CompositesWorld
Issue link: https://cw.epubxp.com/i/919214
JANUARY 2018 10 CompositesWorld DESIGN & TESTING » When the opportunity arises, I like to ask those familiar with composites testing this question: What type of mechanical test do they consider to be the most important for composite mate- rials? After listening to responses, I follow up by asking people what they think is the primary purpose for performing the test and what are its intended applications. I'll share here what I've learned from asking these questions. But before I do, I'd suggest those who read this spend a minute thinking about what their responses would be to these questions. Perhaps it comes as no surprise that there is no single type of mechanical test that's the consensus choice as most impor- tant for composites, especially given the variety of purposes for testing and intended applications. ere are, however, a few test methods that were more often mentioned. ese can be grouped into three general categories and best discussed in terms of the primary purposes for performing the test. One test method mentioned frequently as most important is the short beam shear test, described in ASTM D 2344 1 (Fig. 1). One of the simplest to perform, it uses the smallest test specimen of any mechanical test method for composites. e test consists of a three-point bend test performed using a short loading span relative to the specimen thickness. is results in interlaminar shear failure within the specimen's interior. e measured short beam strength is highly sensitive to problems with porosity, fiber-to-matrix adhesion and the layer-to-layer strength within the composite laminate. erefore, although the measured short beam strength may be considered only an estimate of the interlaminar shear strength, this test method is widely considered an excellent choice for comparative testing purposes. Accordingly, stated purposes for performing this test include material comparison and selection, material and fabri- cation process development and quality control. Not as commonly mentioned as most important, the ±45° tensile shear test, ASTM D 3518 2 , may be used for these purposes as well. e advantage of this test is that it produces a shear stress vs. shear strain curve throughout the loading and, thus, provides a measurement of shear modulus in addition to shear strength. e nonlinear shear stress vs. shear strain curve is analogous to a "fingerprint" that corresponds to a tested material, allowing for a more thorough comparative assess- ment between materials or processing conditions. However, the disadvantage is the required use of bonded strain gages for shear strain measurement and, therefore, significantly greater effort and cost. As a result, the ±45° tensile shear test remains a distant second choice as most favorite for the above stated purposes. What's the most important type of mechanical test for composites? Another common purpose for performing mechanical testing of composites is to determine material properties for use in design and analysis. While many types of material characterization tests are performed with composites, compression strength testing in the fiber direction is often considered most important. e reason? For virtually all carbon fiber-reinforced polymer matrix composites, the compression strength is significantly lower than the tensile strength and, therefore, is the limiting design value. Interestingly, those who identify compression testing as most important sometimes will also identify the most important test environment in which to perform the test. For polymer matrix composites, the most severe environmental condition is often the highest service temperature, coupled with a moisture saturation condition. is "hot/wet" combination is typically specified as the most important environmental condition in which to perform the fiber-direction compression test. FIG. 1 Short beam shear test, ASTM D 2344. Source (all images) | Dan Adams FIG. 2 Combined-loaded ASTM D 6641 test (left) end-loaded Boeing-modified D 695 test (cen- ter) and face-loaded ASTM D 3410 test (right).