Search

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.

Laminated composites materials are mostly used in dynamically loaded structures. The design of these structures with finite element packages is focused on vibrations, elastic deformations and failure control. Damping is often neglected because of its assumed secondary importance and also because of dearth of information on relevant material properties. This trend is prone to change as it is now realised that damping plays an increasingly important role in vibration comfort, noise radiation and crash simulations. This paper shows in a first step how to identify the orthotropic elastic and damping properties of single layer fibre-reinforced composite material sheets using a new extended version of the Resonalyser procedure. The procedure is based on the elastic-viscoelastic correspondence principle and uses a mixed numerical experimental method. In a subsequent step, the complex laminate stiffness values are computed using the identified single layer material properties. To validate this approach, the modal damping ratios of arbitrary laminated plates of different materials at several resonance frequencies are predicted and experimentally verified.

A novel method is introduced for the measurement of a 3D strain field by exploiting the interaction between ultrasound waves and geometrical characteristics of the insonified specimen. First, the response of obliquely incident harmonic waves to a deterministic surface roughness is utilized. Analysis of backscattered amplitudes in Bragg diffraction geometry then yields a measure for the in-plane strain field by mapping any shift in angular dependency. Secondly, the analysis of the reflection characteristics of normal incident pulsed waves in frequency domain provides a measure of the out-of-plane normal strain field component, simply by tracking any change in the stimulation condition for a thickness resonance. As such, the developed ultrasonic strain gauge yields an absolute, contactless and single-sided mapping of a local 3D strain field, in which both sample preparation and alignment procedure are needless. Results are presented for cold-rolled DC06 steel samples onto which skin passing of the work rolls is applied. The samples have been mechanically loaded, introducing plastic strain levels ranging from 2 % up to 35 %. The ultrasonically measured strains have been validated with various other strain measurement techniques, including manual micrometer, longitudinal and transverse mechanical extensometer and optical mono- and stereovision digital image correlation. Good agreement has been obtained between the ultrasonically determined strain values and the results of the conventional methods. As the ultrasonic strain gauge provides all three normal strain field components, it has been employed for the extraction of Lankford ratios at different applied longitudinal plastic strain levels, revealing a strain dependent plastic anisotropy of the investigated DC06 steel sheet.

Already in the early 1980's, it has been conjectured that the pulsed ultrasonic polar scan (P-UPS) provides a unique fingerprint of the underlying mechanical elasticity tensor at the insonified material spot. Until now, that premise has not been thoroughly investigated, nor validated, despite the opportunities this would create for NDT and materials science in general. In this paper, we report on the first-ever implementation of an inverse modeling technique on the basis of a genetic optimization scheme in order to extract quantita- tive information from a P-UPS. We validate the optimization approach for synthetic data, and apply it to experimentally obtained polar scans for annealed aluminum, cold rolled DC-06 steel as well as for carbon fiber reinforced plastics. The investigated samples are plate-like and do not require specific preparation. The inverted material characteristics show good agreement with literature, micro-mechanical models as well as with results obtained through conventional testing procedures.

Purpose Finite element analysis has been used extensively in the study of biomechanical modeling of the breast. However, issues regarding the complexity of material models and the influences of geometric boundary conditions on the accuracy of a breast Finite Element (FE) model are still under debate. This work demonstrates the importance of material modeling in FE models of the breast. Methods A simple hemispherical geometry is used to model the shape of a human breast. Different material models are being investigated to accurately model changes in terms of displacement, stress, and reaction forces distribution. Results The results obtained using nonlinear material models are compared with those obtained employing their linear approximation. Results have shown that differences, in terms of displacement, ranging between 20% and more than 80%, may occur and that large differences are present in terms of maximum principal stresses when the displacement is correctly approximated. Conclusions This study clearly shows that, in a FE model, simulating large deformations material modeling strongly influences the accuracy of the solution.

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.

Local stiffness of Euler–Bernoulli beams can be identified by dividing the bending moment of a deformed beam by the local curvature. Curvature and moment distributions can be derived from the modal shape of a beam vibrating at resonance. In this article, the modal shape of test beams is measured by both scanning laser vibrometry (SLV) and shearography. Shearography is an interferometric optical method that produces full-field displacement gradients of the inspected surface. Curvature can be obtained by two steps of derivation of the modal amplitude (in the case of SLV) or one step of derivation of the modal shape slope (in the case of shearography). Three specially prepared aluminium beams with a known stiffness distribution are used for the validation of both techniques. The uncertainty of the identified stiffness distributions with both techniques is compared and related to their signal-to-noise ratios. A strength and weakness overview at the end of the article reveals that the shearography is the technique that shows the most advantages.

Crack monitoring is crucial for safety, cost-effective design and maintenance of concrete structures. Most stand-alone crack monitoring techniques have some limitations in characterizing concrete cracking. Therefore, this article focuses on a combined use of acoustic emission and digital image correlation techniques for more effective monitoring of cracks in concrete structures at laboratory standard tests level. The combined methodology provides complimentary information from ‘the ear’ aided by the acoustic emission and from ‘the eye’ aided by the digital image correlation. This complimentary approach is applied herein to characterize the mechanical and damage behaviour of prestressed concrete sleepers that plays an important role in the structural mechanism of the railway structure. The obtained acoustic emission datasets were post-processed, in combination with digital image correlation and mechanical information, using parametric analyses to investigate progressive failure of the prestressed concrete sleepers under three-point bending at the rail seat and at the centre section. Digital image correlation clearly identified critical damage areas as a function of loading periods, while acoustic emission was capable of monitoring the damage process and revealed changes in the overall behaviour. It is demonstrated that the combined use of digital image correlation and acoustic emission can enhance practical applications and improve the understanding of concrete cracking behaviour.

Conventionally, the mechanical strength of clinched connections is determined by a single shear lap test and/or a pull-out test. However, in most practical applications a combination of shear and pull-out is exerted on the clinched joint. This paper deals with the development of an Arcan-like device which enables to introduce various shear/tensile ratios in a clinched assembly. An experimental survey of the multi-axial behaviour of a non-cutting single-stroke round clinched connection of two DC05 sheets, which is mild deep drawing steel, is conducted with this modified Arcan setup. These experimental results are used to check the validity of numerical models that predict the strength under multi-axial loading. Since the forming of a clinch is a fairly complex sheet metal operation, a good knowledge of the plastic material properties and the frictional behaviour is of the utmost importance to perform a sufficiently accurate FEA simulation. The impact of these factors on the multi-axial loading behaviour of the DC05–DC05 connection is investigated. ispartof: International Journal of Material Forming vol:6 issue:4 pages:437-451 status: published

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.

Single Point Incremental Forming (SPIF) is a modern and flexible alternative to traditional forming techniques. It thanks its flexibility to the fact that it does not require a dedicated tool set to operate. Numerical simulation of the SPIF process requires an accurate FE model. In the past several attempts have been undertaken to use inverse methods for sheet metal SPIF material model identification based on shearing, tensile and indenting tests. The basic idea of this paper is that the results of inverse methods can be improved by using the SPIF process itself as experimental data source. A SPIF experiment dedicated for material identification on a simple geometry using large step sizes is presented and compared with the FE simulation of the forming process based on an initial guess for the material behavior.

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.

Clinching is a mechanical joining technique which involves severe local plastic deformation of two or more sheet metal parts resulting in a permanent mechanical interlock or joint. The required forming load and energy can be determined with the aid of the finite element method. However, a good knowledge of the elasto-plastic properties is of utmost importance to perform a sufficiently accurate simulation. This paper presents two alternative material tests to identify the hardening behaviour of sheet metal beyond the point of maximum uniform elongation. In addition, the material tests were applied to DC05 and the identified material behaviour is evaluated through the prediction of the forming load during clinching.

The aim of this article is to study the accuracy of finite element simulations in predicting the tool force occurring during the single point incremental forming (SPIF) process. The forming of two cones in soft aluminum was studied with two finite element (FE) codes and several constitutive laws (an elastic---plastic law coupled with various hardening models). The parameters of these laws were identified using several combinations of a tensile test, shear tests, and an inverse modeling approach taking into account a test similar to the incremental forming process. Comparisons between measured and predicted force values are performed. This article shows that three factors have an influence on force prediction: the type of finite element, the constitutive law and the identification procedure for the material parameters. In addition, it confirms that a detailed description of the behavior occurring across the thickness of the metal sheet is crucial for an accurate force prediction by FE simulations, even though a simple analytical formula could provide an otherwise acceptable answer.

Clinching, commonly referred to as press-joining, is a mechanical joining technique which involves severe local plastic deformation of two or more sheet metal parts resulting in a permanent mechanical interlock or joint. This interlock is achieved by using simple tools like a die, a punch and a blank holder. Since no additional elements are used, the strength of the clinched connection is entirely determined by the clinch geometry, and, consequently, by the geometry of die and punch. As a result, for each combination of material and sheet thickness an optimal geometry of the tools can be derived. This can be done either through extensive experimental testing or, more cost-effectively, with the aid of numerical simulations. However, for these results to be useful they have to be able to reproduce the experimental strength of the connection. In this paper we investigate the possibility of predicting the shear and pull-out strength of a clinched sheet metal assembly using FEA. Numerical difficulties associated with these simulations and the preceding forming operation are discussed. A comparison between experimental data and simulation is provided.

Incremental Sheet Forming (ISF) is a relatively new class of sheet forming processes that allow the manufacture of complex geometries based on computer-controlled forming tools in replacement (at least partially) of dedicated tooling. This paper studies the straining behaviour in the Single Point Incremental Forming (SPIF) variant (in which no dedicated tooling at all is required), both on experimental basis using Digital Image Correlation (DIC) and on numerical basis by the Finite Element (FE) method. The aim of the paper is to increase understanding of the deformation mechanisms inherent to SPIF, which is an important issue for the understanding of the high formability observed in this process and also for future strategies to improve the geometrical accuracy. Two distinct large-strain FE formulations, based on shell and first-order reduced integration brick elements, are used to model the sheet during the SPIF processing into the form of a truncated cone. The prediction of the surface strains on the outer surface of the cone is compared to experimentally obtained strains using the DIC technique. It is emphasised that the strain history as calculated from the DIC displacement field depends on the scale of the strain definition. On the modelling side, it is shown that the mesh density in the FE models plays a similar role on the surface strain predictions. A good qualitative agreement has been obtained for the surface strain components. One significant exception has however been found, which concerns the circumferential strain evolution directly under the forming tool. The qualitative discrepancy is explained through a mechanism of through-thickness shear in the experiment, which is not fully captured by the present FE modelling since it shows a bending-dominant accommodation mechanism. The effect of different material constitutive behaviours on strain prediction has also been investigated, the parameters of which were determined by inverse modelling using a specially designed sheet forming test. Isotropic and anisotropic yield criteria are considered, combined with either isotropic or kinematic hardening. The adopted constitutive law has only a limited influence on the surface strains. Finally, the experimental surface strain evolution is compared between two cones with different forming parameters. It is concluded that the way the plastic zone under the forming tool accommodates the moving tool (i.e. by through-thickness shear or rather by bending) depends on the process parameters. The identification of the most determining forming parameter that controls the relative importance of either mechanism is an interesting topic for future research.

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 resonant-based identification of the in-plane elastic properties of orthotropic materials implies the estimation of four principal elastic parameters: E 1 , E 2 , G 12 , and ν 12 . The two elastic moduli and the shear modulus can easily be derived from the resonant frequencies of the flexural and torsional vibration modes, respectively. The identification of the Poisson’s ratio, however, is much more challenging, since most frequencies are not sufficiently sensitive to it. The present work addresses this problem by determining the test specimen specifications that create the optimal conditions for the identification of the Poisson’s ratio. Two methods are suggested for the determination of the Poisson’s ratio of orthotropic materials: the first employs the resonant frequencies of a plate-shaped specimen, while the second uses the resonant frequencies of a set of beam-shaped specimens. Both methods are experimentally validated using a stainless steel sheet.

For simulating the injection stage of the manufacturing process of composite materials, the permeability of the textile is an important input parameter. The permeability of textiles can be determined experimentally. However, this is a time and resource consuming process and no standard procedure is yet available. In this paper, we will present a user-friendly software package to simulate the flow through a textile model and predict the permeability Results are presented for a woven fabric and two different non-crimp fabrics. The computed permeability values are compared with experimental results.

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.

Single point incremental forming (SPIF) is a promising new production technique in which a metal sheet is formed stepwise by a spherical tool. However, the technique still shows some particularities. It is observed that the final geometry of a SPIF part can deviate significantly from the programmed tool path. As illustrated in this paper, elastic springback is only to a minor extent responsible for this phenomenon. The goal of the presented paper is to illustrate the gradual emergence of unintended deviations as measured by means of a Digital Image Correlation (DIC) technique. Two CCD cameras were used to take the necessary in-process images. The mechanism of deformation in function of the forming depth is documented and discussed.

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.

The purpose of this article is to develop an inverse method for adjusting the material parameters for single point incremental forming (SPIF). The main idea consists in FEM simulations of simple tests involving the SPIF specificities (the “line test”) performed on the machine used for the process itself. This approach decreases the equipment cost. It has the advantage that the material parameters are fitted for heterogeneous stress and strain fields close to the ones occurring during the actual process. A first set of material parameters, adjusted for the aluminum alloy AA3103 with classical tests (tensile and cyclic shear tests), is compared with parameters adjusted by the line test. It is shown that the chosen tests and the strain state level have an important impact on the adjusted material data and on the accuracy of the tool force prediction reached during the SPIF process.

In the early 1980’s, the ultrasonic polar scan (UPS) technique was developed to assess the fiber direction of composites in a nondestructive way. In spite of the recognition by several researchers as being a sophisticated and promising methodology for nondestructive testing and materials science, little advance was made during the following 30 years. Recently however, the UPS technique experienced a strong revival and various modifications to the original UPS setup have been successfully implemented. This revival has exposed several powerful capabilities and interesting applications of the UPS technique for material characterization and damage assessment. This paper gives a short historical overview of the UPS technique for characterizing and inspecting (damaged) fiber reinforced plastics. In addition, a few future research lines are given which will further expand the applicability and potential of the UPS method to a broader range of (damaged) materials, bringing the UPS technique to the next level of maturity. ispartof: Applied Sciences vol:6 issue:2 status: published

A simplified method to determine the strain distribution during incremental forming of a cone is proposed in this paper. Because of the symmetry of the deformed part, the strain can be derived using the results obtained from a limited number of consecutive tool contours instead of going through the whole process. Comparisons made between the measured and simulated results show that the proposed method can be applied to determine the strain encountered in such kind of incremental forming process where axi-symmetric parts are formed.

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.

Conventionally, the ultrasonic polar scan (UPS) records the amplitude or time of flight in reflection or transmission using short ultrasonic pulses for a wide range of incidence angles, resulting in a unique fingerprint of the critical bulk wave angles of the material at the insonified spot. Here, we investigate the use of quasi-harmonic ultrasound (bursts) in a polar scan experiment, experimentally as well as numerically. It is shown that the nature of the fingerprint drastically changes, and reveals the positions of the leaky Lamb angles. To compare with experiments, both plane wave and bounded beam simulations have been performed based on the recursive stiffness matrix method. Whereas the plane wave computations yield a pure Lamb wave angle fingerprint, this is no longer valid for the more realistic case of a bounded beam. The experimental recordings are fully supported by the bounded beam simulations. To complement the traditional amplitude measurement, experimental and numerical investigations have been performed to record, predict and analyze the phase of the transmitted ultrasonic beam. This results in the conceptual introduction of the 'phase polar scan', exposing even more intriguing and detailed patterns. In fact, the combination of the amplitude and the phase polar scan provides the complete knowledge about the complex transmission coefficient for every possible angle of incidence. This comprehensive information will be very valuable for inverse modeling of the local elasticity tensor based on a single polar scan experiment. Finally, the polar scan method has been applied for the detection of an artificial delamination. Contrary to the pulsed UPS, the quasi-harmonic UPS (both the amplitude and phase recording) shows a superior sensitivity to the presence of the delamination.

Based on the well-known pin-on-disc test rig, a new test setup for online measuring of wear and friction behaviour of polymer matrix composites has been developed. In contrast to a traditional friction-and-wear test rig, a steel pin and composite disc are used for studying the influence of wear debris and fibre orientation. During sliding, a thin adhesive film is possibly formed on the wear track of a composite disc, consisting of wear debris that is squeezed under the steel pin and that finally smoothens onto the composite surface. By optical microscopy, it was observed that most of the debris particles originate from the edges of the wear track. The thin film deforms continuously, with large and dark wear particles observed at the edge of the wear track. A lower coefficient of friction is achieved when the particles are re-adhered to the mating surface. The film formation mechanism depends on the normal force, sliding velocity, and bulk composite structure: because pultruded composite profiles are prese...

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.

Quantitative shearography applications continue to gain practical importance. However, a study of the errors inherent in shearography measurements, related to calibration of the instrument and correction of the results, is most often lacking. This paper proposes a calibration and correction procedure for the out-of-plane shearography with a Michelson interferometer. The calibration is based on the shearography measurement of known rigid-body rotations of a flat plate and accounts for the local variability of the shearing distance. The correction procedure further compensates for the variability of the sensitivity vector and separates the two out-of-plane deformation gradients when they are coupled in the measurement. The correction procedure utilizes two shearography measurements of the same experiment with distinct shearing distances. The effectiveness of the proposed calibration procedure is demonstrated in the case of a static deformation of a centrally loaded plate, where the discrepancy between experimental and finite element analysis results is minimized.

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 this paper, the hybrid compliance-stiffness matrix method for simulating wave propagation in (delaminated) multilayered media with viscoelastic anisotropy has been confronted with high-quality amplitude and phase experiments on delaminated composites, obtained using the ultrasonic polar scan setup (UPS) in transmission by considering harmonic as well as pulsed ultrasound. Results are presented for multiple thin carbon/epoxy laminates with an artificial edge delamination induced by a foil insert, showing a good agreement between experimental recording and numerical modeling. The obtained results further reveal the feasibility of the harmonic UPS to detect and even locate the depth-position of multiple delaminations in fiber reinforced composites. Considering that the harmonic UPS method does not rely on the detection of different echoes like the classical C-scan, but rather expounds the conditions for efficient stimulation of guided waves in the solid, the method is found to be highly suited for inspecting thin composite materials for the presence of delaminations.

The pulsed ultrasonic polar scan (P-UPS) technique provides a map with characteristic contours reflecting the critical bulk wave angles, which directly relate to the in-plane elastic properties of the insonified material spot. Besides these contours, additional information is captured in a P-UPS experiment which has particular importance for the inspection of anisotropic materials. By extensive post-processing on a single P-UPS experiment, we successfully extracted the local directional (i) critical bulk wave angles, (ii) phase velocity profiles, (iii) slowness curves, (iv) walk-off angles and (v) energy velocity profiles, for the three different polarization states of bulk waves. The procedure is demonstrated and discussed for an isotropic aluminum sample as well as an autoclave manufactured orthotropic [o](8) carbon/epoxy laminate. In addition, a numerical and experimental investigation of the role of the immersion liquid as a lens for the P-UPS result is performed. This lensing effect permits to zoom in/out on the P-UPS fingerprint, hence broadening the class of materials which can be inspected by the P-UPS technique. (C) 2014 Elsevier B.V. All rights reserved.

This study investigates the use of both the amplitude and time-of-flight based pulsed ultrasonic polar scan (P-UPS) for the nondestructive detection and evaluation of fatigue damage in fabric reinforced composites. Several thermoplastic carbon fabric reinforced PPS specimens (CETEX), loaded under various fatigue conditions, have been scanned at multiple material spots according to the P-UPS technique in order to extract material degradation in a quantitative way. The P-UPS results indicate that shear dominated fatigued carbon/PPS goes with a reduction of shear properties combined with large fiber distortions. The P-UPS results of the tension-tension fatigued carbon/PPS samples on the other hand reveal a directional degradation of the stiffness properties, reaching a maximum reduction of −12.8 % along the loading direction. The P-UPS extracted damage characteristics are fully supported by simulations, conventional destructive tests as well as visual inspection. The results demonstrate the excellent capability of the P-UPS method for nondestructively assessing and quantifying both shear-dominated and tension-tension fatigue damage in fabric reinforced plastics.

The ultrasonic polar scan (UPS), either in transmission, reflection or backscatter mode, is a promising non-destructive testing technique for the characterization of composites, providing information about the mechanical anisotropy, the viscoelastic damping, the surface roughness, and more. At present, the technique is merely being used for qualitative purposes. The limited quantitative exploration and use of the technique can be primarily ascribed to limitations of current theoretical models as well as the difficulty to perform accurate, and more importantly, reproducible UPS experiments. Over the last years, we have identified several potential pitfalls in the experimental implementation of the technique which severely deteriorate the accurateness and reproducibility of a UPS. In this paper, we make an inventory of the most important difficulties, illustrate each of them by a real experiment and present a feasible mediation, either numerical or experimental in nature. Once the experimental set-up is fine-tuned to overcome these pitfalls, it is expected that the recording of high-level UPS experiments, in combination with numerical computations, will facilitate the technique to become a fully quantitative non-destructive characterization method. (C) 2014 Elsevier B.V. All rights reserved.

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.

In a conventional ultrasonic polar scan (UPS) experiment, the amplitude or time-of-flight-diffraction (TOFD) values of the transmitted and/or reflected acoustic pulse are recorded for a wide range of incidence angles, in view of gaining knowledge about the elastic properties at the insonified material spot. Here we apply the pulsed UPS technique and investigate the backscattered signal, resulting in the ultrasonic backscatter polar scan (UBPS). It is shown that a UPBS contains a blueprint of geometrically related features of the insonified material spot which can be of particular interest for various industrial applications. We applied the UBPS for (i) the determination of the stacking sequence of a cross-ply composite laminate, (ii) the semi-quantification of a multidirectional microscopic surface corrugation, (iii) the detection of corrosion in an early stage as well as (iv) the detection and the localization of a closed surface breaking crack.

The ultrasonic backscattering technique is employed for the characterization of a 2D surface corrugation which is superposed on or hidden on the backside of a polycarbonate sample. In contrast to previous studies where the incident angle at well-defined and a-priori known symmetry orientations of the surface structure is varied in order to extract the characteristic periodicities, the backscatter polar scan method incorporates an additional variation of the orientation of the vertical insonification plane within the experimental measurement protocol. As such, the characteristic periodicities as well as the surface symmetries can be extracted without any prior knowledge of the surface structure. As a benefit compared to optical methods, we have also validated this extended methodology for the investigation of a 2D subsurface corrugation. Although the diffraction conditions do not change in comparison with a visible 2D surface corrugation, we remark that additional attention is required in the sense that the elastic properties of the substrate material put further restrictions to the range of applicable ultrasonic frequencies. The characterized periodicities and symmetries are in excellent agreement with the design parameters of the (hidden) 2D surface grating.

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.

A mixed numerical experimental approach is the basis of a new method for the identification of the material damping properties of fibre reinforced polymers, which provides an answer to many problems encountered in experimental damping characterization. Experimental modal parameters, measured on a plate specimen, are compared with corresponding results from a numerical calculation, thus allowing to determine the stiffness and damping properties of the material. The relation between the modal parameters (structural parameters) and the material parameters, is obtained by using a numerical model of the specimen in combination with the modal strain energy method.

Shearography is an interferometric method that produces full-field displacement gradients of the inspected surface. In high-technology industry it is often used qualitatively to detect material defects, but quantitative applications are still rare. The reasons for that are the complicated calibration procedure as well as the denoising, unwrapping, the local sensitivity vector estimation and the local shearing angle estimation needed to get quantitative gradient-maps. To validate the technique and its calibration, results obtained from shearography are compared to results obtained from scanning laser vibrometry. Beams are acoustically excited to vibrate at their first resonant frequency and the mode shape is recorded using both shearography and scanning laser vibrometry. Outputs are compared and their properties discussed. Separate inverse method algorithms are developed to process the data for each method. They use the recorded mode shape information to identify the beam’s local stiffness distribution. The beam’s stiffness is also estimated analytically from the local geometry. The local stiffness distributions computed using these methods are compared and the results discussed.