Marco Peroni

Effect of the Strain Rate on the Mechanical Behaviour of Epoxy Adhesives

In this paper the dynamic mechanical behaviour of two epoxy adhesives for structural bonding is investigated. The study has been focused on the influence of the strain rate on the tensile and compressive strength of specimens made of adhesive. The experimental tests have been performed with an hydraulic universal testing machine and a tension-compression Hopkinson bar. The results of the tests show that the σ−ε curve rises significantly increasing the strain rate. The Cowper-Symonds and Johnson-Cook models of strain rate dependence have been used to fit the experimental data with unsatisfactory results, thus a poly-linear fit has been adopted.

Numerical Analysis of Hybrid Joining in Car Body Applications

Nowadays, there are two fundamental issues in car body design. On the one hand, there is the need for weight reduction in order to reduce fuel consumption and, consequently, pollutant emissions, and on the other hand, there are ever more stringent safety requirements. To meet these targets, the trend is towards using hybrid structures made of unconventional materials, like aluminium, polymeric and composite materials. The use of these materials brings about some problems, one of them is associated with the joining techniques because the traditional resistance spot-welding, used in the assembly of a common steel chassis, cannot be used. Among the different alternative solutions, the most promising is the use of structural adhesives. From this perspective, this work aims to model the behaviour of simplified crash box elements made of different types of materials joined together by structural adhesives. In particular, the attention was focused on the adhesive joint modelling with a cohesive element formulation. Starting from experimental results for the characterization of the adhesive, the cohesive parameters were identified. The results were then applied to model the crushing of simplified crash boxes. The crushing axial compression of these components was investigated considering geometrical and loading conditions. The models were developed and verified by comparing the numerical with experimental results on these components. A good correlation was found in all loading conditions and with different geometries and substrate combinations.

The mechanical behaviour of polyurethane foam: multiaxial and dynamic behaviour

In the work, polyurethane rigid foam for energy absorption in automotive applications has been considered. Polyurethane foams are very interesting in these applications because of the possibility to foam them in place filling the required volume space. The material behaviour has been studied by subjecting the foam to different stress state conditions, including uniaxial compression and tension, hydrostatic, and pure deviatoric. To achieve this objective, a series of experimental tests and of test specimens have been designed. The experimental devices have been constructed and the test results have been used to characterise the material. The results obtained from the different tests, allowed defining failure criteria for the foam, in terms of yield and post-yield behaviour. The influence of density (three different densities have been considered in the analysis), repeated loading, and strain-rate effects are also taken into account. For an effective interpretation of the results, the experimental data of all the tests have been interpolated by means of a particular material model: such model permits to describe in a simple way the foam behaviour in different stress conditions, making simpler, at the same time, the description of the global multiaxial behaviour.

Identification of strain-rate sensitivity parameters of steels with an inverse method

Since the second half of the past century, research in the automotive industry focused on vehicle performance, environmental compatibility, and safety improvement. Increasing interest about passive safety and the ever stricter regulations, both in the US and EU, pushed towards more reliable models of the vehicle behaviour and, to achieve this, to a deeper insight into material behaviour. For what concerns crash simulations, one of the most critical uncertainty relates to the influence of the loading speed on the material mechanical properties. This influence can be accounted by means of the strain-rate sensitivity that is substantial for most materials. Strain-rate sensitivity of low-carbon steels, such as the deep-drawing steels of most automotive body, is very well known.