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    Fatigue Fundamentals for Composite Materials

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    As composites are a combination of many organic and inorganic materials already studied, we draw on this information to guide us in strength evaluations, interpretation of results, and design aspects of the new materials. Basically, many micro- and macrostructural differences exist between the materials made of metals and those of reinforced plastics, sandwich constructions, and composites. The microstructure of the metals allows engineers to assume metals are isotropic and hence use simple formulas relating stress and strain. In contrast, composites are a mixture of dissimilar materials ranging from ductile, rubbery, weak, low-modulus materials to brittle, strong, high-modulus reinforcements. The result is a non-homogeneous or anisotropic material involving complicated relations of stress and strain. If the dissimilar materials are arranged in layers and oriented in specific patterns, such as many reinforced plastics and sandwich constructions, the composite may be considered an orthotropic material and treated as such mathematically. In addition to the various strength properties of the component materials, the composite also is affected by environmental factors, fabrication factors, and service factors. For example, temperature, moisture, various liquids, various gases, notches, imperfections, surface conditions, size, damping characteristics, adhesion, fastenings, cold working, orientation of crystals and fibers, and the rate of loading are all factors that affect the time and repetition of load, which is called fatigue. The effect of such factors generally cannot be predetermined mathematically, so that experimental evaluation must be made of each new composite. In evaluating strength and related properties of the composites, experience with well-known materials provided the concepts for testing coupons, models, prototypes, and full-scale structures. Experimental evaluations include the effects of various environmental exposures, repeated and dead loads, various stress ranges, and as many mechanical tests as possible. Current trends are to use accelerated methods of test (such as the Prot, staircase, Probit, and rate process) to assay quickly the material's strength and life and to provide design criteria. Standard strength determinations provide stress-strain relations in tension, compression, and shear. Fatigue determinations provide curves of stress and strain magnitude versus usable life or time to failure. Superimposed on such graphs are the effects of environments in service. Just as there are many facets and ramifications in construction of composites, there are likewise many variables in the testing for establishing design criteria. If established procedures are not feasible, modifications are necessary to evaluate a new product and provide the assurance of safe design loads.


    composite materials, mechanical properties, testing, fatigue, fiber composities, evaluations, tests

    Author Information:

    Boller, K. H.
    EngineerPersonal member ASTM, Forest Products LaboratoryUniversity of Wisconsin,

    Committee/Subcommittee: D30.04

    DOI: 10.1520/STP49819S