Potentials of new ductility criterions in car development with lightweight materials

Not only in today’s car development but also in every safety relevant construction, the material forming behaviour because of working load is highly considered. Depending on the respective application on the one side a dimensional accuracy or on the other side a well-tempered compensation of stress peaks is prefered. To manifest this daily necessity in a quantitative parameter new more valid ductility criterions have to be developed.

At the state of the art the ductility is often used to characterize the forming behaviour until fracture of sheet, cast and extrusion profile metal in a mainly qualitative way. Nevertheless the ductility affects attractive in different points of view:

1) To evaluate the deformation until fracture for primary and secondary forming like rolling, extrusion, wiredrawing and sheet metal forming. Though there is no quantitative correlation between ductility and formability or workability in these processes, more ductile materials often guarantee a better workability.

2) To indicate the ability of the metal to flow plastically before fracture. Although ductility measurements are not used quantitatively in design, a high tensile ductility indicates the capacity to compensate local strain peaks more homogeneous to neighbouring areas.

3) To evaluate sensitively material properties according to the application as a quality criterion.

For the structural designer of aluminium-weight tension structures, ductility is of great importance because of its role in a relief of stress concentrations. To characterize material properties concerning the ductility for forming simulation based on the law of plasticity and crash test basically the yield strength, tensile strength and the ultimate strain are used. In this case it’s only possible to get an impression of a ductile or brittle material behaviour. But in many cases this impression deceives.

Because of this fact a lot of real part testing is indispensable. Today every structural part has to be checked on the forming behaviour correlating the material properties to the part geometry and kind of workload. After testing the separation of the respective share of these parameters to the fracture is practically not feasible. As a result of this, crash tests of different geometries are not comparable related to their ductility. But for structural car body optimization in the stage of vehicle development but also for the running quality assurance in production a proper material characterisation is just as well inevitable as the objective quantification of ductility parameters. Both the objective quantification of ductility and the description in scalar values are the basic requirements for the crash simulation based on the laws of plasticity inclusive the material failure prediction. The researched failure limits are described on the one hand by forced rupture in achievement of the ductility limit which is due to the increase and unification of micro-cracks and -pores and on the other hand by failure because of necking which is due to an insufficient strain hardening an the consequential membrane instability of thin-walled structures. Similar to the formability, the dependence of ductility to the dominant state of stress, temperature, strain rate and the geometry of the specimen has to be considered accurately. The resultating shape of crashed specimen can drastically differ.

General approach in ductility measurement

The simplest approach in ductility measurement uses the ultimate tensile strain from the uniaxial tensile test as the main criterion of ductility measurement. A larger ultimate tensile strain often corresponds with a more ductile material property [ALU06]. In spite of the easiness of this approach there are two disadvantages to distinguish. On the one hand neither the strain hardening nor the kind of necking is considered and on the other hand there can exist clearly different material properties which would be seen subjective with different ductilities but are misinterpreted with the same properties.

Approach In Ductility Measurement According To DIETER/LANGE
The approach according to DIETER/LANGE adopts that materials with large ductility not only are characterized by a large ultimate tensile strain but also posses the ability to compensate local stress peaks to a wider area and because of this fact sustain to stronger necking. In praxis this approach can act as an indicator for slight variation of material properties of similar alloys very well. At alloys with a highly different strain hardening behaviour, many examples with different subjective interpretations but the same ductility values according to this approach are detectable. Because of this the approach according to DIETER/LANGE is not for all relevant cases well-defined.

The limits of previous ductility measurement using the ultimate tensile strain show the necessity of further approaches and definitions.

New Approach In Ductility Measurement

The main aim in developing new ductility criterions consists in improving the prediction of ductile material behaviour and estimating a non time and money expansive measuring methodology. Referring to former researches and made approaches results the consideration of a ductility criterion with regard to the uniform elongation, strain hardening effects, form of necking and the ability to compensate local stress peaks by using objective indicators. With additional consideration of strain hardening and with this the closely linked increase of tensile force, the logarithmic ratio of yield strains until ultimate tensile strain is used. In this place this new approach offers the possibility of inserting the most adequate and material specific flow criterion. In all mentioned examples here the flow criterion according to LUDWIK is inserted. Because of the usage of the necking width and the anisotropy of the material, this proceeding is nearly alike the description of a formability.

Conclusion

The new developed non dimensional ductility value offers the opportunity for objective material characterisation and evaluation. In order to this numerous sheet, cast and extrusion profile metals in different alloys and heat treatments have been researched to their ductility and the correlation between form of failure and the material ductility. Evaluating the ductility by a subjective point of view, the T4 state would be regarded as the more ductile. This is very well reflected by the ductility rating according to this new methodology.

In this way it’s also possible to compare ductility of sheet, cast and extrusion profile metal alloys which were hardly comparable with former approaches in past. Sheet metal alloys are subject to little variations of ductility and are on a higher level as the researched aluminium cast alloys at the same time. It’s conspicuous that the largest variation of ductility appears at extrusion profiles which can be due to the different heat treatments and the larger material thickness.

A further improvement of these considerations can be attained with using the material properties at the most critical state of stress, the plane-strain, instead of the measured properties from the uniaxial tensile test. Because in a dominant state of plane-strain stress materials suffer the smallest strains until failure. Correlating to this the ductility is also at a minimum value. For common aluminium sheet alloys both the forming limit curves and yield loci for the description of formability is extensively available. In this case an analytical calculation of the most critical ductility value in the plane-strain-stress-state is imaginable. Furthermore the creation of a „ductility-map“, so called Ductility Limit Curve (DLC), for all relevant states of stress is desirable and also realisable.

The main advantage in using valid ductility criterions consists on the one hand in a more precise description of material forming behaviour for simulation and on the other hand in the now feasible reduction of real part testing both in car development and later for quality assurance. For guaranteeing failure safety only in the early state of material preselection real part testing has to be made. In dependency of the respective material behaviour, for each geometry and state of stress a minimum tolerable ductility value can be easily defined. A correlation between simulation and the minimum necessary ductility is also imaginable but an experimental evaluation promises more reliable results. In reverse the ductility value can also serve as a design guideline in the early state of part development on the one hand for a specific material preselection and on the other hand for specific preselection of crash profile geometries. After specification of part and load depended minimum necessary ductility value the later measurement of ductility can be done by laboratory tests. The respective test application can also be chosen in comparison to the state of stress of the real work load situation. A test application which is very easy to accomplish is the uniaxial tensile test with a clear stress state. By this laboratory tests the material depended ductility values can be measured for a long spell, documented and be used for analysing quality trends.

This new approach is actually in further validations at AUDI Neckarsulm.

Previous retrospective investigations and testings with existing material data have been done and showed significant possibilities of estimating the ductility properties of sheet, extrusion profile and cast aluminium alloys. Furthermore on the one hand more data has been accumulated by comparing ductility properties with the results of the corresponding crash tests. And on the other hand this approach is used accompanying to the series production in quality assurance and in development of semi-finished products for the new car models.

These ductility approaches offer the possibility of a cost- and time-effective quantitative correlation between workability and ductility for sheet, cast and extrusion profile applications. Thus for the crash-simulation in car development now exists an objective evaluating criterion which has to be seen in addition to the laws of plasticity and with best possible objectivity to forecast a failure because of cracking.

References
[DIE67] Dieter, George E. Introduction to Ductility, Paper präsentiert
beim Seminar der American Society for Metals, Ohio, Okt. 1967
[ALU06] N.N. Alumatter Aluminium Wissensdatenbank;
http://www.alumatter.info; 2006
[LEP03] Leppin, C. Duktilität und Umformbarkeit von
Werkstoffen – Werkstoffcharakterisierung, aber wie?, Forschungsbericht Alcan Technology, 2003
Authors:
Dipl.-Ing. Ralf Schleich
Ph.D. student Hochschulinstitute Neckarsulm (HIN), an academic cooperation between the AUDI AG and the Institute for metal forming technology, University of Stuttgart, exploratory focus on materials and test methods aluminium technology
Dr.-Ing. Manfred Sindel
Head of quality assurance aluminium technology, Aluminium- und Leichtbau-Zentrum AUDI AG Neckarsulm
Prof. Dr.-Ing. Mathias Liewald
Director Institute for metal forming technology, University of Stuttgart
Further Information:
Contact: Ralf Schleich
Mail: Ralf.Schleich@audi.de
Web: www.hin.nsu.de