220 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES area;if this does not occur correctly,then the data point should not be used. Figure 7.6 illustrates examples of acceptable and unacceptable failures,with any failure residing solely in the tabbed or gripped region being considered unacceptable.Due to the very short gauge length,it is likely that the failure location could be near the grip/tab termination region;this is still an acceptable failure. Compression tests are very sensitive to the flatness and parallelism of the specimens and/or tabs,and within the test specifications,the required tolerances are outlined.Gripping of the specimens and system misalignment are generally the biggest cause of data scatter,and of particular relevance to the test methods that use stabilizing lateral supports(SACMA SRM 1R-94 and 3R-94)is the issue of bolt torque.An over-torque of the bolts allows more of the applied load to be carried by the lateral supports through friction,thereby increasing the apparent compressive strength.Generally,the bolts are tightened up to a"finger-tight" level,a fairly arbitrary measurement;however studies'5 have shown that the torque should not exceed approximately 1 Nm. 7.2.3 Flexure A flexure test is,without doubt,one of the simplest types of tests to perform and thus has long been popular(Fig.7.7).The main difficulty is that it does not a) b) Fig.7.6 Failure modes in compression testing:a)unacceptable;b)acceptable.220 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES area; if this does not occur correctly, then the data point should not be used. Figure 7.6 illustrates examples of acceptable and unacceptable failures, with any failure residing solely in the tabbed or gripped region being considered unacceptable. Due to the very short gauge length, it is likely that the failure location could be near the grip/tab termination region; this is still an acceptable failure. Compression tests are very sensitive to the flatness and parallelism of the specimens and/or tabs, and within the test specifications, the required tolerances are outlined. Gripping of the specimens and system misalignment are generally the biggest cause of data scatter, and of particular relevance to the test methods that use stabilizing lateral supports (SACMA SRM 1R-94 and 3R-94) is the issue of bolt torque. An over-torque of the bolts allows more of the applied load to be carried by the lateral supports through friction, thereby increasing the apparent compressive strength. Generally, the bolts are tightened up to a "finger-tight" level, a fairly arbitrary measurement; however studies 15 have shown that the torque should not exceed approximately 1 Nm. 7.2.3 Flexure A flexure test is, without doubt, one of the simplest types of tests to perform and thus has long been popular (Fig. 7.7). The main difficulty is that it does not a) b Fig. 7.6 Failure modes in compression testing: a) unacceptable; b) acceptable