Fracture of aluminium reinforced with densely packed ceramic particles: Link between the local and the total work of fracture

Model composites of pure Al reinforced with 50% ceramic particles are produced by infiltration. The composite fracture energy is measured by J-integral testing. Marked R-curve behaviour is found. The J-R curves exhibit a break in their slope at a well-defined point. This point is shown to denote the onset of macroscopic crack propagation and is used to assess the composite toughness. Toughness reaches values as high as 40 MPam√m. Quantitative metallography and stereoscopic reconstructions of fracture surfaces are used to estimate the local work of fracture in the process zone. The measured (total) fracture energy is about ten times the estimated local fracture energy, for all composites. The main contribution to their total fracture energy is thus from plastic dissipation around the crack-tip; however, the toughness is still dictated by the local fracture energy. This study hence experimentally substantiates the "valve" concept in fracture mechanics.