Continued advancements in the design and manufacture of engineered composites has allowed composite materials to work their way into the products we use everyday. By definition, a composite material is simply any material that contains two or more constituent materials. The materials that make up a composite are chosen either to enhance or supplement the material properties of the individual materials. The typical material combination seen in today's composites is a material that performs well in tension (fiber or rebar) paired with a material that performs well in compression (epoxy, ceramic, or cement). The most common modern advanced composites are fiber-matrix composites and they can be manufactured with polymer, carbon, metal, or ceramic matrix and an extremely wide range of reinforcement fiber including, carbon, graphite, boron, aramid, and glass. Shear is an extremely important mechanism for composite materials, since current load transfer theories for fiber reinforced composites use shear stress at the fiber-matrix bond as the sole load transfer mechanism. The shear strength of a composite material is a function of the interlaminar shear strength, shear properties of the bulk matrix material, and the fiber-matrix bond strength. Shear tests of composite materials require specially designed grips and fixtures to ensure proper alignment of test samples, limit failure to the specific mode and shear plane being tested, and prevent improper load application which could damage the composite microstructure and adversely impact test results.

Since matrix shear strength is the dominating factor in the shear strength of fiber reinforced composites, alignment is not as crucial as with compression and tensile tests. It is necessary to prevent rotation of the composite test specimen, as specimen twisting can result in fiber-matrix debonding, delamination of the composite, or matrix cracking, which will all negatively impact composite shear strength. Test fixtures are typically designed to be used in a wide variety of test environments, which are necessary to recreate the real life application conditions of composite materials. The creation of these environments is achieved through the use of saline baths for testing composites used in marine, biological, and outdoor environments and high temperature environmental chambers for composites used in the aviation, aerospace, and automotive industries. ASTM and ISO have developed standard test methods for testing composite materials in shear. The standards provide methods that can be recreated, ensuring materials are tested in the same manner and conditions and allowing test results validation between manufactures and customers. Popular shear strength test methods are ASTM D3518 for in-plane shear of polymer matrix composites by tensile loading, ASTM D4255 for in-plane shear of polymer matrix composites by rail shear, ASTM D4762 for polymer matrix composites, ASTM D5379 for shear properties by the v-notched beam method, ASTM D5448 for hoop wound polymer matrix composite cylinders, ASTM D7078 for shear properties by v-notched rail shear, ASTM D7616 for overlap splice shear of fiber reinforced polymer matrix composites, ASTM D7617 for transverse shear strength of fiber reinforced polymer matrix composite bars, ISO 3597-4 for interlaminar shear of glass roving reinforced plastics, EN658-4 for interlaminar shear of ceramic composites by compression loading, and EN658-5 for interlaminar shear of ceramic composites by bending.

Most fiber reinforced composites have significantly lower shear strength than compressive or tensile strength. Even with the relatively low shear strength of composite materials, the required force to shear a composite to failure is significant due to the large surface area over which the shear stress is applied. Depending on the individual sample size and composition of the composite, the force requirements can be met with either tabletop or floor standing machines. The machine families below can be matched to the wide range of composite materials being tested. Since grips for composites tend to be custom, the shear fixtures below are designed for specific ASTM or ISO test methods, and other custom grips can be manufactured. Contact a TestResources application engineer to determine the best machine and grip combination for your exact composite testing needs.

Applicable Testing Standards

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