The effects of matrix and fiber properties on the mechanical behavior and acoustic emission in continuous fiber reinforced metal matrix composites

This paper examines and compares the mechanical properties and acoustic emission activity of four metal matrix composites (MMCʹs). The composites were manufactured with either 6061 Al or a high purity aluminum (HPAL) matrix. The two fibers used were the Nextel 440 and 610 fibers made by the 3M Corporation. The two fibers differed in stiffness, strength, composition and geometry. The strengths of the composites were found to increase from the 610/6061 composite to the 440/6061 and 440/HPAL composites to the 610/HPAL composite. The high strength and low ductility of the 6061 matrix led to a rapid propagation of failure in these materials. The increased ductility of the HPAL matrix slowed crack propagation and more fibers broke prior to failure of these composites. The increased stiffness of the 610 fibers led to an increase in residual stresses and earlier yielding of the matrix. A flat fracture surface was observed for all the composites with little fiber pullout indicating a strong fiber/matrix bond. The acoustic emission (AE) events could be separated into two regions for all the composites. Events in region one were attributed to dislocation motion during yielding of the matrix. The events in this region were predominantly low amplitude. Events in region two were attributed to continued dislocation motion and fiber or inclusion breaking (cracking). The events were predominantly lower amplitude with some higher amplitude events occurring. The cracking was more extensive in the 440 fiber composites due to the increased number of fibers breaking prior to failure as indicated by the increase in AE activity of the 440 composites compared with the 610 fiber reinforced composites.