V-notched Rail Shear Fixture
ASTM D7078 covers a shear testing procedure for composites. The V-notched sample is ruptured using the rail shear method. This method enacts a uniform shearing strain onto the test specimen.
ASTM D7078 is similar to the ASTM D5379 Iosipescu shear test. The shearing mechanism is the same, however the tests can yield slightly different results. D7078 is generally used for flexible materials that are subject to edge crushing. D5379 is better to use with more rigid samples whose edges can withstand the compression force.
The sample is similar to the one from the Iosipescu test, however it is slightly more square. The V-notch is made at 90 degrees in the center of both the top and bottom of the sample.
This test requires a Rail Shearing Fixture which is specifc to ASTM D7078. Along with the metal fixtures, plastic centering triangles are also used to align the sample. Different sized rail shear fixtures are available, including those with an increased amount of tightening screws for higher loads.
Maximum shear stress is the most common measure of shear strength and it is divided by the total cross-sectional shear zone. The strain is concentrated in the center of the sample and can be more accurately measured with a strain gauge or laser extensometer.