Elmar Beeh

Influence of Tension-Compression Asymmetry on the Mechanical Behavior of AZ31B Magnesium Alloy Sheets in Bending

Magnesium alloys are promising materials for lightweight design in the automotive industry due to their high strength-to-mass ratio. This study aims to study the influence of tension-compression asymmetry on the radius of curvature and energy absorption capacity of AZ31B-O magnesium alloy sheets in bending. The mechanical properties were characterized using tension, compression, and three-point bending tests. The material exhibits significant tension-compression asymmetry in terms of strength and strain hardening rate due to extension twinning in compression. The compressive yield strength is much lower than the tensile yield strength, while the strain hardening rate is much higher in compression. Furthermore, the tension-compression asymmetry in terms of r value (Lankford value) was also observed. The r value in tension is much higher than that in compression. The bending results indicate that the AZ31B-O sheet can outperform steel and aluminum sheets in terms of specific energy absorption in bending mainly due to its low density. In addition, the AZ31B-O sheet was deformed with a larger radius of curvature than the steel and aluminum sheets, which brings a benefit to energy absorption capacity. Finally, finite element simulation for three-point bending was performed using LS-DYNA and the results confirmed that the larger radius of curvature of a magnesium specimen is mainly attributed to the high strain hardening rate in compression.

Bending Collapse Behaviour of Polyurethane Foam-Filled Rectangular Magnesium Alloy AZ31B Tubes

Quasi-static/dynamic three-point bending tests were conducted to assess the crash performance of magnesium alloy AZ31B extruded and sheet tubes at the German Aerospace Centre (DLR) – Institute of Vehicle Concepts in Stuttgart. Different foam-filled AZ31B beams with a variation of foam density and thickness were fabricated through several manufacturing processes: cold bending, tungsten inert gas welding, cathodic dip painting and polyurethane foam injection. The experimental results were compared with those from mild steel DC04 tubes. It shows that empty magnesium alloy AZ31B outperforms steel DC04 in terms of specific energy absorption for the empty tubes with equivalent volume when subjected to bending loads. It was found that the foam-filled tubes achieved much higher load carrying capacity and specific energy absorption than the empty tubes. Moreover, there is a tendency showing that a foam-filled beam with a higher foam density reaches higher load carrying capacity, but fractures earlier. The foam-filled AZ31B tube with 0.20 g/cm3 foam obtained the highest specific energy absorption, but this outperformance was weakened due to the earlier fracture. In addition, the numerical simulation utilising material model MAT_124 in LS-DYNA explicit FEA package was performed. The simulation results indicate that using calibrated stress-strain curves and failure parameters, material model MAT_124 yields a general good agreement with the experimental results.

Kostenattraktiver Leichtbau in der Vorderwagenstruktur

Zur Senkung des CO2-Ausstoses spielt die Reduzierung der Fahrzeugmasse eine wichtige Rolle. Am Beispiel eines Magnesiumgussteils fur die Vorderwagenstruktur zeigt das DLR-Institut fur Fahrzeugkonzepte, wie durch Anwendungen einer geeigneten Leichtbaustrategie, durch Nutzung der Topologieoptimierung und durch eine neue kostenattraktive Bauweise erhebliche Gewichtseinsparungen erzielt werden konnen.

Dynamic tensile behaviors of AZ31B magnesium alloy processed by twin-roll casting and sequential hot rolling

Abstract The dynamic tensile behavior of twin-roll cast-rolled and hot-rolled AZ31B magnesium alloy was characterized over strain rates ranging from 0.001 to 375 s−1 at room temperature using an elaborate dynamic tensile testing method, and the relationship between its mechanical properties and microstructures. It is observed that the sheet has a strong initial basal fiber texture and mechanical twinning becomes prevalent to accommodate the high-rate deformation. The yield strength and ultimate tensile strength monotonically increase with increasing the strain rate, while the strain hardening exponent proportionally decreases with increasing the strain rate due to twinning-induced softening. The total elongation at fracture distinctly decreases as the strain rate increases under quasi-static tension, while the effect of strain rate on the total elongation is not distinct under dynamic tension. Fractographic analysis using a scanning electron microscope reveals that the fracture is a mixed mode of ductile and brittle fracture.

Dynamic bending behaviour of magnesium alloy rectangular thin-wall beams filled with polyurethane foam

ABSTRACT This study proposes a hybrid structural design concept of polyurethane foam-filled magnesium alloy AZ31B rectangular thin-walled beams which serve as energy absorbing components in automotive applications. Uniaxial tensile and compressive tests, and fracture tests were performed to investigate the material mechanical properties. Dynamic three-point bending tests were performed to study the deformation/fracture modes and energy absorption capacity for the foam-filled AZ31B beams, and to compare these mechanical properties with those for mild steel DC04 beams. Different AZ31B beams were filled with a variation of foam density (0.05, 0.20 and 0.30 g/cm3). It was found that the AZ31B beam filled with 0.20 g/cm3 foam reached the highest specific energy absorption; moreover, it absorbed more energy and reached much higher specific energy absorption than the foam-filled DC04 beam filled with the same foam, although the former one was nearly 54% lighter. The potential advantage of the foam-filled AZ31B beams is possibly associated with the high work hardening rate of AZ31B sheet in compression, which may involve more material in plastic deformation compared with the foam-filled DC04 beams. It has therefore been demonstrated that the hybrid structural design concept of the polyurethane foam-filled AZ31B beam has potential applications in auto-body structures.

Load/Displacement and Energy Dissipation Performances of Aluminum and Magnesium Extrusions Subjected to Quasi-Static and Dynamic Loading under Axial Crush and Cutting Deformation Modes

Findings from quasi-static and impact loading of aluminum (AA6082-T6) and magnesium (AZ31B-F) circular extrusions when subjected to crush and cutting modes of deformation are presented. Circular cross sectional extrusion geometry with a thickness of 1.5 mm, a diameter of 62 mm, and lengths equal to 300 mm were selected. Dynamic loading resulted from impact with a dropping mass of 57 kg at a velocity of 7 m/s. Under cutting deformation, the aluminum alloy extrusions generated lengthy chips ahead of the cutter followed by stable formation of petalled cut side walls. The magnesium extrusions, illustrated the formation of small chips and sides walls which often, although not consistently, fractured. Energy dissipation was noted to be greater and the deformation more stable for the aluminum extrusions. Under a cutting deformation mode, energy dissipation of 0.621 – 0.684 kJ for the magnesium extrusions was found compared to 1.20 – 1.23 kJ for the aluminum extrusions.

Experimental Observations on the Mechanical Response of AZ31B Magnesium and AA6061-T6 Aluminum Extrusions Subjected to Compression and Cutting Modes of Deformation

Cylindrical extrusions of magnesium AZ31B were subjected to quasi-static axial compression and cutting modes of deformation to study this alloy’s effectiveness as an energy absorber. For comparison, the tests were repeated using extrusions of AA6061-T6 aluminum of the same geometry. For the axial compression tests, three different end geometries were considered, namely (1) a flat cutoff, (2) a 45 degree chamfer, and (3) a square circumferential notch. AZ31B extrusions with the 45 degree chamfer produced the most repeatable and stable deformation of a progressive fracturing nature, referred to as sharding, with an average SEA of 40 kJ/kg and an average CFE of 45 %, which are nearly equal to the performance of the AA6061-T6. Both the AZ31B specimens with the flat cutoff and the circumferential notch conditions were more prone to tilt mid-test, and lead to an unstable helical fracture, which significantly reduced the SEA. Axial cutting of AA6061-T6 extrusions has been shown to be an effective, ductile mode of energy dissipation, yielding a repeatable, nearly constant load/deflection response with a crush force efficiency (CFE) up to 96%. In the present tests, the quasi-static cutting deformation of AZ31B extrusions achieved a respectable CFE of 80%, but revealed a load/deflection response with sharp, minute, rapid fluctuations, indicating an undesirable fracturing failure. Additionally, the average specific energy absorption (SEA) of AZ31B was 11 kJ/kg, which is less than half that seen for AA6061-T6 extrusions of the same geometry (24 kJ/kg). An analytical model of the cutting deformation of AA6061-T6 extrusions can predict the steady state cutting force to within 10%. However, the model did not agree well with the experimental results of AZ31B, yielding approximately 150% error. This deviation is likely attributed to the brittle deformation nature of AZ31B that is not accounted for in the model.

Novel Concepts for the Application of Magnesium Sheets and Profiles in Crash Loaded Vehicle Areas

As the lightest structural metal, magnesium alloys have been attractive to reduce vehicle weight and emissions by lightweight design in the automotive industry. Structural crashworthiness is not a physical property itself, but correlates with the material’s ductility and structural design. Magnesium is known to be a material with lower failure strain than other metallic materials. Therefore the use of magnesium in crash-related areas is more challenging compared to steel and aluminum.In structures with a bending load, as in the case of a bumper or the sill, crash properties can be significant improved by filling profiles with a stabilizing core. In order to evaluate the crashworthiness of this hybrid structure under bending loads, both empty and polyurethane foam-filled rectangular section beams were constructed and tested by using the quasi-static/dynamic three-point bending facilities at German Aerospace Centre (DLR) – Institute of Vehicle Concepts.For structures with axial crash loads the normal buckling mode will lead to a very early fracture of the magnesium part. In collaboration with researchers from the University of Windsor and the University of Waterloo, novel technologies for energy absorption which are based on cutting or peeling mechanisms have been developed and investigated, which allow the use of magnesium in these challenging applications. Results of the joint research will be presented.

New Multi-Material Design Concepts and High Integration Light Metal Applications for Lightweight Body Structures

A major motivation for the development of new vehicle structures is, apart from the reduction of fuel consumption, is to decrease emissions which affect the climate. Therefore we also have to look at the reduction of vehicle weight and consequently at various strategies for lightweight construction. In the future steel structure concepts still show lightweight potential. But even more attractive potential for lightweight body in white structures could be realised by new multi-material design concepts and highly integrated light metal applications. Today’s research activities are focussed on the area of multi-material design, with the objective of placing the material with the best properties for the given requirements in the right position. Based on various methods of lightweight construction, techniques and tools, it is possible to find an optimum between lightweight design and costs. These activities will be illustrated by several research examples. One example will be the lightweight concept of the front module developed by the Institute of Vehicle Concepts (DLR) in the European research project -‘Super Light Car’ (SLC). By using aluminium in the front structure and the high pressure die casting strut tower the concept has a weight benefit of 32% compared to a steel reference structure. The methodology for reaching targets and requirements like weight reduction, crash performance and cost targets will be explained. Another example is a concept which is developed in the DLR project ‘Novel Vehicle Structures’. This concept shows the combination of different materials and a new construction method to increase front impact crash performance.

Solutions for Next Generation Automotive Lightweight Concepts Based on Material Selection and Functional Integration

Various aspects of global trends like energy saving or safety, economic and production targets or requirements due to the automation give high and challenging targets for the development of future vehicles. With respect to these requirements weight saving is also a must during an early phase of the concept and structural development process. Within the Next Generation Car project the DLR is looking for solutions for future integrated functions like health monitoring and the combination of passive and active safety functions. New material systems and novel combinations of design methods are also developed. The integrated approach of combining development and optimization methods with tools for the design is the key to the development of concepts for holistic lightweight solutions and new vehicle concepts utilizing highly automated and autonomous driving abilities. Challenges and potentials for next generation lightweight design are shown and solutions and results from the DLR project Next Generation Car are presented.