The plane-strain initiation and growth fracture toughnesses of powder-metallurgy-processed, Sic particulate-reinforced 2009 plate were measured at temperatures from 25°C to 316°C. Initiation toughness from electrical potential monitoring (KII0) is 18 M P a 6 at 25°C. and is nearly constant to 220°C before decreasing sharply to 6 M P a f i at 316°C. Growth toughness, given by the tearing modulus (TR), is less than 3 from 75°C to 125°C. and increases dramatically above 200°C. The magnitude and temperature dependence of initiation toughness depend on detection of the critical fracture event.Standard measures of toughness K,, and K,,, exceed K,,,, and increase to a plateau with increasing temperature. The fracture mode for the composite is microvoid nucleation, growth and coalescence at all temperatures. Void nucleation is associated with Sic; such particles both crack and create stress and plastic strain concentrations that rupture the interface or adjacent matrix, particularly at corners. Matrix plasticity and cavitation increase with increasing temperature. Void growth is regular at all temperatures, but limited by adjacent SIC particles. Both K,,,, and TR are governed by the temperature-dependent crack-tip plastic stress and strain fields, and the intrinsic damage resistance of the composite microstructure. NOMENCLATURE a = crack length E = elastic modulus J = J-integral B, = net specimen thickness J,, = elastic component of J-integral Jlc = engineering value of critical J-integral for fracture initiation Jlc, = lower-bound value of critical /-integral for fracture initiation JpI = plastic component of J-integral K = stress intensity factor K,, = engineering value of critical stress intensity for fracture initiation K,,, = engineering value of critical stress intensity calculated from J,, K,lci = lower-bound value of critical stress intensity calculated from Jlci K,,, = maximum stress intensity in fatigue cycle n = work-hardening exponent P = applied load TR = tearing modulus W = specimen width 6 = load-line displacement E = total true strain v = Poisson's ratio CT = true stress EF =constrained ductility u0 = flow strength cuts = tensile strength cys = yield strength 565 566 B. P. SOMERDAY et al.