Void Damage Research Articles

Influence of damage and texture evolution on limit strain in biaxially stretched aluminum alloy sheets

A numerical code has been developed to calculate limit strains of textured aluminum alloy sheets. This code is based on the Marciniak–Kuczynski (M–K) model, but allows for void nucleation and growth so that both limit strain and fracture strain can be predicted. The strain induced void nucleation model was employed together with the Cocks and Ashby’s void growth model. The influences of initial texture, texture evolution, and void nucleation and growth during deformation on the limit strains of an Al–Mg alloy were all investigated. Satisfactory agreement was obtained between the predictions and measured data. It was also shown that the introduction of void damage into the old M–K model can lead to more reasonable and accurate predictions.

Effect of void nucleation and growth on forming limit diagrams of textured aluminum alloy sheets

A numerical code has been developed to calculate Forming Limit Diagrams (FLDs) of textured aluminum alloy sheets. This code is based on the Marciniak-Kuczynski (M-K) model, but allows for void nucleation and growth so that limit strains and fracture strains can be predicted. The strain induced void nucleation model was employed together with the Cocks and Ashby’s void growth model. The influences of initial texture, texture evolution, and void nucleation and growth during deformation on the FLDs of an Al-Mg alloy were all investigated. Satisfactory agreement was obtained between the predictions and measured data, It was also shown that the introduction of void damage into the old M-K model can lead to more reasonable and accurate predictions.

Deformation and fracture of metal matrix particulate composites under combined loadings

Flow and fracture behaviour of 17% and 25% SiC particulate reinforced 2124 aluminium is examined from combining torsion with either tension or compression. Incremental loadings were applied along both radial and stepped paths. The results of all tests show that the yield locus for the material obeys a von Mises description. The observed flow behaviour is compared with that predicted from the Prandtl-Reuss incremental plasticity theory. Regions of elasticity are apparent for both paths. Flow beyond the yield point under each path revealed a limited amount of parabolic hardening consistent with the normality rule. Pure torsion, tension and compression tests were also conducted to establish the yield and hardening behaviour for the mmc in its low and higher strain ranges. Void damage renders these materials brittle under all loadings except compression. This points to a more appropriate flow theory modified with a suitable damage parameter sensitive to the sense of the applied stress.

The classical pressure vessel problem for void damage materials

In this paper, the classical pressure vessel problem for void damage materials is studied from the theory of microstructure in linear elasticity. The solutions are quasistatic. The stress distribution is predicted by isotropic linear elasticity. The displacement and damage fields exhibit a volumetric viscoelasticity induced by considering material damage.

The plastic instability behaviour of laser-textured steel sheet

As the production of a new technique that can offer both good formability and high image clarity for texturing metal sheet, laser-textured sheet has attracted the attention of many manufacturers and users. Among the many subjects to be studied, plastic instability behaviour of the laser-textured sheet is one of most important to understand its ability in extending material ductility and to appropriately control this technique. Experimental investigations are carried out in this paper to study the macroscopic behaviour and microstructural mechanism of the laser-textured sheet, and comparison is made with the normal sheet taken from the same coil of metal sheet. It is demonstrated that, the difference in the behaviour of plastic instability obviously shows tendency to delay strain localization and the onset of thickness necking. Shear banding and internal void damage are spread to a much wider region in the sheet being laser-textured. The prestrained microcraters enforced on the surface of the textured sheet act as hardening spots, which are likely to share out deformation and inhibit the increasing rate of voiding, and eventually favouring the ductility of the material used.

Numerical analyses for the material bifurcation in plane sheet under tension

The various patterns (shear banding, surface wrinkling and necking) of material bifurcation in plane sheet under tension are investigated in this paper by means of a numerical method. It is found that numerical analysis can provide better ground for searching for the lowest critical loads. The inhomogeneity caused by void damage and the nonuniformity in the stress distribution across sheet thickness are proved to have detrimental effects on the material bifurcation. Nevertheless, material stability can be promoted by any means of depressing void damage or alleviating stress, even locally across the thickness. Besides, the peculiar behaviour of material bifurcation under slight biaxiality state is demonstrated.

An exponential ductile continuum damage model for metals

In the frame of continuum damage mechanics an isotropic ductile plastic damage model is derived. The model is based on void damage variable, defined using effective stress concept and thermodynamics. The damage evolution from this model is exponential with equivalent plastic strain as experienced in some low carbon steel like AISI 1015. The damage model is sensitive to stress triaxiality and emulates the damage evolution as recorded in experiments conducted on such metals. The model is validated by comparison with the Rice-Tracey model and other experimental results published in the literature. This model can be used to study the growth and coalescence of micro voids, influence of stress triaxiality on strain to rupture and crack initiation phenomena.

Characteristics of shear banding in dual phase steel

AbstractThe evolution process of shear banding in a ferrite–martensite dual phase steel has been investigated via in situ tensile testing in a scanning electron microscope. Shear band type deformation localisation occurs at the maximum loading point of uniaxial tension. Necking occurs simultaneously and locally. Voids nucleate in ferrite domains and on the interfaces between the two component phases or grain boundaries. The void volume fraction is greater within the shear band than away from the band and is greater in the interior than at the specimen surface. The results also show that void damage promotes the initiation of shear bands and the development of shear banding stimulates further void damage.MST/1819

Characteristics of shear banding in dual phase steel

The evolution process of shear banding in a ferrite–martensite dual phase steel has been investigated via in situ tensile testing in a scanning electron microscope. Shear band type deformation localisation occurs at the maximum loading point of uniaxial tension. Necking occurs simultaneously and locally. Voids nucleate in ferrite domains and on the interfaces between the two component phases or grain boundaries. The void volume fraction is greater within the shear band than away from the band and is greater in the interior than at the specimen surface. The results also show that void damage promotes the initiation of shear bands and the development of shear banding stimulates further void damage.MST/1819

CRACK TIP BEHAVIOUR AND CRACK PROPAGATION IN DUCTILE MATERIALS

AbstractDilatational plastic equations, which can include the effects of ductile damage, are derived based on the equivalency in expressions for dissipated plastic work. Void damage developed internally at the large‐strain stage is represented by an effective continuum being strain‐softened and plastically dilated. Accumulation of this local damage leads to progressive failure in materials. With regard to this microstructural background, the constitutive parameters included for characterizing material behaviour have the sense of internal variables. They are not able to be determined explicitly by macroscopic testing but rather through computer simulation of experimental curves and data. Application of this constitutive model to mode‐I cracking examples demonstrates that a huge strain concentration accompanied by a substantial drop of stress does occur near the crack tip. Eventually, crack propagation is simulated by using finite elements in computations. Two numerical examples show good accordance with experimental data. The whole procedure of study serves as a justification of the constitutive formulation proposed in the text.

INFLUENCE OF SECONDARY VOID DAMAGE IN THE MATRIX MATERIAL AROUND VOIDS

Abstract— The mechanism of ductile damage caused by secondary void damage in the matrix around primary voids is studied by large strain, finite element analysis. A cylinder embedding an initially spherical void, a plane stress cell with a circular void and plane strain cell with a cylindrical or a flat void are analysed under different loading conditions. Secondary voids of smaller scale size nucleate in the strain hardening matrix, according to the requirements of some stress/strain criteria. Their growth and coalescence, handled by the empty element technique, demonstrate distinct mechanisms of damage as circumstances change. The macroscopic stress‐strain curves are decomposed and illustrated in the form of the deviatoric and the volumetric parts. Concerning the stress response and the void growth prediction, comparisons are made between the present numerical results and those of previous authors. It is shown that loading condition, void growth history and void shape effect incorporated with the interaction between two generations of voids should be accounted for besides the void volume fraction.

Surface topology changes during electromigration in metallic thin film stripes

Electromigration damage in silver and aluminum thin film stripes was observed by scanning electron microscopy. Early formation of voids and surface growths occured in regions of maximum temperature and in the same cross-section. Once formed, void damage was comparatively stable, and failure ultimately occurred at the cathode end of the stripes. The growths in the aluminum stripe were nucleated at grain boundaries and were restricted to the boundary. The flux of atoms into one growth was approximated to be 2 X 10 8 atoms h -1. Growths in the silver stripe were not restricted by the grain size but had the appearance of surface undulations. Large protrusions appeared in coincidence with a catastrophic upswing in stripe resistance. It is felt that local failure of a thin oxide and subsequent “extrusion” may be responsible for this behavior.