A. Chateauminois

In situ detection of damage in CFRP laminates by electrical resistance measurements

This work deals with the possibility of in situ detection of damage in unidirectional CFRP by means of electric resistance measurements. In the first stage, the conducting paths in unloaded specimens were investigated by changing the electrode location. Results showed that conduction occurred both along the fibre and in the transverse direction by virtue of fibre-to-fibre contacts. Monotonic and cyclic flexural tests were subsequently carried out with unidirectional carbon/epoxy laminates of different fibre volume fractions. Owing to the modifications of the conduction paths during the development of damage, strong changes in the resistance were measured. This method proved to be suitable for the detection of very low damage levels in terms of fibre breaks.

In-situ monitoring of damage in CFRP laminates by means of AC and DC measurements

Abstract Periodical maintenance NDT-based inspections are generally used for almost all complex technological structures today. The idea of using specific sensors integrated in the structure for structural real time monitoring (or health monitoring) is now in progress. Carbon-fibre-reinforced polymers (CFRP) are increasingly used as structural parts, especially in the aircraft industry. This work deals with the possibility of detecting in-situ damage in CFRP by the use of DC or AC electrical property measurements. Monotonic tests under post-buckling bending conditions are being performed on cross ply [0/90] s and [90/0] s carbon/epoxy laminates. The monitoring of electrical resistance and capacitance changes, linked to the modifications of the conduction paths in the composite, and occurrence of voids during loading, allowed the detection of damage growth. It seems that DC electrical conduction allows to detect fibre failure, but AC measurements are more suitable for monitoring matrix cracks (delamination, fibre/matrix debonding or transverse cracks).

Fretting wear behaviour of polymethylmethacrylate under linear motions and torsional contact conditions

The fretting wear behaviour of PMMA against a rigid counterface has been investigated under various contact zone kinematic conditions. A specific device has been used in order to achieve load axis spin or stationary rolling motions in a contact between a PMMA flat and a steel ball. Wear processes under such conditions have been investigated by means of laser profilometry and in-situ optical observations of the contact area during tests. Very different wear patterns were produced depending on the contact kinematics. For stationary rolling conditions, the progressive accumulation and compaction of debris induced the formation of a single ripple located in the middle of the contact. Very little debris was found to be eliminated from the contact and the resulting wear was quite low. On the other hand, little accumulation of debris was observed for torsional contact conditions and the wear was drastically enhanced. These results are analysed by considering the effects of contact zone kinematics on particle detachment and third body elimination.

Interactions between hygrothermal ageing and fatigue damage in unidirectional glass/epoxy composites

Abstract This study was directed toward the analysis of the effects of hygrothermal ageing on the fatigue behaviour of a unidirectional glass/epoxy composite. In the first stage, the fatigue properties of the unaged composite were studied in various environments involving moisture and temperature. Local interactions between the surrounding moisture and the crack tip were found to induce significant losses in lifetimes at the most elevated temperature (70 °C). In the second stage, fatigue properties were investigated after a preliminary ageing step. Hygrothermal defects created in the bulk composite during the water-sorption step induced a significant decrease in fatigue properties, especially after ageing in immersion. These overall losses of properties were linked to the drop in the statistical distribution of the fibre strength after ageing. The latter was measured by means of a micro-mechanical analysis of the first fibre breaks during mechanical loading. This analysis was performed in situ, i.e. in the real physico-chemical environment encountered by the glass fibres during ageing. A strong correlation was found between the physico-chemical degradation of the matrix and the fibre weakening.

Quantitative analysis of the micro-indentation behaviour of fibre-reinforced composites: development and validation of an analytical model

Abstract A shear-lag model has been developed in order to assess interfacial shear strength from micro-indentation experiments. The treatment of the experimental load/displacement curves is based upon the quantification of the two components of the indenter displacement, i.e. (i) the elasto-plastic indentation of the fibre surface by the Vickers indenter, and (ii) the displacement of the fibre surface due to its compression and to the deflection of the matrix. The latter component has been modelled by using an analytical shear-lag model which takes into account both fibre/matrix debonding and fibre sliding in the debonded areas. Some of the basic hypothesis of this analytical approach have been checked by finite-element (FE) simulations with appropriate model configurations. By means of this model, the critical shear debonding stress can be obtained from the experimental indentation curves. Moreover, the data-reduction scheme takes into account separately the local environment of each indented fibre. This method has been successfully applied to experimental data by using a unidirectional E-glass/epoxy composite.

Application of a stress-corrosion-cracking model to an analysis of the durability of glass/epoxy composites in wet environments

A stress-corrosion-cracking (S.C.C.) model has been applied to an analysis of the flexural fatigue behaviour of water-aged unidirectional glass/epoxy composites. The approach has been restricted to the initial stages of the fatigue life, i.e. when the damage consists mainly in the accumulation of broken glass fibres at the microscopic level. The fatigue behaviour of the aged material has been investigated at different strain levels, frequencies and strain ratios. The results demonstrate that the stiffness loss exhibits some of the main characteristic features which can be deduced from the delayed failure of a statistical population of glass fibres by a S.C.C. mechanism. This approach was supported by microscope observations which revealed that, within the investigated time range, the stiffness loss is proportional to the density of broken fibres in the vicinity of the loading point.

Experimental and numerical investigation of the sliding behaviour in a fretting contact between poly (methylmethacrylate) and a glass counterface

Abstract The fretting conditions in a contact between poly (methylmethacrylate) and a rigid counterface have been investigated using both experiments and finite elements computations. The computation of the microdisplacements in the contact area during a tangential loading allowed the determination of the critical displacement for transition from partial sliding to gross sliding conditions. These conditions were mapped in friction maps as a function of the contact loading parameters (i.e., normal load and displacement amplitude) and the friction coefficient. This analysis was performed assuming that the polymer behaved elastically and that the friction obeyed Coulombs law. Experimental results were found to be in accordance with the numerical predictions, despite the fact that some plastic deformation of the polymer surface occurred during tangential loading.

Analysis of Fretting Damage in Polymers by Means of Fretting Maps

The features of the fretting-wear behavior of glassy polymers against rigid counterfaces are reviewed through fretting maps concepts. The suitability of an elastic analysis to describe the fretting conditions in Running Condition Fretting Maps is discussed on the basis of numerical and experimental investigations of the contact zone kinematics. Material response in terms of cracking is subsequently analyzed in relation to the contact loading and to the plain fatigue properties of the polymer.

Surface damage of poly(methylmethacrylate) under fretting loading

The initial fretting damage in a glass/PMMA contact was investigated by means of experiments and numerical (F.E.M.) simulations. Both micro-crack nucleation at the contact edges and particle detachment were identified on the PMMAs surface. Micro-crack initiation was related to the combination of high tensile stresses and positive hydrostatic pressures which are known to enhance crazing. During the early stages of the fretting tests, the distribution of the detached particles within the contact was correlated to the spatial distribution of the cumulative interfacial energy dissipated by friction. As the number of cycles was increased, it was observed that detached particles moved toward the middle of the contact. On the basis of FEM simulations, this particle displacement within the contact was attributed to the existence of differential micro-displacements during the fretting cycle.

Development of a micro-indentation model simulating different mechanical responses of the fibre/matrix interface

An analytical shear-lag model has been developed for quantifying the interfacial shear strength of glass-reinforced composites from micro-indentation experiments. The model takes into account the local fibre environment, together with the occurrence of debonding and fibre sliding. In order to simulate the experimental indentation curves, various interfacial laws have been implemented. In a first approach, it was assumed that the shear stress in the debonded part of the interface was constant and proportional to the debonding stress. A more refined generalised interface law relating the shear stress to the fibre displacement was subsequently introduced to describe a progressive transition from an adhesive to a sliding state. The model has been successfully applied to the analysis of experimental reduced indentation curves giving the displacement of the fibre surface as a function of the applied load.