The mechanism of the shear mode of ductile fracture has been investigated in sheet tensile specimens of an α brass containing few inclusions. The techniques of optical metallography, scanning electron microscopy and transmission electron microscopy of both thin foils and replicas were all used in order to obtain as complete a picture of the fracture mechanism as possible. It was found that a zone of shear deformation, made up of many intense shear bands, developed across the specimen neck and fracture occurred within this zone along the shear bands. These shear bands consisted of material with a fine subgrain structure. No voids were found in the necked region of either deformed or fractured specimens yet the fracture surfaces were covered with elongated dimples. It is therefore suggested that the dimples on the fracture surface are not a precursor of the fracture but form only during the final process of separation.
Abstract The influence of superimposed hydrostatic pressures up to 600 MPa on the tensile fracture mechanism of sheet specimens of α-brass, α-β brass and copper has been studied. Particular attention was paid to the mechanisms leading to the initiation of fracture. At low pressures a common fracture mechanism occurred in all materials which involved the formation of a zone of intense shear deformation followed by fracture due to the development of void-sheets within this zone. At a higher pressu...
An investigation has been made into the effects of hydrostatic pressures up to 600 MPa on the tensile deformation and fracture mechanisms of commercial copper. Fracture occurred by the normal tensile fracture mechanism involving void coalescence followed by shear tearing at pressures up to 300 MPa. At pressures in excess of 300 MPa fracture occurred entirely by a mechanism involving highly localized bands of intense shear deformation. This mechanism resulted in fracture at a chisel-point so that...
Abstract Single crystal silver ribbons were strained to fracture with a precision tensile stage inside of a high voltage electron microscope. Direct observations of the dynamical fracture processes have permitted a detailed microstructural analysis of the basic ductile fracture mechanisms involved. Specimens were found to undergo extensive necking during elongation which resulted in lattice rotation and tensile axis reorientation. Cracks initiated at one of the edges of the specimens in the neck...
#2P.F Weinrich(Australian Department of Defence)H-Index: 2
Abstract A detailed study has been made of the effects of hydrostatic pressures in the range 0.1–600 MPa on the tensile fracture strain of an α-brass. Fracture strain has been found to increase linearly with pressure up to 350 MPa, to decrease between pressures of 350 and 375 MPa and to then increase to a constant value at pressures greater than 450 MPa. The abrupt change from linearity of the fracture strain-pressure relationship at a pressure of 350 MPa was found to coincide with the suppressi...
Fracture surfaces of 304 stainless steel were examined with a scanning electron microscope and inter void and inter particle spacings were measured. A significant relationship between inter void spacings and fracture stress was found. It was concluded that voids preceding ductile fracture could be initiated without the presence of secured phase particles in pure metals and 'clean' alloys. (Author)
Abstract Microscopic examinations have been made of the necked regions of pure aluminium and copper in single crystal and polycrystalline form. The present experiments suggest that cracks initiate on the coarse slip bands which are characteristic of the deformation in the necked regions. As these cracks do not nucleate at inclusions, it is considered that a dislocation mechanism can contribute to ductile fracture.
Abstract Cup-and-cone fracture in single-phase ductile metals appears to originate at holes formed by drawing away of material from non-metallic inclusions, as suggested by Tipper. In copper, the holes expand under the triaxial stresses in the neck and coalesce in a macroscopic fissure; in α iron fine cracks are formed by the stress concentrated at the holes. In coarsegrained material shear cracks are formed on the surface of the neck. Pure polycrystalline aluminium separates at the neck of a te...
#2Thomas Linse(TUD: Dresden University of Technology)H-Index: 11
Usually the safety margin against failure for precracked components is calculated with fracture mechanics approaches. Due to several severe limitations of these approaches, it was searched for alternative calculation models. Starting with McClintock and Berg in the sixties, so-called damage models have been developed for describing ductile fracture on the basis of micromechanical processes. The development of such kind of models is in progress now for nearly 50 years, but until today no model is...
#1G. Sanyal(BARC: Bhabha Atomic Research Centre)H-Index: 8
#2Arpan Das(BARC: Bhabha Atomic Research Centre)H-Index: 18
Last. J.K. Chakravartty(BARC: Bhabha Atomic Research Centre)H-Index: 21
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Fracture is the ultimate continuum of a material. Hence, it is possible that the fracture features contain the signature of the whole deformation process undergone due to variation in triaxial state of stress under tensile deformation. The two dimensional fracture features were correlated with the stress triaxiality ratio where initial inclusion content was unaltered. In order to test that, many tensile experiments of a superalloy were carried out at different notch geometries and fractographic ...
Abstract The effects of heat treating Inconel 718 on the ballistic impact response and failure mechanisms were studied. Two different annealing conditions and an aged condition were considered. Large differences in the static properties were found between the annealed and the aged material, with the annealed condition having lower strength and hardness and greater elongation than the aged. High strain rate tests show similar results. Correspondingly large differences were found in the velocity r...
Abstract Ductile failure in polycrystalline oxygen-free, high-conductivity copper is explored through uniaxial tension experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution are examined through quantitative microscopy to discern the mechanisms of failure and the local strain evolution. The formation of a single rectangular prismatic channel-like cavity, and its subsequent growth as a rectangular cavity, are demonstrated. Fractographic ob...
In the present article novel approaches to account for localized necking, post-necking and ductile fracture in the numerical simulation of aluminium sheet forming are presented. The modelling concept was implemented in a commercial finite element software and is applied to an aluminium alloy sheet subjected to different forming operations. A validation of the simulation results by means of experiments is employed. It is shown that the proposed modelling framework is capable of accurately predict...
Abstract The prediction of the forming limit curve at fracture (FLCF) is of industrial importance, particularly in sheet metal stretching where a successful process is controlled by fracture instead of necking. In this work an analytical model is proposed to predict FLCF for sheet metals which are biaxially stretched under various loading paths. The model considers growth and coalescence of voids nucleating at inclusions in a sheet metal matrix characterized by strain hardening and normal anisot...
Abstract This paper investigates mechanisms that cause surface microstructure to progressively slip apart during ongoing plastic flow. Four processes of localized slip can separate microstructure at a free surface while leaving underlying microstructure connected. None can operate in a sustained manner without being directly coupled to sample-scale shape change. A mechanism connects rate of surface failure to ongoing sample deformation. Different mechanisms operate in different geometries of met...
Abstract This study is a quantitative, experimental study of development of shear band formation during plane strain extension of ferrite-austenite sheet. The position of aligned austenite islands was used to measure internal shape change throughout the development of flow localization to sheet failure. Through-thickness shearing flow and rotation of material by an angle φ develop during nonuniform thinning in order to maintain compatibility between deforming subelements of sheet. Surface grains...
#2M.G.S. Naylor(OSU: Ohio State University)H-Index: 1
Last. L.K. Ives(NIST: National Institute of Standards and Technology)H-Index: 3
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Abstract Tribological processes cause large plastic strains and plastic strain gradients adjacent to the interface between the interacting materials. For sliding wear and erosion test specimens, the dislocation structures observed in the plastically deformed material are consistent with those expected when energy is minimized. Studies of these structures can contribute to our knowledge of deformation and dislocation substructures produced at large strains under other test conditions. Tribologica...
