L. Schillemans

The effects of motion on thermoelastic stress analysis

Abstract An indepth examination of the effects of specimen motion on the technique of thermoelastic stress analysis is presented. It is shown that the presence of a non-uniform temperature distribution, coupled with the material motion due to cyclic loading, can greatly corrupt the measured data. This is particularly important when analysing composite materials in which heat is generated due to viscoelastic processes and/or to internal friction at delamination sites. A series of experiments using delaminated composite specimens as well as an externally heated specimen are presented to illustrate this problem and a mathematical model is developed to account for this phenomenon. Methods to remedy this problem are suggested and in the case of symmetrical conditions, it is shown that this effect can be effectively removed by appropriate post-processing of the data.

Thermoelastic Stress Analysis of Fibre Reinforced Composite Systems

The thermoelastic effect first analysed by Lord Kelvin is governed by a simple relation between amplitude of temperature and amplitude of the sum of principle stress change as long as adiabatic conditions are maintained. Although valid for isotropic materials it is shown that this simple relation leads to very poor results for anisotropic materials. A non-adiabatic theory taking into account the interlaminar heat transfer shows a better agreement between theory and experiment.

Defect-identification in composite materials using pattern recognition techniques on ultrasonic data

One of the methods most widely used in non-destructive testing of materials is ultrasonics. However, in many applications the result is treated as a binary value (good or bad) regardless of the enormous amount of information embedded in an ultrasonic A-scan. A small number of users can go beyond this binary threshold by identifying the defect based on years of practical experience. However, when material properties differ from the isotropic ones, the complexity of the total system becomes such that identification by human judgment becomes practically impossible even though this information is available in the signal. Pattern recognition techniques applied on these ultrasonic signals have proved the ability to identify defects with a much lower error rate (compared with the statistical one) when no prior information is available. Thus providing a non-binary result by high-level expertise of the signal. At first, the time domain signal (A-scan) is digitized and stored. Based on these data, numerical algorit...

A Finite Element Approach for Thermal-Structural Response in Fibre Reinforced Composite Systems

This paper presents a finite element analysis of coupled and semi-coupled thermal structural response of composite laminated plates. Our purpose is to use the results in order to quantify and’ calibrate’ the SPATE ( Stress Pattern Analysis by Thermal Emission ) experimental procedure. SPATE uses the thermoelastic effect, first quantified by Lord Kelvin [1], in order to obtain the stress distribution in a structure. Unfortunately the application of SPATE as NDT method on composite materials has so far been limited. The main problem has been identified to be associated with the existence of true adiabatic conditions, therefore the valid application of Kelvin’s law and the specimen motion due to the cyclic loading, known as’ motion effect’ [2-4].

A Computerized Test Setup for the Determination of the In-Plane and Out-of-Plane Shear Modulus in Orthotropic Specimens

An automated torsion test is proposed for the determination of the in-plane and out-of-plane shear modulus. The obtained results show that for some composite systems the out-of-plane shear modulus is much different from the in-plane shear modulus, certainly sufficiently to be considered in some design application.

Comparative Study Between Ultrasonic Inspection (C-scan) and Vibrothermography (SPATE) for the Assessment of Damage in Carbon Fiber Reinforced Plastics (CFRP)

In today s research, damage analysis is the burning issue. A lot of papers are dedicated to this subject. In the present study we will compare two different methods to visualize damage and damage growth in composite materials. T h e first method is ultrasonic inspection which is a well established method to detect damage. The second uses the change of temperature as an indicator for damage development and damage accumulation. Results are given for bo th methods in a qualitative colour map. INTRODUCTION Fiber reinforced composite materials are being used increasingly for primary as well as secondary load bearing structures in the aerosnace and other advanced industries. The economic advantages of such high strength—low density materials coupled with the oppor tun i ty to tailor structures to suit specific requirements are well known. The stiffness and strength of these materials are usually not the limiting factors; rather the tolerance to damage and the evolution of properties over the life-span are not fully resolved. Because of the nonhomogeneous and anisotropic nature of fibre-reinforced materials the detection of damage or the assessment of the quality of the materials is not an easy problem to solve. A great number of mechanisms may occur. The most f requent ly observed are matrix crazing, transverse cracking, debonding and delamination. This complexity has given rise to many non-destructive control techniques. Among the many methods available, ultrasonic inspection is certainly one of the most widely used. Many of the inspection problems posed by composites can be dealt with effectively using ultrasonics. Besides ultrasonics, a relatively new technique, vibrothermography, has been proposed for application to composites as well as to the more usual engineering materials. This method uses a low amplitude vibration to induce localized heating in the material. The frequency of the vibration can range f rom 5 Hz up to 20 KHz. A high resolution infra-red detector together with a scanning mechanism records

The Torsional Fatigue Behaviour of the In-Plane Shear Modulus of Composite Materials

Static torsion tests and fatigue torsion tests are carried out on quasi-isotropic carbon epoxy laminates consisting of 16 plies with different stacking sequences in order to determine the influence of the stacking sequence on the value of G12. Therefore, an automated torsion setup developed by COSARGUB, was used. The results are compared to those obtained with CLT (Classical Laminate Theory). Based on the conclusions in [2], the axial displacement was also monitored during the fatigue tests because this parameter seems to be more sensitive than the evolution of G12 in order to describe the torsional fatigue behaviour.

Static and Dynamic Characterization of Composites, Using a Fourth Generation Programming Language on a Macintosh

This paper will present two applications of a fourth generation programming language, called Labview.

Interphase Region in Composite Systems: Analysis, Characterisation and Influence on Long Term Behaviour

In a composite continuum we have at least two distinct continua with thermomechanical characteristics available on macroscopic scale. Generally one of the continua has an outer surface with some measurable surface characteristics. The second phase, due to the presence of the first one, cannot fully develop his bulk properties during the processing in the region close to the geometrical interface. This “constraint”, together with some surface changes of the first phase, results in the existence of a third phase, the transition region between the two original continua or interphase region.

The Use of Vibrothermography for Monitoring Damage Evolution in Composite Materials

For the non-destructive evaluation of materials, several methods are well known, for instance the ultrasonic, the holographic, the radiographic method and others. For the visualization of stress concentrations on the other hand, Moire interferometry and photoelasticity are common practice. In the past few years however, a new method, called Stress Pattern Analysis by Thermal Emission (SPATE), is more and more known and used. A number of papers about the research on the value and interpretation of this method are already available but they are all especially concentrating on isotropic homogeneous materials. The basic thermoelastic theory for isotropic materials will be explained. However, because of the fact that the use of SPATE on composite materials has not been examined very well, the research being done at the Free University of Brussels is focusing on this domain. In this case, the deduction of the thermal emission — stress law must be reviewed and will be presented. A difficulty arises when at certain load levels, a phase shift is encountered between the upper and lower part of the hole. A basic theoretical explanation of this phenomenon will be put forward. Notification will be made that this is a general effect, valid for both isotropic and anisotropic materials. However, the magnitude of this effect is in general not noticeable on isotropic materials. On the other hand, this effect is in laminate materials enhanced and even of the same magnitude as the expected SPATE signal itself. As a result, SPATE scans of stressed laminated materials are much more difficult to interpret and evaluate correctly.