Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

A sustainable future requires a responsible handling of our material and energy resources. However, our modern products are becoming increasingly complex with respect to the material combinations and their linkages. While we engineer multi-material structures against failure for the use phase, we also need them to be dismantled in the end-of-life phase during recycling. This describes a main functional contradiction of the structure under the scope of a circular economy and sustainability.
To achieve good material-specific recovery rates, materials locked in joints have to be liberated, which is typically achieved by breaking materials and joints in mechanical shredding processes. Unfortunately, no adequate models exist currently to describe these processes, which constitutes a missing link for a recycling-oriented design.
The presented approach models the shredding of multi-material structures with adhesion joint through numerical simulations using the finite element method (FEM). For shredding, a rotary shear is employed as usual first process stage in recycling. A rotary shear consists of two counter-rotating shafts with discs and V-shaped teeth turning at a fixed speed (circumferential velocity at teeth <0.5 m/s) and exerting tensile stresses in conjunction with bending and torsion (tearing stresses) on specimens. An A-frame dummy specimen for lightweight automotive applications was used consisting of a sheet steel top-hat profile with a glass fibre-reinforced polyamide composite layer and polyamide rib structure glued to it.
LS-DYNA software was used for explicit FE analysis as well as material models that consider the plasticity and failure of different materials and their interfaces. Furthermore, simulations were performed for different load cases, representing different orientations of the test specimen relative to the rotary shear as observed in experiments. A model evaluation workflow was developed in Python and R to quantify the shredding performance in terms of the metrics liberation degree, particle sizes and energy consumption.
Simulation results show high qualitative and quantitative agreement regarding deformation, fracture and liberation phenomena observed in previous experiments, e.g., brittle breakage of polymers into many fragments, partial to full detachment of adhesion joint, as well as high degree of plastic deformation of steel that sometimes even clamped-in polymer material thus forming new form-locking joints. One highlight is the realistic estimation of the mechanical energy consumption required for shredding. However, mass losses occur due to element deletion at failure, which are observed with increasing element size of the mesh. In addition, the model underestimates the number of generated fragments especially in the small size range (< 5 mm). Better results are expected by incorporating strain-rate dependent material behaviour in the future.
The developed simulation process could be integrated into a new design assistance tool for the conceptual design phase of multi-material structures with two main outcomes. First, to provide quantitative metrics linking the design and the failure behaviour during shredding of such structures, and consequently, to estimate the impact of design decisions on the recycling phase of a product enabling a recycling-oriented design.