The main goal of this research was to improve the three-rail shear test for studying the in-plane shear behavior of composite materials in static and fatigue loading. This method is currently a standard guide in the ASTM Book of Standards. Presently over 30 test methods exist that are briefly reviewed. The three-rail shear test was chosen for its ability to produce both strength and modulus values in both static and fatigue testing, as well as for producing a large uniform region of pure shear away from the free edges. The disadvantages of this test method include free edge effects, which cause stresses to differ from pure shear in that area, and premature failure of the specimen initiating at the rail edges. Thus this testing method was improved to eliminate these problems.
In order to fully examine the three-rail shear test method as prescribed in the ASTM Standard Guide for Testing In-Plane Shear Properties of Composite Laminates (D 4255), both the fixture and specimen were examined. Recently, the authors improved the three-rail specimen by performing analytical and experimental studies of the specimen . The analytical results were verified by the three-rail shear test for both the standard un-notched and the new notched specimens. In this research, the fixture itself was examined experimentally and found to be inadequate for fatigue loading. The fixture was thus modified for overall strength and a bottom grip was added for fatigue loading. To eliminate out-of-plane motion of the middle rail, the fixture was again modified to be restrained at the top and bottom while preventing fretting during fatigue testing by using an Oilite bronze lining at the contact surfaces. Thus a new modified fixture was designed to consolidate all necessary modifications, making the fixture suitable for static and fatigue loading. The test results from the fixture show great improvement in both static strength and fatigue life.