Papers by Author: Dan Mihai Constantinescu

Abstract: Experimental and numerical investigations were conducted to characterize the joint strength and failure modes in adhesively bonded double lap shear type specimens. As bonded joints are more efficient when designed for pure shear, the used specimens consisted of double lap joints to be used for moderately thick to thick foam core sandwich components. The foam thickness and overlap length are parameters investigated hereby. For such an experimental testing setup, in which peeling is reduced considerably, we combined finite element (FE) simulations and digital image correlation (DIC) for obtaining a more complete insight on the peeling and shear strain distribution in the foam, at the interface with the adherend and in the whole overlap region.

Abstract: Many efforts have been made recently to determine the fracture toughness of different types of foams in static and dynamic loading conditions. Taking into account that there is no standard method for the experimental determination of the fracture toughness of plastic foams, different procedures and specimens were used. This paper presents the polyurethane foam fracture toughness results obtained experimentally for three foam densities. Asymmetric four-point bending specimens were used for determining fracture toughness in mode I and in a mixed one, and also the influence of the loading speed and geometry of the specimen were investigated.

Abstract: Epoxy-based nanocomposites with multi-wall carbon nanotubes (MWNT) and nanopowders were fabricated and subjected to monotonic uniaxial testing. Three procedures used to fabricate these nanocomposites are presented, as the dispersion of the MWNTs and nanopowders in the epoxy matrix is a significant challenge. Optical microscopy is used to observe the degree of mixing of the nanofillers in the resin. The inclusion of MWNT and nanopowders don't affect significantly the static properties, but lead to a slight increase of the strength and stiffness of the nanocomposite relative to the neat epoxy.

Abstract: In a recent study of the corresponding author, it was found that PEEK bearing elements revealed high (irreversible) surface strains if they were loaded between steel sheets. Since this reflects the conditions in the practical application and because the rolling properties are dominated by the surface material, a more detailed analysis of highly strained PEEK was required. Hence, fatigue tests in the high stress tensile regime were conducted. The experiments were carried out on servo-hydraulic testing machines and during the tests the mechanical response of the specimens was recorded. Two material modifications of PEEK were investigated in the research: untreated PEEK (without heat treatment) and annealed PEEK which was modified using defined thermal conditions. The analysis of the recorded test data aimed on the distinction between cumulative material response (creep deformation, material hardening / softening) and spontaneous material response (material hardening / softening). At the highest stress levels, the cumulative response pretended material softening with increasing number of cycles. However, by examining the spontaneous material response which became stiffer with increasing number of cycles, it was shown that the cumulative softening was caused by time-dependent deformation processes.

Abstract: Sandwich panels are important components of advanced structures used in aerospace, automotive, railway, civil engineering etc. They are subjected to high and repeated variations of temperature which induce additional stresses as the core and the face sheets are from different materials having different coefficients of thermal expansion and moduli of elasticity. Therefore it is important to evaluate both mechanical and thermal stresses. In the literature one can find thermo-mechanical analyses of sandwich panels with metallic or composite face sheets and having a honeycomb or compact core made from polyurethane foam. In this paper was analysed a plane sandwich panel made from a cellular rigid polyurethane core, having a chiral configuration and auxetic properties (negative Poissons ratio) exposed to a stationary temperature field with a linear variation from +25 °C on one sheet to-50 °C on the opposite sheet. Two boundary conditions were considered in the thermo-mechanical evaluation: the free panel and the panel simply supported around the edges.

Abstract: In this paper, starting from analytic relations presented in literature, a more general approach was obtained in order to evaluate the load capacity of single strapped joints in the cases when the adherends are right or tapered in the overlap zone. Two distinct loading cases, in tension and in bending, are considered. The analytical relations were validated by nonlinear finite element analyses. The comparison with the similar results of the linear analyses emphasized that only the nonlinear approach is appropriate to study the stress states in unsymmetrical joints, even if the adhesive can be considered as an isotropic and homogenous material. For design purposes, the influence of some geometrical parameters on the stress state in the joint was also evaluated.

Abstract: Static loading and impact tests of ceramic alumina tiles have shown a very interesting behaviour of tough, brittle, and high energy absorbing materials. Strain gauges techniques and data acquisition were used in previous research for static and low velocity impact testing. A static-dynamic equivalence was done through a calibration procedure of the measuring system. The research is continued in this paper by using the digital image correlation method with the complete displacement and strain history acquired till the failure of the ceramic tiles of different compositions, proving the capacity of such materials to be used for particular purpose applications.

Abstract: Static loading and impact tests of ceramic alumina tiles have shown a very interesting behaviour of a tough, brittle, and high energy absorbing material. Strain gauges techniques and data acquisition were used throughout this research. A static-dynamic equivalence is done through a calibration procedure of the measuring system. The experimental strain history is completely acquired till the failure of the ceramic tile. Numerical simulation of impact tests gives additional insight on the complicated phenomena. Low velocity impact testing revealed different patters of failure, depending on the conditions of impact, and the capacity of this material to be used in particular purpose designed applications.

Abstract: . Polyurethane foam materials are widely used as cores in sandwich composites, for packing and cushioning. The main characteristics of foams are light weight, high porosity, high crushability and good energy absorption capacity. The paper presents the experimental results obtained for the mechanical properties of polyurethane foams in different loading conditions and the influence of impregnation on the mechanical properties. A 200 kg/m3 density polyurethane foam was investigated in the experimental program in three different Strength of Materials laboratories from Lublin, Bucharest and Timisoara. The paper assesses the possibility to describe the polyurethane foam behaviour trough compression tests, micromechanical models and Finite Element Analysis (FEA). The micromechanical models and Finite Element Analysis could be used successfully for representing the engineering stress – strain behaviour if the compression tests provide reliable material parameters.

Abstract: Interface damage characterization and interlaminar failure of sandwich specimens with two initial interlaminar defects (inserts) is done by using the digital image correlation method. Mode I tests reveal interesting particularities on damage localization and unstable crack propagation. After analyzing the experimentally obtained results, we propose as a failure parameter the local strain at the crack tip or, alternatively, the crack tip opening displacement which quantifies the non-linear phenomena.