CompositesWorld
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JULY 2016 14 CompositesWorld DESIGN & TESTING hole in the composite laminate also can produce signifcant varia- tions in bearing strength. Tus, it is important that the holes in the composite laminate be machined accurately, without damaging the laminate. Single- and double-fastener bearing tests are commonly used, but the latter, especially those performed using a single-shear confgura- tion, provide the best representation of the mechanical joints typi- cally found in composite structures. Of the test methods discussed, however, only ASTM D 5961 Procedure B (Fig. 2, p. 12), includes a double-fastener confguration. In general, double-fastener testing can lead to uncertainty in the bearing stress produced at each fastener hole, due to diferences in clearances between the pin and hole and following the onset of damage formation. A fnal diference in bearing tests is whether the load applied to the specimen is tensile or compressive. While both loadings lead to the same localized bearing stresses produced as the fastener compression loads the edge of the hole, the global stress state changes from tension to compression and the interaction between the fastener hole and the end of the specimen changes as well. Perhaps most importantly, the bending deformation produced in a single-shear confguration is more pronounced under compression loading, producing diferences in the resulting bearing strength. Procedure B of ASTM D 5961 describes a support fxture (Fig. 2) that can be used to reduce the bending deformation of two-piece REFERENCES 1 ASTM D 953-10, "Bearing Strength of Plastics," ASTM International (W. Conshohocken, PA, US), 2010 (frst issued in 1948). 2 ASTM D 5961-13, "Bearing Response of Polymer Matrix Composite Laminates," ASTM International (W. Conshohocken, PA, US), 2013 (frst issued in 1996). 3 SACMA SRM 9-89, "SACMA Recommended Test Method for Bearing Strength Properties of Oriented Fiber-Resin Composites," Suppliers of Advanced Composite Materials Assn. (Washington, DC, US) 1989. 4 Sawicki, A., "On the Proliferation of Standard Tests for Composite Bearing Strength," Proceedings of the 19 th International Conference on Composite Materials, July 28-Aug. 2, 2013, Montreal, ON, Canada. ABOUT THE AUTHOR Dr. Daniel O. Adams is a professor of mechanical engineering and has been the director for 19 years of the Composite Mechanics Laboratory at the University of Utah and vice president of Wyoming Test Fixtures Inc. (Salt Lake City, UT, US). He holds a BS in mechanical engineering and an MS and Ph.D in engineering mechanics. Adams has a combined 36 years of academic/industry experience in the composite materials feld. He has published more than 120 technical papers, presents seminars and chairs both the Research and Mechanics Divisions of ASTM Committee D30 on Composite Materials and the Testing Committee of the Composite Materials Handbook (CMH-17). He regularly provides testing seminars and consulting services to the composites industry. composite specimens loaded in single-shear. Additionally, ASTM D 5961 Procedure D describes a double-shear test confguration specifcally designed for compression loading.