Fatigue behavior of composite materials is a particularly important area of composites research, as we attempt to understand the mechanisms of fatigue failure. While researchers continue work on the mechanics of composite fatigue damage, industry must test their products for fatigue performance to ensure customer safety and satisfaction. Aircraft are one of the best examples of composite materials undergoing cyclic loading that could lead to fatigue damage or failure. Aircraft are designed to undergo tens of thousands of cabin pressurization, take-off, and landing cycles over their lifespan. In each pressurization cycle the airplane fuselage undergoes an expansion and subsequent contraction, applying and relieving stresses on the fuselage skin, stringers, and fasteners. Take-off and landing cycles change the loading on the wing spars, struts, skin, and root from supporting its own gravitational weight while on the ground to holding the entire weight of the aircraft and payload while in flight. Lightweight polymer matrix composites, such as carbon-fiber epoxy, have demonstrated a promising fatigue life and great strength to weight properties, resulting in their increased use for airframe structural components. As with static tests, composite fatigue testing is sensitive to specimen alignment and load application, due to material anisotropy. Specimen alignment and orientation may not be as crucial for non-aligned short fiber composites, as many exhibit quasi isotropic behavior. However, non-aligned short fiber composites should still be carefully aligned and tested in multiple orientations to ensure that manufacture processes do not induce undesired fiber alignment. All aligned fiber composites will exhibit different mechanical properties in different orientations. ASTM and ISO have developed standard test methods for testing the fatigue properties of composite materials. 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 composite fatigue test methods are ASTM D3479 for tension-tension fatigue of polymer matrix composites, ASTM D4762 for polymer matrix composites, ASTM D6115 for mode I fatigue delamination growth of unidirectional fiber reinforced polymer composites, ASTM D6873 for bearing fatigue response of polymer matrix composites, ASTM D7615 for open-hole fatigue of polymer matrix composite laminates, and ISO 13003 for fatigue properties of fibre reinforced plastics under cyclic loading. Advanced composite materials used in fatigue applications tend to be high modulus and high strength materials. Therefore, most composites fatigue testing requires high force capacity machines and precision control systems. TestResources line of electrodynamic test machines is well suited for composite materials testing requirements and can be configured with a vast assortment of grips and fixtures designed to meet the needs of fatigue testing. Contact a TestResources application engineer to help determine the best fatigue test system for your composite testing requirements.

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