Two damage models were implemented into the finite element program ADINA to study the correlation between microscopical damage and macroscopical material failure. In the first model, based on the Gurson yield function the nucleation, growth and the coalescence of voids were incorporated into the constitutive relations. In the second model the void growth was determined according to the Rice and Tracey model using the von Mises yield function, and material failure was simulated by eliminating the elements where the critical void growth ratio was exceeded. The numerical results for the local and global behaviour of the specimens were compared with experiments. The generality of the damage parameters was checked by investigating several specimen geometries. Both damage models deliver qualitatively consistent results with regard to the influence of the stress triaxiality on the void growth and on the beginning of the material failure. However, the Gurson model gives a more accurate numerical simulation because the damage development and the stress drop continue after the onset of void coalescence while the critical void growth model causes less convergence problems in the simulation of large crack extension. The J,-curve was estimated on the basis of both models. NOMENCLATURE a = crack length Aa = crack extension Ad = change of diameter B, D = coefficients for void nucleation E = elastic modulus E, = current tangent modulus cF, = equivalent plastic strain tN = mean strain for void nucleation f = void volume fraction f , = critical void volume fraction at void coalescence ff = void volume fraction at rupture f N = volume fraction of void forming particles fa = initial void volume fraction f,, = coefficient for void coalescence G = shear modulus J = J-integral K =bulk modulus tij = strain tensor q , = coefficient in Gurson yield condition R = void radius R, = initial void radius p = notch radius S , = standard deviation ue = equivalent stress u . = Cauchy stress tensor 8: = Jaumann stress rate tensor urn = flow stress of the material uy = yield stress of the material u, = maximum principal stress ukk,3 = mean stress 20 I 202 D.-Z. SUN et al.

The ductile fracture behavior of different specimens is analyzed by continuum damage-mechanics techniques. A model introduced by Gurson and modified by Needleman and Tvergaard has been implemented in the finite element program package, ADINA. The damage parameters of the model are measured and calculated from smooth tension tests, and the characteristic material distance is estimated from compact tension experiments. A steel, ASTM A710, and a weld metal for the steel, ASTM A508, are investigated. The damage parameters determined from the smooth bars are used to predict the deformation and fracture behavior of notched round bars and of sidegrooved compact specimens. For the weld metal, a side-grooved WOL-X-specimen is also simulated. In every case, a satisfactory agreement of prediction and experiment is observed. In order to investigate the influence of the stress state (constraint) in cracked specimens, a series of numerical computations of different specimen geometries and loading situations is performed utilizing the same set of parameters of the ASTM A710 steel. The slopes of the predicted J-resistance curves increase with increasing ratio of tension versus bending load and with decreasing relative crack length.

A new methodology for measuring galling wear severity in high strength steels, Wear, http://dx. AbstractWith the increased usage of Advanced High Strength Steels, galling wear has become a significant challenge for sheet metal stamping industries. Galling, in particular, can have a large economic impact due to the high costs and lost productivity associated with manual monitoring, refinishing/resurfacing damaged tooling and formed parts, and the need to apply expensive treatments or coatings to tool surfaces. This has led to a push for automated galling wear detection systems. However, developing such systems requires an accurate measurement of galling wear severity that can be easily implemented in industrial situations. Parameters used for measuring galling wear are often difficult to collect in large industrial style trials, and can be inaccurate as they are not targeted towards characterising the localised features associated with galling wear damage. In this study, a new galling wear characterisation and measurement methodology is introduced that accurately measures galling wear severity by targeting the localised features on sheet metal parts. This methodology involves calculating Discrete Wavelet Transform detail coefficients of 2D surface profiles. A case study on a series of deep drawn channel parts demonstrates the accuracy of the Discrete Wavelet Transform methodology when compared to visual assessment of galling wear severity. Based on comparison to visual assessment the presented Discrete Wavelet Transform galling wear measurement methodology outperforms other commonly used wear measures. The methodology provides a targeted, repeatable and non-subjective measure of galling wear severity. The specific outcome of this work provides an important tool for research into galling wear monitoring and detection systems in sheet metal forming, and the study of galling wear and its prevention in general.

Traditional quality control of resistance spot welds by analysis of the dynamic resistance signature (DRS) relies on manual feature selection to reduce the dimensionality prior to analysis. Manually selected features of the DRS may contain information that is not directly correlated to strength, reducing the accuracy of any classification performed. In this paper, correlations between the DRS and weld strength are automatically detected by calculating correlation coefficients between weld strength and principal components of the DRS. The key features of the DRS that correlate to weld strength are identified in a systematic manner. Systematically identifying relevant features of the DRS is useful as the correlations between weld strength and DRS may vary with process parameters.