Materials with variable stiffness

Paper in proceeding, 2015

In this study different concepts to attain a material that can reduce its stiffness upon external stimulation are evaluated. All concepts rely on resistive heating of a carbon fibre reinforcement upon application of electric current through the fibres. The stiffness reduction relies on a phase transformation due to heating of the material. During the European project ENLIGHT, four different stiffness-modifiable materials were investigated. The materials differ in their thermal, mechanical and processing properties as well as the achievable stiffness reduction. The materials have many potential applications, but are of special interest for road safety. Within ENLIGHT, the material was considered for application in a softening car hood. Requirements for such an application were defined with regards to the mechanical and thermal performance, as well as the activation time. It is crucial for active safety systems in cars to be activated within a short time. According to investigations performed at Honda, the time interval between impact at the front of the car and head impact on the hood is 60 ms for children (more for adults). Therefore, upon activation at front impact, the hood stiffness needs to be reduced within 60 ms in order to soften the impact with the hood. Longer activation time can be allowed if the stiffness-reduction is activated by a collision warning system. Such a solution would make sense in situations where the time between alert and collision is not sufficient to stop the vehicle (stopping time is e.g. ≥1 s at 40 km/h and ≥2.5 s at 70 km/h). Calculations performed at Swerea SICOMP revealed that activation times in the range of 500 ms – 1 s can be achieved with the power sources already available in electric vehicles. However, an activation time of 60 ms cannot be achieved for the amount of material required for efficient stiffness-reduction in a hood. Hence, the technology needs to be combined with a collision warning system. The ability of the material to reduce its stiffness was verified by dynamic mechanical analysis (DMTA), in-plane shear tests (IPS) at different temperatures and 3-point-bending tests. Verification of the possibility to achieve stiffness reduction upon application of current within the required time was performed using a 3-point-bending set-up where specimens were heated with short voltage pulses while monitoring stiffness. To date stiffness-reductions between 50% and 90% have been demonstrated for the different materials.