Bayesian estimation of defect patterns in composite materials using through-thickness dielectric measurements

A non-destructive testing method using ultrasonic waves for composite materials has been studied. In this paper, a computer program of three dimensional image processing was developed to evaluate the defects in composite materials. The developed computer program has two particular functions, which are smoothing of scan data and decision of defect size. The size of defect was obtained by a gradient of echo level. As the examples, this method was applied to GFRP and CFRP laminates with artificial defects, and the effect of processing on the size of defects was investigated. As a result, the shape, size and location of defects in composite materials were obtained by the three dimensional image processing. It is also revealed that the detectability of defects is improved by this method. From these results, it can be shown that the method proposed in this paper is very useful to evaluate defects in composite materials.

A technique for an integrated study of defects in composite materials using brittle strain-sensitive coatings and acoustic emission is described. Strain-sensitive coatings make it possible to detect hidden defects that affect the stress–strain state of the product at the early stages of loading. Acoustic emission monitoring conducted during further loading provides a more accurate determination of the position of potentially dangerous defects. The technique is optimized using flat samples with a central hole simulating a defect under conditions of uniaxial tension.

With the increasing applications of composite materials in various engineering structures, failure analysis on crack induced fracture problems in composite materials has attracted more and more attention among researchers. In the current work, with the plastic zone corrections, the elastic-plastic fracture behavior investigations on crack problems in different configurations have been carried out. Two approaches of plastic zone correction are demonstrated. The first one is the mixed-mode Dugdale model for interface cracks and the second one is the generalized Irwin model which is suitable for matrix cracking. The introduction of plastic zone can greatly improve the accuracy of fracture prediction in fiber-reinforced composite materials with ductile matrix. Based on the two models, the problem of a crack lying at the interface of two semi-infinite plate, and crack-coated inclusion interactions have been studied. Numerical examples are given to show the influence of different kinds of parameters on the plastic zone size and crack tip opening displacement of each problems. Some useful conclusions have been figured out after the investigation.

Lock-in thermography represents a valid and consolidated techniques capable of detecting and quantifying defects in composite materials. This technique in industry is still not widespread especially for large structures, mainly because of the overall lengthy time required for testing due to the necessary scanning approach.

Key stages of adhesively bonded repairs

In this work microwave sources were used as heating sources to evaluate the damages areas in composite materials using thermographic techniques. In particular, lock-in thermography and microwave lock-in thermography tests were carried out on CFRP damaged specimens. A new data analysis was developed to processing the thermographic data obtained by microwave tests and various algorithm were used to processing lock-in thermography tests. Data obtained were compared with non destructive tests performed with other techniques such as x-ray. A numerical model was developed for microwave heating simulation.

The problem of inspecting composite materials to detect internal defects is felt in many industrial contexts both for quality controls through production lines and for maintenance operations during in-service inspections. The analysis of the internal defects (not detectable by a visual inspection) is a difficult task unless invasive techniques are applied. For this reason in the last years there has been an increasing interest for the development of low cost non-destructive inspection techniques that can be applied during normal routine tests without damaging materials but also with automatic analysis tools. In this paper we have addressed the problem of developing an automatic signal processing system that analyzes the time/space variations in a sequence of thermographic images and allows the identification of internal defects in composite materials that otherwise could not be detected. First of all a preprocessing technique was applied to the time /space signals to extract significant information, then an unsupervised classifier was used to extract uniform classes that characterize a range of internal defects. The experimental results demonstrate the ability of the method to recognize different regions containing several types defects.

In a field operation that uses coiled tubing in its applications, fibre-reinforced polymer matrix composite tubing is seldom used. Fibre-composite coiled tubes offer advantages, compared to steel material, through a reduction in weight and improvement in fatigue life. The stiffness of composite material degrades progressively when increasing the number of cyclic loading. The fatigue damage and failure criteria of fibre-reinforced composite coiled tubes are more complex than that of steel; hence, failure predictions are somewhat unreliable. Among the defects in composite materials, interlaminar delamination is the foremost problem in fibre-reinforced composite material, and it leads to a reduction in strength and stiffness especially in cyclic-load conditions. Delamination causes a redistribution of the load path along the composite structure, which is unpredictable; therefore, delamination in a composite coiled tube in an oil and gas field eventually leads to final failure, which could be catastrophic. A-ply-by-ply mathematical modelling and numerical simulation method was developed to predict interlaminar delamination of filament-wound composite coiled tubes under a combination of different loading scenarios with consideration to low-cycle fatigue. The objective of this paper is to explain interlaminar delamination as an initial crack and source of stress concentration in composite coiled tubes in the framework of meso-cracking progression of matrix damage modelling of composite laminates. The paper focuses on delamination failure because the largest span of the composite lifecycle is at the crack propagation phase, which manifests itself in the form of delamination. The analysis shows that the crack front tip is not uniform, and also shows that carbon fibre possesses higher stiffness values compared to glass fibre. The paper confirms that 2D modelling cannot express the real release strain energy rate at the crack front tip. Mode-I testing, however, showed that the double cantilever beam (DCB) only represents the normal stress from the release strain energy rate. The results also indicated that there were other sources contributing to the strain energy release rate, such as inter-layer frictions and normal stress in the end notched flexure (ENF) testing mode.

The paper presents a theoretical and experimental study of the influence of damage accumulation in composite material on the elastic fourth-rank tensor and on the strength under quasi-static, cyclic and dynamic loads. Layered polymer composites (glass or carbon fiber-reinforced plastic) are considered. This work aims to develop a mathematical model that accounts for the impact of damage in the composite material on the fourth-rank tensor of elastic properties. The constitutive relations for an orthotropic composite material are proposed, taking into account the accumulation of damage during tensile or shear loading. A numerical simulation using the finite element method is done for a periodicity cell for composite variants (unidirectional, layered, etc.) to determine the effective elastic properties. Quasi-static loading experiments were conducted on composite samples (carbon plastic) to validate the model and the degradation of elastic properties was determined by measuring the longitudinal sound speed. The full-scale experiments confirmed that damage accumulation in carbon fiber-reinforced plastic leads to the degradation of its effective elastic properties. In this connection, a technique was developed to take into account irreversible damage in an orthotropic composite material through the components of the ductility tensor, which may be helpful for the design and analysis of composite structures. The findings of the study could help with the creation of new materials with enhanced mechanical characteristics, as well as with raising the quality of already-existing materials. The developed mathematical model of orthotropic composite material can be used to enhance the structural strength properties and boost the application safety in a variety of industry.

Processing of thermal images for the detection and enhancement of subsurface flaws in composite materials : Maldague, X.; Krapex, J.C.; Cielo, P.; Poussart, D. Signal Processing and Pattern Recognition in Nondestructive Evaluation of Materials, Proceedings of the NATO Advanced Research Workshop, Lac Beauport, Quebec, Canada, 19–22 Aug. 1987. Edited by C.M. Chen, pp. 257–286. Springer-Verlag (1988)

A focusing transducer utilizing trapped energy modes can be easily fabricated by plating a number of concentric rings of electrodes on a suitable piezoelectric plate of uniform thickness. The concentric ring structure acts as a Fresnel lens and can be used to obtain excellent lateral focusing of ultrasonic waves. Several transducers operating in the 2–5 MHz range have been produced using PZT-7A as the piezoelectric material. The near field radiation pattern has been observed directly in front of the plate using the first order diffraction peak from a laser probe. It is confirmed that there is good acoustic isolation between the rings and that they radiate in the trapped energy mode. The ultrasonic radiation pattern of the Fresnel lens has been observed at various distances from the plate and compared with computed results which show excellent agreement. Furthermore, the axial diffraction pattern of the lens can be optimized by adjusting ring spacings. During the course of this study a new, previously unreported mode of energy trapping has been discovered. The explanation for this method of trapping is briefly discussed. Finally an acoustic through-transmission imaging system incorporating one focusing transducer is used for imaging of flaws in composite materials.

Processing of thermal image for the detection and enhancement of subsurface flaws in composite materials : Maldague, X.; Krapez, J.C.; Cielo, P.; Poussart, D. Signal Processing and Pattern Recognition in Nondestructive Evaluation of Materials, Proceedings of the NATO Advanced Research Workshop, Lac Beauport, Quebec (Canada) 19–22 Aug. 1987, pp. 257–286 Springer Verlag, Berlin Heidelberg (1988)

Thermal Signal Reconstruction (TSR) is a well-known nondestructive evaluation technique. It enhances the locations of defects in composite materials in aerospace industry, and has been applied in infrared imaging of human hands recently to locate microvasculature under the skin. In this work we reinvestigated the new application and explored the relationship between the temporal resolutions in TSR and the quality of reconstructed images. The goal is to optimize the variable to improve the detection rate of microvasculature under difficult situations.

Ultrasound is a common tool in the nondestructive evaluation of composite material integrity. Echoes from high frequency (5-10 MHz) sound waves vary with subsurface flaws and delaminations. To improve detection of internal defects in composite materials, a linear Bayes classification is applied to the wavelet transform coefficients of ultrasound scan lines. Using a Daubechies basis, wavelet transforms are taken of the ultrasound signals. A subset of the coefficients is then used as features for the classifier. A forward sequential feature selection (FSFS) algorithm was used to determine the optimal features. The training set was comprised of scanned signals from both damaged and undamaged samples. Performance statistics for classification of damaged materials were calculated using a separate set of test samples. Application of a shift-invariant wavelet transform removed some of the variability of the wavelet coefficients and improved the classification.

Manufacture and in-service assessment of composite materials can be challenging since there is, yet, no standard method for testing them. Nevertheless, regular inspection is necessary to maintain the structural integrity. This paper describes a nondestructive, broadband noise mapping method that uses a millimeter-wave radiometer to detect defects in composite materials. The $W$ -band system is configured for transmission mode measurement in which an amplified photonic source illuminates a glass fiber reinforced polymer containing machined defects. A high sensitivity radiometer is used for imaging the sample. The radiometer consists of a Schottky diode-based frequency tripler and a fundamental mixer.