Fracture toughness

About: Fracture toughness is a research topic. Over the lifetime, 39642 publications have been published within this topic receiving 854338 citations.

Toughness Properties of a Silicon Carbide with an in Situ Induced Heterogeneous Grain Structure

Abstract: Toughness characteristics of a heterogeneous silicon carbide with a coarsened and elongated grain structure and an intergranular second phase are evaluated relative to a homogeneous, fine-grain control using indentation–strength data. The heterogeneous material exhibits a distinctive flaw tolerance, indicative of a pronounced toughness curve. Quantitative evaluation of the data reveals an enhanced toughness in the long-crack region, with the implication of degraded toughness in the short-crack region. The enhanced long-crack toughness is identified with crack-interface bridging. The degraded short-crack toughness is attributed to weakened grain or interface boundaries and to internal residual stresses from thermal expansion mismatch. A profound manifestation of the toughness-curve behavior is a transition in the nature of mechanical damage in Hertzian contacts, from classical single-crack cone fracture in the homogeneous control to distributed subsurface damage in the heterogeneous material.

Evaluation of Toughness in AISI 4340 Alloy Steel Austenitized at Low and High Temperatures

Abstract: It has been reported for as-quenched AISI 4340 steel that high temperature austenitizing treatments at 1200°C, instead of conventional heat-treatment at 870°C, result in a two-foldincrease in fracture toughness,KIc, but adecrease in Charpy impact energy. This paper seeks to find an explanation for this discrepancy in Charpy and fracture toughness data in terms of the difference betweenKIc and impact tests. It is shown that the observed behavior is independent of shear lip energy and strain rate effects, but can be rationalized in terms of the differing response of the structure produced by each austenitizing treatment to the influence of notch root radius on toughness. The microstructural factors which affect this behavior are discussed. Based on these and other observations, it is considered that the use of high temperature austenitizing be questioned as a practical heat-treatment procedure for ultrahigh strength, low alloy steels. Finally, it is suggested that evaluation of material toughness should not be based solely onKIc or Charpy impact energy values alone; both sharp crack fracture toughness and rounded notch impact energy tests are required.

Microbranching instability and the dynamic fracture of brittle materials.

Abstract: We describe experiments on the dynamic fracture of the brittle plastic, PMMA. The results suggest a view of the fracture process that is based on the existence and subsequent evolution of an instability, which causes a single crack to become unstable to frustrated microscopic branching events. We demonstrate that a number of long-standing questions in the dynamic fracture of amorphous, brittle materials may be understood in this picture. Among these are the transition to crack branching, ``roughness'' and the origin of nontrivial fracture surface, oscillations in the velocity of a moving crack, the origin of the large increase in the energy dissipation of a crack with its velocity, and the large discrepancy between the theoretically predicted asymptotic velocity of a crack and its observed maximal value. Also presented are data describing both microbranch distribution and evidence of a new three-dimensional to two-dimensional transition as the ``correlation width'' of a microbranch diverges at high propagation velocities. \textcopyright{} 1996 The American Physical Society.

On the toughness of ductile adhesive joints

Abstract: Crack propagation along one of the interfaces between a thin ductile adhesive layer and the elastic substrates it joins is considered. The layer is taken as being elastic-plastic, and the fracture process of the interface is modeled by a traction-separation law, characterized by the peak separation stress 6 and the work of separation per unit area To. Crack growth resistance curves for mode I loading of the adhesive joint are computed, with emphasis on steady-state toughness, as a function of three extrinsic effects : layer thickness, layer-substrate modulus mismatch, and initial residual stress in the layer. Conditions under which separation first occurs well ahead of the initial crack tip are discussed. 1. SPECIFICATION OF THE MODEL This paper continues the study begun by Tvergaard and Hutchinson (1994) in which an embedded fracture zone model is applied to the mode I fracture of an adhesive joint comprised of a thin elastic-plastic metal layer joining two elastic substrates. The present work employs the model to investigate the influence on joint toughness of both the elastic mismatch between the layer and the substrates and the residual stress in the layer. As in the earlier study, the thickness of the ductile layer is another extrinsic variable which comes into play. The approach adopted was first introduced by Needleman (1987) to study particle debonding in metal matrices and subsequently by Tvergaard and Hutchinson (1992, 1993) to model crack growth resistance in homogeneous solids and along interfaces. A traction-separation law simulating the fracture process is embedded within an elastic-plastic continuum as a boundary condition along the line extending ahead of the crack. In the case of an interface joining dissimilar materials, the separation law necessarily involves both the normal and shear tractions and the two associated relative displacements of the surfaces across the interface.