Key Engineering Materials Vols. 410-411

Abstract: This work covers the finite element analysis of geometric and process parameters in hydraulic bulge tests in terms of the accuracy of the evaluated flow curve. The important parameters are identified and varied to cover the whole range of possible uses. The effects of these parameters are analyzed for three representative materials: aluminium, mid-strength steel, and high-strength steel. The flow curves of the materials for each set of parameters are calculated by using the results of the simulations and the membrane theory. It is seen that even with simulation results, it is not always possible to obtain the input flow curve, especially towards the end of the test. The dimensions of the sheet and the tooling affect the plastic strain development and geometry of the bulge, leading to errors in computed flow curves. In order to observe the effect of the material flow from the flange on the determined yield stresses, the function and position of the drawbeads are also examined. These parameters, together with the method used to calculate the radius of the bulge, determine the accuracy of the calculated flow curve. Guidelines for an accurate flow curve determination regarding the test set-up and calculation methods are given.

Abstract: The UltraSTEELTM process, developed by Hadley Industries Plc (Hadleys), is a novel surface dimpling process used on steel strip prior to cold roll forming. This dimpling process increases the strength of the final rolled products and enhances other product properties such as fire test performance and screw retention. Reported in this paper are the results of finite element analysis conducted to simulate the application of the spatially distributed dimple pattern to the metal sheet prior to the cold roll forming process. The model contains a representation of the two rotating rolls that plastically deform, imparting the spatially distributed dimple pattern on the sheet as it moves between them. The simulation results are compared with industrially processed UltraSTEELTM steel sheet. Consideration is given to the plastic region developed in the dimples and the effect of rolling action on the regions. The dimensional change of the metal sheet is also discussed.

Abstract: This paper studies the effects of sheet thickness on the forming limits of high strength aluminum alloys commonly used in the aircraft industry. The selected materials are 2024-T3 and 7075-T6 representing 2000 and 7000 series aluminum alloys. Two sets of experiments are carried out to identify the effects of sheet thickness on the forming behavior of the selected alloys. The first set of the experiments is tensile testing. The tensile properties of sheets with different thickness and different materials including the plasticity parameters are determined in the first set of experiments. The second set of the experiments is air bending. The minimum bending radius of the different series of materials is determined in the second set of experiments. The results of the tensile testing and air bending are studied both separately and in comparison with each other to identify the trends and to understand the mechanisms governing the observed trends. It is shown that the behavior of the studied alloys is to some extent different from the behavior of more ductile aluminum alloys and mild steels.

Abstract: The finite element method is a widely used tool in sheet metal forming. The quality of the results of such an analysis depends largely on the applied constitutive model and its material parameters, which have to be determined experimentally. These data are relevant on the choice of the yield criterion among the wide range of options available in the commercial applications implementing the finite element method. Since the accuracy of material parameters estimation is therefore crucial, investigations were performed with an Al-Mg sheet alloy and a mild steel sheet to optimize a Miyauchi-based simple shear test. This method is one of the basic ways to investigate the plastic properties of a sheet metal up to large strains, which is very important for numerical analysis of sheet metal forming processes. Aim of the test is to determine the shear stress-strain correlation. In order to enhance the quality of the experimental results the detection of the deformation’s field, trough an optical measurement system, and the methodology for its evaluation are focus of the present study.

Abstract: This work deals with the formability of metal foams and it is focused on three point bending of aluminum foam sandwich panels. In this study bending can be considered as both a process for shaping of foamed panels and a mean for metal foam testing. Several tests were carried out by varying bending conditions and collecting load-displacement data. Specimens showed an interesting behaviour during bending since the sample deformation was related to the occurrence of foam cells failure or collapse. Moreover, once the process is terminated specimens retained a significant bending strength. During the experimental campaign several samples showed irregular fracture behaviour. In these cases cells fracture often starts in a generic part of the specimen (in correspondence of some non-eligible material defects) and propagates through the foam. The reasons of this problem and the possibility of failure prediction were investigated using different approaches. In particular, thickness measurements (using a ultrasound feeler) and X-ray analysis were carried out for this purpose. In addition, a study based on foam density showed a remarkable data scatter that can be considered as a characteristic of the state of the art in foamed panels manufacturing (in terms of process control and product variability). Finally, load-stroke curves were taken into account for this purpose. Computer simulations of the experiments were performed using the commercial FEM code (Deform 2D). Foam compressibility was simulated using a porous material model and the onset of foam instability was simulated by means of a specific damage criterion. Good agreement between simulative and experimental results was found.

Abstract: A pilot study has been carried out to examine the effect of specimen dimensions on the obtained tensile properties of aluminium and steel sheet. The materials used were DP600 grade steel and AA5754 grade aluminium sheet. Four types of dog-bone samples with varying dimensions were tested for both materials. Standard tensile test procedures were performed using a universal test machine together with contacting extensometry. The GOM Aramis photogrammetric 2D strain mapping technique was also applied. The results suggest that for both steel and aluminium sheet materials, differing specimen dimensions have little effect on the obtained mechanical properties. Depending on the gauge length of extensometer chosen and the position at where necking occurred on the sample, the extensometry results and 2D strain mapping results slightly differed towards to the end of stress-strain curve. The failure mode between the chosen grades of steel and aluminium samples was observed to differ, as did the percentage of failures that occurred within the gauge length. All steel samples fractured across the specimen perpendicularly to the test direction; whilst fracture of aluminium samples occurred approximately 30 degrees from the perpendicular.

Abstract: The development of manufacturing methods for producing plastic-metal hybrid structures has already opened new possibilities for lightweight design. Contrarily to the existing technologies i.e. Insert, Outsert and Hybrid Technology, the new forming process “Polymer Injection Forming” (PIF) offers the advantages associated with injection moulding technology and hydroforming technology in a way that hybrid structures can be produced in a single step. The polymer which is used as pressure medium in a melted state to form geometrical features in the sheet metal remains as a functional part in the final hybrid structure. This paper focuses on the experimental investigation of Polymer Injection Forming. Particularly, the interaction between process parameters of injection moulding including injection pressure, cavity pressure, volume flow rate, melt temperature and the resulting part properties e.g. shape and strain distribution of the sheet metal structure from the preliminary results are discussed. The experiments comprise the bulging of a (free form) dome geometry and simple cup geometry of Aluminium and steel sheets by using thermoplastic Polypropylene (PP) as working medium.

Abstract: In this study, an easy and innovative technique for laser welded butt joint characterization is proposed. AISI 304 sheets, 1.25 mm in thickness, were welded in the butt configuration by means of a high power diode laser (HPDL). Different combinations of the process parameters were considered. For each combination, during the process different thermal cycles were induced in the material generating heat affected zones, which were subsequently tested by the double indentation test method. This test was performed on the sheets by means of two co-axial flat indenters, 1 mm in diameter, made of tungsten carbide (WC); the upper indenter penetrates at a constant rate (0.1 mm/min) into the material, whereas the lower indenter acts as a support. On each side of the seam, 10 indentations were performed at fixed positions with different distance from the centre of the seam, to obtain a load map correlated to the mechanical properties. By comparing the maps of the different laser welded joints, a clear effect of the laser scan speed and the power was found. Furthermore, a wake field effect is recognizable, in fact the loads are symmetric as regards the seam, and there is a clear trend in the direction of the laser path. These results confirm the effectiveness of this method, which is also suitable for on-line application because a very small indentation is left on the sheet.

Abstract: The paper presents an analysis of a recently proposed failure criterion for thin sheets. According to Aretz [1], this criterion becomes numerically unstable for yield surfaces with locally constant exterior normal fields. Here we make more precise statements about the nature of this instability, asses the predictive capabilities of the criterion, and introduce a fitting parameter for its plane strain calibration.

Abstract: Vehicle safety has increasingly become an economical factor for vehicle manufacturers and this has become most apparent in car safety [1-4]. Manufacturers are now spending considerable resources on safety research. Government requirements on safety have compelled manufacturers to carry out considerable number of crash tests to validate the safety of their cars [6-7]. The data from these tests is important in the development of simulation models employing finite element (FE) software. Many companies predict crashworthiness using commercially available software such as PAMCARSH and LS-DYNA. These simulations are based on mathematical constitutive equations and hence any simulation created is only as representative as the constitutive equations used. This project has studied the reliability of the material models used by LS-DYNA. Material models selected for analysis are used extensively by impact simulations software and were namely: Power Law Plasticity and Cowper/Symonds. Piecewise Linear Plasticity was also selected because it is based on a true stress/strain and is expected that the simulation would be representative. The models were developed using Belytschko-Lin-Tsay shell elements and were compared with experimental tests employing uni-axial tension strips carried out on three materials – aluminium, high strength steel and mild steel. The tests were carried out using a DARTEC tensile testing machine (up to strain rate of 2.0s-1) at UCE in Birmingham. Testing for the higher strain rates (aluminium up to 269.1s-1, mild steel up to 460s-1, and high strength steel up to 456.9s-1), were carried out at The Royal Military College, Shrivenham using a ROSAND tester.