Application of Photometric Data to Estimation of the Deterioration of Fatigue Properties of an AMg6 Alloy*

Abstract

The method of constructing the fatigue curves by photometric data obtained in surface analysis of three-four flat specimens, fatigue-tested in cantilever bending, was examined using an AMg6 aluminum alloy as an example. Before and after experiments the test portions of each specimen are scanned, and the durability of their fragments is assessed at known stress amplitudes by the generated data.Introduction. Fatigue testing of structural materials is one the most labor-and time-consuming mechanical treatments. For plotting a fairly representative fatigue curve based on 10 7 cycles, 15-50 specimens and no less than 5000-5500 h would be required. The time can be significantly reduced only by employing several test units. On the other hand, the fatigue resistance of materials is their important characteristic since the majority of machines and engineering constructions are subject to variable loads in their service. However, the theory of fatigue fracture of metals and alloys, considering the whole body of existing information on their behavior under cyclic loading and creating the basis for engineering strength calculations, has not been elaborated up to now [1]. The attempts to analyze fatigue fracture on the basis of fracture mechanics approaches [2] were not very successful because they are practically inapplicable for analysis of the conditions of fatigue crack nucleation and their extension to critical sizes. Thus, further accumulation of fatigue test data is currently central. Considering the above, it should be noted that development of fatigue test methods is quite appropriate, which allowed their volume to be sharply cut down without impairing informativity and reliability with simultaneous reduction in their costs. Such is indeed the method of accelerated fatigue tests built upon the information on the structural evolution of the material at cyclic loads. The approach to fatigue damage assessment, elaborated by Baikov Institute of Metallurgy and Materials Science in cooperation with Bauman Moscow State Technical University, is based on the photometric diagnostics of structural damage of specimen surfaces, fatigue-tested in cantilever bending with a rigidly fixed specimen grip section.Materials and Methods. Blade-type flat specimens from an AMg6 aluminum alloy were the object of investigation, their chemical composition is presented in Table 1.The specimens l =149 mm long, b =10 mm wide, and d =1 mm thick were used. The tests were performed on electrodynamic vibrotables in the resonant excitation mode of specimen vibrations at frequencies close to the frequency of the first harmonic of their natural vibrations. Parameters of specimen tests and their results are summarized in Table 2. The test portions of the specimens are first divided into equal-sized fragments and scanned. The division of a computerized specimen image into fragments is exemplified in Fig. 1.