A mechanical behavior law for the numerical simulation of the mushy zone in welding

The aim of this work is to propose a mechanical behavior law dedicated to the mushy zone located between the solid phase and the weld pool in welding. The objective is to take into account of the influence of the mushy zone in the simulation of welding in order to improve the computation of induced effects such as residual stresses

Chaining of welding and finish turning simulations for austenitic stainless steel components

The chaining of manufacturing processes is a major issue for industrials who want to understand and control the quality of their products in order to ensure their in-service integrity (surface integrity, residual stresses, microstructure, metallurgical changes, distortions,…). Historically, welding and machining are among the most studied processes and dedicated approaches of simulation have been developed to provide reliable and relevant results in an industrial context with safety requirements. As the simulation of these two processes seems to be at an operationnal level, the virtual chaining of both must now be applied with a lifetime prediction prospect. This paper will first present a robust method to simulate multipass welding processes that has been validated through an international round robin. Then the dedicated “hybrid method”, specifically set up to simulate finish turning, will be subsequently applied to the welding simulation so as to reproduce the final state of the pipe manufacturing and its interaction with previous operations. Final residual stress fields will be presented and compared to intermediary results obtained after welding. The influence of each step on the final results will be highlighted regarding surface integrity and finally ongoing validation works and numerical modeling enhancements will be discussed

A new technique to measure the true contact area using nanoindentation testing

Nanoindentation technique requires the determination of projected contact area under load for calculation of modulus and hardness of materials. This projected contact area is usually calculated by models which take into account the pile-up or sink-in phenomena around the tip. The most commonly used model was developed by Oliver and Pharr [1] which can precisely model the sink-in around the tip, but cannot account for pile-up. Another model developed by Loubet et al can be used [2]. It can take into account the pile-up and the sink-in phenomena and can precisely measure the projected contact area for a large range of materials, except for materials with high strain hardening exponent. Other techniques, like post mortem measurements, can be used. However these measurements do not take into account the elastic recovery during unloading. A new technique to estimate the true projected contact area will be presented. It consists of combining two models (The Dao et al. model and the Kermouche et al. model) that are used normally to calculate the representative stress and the representative strain in indentation. Consequently, the projected contact area calculation does not depend on any contact area model. Moreover, it can account for the pile-up or sink-in phenomenon and the strain hardening of the material, which is not possible with the actual models used. This new technique requires measuring indentations parameters like the maximum load, the contact stiffness and the loading curvature. It requires also the use of two tetrahedral indenters: a Berkovich tip and a tetrahedral tip where the included semi-angle is 50°. The method was tested on three different samples: glass, PMMA and 100C6 steel. For indentations on glass and PMMA samples, the projected contact area was precisely measured. For indentations on 100C6 steel sample, the method was adapted to take into account the Indentation Size Effect observed at small indentation depths. The projected contact area values measured with this new technique will be presented and compared to the values calculated with classical literature models. Also, the limits of the technique will be discusse

Simulation of material consequences induced by fsw for a trigonal pin

The numerical simulation of Friction Stir Welding processes involves the coupling of a solid mechanics approach under large strains and large strain rates and heat transfer. The eulerian formalism leads to specially efficient finite element simulations of the matter flow under steady conditions. But with such a formulation, the calculation of the consequences induced by the stirring on the material (stirred state, microstructure, etc.) requires the coupling of advection equations for integrating the associated state variables. In this paper, a moving mesh strategy is proposed for the numerical simulation of Friction Stir Welding and material consequences, for complex pin’s geometries. The numerical processing is detailed and the efficiency of the proposed method is discussed on a Friction Stir Welding simulation of 7075 series aluminum alloy

New smoothing procedures in contact mechanics

AbstractThis paper presents recent methods to improve numerical simulation of contact problems by smoothing. The main idea is to combine contact surfaces regularization with an automatic adjustment of both penalty parameter and load step. The underlying goal is to provide handle situations frequently met in an industrial context

Numerical study of scratch velocity effect on recovery of viscoelastic-viscoplastic solids

International audienceThe scratch test is a classical way to investigate the abrasive resistance of coatings and substrates. Because of the complex phenomena involved, the use of refined finite element analysis is often required to analyze the influence of specific parameters. In this paper, the influence of the tip velocity on the scratch recovery of polymer-like time-dependent solids is qualitatively investigated. More precisely the response of three constitutive models is analyzed: an elastic-viscoplastic model, a linear viscoelastic model and finally a viscoelastic-viscoplastic model. This last model is an original assembly based on the connection in series of the elastic-viscoplastic model and the linear viscoelastic model. For that, a new method allowing the connection in series of two different rheological models in a FE code is presented. To analyze the numerical results, the concept of representative stress and representative strain rate of a scratch test is introduced

Indentation relaxation test: Opportunities and limitations

Small scale characterization of material’s mechanical behavior has been performed for fifty years using indentation tests. Many developments have been made in order to improve the reliability of both measurements and interpretations. However, determination of material’s time dependent mechanical properties by means of nanoindentation techniques is still to be enhanced 1. It is proposed to investigate the indentation relaxation – i.e. constant displacement – test as an alternative to the commonly used indentation creep – i.e. constant load – test. Effects of loading strain rate on the measured relaxation behavior are studied, analytically, from a linear viscoelastic model. It is shown that constant strain rate loading guarantees a depth-independent measure of the relaxation behavior. Moreover, indentation strain rate (ISR) affects the relaxation spectrum 2 up to a critical time constant 3 (see figure 1). These effects, highlighted analytically, are confirmed experimentally on PMMA. Limitations of the indentation relaxation test are also discussed. Two main difficulties arise from this kind of experiment. Acquisition of reliable measurements is limited, for long time characterization, by the system drift and, for short time, by the displacement control loop. A particular care has been taken in tuning the control feedback gains to limit displacement overshoot. Very low drift rate has been attained – under 0.015 nm.s-1 – This allowed for measurements at constant displacement up to 600 s. Please click Additional Files below to see the full abstract

Quantification of mechanical properties gradient by nano-indentation and microcompression testing on mechanically-induced transformed surfaces

In the industry, there are several techniques which improve the service lifetime of materials by increasing the local mechanical properties in the near-surface. In the case of mechanical surface treatments (such as impact-based), the material is exposed to repeated mechanical loadings, producing a severe plastic deformation in the surface, and then leading to a local refinement of the microstructure into the affected zone (Tribologically Transformed Surfaces - TTS). The microstructure’s transformation is characterized by a progressive increment of the grain size from the surface until the bulk material. Consequently, very interesting physical properties such as high hardness and better tribological properties are exhibit in these mechanically-induced transformed surfaces. Nowadays, it is well-known that the grain size gradient generated provokes an evolution on the mechanical properties in the impacted zone over a few tens of microns. However, a simple micro-hardness test is not quite enough to quantify precisely the engendered variation of mechanical properties due to the heterogeneity of the transformed surface. The main issue of this work is to assess and describe precisely the elastic-plastic behavior and the distribution of mechanical properties on deformed zones of a model material (pure iron). In our project, a characterization of the transformed microstructure, as well as a statistics measurement of the grain size distribution on the cross-section of the sample is presented firstly. Afterwards a methodology based on nano-indentation tests (Fig.1) and in-situ micro-pillars compression tests (Fig.2) is implemented to quantify the evolution of mechanical properties starting from the near-surface. A relation between the hardness gradient and the microstructure evolution is established, as well as a comparison between the properties measured by both techniques is discussed

Two experimental set-ups designed for investigation of friction stir spot welding process

International audienceThe effects of positioning and clamping conditions of a specimen of friction stir spot welding are investigated in this paper in terms of axial force and torque generated during the process. For this purpose, two special designs of experimental set-ups embedding different positioning and clamping conditions are presented. A four-component mechanical sensor is used for the measurements. First, the effects of the rotational speed of the spindle and the plunge depth of the tool on the axial force and torque are studied. Second, the effects of positioning and clamping conditions are investigated through both set-ups designed, varying the spindle rotation speed. It is shown that the axial force and torque exhibit an important dependence with respect to the rotation speed of the tool and that their maxima depend on positioning and clamping conditions of the specimen