Lovre Krstulović-Opara

Mechanical Properties of Advanced Pore Morphology Foam Elements

Advanced pore morphology (APM) foam, consisting of sphere-like metallic foam elements, exhibits some particular mechanical properties with unique application possibilities. The article presents the results of experimental and computational testing of APM foam elements to determine their mechanical behavior under quasi-static and dynamic compressive loading conditions. Additionally, an infrared thermal imaging camera has been used during experimental testing. Evaluated mechanical properties give better insight into the behavior of single APM foam elements under different types of loading and provide a good base for further studies of the topology and morphology influence on the global behavior of composite structures, based on APM foam elements.

Behavior of composite advanced pore morphology foam

The mechanical characterization of advanced pore morphology (APM) foam, consisting of sphere-like metallic foam elements, is very limited since APM foam has been developed only recently. The purpose of this research was thus to determine the behavior of APM spheres and its composites when subjected to compressive loading. Single metallic APM spheres have been characterized with experimental testing and computational simulations, providing the basic properties and knowledge for an efficient composition of composite APM foam structures. Then, the APM foam elements were molded with epoxy matrix resulting in new composite structures. These composites have been adhered together with the epoxy resin achieving partial and syntactic morphology. The mechanical characterization of composite APM foam structures was based on experimental testing results with free and confined boundaries. The results of the performed research have shown valuable mechanical properties of the composite APM foam structures. Furthermore, they offer new possibilities for their use in general engineering applications.

THE MOVING FRICTION CONE APPROACH FOR THREE-DIMENSIONAL CONTACT SIMULATIONS

A finite element contact approach based on the Moving Friction Cone (MFC) formulation is presented herein. The formulation is based on the contact constraint described using a single gap vector. Such a simplification, in comparison with the standard approach where normal and tangential gap vectors are used, results in significantly simpler, shorter and faster element code. The associated penalty is formulated to include large deformations and displacements. Within this approach a triangular contact element is developed using a high abstract mathematical level of symbolic description. Using this technique, a consistent linearization is obtained which leads to quadratic rates of convergence. Furthermore, the new technique results in algorithmic robustness, fast evaluation time, as well as a compact element code.

Experimental and Numerical Evaluation of the Mechanical Behavior of Strongly Anisotropic Light-Weight Metallic Fiber Structures under Static and Dynamic Compressive Loading

Rigid metallic fiber structures made from a variety of different metals and alloys have been investigated mainly with regard to their functional properties such as heat transfer, pressure drop, or filtration characteristics. With the recent advent of aluminum and magnesium-based fiber structures, the application of such structures in light-weight crash absorbers has become conceivable. The present paper therefore elucidates the mechanical behavior of rigid sintered fiber structures under quasi-static and dynamic loading. Special attention is paid to the strongly anisotropic properties observed for different directions of loading in relation to the main fiber orientation. Basically, the structures show an orthotropic behavior; however, a finite thickness of the fiber slabs results in moderate deviations from a purely orthotropic behavior. The morphology of the tested specimens is examined by computed tomography, and experimental results for different directions of loading as well as different relative densities are presented. Numerical calculations were carried out using real structural data derived from the computed tomography data. Depending on the direction of loading, the fiber structures show a distinctively different deformation behavior both experimentally and numerically. Based on these results, the prevalent modes of deformation are discussed and a first comparison with an established polymer foam and an assessment of the applicability of aluminum fiber structures in crash protection devices is attempted.

Convergence Studies for 3D Smooth Frictional Contact Elements Based on the Quartic Bézier Surfaces

A 3D smooth triangular frictional node to surface contact element is developed using an abstract symbolical programming approach. Such an element is used in combination with tetrahedral continuum elements suitable for the automatic mesh generation. The C1-continuous smooth contact surface description is based on the six quartic Bezier surfaces. The approach is based on the elastic-plastic tangential slip vector decomposition, non-associated frictional law, penalty method and weak formulation. The presented convergence studies provide information about the benefit of using the smooth contact approach.

Application of gradient based IR thermography to the GRP structures inspection

The article presents application of non destructive testing method based on the pulse heating infrared thermography used to detect material anomalies for the case of glass reinforced polymer structures. The goal of presented research, based on the thermal gradient approach, is to establish the procedure capable of filtering out anomalies from other thermal influences caused by thermal reflections of surrounding objects, geometry influences and heat flows for observed object.

Multiscale Modeling of Metal Foams Using the XFEM

Irregular open cell cellular structures such as metal foams play an important role in light weight structures and shock absorption components. They are tested experimentally in quasi-static and dynamic compression tests, and they are modeled numerically with finite elements and geometrical information from CT scans. Filler material within the metal foams enhances the energy dissipation properties which are important for shock absorbers. For comparison reasons the same geometrical structure of the foam needs to be modeled with and without filler material. One way to do that is the application of level sets and the XFEM to account for the jump in the strain field within an element. Since only small parts of the foam structure can be modeled, a representative volume element of the structure is used for the calculations. A comparison of the experiments as well as the numerical results of the model with and without filler material is shown.

The quadrilateral parametric contact element based on the moving friction cone formulation

Contact between bodies is most commonly analyzed using quadrilateral contact elements that are based on 8-node brick (hexahedral) continuum finite elements. As a quadrilateral contact surface, in comparison to a triangular contact surface (tetrahedral continuum elements), is not necessarily flat, or it deforms as deformable body deforms, contact formulation turns to be a complex problem. Recent developments in contact routines based on the Moving Friction Cone (MFC) approach for flat triangular contact elements enabled significant simplifi-cations in the element formulation, what is used herein. The MFC formulation of contact is based on the single gap vector, instead of two vectors (slip and stick one). The curved contact surface is defined in a parametric form, thus enabling fi-nite deformations and a Lagrangian definition of contact.