For technical use as dampers or protectors, materials are needed that deform with a delay when a load is applied and gradually dissipate the deformation energy. We are particularly interested in how this time dependence manifests itself in the case of very short, shock-like loads.
Soft elastomers can dissipate the shock waves generated at high strain rates and thus damp the pressure pulse. We develop material models that capture not only viscoelasticity but also local microstructure. For example, we investigate open-cell polymer foams and optimize their damping properties.
Phase field methods for multi-field problems
Solid metallic mixtures are inhomogeneous and form phases of different compositions, which can change due to diffusion, stresses and electric fields. Examples include solder alloys in microelectronic devices and lithium battery anodes during charging and discharging. Here, numerical simulations help to predict the mechanical properties and avoid premature failure.
We calculate such processes by phase field methods, where the Cahn-Hilliard equations of diffusion are coupled with the equations for the mechanical, electric and/or temperature field problem. Due to their structure and nonlinear coefficients, the resulting 4th order differential equations place high demands on the numerical solution procedure. With the help of isogeometric NURBS approaches for the FE basis functions, promising results result.
Calculation of fracture and fragmentation
The numerical calculation of crack initiation and propagation is currently the subject of much research. For this purpose, we use the phase field method and peridynamics in addition to the cohesive element technique, which can now almost be described as classical.
Phase field models regularize the discontinuous problem of crack propagation and thus allow efficient and numerically stable simulations. Our current work deals, for example, with the formulation of polyconvex material models for the crack-inducing distortion energy function, with more accurate computation through adapted local refinement, and with continuum mechanics formulations in peridynamics.
Dynamic loading and split Hopkinson pressure bar (SHPB) experiments.
In addition to two servo-hydraulic traction machines, we have two self-designed bar impact devices in the laboratory of the Chair of Solid Mechanics. Here, experiments on dynamic material testing, in particular on dynamic fracture, are carried out. In the meantime, we have optimized the test setup in such a way that material data can be determined even for relatively soft specimens. Our numerical algorithms for the calculation of dynamic fracture are used in the sense of an inverse analysis. Currently, for example, we are investigating open-cell lattice structures under shock wave loading.