Papers by Author: Marin Sandu

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: 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: Because of the great amount of hard dental tissues loss, most of non-vital teeth must be prosthetically restored using a post. The anchorage of these posts is based on enlarging the root canal, so the question is what type of endodontic post is more appropriate to use in order to ensure a favorable prognosis of these teeth, avoiding as long as possible their extraction. Loss of teeth is affecting not only the oral cavity and the dento-maxillary system, but also the quality of life of the patients, with both psychological and general health consequences. Even treated, the edentulous state determines major changes in diet and can be a cause of gastrointestinal disorders. This in vitro experiment tested the resistance to fracture of non vital teeth restored with different types of posts, in order to find out which type is best to use for their restoration. From the of this in vitro experiment it has been observed that in the case of teeth restored with metal posts, regardless of the diameter of the post, the failure of the sample was represented by root fracture. In the case of the teeth restored with fiberglass posts, the failure was caused by the fracture of the post or the fracture of the coronary part of the tooth.

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: 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.

Abstract: Adhesive bonding is a particularly effective method of assembling complex structures, especially those made from dissimilar materials. If the joint is well designed and correctly executed, the adhesive bond ought to be one of the strongest components of the structure and most certainly should not be the reason for reducing the load capacity or fatigue life. The major factors determining the integrity of an adhesive bond are selection of the most appropriate adhesive, joint design, preparation of the bonding surfaces, strict quality control in production and monitoring in service. This work focuses on the evaluation of the load capacity of some configurations of adhesively bonded single-strapped joints based on finite element analyses. The adhesive layer thickness, the overlap length, the adherent and strap thicknesses were varied as well as the materials properties.

Abstract: Low velocity impact is a frequent and inevitable in-service event, with higher occurrence in transportation structures. The damages following such an event are more diverse, extended and with more severe consequences in the case of composite materials and structures. The research work presented here concerns fibre reinforced polymeric composites in the forms of plates and pipes. It is continuing an effort meant to allow customers exploiting such structures to have a short cut in monitoring the integrity of this kind of structures. To this end, it is proposed a careful following of the impact force history recording, which can offer valuable and more direct information about the damage level produced under this insidious loading.