CompositesWorld
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JULY 2015 8 CompositesWorld DESIGN & TESTING ยป Unlike metals and plastics, for which tensile and compressive properties are the same or very similar, unidirectional composites exhibit signifcantly diferent mechanical properties, such that both tension and compression testing must be performed. Te relationship between composite tensile and compressive properties is complex. Te tensile and compressive modulus of elasticity of unidirectional composites in the fber direction, E1, are similar, but diferences of 10% or more are commonly observed for carbon/epoxy composites, with the compressive modulus typically lower. Te reason for these diferences is not well understood. More importantly, however, the tensile and compressive strengths of unidirectional composites in the fber direction difer greatly, with the compressive strength often signifcantly lower than the tensile strength. Tis strength diference results from the diferent failure modes that occur for each: fber tensile failure under tension loading vs. fber micro-buckling under compression loading. Tis lower compressive strength has important consequences for design, especially in applications that involve fexural loading, in which equal magnitudes of tensile and compressive stress are produced. Unfortunately, it's impos- sible to predict the unidirectional compressive strength of composites based on fber and matrix properties, even when tensile strength is known. Tus, compression testing typically is performed in addition to tension testing, and compres- sive strength is widely considered the more critical property. Measuring compressive strength of unidirectional compos- ites is among the most difcult tasks in composites testing. In my January 2015 column (short.compositesworld.com/DandT0615), I summarized the difculties associated with obtaining the tensile strength of unidirectional composites. Tese stem from the need to introduce a relatively large load into the specimen and produce failure in the central gage section before failure occurs elsewhere. Te same is true for compression testing, and potential specimen buckling/bending make compression testing even more difcult. To get the most out of the compression test, we must frst select a test method. Te primary diference among the commonly used methods is the manner in which compressive load is introduced into the specimen (Fig. 1): through shear loading of the tabbed specimen surfaces, through direct compression loading of the specimen ends, or a combination of the two. Regardless of the loading method used, however, tabbed specimens are typically required to achieve the desired results. Typically glass fabric/epoxy Optimum unidirectional compression testing of composites FIG. 1 Specimen loading methods for compression testing. Source (all images) | Dan Adams printed circuit board material is used for tabbing compression specimens, because it is readily available at low cost, has low stif- ness but high strength and can be machined in the same manner as the tested composite material. Te most commonly used shear- loaded compression test method is ASTM D 3410 1 . Its test fxture (Fig. 2) includes fat wedge grips that are bolted onto the tabbed surfaces of the specimen, with the assembly placed into mating cavities with wedge-shaped spacers. Alignment rods and linear bearings provide the required alignment between fxture halves. Upon loading, wedge-action gripping is produced in a manner similar to mechanical wedge grips used in tensile testing. For unidirectional composites, the test method specifes a 13-mm wide specimen that is 140 mm long with a 13-mm gage length between the tabs. However, greater specimen widths and gage lengths are permissible, if desired. Although popular in the 1990s, ASTM D 3410 is used less frequently now, due to the large fxture's high cost and the development of other more popular test methods. End loading of the specimens is another option, but tabs are required here as well to achieve a suitable compression failure in the gage section prior to crushing or splaying at the specimen ends. Additionally, lateral supports are needed to prevent buckling during loading. Te most popular end-loaded compression test method is the Modifed ASTM D 695 test method (Fig. 2). Tere is actually no ASTM standard governing this method, but it is defned in the SACMA Recommended Test Method SRM 1R-94 2 . ASTM D 695 specifes a shorter 4.7-mm gage length. Tis reduces the risk of specimen buckling for a given specimen thickness. But there's a downside: this gage length is too short to permit the use of strain gages or extensometers and, therefore, a separate set of untabbed specimens must be tested for elastic modulus determination. Te third method of loading combines both shear loading and end loading. Te Combined Loading Compression (CLC) test method, ASTM D 6641 3 , was standardized by ASTM in 2001, and has become the most commonly used compression test method for composites. Its test fxture (Fig. 2) consists of four steel blocks with specimen gripping surfaces coated with tungsten carbide particles. Te upper and lower pairs of fxture blocks are bolted FIG. 2 Three types of compression test fxture (from left to right): the IITRI compres- sion test fxture (ASTM D 3410), the Modifed ASTM D695 test fxture, and the combined loading compression test fxture (ASTM D 6641). . Shear End Combined Loading Loading Loading