Fatigue testing of hybrid joined component-like bowl specimens in multi-material construction.

Fatigue strength, hybrid joint connection, self-piercing riveting, adhesive, lightweight, multi-material construction

Component-like bowl specimens CAD- und CAE-model. (© Fraunhofer LBF)

The fatigue strength of joined multi-material thin sheet metal structures is becoming increasingly important in the context of lightweight design. The use of component-like bowl specimens provides practical insights into the fatigue strength properties of structural components, which cannot be determined in this way using the usual simply overlapping standard specimens. The fatigue strength tests serve as a basis for the validation of existing numerical methods for fatigue strength estimation and allow insights into real damage mechanisms.

Experimental tests with steel and aluminum specimen

Within the framework of the EU-funded research project "ALLIANCE" on the topic of lightweight design and CO2 reduction, innovative numerical fatigue strength estimations for multi-material joining techniques were developed at the car manufacturer Opel, based on fatigue tests of shear and peel specimens. Adhesively bonded, riveted and hybrid joined variants were tested. In order to enable a validation of the numerical fatigue strength estimations, experimental tests were carried out at the Fraunhofer Institute for Structural Durability and System Reliability LBF on component-like bowl specimens with the material pairing Steel-Aluminium. The knowledge gained from these tests helps to clarify the question whether the fatigue properties of simple shear and peel specimens can be transferred to complex components.

For the bowl specimen, a deep-drawn bowl is connected to a firmly clamped flat base plate in the area of the formed flange of the bowl.

Both forces and moments can be introduced into the bowl in any direction in the test set-up and thus a defined stress state (combination of shear and peel stress) can be set. Thus, this specimen form allows the fatigue strength properties of structural components to be investigated as close to practical conditions as possible and yet still on a laboratory scale.

To determine the lightweight potential of structural components in multi-material construction, fatigue tests were carried out on adhesively bonded, riveted and hybrid joined bowl specimens. In comparison of the test results, the adhesively bonded bowl specimens show significantly higher fatigue load capacities compared to the riveted bowl specimens. A similar behaviour can also be identified in the test results of the shear specimens. The hybrid joined bowl specimens, however, show lower fatigue load capacities than the adhesively bonded bowl specimens, in contrast to the shear specimens. One of the possible reasons for this behaviour is that due to the hybrid manufacturing process which has not yet been optimized, in an improper bonding of both joining partners results. This assumption can also be supported in the comparison between the different batches of the hybrid joined bowl specimens. In the case of the second batch, the areas between the set rivets were additionally fixed with steel clamps, which minimized the gap formation between the base plate and the bowl, caused by the joining process of the rivets. This significantly improved the joining quality and thus the fatigue strength.

During the fatigue tests, two different approaches were additionally used to determine the crack propagation behaviour of the bowl specimens. On the one hand, the dynamic stiffness and on the other hand the stress distribution at the surface of the bowl specimens were recorded visually by means of thermo-elastic stress analysis. The resulting findings on the correlation between the crack propagation behaviour and the stiffness curves are used to define relevant failure criteria for structural components. These form the input for a mechanism-based fatigue strength evaluation.

Figure 1: Test results of shear and peel specimens.
(© Fraunhofer LBF)

Figure 2: Test results of component-like bowl specimens.
(© Fraunhofer LBF)

Figure 3: Exemplary representation of a stiffness curve in correlation with thermoelastic stress analysis. (© Fraunhofer LBF)

Significant potential for the automotive industry established

The test results offer first insights into the complex world of fatigue strength of hybrid joined multi-material structures and give an idea of the potential of this joining method, especially for future lightweight construction concepts in the automotive industry. However, for an industrial application it is absolutely necessary to develop optimized manufacturing processes in order to exploit the full potential.

Sponsors and partners

Six leading European automobile manufacturers (Daimler, Volkswagen, Fiat-Chrysler Forschungszentrum, Volvo, Opel und Toyota) have joined forces with four suppliers (Thyssenkrupp, Novelis, Batz, Benteler) and eight scientific partners (Swerea, Inspire, Fraunhofer LBF, RWTH-IKA, KIT-IPEK, Universität Florenz, Bax & Company, Ricardo) to form the ALLIANCE consortium. The ALLIANCE initiative is supported by EUCAR and EARPA.

Supported by:
European Union Horizon 2020

“One of the goals of the Alliance project was to develop an application-oriented simulation method for the reliable structural durability evaluation of bonded and self-piercing riveted sheet metal joints. The test results from the bowl specimen developed and tested at Fraunhofer LBF were extremely helpful for validating the method in combined load states.” Dr. Boris Künkler, Group Leader of Body/Exterior/Hang-on-Parts CAE, Opel Automobile GmbH, Rüsselsheim