Ştefan Sorohan

Extraction of dispersion curves for waves propagating in free complex waveguides by standard finite element codes.

This paper presents a fast and reliable method, for obtaining all the range of dispersion curves for wave propagation usually used in practice, by numerical simulation only, via common commercial finite element codes. Essentially, the method is based on a simple and robust approach, consisting in a few series of modal analyses for a representative part of the inspected structure. In this way, for different wave lengths, one can find the mode shapes and corresponding natural frequencies by solving some real, symmetric and well numerically conditioned eigenvalue problems. The method allows the extraction of propagating modes only and, in spite of not producing continuous dispersion curves, it is not susceptible to aliasing effects, as some similar methods are. Additionally, complete graphical representations of guided waves are possible with some minor calculus effort.

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.

Mixed Domain Macromodels for RF MEMS Capacitive Switches

A method to extract macromodels for RF MEMS switches is proposed. The macromodels include both the coupled structural-electric behavior of the switch as well as its RF behavior. The device with distributed parameters is subject to several analyses from which the parameters of the macromodel are extracted, by model reduction. From the coupled structural-electrostatic analysis the parametric capacitance and the effective stiffness coefficients of the switch are extracted. From the RF characteristics in the up stable state, the transmission line parameters are extracted. Finally, all parameters are combined in a Spice circuit model, which is controlled by the MEMS actuation voltage and is excited with the RF signal. The procedure is applied to a capacitive switch. Relative modeling errors with respect to the non-reduced model, considered as reference, of less than 3 % for the RF characteristics and less than 1 % for the mechanical characteristics are obtained.

Results of Nondestructive Inspection of Layered Composites Using IR Thermography and Ultrasonics

Polymeric layered composites exhibit a variety of damages following in service loading conditions, like delamination, matrix cracking or even fibre breaking. Detection of such damages and assessing their extension and severity is vital during maintenance cycle, in view of keeping the normal operational reliability. For local inspections, IR thermography and ultrasonic scanning are among the best valued NDT methods. The paper describes the inspections performed by IR active thermography, in different variants, and pulse-echo ultrasonic scanning on GFRP. A variety of layered composites and defects/damages were inspected and the results are evaluated independently, in some cases being compared each other, with valuable conclusions for the users of the mentioned NDI techniques.

Linking the Impact Force History with Residual Mechanical Characteristics and NDI - A Smart Way to Perform SHM in Composites

Structural integrity monitoring (SHM) and evaluation of residual mechanical performance are highly needed in assessing the post-impact behaviour of composite materials and structures. The link between impact force history and the damage level was not followed enough in research studies upon the SHM of composites. The authors put in evidence a clear link in this matter in a variety of layered composite materials. The link was assessed by evaluating the residual mechanical performance and by nondestructive inspection (NDI) – ultrasonics and infrared thermography (IRT) - on the impacted samples. Such a link may prove a very useful and reliable shortcut for backing the online SHM and condition based maintenance.

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...

Assessment of Effective Elastic Properties of Honeycomb Cores by Modal Finite Element Analyses

The main issue in analyzing honeycomb structures is the substantial computational effort that has to be spent in modeling and analyzing them with a multi-cell construction core by maintaining the actual geometry. Therefore, the common practice in the finite element modeling is to replace them by an equivalent orthotropic material. The determinations of these equivalent properties of the homogenized core are based on analytical or numerical relationships usually obtained from pure axial and shearing loads. By using these initial estimations of the equivalent properties, a homogenized model works well for in-plane deformations but may give large errors for out-of plane deformations which generally take place in panel or sandwich homogenized structures. This paper presents a numerical method that can be used to correct some of the equivalent elastic properties of the homogenized core to work properly in bending and torsion based on some modal analyses applied first for a real honeycomb panel, to obtain a reference solution, and then on the homogenized orthotropic panel where some of the equivalent elastic constant are iteratively improved by an optimization algorithm to fit the reference solution. The analyzed types of honeycomb cores are the commercial ones in three cell configurations: square, regular hexagonal and re-entrant shapes. The calculations show that some of the equivalent elastic properties obtained in the classical approach must be corrected with factors larger than five as to obtain correct results in bending and torsion loads.

Evaluation of the structural behaviour of a port chamber following damages produced in the refractory lining

The port chamber used in oil refineries is an exhaust chamber mounted after the catalytic reactor for retaining and unifying the exhaust hot process gases. The thick steel shell has a refractory lining made of refractory cement called gunite or shotcrete. Damages produced in the refractory lining have serious effects on the thermal generated stresses in the steel shell. The research work aimed to find the level of these stresses in a numerical/experimental approach and to evaluate its consequences on the service life of this important component of the oil refinery. A complex sequentially coupled physics thermal-structural analysis was performed using the ANSYS code in order to get the stresses generated by a deep crack in the shotcrete layer. The results concerning the thermal mapping were based on previous inspections made with infrared thermography (IRT) and made possible an estimation of the remained service life of the port chamber.

Particular Aspects Concerning Low Velocity Impact on Composite Tubes

The study had in view various aspects which can arise during the low velocity impact tests made on composite pipes/tubes. It implied numerical simulations, made by ANSYS and LS-DYNA codes, on glass fiber/epoxy composite pipes. The geometry and material characteristics were taken from real pipes, which have been experimentally tested in parallel, using a drop weight impact tower. The main parameter in view was the impact force history, which gives most information upon the impact event and, accordingly, is used by most of researchers for characterizing the damages produced on the impacted body and for assessing the impact installation.