Elevated Temperature Fracture of Particulate‐reinforced Aluminum Part Ii: Micromechanical Modelling

Abstract

Micromechanical fracture-toughness models are applied to experimental results for a metalmatrix composite (2009/SiC/20p-T6) to understand the temperature dependencies of toughness and fracture mechanisms, as well as to test quantitatively a continuum fracture-mechanics approach. Models which couple the crack-tip strain field, characteristic fracture-process distance and measured intrinsic microvoid-fracture resistance predict the temperature dependencies of fracture-initiation (KJla) and crackgrowth (&) toughnesses from 25°C to 316°C. The temperature dependencies of KJla and TR result from the interplay between the fracture resistance and the crack-tip strain field, each being temperaturedependent. Strain-based models are equally valid for void nucleation-or growth-controlled fracture. A scenario for fracture is nucleation-controlled damage within Sic-particle clusters, corresponding to KnCi, followed by cluster-damage growth to coalescence under increasing stress intensity. Void growth is stabilized increasingly at elevated temperatures. NOMENCLATURE a = crack length a* = Sic-particle flaw size C1 = curve-fitting constant for crack-tip HP distribution C2 = curve-fitting constant for crack-tip Bp distribution d, = variable in relationship between 3 and 6, D = average void-nucleating-particle diameter e = natural-logarithm base E = elastic modulus f, = average void-nucleating-particle volume fraction 3 = 3-integral J,, = standard value of critical 3 for initial crack extension K = stress-intensity factor Klc = standard value of critical K for initial crack extension Kna = lower-bound value of critical K for initial crack extension calculated from 3 K,,,-eps = K,,, predicted from strain-controlled model using experimental E;* K,,,-nuc = K,,, predicted from strain-controlled model using modelled E;* KjIa-RJ = K,,, predicted from Rice-Johnson model I* = characteristic fracture-process distance m = strain rate-hardening exponent n = strain-hardening exponent r = radial distance from crack tip r, = distance behind crack tip at which 6, is defined TR = tearing modulus Z = variable in expression for 6, a = constant which defines average stress elevation around particle a, = constant which defines ''local'' stress elevation near particle comer B = constant in expression for 6 , 6, = critical value of 6 , 6, = stationary-crack-tip opening displacement 103 1 1032 B. P. SOMERDAY et al. 6, = moving-crack-tip opening displacement 8 = effective plastic strain Ef = critical crack-tip process-zone effective plastic fracture strain K = Variable in expression for S , 1 = mean spacing of void-nucleating particles v = Poisson's ratio 62 = material-property parameter in model prediction for62 * = value of 62 at "brittle"-to-"ductile" fracture transition ufl = Bow stress ufnt = ootched-tensile fracture stress u, = local stress near particle u,,, = mean stress u,, = Rambergasgood reference stress us = stressconcentration component of u, bum = ultimate tensile stress uy, = 0.2%-offset yield stress ur = critical stress for particle frac...

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