Marin Sandu

Failure analysis of dissimilar single-lap joints

Single-lap joints made of aluminium and carbon fibre adherends of different thickness are tested to understand better the behaviour of such dissimilar joints. The overlap length and the thickness of the adhesive are kept constant. Local deformation fields are monitored by using the digital image correlation method. Peeling and shearing strains are investigated, emphasizing that peeling is important in the region where failure is initiated, towards an extremity of the overlap region. The use of only carbon fibre adherends is not recommended for a smaller thickness as an additional interface failure is produced and compromises the integrity of the lap joint. However, a dissimilar joint (aluminium-carbon) with smaller thickness adherends succeeds to maintain the stiffness of the assembly, but its strength is diminished. The obtained results are suggesting that a complete monitoring of the failure processes in the overlap region can be fully understood only if local deformation measurements are possible.

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.

The Effect of Geometry and Material Properties on the Load Capacity of Single-Strapped Adhesive Bonded Joints

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.

Links between Low Velocity Impact Results and the Residual Mechanical Performance of Sandwich Composite Materials

The paper presents results obtained during low velocity impact tests, performed on sandwich materials of different types. The common force history curves are discussed along with the visual inspections, in order to asses, in a first stage, the structural integrity of the materials subjected to this aggressive and hazardous event.

Some Aspects Concerning the Mechanical Characterization versus Manufacturing Technology of Sandwich Composite Materials

The paper presents aspects observed during classic three-point test bending of various sandwich materials. These aspects outline the need to spare special attention to such test, in connection with the particular material and service life conditions it will endorse. Such special care is needed in conditions of scarcity of dedicated standards and some evasive formulations in the existing ones.

An Analytical-Numerical Approach Concerning the Stresses and Deformations in Single-Strapped Adhesively Bonded Joints

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.

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.

Experimental evaluation of the response of sandwich panels in low-velocity impact

Sandwich panels with aluminum or glass fiber composite facesheets and polyurethane or polystyrene foam core were tested in impact by using an Instron Ceast 9340 impact tower at speeds from 1.5 to 4.5 m/s. The influence of the initial velocity of impact and kinetic energy is analyzed for all types of panels. Particularities of the impact response of the sandwich panels were observed and explained. The facesheet type influence on the damage and penetration of the panels during impact is discussed. If the absorbed energy of the panels is a priority, then the aluminum facesheets and the polystyrene foam core are a good combination. If minimum deformation is required, then composite facesheets and the more rigid polyurethane foam core are a strong option for the sandwich panel design.

Low velocity impact of 6082-T6 aluminum plates

The low velocity domain covers vehicle impacts, ship collisions and even accidentally tool drops. Even though more and more research is needed into these fields, most of the papers concerning impact problems focus on impact at medium and high velocities. Understanding the behavior of structures subjected to low velocity impact is of major importance when referring to impact resistance and damage tolerance. The paper presents an experimental and numerical investigation on the low velocity behavior of 6082-T6 aluminum plates. Impact tests were performed using an Instron Ceast 9340 drop-weight testing machine. In the experimental procedure, square plates were mounted on a circular support, fixed with a pneumatic clamping system and impacted with a hemispherical steel projectile. Specimens were impacted at constant weight and different impact velocities. The effect of different impact energies was investigated. The impact event was then simulated using the nonlinear finite element code LS_DYNA in order to determine the effect of strain rate upon the mechanical behavior of the aluminum plates. Moreover, in order to capture the exact behavior of the material, a special attention has been given to the selection of the correct material model and its parameters, which, in large extent, depend on the observed behavior of the aluminum plate during the test and the actual response of the plate under simulation. The numerical predictions are compared with the experimental observations and the applicability of the numerical model for further researches is analyzed.The low velocity domain covers vehicle impacts, ship collisions and even accidentally tool drops. Even though more and more research is needed into these fields, most of the papers concerning impact problems focus on impact at medium and high velocities. Understanding the behavior of structures subjected to low velocity impact is of major importance when referring to impact resistance and damage tolerance. The paper presents an experimental and numerical investigation on the low velocity behavior of 6082-T6 aluminum plates. Impact tests were performed using an Instron Ceast 9340 drop-weight testing machine. In the experimental procedure, square plates were mounted on a circular support, fixed with a pneumatic clamping system and impacted with a hemispherical steel projectile. Specimens were impacted at constant weight and different impact velocities. The effect of different impact energies was investigated. The impact event was then simulated using the nonlinear finite element code LS_DYNA in order to det...

Study upon the Damage Tolerance of Thick Sandwich Materials

Sandwich composite materials are widely used in various applications, due to their advanced flexibility in responding to special design requirements. This paper presents the evaluation of thick sandwiches, aimed to be imbedded in platforms of a green energy unit, accommodating storage water tanks. The evaluation of the damage tolerance was made having in view previous studies on similar materials and covered assessment of results obtained during low velocity impact tests and post-impact tests, aimed to establish the residual mechanical performance. Ways to increase the damage tolerance, by diminishing the invasive effect of low velocity impact, were also explored.