Role of matrix properties on the toughness and notch-strength of aluminum-alumina fiber composites

Preliminary results on the fracture behavior of continuous-fiber unidirectional aluminum matrix composites are reported. The composites are reinforced with 50-60 vol% Nextel(TM) 610 Ceramic Fibers. The fibers are pure alpha-alumina 10-12 mu m diameter. The aluminum matrix alloys investigated are 6061-T6, Al-2Cu-T6 and pure Al, each resulting in a strong interfacial bond while offering a range of matrix yield strengths. Results of monotonic mode I testing of both compact tension specimens, center-cracked panels and center-notched (drilled holes) panels show a strong dependence of fracture behavior on the matrix yield strength. High values of yield strength are typified by the 6061-T6 alloy, which shows mode I cracking across the fibers accompanied by moderate toughness levels (25-35 MPa root m). The lower yield strength matrices result in a strong tendency toward mode II crack propagation along the fiber direction. Such splitting initiates and propagates at relatively high values of the applied stress intensity factor (50-65 MPa root m). The low notch sensitivity of this class of metal matrix composites is attributed to the splitting mechanism. These results are in contrast to a recent study on Al/Al2O3 composites, in which mode I cracking was observed for relatively low matrix yield strengths. It is clear that the fracture behavior of this class of composites requires further study and modeling. In particular, the focus of future study will focus on the initiation and propagation of mode II splitting cracks.