Materials Science Forum Vols. 636-637

Abstract: Shape Memory Alloys (SMA) belong to a special group of metallic materials, which are capable of returning to a pre-determined shape or size when submitted to an appropriate thermal cycle. Generally, these alloys can be easily deformed at a relatively low temperature and, when exposed to a higher temperature, can return to their original shape, that is, to the shape they had before the mechanical deformation was imposed. Although there are a vast variety of materials that can achieve the Shape Memory Alloy effect, only those in which it is possible to have a significant recovery of the deformation – or in which it is possible to generate an important force during the shape's change – can be functionally and commercially interesting. The Ni-Ti alloys, which are one type of SMA, were numerically modelled through Ansys software and these studies are presented in this paper. These special alloys, among many others applications, could be used, for instance, in the form of wires (fibres), in smart composites, as actuators to recover partially the structural integrity of a matrix with cracks. Some 2D and 3D cracked plates were modelled through finite elements and the Stress Intensity Factor, in Mode I, KI, at each crack tip, was determined and compared with the result obtained through the analytical solution. Wires of Ni-Ti, with very small diameters and with different length/diameter ratios, were modelled and mechanical load cycles, at different temperatures, were applied and the material’s behaviour/response was obtained. The thermo-mechanical behaviour of the material was defined based on results published by other authors. Also, a composite material with Ni-Ti fibres embedded was modelled and a mechanical load was applied to it.

Abstract: The aim of this work is to develop a heat transfer mathematical model based on the finite difference method in order to simulate temperature fields in the laser surface remelting process. Convective heat transfer in the remelted pool is taken into account by using the effective thermal conductivity approach. Experiments of laser surface remelting of AISI 304 austenitic stainless steel samples were carried out in the present investigation, and numerical simulations were applied for the CO2 laser machine operating parameters. The work also encompasses the analysis of microstructural and microhardness variations throughout the resulting treated and unmolten zones. This study permits to conclude that numerical simulation is a useful tool in setting the laser operating parameters, enabling pre-programming of the extent of the treated area.

Abstract: Phenomenological yield criteria are generally described by many material parameters. A technique to identify these parameters is required to find the best fit to the results of the mechanical tests. The parameter identification by the classical simulated annealing technique is presented in this paper. This algorithm, based on works by Metropolis et al, is a global optimization method that distinguishes between different local optima to reach the global optimum. The anisotropic model used in this study is the one proposed by Cazacu et al. To prove the efficiency of the proposed algorithm, the material parameters of Ti6Al4V titanium alloy are identified and compared with those obtained using different identification procedures and the same experimental data.

Abstract: The objective of this work is to develop analysis methods based on 3D-FEM simulations for optimum design of the diamond cutting tools under various loading conditions, considering the sintering process of diamond–metal matrix to originate residual stresses. The work concerns the use of finite element simulation for modelling of thermal residual stresses generated during the sintering process of metal matrix diamond tools normally employed by the industry. Stress distribution fields were determined for the diamond shape using a 4-node, reduced integration ABAQUS solid element type C3D4. The residual stress fields in the nearby region of a diamond particle are examined to study the effects of the sintering temperatures, the stress–strain behaviour of the metal matrix and the compression pressure on the upper surface of the metal matrix. Through the simulations of the cutting forces on the diamond, it is demonstrated that the diamond retention capacity induced by the metal matrix (important for extending the life of a diamond tool) is principally dependent on the sintering process. Optimum design of the diamond cutting tools can be achieved by selecting the appropriate sintering temperatures, the stress–strain behaviour of the metal matrix and the compression pressure on the upper surface of the metal matrix during the sintering process.

Abstract: Initiation of intragranular cracks during low cycle fatigue is governed by complex microstructural phenomena. Depending on the loading amplitude, number of cycles, lattice structure and/or chemical composition, different dislocation structures (veins, cells or Persistent Slip Bands) develop and induce heterogeneous localization of strain and stress in the material. For a better comprehension of crack initiation in 316LN stainless steel, low cycle fatigue tests and numerical simulations were performed. Specimens of 316LN steel with polished shallow notch were cycled with constant loading amplitude and Persistant Slip Bands were identified by SEM observations. In parallel, numerical studies were carried out with the model of cristalline plasticity CristalECP. Simulations were performed on 3D polycristalline aggregates of 316LN steel with the finite elements code Abaqus® and Cast3m®. The results show a heterogeneous localization of strain in bands. For a more precise computation of the mechanical fields and to introdruce a grain size effect, Geometrically Necessary Dislocations were introduced in CristalECP. The GNDs are directly related and computed with the lattice curvature.

Abstract: In this study, after a brief introduction to recent investigations on syntactic foam, the free vibration of sandwich structures with syntactic foam as a functionally graded flexible core based on higher order sandwich panel theory is investigated. The formulation uses the classical beam theory for the face sheets and an elasticity theory for the functionally graded core. In the following a numerical study of free vibration of a simply-supported sandwich beam is carried out and corresponding eigenmodes are obtained.

Abstract: The task of obtaining suitable welding parameters for the friction stir welding process is often a difficult one, due to the lack of published data and the fact that the exact mechanism by which the process operates has not yet been fully determined. Therefore, suitable welding parameters often need to be obtained by using extensive, time consuming and expensive experimental methods. The work detailed in this paper pertains to the use of the Taguchi method as a mean to reduce the disadvantages of these experimental methods, more specifically, their cost. The Taguchi method accomplishes this task by substantially reducing the number of welding trials that are needed to obtain suitable welding parameters. This reduction leads to the parameters being obtained more rapidly and at a substantially smaller cost. In this paper a procedure for applying the Taguchi method to the friction stir welding process is presented as well as its application to the welding of a specific component. The method was applied to the welding of 4mm thick AA5083-H111 plates in a butt joint configuration, which constitutes one of the most common industrial welding scenarios. The purpose of the experimental tests was to maximize the welding speed whilst ensuring an acceptable welding quality. The quality of the welds was determined through visual inspection and tensile and bending tests. The application of the Taguchi method allowed, with a relatively small number of experimental welds, to provide some insight into the manner by which the parameters should be altered in order to optimize the process.

Abstract: Polyolefins are especially difficult to bond due to their non-polar, non-porous and chemically inert surfaces. Acrylic adhesives used in industry are particularly suited to bond these materials, including many grades of polypropylene (PP) and polyethylene (PE), without special surface preparation. In this work, the tensile strength of single-lap PE and mixed joints bonded with an acrylic adhesive was investigated. The mixed joints included PE with aluminium (AL) or carbon fibre reinforced plastic (CFRP) substrates. The PE substrates were only cleaned with isopropanol, which assured cohesive failures. For the PE CFRP joints, three different surfaces preparations were employed for the CFRP substrates: cleaning with acetone, abrasion with 100 grit sand paper and peel-ply finishing. In the PE AL joints, the AL bonding surfaces were prepared by the following methods: cleaning with acetone, abrasion with 180 and 320 grit sand papers, grit blasting and chemical etching with chromic acid. After abrasion of the CFRP and AL substrates, the surfaces were always cleaned with acetone. The tensile strengths were compared with numerical results from ABAQUS® and a mixed mode (I+II) cohesive damage model. A good agreement was found between the experimental and numerical results, except for the PE AL joints, since the AL surface treatments were not found to be effective.

Abstract: In contrast with conventional tools, laser processing allows hardening of a restricted zone while keeping the structural properties of the steel bulk. This last quality indicates that, in order to verify a laser hardening, only some specific information of the quenched area is required. The aim of this article is to analyse the reliability of finite element numerical simulation by comparing numerical and experimental outcomes. To do that, we define some magnitudes: the maximum width, the real depth of the laser penetration, the maximum hardness and the hardness versus depth profiles (Jominy’s curves). The tests show the good behaviour of the model and how this contributes important information to the choice of the laser parameters.

Abstract: A model to determine Stress Intensity Factors (SIFs) and simulate the fatigue crack growth in stiffened structures taking into consideration residual stresses is presented in this paper. The stress field required to estimate the SIF was calculated using the Finite Element Method (FEM) considering the residual stress as an initial condition. The residual stress field redistribution as a function of crack growth is taken into account using the Abaqus software. Specimens without and with residual stresses, resulting from different welding techniques, were considered for the present study. The residual stress fields can significantly deteriorate or improve the fatigue life of the structure, depending upon the location of the initial crack; consequently these effects should be analyzed and modelled in order to better understand the consequences of the application of the considered manufacturing processes.