Advanced High Strength Steels (AHSS) are under constant development to meet increasing demands regarding lightweight and improved properties such as strength and ductility. In the automotive industry, AHSS are nowadays used for nearly every new vehicle design. Around half of the body-in-white consists of AHSS making the vehicles lighter and enhancing safety (World Steel Association). Large research efforts have, therefore, been devoted in the last decade to further develop AHSS for improved properties.
Crashbox testing using stereo high-speed imaging and Digital Image Correlation (DIC)
In order to evaluate the crashworthiness and energy absorption of safety parts, axial compression- and bending tests are often considered. In the frame of FormPlanet project, a service – Crashworthiness –offering axial testing of crashboxes has been developed. In order to achieve typical speeds present in real crash situations, the crashboxes are compressed in a high-speed machine, Instron VHS160/100-20, with a maximum speed of 20 m/s.
To complement the typical outcome in terms of force vs intrusion, two high-speed cameras are added to capture the deformation process. By applying a speckle pattern on the crashbox, the evolution of the deformation field is measured by using the stereo high-speed imaging together with Digital Image Correlation (DIC). The level of plastic straining can thereby be followed as the crashbox is compressed. Furthermore, potential cracks can be detected as they initiate and their propagation can be tracked. In Figure 1, an example of a plastic strain field is shown.
Figure 1: Example of a plastic strain field
Prediction and verification of crashworthiness using Finite Element Modelling
By using the service Smart material model calibration, relevant material data to simulate the crash tests are provided. In Figure 2, the computed plastic strain is shown at the same level of intrusion as the experimentally obtained field presented in Figure 1. The simulation shows a similar fold formation and level of plastic strain as the corresponding measurements. Hence, by utilising the services Crashworthiness and Smart material model calibration, crashworthiness, energy absorption and potential crack formations can be predicted and experimentally verified.
Figure 2: Computed equivalent plastic strain fields at an intrusion of approx. 90 mm.
Simon Jonsson
MSc in Engineering Physics and Electrical Engineering. He is a PhD student in Solid Mechanics at Luleå University of Technology. He investigates the contribution from the intrinsic material property – fracture toughness – to the crash resistance. Furthermore, he develops fracture mechanical based (energy based) criteria to predict crack growth and energy absorption in safety parts.
