CompositesWorld
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CompositesWorld.com 11 In my July 2015 column, I discussed the variety of test methods available for determining the compression strength of composites. e primary way in which they differ is the method of load intro- duction into the specimen. e three most commonly performed tests (Fig. 2) are the combined-loaded ASTM D 6641 3 test, the end-loaded Boeing-modified D 695 4 test and the face-loaded ASTM D 3410 5 test. e final category of test methods that have received a consid- erable number of most important votes are those used to develop structural design allowables. For this purpose, the open-hole compression test, ASTM D 6484 6 , and the compression-after- impact test, ASTM D 7137 7 , have been named most important, particularly for aerospace applications. e open-hole compression test (Fig. 3a) uses a 36-mm- wide specimen with a central 6-mm-diameter hole, producing a specimen width-to-hole diameter (w/D) ratio of 6:1. To prevent buckling during compression loading, a support fixture is bolted to the 300-mm-long specimen. e ASTM D 7137 compres- sion-after-impact test uses a 100-mm-wide by 150-mm-long specimen that is loaded in edgewise compression while it is supported to prevent out-of-plane bending along the vertical edges (Fig. 3b). Prior to compression testing, the specimen is subjected to a drop-weight impact to produce damage in the spec- imen's central region. Note that when structural design allowables are generated using either test method, the composite laminate used in the test specimen is representative of the laminate that will be used in the intended application. For compression-after- impact testing, the impactor shape and impact energy used to produce the damage are also selected to represent impact threats for the application of interest. In addition to producing critical structural design allowables for a variety of applications, these two test methods share other important features. First, both are performed under compression loading because the resulting strengths typically are lower than those under tension loading. Second, both tests produce design FIG. 3a Open-hole compression test, ASTM D 6484. allowables that typically are more critical than those resulting from the strength-based material properties discussed in the previous category. As a result, both the open-hole-compression and compres- sion-after-impact tests also are commonly used in the material selection process for an intended application. Because a consistent laminate must be used when testing to compare candidate materials, the use of a quasi-isotropic laminate is suggested. ABOUT THE AUTHOR Dr. Daniel O. Adams is a professor of mechanical engineering and has been the director for 21 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 38 years of academic/ industry experience in the composite materials field. He has published more than 120 technical papers, is vice-chair of ASTM Committee D30 on Composite Materials and co-chair of the Testing Committee for the Composite Materials Handbook (CMH-17). He regularly provides testing seminars and consulting services to the composites industry. References 1 ASTM D 2344/D 2344M-16, "Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates," ASTM International (W. Conshohocken, PA, US), 2016 (first issued in 1965). 2 ASTM D 3518/D 3518M-13, "Standard Test Method for In-Plane Shear Response of Polymer Matrix Composite Materials by Tensile Test of a ±45° Laminate," ASTM International (W. Conshohocken, PA, US), 2013 (first issued in 1994). 3 ASTM D 6641/D 6641M-16, "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), 2016 (first issued in 2001). 4 Modified ASTM D 695 Compression Test Fixture, Wyoming Test Fixtures Inc., http://www. wyomingtestfixtures.com/Products/b2.html. 5 ASTM D 3410/D 3410M-16, "Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials with Unsupported Gage Section by Shear Loading," ASTM International (W. Conshohocken, PA, US), 2016 (first issued in 1975). 6 ASTM D 6484/D 6484M-14, "Open-Hole Compressive Strength of Polymer Matrix Composite Laminates," ASTM International (W. Conshohocken, PA, US), 2014 (first issued in 1999). 7 A STM D 7137/D 7137M-17, "Compressive Residual Strength Properties of Damaged Polymer Matrix Composite Plates," ASTM International (W. Conshohocken, PA.), 2017 (originally published 2005). FIG. 3b Compression-after-impact test, ASTM D 7137. Mechanical Testing