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11 REFERENCES 1 ASTM D3410/D3410M-03(2008), "Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials with Unsupported Gage Section by Shear Loading," ASTM International (W. Conshohocken, PA, US), reapproved 2008 (frst issued in 1975). 2 SACMA Recommended Method SRM 1R-94, "Compressive Properties of Oriented Fiber-Resin Composites," Suppliers of Advanced Composite Materials Assn. (Arlington, VA, US), 1994. 3 ASTM D6641/D6641M-09, "Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials Using a Combined Loading Compression (CLC) Test Fixture," ASTM International (W. Conshohocken, PA,US), 2009 (frst issued in 2001). 4 P.M. Wegner and D.F. Adams, "Verifcation of the Combined Load Compression (CLC) Test Method," Final Report DOT/FAA/AR-00/26, Federal Aviation Admin. (Washington, DC, US), August 2000. Optimum Unidrectional Compression Testing specimen thickness to prevent buckling, it's recommended to increase the specimen thickness an additional 10-20% above the calculated value. Additionally, because buckling cannot be detected visually during testing or from failed specimens, the use of back-to-back strain gages on the specimen faces during testing is required to determine that buckling is not occurring. Buckling results in a sudden divergence of the strains on the specimen faces (Fig. 3), and can signifcantly reduce the measured compressive strength. Terefore, it is recommended that back-to-back strain gages be used on the initial set of compression tests and that the strains be analyzed before proceeding with additional testing. Similar to buckling, out-of-plane bending is of concern when compression testing because the variation of strains and stresses across the specimen thickness can lead to reductions in measured compression strength. Bending is commonly traced to problems in specimen fabrication and machining that result in thickness variations. However, bending also can be the result of misalign- ment in the test fxture or even the testing machine. Similar to buckling, specimen bending may be detected using back-to-back strain gages during testing. However, bending produces a difer- ence in the slopes of the stress vs. strain diagrams from the two gages throughout the test (Fig. 3) rather than a sudden diver- gence. Let's note, here, that the ASTM standards mentioned above limit bending to less than 10% during the entire test. Interestingly, research results 4 suggest that higher levels of bending (up to 30%) do not signifcantly reduce compressive strength. Tus, although eforts must be made to minimize misalignments that lead to out- of-plane bending, buckling remains the greater concern. ABOUT THE AUTHOR Dr. Daniel O. Adams is a professor of mechanical engineering, the director 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 35 years of academic/industry experience in the composite materials feld and chairs both the Research and Mechanics Div. of ASTM Committee D30 on Composite Materials and the Testing Committee of the Composite Materials Handbook (CMH-17). WHETHER YOU'RE BUYING 2 OR 200... ...THERE'S A SUPERIOR TOOL IN YOUR FUTURE! Tools for Composite, Aluminum, Titanium, Steel. Quick Turnaround on Tools & Coating. ORDER RAPID CUSTOM MADE: 800.428.TOOL (8665) TO BUY ONLINE STOCK: SUPERIORTOOLSERVICE.COM