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.

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.

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.

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.

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.

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.

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.

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...

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.

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 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.

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.

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.

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.

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 MLUT is an alternative compression test that is based on the stacking of small circular specimens on top of one another. This test was designed in order to identify frictional and material behavior under severe local forging of sheet metal as encountered during a clinching operation. 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 behavior 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 behavior of sheet metal where standard test specimens cannot be prepared due to size limitations of the specimen.

A fast and robust mixed numerical/experimental technique is presented, that allows with a single experiment to determine all components of the in-plane permeability tensor, needed for process simulation of resin transfer molding. This technique combines the highly automated “PIERS” (permeability identification using electrical resistance sensors) central injection rig with a customized numerical simulation of the experiment, and uses an iterative inverse method to identify permeability values for which the simulation agrees with the experiment. It is shown that, for reinforcements that are not approximately isotropic, the numerical simulation in the inverse method significantly improves the precision of the parameter identification, compared to inverse methods with models that are valid only until the injected fluid reaches an edge of the reinforcement.

The accuracy of a Finite Element Simulation for plastic deformation strongly depends on the chosen constitutive laws and the value of the material parameters within these laws. The identification of those mechanical parameters can be done based on homogeneous stress and strain fields such as those obtained in uni-axial tensile tests and simple shear tests performed in different plane material directions. Another way to identify plastic material parameters is by inverse modeling of an experiment exhibiting a heterogeneous stress and strain field. Material parameter identification methods, which integrate optimization techniques and numerical methods such as the finite element method (FEM), indeed offer an alternative tool. The most common approach is to determine the optimal estimates of the model parameters by minimizing a selected measure-of-fit between the responses of the system and the model, In the present study a method is proposed for the identification of the initial yield stress, the two parameters of a Swift isotropic hardening law and the four parameters of the Hil148 yield surface, based on the full-field surface measurements of a cruciform specimen subjected to biaxial tensile loading. Experimental forces and strains are in this case compared to the simulated values. A finite element model of the perforated specimen serves as numerical counterpart for the experimental set-up. The difference between the experimental and numerical strains (e x, e y and e xy) is minimized in a least squares sense by updating the values of the different parameters simultaneously. The sensitivities used to obtain the parameter updates are determined by finite differences, using small parameter perturbations. The optimization routine used, is based on a constrained Newton-type algorithm.

The simulation of a resin flow through a porous medium by FE-models has become a very important aspect for the design of a high-performance RTM produced composite part. The key parameters to perform RTM flow simulations are the permeability values of the fibre reinforcement. The measurement of this material parameter is still not standardized and thus many different set-ups have been proposed. This paper presents an inverse method, or a so-called mixed numerical/experimental method, for the identification of the permeability values.

Textile reinforcements are characterised by a hierarchy of structure, which allows a general structural approach to their behaviour and their use in composite processing. The hierarchical approach led to the development of a software tool for describing the geometry and mechanics of textiles. This tool is used as a pre-processor for meso-mechanical and permeability models for textile composites. Together the pre-processor, permeability and meso-mechanical models constitute an integrated design tool for textile composite modelling on a meso-scale level. The permeability-related part of this tool is presented in this paper. The focus is on the strategy and numerical approach to permeability modelling of textiles. The proposed strategy allows integrating in a natural way the different processing steps of composite manufacturing, while a numerical approach based on the lattice Boltzmann method allows dealing with the complexity of flow in textiles. Comparison with model test cases and with experimental data on permeability of woven fabrics is provided.

Complex moduli are used to characterize the dynamic material behaviour of fibre reinforced polymers (FRP). This paper presents an efficient method to identify the in-plane complex moduli of thin orthotropic FRP plates, based on a mixed numerical experimental approach. The required experimental data are the modal parameters of a set of specific eigenmodes of a plate specimen. In combination with a numerical model for the calculation of the modal parameters of the specimen, all the independent complex moduli of the material are derived.

This paper presents some further developments and results of the mixed numerical/experimental method for material identification, presented at CADCOMP ’90 in Brussels (see proceedings, page 267).

This paper describes a new method for the characterization of the engineering constants of linear visco-elastic orthotropic materials.

General Lectures.- Identification of the Rigidities of Composite Systems by Mixed Numerical/Experimental Methods.- Measurement of Complex Moduli of Composite Materials and Discussion of some Results.- Moire Interferometry for Composites.- A Damage Approach for Composite Structures: Theory and Identification.- Contributions.- Identification.- Identification of Temperature Dependence for Orthotropic Material Moduli.- Experimental Identification of Complex Stiffnesses of Composite Materials by Ultrasonic Wave Propagation.- A Hybrid Method to Determine Material Parameters of Composites.- Non Determined Tests as a Way to Identify Wood Elastic Parameters: the Finite Element Approach.- Measurement of Laminate bending Elastic Parameters from Non-Uniform Strain Fields.- Identification of the Shear Properties of the Lamina in the Framework of Plasticity.- Parametric Identification of Mechanical Structures: General Aspects of the Methods used at the LMA.- Static and Dynamic Characterization.- Material Parameters in Anisotropic Plates.- Evaluation of Global Composite Laminate Stiffnesses by Structural Wave Propagation Experiments.- Dynamic Measurements of Elastic Properties of Filament-Wound Cylindrical Shells.- Torsion Response Analysis of T300/914 and T800/914 Unidirectional Specimens.- A Computerized Test Setup for the Determination of the In-Plane and Out-of-Plane Shear Modulus in Orthotropic Specimens.- Some Experience from the Application of the Iosipescu Shear Test.- Constitutive Prediction for a Non-linear Orthotropic Media.- Creep and Relaxation of Coated Fabrics under Biaxial Loading.- Joints and Assemblies of Composite Material Structural Elements: Characterization of adhesives in bulk form and in composite structures.- Static and Dynamic Characterization of Composites using a Fourth Generation Programming Language on a Macintosh.- Impact Behaviour and Damage Characterization.- Use of the Shadow Moire Technique for the Investigation of the Impact Behaviour of Composite laminates.- Delamination Detection via Holographic Interferometry Techniques.- Dynamic Failure Processes in Fibre-Reinforced Composites.- Effect of Strain Rate on the Compressive Strength of Woven Glass-Reinforced/Epoxy Laminates.- Characterization of Highly Anisotropically Reinforced Solids in High Velocity Tension.- Micromechanics and Interfaces.- Determination of the Effective Mechanical Response of Polymer Matrix Composites via Microstructural Data.- A microstructural Method for Predicting the Damping of Lamina.- Theoretical and Experimental Study Technique of the Mechanical Properties of Fiber-Reinforced Laminates and of the Strain Features of Structural Elements.- Dynamic Properties of Layer Reinforced by Two Families of Fibres.- Micromechanical Modelling of Unidirectional Glassfiber Reinforced Polyester: Effect of Matrix Shrinkage.- Plastic Failure of Unidirectional Fibrous Composite Materials with Metal Matrix in Compression.- Analytical Model of Fibre Pull-Out Mechanism.- Nonlinear Viscoelastic Behavior of the Fibre-Matrix Interphase: Theory and Experiment.- Empirical Model Building and Surface Response Methodology Applied to Sclerometrical Investigation for Refractories Concrete.- Fracture and Fatigue Damage Mechanics.- Development of Fatigue Damage Mechanics for Application to the Design of Structural Composite Components.- Influence of Fibre Orientation on the Cracking and Fracture Energy in Brittle Matrix Composites.- Application of the Method of Caustics to Fibre-Reinforced Materials.- Modelling of Orthotropic Layered Structures Containing Cracks in the Interfaces.- A Criterion of Mixed Mode Delamination Propagation in Composite Material.- Fracture and Fatigue Damage in a CSM Composite.- Damage Modelling and Non-Destructive Testing.- Non linear Behaviour of Ceramic Matrix Composites.- Notched Strength and Damage Mechanism of Laminated Composites with Circular Holes.- Damage Modeling and Experimental Tools for 3D Composites.- Increasing Energy Absorption by 'Rope Effect' in Brittle Matrix Composites.- Specific Damping of a Carbon/Epoxy Laminate under Cyclic Loading.- The Application of automated Ultrasonic Inspection for Damage Detection and Evaluation in Composite Materials.- The Use of Vibrothermography for Monitoring Damage Evolution in Composite Materials.- Acoustic Emission Source Location in Anisotropic Plates.- List of Participants.- Index of Contributors.

The simulation of a resin flow through a porous medium by FE-models has become a very important aspect for the design of a high-performance RTM produced composite part. The key parameter to perform RTM flow simulations is the fibre reinforcement permeability. Unfortunately, permeability measurements are not yet fully standardized and thus many different set-ups have been proposed. A major problem in comparing different set-ups has always been the fact that there exists no reliable reference material on which a comparative permeability measurement can be performed. This paper presents a solid test specimen, produced with a stereolithography technique, which can be used as a reference sample for calibration of test rigs and for comparison of results from different test rigs. Since the permeability properties of the specimen do not vary from test to test, an excellent repeatability of the experiments is obtained and any measured difference must be attributed to the set-up.

The resonalyser method is a material identification technique which is based on the measurement of resonance frequencies of freely suspended rectangular test plates, combined with numerical simulations. By adjusting the ratio of the width to the length of the test plate, the resonance frequencies can be made very sensitive for small variations of Poisson’s ratio. This study examines a fabric-reinforced composite material with a very small value of Poisson’s ratio. The material on which the experiments are performed is a carbon fabric-reinforced polyphenylene sulphide. The accurateness of the determined values of the in-plane elastic properties of the test plates is validated with static tensile tests. First, the four orthotropic elastic properties, Young’s moduli E11 and E22, the in-plane shear modulus G12 and Poisson’s ratio m12, are identified using the resonalyser technique. Next, the obtained values for Young’s moduli and Poisson’s ratio are validated with static uni-axial tests. It can be concluded that the results derived from both measurement methods corresponded very well. � 2007 Elsevier Ltd. All rights reserved.