J.W. Hancock

On the mechanisms of ductile failure in high-strength steels subjected to multi-axial stress-states

Abstract The strain required to initiate ductile failure in three low-alloy, quenched and tempered steels has been determined in multi-axial stress-states. The ductility was found to depend markedly both on the orientation of the stress system with respect to the rolling direction and on the tri-axiality of the stress-state. In some cases, ductile failure occurred at plastic strains which were only a few times the yield strain. Metallographic studies have been used to compare the size, shape and orientation of the holes which cause failure initiation with the isotropic continuum analysis of F.A. Mc Clintock (1968). The application of ductile-fracture models to directional steels is discussed with particular reference to the effects of directionality and stress-state on the condition for flow localization to occur between holes.

On the influence of state of stress on ductile failure initiation in high strength steels

Abstract The effect of stress state on the effective plastic strain to initiate ductile failure in three high strength steels is investigated. Circumferentially notched tension specimens were used, and failure initiation strains were correlated with a parameter which is a measure of the “triaxiality” of the stress state. Results are given for both in-plane and through-thickness directions in rolled plate. The failure initiation data, together with appropriate stress-strain fields, and estimates of characteristic lengths over which failure initiates, have been used to predict failure initiation at notches and at crack tips; conventional fracture mechanics parameters, such as KI at crack extension in small scale yielding are estimated for one of the high strength steels.

THE EFFECT OF NON-SINGULAR STRESSES ON CRACK-TIP CONSTRAINT

The effect of the T-stress on the small-scale yielding field of a crack in plane strain conditions has been examined using modified boundary layer formulations. The numerically calculated stresses at the crack tip are represented by slip line fields for small-strain theory. Positive T-stresses cause plasticity to envelop the crack tip and exhibit a Prandtl field, corresponding to the limiting solution of the HRR field for a nonhardening material. Moderate compressive T-stresses reduce the direct stresses within the plastic zone by decreasing the hydrostatic stress by T. This causes a loss of J-dominance, and a stress distribution represented by an incomplete Prandtl field.

J-Dominance of short cracks in tension and bending

Abstract Edge-Cracked bars with a W ratios less than 0.3 in bending and 0.5 in tension are shown to lose Jdominance. The loss of single parameter characterization is associated with the development of plasticity to the cracked face. Deeper cracks, for which plasticity develops through the ligament without spreading to the cracked face maintain J-dominance into large scale plasticity. The loss of J-dominance is attributed to compressive T-stresses, while geometries which exhibit tensile T-stresses retain J-dominance in accord with modified boundary layer formulations. The solutions for all the geometries can be characterized by J and Tinto large-scale plasticity.

On the role of strain and stress state in ductile failure

Abstract U sing plane strain and axisymmetric notched tensile specimens in combination with finite deformation stress analysis, the strain to initiate failure in a range of structural steels is shown to be co-related by the state of stress for both axisymmetric and plane states of strain. The implications of this result for ductile failure terminated by flow localisation is discussed in the light of the theoretical work on localised flow.

Interfacial slip and damping in fibre reinforced composites

The effect of interfacial slip on the stress—strain properties of a rubbery polymer reinforced with short fibres is discussed with particular reference to hysteresis. The amount of energy dissipated by interface sliding and by the viscoelastic response of the matrix is calculated in the light of a simple model. This is compared with the dynamic properties of the composites, both with a view to designing composites with a useful combination of stiffness and damping, and also with the possibility of evaluating the integrity of the fibre-matrix interface from dynamic measurements.

J dominance in mixed mode loading

Elastic-plastic plane-strain crack problems subject to combined mode I and mode II loadings have been analysed with modified boundary layer formulations using the first two terms, K and T of the asymptotic elastic field. Corresponding full field calculations have been performed on geometries in which the mode I component arises largely from bending or tension and in which the T stress varies from tensile to compressive. The conditions for J dominance have been considered in terms of the effect of the T stress on the asymptotic field. As in related work on the pure mode I problem, positive T stresses are shown to favour J dominance, while compressive T stresses cause the stresses to fall from the HRR field.

A family of plane strain crack tip stress fields for interface cracks in strength-mismatched elastic-perfectly plastic solids

Abstract Asymptotic crack tip stress fields are developed for a plane strain crack located on the interface between two incompressible, elastic–perfectly plastic solids, with matching elastic properties, but mismatched yield strengths. The complete range of normal versus shear tractions on the interface at the crack tip is analyzed. In the limit as the yield strength mismatch vanishes the asymptotic stress fields for the interface crack approach established crack tip fields for homogeneous bodies. The crack tip stress fields are assembled from combinations of centered fans, constant stress sectors and elastic sectors. All the stresses within each material region are continuous while traction continuity is maintained across the interface. These asymptotic stress fields are verified by comparison with small-scale yielding finite element solutions obtained for various remote load mixity using boundary layer formulations ( Rice, 1967 ) .

The stress intensity factors of semi-elliptical cracks in a tubular welded T-joint under axial loading

Abstract The stress intensity factors K I , K II and K III of semi-elliptical cracks located near the chord-brace intersection of a tubular welded T-joint have been determined under axial loading conditions. A finite element analysis technique has been used, based on the line-spring concept of Rice and Levy to represent the crack, while the structure has been represented by shell elements. The stiffness of the joint is maintained until the crack penetrates the chord wall when the stiffness decays. The study provides a basis for a fracture mechanics analysis of the fatigue life of tubular welded joints.

The re-characterisation of complex defects: Part I: Fatigue and ductile tearing

Abstract Defect assessment codes idealise complex defects as simple shapes which are amenable to analysis in a process known as re-characterisation. The present work examines the re-characterisation of complex defects which extend by fatigue, ductile tearing or cleavage. A family of representative defects were analysed numerically, while a related experimental programme investigated defect interaction and failure. Part I of the paper focuses on fatigue and ductile tearing. Part II examines cleavage. The numerical and experimental results are discussed within the context of the re-characterisation procedures described in BS 7910 (Guidance on methods for assessing the acceptability of flaws in metallic structures. London, UK: British Standard Institution; 1999 [Chapter 7]) and R6/4 (Assessment of the integrity of structures containing defects. Gloucester: British Energy Generation Ltd.; 2001 [Revision 4, Chapters I and II.3]). The level of conservatism of the re-characterisation procedures for fatigue and ductile tearing are discussed. A possible non-conservatism of the re-characterisation for cleavage is discussed in Part II, within the framework of constraint based statistical fracture mechanics.