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Fatigue data are normally obtained from three general types of tests, which may be designated as the material type, the structural type, and the actual service type. The material type of fatigue test is the conventionalized or in some instances standardized one, using rather small specimens of various sizes and shapes, depending on the mode of stressing to be imposed and the testing machine involved. The structural type of fatigue test for this classification should be broadly interpreted to include machine parts and assemblies. The actual service type of fatigue test is, as its name implies, a test under actual service conditions. The results obtained from the material type fatigue test are generally considered to be indicative of the inherent fatigue strengths of the materials tested, when subjected to specific types of repetitive loading and environment. In making such tests major attention is accorded the material, but a number of other factors are involved whose effects upon the results obtained must be considered for proper interpretation of the test data. The more important of these factors include: 1. Specimen geometry (size and shape), 2. Specimen preparation (machining, polishing), 3. Testing machine (design, mechanical condition, operation), 4. Testing technique, 5. Speed of testing, 6. Type of stressing, and 7. Environment. Consideration of these factors is important because frequently minor differences in them can cause appreciable differences in fatigue results. Tolerable discrepancies in these factors are generally smaller than those considered satisfactory for static tests of the materials. Data from the material type of fatigue test may be advantageously used for the following purposes: 1. Comparing the behavior of different materials subjected to repeated stresses, 2. Comparing the effects of various manufacturing practices or processes upon the fatigue properties of materials, 3. As a guide in developing new materials, or manufacturing processes, 4. Comparing the behavior of materials in various environments, while subjected to repeated stresses, 5. Comparing effects of various simple geometrical factors such as different sizes and shapes of notches and different surface finishes, 6. Establishing correlations with other mechanical properties, different types of stressing, chemical composition, etc., 7. Checking the quality of different lots of a given material, and 8. Evaluating the effects of surface treatments such as case hardening, decarburization, nitriding, shot peening, and plating upon the fatigue properties of materials.
Templin, R. L.
Assistant Director of Research and Chief Engineer of Tests, Aluminum Company of America, New Kensington, Pa.