Measurement of residual stresses in graphite-Al and graphite-Mg composites by acoustic emission techniques

Abstract

Residual stresses occur in graphite-reinforced metal matrix composites (MMCs) as a result of a mismatch in the coefficient of thermal expansion of the fiber (negative) and the matrix (positive) during cooling after consolidation. Generation of residual stresses can alter subsequent thermomechanical properties and damping characteristics of MMCs. It is imperative to know the magnitude of residual stresses in order to design and to predict the thermomechanical behavior of these composites. Acoustic emission was used in this study to determine the amount of residual stress. A two peak r.m.s. behavior was observed in precursor wire specimens, the low amplitude peak being attributed to matrix yielding and interfacial degradation, while the high amplitude peak was due to fiber breakage. Detection of the onset of matrix yielding in graphite-aluminum composites by acoustic emission techniques was used to determine the magnitude of residual stresses. Using this approach, residual stresses in [0°] Pitch-55 6061 graphite-aluminum alloy was found to be between 35.2 and 124.8 MPa (5.1 and 18.1 klbf in −2), depending upon whether the aluminum alloy 601 matrix when incorporated in the composite was in the 0 or T4 condition. Similar techniques for graphite-Mg composites determined that much lower levels of residual stress of 24.6 MPa (3.53 klbf in −2) were observed.