Why does necking ignore notches in dynamic tension?

Recent experimental work has revealed that notched tensile specimens, subjected to dynamic loading, may fail by growing a neck outside of the notched region. This apparent lack of sensitivity to a classical stress concentration case was reported but not explained or modeled.The present paper combines experimental and numerical work to address this issue. Specifically, it is shown that the dynamic tensile failure locus is dictated by both the applied velocity boundary condition and the material mechanical properties, specifically strain-rate sensitivity and strain-rate hardening.It is shown that at sufficiently high impact velocities, the flows stress in the notch vicinity becomes quite higher than in the rest of the specimen, so that while the former resists deformation, it transfers the load to the latter. The result will be the formation of a local neck and failure away from the notch.This effect is shown to be active when the material properties are perturbed only at the local level, as in the case of machining of the notch, which in itself may again be sufficient to stabilize the structure under local failure until a neck forms elsewhere.While the physical observations are quite counterintuitive with respect to the engineering views of stress concentrator's effect, the present work rationalizes those observations and also provides information for the designers of dynamically tensioned structures that may contain notches or similar flaws.     

Effect of annealing prior to cold rolling on magnetic and mechanical properties of low carbon non-oriented electrical steels

The effects of annealing prior to cold rolling on the microstructure, magnetic and mechanical properties of low-C grain non-oriented (GNO) electrical steels have been investigated. The grain structure of hot-rolled electrical steel strips is modified by annealing at temperatures between 700 and 1050 °C. Annealing at temperatures less than the ferrite to austenite+ferrite transformation temperature on heating (Ac₁) causes a marginal effect on the grain size. However, annealing in the intercritical region at temperatures between Ac₁ and Ac₃ (the ferrite+austenite to austenite transformation temperature on heating) causes rapid decarburization and development of large columnar ferrite grains free of carbide particles. This microstructure leads, after cold rolling and a fast annealing treatment, to carbide free, large ferrite grain microstructures with magnetic and mechanical properties superior to those observed typically in the same steel in the industrially fully processed condition. These results are attributed to the increment in grain size and to the {1 0 0} fiber texture developed during the final annealing at temperatures up to 850 °C. Annealing at higher temperatures, T>Ac₃, results in a strong {1 1 1} fiber texture and an increase of the quantity of second phase particles present in the microstructure, which lead to a negative effect on the final properties. The results suggest that annealing prior to cold rolling offers an attractive alternative processing route for the manufacture of fully processed low C GNO electrical steels strips.     

An elastoplastic damage model for metal matrix composites considering progressive imperfect interface under transverse loading

An elastoplastic damage model considering progressive imperfect interface is proposed to predict the effective elastoplastic behavior and multi-level damage progression in fiber-reinforced metal matrix composites (FRMMCs) under transverse loading. The modified Eshelby’s tensor for a cylindrical inclusion with slightly weakened interface is adopted to model fibers having mild or severe imperfect interfaces [Lee, H.K., Pyo, S.H., 2009. A 3D-damage model for fiber-reinforced brittle composites with microcracks and imperfect interfaces. J. Eng. Mech. ASCE. doi:10.1061/(ASCE)EM.1943-7889.0000039]. An elastoplastic model is derived micromechanically on the basis of the ensemble-volume averaging procedure and the first-order effects of eigenstrains. A multi-level damage model [Lee, H.K., Pyo, S.H., 2008a. Multi-level modeling of effective elastic behavior and progressive weakened interface in particulate composites. Compos. Sci. Technol. 68, 387–397] in accordance with the Weibull’s probabilistic function is then incorporated into the elastoplastic multi-level damage model to describe the sequential, progressive imperfect interface in the composites. Numerical examples corresponding to uniaxial and biaxial transverse tensile loadings are solved to illustrate the potential of the proposed micromechanical framework. A series of parametric analysis are carried out to investigate the influence of model parameters on the progression of imperfect interface in the composites. Furthermore, a comparison between the present prediction and experimental data in the literature is made to assess the capability of the proposed micromechanical framework.     

A first-principle study of Os-based compounds: Electronic structure and vibrational properties

The electronic structure, elastic, and phonon properties of OsM (M=Hf, Ti, Y and Zr) compounds are studied using first-principles calculations. Elastic constants of OsY and specific heat capacity of OsM (M=Hf, Ti, Y, and Zr) are reported for the first time. The predicted equilibrium lattice constants are in excellent agreement with experiment. The calculated values of bulk moduli are considerably high but are much smaller than that of Osmium, which is around 400 GPa. The phase stability of the OsM (M=Hf, Ti, Y and Zr) compounds were studied by DOS calculations and the results suggest that OsY is unstable in the B2 phase. The brittleness and ductility properties of OsM (M=Hf, Ti, Y and Zr) are determined. OsM (M=Hf, Ti, Y and Zr) compounds are predicted to be ductile materials. The electronic structure and phonon frequency curves of OsM (M=Hf, Ti, Y and Zr) compounds are obtained. The position of Fermi level of these systems was calculated and discussed in terms of the pseudo gaps. The finite and small DOS at the Fermi level 0.335, 0.375, 1.063, and 0.383 electrons/eV for OsHf, OsTi, OsY, and OsZr, respectively, suggest that OsM (M=Hf, Ti, Y and Zr) compounds are weak metals.     

Effects of superimposed hydrostatic pressure on sheet metal formability

The effect of superimposed hydrostatic pressure on sheet metal formability is studied analytically and numerically. A tensile sample of power-law hardening material under superimposed hydrostatic pressure is first analyzed using the classical isotropic plasticity theory. It is demonstrated that the superimposed hydrostatic pressure p lowers the true tensile stress level at yielding by the amount of p, while material work-hardening is independent of p. It is showed, based on the Considère criterion, that the superimposed hydrostatic pressure increases the uniform strain. The effect of superimposed hydrostatic pressure on sheet metal formability is further assessed by constructing the Forming Limit Diagram (FLD) based on the M-K approach. It is found that the superimposed pressure delays the initiation of necking for any strain path. The difference in predicted FLDs between the superimposed hydrostatic pressure and the stress component normal to the sheet plane is demonstrated. Finally, the effect of superimposed hydrostatic pressure on fracture in round bars under tension is studied numerically using the finite element method, based on the Gurson damage model. The experimentally observed transition of the fracture surface, from the cup-cone mode under atmospheric pressure to a slant structure under high pressure, is numerically reproduced.     

Generalised forming limit diagrams showing increased forming limits with non-planar stress states

The forming limit diagram and its associated analytical and experimental techniques has been widely used for 40 years with the assumption that sheet deformation occurs inplane-stress. Some hydro-forming type processes induce significant normal stress across the workpiece and this has led to a small number of extended formability analyses. However, recent work on the incremental sheet forming process which is known to give higher formability than conventional sheet pressing has shown that the repeated passage of a tool over the sheet leads to significant through-thickness shear strains being induced in the workpiece. Accordingly this paper explores the forming limits of sheet forming processes which induce any possible proportional loading, including all six components of the symmetric stress tensor. Marciniak and Kuczyinski’s famous (1967) analysis is extended to allow such loading, and a new generalised forming limit diagram (GFLD) is proposed to allow visual representation of the resulting forming limit strains. The GFLD demonstrates that forming limits can be increased significantly by both normal compressive stress and through-thickness shear. This increased formability is confirmed by experiments on a specially designed ‘linear paddle testing’ apparatus in which a conventional uniaxial test is augmented by the action of a paddle that ‘strokes’ the sample while also applying a normal force. Tests on the rig show that the paddle action leads to enhanced engineering strains at failure up to 300%. The insight gained in this paper is significant for process analysts, as it may explain existing discrepancies between prediction and experience of forming limits, and is important for designers who may be able to use it to expand process operating windows.     

Fragility analysis of bridges under ground motion with spatial variation

Seismic ground motion can vary significantly over distances comparable to the length of a majority of highway bridges on multiple supports. This paper presents results of fragility analysis of highway bridges under ground motion with spatial variation. Ground motion time histories are artificially generated with different amplitudes, phases, as well as frequency contents at different support locations. Monte Carlo simulation is performed to study dynamic responses of an example multi-span bridge under these ground motions. The effect of spatial variation on the seismic response is systematically examined and the resulting fragility curves are compared with those under identical support ground motion. This study shows that ductility demands for the bridge columns can be underestimated if the bridge is analyzed using identical support ground motions rather than differential support ground motions. Fragility curves are developed as functions of different measures of ground motion intensity including peak ground acceleration, peak ground velocity, spectral acceleration, spectral velocity and spectral intensity. This study represents a first attempt to develop fragility curves under spatially varying ground motion and provides information useful for improvement of the current seismic design codes so as to account for the effects of spatial variation in the seismic design of long-span bridges.     

Spatiotemporally inhomogeneous plastic flow of a bulk-metallic glass

With geometrically-constrained specimens, the spatiotemporally inhomogeneous deformation of a Zr-based bulk-metallic glass in uniaxial, quasistatic, compression was investigated. Decreasing the height/width ratio of specimens from 2 to 0.5 significantly increases the plastic strain from 2% to about 80%. Using an infrared camera, we first observe in situ dynamic shear-banding operations during compression at various strain rates. The shear banding is highly dependent on strain rates, either intermittent at the lower strain rate or successive at the higher strain rate. Scanning electron microscopy observations show the spatiality of the rate-dependent shear banding. The serrated plastic flow is a result of the shear-banding operations. At the lower strain rate, more simultaneous shear-banding operations result in more obvious serrations, while at the higher strain rate, fewer simultaneous shear-banding operations cause less obvious serrations.