Creep of laminated aluminum composites
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FIBERreinforced composites are designed to provide better mechanical properties, such as specific modulus and specific strength, than unreinforced matrix material, using the superior properties of the fibers as the load carrying component. These materials are anisotropic and the advantages in mechanical behavior are predominantly confined to the longitudinal axis of the composite; typically the longitudinal tensile strength can be up to 30 times the transverse strength. 1This anisotropy of behavior occurs over a large range of temperature and at temperatures where creep deformation is important the differences between longitudinal and transverse creep characteristics can be significant. The creep behavior of unidirectional composites is governed primarily by the creep of fibers when longitudinally loaded and by creep of the matrix when transversely loaded. Examination of the literature on creep of unidirectionally bonded fiber composite systems, particularly systems based on boron fibers-aluminum alloys2-5 shows that longitudinal creep is controlled by the fibers; conversely the creep of transversely stressed boron-aluminum composites is typical of matrix creep behavior. The detailed evaluation of Toth and co-workers5,6on unidirectional and cross-ply boron-aluminum composites show that as the direction of applied stress deviates from the longitudinal fiber axis the creep resistance decreased rapidly. Small but worthwhile improvements in creep behavior in off-axis directions may be obtained with cross-ply configurations. Ericksen's 7 work on transversely stressed boronaluminum showed that creep rates at 300 K were generally less than those of the unreinforced matrix at the same stress. A wide-ranging study on boron and Borsic fibers in various aluminum alloy composites by Breinan and Kreide: indicated that the transverse creep behavior was dependent on the strength of the matrix such that the composite creep strength could be more or less than that of the unreinforced matrix and that fiber splitting was a common mode of failure. A W. MOORE, formerly Postgraduate Research Student, Department of Metallurgy, University of Manchester/UMIST, is now Marketing Manager, Metal Treatment Division, Foseco Steelmills International, Ltd., Birmingham, United Kingdom. T. J. DAVIES is Lecturer, Department of Metallurgy, University of Manchester/ UMIST, Manchester, United Kingdom. Manuscript submitted December 14, 1979.
reduction in primary creep rates of transversely loaded boron-aluminum alloy composites was observed by Lucas and McNelley.9 These workers observed a transition stress above which the strain to failure of their composites decreased; the.temperature dependence of the secondary creep stage was similar for composite specimens and laminated matrix element specimens. In all cases, Lucas and McNelley attributed failure of the composites to debonding at fiber matrix interfaces and concluded that this debonding governed the ultimate strain to failure. An alternative reinforcing system is the laminate assembly in which the d
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