UNITED STATES DEPARTMENT OF AGRICULTURE - FOREST SERVICE - FOREST PRODUCTS LABORATORY - MADISON, WIS.
COMPARISON OF BLOCK SHEAR METHODS FOR DETERMINING SHEARING STRENGTH OF SOLID WOOD
February 1964
FPL-030
COMPARISON OF BLOCK SHEAR METHODS
FOR DETERMINING SHEARING STRENGTH OF SOLID WOOD
HOWARD C. HILBRAND, Engineer
1 Forest Products Laboratory, Forest Service U.S. Department of Agriculture
Summary
Information concerning the relative magnitudes of shear strength as determined by using cube-shaped or notched specimens in roller or nonroller testing apparatus has often been contradictory. This describes a comparative study, conducted at the U.S. Forest Products Laboratory and the Australian C.S.I.R.O. Division of Forest Products on six different specimen-apparatus combinations using matched specimens of southern yellow pine and overcup oak.
Several of the combinations yielded estimates of shear strength that were significantly different, but standard deviations of shear strength were generally independent of the test method. Significant interaction of growth ring orientation and test type was apparent and may have contributed to previous conflicting views. In general, cube specimens yielded higher estimates of shear strength than notched specimens, and nonroller apparatus yielded higher estimates than roller-equipped apparatus.
Introduction
A Timber Mechanics Conference sponsored by the Standing Committee on Forest Products Research of the British Empire Forestry Conference met for 2 weeks each in Ottawa, Ontario, and Madison, Wis., during September and October 1948. Staff members of the U.S. Forest Products Laboratory were invited to participate with staff members of
1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin.
FPL-030
----
By
v a r i o u s Forest Products Laboratories of the British Empire. The conference was primarily concerned with standardization of methods of determining the mechanical and physical properties of woods. During the course of these meetings, considerable discus- sion was devoted to shear test methods for small, clear specimens and comparability of shear strength values derived.
Information concerning the relative magnitudes of shear strength as determined by the standard ASTM block shear test and shear tests involving a cube-shaped specimen was often contradictory. Furthermore, it was realized that shear strength as determined by similar apparatus at different laboratories did not always agree. As a result, the confer- ence recommended further study and exchange of specimens as a means of reconciling some of the difficulties and differences in the various shear test methods. The U.S. For- est Products Laboratory and the Australian C.S.I.R.O. Division of Forest Products, Melbourne, undertook a cooperative study for the purpose of implementing that recom- mendation.
On December 1, 1949, a proposed working plan for the cooperative shear study was sent to Melbourne. On February 8, 1950, the working plan had been modified and agreed upon by the two laboratories. The plan required, in part, that sets of matched material be tested on identical tools located in the two laboratories and that a statistical analysis be employed to show significance of differences. The plan further provided that three dif- ferent test methods would be studied at each laboratory with the specific types of test to be decided at a later time.
Types of Shear Tests
The type of specimen and the type of test tool involved in determining shear strengths were considered to be the most important factors influencing the result of the shear test. The cube-shaped specimen and the notched specimen were of particular interest as well as the tools equipped with roller bearings and those without such bearings.
2 3 4 Six test methods, including some standard methods, and some nonstandard
methods, were proposed for study. The description of each is given in table 1, and the specimens and apparatus are shown in figures 1, 2, and 3. An examination of table 1 shows that test types A and C differ only in the location of the test. The standard British shear test used in 1929 was revised to the present type of test in 1938. It was intended at the outset of the study that test types B and E also differ only in location, but a misunder- standing as to type of apparatus led to the difference indicated.
3 4
2 American Society for Testing and Materials. Standard methods of testing small clear specimens of
3 British Standards Institution. British standard specification for methods of testing small clear speci-
4 British Standards Institution. British standard methods of testing small clear specimens of timber.
timber. ASTM D 143-52. Philadelphia.
mens of timber. Standard 373-1929. London.
Standard 373-1938. London.
FPL-030 -3-
M 35571 F
Figure 1.--British standard shear cube (in 1938) and apparatus used for test types A and C.
M 124 168
Figure 2.--British shear specimen (in 1929) and apparatus used for test type D.
FPL-030 -4-
M 14302 F
Figure 3.--ASTM standard shear specimen and apparatus (D 143-52). Specimen used for test types B and E. Apparatus used for test type B.
Figure 4.--Typical cutting diagram used for obtaining matched shear specimens.
FPL-030 -5-
Specimen Preparation
Specimens were prepared from overcup oak and southern yellow pine. Fifteen radial and 15 tangential specimens were provided for each of the six different test types, making a total of 180 specimens for each species. Thirty sticks of each species were selected from a larger number of pieces cut from green material at the Madison laboratory. Each piece was scribed for grain direction and selected on the basis of uniformity of structure and lack of defects. The sticks were air dried to about 13 percent moisture content. Each stick was then machined to a size of 2 by 2-1/4 by 18 inches, care being taken to keep the long edges parallel to the grain.
Numbers 1 to 30, inclusive, were assigned at random to the pine sticks, and numbers 51 to 80, inclusive, to the oak sticks. Odd-numbered sticks (1, 3, 5, etc.) were designated to provide radial specimens and even-numbered sticks (2, 4, 6, etc.) tangential specimens. The proposed shearing plane was then clearly indicated on each stick. The six test types, as described in table 1, were assigned the letters A through F, inclusive, and a table of random numbers was used to determine the relative position of a test type within each stick. Figure 4 shows a cutting plan from which six end-matched test specimens were taken (one for each of the six test types).
The specimens were cut approximately to size and conditioned at the U.S. Forest Products Laboratory at 75° F. and 64 percent relative humidity. This condition produced an equilibrium moisture content of approximately 12 percent. On April 23, 1951, the spec- imens to be tested at the Australian laboratory were shipped via railway and ocean express to Melbourne. The specimens were further conditioned at 75" F. and 64 percent relative humidity in Australia. Immediately before testing at each laboratory, the specimens were machined to final dimensions using a jointer on all four sides and a sharp saw on the end grain.
Test Procedures
In testing at the U.S. Forest Products Laboratory, specimens were seated in the shear tool with an initial load of 200 pounds and then loaded at constant speed of about 0.024 inch per minute until failure occurred. The maximum load and the elapsed time from initial load to maximum load were recorded for each specimen. The weight and volume of each specimen were measured immediately before testing as a basis for specific gravity deter- mination. Moisture content was determined by weighing and ovendrying the smaller piece of the broken shear specimen immediately after the test.
Similar procedures were followed at the C.S.I.R.O. laboratory, except that elapsed time was not recorded.
FPL-030 -6-
Presentation and Analysis of Results
Table 2 is a summary of the results obtained at the two laboratories. Included in the table are the average, standard deviation, and coefficient of variation of shear strength values, together with corresponding values of moisture content and specific gravity. Indi- vidual specimen results are given in Appendixes I and II.
Figures 5 and 6 are bar graphs showing average shearing strength, specific gravity, and moisture content. Figures 7 and 8 are bar graphs showing average shearing strength, range of shearing strength, and a range of ±1 standard deviation from the average. The small figures above the bar indicate the number of individual specimens yielding results in the corresponding region of the bar.
It may be seen from figures 5 and 6 that specific gravity is quite uniform with respect to comparative groups, and that differences in moisture content are generally small. This indicates reasonably good matching and hence reported strength values are not adjusted for minor specific gravity and moisture content differences.
It is useful in a comparative study of this type to provide some reasonable definitive basis for comparison expressed in terms that are generally understood. It is also useful to provide a basis for comparing group means that takes into account the variability in the individual test results that contribute to those means. Limited statistical analysis was undertaken as a means of contributing to both of those bases.
An analysis of variance of all the data for each of the two woods was designed to test the null hypothesis that there is no significant difference between results of tests involving the two growth ring orientations (radial or tangential), no significant difference between the results of the various types of test, and no significant interaction between the effect of ring orientation and that of test type. In comparing the results of the various types of tests, particular attention was paid to the significance of difference between results of tests involving the cube specimen in a roller jig conducted in Madison and in Melbourne, and the significance of difference between the combined results of those tests and the re- sults of tests involving the ASTM notched specimen and ASTM tool conducted in Madison.
As a further aid in comparing means of the various groups, values of least significant difference at the 5 percent level of probability were computed
Results of the analysis of variance and values of least significant differences are pre- sented in Appendixes III and IV.
Table 3 provides a basis for overall comparison of cube and notched specimens and of roller and nonroller tools.
FPL-030 -7-
Figure 6. --Average moisture content, specific gravity, and shear strength for overcup oak specimens.
Z M 124 171
FPL-030 -9-
Figu
re
7.--
Ave
rage
an
d ra
nge
of
shea
r st
reng
th
of
sout
hern
ye
llow
pi
ne
spec
imen
s.
Shad
ed
porti
on
of
the
bar
repr
esen
ts
a ra
nge
of
±1
stan
dard
de
viat
ion
from
th
e m
ean.
Sm
all
figur
es
abov
e th
e ba
r in
dica
te
the
num
ber
of
indi
vidu
al
spec
imen
s yi
eldi
ng
resu
lts
in
the
corr
espo
ndin
g ra
nge.
Z M
12
4 17
2
Figu
re
8.--
Ave
rage
an
d ra
nge
of
shea
r st
reng
th
of
over
cup
oak.
Sh
aded
po
rtion
of
th
e ba
r re
pres
ents
a
rang
e of
±1
st
anda
rd
devi
atio
n fr
om
the
mea
n.
Smal
l fig
ures
ab
ove
the
bar
indi
cate
th
e nu
mbe
r of
in
divi
dual
sp
ecim
ens
yiel
ding
re
sults
in
th
e co
rres
pond
ing
rang
e.
Z M
124
173
FPL-030 -12-
Discussion of Results
This study is intended only to show what differences in results may be expected by using the various test methods and to indicate the variation inherent in each. It is not the pur- pose to explain the reasons for the different shear strengths obtained by the various test methods used, nor to establish the best method of shear testing. However, the results re- ported herein are considered to be worthy of incorporation into a study of that nature if and when it is deemed desirable to do so.
It is apparent that the various specimen-apparatus combinations produced different estimates of shear strength that may be associated with the test method. Figures 7 and 8 show that test types F and B (both having nonroller bearing tools) generally yielded the highest shear strength values. Test types A and C (both standard British cube shear spec- imens and roller bearing tools) yielded intermediate values of shear strength. Test types E and D (both involving a notched specimen and roller bearing tools) yielded the lowest values of shear strength. Standard deviation of shear strength, while somewhat lower for the oak than for the pine, is apparently independent of shear strength. This implies that, in general, the coefficients of variationof shear strength of the six test types are inversely related to their average shear strengths, which implication may be verified by inspection of columns 5 and 9, table 2.
Test types A and C were expected to compare almost identically since each is the standard British cube shear Lest with type A tested in Madison and type C tested in Mel- bourne. Table 2 indicates that the results are similar, while the analysis in Appendixes III and IV shows no significant difference between these tests at the 1 percent level of prob- ability. These two test types were therefore combined for comparisons with other test types.
Comparison of the standard British cube shear test and the ASTM notched shear test was one of the primary objectives of this study. This was carried out by comparing the combined results of test types A and C with those of test type B. Table 2 shows that the ASTM test (B) yields somewhat higher shear strength values than the standard British cube test (A and C). However, the analysis of variance (Appendixes III and IV) indicates that this difference is significant at the 1 percent level for the overcup oak, but not for the southern pine.
The analysis of variance (Appendixes III and IV) indicates that results of shear tests in the radial plane differ from those in the tangential plane at the 1 percent level. Further- more, for the pine specimens radial strength is generally higher than tangential strength, while for oak the tangential strength is generally higher than radial strength. The analysis also indicates that there is a significant interaction between the effects of growth ring
FPL-030 -13-
orientation and the effect of test type. Such interaction could explain the apparently contradictory information acquired before this study was undertaken. In two instances, the differences of means for radial specimens differ in sign from those for tangential speci- mens within the same group. In one other case, the difference between radial specimens of two groups is statistically significant while that for tangential specimens is not, though both are of the same sign.
Supplementary comparisons indicate that results with the 1929 BESA test (type D) were significantly lower than the combined A and C results. Further, the ASTM notched speci- men yielded a significantly higher value when tested in the standard ASTM tool (type B) than when tested in the roller modified tool (type E).
Comparisons of the above effects by means of least significant difference agree with those based on an analysis of variance. The calculated values of least significant differ- ence in Appendixes III and IV also provide a basis for further comparison of the effects of type of test, growth ring orientation, and their interaction.
Direct comparisons of the cube or notched specimens and the roller or nonroller type jigs have indicated a general trend for these factors without regard to specific dimensions of the specimens or jigs. Therefore, these factors were grouped for overall analysis as shown in table 3 using average shear strength values derived from table 2. The average
FPL-030 -14-
values shown in table 3 indicate that the cube specimens generally yielded higher esti- mates of shear strength than the notched specimens. Furthermore, a nonroller type shear tool yielded higher estimates of shear strength than a roller type tool. These comparisons are consistent and displayed by both species tested,
Conclusions
Evaluation of the results of this series of comparative shear tests leads to the following conclusions:
(1) Good compatibility of results was obtained by the two laboratories using a cube spec- imen and roller apparatus. Unfortunately, there is no basis within this study for evalua- ting compatibility of other specimen-apparatus combinations.
(2) Various specimen-apparatus combinations may be expected to yield estimates of shear strength that are significantly different from one another, Cube specimens yield higher estimates of shear strength than do notched specimens and nonroller apparatus yield higher estimates than do roller equipped apparatus.
(3) Significant interaction of growth ring orientation (radial or tangential) and test type may have contributed to the previous conflicting views regarding comparability of shear strength values yielded by various specimen-apparatus combinations.
(4) Standard deviation of the shear test results appears to be generally independent of shear strength, so that coefficients of variation are inversely related to corresponding shear strengths.
FPL-030 -15- 1.5-25
APPENDIX I
-16- (Sheet 1 of 2) FPL-030
FPL-030 -17- (Sheet 2 of 2)
(She
et
1 of
4)
(She
et
2 of
4)
(She
et
3 of
4)
(She
et
4 of
4)
APPENDIX III
FPL-030 -22-
STATISTICAL ANALYSIS OF SHEAR TEST RESULTS FOR SOUTHERN YELLOW PINE
APPENDIX I V
STATISTICAL ANALYSIS OF SHEAR TEST RESULTS FOR OVERCUP OAK
FPL-030 -23-
SUBJECT LISTS OF PUBLICATIONS ISSUED BY THE
FOREST PRODUCTS LABORATORY
The following are obtainable free upon request from the Director, F o r e s t P r o d u c t s L a b o r a t o r y, Madison, Wisconsin 53705.
List of publications on Box and Crate Construction and Packaging Data
List of publications on Mechanical Properties and Structural Uses of Wood and Wood Products
List of publications on Chemistry of Wood and Derived Products
List of publications on Pulp and Paper
List of publications on List of publications on
Drying of Lumber
List of publications on Fire Performance
List of publications on
Structural Sandwich, Plastic Laminates, and Wood-Base Aircraft Components
List of publications on Thermal Properties of Wood
Fungus Defects in Forest Products and Decay in Trees
List of publications on Wood Finishing
List of publications on Glue, Glued Products, and Veneer
List of publications on Growth, Structure, and Identification of Wood
List of publications on Logging, Milling, and Utiliza- tion of Timber Products
List of publications on Wood Preservation
Partial list of publications for Architects, Builders, Engineers and Retail Lumbermen
Partial list of publications for Furniture Manufacturers, Woodworkers, and Teachers of Woodshop Practice
Note: Since Forest Products Laboratory publications are so varied in subject no single list is issued. Instead a list is made for each Laboratory division. Twice a year, December 31 and June 30, a list is made showing new reports for the previous 6 months. This is the only item sent regularly to the Laboratory’s mailing list. Anyone who has asked for and received the proper subject lists and who has had his name placed on the mailing list can keep up to date on Forest Products Lab- oratory publications. Each subject list carries a listing of all other subject lists.
