Xinjian Duan

A Unified Finite Element Approach for the Study of Postyielding Deformation Behavior of Formable Sheet Materials

Deformation and fracture behavior of several formable automotive aluminum alloys and steels have been assessed experimentally at room temperature through standard uniaxial tension, plane strain tension, and hemispherical dome tests. These materials exhibit the same deformation sequence: normally uniform elongation followed by diffuse necking, then localized necking in the form of crossed intense-shear bands, and finally fracture. The difference among these alloys lies primarily with respect to the point at which damage (i.e., voiding) starts. Damage develops earlier in the steel samples, although in all cases very little damage is observed prior to the onset of shear instability. A unified finite element model has been developed to reproduce this characteristic deformation sequence. Instability is triggered by the introduction of microstructural inhomogeneities rather than through the commonly utilized Gurson-Tvergaard-Needleman damage model. The predicted specimen shape change, shear band characteristics, distribution of strain, and the fracture modes for steels and aluminum alloys are all in good agreement with the experimental observations.

Failure Behaviour of Notched Sheet Specimen Under Plane Strain Deformation

The failure behaviour of notched AA5754 specimen subject to plane strain deformation is examined by the use of a heterogeneous finite element model. A ductile damage indicator triggered model is applied together with the use of element deletion technique for virtualization of fracture evolution. The predicted characteristics of the failure process under a wide range of triaxiality (0.33 to 2.5) fits well with those experimental observations in the literature.

Numerical Simulation of Sample Scale Intense Shear Bands in Uniaxial Tension Test of Aluminum Alloys

The present work focuses on the study of the development of shear localization through the use of the conventional finite element method which does not involve the non-associative flow rule, yield surface vertex or void growth model. The various aspects, i.e. geometry idealization, element type, solution procedure, convergence, and particularly the representation of true-stress and plastic strain relationship, are emphasized. The predicted profile of the deformed specimen, shear band angle and strain distribution show an excellent agreement with experimental observations.Copyright

Mechanical Behaviour of Alloy 800 Steam Generator Tube With a Circumferential Crack-Like Throughwall Flaw

This paper presents an experimental and numerical study of the mechanical behaviour of Alloy 800 tubes with a circumferential crack-like throughwall flaw. The crack-like throughwall flaw was simulated as a slot fabricated by Electron Discharge Machining (EDM). A digital image correlation (DIC) method is employed to measure the distribution of strain and the deformed EDM slot profile during the tension test. A Finite Element Analysis (FEA) model in ANSYS is then developed to predict the mechanical behaviour of Alloy 800 steam generator (SG) tubing in the tension tests. The FEA model with a verified material stress-strain curve and the calibrated failure criterion is applied to simulate the burst test of SG tubing with a circumferential crack-like throughwall flaw. The maximum allowable size for a circumferential throughwall crack is recommended based on FEA simulations.Copyright

Elastic-Plastic Limit Analysis for Perforated Plates With Triangular Array of Circular Holes

The plastic limit load design for a perforated structure that contains a large amount of circular holes has attracted much attention from power industry in recent years. Most of the previous analyses have been built on the elastic-perfectly plastic material model with small strain finite element formulation. In the present work, a series of newly developed heterogeneous unit cells based on large strain finite element formulation are applied to consider the effect of work hardening rate and the unit cell size on the computed limit load and failure modes. The results indicate that as the unit cell size increases, the unit cell tends to localize early. Also, we found that for the pre-work hardened materials with a strain-hardening coefficient of less than 0.1, work hardening rate has less effect on the computed limit load, but substantial impact on the calculated local strain magnitude.Copyright

Plastic Limit Analysis of Perforated Material Under Finite Deformation

In this paper, the fracture pattern of a perforated aluminum sheet is studied experimentally and numerically using finite element models on two different length scales: a full-scale structural and a local cell models based on the large deformation theory. Through appropriate application of boundary conditions, the more efficient local cell model is shown to produce almost the same results as the full structural model. It is also found that the failure path is significantly affected by the loading conditions (uniaxial vs. biaxial) and the hole distribution pattern. By plotting the instantaneous contour of plastic strain rate, the fracture path could clearly be distinguished by the time that the overall engineering strain had reached 3%.Copyright

A Novel Approach for the Prediction of Shear Localization in Sheet Forming

In this work, a novel approach has been proposed to predict shear band formation under uniaxial tension. The approach postulates a heterogeneous distribution of mechanical properties through the starting material. The model is used to study the effect of such heterogeneity on the development of shear localization with increasing strain. A correlation between diffuse and localized necking, and strain‐hardening rate has been found. The predicted force‐displacement curves and the limit strains to failure agree well with experimental observations.

Influence of Yield Criteria on the Prediction of Shear Localization Considering the Inhomogeneous Distribution of Microstructure

The occurrence of shear localization in structural materials is often associated with bifurcation in continuum solid mechanics. Many approaches such as J2 corner theory and the void model have been proposed to simulate this phenomenon by the use of FEM. In this paper, a new approach with a basis in microstructural inhomogeneity has been proposed and successfully applied to simulate large strain deformation in uniaxial tension of aluminium alloys. The method, in addition, takes advantage of a more suitable hardening law for Al alloys — namely the Voce equation. Further, the influence of various yield criteria (i.e. von Mises, Hill’s 1948 and Barlat’s 1991) on the prediction of shear localization is discussed in the present work. The predicted shear band angle is also compared with the measured value.Copyright