Mechanical Testing of Timber Specimens

Two kinds of test were undertaken; static bending and compression parallel to the grain. Static bending tests allowed the modulus of rupture and modulus of elasticity to be calculated, the compression parallel to the grain tests gave maximum crushing strength. All tests were done at the N.S.W. Forestry Commission laboratory at West Pennant Hills, to take advantage of automated testing procedures and because the laboratory is air conditioned to the standard conditions of 68% relative humidity and 15'C. The temperature range in the timber testing laboratory at the ANU was too great.

All specimens prepared from the Scottsdale wood were tested that is there were 91 values for each property in the unseasoned timber and 62 in the seasoned timber.

Not all specimens prepared from the Toolangi wood were tested, the number being a compromise between the precision required, the availability of test specimens and the time available for testing.

Pearson and Williams (1965) estimated the precision of results from mechanical testing as shown in Table 3.6.

Table 3.6. Precision of results from mechanical tests for strength properties

No. of trees Number of specimens per tree 95% confidence half range [%]1

9.5

8

7.3 6.5 Source - Pearson and Williams (1965)

The required precision, expressed as the 95% confidence half range of the species mean value for 1939 regrowth E. regnans was not accurately known. The confidence half range of the species mean value for MOR of mature E. regnans was calculated Table 3.7 using data in Bolza and Kloot (1963) and for the Scottsdale timber already tested in this study (See 3.4.2.1.1).

Table 3.7. Precision of results for MOR of mature E. regnans and for 1927 regrowth E. regnans from Scottsdale.

S tatistic Mature Regrowth

Species mean [MPa] 63.2 52.6

95% Confidence half 6.2 7.2

range [%]

Thus if the required confidence half range was determined as the same as for mature E. regnans i.e. 6.2 %, the number of specimens per tree would be about 24 and if the same as the 1927 regrowth E. regnans from Scottsdale, i.e. 7.2%, the number of specimens per tree would be 9. The number of specimens prepared from each log of Toolangi wood ranged from 2 to 6; the mean number of logs per tree was 4 and 9 trees were selected. Since the number of specimens tested should be spread evenly over the number of logs, it would be appropriate to select one or two specimens per log. One specimen per log, gives 4 specimens per tree; 36 specimens

l

for each test, if two per log than 72 specimens in all. If 4 specimens were tested from each tree the confidence half range for the species mean value was estimated as about 7.5%, with 8 specimens about 7%. It was decided to allocate the specimens into sets with one specimen per sawlog, giving a confidence half range of about 7.5 %, not much less than that of the 1927 regrowth E. regnans. Six specimens were not available from every log so that some sets did not include 36 specimens. The number of specimens in each set is shown in Table 3.8.

Table 3.8. The number of specimens in each set re-sampled randomly to give one specimen per log from 1939 regrowth E. regnans from Toolangi, V ictoria.

Moisture Number of specimens

Test

condition Set 1 Set 2 Set 3 Set 4 Set 5 set 6

S ta tic green 36 36 35 32 20 8

Bending air dry 36 36 33

Compression green 36 36 35 32 20 8

parallel to the grain

air dry 36 36 33

Mechanical testing for strength grouping was conducted with set number 1 which included 36 specimens for every kind of test. To provide a larger data set to analyse for variability in terms of MOR values 72 specimens of unseasoned timber and 115 of seasoned timber were tested.

3.4.1.4.1. Static Bending Test

Static bending tests were done on a SHIMADZU timber testing machine, set up according to the Australian standard testing method (See Plate 1, Mack (1979)). The span was 280 mm using centre point loading at a constant rate of movement of the load table. The machine was connected to a data recording and processing system and the results of the tests were computed immediately. The computer network included a plotting facility and a plot of load against deflection was drawn for every specimen as the tests proceeded. This was found very useful in checking for test results. Examples of the data print out are shown in Appendix 6.

The MOE values were measured first in batches. Specimen number, depth and width were typed into the data storage system. The specimen was positioned in the machine and a digital meter was set to measure the deflection due to load. Loading was continued to less than the limit of proportionality and then released. The modulus of elasticity was then calculated by the computer and printed out.

More tests were undertaken in batches after MOE tests. Before each specimen was tested, number, depth and width were again typed into the data storage system. The specimen was then positioned and testing begun. Load was applied until the specimen broke. The plotter drew a curve for load against deflection throughout the test. After each test the modulus of rupture was calculated by the computer and printed out.

The MOE and MOR tests were done separately for the following reasons:

(1) To avoid frequent removal of the digital meter. (2) Separate computer programmes were involved. (3) Plotting scales were different.

(4) To save time the MOR tests were all done at a test rate of approximately 3 mm per minute whereas the MOE tests were done at the standard rate of approximately 1 mm per minute.

The faster rate was adopted because hundreds of tests were envisaged to examine the variability of the wood at different sites, because there was limited time available for testing in the laboratories of the Forestry Commission of New South Wales in Sydney and because comparative results of material from the various sites and trees was a major interest. The practical effects of the faster rate were assumed to be small on the basis that the multiplication factors for duration of load, adopted in AS1720 1975, are 1.75 for both 5 second and 5 minute duration and 1.70 for 5 hours. The faster rate reduced the testing time from about 15 minutes to 5 minutes.

(5) The test configurations were also slightly different as shown in plates 3.1 and 3.2, and 3.3 and 3.4. Appendix 7.

3.4.1.4.2. Compression parallel to the grain test

The tests for compression parallel to the grain were done on an AVERY timber testing machine according to the Australian standard testing method. Movement of the testing head was at a constant rate of approximately 0.18 mm per minute. After each specimen was tested, the maximum crushing strength was immediately computed and printed out. The test configuration is shown in Plates 3.5 and 3.6. (Examples of the results are also shown in Appendix 8).