CompositesWorld
Issue link: https://cw.epubxp.com/i/576646
OCTOBER 2015 14 CompositesWorld DESIGN & TESTING » Many test methods are available for measuring the shear properties of composite materials. Te V-notched methods are currently among the most popular, in part because they can produce a relatively pure and uniform shear stress state in the test specimen, and they can measure the shear modulus and shear strength for the in-plane and interlaminar material orienta- tions. Te original V-notched shear test for composites, commonly referred to as the Iosipescu shear test, was introduced in the early 1980s 1 and became an ASTM standard (ASTM D 5379 2 ) in 1993. In more recent years, the V-notched rail shear test was developed and standardized (ASTM D 7078 3 ); a V-notched, combined loading shear test has been developed as well. Te test fxtures and specimens used for the three methods are compared in Fig. 1. All three apply an asymmetrical four-point bend loading to the specimen. As illustrated in Fig. 2, this type of loading can be visualized (and experienced) by the reader using a pencil or pen as the specimen, and applying the load, using the index fnger and thumb on both hands. For starters, if the index fngers are placed on the outer bottom surfaces of the specimen and the thumbs placed at the inner top surfaces (Fig. 2a), signif- cant bending stresses are produced in the central section between the reader's thumbs. Tis is the loading method used in conven- tional four-point fexure testing. However, if the reader simply rotates one hand by 180°, such that the thumb loads the bottom and the index fnger loads the top of the specimen (Fig. 2b), the resulting asymmetric four-point loading changes from bending to shear in the central section. Te 90° V-notches machined into the specimens are used to produce a uniform state of shear stress in the central test section between the notch tips. Without notches, the shear stress distri- bution varies parabolically across the width of the specimen, ranging in magnitude from zero at the top and bottom specimen edges to a maximum at the horizontal midplane. Te V-notches efectively remove the outermost regions of the specimen under relatively low shear stress, leaving the central section between the notches with a surprisingly uniform state of shear stress. Using V-notches to produce a uniform stress state may seem counterin- tuitive for readers who are unfamiliar with this type of shear test, because notches are known to produce large stress concentrations and nonuniform stress distributions in axially loaded specimens. However, the 90° V-notches in the shear specimens, with depths of 20-23% of the total specimen width, produce minimal stress concentrations. Although the same asymmetrical bend loading (Fig. 2b) is used in all three V-notched shear tests, the specimen surfaces where the load is applied and the size and shape of the test speci- mens difer between them. In the Iosipescu shear test method (Fig. 1a), the fxture loads the top and bottom edges of the 76-mm long by 19-mm wide specimen. Although limited load can be applied through the specimen edges without causing crushing, it is adequate to produce failure in unidirectional or cross-ply composite specimens, such that the shear strength and shear modulus of the composite material can be measured. Although it is still common practice to test 0° unidirectional composite specimens with fbers oriented lengthwise, there are signifcant advantages to using a [0/90] ns cross-ply laminate for in-plane shear testing. Cross-ply specimens are not only less fragile, but also produce a more uniform state of shear stress and The original V-notched shear test for composites, the Iosipescu shear test, was introduced in the 1980s. V-notched shear testing of composites FIG. 1 V-notched shear testing of composites test fxtures/specimens. c) Combined loading shear test a) Iosipescu shear test (ASTM D 5379) b) V-notched rail shear test (ASTM D 7078) a) Symmetric four-point fexure loading. b) Asymmetric four-point shear loading. FIG. 2 Four-point loading confgurations of beam specimens.