A new technology for a novel material inherent load sensor for sheet metal components of metastable austenitic steel was developed and investigated in the framework of the subproject S2 at the Institute of Forming Technology and Machines (IFUM) (Figure 1). The aim was to qualify the sheet metal material of components as a load sensor. The material should store information concerning mechanical load that was applied to a component in the past or is currently impacting the component. The advantage of such a technology is that mechanical load can be detected without external force sensors that need to be applied to a component.   

The so called „load-induced martensite transformation“, an effect that occurs in the material of metastable austenitic stainless steels, was the basis for the novel inherent load sensor. Depending on the temperature, the pressure as well as the state of stress and strain, the micro structure changes partially from retained austenite (paramagnetic properties) to α‘-martensite (ferromagnetic properties). Due to the change of the magnetic properties the phase portion of martensite can be measured by non-destructive eddy current testing and conclusions regarding the mechanical load that caused the phase transformations can be drawn. As the phase transformation can be reversed only with help of heat treatment, information concerning load that was applied to the material in the past is stored by means of the martensite phase portion.

In metastable austenitic sheet metals, as they are offered in the market, this effect is restricted to high stress and strains and is thus not suited to be used for the novel sensor. But by means of local forming such as embossing, the sensitivity of the material related to mechanical load can be increased. Due to the forming, the threshold for mechanical load to cause the phase transformation can be locally decreased making the effect useful for the purpose that is discussed here. The technology mentioned above was researched and developed within the framework of the subproject S2. A model to consider the martensite transformation in numerical simulations was developed. Furthermore, sensorfields that consist of load sensitive stampings that are surrounded by embossed stiffening ribs were developed. With the help of sensor fields, the level and the direction of mechanical load can be classified (Figure 2). The technology was validated by means of experiments. The subproject has ended with the second period of funding. The basic research related to load-induced martensite transformation will be partially continued within the subproject S3 in the third period of funding the CRC 653.