Ductile Failure Criterion Research Articles

Role of stress triaxiality in elastoplastic characterization and ductile failure prediction

The triaxiality of the stress state is known to greatly influence the amount of plastic strain which a material may undergo before ductile failure occurs. During tensile load histories, the necking induces significant stress triaxiality modifications which in turn affect the experimental stress–strain measurements needed for the characterization of ductile metals. In this paper, the recently proposed “MLR” model of necking effect is used to obtain the flow curves of various metals by correcting the experimental data of tensile tests. Finite elements simulations of the experimental tests are performed to calculate the stress triaxiality evolution on various notched and unnotched specimens. A ductile failure criterion, due to Bao and Wierzbicki, is then applied to evaluate the material damage and predict failure. This procedure is applied to a set of 20 specimens series made of six metals with 10 different notch shapes. The damage calculations also indicate the material points where failure initiates. These predictions are confirmed by micrographic observation of voids on polished fragments of the broken specimens.

A method for the prediction of ductile fracture by central bursts in axisymmetric extrusion

A new approach for the prediction of central burst defects in extrusion and wire drawing is proposed. Central burst defects are troublesome in industry as it is impossible to detect these defects by simple visual examination alone. It is therefore important to identify the combination of process parameters that will reduce the probability of these defects occurring. The proposed approach is capable of accurately predicting when and where a central burst is likely to occur. The influence of die geometry on damage development and subsequent central burst formation is examined using the modelling capabilities of Abaqus finite element software. The ductile fracture criteria of Cockcroft and Latham, Oyane and Chaouadi were employed using a Fortran subroutine, and their ability to predict the onset and evolution of the central burst defect was examined. For relatively lower area reductions and die angles the considered criteria are capable of accurately predicting the morphology of the defect. Under these conditions, the proposed approach shows good agreement with experimental results, confirming its effectiveness and suitability for industrial application. The failure criteria are inadequate in predicting central burst defect formation during conditions of large reduction in area and large semi-die angles. Further development of ductile failure criteria is necessary to accurately simulate defect evolution for all die angles and area reductions.

Comparison of ductile failure models using a simple elastic–plastic based degradation model

In material fracture, crack initiation and growth are regarded as a characteristic outcome from a gradual loss of material integrity. In this paper the aforementioned loss of integrity is modelled using an approach to ductile fracture, whereby the elastic properties of the material are modified to simulate material degradation. This is achieved by reducing the element stiffness to zero once the damage variable has reached a critical failure index. A Fortran program has been written and incorporated into ABAQUS finite element software to determine and monitor the prevailing damage in the material. To assess the usefulness of this model, a comparison of two ductile failure criteria were carried out on a one-stage wire drawing configuration. The results are compared with published results, which highlights the applicability in view of its practical implementation to simulate forming processes.

Response and failure of a double-shear beam subjected to mass impact

Abstract Experimental results are reported for the response and failure of a notched double-shear (DS) beam made from BS970/En24 medium carbon steel alloy when subjected to a mass impact loading. Mechanical properties and constitutive equations were obtained from quasi-static, dynamic and temperature tests on the beam material. Response and failure features were recorded for impact velocities between 30 and 110 m s −1 . A failure mode transition was observed between a ductile tensile rupture failure and an adiabatic shear banding failure. This test method could be used for studying shear stress–strain relations over a broad range of strain rates and for investigating different failure modes and their transitions. Finite-element numerical simulations were conducted for the response and failure in the notch section of the DS beam. Calibration factors were obtained, which may be used to calculate the actual stress and strain at the mid-point of a notch section in terms of the nominal values. Isothermal and adiabatic conditions obtained from a dimensional analysis were examined with the aid of FE simulations. An analytical model is used to predict the central block motions of DS beams and gives good agreement with experimental results. Various ductile fracture failure criteria were examined, among which the generalised plastic work density failure criterion leads to successful predictions for the ductile tensile failure within the notch section.

Softened zone formation and joint strength properties in dissimilar friction welds

The mechanical properties of dissimilar MMC/AISI 304 stainless steel friction welds with and without silver interlayers were examined. The notch tensile strengths of MMC/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds increased when high friction pressures were applied during the joining operation. The higher notch tensile strengths of dissimilar MMC/AISI and MMC/Ag/AISI 304 stainless steel friction welds resulted from the formation of narrow softened zones in MMC material immediately adjacent to the bondline. The influence of softened zone width and hardness (yield strength) on the notch tensile strengths of dissimilar welds was analysed using finite element modelling (FEM). FEM in combination with the assumption of a ductile failure criterion was used to calculate the notch tensile strengths of dissimilar joints. The key assumption in this work is that dissimilar weld failure wholly depended on the characteristics (mechanical properties and dimensions) of the softened zone formed in MMC material immediately adjacent to the bondline. The modelling results produced based on this assumption closely correspond with the actual notch tensile strengths of dissimilar MMC/Ag/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds.

Rheological control on the initial geometry of the Raft River detachment fault and shear zone, western United States

The strain, exhumation history, and field orientation of a well‐exposed shear zone and detachment fault in the Raft River Mountains of northwestern Utah, a Cordilleran metamorphic core complex, have been studied to determine the kinematics of ductile shearing and initial orientations of the shear zone and detachment fault. Mapping and strain and kinematic analysis indicate that the top‐to‐the‐east Raft River shear zone initially developed parallel to an unconformity separating Archean rocks from overlying Proterozoic quartzite and schist for at least 24 km in the shear direction. Experimental rock deformation data from lithologies similar to the Archean and Proterozoic rocks suggest the unconformity represented a significant rheological boundary at the deformation temperatures; the base of the shear zone was localized along the boundary between relatively weak quartzite above and stronger monzogranite below. An extensive thermochronological database is used to reconstruct the position of the basement unconformity in temperature‐lateral distance coordinates. The initial average dip of the shear zone and basement unconformity is estimated between 7° and 30°, assuming subhorizontal isotherms and geothermal gradients of 20°–40°C/km. The east dip of the unconformity at the onset of Miocene extension is interpreted to have resulted from late Eocene unroofing and flexure beneath a top‐to‐the‐WNW extensional shear zone in the western Raft River, Grouse Creek, and Albion Mountains. The observations from the Raft River shear zone suggest that the orientation of some midcrustal shear zones may not reflect the predicted orientation for ductile faults according to ductile failure criteria but, rather, the orientation of rheological boundaries along which deformation is localized. Furthermore, detachment faults that are superimposed on mylonite during progressive displacement and footwall unroofing may use an inherited mechanical anisotropy from the mylonite, and their orientations may not reflect the predicted orientation of shear fractures in isotropic rock. The common parallelism between detachment faults and mylonitic foliation may indicate a mechanical and kinematic preference for localization of throughgoing brittle faults parallel to preexisting mylonitic foliation. Because of this preference, studies restricted to detachment faults which lack footwall mylonite or restricted to structural levels between the breakaway and mylonitic front have more bearing on the question of the initial dip of normal‐sense shear fractures (faults) within the seismogenic crust.

A Study of Burr Formation Processes Using the Finite Element Method: Part I

A finite element model of orthogonal metal cutting including burr formation is presented. A metal-cutting simulation procedure based on a ductile failure criterion is proposed for the purpose of better understanding the burr formation mechanism and obtaining a quantitative analysis of burrs using the finite element method. In this study, the four stages of burr formation, i.e., initiation, initial development, pivoting point, and final development stages, are investigated based on the stress and strain contours with the progressive change of geometry at the edge of the workpiece. Also, the characteristics of thick and thin burrs are clarified along with the negative deformation zone formed in front of the tool edge in the final development stage. [S0094-4289(00)00702-7]

Finite Element Modeling of Burr Formation Process in Drilling 304 Stainless Steel

A 3D finite element model was developed for the simulation of drilling burr formation processes of 304L stainless steel. The nonlinear thermo-elastic-plastic model simultaneously accounts for dynamic effects of mass and inertia, strain hardening, strain rate, automatic mesh contact with friction capability, material ductile failure and temperature-mechanical coupling. Material ductile failure criteria were proposed to simulate drilling burr formation. Based on a series of stress contours and the progressive deformation of the workpiece edge obtained from simulation, a drilling burr formation mechanism was proposed and divided into four stages: initiation, development, pivoting point, and formation stages with cap formation. The burr thickness is largely determined by the distance between the pre-defined machined surface and the pivoting point, while the burr height is determined by the positions of the pivoting point and the cap formation. The FEM simulation demonstrates the dominant roles of negative shearing and bending mechanisms in the drilling burr formation process. The simulation results coincide with phenomenological observations of burr geometry from drilling 304 stainless steel and plasticine work materials. [S1087-1357(00)01202-8]

A homologous ductile failure criterion for generalized stress states

The Hancock-Mackenzie failure criterion is here expressed in homologous form and is thereby shown to apply to a range of ductile materials. The method is easily implemented in a finite element code such as DYNA2D/3D, using only a normalizing strain as input. The method is illustrated for the standard “uniaxial” tensile test of a smooth bar; fracture initiates in a region where the stress state is highly triaxial. By DYNA2D simulations of the test, and from the measured fracture strain, the normalizing strain of the homologous criterion is determined. It is also shown that the stress-strain curve derived from experimental data alone does not adequately describe behavior beyond necking, because of the radial variation of stress. An analytical approach, using trial-and-error, is used to obtain the true stress-strain curve.

A numerical study of void distribution effects on dynamic, ductile crack growth

Rapid crack propagation through a material containing two populations of second phase particles is analysed numerically. An elastic-viscoplastic model of a ductile porous material is used to represent small second phase particles that nucleate cavities at large strains as well as larger inclusions with low strength, which result in large voids near the crack tip at an early stage. Various random distributions of the larger inclusions in front of the crack tip are considered, while the small second phase particles are taken to be uniformly distributed. Analyses are carried out for a plane strain double edge cracked specimen under symmetric impulsive loading at the ends and adiabatic heating and the resulting thermal softening are accounted for. The predicted crack growth velocities are entirely based on the ductile failure criterion incorporated in the material model, and thus the present results give a direct estimate of the effect of various random inclusion distributions, compared to results for a uniform distribution. The resistance to crack growth increases and the average crack speed decays with increasing deviation from a uniform inclusion spacing. The dependence of the average crack speed on the root mean square particle spacing is found to be approximately linear over the range considered.