76.2 44.4 25.2 00 SPECIMEN 50.8 6 holes STRAIN GAGE 12.7mm diameter 152 RAILS Strain Gages LOAD PLATE 50.8 TENSILE FIXTURE 15.9 (a) (b) FIGURE 7.10 Two-rail shear test method:(a)fixture configuration,and (b)specimen geometry (all in mm). The two-rail shear test specimen is shown in Figure 7.10(b).As indicated, the specimen is 76.2 x 152.4 mm,thus consuming eight times more test material than the losipescu shear specimen.Note also that there are six holes in the otherwise simple rectangular specimen.These are clearance holes for the six bolts that clamp the specimen to the rails.Not only do these holes potentially introduce stress concentrations in the test specimen,there is always some inherent concern about making holes in a composite material without introducing auxiliary damage.In addition,for very high shear strength composites the clamping forces have to be very high to avoid slipping of the rails during the test.A bolt torque of 100 N.m,which is a very high torque for the 9.5-mm-diameter bolts,is specified [4]. Despite these current deficiencies of the two-rail shear test method,there are distinct positive attributes of the method as well.The loading mode is actually much like that for the losipescu shear test method.That is,an essentially pure shear loading is applied to the gage section of the specimen (the 12.7-mm-wide portion of the specimen exposed between the rails).The shear stress along the length of the specimen(parallel to the rails)is relatively constant,except near the ends(which must be at a zero shear stress because they are free surfaces).Some extraneous normal(tensile and compressive) stresses are introduced by the presence of the rails,particularly near the boundaries of the gage section.Finite element analyses have been conducted to characterize these stresses [10,15],and undoubtedly specimen and fixture modifications can be made to significantly reduce,if not eliminate,them. Studies are currently in progress. One previously stated advantage of the losipescu shear test method is the relatively small specimen size.Seemingly contradictory is the potential ©2O03 by CRC Press LLCThe two-rail shear test specimen is shown in Figure 7.10(b). As indicated, the specimen is 76.2 × 152.4 mm, thus consuming eight times more test material than the Iosipescu shear specimen. Note also that there are six holes in the otherwise simple rectangular specimen. These are clearance holes for the six bolts that clamp the specimen to the rails. Not only do these holes potentially introduce stress concentrations in the test specimen, there is always some inherent concern about making holes in a composite material without introducing auxiliary damage. In addition, for very high shear strength composites the clamping forces have to be very high to avoid slipping of the rails during the test. A bolt torque of 100 N·m, which is a very high torque for the 9.5-mm-diameter bolts, is specified [4]. Despite these current deficiencies of the two-rail shear test method, there are distinct positive attributes of the method as well. The loading mode is actually much like that for the Iosipescu shear test method. That is, an essentially pure shear loading is applied to the gage section of the specimen (the 12.7-mm-wide portion of the specimen exposed between the rails). The shear stress along the length of the specimen (parallel to the rails) is relatively constant, except near the ends (which must be at a zero shear stress because they are free surfaces). Some extraneous normal (tensile and compressive) stresses are introduced by the presence of the rails, particularly near the boundaries of the gage section. Finite element analyses have been conducted to characterize these stresses [10,15], and undoubtedly specimen and fixture modifications can be made to significantly reduce, if not eliminate, them. Studies are currently in progress. One previously stated advantage of the Iosipescu shear test method is the relatively small specimen size. Seemingly contradictory is the potential FIGURE 7.10 Two-rail shear test method: (a) fixture configuration, and (b) specimen geometry (all in mm). TX001_ch07_Frame Page 112 Saturday, September 21, 2002 4:58 AM © 2003 by CRC Press LLC