Key Engineering Materials Vols. 651-653

Abstract: Tailor Heat Treated Profiles (THTP) are profiles that exhibit local different mechanical properties optimized for a subsequent forming operation. The property distribution is realized by short term heat treatment before a forming operation. Based on the interaction of soft and hard areas the material flow can be improved and the formability can be enhanced. Prerequisite for a successful application of the technology is a comprehensive material characterization. Therefore, within this paper the influence of short term heat treatment on the mechanical properties of profiles will be presented. In particular, different heating technologies based on heat conduction and laser radiation are compared. Based on the results, a process window will be derived. All investigations were performed using the precipitation hardenable aluminum alloy EN AW 6060.

Abstract: The friction conditions are responsible of the thickness distribution in a part realized by tube hydroforming. Then it is essential to have a good evaluation of the friction coefficient for running predictive finite element simulations. The tube expansion in a square die is one of tests proposed for the friction evaluation. In the literature, several analytical models have been developed for this specific test. The present paper concentrates on one of this model and results obtained from the analytical analysis, FE simulations and experiments are compared. The repartition of the thickness over the shaped tube and its evolution during the process are studied. The tendencies are in agreement but some complementary evaluations are proposed for using the proposed approach for the evaluation of the friction coefficient with the analytical model.

Abstract: Wrinkling is one of the primary failure modes in deep drawing of sheet metal parts. Previous studies showed that the second principle stress can be a measure for the initiation and growth of wrinkles. The wrinkling analysis is usually made with using conical cup geometries. Recent experiments and numerical simulation results at the Institute for Metal Forming Technology (IFU) showed that the wrinkling analysis using simple conical cup geometries is not suitable for description of complex wrinkling conditions for real deep drawing processes. In the presented experimental results, fender shaped geometry was chosen as an example. During deep drawing of this geometry, different wrinkling formulation mechanisms were observed. Regarding these wrinkling mechanisms, a new wrinkling limit curve can be determined. By use of this new wrinkling limit curve, it is possible to detect the occurrence of wrinkles in each area of the formed sheet metal part until the wrinkle is finally formed.

Abstract: In this work experimental tensile tests were performed on a DP1000 steel at different temperature levels in the range (293-600) K. This experimental database was used for the identification of a Johnson-Cook constitutive model taking into account temperature effects. A critical void area fraction and a corresponding critical equivalent plastic strain were also identified from each temperature using a metallographic analysis previously described. This critical equivalent plastic strain was considered as a fracture criterion instead of the traditional fracture strain of the Johnson-Cook model and a corresponding damage parameter was determined in these specific conditions. Finally numerical simulations were carried out to analyze the influence of the temperature on the macroscopic critical fracture during a forming operation by bending.

Abstract: In-plane tension/compression tests of a cold rolled interstitial-free (IF) steel and sheet a 980MPa dual phase high strength steel sheet (980DP) were carried out to investigate the work-hardening behavior under two-stage loading paths. The two-stage loading paths consist of the uniaxial tension/compression for the rolling direction (RD) followed by unloading and subsequent uniaxial tension/compression in the 0°, 45° and 90° directions from the first loading direction (0°-, 45°- and 90°-loading). The work hardening behavior in the second loading was different between the 980DP and the IF steel. It was found that the work hardening behaviors were significantly affected by the inner product of the strain rate mode tensors for the first and second loading and that the effect of the deformation mode (tension/compression) was small.

Abstract: We reconsider here the FEM-algorithm for solving the initial and boundary value problems performed within the viscoplastic constitutive framework and proposed in our paper [1]. The problems concerning the deformation of a sheet composed of a single fcc-crystal, generated by different slip systems simultaneously activated, are solved numerically for an in-plane stress state. The variational formulation is associated to the incremental equilibrium equations and is coupled with an update procedure for the state variables, which are described by the differential type equations, as well as for the non-local evolution equations of the dislocation densities. The length scale parameter is introduced into the model through the diffusion-like parameter which enters the evolution equations for dislocation densities. For more accuracy of the simulation, the shape functions have been chosen polynomials with higher than one degree. We do not consider that once a slip system was activated it remains active for the rest of simulation. The activation condition is a key point in the numerical algorithm. As a numerical example, we perform a tensile test of a rectangular and non-rectangular metallic sheet, comparring the results of the simulation when two, respectively eight slip systems are considered.

Abstract: This paper is focused on the performance evaluation of two theoretical models that can be used to predict the Forming Limit Curve (FLC) for an AA6016-T4 aluminium alloy sheet. The FLC is calculated based on the Marciniak-Kuczynski (M-K) model and the Modified Maximum Force Criterion (MMFC) using the Hill '48, Barlat '89 and BBC 2005 yield criteria, the latter identified in three variants, namely with 6, 7, and 8 material parameters. The performance assessment of the M-K and MMFC models combined with different yield functions is based on the comparison between the theoretical predictions and the experimental data provided by the Nakazima test (ISO 12004: 2008) as well as by an experimental procedure recently developed by the authors for the FLC determination.

Abstract: The purpose of present study is to present experimental results and a mathematical model for the evolution of surface waviness parameters with plastic strain of Interstitial Free - IF steel sheet under uniaxial and biaxial stretching tests. Roughness and waviness are very important quality parameters to be evaluated in sheet metal forming. Various waviness profile parameters such as the arithmetic average waviness Wa, the total height peak-valley waviness Wt, maximum peak height Pp and maximum valley depth Pv were measured during uniaxial and biaxial tests. Tensile test specimens at 0º, 45º and 90º to the direction of rolling and Nakazima type specimens of IF steel were fabricated. After preparing the test specimens, incremental simple tensile and Nakazima biaxial tests with flat punch were performed to characterize the negative and positive quadrant of the Map of Principal Surface Limit Strains, MPLS, of IF steel sheet. Measurements of waviness parameters of the specimen surface at incremental plastic strain stages were performed at the same surface site. Also, during the uniaxial and biaxial tests, the following plastic strains were calculated from printed circular mesh at each incremental step: ε₁ longitudinal major strain and ε₂ transverse minor strain. From these data, curves of waviness parameters versus equivalent strain were plotted to obtain a phenomenological equation of 4th or 3rd degree polynomial type. Furthermore, the growth rates of Wa and Wt parameters with the equivalent plastic strain were assessed. From the growth rate curves, it was possible to verify how the sheet thickness imperfections evolves during straining, being possible to predict the influence of plastic strain on the waviness values of IF steel sheets. From the analysis of Wa and Wt growth rates during straining, it was possible to proposed a criteria for the onset of local necking or limit strains in the MPLS. The waviness parameters Wt is the best for characterizing the onset of local necking in sheet metal forming.

Abstract: The present work aims at studying the influence of strain rate on the frictional behaviour of AA7075 aluminium alloy in the O-annealed temper state. To this purpose, ring compression tests were performed both under quasi-static and dynamic loading conditions. The high strain rate tests were carried out by means of the Split Hopkinson Tension-Compression Bar in the direct version. In both cases, hollow cylindrical samples, characterised by an initial outer diameter to inner diameter to height ratio of 6:3:2, were tested under dry condition and by lubricating with molybdenum disulphide grease. The different frictional behaviour exhibited by AA7075-O under quasi-static and dynamic loading conditions can be attributed to the strain rate effect both on the plastic flow behaviour of the deformed material, and on the thickness of the lubricant film.

Abstract: The aim of this work is to study the mechanical properties of alloy AA7075 in both T6 and O temper states, in terms of visco-plastic and fracture behavior. Tension and compression tests were carried out starting from the quasi-static loading condition 10^-3 up to strain rates as high as 2 x 10³ s^-1. The high strain rate tests were performed using a Split Hopkinson Tension-Compression Bar (SHTCB) apparatus. The tensile specimens were also subjected to micro-fractography analysis by Scanning Electronic Microscope (SEM) to evaluate the characteristics of the fracture. The results show a different behavior for the two temper states: AA7075-O showed a significant sensitivity to strain rate, with a ductile behavior and a fracture morphology characterized by coalescence of microvoids, whilst AA7075-T6 is generally characterized by a less ductile behaviour, both as elongation at break and as fracture morphology. Brittle cleavage is accentuated with increasing strain rate. The Johnson-Cook viscoplastic model wad also used to fit the experimental data with an optimum matching.