Fabrice Morestin

On the necessity of taking into account the variation in the Young modulus with plastic strain in elastic-plastic software

Work hardening of steel involves modifications of the elastic properties of the material, e.g. an increase in its yield stress. It can also be the cause of an appreciable decrease in the Young modulus. However, this property diminishes as the plastic strain increases. Experiments with a microcomputer-controlled tensile test machine indicate that the diminution can reach more than 10% of the initial value after only 5% plastic strain. In spite of this fact, a lot of elastic-plastic software does not take into account the decrease in the Young modulus with plastification even though it may lead to obvious differences among results. As an application we have developed a software which computes the deformation of steel sheet in press forming after springback. This software takes into account the decrease in the Young modulus and its results are very close to experimental values. Quite arbitrarily, we noticed a recovery of the Young modulus of plastified specimens after few days, but not for all steels tested.

A method for vector displacement estimation with ultrasound imaging and its application for thyroid nodular disease

Ultrasound elastography is a promising imaging technique that can assist in diagnosis of thyroid cancer. However, the complexity of the tissue movements under freehand compression requires the use of a parametric displacement model and a specific estimation method adapted to sub-pixel motion. Therefore, the aim of this study was to develop a motion estimation method for ultrasound elastography and test its performances compared to a classical block matching technique. The proposed method, referred to as Bilinear Deformable Block Matching (BDBM), uses a bilinear model with eight parameters for controlling the local mesh deformation. In addition, a technique of motion initialization based on a triangle scan of the images adapted to ultrasound elastography is proposed. The BDBM method includes an iterative multi-scale process. This iterative approach is shown to decrease the absolute error of the displacement estimation by a factor of 1.4 when passing from 1 to 2 iterations. The method was tested on simulated images and the results show that absolute displacement estimation error was reduced by a factor of 4 compared to classical block matching. We applied the BDBM method on three experimental sets of data. In the first data set, a phantom designed for ultrasound elastography was used. The two other sets of data involve the thyroid gland and were acquired using freehand tissue compression by ultrasound probe of a clinical ultrasound scanner modified for research. A similarity measurement based on local cross-correlation shows that, for experimental data, the BDBM method outperforms the usual block matching.

Characterization of Flow Stress for Commercially Pure Titanium Subjected to Electrically-Assisted Deformation

Uniaxial tension tests were conducted on thin commercially pure titanium sheets subjected to electrically-assisted deformation using a new experimental setup to decouple thermal-mechanical and possible electroplastic behavior. The observed absence of stress reductions for specimens air-cooled to near room temperature motivated the need to reevaluate the role of temperature on modeling the plastic behavior of metals subjected to electrically-assisted deformation, an item that is often overlooked when invoking electroplasticity theory. As a result, two empirical constitutive models, a modified-Hollomon and the Johnson-Cook models of plastic flow stress, were used to predict the magnitude of stress reductions caused by the application of constant DC current and the associated Joule heating temperature increase during electrically-assisted tension experiments. Results show that the thermal-mechanical coupled models can effectively predict the mechanical behavior of commercially pure titanium in electrically-assisted tension and compression experiments.Copyright

Modelling the deep drawing of a 3D woven fabric with a second gradient model

Experimental testing on dry woven fabrics exhibits a complex set of evidences that are difficult to completely describe using classical continuum models. The aim of this paper is to show how the introduction of energy terms related to the micro-deformation mechanisms of the fabric, in particular to the bending stiffness of the yarns, helps in the modelling of the mechanical behaviour of this kind of materials. To this aim, a second gradient, hyperelastic, initially orthotropic continuum theory is proposed to model fibrous composite interlocks at finite strains. In particular, the present work explores the relationship between the onset of wrinkling during the simulation of the deep drawing of a woven fabric and the use of a second gradient model. It is shown that the introduction of second gradient terms accounting for the description of in-plane and out-of-plane bending rigidities decreases the onset of wrinkles during the simulation of deep drawing. In this work, a quadratic energy, roughly proportional to the square of the curvature of the fibres, is presented and implemented in the simulations. This simple constitutive assumption allows the effects of the second gradient energy on both the wrinkling description and the numerical stability of the model to be clearly shown. The results obtained in second gradient simulations are descriptive of the experimental evidence of deep drawing whose description is targeted in this work. The present paper provides additional evidence of the fact that first gradient continuum theories alone cannot be considered fully descriptive of the behaviour of dry woven composite reinforcements. On the other hand, the proposed second gradient model for fibrous composite reinforcements opens the way both to the more accurate simulation of complex forming processes and to the possibility of controlling the onset of wrinkles.

Characterization of tensile and compressive behavior of microscale sheet metals using a transparent microwedge device

The cyclic and compressive mechanical behavior of ultrathin sheet metals was experimentally investigated. A novel transparent wedge device was designed and fabricated to prevent the buckling of thin sheets under compressive loads, while also allowing full field strain measurements of the specimen using digital imaging methods. Thin brass and stainless steel sheet metal specimens with thicknesses on the order of 10–100 μm were tested using the microwedge device. Experimental results show that the device can be used to delay the onset of early buckling modes of a thin sheet under compression, which is critical in examining the compressive and cyclic mechanical behavior of sheet metals.

On the use of cyclic shear, bending and uniaxial tension–compression tests to reproduce the cyclic response of sheet metals

Simple shear, uniaxial tension–compression and bending tests were used to determine the cyclic behaviour of two sheet metals: DP600 and AKDQ. The Yoshida–Uemori two-surface model along with Hill’s quadratic yield function was used to simulate the behaviour of these two materials in each test. For each test, a set of material constants was identified such that the error between the simulated and experimental responses is minimized. Using the material constants obtained from one test, the other tests were simulated to see whether the set of constants obtained from this test is able to describe the material response in the other tests. The results show that depending on the material, the set of constants obtained from one test may or may not be able to reproduce the material response in the other tests. Finally, each set of constants was used to simulate the springback of a U-shaped part formed in a channel draw process. The predicted springback profiles obtained from each set of constants were compared with the experimental profile. It was found that all three tests are suitable to characterize the behaviour of DP600 sheets in view of predicting the springback of channel sections. For AKDQ, however, the error between the predicted and experimental springback profiles was significant regardless of the type of characterization test performed. But for this channel draw process, simulations based on material data obtained from the reverse bending test provided the best prediction of springback.

Friction of Rough Soft Matter Contacts: Local Investigations Through Image Correlation Technique

The friction induced in contacts is a key feature concerning functionality of mechanisms, reliability of systems, energy consumption… Friction on soft matter occurs in many applications (tire/road contacts, touch-sensitive exploration, micro-manipulation of biological items…) as well as in nature. The latter offers various examples of how a topographic surface pattern may control friction. The result is a complex combination of phenomena: adhesion, elastic ratio of bodies in contact, viscous flow, plasticity occurrence, and topography interaction. The role of this latter phenomenon essentially lies in the splitting of the contact area between the two contacting materials and plays an important role on friction response when coupled with adhesion.© 2014 ASME

Characterization of Tensile and Compressive Behavior of Microscale Sheet Metals Using a Transparent Micro-Wedge Device

The cyclic and compressive mechanical behavior of ultra-thin sheet metals was experimentally investigated. A novel transparent wedge device was designed and fabricated to prevent the buckling of thin sheets under compressive loads, while also allowing full field strain measurements of the specimen using digital imaging methods. Thin brass and stainless steel sheet metal specimens were tested using the micro-wedge device. Experimental results show that the device can be used to delay the onset of early buckling modes of a thin sheet under compression, which is critical in examining the compressive and cyclic mechanical behavior of sheet metals.Copyright

Spring back and ‘rebound’ phenomenon analysis with the software PLIAGE

Publisher Summary This chapter illustrates that PLIAGE gives bad spring back results for die radius on thickness ratio lower than 3, bad results that are because the rebound phenomenon produced by the parasitic radius Rdefect. Errors from experiment are not because the plane stresses state assumption. However, PLIAGE can be used for die radius on thickness ratio lower than 3 by adding the right value of Rdefect. This is not easy because this radius depends on the material, the gap-thickness ratio. A solution consists in using ABAQUS like a preprocessor for the analytical software PLIAGE to obtain the fight identical strain area. This procedure has been used to calculate the residual radius of a blank introduced in a drawbead simulator and it has given very good results.