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Both plywood and medium density fiberboard (MDF) are often intended to be applied in conditions with fungal decay risk, which can lead to a decrease in physical and mechanical properties. It is thus essential to understand the detailed performance of the panels under fungal decay over time. In the presented research, the specimens were exposed to wet vermiculite inoculated with Pleurotus ostreatus for 14 weeks. A non-destructive flexural vibration method (resonalyser) was used to measure the Young's modulus (MOE) of the specimens, whereas the internal structure and mass loss distribution were monitored with X-ray CT scanning. The significant MOE decrease of MDF for interior use is a result of internal cracking. MDF produced with acetylated fibers showed only 1.5% mass loss and 10% MOE decrease. Plywood specimens exhibit a MOE decrease of approximately 12% at incipient fungal decay partially resulting from water dynamics and structural changes. There is a linear relationship between MOE decrease and mass loss, yet different for different types of plywood. Most mass loss occurs in the surface layers causing significant MOE decrease of entire plywood specimens, whereas the mass loss of the inner layers is influenced by structural properties of the veneers such as grain direction.

This article describes an inverse method for the identification of the plastic behavior of aluminum plates subjected to sudden blast loads. The method uses full-field optical measurements taken during the first milliseconds of a free air explosion and the finite element method for the numerical prediction of the blast response. The identification is based on a damped least-squares solution according to the Levenberg–Marquardt formulation. Three different rate-dependent plasticity models are examined. First, a combined model based on linear strain hardening and the strain rate term of the Cowper–Symonds model, secondly, the Johnson–Cook model and finally, a combined model based on a bi-exponential relation for the strain hardening term and the strain rate term of the Cowper–Symonds model. A validation of the method and its sensitivity to measurement uncertainties is first provided according to virtual measurements generated with the finite element method. Next, the plastic behavior of aluminum is identified using measurements from real free air explosions obtained from a controlled detonation of C4. The results show that inverse methods can be successfully applied for the identification of the plastic behavior of metals subjected to blast waves. In addition, the material parameters identified with inverse methods enable the numerical prediction of the material’s response with increased accuracy.

Clinching is a mechanical joining technique by which sheet metal parts can be assembled without the use of additional material inserts. The strength of such a connection highly depends on the final geometry of the permanent joint. The assessment of the mechanical strength is conventionally done by conducting simple experiments such as a single shear lap test. However, it is generally recognized that the axial strength of this type of connections is considerably lower than its strength in shear loading. As a consequence, it is clear that applications where the joint is loaded in the axial direction should be handled carefully. The so-called pull-out strength can be investigated by dedicated experiments or sophisticated finite element calculations. In this paper an analytical approach to estimate the pull-out strength is presented. The first part describes the method along with the necessary assumptions. The remainder of this contribution validates this analytical method through a new experimental setup and a finite element model. In addition, a correction curve is derived which enables to adapt the analytical solution accounting for bending effects. Finally, the alternative method is applied to a joint connecting two DC05 sheets.

In this international permeability benchmark exercise, in-plane permeability data for two reinforcement fabrics, obtained using a total of 16 different experimental procedures, were compared. Although, for each procedure, the results appear consistent, different procedures result in a scatter of up to one order of magnitude in principal permeability values for each fabric at any given fibre volume fraction. The ratio of the principal permeability values varies by factors of up to 2. While experimental uncertainties and variability of the specimens affect the scatter in results for any single series of experiments, it is suspected that the main source of scatter in results from different procedures is related to human factors. Aiming at standardisation of measurement methods and interchangeability of results, "good practice" guidelines will be formulated in order to eliminate sources of scatter. (C) 2011 Elsevier Ltd. All rights reserved.

The most common way to evaluate the stress–strain relation of sheet metal is by performing standard tensile tests. However, those tests only allow the identification of the hardening behaviour up to the point of maximum uniform elongation, at least when using standard measuring equipment and simple analytical formulas to derive the relation between force and stress, and, elongation and strain. Usually the hardening behaviour beyond this point is estimated by extrapolation of the hardening behaviour before the point of maximum uniform elongation. Such procedure may yield very different results, depending on the hardening law which is fitted to the available experimental data. Other methods which deal with the problem of extended yield curve identification have been proposed and the most applied among them are finite-element based inverse approaches. This paper presents an alternative method to identify the post-necking hardening behaviour of sheet metal without using a finite element model. The key point in the presented method is the minimization of the discrepancy between the internal and external work in the necking zone during a tensile test. The main focus of this paper is on the presentation of the method, the underlying assumptions and the experimental validation. In addition, the method was applied to identify two common hardening laws for DC05, which is a mild deep drawing steel, using experimental data from the pre- and post-necking region.

This paper describes the identification of hardening parameters of DC05 sheet metal and contact friction coefficients using a multi-layered upsetting test (MLUT), the modified two specimen method (MTSM) and a finite element based inverse method. The MTSM is adopted in order to identify the friction coefficient between the tools and the stacked circular specimens which are cut from the base material by spark erosion. Next, the hardening behaviour is identified inversely by combining the results of a MLUT and finite element simulations of the test setup. Finally, the results are compared with standard tensile tests and it is shown that the MLUT is a viable alternative for the identification of the local hardening behaviour of sheet metal where standard test specimens cannot be prepared due to size limitations of the specimen.

The capability to simulate the flow of resin through a porous reinforcement by FE modelling has become very important for designing RTM production processes for high-performance composite parts. The key parameter in RTM flow simulations is the fibre reinforcement permeability. The measurement of this material parameter is still not standardized and many different set-ups have been proposed. Recently, a stereolithography technique was used to produce a textile-like solid specimen with anisotropic permeability, designed as a reference for calibration and comparison of permeability measurement set-ups and for validation of numerical permeability computation software. Unlike real textiles, the permeability properties of such reference specimens do not vary from test to test. Excellent repeatability of the experiments is achieved. When used for benchmarking, any discrepancy between different measurements on this specimen must be attributed to the set-up and data processing. This paper presents the first experimental measurements of the permeability of such reference specimens, obtained with a 2D central injection rig. The measured values, with principal components of the 2D permeability tensor of 2.47 ± 0.09 × 10−9 m2 and 5.44 ± 0.22 × 10−9 m2, are in good agreement with the values predicted using numerical permeability computation software. Statistics from a series of measurements using the same set-up show that the precision of the permeability identification depends on the data processing procedure. An approach which incorporates a numerical model that is also valid after the fluid reached an edge of the reinforcement performs significantly better than the approach, based on an analytical approximation, which was used before.

The permeability of textile reinforcements is a crucial input for the simulation of the impregnation stage of a composite material fabrication process. In this paper, we present a fast and accurate simulation method for the permeability of a textile reinforcement, based on a finite difference discretisation of the Stokes equations. Results for single layer, multi-layer and sheared models are discussed. The influence of intra-yarn flow and periodic or wall boundary conditions are considered. We compare the numerically computed permeability values with experimental data.

Inverse methods offer a powerful tool for the identification of elasto-plastic material properties of metals. The basic principle of the inverse method we are studying, is to compare an experimentally measured strain field with a strain field computed by a finite element (FE) model. The material parameters in the FE model are iteratively tuned in such a way that the numerically computed strain field matches the experimentally measured field as closely as possible. One of the building blocks in this identification procedure is the optimization algorithm for the material parameters in the numerical model. The key problem of this optimization algorithm is the determination of a sensitivity matrix, which expresses the sensitivities of the strains with respect to the material parameters. This paper presents an analytical method for the calculation of this sensitivity matrix in the case of a tensile test with non-rotating principal axes of strain.

This paper presents a mixed numerical–experimental method for the identification of the four in-plane orthotropic engineering constants of composite plate materials. A biaxial tensile test is performed on a cruciform test specimen. The heterogeneous displacement field is observed by a CCD camera and measured by a digital image correlation (DIC) technique. The measured displacement field and the subsequently computed strain field are compared with a finite element simulation of the same experiment. The four independent engineering constants are unknown parameters in the finite element model. Starting from an initial value, these parameters are updated till the computed strain field matches the experimental strain field. Two specimen geometries are used: one with a centered hole to increase the strain heterogeneity and one without a hole. It is found that the non-perforated specimen yields the most accurate results.

Numerical models of a railway track rarely take the nonlinear dynamic component behaviour of the railpad into account due to three important problems: first, the considerable amount of calculation time consumed by nonlinear numerical models; second, the lack of validation tools for nonlinear numerical models; third, the lack of data on the nonlinear properties of those components. The ever-increasing power of computers will solve the first problem. Scale models can be a solution for the validation problem. This paper discusses a practical solution for the third problem. The paper will present an alternative set-up for railpad testing allowing for the measurement of stiffness and damping values between 20 and 2500 Hz with variable preload. It shows the important variation of the dynamic stiffness and the loss factor throughout the chosen frequency range. It is also found that the nonlinear dynamic behaviour of this railway track component certainly affects the dynamic behaviour of the complete track.

Mixed numerical-experimental techniques (MNETs) combine experimental test results and numerical modelling techniques with the goal of identifying physical properties. Experimental results always come with a level of uncertainty. This input uncertainty will migrate through the MNET routine, and will result in an uncertainty on the identified parameters. Therefore, MNET procedures should not only provide an estimated value for the physical properties, they should also provide additional information about the reliability of the results obtained. This paper presents a routine that is able to transform the uncertainty on the input parameters of a MNET into the uncertainty on the output parameters. The approach does not require any change of the initial MNET routine, it is just an additional computational step that has to be performed after the MNET has identified the deterministic values of the unknown physical properties. The routine is demonstrated on a vibration based MNET used to identify elastic material properties.

Sandwich structures are extensively used in engineering because of their high specific stiffness and strength. The modelling of sandwich structures has been studied extensively, but less attention has been paid to their material identification. This paper proposes an inverse method for the material identification of sandwich beams by measured flexural resonance frequencies. The procedure is illustrated with numerically generated test data and also applied on experimentally measured data taken out of literature. An error estimation procedure is conducted to discover and discuss the main error sources.

Many papers have already been presented about identification of 2D in plane elastic engineering constants E1, E2, v12 and G12 by inverse methods. Most of the described methods are based on measured resonance frequencies on plate specimens. Less attention has been paid to the identification of transverse shear moduli. Especially, validation of the obtained results and error bound estimations are often lacking. This paper presents an inverse method for the identification of the transverse moduli of test beams by measured flexural resonance frequencies. The procedure is first illustrated with numerically generated test data and next applied on experimentally measured results. The sensitivity of the flexural resonance frequencies for variations of the shear modulus is discussed as well as an estimation of the uncertainty intervals on the obtained moduli. The results are validated by using the obtained shear moduli for the prediction of torsional resonance frequencies.

Time consumption is the major drawback of many existing set-ups for permeability measurement. This drawback has prevented many researchers from studying the statistical distribution of experimentally obtained permeability values. To overcome this problem—while maintaining the necessary accuracy—an automated central injection rig for permeability identification called ‘PIERS set-up’ (permeability identification using electrical resistance sensors) was developed. The PIERS set-up was used to characterize two typical glass fiber reinforcements. The test results demonstrated the presence of large scatter in the identified permeability values and hence the necessity to consider a statistical distribution of permeability values. The existence of a permeability distribution implies that a small number of measurements will not be sufficient to fully characterize the material. This paper discusses the necessary number of measurements to adequately characterize the statistical distribution and investigates the origins of the observed scatter. The paper also shows that the nesting of layers during stacking and mold closing is a major source of the observed variations in permeability values.

Vibration control of structures is confronted with many problems like the proper selection of modelling methods, controllability and observability of models, model size and model reduction methods. In this paper a practical procedure is proposed to overcome technical problems of structure control due to a large model size. Firstly, an accurate numerical model is derived by using the finite element method. This model is verified by modal analysis experiments. Secondly, a new model reduction procedure is proposed to reduce considerably the full order model. This reduction makes the reduced order model controllable and observable. In a further step, a controller is designed based on the verified reduced order model. Finally, a real-time control system is set up. The controlled and uncontrolled impulse responses at the free tip of the cantilever beam with active constrained layer damping treatments are compared both in time domain and frequency domain. The results clearly show the efficiency of the proposed procedure. The procedure proposed in this paper can be extended towards more complex structures.

Railway-induced vibrations are a growing matter of environmental concern. The rapid development of transportation, the increase of vehicle speeds and vehicle weights have resulted in higher vibration levels. In the meantime vibrations that were tolerated in the past are now considered to be a nuisance. Numerous solutions have been proposed to remedy these problems. The majority only acts on a specific part of the dynamic behaviour of the track. This paper presents a possible solution to reduce the noise generated by the ‘pinned–pinned’ frequencies. Pinned–pinned frequencies correspond with standing waves whose nodes are positioned exactly at the sleeper supports. The two first pinned–pinned frequencies are situated approximately at 950 and 2200 Hz (UIC60-rail and sleeper spacing of 0.60 m). To attenuate these vibrations, the Department of MEMC at the VUB has developed a dynamic vibration absorber called the Double Tuned Rail Damper (DTRD). The DTRD is mounted between two sleepers on the rail and is powered by the motion of the rail. The DTRD consists of two major parts: a steel plate which is connected to the rail with an interface of an elastic layer, and a rubber mass. The two first resonance frequencies of the steel plate coincide with the targeted pinned–pinned frequencies of the rail. The rubber mass acts as a motion controller and energy absorber. Measurements at a test track of the French railway company (SNCF) have shown considerable attenuation of the envisaged pinned–pinned frequencies. The attenuation rate surpasses 5 dB/m at certain frequency bands.

This paper compares results of three different methods to determine the in-plane elastic properties of sheet materials. Results obtained with standard resonant beam and tensile tests are used to assess a mixed numerical–experimental technique developed to determine the in-plane elastic properties of orthotropic plates from the resonance frequencies of rectangular plate samples (the so-called ‘Resonalyser’ technique). Test materials were selected on the basis of an expected low degree of elastic anisotropy in order to put the accuracy and sensitivity of the different techniques to assess anisotropic materials to a test. Therefore, aluminium alloy and stainless steel samples were prepared from hot-rolled plates, deliberately avoiding pronounced cold-rolling textures. The differences between the results obtained with the three experimental approaches are critically evaluated. In the case of very thin plates, the existing mixed numerical–experimental Resonalyser procedure succeeded in accurately identifying the elastic material properties. A slightly adapted procedure is proposed to extend the applicability of the Resonalyser procedure to thicker plates.

Resin transfer molding (RTM) is becoming increasingly important as a production technique for FRPs. However, the design of molds that assures the production of parts with acceptable quality remains a difficult task. Simulation codes developed for design need reliable input data such as reinforcement permeability. The measurement of this material parameter is still not standardized and thus many different set-ups have been proposed. In this paper a new method will be presented that is much faster and much less operator-dependant than other set-ups. It is a 2D central injection set-up, based on the automatic detection of the flow front position as a function of time. For this purpose, simple electrical sensors were designed and an automated calculation procedure was programmed on a PC-based system. As will be shown, the elements of the permeability tensor are not single valued at a particular fiber volume fraction. Rather, each element displays a broad distribution. More importantly, the variations in the principal permeability values are not correlated since their ratio, the anisotropy, also displays a broad distribution.

In liquid composite moulding (LCM) techniques, the liquid resin has to flow a long distance to impregnate the dry fibres. The measure for the resistance of the fibre preform to the resin flow is the permeability of the fibre preform. Because of the dual-scale porous structure of the textile preforms, test fluid can influence the unsaturated permeability values through the interaction of the fluid and fibres. In this study, the influence of test fluid on the permeability measurement of several types of textile reinforcements is investigated. First the contact angle of various fluids and fibres was measured. Then the permeability measurement of the textile reinforcements was carried out. The results showed that the influence of test fluid is small under the test conditions.

The characterization of the stiffness properties of fibre reinforced composite (FRC) materials presents more difficulties than the characterization of traditional isotropic materials. This paper first describes the difficulties that can arise and next presents a nondestructive method that offers a possible solution to the problems. The proposed method is a so-called ‘Mixed Numerical/Experimental Technique’ (MNET). From an experimental point of view, the method requires the measurement of resonant frequencies of freely suspended rectangular test plates. From the numerical side, an accurate model of the test plate must be available. The stress field associated with the different resonant frequencies is multi-axial. The measurement procedure however is very simple and requires little specimen preparation. The equipment is inexpensive and the measurement procedure can be controlled by a PC program. The proposed MNET allows the simultaneous determination of all the anisotropical properties and provides an estimation of their statistical distribution.