Search

This paper aims to statistically estimate the dynamic fatigue strength in brittle materials under a wide range of stress rates. First, two probabilistic models were derived on the basis of the slow crack growth (SCG) concept in conjunction with two-parameter Weibull distribution. The first model, Model I, is a conventional probabilistic delayed-fracture model based on a concept wherein the length of the critical crack growth due to SCG is enough larger than the initial crack length. For the second model, Model II, a new probabilistic model is derived on the basis of a concept wherein the critical cracks have widely ranging lengths. Next, a four-point bending test using a wide range of stress rates was performed for soda glass and alumina ceramics. We constructed fracture probability-strength-time diagrams (F-S-T diagrams) with the experimental results of both materials using both models. The F-S-T diagrams described using Model II were in good agreement with plots of the fracture strength and the fracture time of both materials more so than Model I. [ABSTRACT FROM AUTHOR]

This paper presents the results of a combined experimental and theoretical study of the ductile layer toughening of brittle intermetallics. Following a brief description of small- and large-scale bridging models for the estimation of the crack-tip shielding due to small- and large-scale bridging, the effects of ductile layer toughening are examined in a gamma-based titanium aluminide alloy (Ti-48Al) reinforced with TiNb strips; MoSi2 reinforced with Nb layers, and NiAl reinforced with V layers with different thicknesses. In all cases, the measured resistance-curves are well predicted by existing small- and large-scale bridging models. The intrinsic specimen-independent toughness values are also shown to increase with increasing layer thickness, in the case of the NiAl/V composites. Finally, the general implications of the results are discussed for the toughening of brittle matrix composites under monotonic and cyclic loading. [ABSTRACT FROM AUTHOR]

The brittleness of polystyrene (PS) and the toughness but notch sensitivity of polycarbonate (PC) have been studied by the detailed finite element analyses of the stress and strain fields in a notched tensile bar with a minor defect. The defect represented a flaw or imperfection, generated during the test specimen production. The large-strain mechanical responses of both materials were approximated by an accurate elasto-viscoplastic constitutive model with appropriate material parameters. It was assumed that failure occurs instantaneously once the dilative stress exceeds a certain critical craze-initiation stress. The analyses show that the unstable post-yield mechanical response of both materials results in localisation of stresses and strains near the defect at a very low macroscopic strain (0.16%). As a result, a strong dilative stress concentration is formed just below the surface of the defect. For the polystyrene specimen, the critical stress is reached at the defect. For the polycarbonate, however, the effect of the stress concentrating defect was counteracted by a higher craze-initiation stress and stronger strain hardening. The PC craze-initiation resistance, however, did not suffice to overcome the dilative stress concentration raised by the notch tip. [ABSTRACT FROM AUTHOR]