Indirect shear stress

California State University, Fresno Civil Engineering

CE121L Mechanics of Materials Laboratory

Lab9. Shear Test of Wood 1. PRELIMINARY READING Read the section in the Mechanics of Materials textbook that deals with wood and wood testing and answer the Prelab questions. 2. OBJECTIVE The purpose of this experiment is to observe and compare the wood strength subjected to direct and indirect shear stresses. 3. EQUIPMENT/MATERIAL REQUIRED Universal Testing Machine Automatic Data Acquisition System Direct Shear Block Testing Apparatus (Figure 1a) Specimen Support and Bearings for Indirect Shear Test (Figure 1b) Tape Measure

a) Direct shear test b) Indirect shear test FIG. 1 Picture of testing apparatus for shear test of wood

(Photo courtesy: Dr. Jesus Larralde)

4. PROCEDURE 4.a Direct Shear Test (Figure 2a):

1. Measure the dimensions of the specimen to be tested. Make sure to determine the dimensions of the shear plane where shear stress will occur.

2. Place the specimen in the direct shear block testing apparatus. 3. Apply the load gradually and record the maximum load at failure. 4. Repeat the direct shear test with a saturated specimen.

4.b Indirect Shear Test (Figure 2b):

1. Determine the actual dimensions of the 12”-2x2 specimen. 2. Make a pencil mark at 1 inch from each bottom corner of the specimen, showing

the supports, and one additional mark at 3 inches from one of the top corners, indicating the load application.

3. Load the specimens into the UTM with the bottom marks resting on supports with bearing plates. The grain of the wood should be as close to 90 to the applied load as possible.

4. Apply the load gradually and record the maximum load at failure.

a) Direct shear test b) Indirect shear test (horizontal shear test) FIG. 2 Shear test of wood

5. ASSIGNMENT Prepare a Memo Lab report including the following results:

a) Divide the maximum load at failure for each shear block by the area of the shear plane. This value is the direct shear strength of the specimens tested in 4.a.

b) Calculate the ratio of the shear strength for the saturated block divided by the shear strength for the dry block.

2 in. 8 in.

c) Calculate the indirect shear stress that caused failure in the specimen from the specimen tested in 4.b.

d) Compute the percent difference between the shear strength from the dry direct shear test and the indirect shear test.

6. QUESTIONS/DISCUSSION TOPICS  Express the relative magnitudes of the strength of the saturated and dry blocks.

Was this ratio as expected? Why or why not?  Comment on the similarity or dissimilarity of the results of the shear strength

from the dry direct shear test and the indirect shear test.