Manfred Sindel

Failure Prediction in Sheet Metal Forming Depending in Pre-Straining and Bending Superposition

The automotive industry nowadays, uses numerical simulation systems to determine process safety of car body parts. Forming simulations are usually used to predict local necking and cracks during the deep-drawing operation or to calculate the spring-back behaviour. Furthermore, FEA is also used for optimizing the hemming process. In this contribution, further development and the use of an enhanced failure criterion for the evaluation of flanging and hemming processes are shown. This criterion describes material failure caused by incipient surface cracks on the bending edge keeping the predominant bending load conditions in consideration. The investigations of the bending conditions in this criterion include loads from previous forming operations and geometrical aspects, such as bending radii. The approach presented in this contribution can deliver a more reliable prediction regarding the expected material failure.

Investigations on Bendability of Lightweight Materials for Various Load Paths

This contribution discusses material failure caused by incipient surface cracks on bending edges during predominant bending processes such as those that occur, e.g. when flanging and hemming outer car body parts. In this regard, the determination of bendability for modern lightweight materials is particularly important. Frequently, the deep drawing operation is already superimposed by bending loads; nevertheless the majority of bending stress is applied in the following bending and hemming operation. Forming analysis of deep drawing operations on real car body parts have shown that in hemming regions a variety of pre‐loads occur. These pre‐loads vary in load condition and its intensity. Based on these results this paper describes the experimental investigations on the bending properties for these pre‐load conditions, especially the bending limit after various pre‐load operations. The influence of different pre‐load operations has been investigated by using the established Three‐Point‐Bending‐Test [1]. Ben...

Erweiterte Werkstoffprüfverfahren zur Charakterisierung von Leichtbaublechwerkstoffen im Hinblick auf die Kantenrisssensitivität

Kurzfassung Im vorliegenden Beitrag wird über zwei neu entwickelte Prüfverfahren berichtet, mit dem die Kantenrissempfindlichkeit von hochfesten Stahlblechwerkstoffen quantifiziert werden kann. Basierend auf konventionellen Biegetests wurde das neue Prüfverfahren so modifiziert, dass eine maximal mögliche Kantenbelastung aufgebracht wird. Im Beitrag wird auf die standardisierten und genormten Werkstoffcharakterisierungmethoden und -prüfverfahren eingegangen und es werden Prüfverfahren vorgestellt, die eine Aussage über die Biegbarkeit von hochfesten Stahlblechwerkstoffen über deren Kantenrissempfindlichkeit vor allem liefern können.

Neue Duktilitätskriterien für die Qualitätsbewertung von Leichtbauwerkstoffen

Kurzfassung Dieser Beitrag zeigt auf der Basis bestehender Ansätze Möglichkeiten neuer Duktilitätsdefinitionen auf. Die dabei neu erstellten Ansätze bieten im besonderen Maße für umformkritische Leichtbaublechwerkstoffe die Möglichkeit zur differenzierten Werkstoffcharakterisierung. Die Beschreibung der Duktilität eines Werkstoffes dient nicht nur der Versagensvorhersage in der Bauteilsimulation, sondern zeigt sowohl dem Karosserieentwickler weitere Optimierungspotenziale als objektives Bewertungskriterium, als auch der Qualitätssicherung zusätzliche Prüfkriterien auf.

Investigation on bending failure to characterize crashworthiness of 6xxx-series aluminium sheet alloys with bending-tension test procedure

As lightweight design as well as crash performance are crucial to future car body design, exact material characterisation is important to use materials at their full potential and reach maximum efficiency. Within the scope of this paper, the potential of newly established bending-tension test procedure to characterise material crashworthiness is investigated. In this test setup for the determination of material failure, a buckling-bending test is coupled with a subsequent tensile test. If prior bending load is critical, tensile strength and elongation in the subsequent tensile test are dramatically reduced. The new test procedure therefore offers an applicable definition of failure as the incapacity of energy consumption in subsequent phases of the crash represents failure of a component. In addition to that, the correlation of loading condition with actual crash scenarios (buckling and free bending) is improved compared to three- point bending test. The potential of newly established bending-tension test procedure to characterise material crashworthiness is investigated in this experimental studys on two aluminium sheet alloys. Experimental results are validated with existing ductility characterisation from edge compression test.As lightweight design as well as crash performance are crucial to future car body design, exact material characterisation is important to use materials at their full potential and reach maximum efficiency. Within the scope of this paper, the potential of newly established bending-tension test procedure to characterise material crashworthiness is investigated. In this test setup for the determination of material failure, a buckling-bending test is coupled with a subsequent tensile test. If prior bending load is critical, tensile strength and elongation in the subsequent tensile test are dramatically reduced. The new test procedure therefore offers an applicable definition of failure as the incapacity of energy consumption in subsequent phases of the crash represents failure of a component. In addition to that, the correlation of loading condition with actual crash scenarios (buckling and free bending) is improved compared to three- point bending test. The potential of newly established bending-tension test...

Investigation on local ductility of 6xxx-aluminium sheet alloys

Within the scope of this paper influence of localization of loading conditions on the ductility of two different 6xxx-aluminium sheet alloys is investigated. In order to improve the prediction of sheet material crash performance, material parameters based on uniaxial tensile and notched tensile tests are determined with varying consolidation areas. Especially evaluation methods based on the localized necking behaviour in tensile tests are investigated. The potential of local ductility characterisation is validated with results of Edge-Compression Tests (ECT) which applies load conditions that occur in actual crash events.

Quality assurance of adhesive processes in the body shop

The need for a suitable joining technology for multi-material combinations in car bodies of the future has been constantly growing. Here, adhesive technologies offer a number of advantages as well as a great development potential. In order to ensure the performance of the entire vehicle, especially in relation to crash safety, rigidity and corrosion, quality assurance methods are required that ensure the efficiency of adhesive bonds, both in applicability and costs. Present-day conventional destructive test methods must be supplemented by non-destructive methods. A preventive quality management which is already used in the early stages of development by means of simulation methods can help reduce inspection costs and expensive reworking in the prototype phase. For this purpose it is necessary to analyse the processes employed in the manufacturing process of the car body in order to improve simulation with the aid of appropriate laboratory experiments. Initially, basic reaction kinetics tests are presented to demonstrate the heating rate dependency of glass transition temperature and the reaction process of a 1K epoxy adhesive. Finally, the requirements for an advanced testing methodology are defined in order to ensure and improve the quality of adhesive bonding in vehicles.

Experimental study of unstretched fiber shifting during hemming processes for automotive aluminum alloys

A typical processes chain for today’s hang-on parts of passenger car bodies such as hoods, bonnets or doors consists of a deep-drawing and cutting process, followed by flanging and hemming. The trend towards smaller bending radii of hemmed exterior parts in automotive industry and increasing use of lightweight materials, such as aluminum for vehicle hang-on parts, requires a further understanding of bending and hemming processes. In bending theory, the unstretched fiber describes a circular arc in a section of a bent part, which uncoiled length corresponds to length in unbent sheet. The unstretched fiber moves from middle of the section towards inner bending radius during bending operation. In radially outward direction, tensile stresses increase due to the increasing strain. The small bending radii, typical in the automotive industry, lead to relatively large tensile stresses in outer fiber. On the one hand, unstretched fiber shifting mainly affects the radius of the bent or hemmed part; on the other hand, position of the unstretched fiber during bending has a great influence on material loads. In this experimental study, shifting of the unstretched fiber during hemming operations is determined and quantified. Therefore, a new method for determination of present position of different fibers or layers in a bent section is introduced.

Development of a New Gradient Based Strain-Criterion for Prediction of Bendability in Quality Assurance and FEA

Numerical simulation systems are more and more used in process development of car bodies. Nowadays, also the hemming process is optimised in FEA. Thus, the analysing of process robustness calls for a failure criterion for the specific bending and hemming load condition. For that purpose the experimental determination of bendability under various pre‐load conditions that occur in real production, e.g. during deep drawing in press shop, is content of this contribution. Using these experimental results, a new approach for a strain‐gradient based failure criterion for bending operations is presented to optimise bendability prediction. The bending‐strain‐gradient approach can be used both in production related departments of quality assurance as well as for simulative process design or process validation for vehicle manufacturing planning.

Application Of A New Semi‐Empirical Model For Forming Limit Prediction Of Sheet Material Including Superposed Loads Of Bending And Shearing

The use of lightweight materials offers substantial strength and weight advantages in car body design. Unfortunately such kinds of sheet material are more susceptible to wrinkling, spring back and fracture during press shop operations. For characterization of capability of sheet material dedicated to deep drawing processes in the automotive industry, mainly Forming Limit Diagrams (FLD) are used. However, new investigations at the Institute for Metal Forming Technology have shown that High Strength Steel Sheet Material and Aluminum Alloys show increased formability in case of bending loads are superposed to stretching loads. Likewise, by superposing shearing on in plane uniaxial or biaxial tension formability changes because of materials crystallographic texture. Such mixed stress and strain conditions including bending and shearing effects can occur in deep-drawing processes of complex car body parts as well as subsequent forming operations like flanging. But changes in formability cannot be described by using the conventional FLC. Hence, for purpose of improvement of failure prediction in numerical simulation codes significant failure criteria for these strain conditions are missing. Considering such aspects in defining suitable failure criteria which is easy to implement into FEA a new semi-empirical model has been developed considering the effect of bending and shearing in sheet metals formability. This failure criterion consists of the combination of the so called cFLC (combined Forming Limit Curve), which considers superposed bending load conditions and the SFLC (Shear Forming Limit Curve), which again includes the effect of shearing on sheet metal’s formability.