Stefan Sorohan

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.

Extraction of lumped structural parameters from multiphysics field simulations for MEMS switches

This paper investigates the extraction of lumped parameters (elastic coefficient and effective mass) for RF MEMS switches. The extraction starts from field simulations obtained with the finite element method, which solves strongly coupled structural-electrostatic field problems, and is based on least square fitting. The elastic coefficients are extracted from static coupled simulations, whereas the effective mass is extracted from a dynamic, without damping, unforced, transient simulation. The degree of system nonlinearity can be estimated from the static simulation. If the system is close to linearity with respect to its structural behavior, then a simple algorithm to extract the effective mass is proposed. The validations are carried out by comparing the initial field results with the results obtained from the reduced order models. Relative errors less than 8 % are obtained for the pull-in voltage for all the structures investigated.

Designing Particulate Composites: The Effect of Variability of Filler Properties and Filler Spatial Distribution

A new perspective on structural design of particulate composites is presented in this chapter. The central concept is that by controlling multiple parameters describing the stochastic microstructure, such as allowing the filler properties to vary from filler to filler, or constructing spatially correlated filler distributions, significantly expands the design space which, in turn, is likely to lead to the development of more performant composites. We investigate the effect of two such parameters on the elastic-plastic and damping behavior of the composite. First, we consider microstructures containing fillers of same properties but which are spatially distributed in a correlated way. It is observed that composites with spatially correlated filler distributions are stiffer, strain harden more and lead to larger damping ratios relative to microstructures with random, uncorrelated filler distributions of same volume fraction. In the second part of the study we consider composites in which filler properties vary from filler to filler. It is observed that the composite modulus and its strain hardening rate decrease as the variance of the probability distribution function of filler elastic constants increases, while the mean of the distribution is kept constant. The damping ratio of the composite is not sensitive to the higher moments of the distribution function of damping coefficients within inclusions.

Investigation of a Mechanism Through a Transient Thermal Analysis and an Equivalent Steady-State Thermal Analysis

The thermal analysis of a rotary engine mechanism requires taking into consideration the transfer of heat from the combustion gas to the engine parts, which include rotating parts and fixed parts, as well as the transfer of heat to the environment. During an engine mechanism rotation, the conditions of convective heat transfer are variable, and the surfaces of fixed parts exposed to combustion gas are continuously changing. In this case, the transient thermal analysis using the finite elements method is very complex because of the permanent modification of surfaces covered with combustion gas, as a consequence of mechanism rotation. Therefore, in the current paper, an equivalent model for steady-state thermal analysis is developed, so that the same results are obtained as in the long transient thermal analysis, but with significantly smaller requirements of time and computational resources. The transient thermal analysis performed for a large number of rotations, which provides the stationary thermal conditions of mechanism parts, is compared with the equivalent steady-state thermal analysis performed using the equivalent film coefficients and the equivalent convection temperatures. The distributions of fixed part temperature and heat flux obtained from the steady-state thermal analysis are compared to those obtained from the transient thermal analysis, and very good similarities are ascertained. In conclusion, the equivalent steady-state thermal analysis provides similar results, compared with the transient thermal analysis, but with significantly lower computational effort.

Parametric multiphysics static models for a bridge type MEMS capacitive switch

The design of RF MEMS switches is a challenging task due to the various multiphysics coupled phenomena that have to be taken into consideration. In order to reach a configuration with specific requirements, dynamical compact models are needed, that should be accurate enough and have few degrees of freedom, so that the simulation of an entire system in which such a device is embedded need reasonable computer resources, measured in time and memory requirements. In order to allow the design and optimization tasks, these models have to be correlated with design parameters such as dimensions and material properties. This paper focuses on the path from coupled structural-electrostatic models to the extraction of relevant information for the design, namely the static pull-in voltage and the effective stiffness coefficient, for a capacitive switch of bridge type. The influence of the width of the actuation pads and the length of the bridge is investigated.

Evaluation of Thermal Stresses in Sandwich Panels with Chiral Cellular Cores

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.

Dynamic Analysis of Composite Plates

The paper presents comparatively the measured and estimated natural frequencies and mode shapes of a rectangular orthotropic panel. The experimental tests were performed using a shaker. The plate was fixed in horizontal position directly on the shaker armature using a rigid rod. The experimental modes shapes were visualized using sand particles. The measured natural frequencies and also the modes shapes correspond to the whole system in motion. So, the analytic model of the plate must include the effect of the vibrator connected to the plates. The purpose of this experimental measurements and analytic modeling of such plates is to further developing a methodology to estimate the material properties of the composite panels. At this research stage, an orthotropic plate with unknown material properties was investigated. Using an adequate finite element model, the mechanical properties of the material were estimated. Using these properties it is possible to numerically estimate the dynamic behavior of the plate for additional sets of boundary conditions.Copyright

Improvement of the Dynamic Behavior of a Fan Using Finite Element Method

The first part of this article presents a dynamic analysis of the fan of a generator connected by rigid coupling to a diesel engine. The purpose of this analysis is to correlate the main components of the fan’s frequency response spectrum with the ones of the excitation source, i.e., the diesel engine. The second part describes a finite element analysis of the fan in order to find the best design solution.Copyright