Optimum unidirectional compression testing of composites
Dr. Daniel O. Adams, a professor of mechanical engineering, the director of the Composite Mechanics Laboratory at the University of Utah, and VP of Wyoming Test Fixtures Inc. (Salt Lake City, UT, US) discusses the complex relationship between composite tensile and compressive properties, why that makes it necessary to perform both tension and compression testing, and how best to perform the latter.
Unlike metals and plastics, for which tensile and compressive properties are the same or very similar, unidirectional composites exhibit significantly different mechanical properties, such that both tension and compression testing must be performed.
The relationship between composite tensile and compressive properties is complex. The tensile and compressive modulus of elasticity of unidirectional composites in the fiber direction, E1, are similar, but differences of 10% or more are commonly observed for carbon/epoxy composites, with the compressive modulus typically lower. The reason for these differences is not well understood. More importantly, however, the tensile and compressive strengths of unidirectional composites in the fiber direction differ greatly, with the compressive strength often significantly lower than the tensile strength. This strength difference results from the different failure modes that occur for each: fiber tensile failure under tension loading vs. fiber micro-buckling under compression loading. This lower compressive strength has important consequences for design, especially in applications that involve flexural loading, in which equal magnitudes of tensile and compressive stress are produced. Unfortunately, it’s impossible to predict the unidirectional compressive strength of composites based on fiber and matrix properties, even when tensile strength is known. Thus, compression testing typically is performed in addition to tension testing, and compressive strength is widely considered the more critical property.
Measuring compressive strength of unidirectional composites is among the most difficult tasks in composites testing. In my January 2015 column (see "Tensile testing of composites: Simple in concept, difficult in practice," under "Editor's Picks," at top right), I summarized the difficulties associated with obtaining the tensile strength of unidirectional composites. These 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. The same is true for compression testing, and potential specimen buckling/bending make compression testing even more difficult.
To get the most out of the compression test, we must first select a test method. The primary difference 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 printed circuit board material is used for tabbing compression specimens, because it is readily available at low cost, has low stiffness but high strength and can be machined in the same manner as the tested composite material. The most commonly used shear-loaded compression test method is ASTM D 34101. Its test fixture (Fig. 2) includes flat 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 fixture 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 specifies 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 fixture’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. The most popular end-loaded compression test method is the Modified ASTM D 695 test method (Fig. 2). There is actually no ASTM standard governing this method, but it is defined in the SACMA Recommended Test Method SRM 1R-942. ASTM D 695 specifies a shorter 4.7-mm gage length. This 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.
The third method of loading combines both shear loading and end loading. The Combined Loading Compression (CLC) test method, ASTM D 66413, was standardized by ASTM in 2001, and has become the most commonly used compression test method for composites. Its test fixture (Fig. 2) consists of four steel blocks with specimen gripping surfaces coated with tungsten carbide particles. The upper and lower pairs of fixture blocks are bolted to the tabbed specimen, and the assembly is loaded between flat platens. The amount of shear loading is controlled by the torque applied to the bolts that connect the fixture blocks, with a goal of providing sufficient shear loading to avoid crushing at the specimen ends. The specimen dimensions are the same as those specified in ASTM D 3410, although specimen widths up to 30 mm can be accommodated. Advantages of the CLC test method include the relatively small, simple and inexpensive test fixture and the potential to obtain compressive strength test results with less data scatter — a benefit of combined shear and end loading.
Regardless of which test method is selected, the most important consideration is proper specimen design. As in tensile testing, relatively thin specimens are of interest, and bonded tabs are required. Unlike tensile testing, however, the use of a shallow tab taper angle leading into the gage section isn’t possible due to the risk of buckling. In general, a thicker specimen and shorter test section are needed to prevent specimen buckling prior to compressive failure. Typically, the shortest practical test section length is selected, and the required minimum specimen thickness to prevent buckling is determined. The two popular ASTM test methods for compression testing of composites, ASTM D 3410 and D 6641, provide an equation for calculating the required specimen thickness, h, to prevent bucking of an orthotropic specimen of rectangular cross section, which can be written as