J. F. Knott

The influence of compositional and microstructural variations on the mechanism of static fracture in aluminum alloys

The object of the paper is to examine the effects of alloy purity and state of aging on the fracture mechanism and resultant toughness of pure Al-Cu alloys, and commercial duralumin. In pure alloys, the transition from a shear to an intergranular mode of fracture with overaging is associated with changes in the nature and size of the matrix precipitate, which affect the slip character. In the corresponding commercial purity alloys, no such fracture mode transition occurs. The presence of second-phase dispersoids inhibits planar slip, and in the overaged state inclusion-matrix interfaces present a suitable alternative site to the grain boundaries for strain accumulation, resulting in debonding leading to the initiation of voids, which subsequently grow and coalesce. The fracture toughness, as conventionally measured, indicates the material’s resistance to crack initiation rather than propagation and is effectively independent of fracture mode. The work hardening capacity has a marked effect on void size, and is shown to be a sensitive indicator of fracture toughness in both pure and commercial alloys. Based on a simple model, good agreement is obtained between experimental results of toughness and those predicted from a knowledge of the tensile properties.

The fracture behaviour of PMMA in mixed modes I and II

Abstract The brittle fracture behaviour of PMMA under mixed mode I/II loading has been investigated using pre-cracked bend bar specimens loaded in antisymmetric and symmetric four point loading. The accuracy of the antisymmetric configuration was verified by a photoelastic investigation, and by comparison with results obtained using other mixed mode load arrangements. The results are used to test and discuss three mixed mode fracture criteria. It is concluded that mixed mode I/II brittle fracture of PMMA at room temperature is described best by a maximum tangential tensile stress criterion based on the linear elastic stress field. This is found to be consistent with the present understanding of the micro-mechanisms involved in fracture of PMMA.

Micromechanisms of brittle fracture

Mechanical processes operating in materials on the scale of the microstructure have come to be called “micromechanisms.” The fundamental science and the micromechanisms of brittle fracture are reviewed here, with particular emphasis on cleavage and intergranular fracture. Extant micromechanisms for these fracture types are evaluated. The role of solutes, particularly in intergranular fracture, is also discussed in terms of the fundamentals of brittle fracture.

The mixed mode I/II fracture behaviour of lightly tempered HY130 steel at room temperature

Abstract The mixed mode I/II fracture behaviour at room temperature of HY130 steel tempered at 350°C has been investigated using edge-cracked bend bar specimens loaded in anti-symmetric and symmetric four point bend configurations. In all cases fracture occurred by a localized shear decohesion mechanism that could not be characterized by the stress intensity factors, K I and K II , but for which the crack tip displacements, δ I , and δ II , appear to provide a first level of characterization. The results suggest that fracture is described by a maximum shear criterion, and this is consistent with the present understanding of fibrous fracture micro-mechanisms in the material.

A numerical investigation of the interaction and coalescence of twin coplanar semi-elliptical fatigue cracks

Abstract The interaction and coalescence of twin coplanar semi-elliptical cracks under both tension and bending loadings are investigated by numerical methods. Crack advance is computed on a step-by-step basis from the Paris equation using stress intensity factors calculated by the finite element method. Simplified computations are also performed in which coalescing cracks are recharacterized as single cracks. The fatigue crack growth curves predicted by the numerical methods are compared with experimental results.

Fatigue crack propagation of coplanar semi-elliptical cracks in pure bending

Abstract The first part of this paper presents stress intensity factor solutions for coalescing coplanar semi-elliptical cracks subjected to pure bending. Crack shapes of “beachmarked” test pieces were modelled using a finite element method and stress intensity factors were obtained using an elastic strain energy method and a displacement extrapolation method. The second part presents the results of an experimental investigation of the fatigue propagation of coplanar semi-elliptical cracks. Fatigue tests were carried out for stress ratios of 0.2, 0.5, and 0.7, and for initial crack separations of 4 mm, 8 mm, and 16 mm. In part III various models are employed for the prediction of fatigue crack growth of coalescing coplanar cracks. The implications for fatigue life assessment are discussed.

Effect of mixed mode I and II loading on the fracture surface of polymethyl methacrylate (PMMA)

In this paper studies conducted on Polymethyl methacrylate (PMMA) under combined bending and shear loading are described. A strong dependency of fracture surface features on the mixed mode stress state is observed. Close to pure mode I, the fracture surface is ‘mirror-like’ in appearance. With increasing mode II component the fracture surface becomes ‘misty’ and parabolic markings appear on the fracture surface. These observations indicate that the level of stress ahead of the crack tip increases with increasing mode II component. The mixed mode specimens are also observed to fracture at much higher stresses than the pure mode I specimen, contrary to the predictions of the fracture criteria based on linear elastic fracture mechanics (LEFM). The fracture surface features and the higher stresses at fracture in the mixed mode specimens are explained in terms of the increase in stiffness (which has been related to an increase in the effective stress intensity factor per unit opening displacement) with the introduction of a mode II component and the geometry of the 3-dimensional crack tip.

Modelling the impact energy in the ductile/brittle transition region of C-Mn multi-run welds

Contrary to a commonly made assumption, the statistical analysis of Charpy impact tests of C-Mn multi-run welds revealed that the impact energy at a specified test temperature is not normally distributed. Test results from specimens with a notch located in different microstructural zones (e.g. as-deposited and reheated) are characterised by distinct distributions of the impact energy values. The resultant distribution of the impact energy is a mixture of several distributions scaled by the probabilities of sampling the existing microstructural zones. Equations regarding the variance of the mixed impact energy distribution have been derived and a weighted regression model has been applied to fit the systematic variation of the mean impact energy in the transition region of C-Mn multi-run welds. The central zone, the bulk of the as-deposited zone and the reheated zone are characterised by distinct variations of the mean impact energy.

Fatigue in SiC-particulate-reinforced aluminium alloy composites

The fatigue behaviour in SiC-particulate-reinforced aluminium alloy composites has been briefly reviewed. The improved fatigue life reported in stress-controlled test results from the higher stiffness of the composites; therefore it is generally inferior to monolithic alloys at a constant strain level. The role of SiC particulate reinforcement has been examined for fatigue crack initiation, short-crack growth and long-crack growth. Crack initiation is observed to occur at matrix-SiC interface in cast composites and either at or near the matrix-SiC interface or at cracked SiC particles in powder metallurgy processed composites depending on particle size and morphology. The da/dN vs ΔK relationship in the composites is characterized by crack growth rates existing within a narrow range of ΔK and this is because of the lower fracture toughness and relatively high threshold values in composites compared with those in monolithic alloys. An enhanced Paris region slope attributed to the monotonic fracture contribution are reported and the extent of this contribution is found to depend on particle size. The effects of the aging condition on crack growth rates and particle size dependence of threshold values are also treated in this paper.

Size effects on the microscopic cleavage fracture stress, σ F * , in martensitic microstructures

Effects of testpiece size on the microscopic cleavage fracture stress, σF*, have been investigated for as-transformed, autotempered martensitic microstructures, by employing geometrically similar (scaled) testpieces. Both fine- and coarse-grained martensites have been examined. For a coarsegrained martensitic microstructure, significant effects on cr* attributable to sampling volume have been observed,i.e., reduced values of σF* are obtained as the testpiece size is increased. For a fine-grained martensitic microstructure the effects on σF* attributable to sampling volume are reduced, but significant reductions in σF* occur due to the lack of depth hardenability in large testpieces. The values of σF* observed are consistent with a micromechanism of failure involving the propagation of microcracks from autotempered carbides. For the coarse-grained condition the most potent autotempered carbides lie on embrittled prior austenite grain boundaries.