Dragos Alexandru Apostol

A Combined Cohesive Elements—XFEM Approach for Analyzing Crack Propagation in Bonded Joints

Cohesive zone modelling (CZM) and extended finite element modelling (XFEM) available in Abaqus® are used together to simulate the behavior and strength of a single lap adhesively bonded joint with an initial delamination in the adhesive. Depending on the thickness of the adhesive and the position of the initial delamination, the crack initiated and propagated by XFEM changes its trajectory following the principle of local symmetry reaching or not the interface. Meanwhile the failure of the interface between the adherent and the adhesive is initiated through CZM and crack propagation at the interface leads to the final failure of the single lap joint assembly. The two simulation models can be used successfully together.

Mixed-Mode Testing for an Asymmetric Four-Point Bending Configuration of Polyurethane Foams

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.

Analysis of Deformation Bands in Polyurethane Foams

Closed cell polyurethane foams with densities of 100, 160 and 301 kg/m3 were tested in compression at speeds of 1 and 5 mm/min. Digital image correlation (DIC) is used to determine the engineering characteristic curve, modulus of elasticity, Poissons ratio and the deformation bands that appear during deformation and prior to the final failure of the specimens. By using this procedure both global and local phenomena are observed and analyzed. While each specimen is compressed the damage behaviour of the foams is directly observed in different stages, as being in the linear elastic domain, in the plateau region, and in the densification region. Several observations, characteristic for each foam density are discussed. As damage mechanisms are different, DIC allows the direct monitoring of the formation of the deformation bands and their propagation till the final failure of the foams, as long as calculations of the local strains are possible. Maps of the vertical displacements and local Mises strains are presented and comments on the characteristics of the deformation bands are done.

Strength Response and Failure Analysis of Ceramic Alumina Tiles

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.

Behaviour of Epoxy Silica Nanocomposites Under Static and Creep Loading

Abstract Specific manufacturing technologies were applied for the fabrication of epoxy-based nanocomposites with silica nanoparticles. For dispersing the fillers in the epoxy resin special equipment such as a shear mixer and a high energy sonicator with temperature control were used. Both functionalized and unfunctionalized silica nanoparticles were added in three epoxy resins. The considered filling fraction was in most cases 0.1, 0.3 and 0.5 wt%.. The obtained nanocomposites were subjected to monotonic uniaxial and creep loading at room temperature. The static mechanical properties were not significantly improved regardless the filler percentage and type of epoxy resin. Under creep loading, by increasing the stress level, the nanocomposite with 0.1 wt% silica creeps less than all other materials. Also the creep rate is reduced by adding silica nanofillers.

Evaluation of Shear Debonding in Double Lap Joints of Sandwich Type

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.

Evaluation of Interlaminar Damage and Crack Propagation through Digital Image Correlation Method

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.