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The expanded polystyrene foam is widely used as a protective material in engineering applications where energy absorption is critical for the reduction of harmful dynamic loads. However, to design reliable protective components, it is necessary to predict its nonlinear stress response with a good approximation, which makes it possible to know from the engineering design analysis the amount of energy that a product may absorb. In this work, the hyperfoam constitutive material model was used in a finite element model to approximate the mechanical response of an expanded polystyrene foam of three different densities. Additionally, an experimental procedure was performed to obtain the response of the material at three loading rates. The experimental results show that higher densities at high loading rates allow better energy absorption in the expanded polystyrene. As for the energy dissipation, high dissipation is obtained at higher densities at low loading rates. In the numerical results, the proposed finite element model presented a good performance since root mean square error values below 9% were obtained around the experimental compressive stress/strain curves for all tested material densities. Also, the prediction of energy absorption with the proposed model was around a maximum error of 5% regarding the experimental results. Therefore, the prediction of energy absorption and the compressive stress response of expanded polystyrene foams can be studied using the proposed finite element model in combination with the hyperfoam material model.

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Ball burnishing is a surface improvement process that provides compressive residual stresses and increment the hardness in the surface layer of workpieces. Actually, in the state of the art some papers use FEM to simulate the burnishing process in the most cases without consideration the roughness of the workpiece, in other cases the roughness is considered only as two-dimensional semicircular or triangular periodic pattern. In other studies to simulate three-dimensional roughness, the two dimensional case is extruded. In this paper, a new way to simulate three-dimensional random roughness was used to simulate the ball burnishing process obtaining the final roughness and residual stresses distribution in the surface and below it. For the simulation, a commercial explicit FEA Software was used considering a bilinear material model. The finite element model of ball burnishing process was validated with experimental test and the methodology of this simulation process is presented. Keywords: burnishing, surface roughness, FEM simulation Streszczenie: Nagniatanie jest procesem stosowanym w celu poprawy powierzchni, który zapewnia ściskające naprężenia szczątkowe i wzrost twardości w warstwie wierzchniej przedmiotów obrabianych. Według aktualnego stanu wiedzy w kilku przypadkach do symulacji procesu nagniatania używano MES, jednakże w większości przypadków nie uwzględniono chropowatości powierzchni przedmiotu obrabianego, a w innych chropowatość była traktowana jako dwuwymiarowy powtarzalny kształt półokrągły lub trójkątny. W innych badaniach, do symulacji chropowatości trójwymiarowej użyto modelu powstałego przez wyciągnięcie przypadku dwuwymiarowego. W niniejszej pracy użyto nowego sposobu