Michelle Salvia

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).

In situ study of the epoxy cure process using a fibre-optic sensor

Real-time, in situ monitoring for quality control of the polymer cure process is of great interest. In fact, high-performance composites, made with polymer matrixes, are used extensively in high-tech areas, such as the aircraft, space and automobile industries. In particular, epoxy resin reinforced with fibre is a system with good mechanical properties and low density. In this paper, a fibre-optic sensor to monitor the cure of an epoxy resin is studied. Optical fibres are, in fact, compatible with the reinforcing fibre in laminate composites. This sensor is based on the measurement of the angular distribution of light transmitted through an optical fibre inside the cured polymer. The light guiding properties of this sensor are linked to the difference between the refractive index of the core and that of the cladding. So, by partially removing the cladding and placing a sample of curing epoxy around the stripped region, it is possible to monitor the extent of the cure. In fact, the refractive index of the DGEBA/amine system varies from 1.54 to 1.58, mainly due to the increase in density during the cure. For manipulation, an experimental fibre made of special glass was prepared so as to have a high refractive index core (n = 1.62). The sensitivity to the refractive index was tested with aniline (n = 1.58) mixed with various concentrations of toluene (n = 1.50). The response of the sensor to this liquid test showed that it would be possible to measure the refractive index in the index range of epoxy during cure. Tests with an epoxy system were also performed. In order to understand the angular distribution of the transmitted light, a model based on geometrical optics was developed using the reflection coefficient of the optical ray on a boundary between two dielectrics. This allowed us to obtain real and imaginary parts of the cladding refractive index, thus the cross-link density during the epoxy cure can be monitored with a single fibre-optic sensor.

Comments on the physical basis of the active materials concept

During the last decade constant improvements have been made in materials and structures design and control. But now some performance objectives cannot be achieved using classical technologies and require the use of the smart materials concept. But it is at the actuation end of the equation that smart materials and structures present the greatest challenge. It is here in particular that improved and even new materials have a leading role to play. Piezoelectrics, electrostrictives, photostrictives, magnetostrictives, electroactive polymers, shape memory materials, carbon nanotubes, rheological fluids,...all have their important contributions to make. So this paper aims to perform a brief review of the physical basis of the active materials behaviour.

Surface morphology and wettability of sandblasted PEEK and its composites

PolyEtherEtherKetone (PEEK) is an advanced high-performance thermoplastic polymer, and its composites are used extensively in the aeronautical industry. This paper presents an experimental approach to determine the role of sandblasting treatment on surface morphology modifications of PEEK and its composites, with the aim of developing a topographic characterization in order to propose pertinent parameters that correlate with contact angles from wettability measurement. Sandblasting (fine abrasive particle projection) was selected as the surface treatment, in order to obtain various morphologically quasi-isotropic surfaces. Two surface metrological approaches to topographical characterization were used to correlate the wettability behavior with the surface roughness parameters, the first based on 2D profile analysis and the second on 3D topography analysis. Two different unreinforced grades of PEEK and four composites: discontinuous carbon fiber or glass fiber-reinforced, oriented, and unoriented, were studied. The experimental results indicated the sandblasting process duration necessary to reach a morphological steady state. It was stated that one of the pertinent parameters is the mean slope of roughness motif in 2D profile characterization, as confirmed by previous findings for anisotropic morphologies. However, for all cases, a new topographic parameter Sr , combining the surface amplitude and the summit density distribution, is proposed as a factor well-correlated with wettability characteristics.

Effects of Electron Beam Irradiation on Charpy Impact Value of GFRP

Electron beam irradiation has been reported to be effective for polymer surface modification and on strengthening of inorganic glass. In the present research effects of electron beam irradiation on impact value of glass fiber reinforced polymer (GFRP) were studied. The electron beam irradiation enhanced Charpy impact value at every fracture probability.

Role of fiber/matrix interphases on dielectric, friction, and mechanical properties of glass fiber-reinforced epoxy composites

This study compares the mechanical, tribological, and dielectric properties of glass fiber-reinforced epoxy (GFRE), presenting various fiber/matrix adhesions. Three GFRE composites were studied. The only difference between them is in the initial preparation of fibers which is intentionally simplified compared to the complex sizings used in industry. Thus, fibers treated with aminosilane or silicone coupling agents were compared with fibers simply washed with deionized water. The dielectric measurements show the leading role of interfacial bonding in the trapping or diffusion of electric charges. To obtain high mechanical properties, a fiber treatment that contributes to the diffusion of electric charges along the fiber/matrix interface is preferred. In addition, after friction, a modification of the dielectric properties in all materials is observed, and the trapping or the diffusion of the charges along the interfaces can make it possible to lower the friction coefficient, or the wear.

Health monitoring of smart composite structures using ultrasonic guided waves

The health of a structure depends on both the homogeneously distributed degradation of its mechanical properties during its life cycle and the presence of localised defects such as cracks or delaminations. The proposed non-destructive health monitoring method allows to recover both kinds of information using ultrasonic waves. To avoid traditional techniques limitations, such as coupling reproducibility for instance, we propose here to integrate a piezoelectric element into the plate-like composite structure. The element dimensions are determined in order to uncouple the frequency ranges of the thickness and radial vibration modes. The thickness mode is used to monitor the homogeneous ageing of the structure through electrical impedance measurement. As for the radial vibrations, they are used to generate and detect Lamb waves, which have the advantage of propagating over long distances and offering specific sensitivity of various modes to different kinds of defects. The present work focuses on this last application and studies the ability of the proposed technique to detect and identify defects such as low speed impact-induced delaminations and cracks incomposite plate-like structures.

Effect of Fiber Treatment on Fiber Strength and Fiber/Matrix Interface of Hemp Reinforced Polypropylene Composites

The use of natural fibers as reinforcement in composites is emerging. Several studies are underway to improve the mechanical characteristics of these fibers and its matrix interface properties for better load transfer. However, the treatments generally used are relatively expensive and complicated to apply. This work deals with the effect of new Fibroline process on tensile and interfacial properties of hemp fiber reinforced in polypropylene. Fibroline is a dry powder impregnation method which consists of submitting fibers and polymer powder under strong alternating electric field. Morphology and tensile properties of hemp fibers after different surface treatments (raw, dried, raw and Fibroline-treated, dried and Fibroline-treated) are evaluated. Interface properties of treated hemp fibers on polypropylene matrix are then characterized by fragmentation test of monofilament composites. Results showed the Fibroline treatment reduces the fiber mechanical properties but improves the load transfer efficiency due to random generation of surface cracks and better fiber/matrix adherence, respectively. For the case of dried and Fibroline-treated hemp fibers, large decrease in mechanical and interfacial properties was observed.

Carbon fibres: sensor components for smart materials

Recently, smart composites have appeared as new materials. But, these materials require the integration of functional elements within the composite structure which generates some problems. This paper demonstrates how carbon fibers are not only the reinforcement but also the sensor to detect in-situ damages in CFRP laminates. (+/- 45 degree(s)) laminates were subjected to monotonic and creep tests under three-point bending conditions. The in-situ monitoring of electrical resistance variation and AE activity allowed the detection and the identification, at different stages, of various types of damage mechanisms (not only fiber fractures but also matrix cracks, debondings and delaminations).

Local strain and damage measurements on a composite with digital image correlation and acoustic emission

Non-linear stress–strain behaviour under monotonic loading can be caused by continuous damage. In order to understand this non-linearity, simultaneous strain and damage measurements were taken on composite specimens using digital image correlation and acoustic emission as these techniques give valuable local information. The latter is essential in the case of inhomogeneous or anisotropic materials, such as continuous fibre composites. On the one hand, digital image correlation gives access to full field strain and on the other hand, acoustic emission recording can be used for damage monitoring and location if, at least two sensors are placed on the specimen under loading. In this work, these two techniques were combined to correlate strain measurements and damage location on a complex composite material during a monotonic tensile test. The composite is a continuous fibre-reinforced friction material used in car clutches. These measures were used to understand the non-linearity of the stress–strain curve of the as-received material as well as detect volume damage after thermal cycling. In depth study of the strain field and acoustic emission events location revealed a correlation between non-homogeneous damage kinetics throughout the specimen and the evolution of the strain distribution.