FRACTURE OF FIBROUS METAL MATRIX COMPOSITES .4. PLASTIC ZONES, LOCAL STRESSES AND FRACTURE STRENGTH

An analytical procedure for prediction of fracture strength in unidirectionally reinforced, notched metal matrix composite (MMC) plates is described. As in Parts I-III [Engng Fracture Mech. 34, 87-104 (1989); 34, 105-123 (1989); 37, 1207-1232 (1990)], the objective is to evaluate the tension stress in a small volume of material ahead of the notch tip, while the composite plate is loaded in simple tension. In the composite systems considered in this study, fracture is typically preceded by formation of long, discrete plastic shear zones aligned in the fiber direction. The length of such zones is evaluated here by the crack interaction scheme proposed recently by Benveniste et al. [Int. J. Solids Structures 25, 1279-1293 (1989)] and a Dugdale-type condition imposed at the end of the plastic zone. Then, the local tension stress ahead of the notch is found by superposition of a ligament stress and the stress induced by the shear tractions in the plastic zone. Comparison with numerous fracture strength measurements on notched B/Al and FP/Al plates confirms that the onset of fracture is associated with a critical magnitude of the local stress in a small volume of the composite material. The present procedure provides closed-form expressions for evaluation of the local stresses, and for fracture strength predictions in unidirectional MMC plates.