Pierre-Yves Manach

Shear and tensile thermomechanical behavior of near equiatomic NiTi alloy

The industrial development of devices using the intriguing properties of shape memory alloys involves accurate prediction of their thermomechanical behavior. This may be achieved using Computer Aided Design together with Finite Element programs. Reliable constitutive laws are needed for the execution of such programs. Several tensorial constitutive laws have been proposed to model the unusual thermomechanical properties of shape memory alloys. However, for all these tensorial models, it is necessary to make assumptions which cannot be verified when only tensile property data are available. The purpose of this paper is to present a new set of experimental mechanical data, including tensile and simple shear tests performed on sheet samples of near equiatomic NiTi alloy. Both mechanical behaviors are compared for a large temperature range (from below Mf to above Af). It is shown that the von Mises assumptions usually made in the establishment of tensorial constitutive equations are not always valid. Other yield locus forms are proposed to model the typical tension and simple shear thermomechanical behavior of shape memory alloys.

Prediction of damage in the hole-flanging process using a physically based approach

The aim of this work is to identify the limits of the hole-flanging process experimentally and numerically by a physically based approach of damage for two different aluminium alloy sheets. Two hole-flanging conditions were considered, namely hole-flanging without ironing in which the flange is formed by edge stretching, and hole-flanging with ironing in which the metal is squeezed between the punch and the die. The forming defects were characterized experimentally by scanning electron microscope observations performed in critical zones of the flanged parts. The forming defects were also predicted numerically by a 3D finite element model based on Gurson-Tvergaard-Needleman constitutive equations. To evaluate the accuracy of the developed finite element model, material damage distribution within the workpiece during the process was studied and compared with experimental observations. Furthermore, the effects of the clearance-thickness ratio on the damage behaviour were investigated for different diameter of the initial hole. This study provides relevant and new results in the design of many parts obtained by hole-flanging. Results showed that the model predicts accurately all types of failures (orange peel aspect, necking, microvoids, tear) for different conditions and both materials. Moreover, despite the large deformation induced by ironing, such conditions decrease the damage, due to void closure effects, that is in agreement with experimental results.

Mechanical Properties Involved in the Micro-forming of Ultra-thin Stainless Steel Sheets

The objective of this paper is to characterize the mechanical behavior of an ultra-thin stainless steel, of 0.15-mm thickness, that is commonly used in the manufacturing of miniature connectors. The main focus is the relationship between some microstructural features, like grain size and surface roughness, and the macroscopic mechanical behavior investigated in uniaxial tension and simple shear. In tension, adaptations to the very small sheet thickness, in order to hold the specimen under the grips, are presented. Yield stress, initial elastic modulus, and evolution of the loading–unloading slope with plastic deformation were evaluated. Moreover, the kinematic contribution to the hardening was characterized by monotonic and cyclic simple shear test and reproduced by a mixed hardening law implemented in Abaqus finite element code. Then, the evolution of surface roughness with plastic strain, both in tension and simple shear, was analyzed. It was shown that in the case of an ultra-thin sheet, the stress levels, calculated either from an average thickness or when considering the effect of the surface roughness, exhibit a significant difference. Finally, the influence of surface roughness on the fracture of a tensile specimen was also investigated.

Benchmark 3 - springback of an Al-Mg alloy in warm forming conditions

Accurate prediction of springback is a long-standing challenge in the field of warm forming of aluminium sheets. The objective of this benchmark is to predict the effect of temperature on the springback process through the use of the split-ring test [1] with an Al-Mg alloy. This test consists in determining the residual stress state by measuring the opening of a ring cut from the sidewall of a formed cylindrical cup. Cylindrical cups are drawn with a heated die and blank-holder at temperatures of 20, 150 and 240°C. The force-displacement response during the forming process, the thickness and the earing profiles of the cup as well as the ring opening and the temperature of the blank are used to evaluate numerical predictions submitted by the benchmark participants. Problem description, material properties, and simulation reports with experimental data are summarized.

Mechanical Behaviour and Springback Study of an Aluminium Alloy in Warm Forming Conditions

This study deals with the mechanical behaviour and material modelling of an AA5754-O alloy at elevated temperature. Experimental shear tests were performed from room temperature up to 200°C, and the material behaviour has been identified with both shear and tensile tests, as a function of temperature. To analyse the influence of temperature during forming over springback, a split-ring test is used. Experimental results are obtained and compared to numerical simulations performed with the finite element in-house code DD3IMP. The numerical process of ring splitting is performed with the in-house code DD3TRIM. The main observed data are force-displacement curves of the punch during forming, cup thickness at the end of forming, and ring gap after splitting. It is shown that all these parameters are strongly dependent on the forming temperature. A correlation is obtained between experimental data and numerical simulation for the evolution of punch force and opening after springback as a function of temperature. The distribution of the tangential stress in the cup wall is the main factor influencing the springback mechanism in warm forming condition.

Springback study in aluminum alloys based on the Demeri Benchmark Test : influence of material model

Springback is a serious problem in sheet metal forming. Its origin lies in the elastic recovery of materials after a deep drawing operation. Springback modifies the final shape of the part when removed from the die after forming. This study deals with Springback in an Al5754‐O aluminum alloy. An experimental test similar to the Demeri Benchmark Test has been developed. The experimentally measured Springback is compared to predicted Springback simulation using Abaqus software. Several material models are analyzed, all models using isotropic hardening of Voce type and plasticity criteria such as Von Mises and Hill48’s yield criterion.

Reverse Deep Drawing: Experimental and Numerical Simulation Results

This paper presents a whole study of the drawing and redrawing of a cylindrical cup in two opposite directions. A device has been developed in order to perform deep drawing tests on a classical tensile machine and the test is simulated with the dynamic explicit finite element code Pam-Stamp. The main reported results are the punch force-displacement curves and the thickness distributions in the cup wall after the first and second stage for several orientations to the rolling direction. Experimental and numerical results are compared.

Surface Defects in Sheet Metal Forming: a Simulative Laboratory Device and Comparison with FE Analysis

Surface defects are small concave imperfections that can develop during forming on outer convex panels of automotive parts like doors. They occur during springback steps, after drawing in the vicinity of bending over a curved line and flanging/hemming in the vicinity of the upper corner of a door. They can alter significantly the final quality of the automobile and it is of primary importance to deal with them as early as possible in the design of the forming tools. The aim of this work is to reproduce at the laboratory scale such a defect, in the case of the flanging along a curved edge, made of two orthogonal straight part of length 50 mm and joint by a curved line. A dedicated device has been designed and steel samples were tested. Each sample was measured initially (after laser cutting) and after flanging, with a 3D measuring machine. 2D profiles were extracted and the curvature was calculated. Surface defects were defined between points where the curvature sign changed. Isovalues of surface defect de...

Identification strategy influence of elastoplastic behavior law parameters on Gurson–Tvergaard–Needleman damage parameters: Application to DP980 steel

This work adopts elastic–plastic/damage coupling in order to describe tensile behavior with validation on the deep-drawing test of a DP980 Dual Phase steel sheet. The damage model used is the Gurson–Tvergaard–Needleman (GTN model). The hardening laws used are those of Swift (non-saturating law), Voce (saturating law), Hockett-Sherby (saturating law) and the two combined laws Swift/Hockett-Sherby and Swift/Voce. An identifying method for elastic–plastic parameters and GTN damage model parameters is presented using the software modeFRONTIER. This method based on the inverse analysis is also proposed for the identification of weighting coefficient α of the Swift/Hockett-Sherby combined hardening law. Finally, a parametric study was carried out to show that the plastic modulus can be considered as another criterion for the choice of a hardening law. Dependence of the damage model parameters to the hardening law is clearly established. The different behavior laws are introduced via a VUHARD type subroutine in the calculation code Abaqus.

Influence of prestrain on the occurrence of PLC effect in an Al-Mg alloy

In the present work, the jerky flow in an Al-Mg alloy during simple shear tests for various strain paths and temperatures is studied. Direct observations of the sample surface using digital image correlation is used to investigate the type and the dynamics of Portevin-Le Châtelier (PLC) bands as a function of shear strain and temperature. The influence of strain path changes on the occurrence of PLC effect is evidenced through cyclic shear tests, composed of a loading up to several values of the shear strain followed by a reloading in the opposite direction. It is shown that the occurrence of PLC effect modifies the transient behavior after strain path changes.