The effect of cryogenic cooling on the tensile properties of metal-matrix composites
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THE
high cost of oriented f i b e r composite materials, together with the need for high performance which iusrifles t h e i r use, make it mandatory that these materials come as close to developing t h e i r full potential in mechanical properties as is possible. The elastic modulus and elastic limit (or yield strength) are important mechanical properties to those who must design with them. Thus, the attainment of high values for b o t h of these properties takes on critical importance. The intrinsic nature of the components of the comp o s i t e (high strength, high modulus fibers, and lower strength, l o w e r modulus matrices) dictate a t h r e e stage response of these materials t o mechanical loading-totally elastic, elastic fibers with plastic matrix, and totally plastic (if the fibers are capable of plastic flow). The elastic l i m i t is determined by the onset of plastic flow in the m a t r i x component, while the yield strength is taken as the s t r e s s which produces some predetermined m i n i m a l amount of plastic flow in the matrix. Unfortunately, it is almost n e v e r possible to achieve the m a x i m u m elastic response of which composites are capable because of the presence of residual stresses which result from fabrication. These stresses may be sufficiently high that the totally elastic response is n e v e r observed in some composites. The situation is further complicated by the fact that the standard s t r e s s relieving operations which are effective in homogeneous materials a r e almost completely ineffective on composite materials. Hence, the presence of residual stresses constitutes a s e v e r e limitation t o the use of these materials. While the residual s t r e s s e s can n e v e r be removed from composite materials, it has been amply demonstrated that t h e i r relative intensities can be a l t e r e d in given directions by the addition of s m a l l amounts of prestrain. 1-3 As a m a t t e r of fact, it is possible to a l t e r the relative intensities to the d e g r e e that the deviatoric component of the residual s t r e s s s t a t e b e c o m e cornJOHN GAYDA and L. J. EBERT are GraduateStudent and Professor of Metallurgy, respectively, Case Western Reserve University, Cleveland, OH 44106. Manuscript submitted August 9, 1978. METALLURGICAL TRANSACTIONS A
pressive in the direction which will be subjected to tension in use. Hence, the residual stresses, as alt e r e d by prestrain, can actually be beneficial, r a t h e r than deleterious. Further, it has been shown that the beneficial effect of prestrain can be predicted by rigorous analytical methods. While mechanical prestrain has been shown t o be quite effective in reducing or removing the a d v e r s e effect of residual stresses, it is not always possible or practical t o prestrain composite hardware. T h e r e f o r e , other m e a s u r e s must be found if composite mat e r i a l s are t o achieve t h e i r full potential as high performance materials. In seeking other r e m e d i a l m e a s u r e s , the signi
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