Center rail slides through guide 9 holes 12.7mm dia 00 00 ◎ 回 ◎ Strain Gages 52 @ ◎ 417 51 -51 136 (a) ) FIGURE 7.11 Three-rail shear test method:(a)fixture configuration,and(b)specimen geometry (all in mm). 7.3 Three-Rail Shear Test Method(ASTM D 4255) Although the in-plane shear stress state induced in the three-rail shear test specimen is generally similar to that induced in the two-rail shear test speci- men,there also are significant differences between the two test methods.In particular,the test fixtures are quite different.A sketch of the three-rail shear fixture is shown in Figure 7.11.The standard fixture shown is designed to be loaded in compression between the flat platens of a testing machine. Tensile loading is also permissible if the fixture is modified to permit attach- ment to the base and crosshead of the testing machine.However,in practice this is not commonly done.In fact,the fixture drawings available from ASTM only include the compression-loaded configuration. Unlike the two-rail shear fixture,the three-rail shear fixture does shear load the specimen along its geometric axes.However,nine rather than six clearance holes must be cut into the specimen,and the size of the standard specimen is 136 x 152 mm rather than 76 x 152 mm,i.e.,1.8 times larger. These are both distinct disadvantages,i.e.,causing increased specimen preparation time (and hence cost)and increased test material consumption, respectively.The(two)gage sections are each 25.4 mm wide,twice as wide as for the(single)gage section of the two-rail shear specimen,which could be an advantage in some cases,as discussed relative to the two-rail shear fixture.However,the gage width of the two-rail shear specimen could also simply be increased to 25.4 mm,if desired.In any case,the potential for ©2003 by CRC Press LLC7.3 Three-Rail Shear Test Method (ASTM D 4255) Although the in-plane shear stress state induced in the three-rail shear test specimen is generally similar to that induced in the two-rail shear test speci￾men, there also are significant differences between the two test methods. In particular, the test fixtures are quite different. A sketch of the three-rail shear fixture is shown in Figure 7.11. The standard fixture shown is designed to be loaded in compression between the flat platens of a testing machine. Tensile loading is also permissible if the fixture is modified to permit attach￾ment to the base and crosshead of the testing machine. However, in practice this is not commonly done. In fact, the fixture drawings available from ASTM only include the compression-loaded configuration. Unlike the two-rail shear fixture, the three-rail shear fixture does shear load the specimen along its geometric axes. However, nine rather than six clearance holes must be cut into the specimen, and the size of the standard specimen is 136 × 152 mm rather than 76 × 152 mm, i.e., 1.8 times larger. These are both distinct disadvantages, i.e., causing increased specimen preparation time (and hence cost) and increased test material consumption, respectively. The (two) gage sections are each 25.4 mm wide, twice as wide as for the (single) gage section of the two-rail shear specimen, which could be an advantage in some cases, as discussed relative to the two-rail shear fixture. However, the gage width of the two-rail shear specimen could also simply be increased to 25.4 mm, if desired. In any case, the potential for FIGURE 7.11 Three-rail shear test method: (a) fixture configuration, and (b) specimen geometry (all in mm). TX001_ch07_Frame Page 114 Saturday, September 21, 2002 4:58 AM © 2003 by CRC Press LLC