII ____ 1;1
NNW
490 -... le ormEl•EnnMa ti lmm il
liill
11: Z
•
k,j'Ll 14 90 i
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144 FA'
111
c 80
80-F.30 PERCENT RELATIVE Hum/DIT
F? ,t,J
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o
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20
25
30
35
4-0
Figure 16.--Effect of exposure to high or low relative humidit y 1-4 the shearing strength of block shear
specimens glued after attaining equilibrium moisture content at 71 , ' F. and f4 percent
relative hum-Mitt.
glued to 0.1, 1M-T ,ifP YFLIeW nIFIcU_rivw01,1]
Series III, Group C-- with grain of fares at right angles and grain of Sirka spruce parallel
to the direction of loading.
F
n
n
111111
80 F, 90
80 PERCENT RELATIVE HUMIDITY
n
nMMIPPL
TIME (DAYS)
Z M 58461
n
8
:-i..1
90
El
1111 n
-. 0
30 PERCENT RELATIVE HUMIDITY mow- mi
miliinm•Iii....
:mmmil=mm'aemmimmsmomm•mim
. 7.1.nnnn•••--.4se,n•••n•n=1••nn
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4•10c
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E
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PERCENT RELATIVE HUMIDITY
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and shearing strength values from U. S. Dept. Agr. Bulletin
No. 479, corrected to the average specific gravity of the
test specimens, are shown by a solid line for each species.
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Figure M.—Effect of exposure to /ow relative humidity on the ultimate strength of typical rib-tn-spar fastenings glued at 12 percent moisture content
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Figure al.—Dimensional change and moisture content of unglued parts conditioned with the typical rib-to-spar fastenings of Series V/11, grou p A.
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and subsequently conditioned to 18 percent.
Top
-- Flat-sawed Sitka spruce glued at right angles.
Center -- Flat-sawed Sitka spruce glued to flat-sawed hickory.
Bottom -- Quarter-sawed Sitka spruce glued to flat-sawed hickory.
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