Delamination Detection By Thermography (original) (raw)
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A review of thermographic techniques for damage investigation in composites
Frattura ed Integrità Strutturale
The aim of this work is a review of scientific results in the literature, related to the application of thermographic techniques to composite materials. Thermography is the analysis of the surface temperature of a body by infrared rays detection via a thermal-camera. The use of this technique is mainly based on the modification of the surface temperature of a material, when it is stimulated by means of a thermal or mechanical external source. The presence of defects, in fact, induces a localized variation in its temperature distribution and, then, the measured values of the surface temperature can be used to localize and evaluate the dimensions and the evolution of defects. In the past, many applications of thermography were proposed on homogeneous materials, but only recently this technique has also been extended to composites. In this work several applications of thermography to fibres reinforced plastics are presented. Thermographic measurements are performed on the surface of the specimens, while undergoing static and dynamic tensile loading. The joint analysis of thermal and mechanical data allows one to assess the damage evolution and to study the damage phenomenon from both mechanical and energetic viewpoints. In particular, one of the main issues is to obtain information about the fatigue behaviour of composite materials, by following an approach successfully applied to homogenous materials. This approach is based on the application of infrared thermography on specimens subjected to static or stepwise dynamic loadings and on the definition of a damage stress, D , that is correlated to the fatigue strength of the material. A wide series of experimental fatigue tests has been carried out to verify if the value of the damage stress, D , is correlated with the fatigue strength of the material. The agreement between the different values is good, showing the reliability of the presented thermographic techniques, to the study of composite damage and their fatigue behaviour.
A review of thermographic techniques for damage investigation in composites.PDF
The aim of this work is a review of scientific results in the literature, related to the application of thermographic techniques to composite materials. Thermography is the analysis of the surface temperature of a body by infrared rays detection via a thermal-camera. The use of this technique is mainly based on the modification of the surface temperature of a material, when it is stimulated by means of a thermal or mechanical external source. The presence of defects, in fact, induces a localized variation in its temperature distribution and, then, the measured values of the surface temperature can be used to localize and evaluate the dimensions and the evolution of defects. In the past, many applications of thermography were proposed on homogeneous materials, but only recently this technique has also been extended to composites. In this work several applications of thermography to fibres reinforced plastics are presented. Thermographic measurements are performed on the surface of the specimens, while undergoing static and dynamic tensile loading. The joint analysis of thermal and mechanical data allows one to assess the damage evolution and to study the damage phenomenon from both mechanical and energetic viewpoints. In particular, one of the main issues is to obtain information about the fatigue behaviour of composite materials, by following an approach successfully applied to homogenous materials. This approach is based on the application of infrared thermography on specimens subjected to static or stepwise dynamic loadings and on the definition of a damage stress, D , that is correlated to the fatigue strength of the material. A wide series of experimental fatigue tests has been carried out to verify if the value of the damage stress, D , is correlated with the fatigue strength of the material. The agreement between the different values is good, showing the reliability of the presented thermographic techniques, to the study of composite damage and their fatigue behaviour.
Damage assessment of composite materials by means of thermographic analyses
Composites Part B: Engineering, 2013
Composite materials are largely used for structural applications, thanks to their high strength-to-weight ratios. However, it is difficult to make accurate estimations on their mechanical behavior, as it is affected by several factors, involved both in the manufacturing process and in the experimental testing. In this study, GFRP laminates, with different stacking sequences, are tested under static loading conditions. During testing, thermal analyses are also performed by means of a thermal camera, obtaining an energetic parameter (i.e. the temperature) useful for the evaluation of damage. The thermographic method allows both qualitative and quantitative analyses to be performed in a relatively short time. Besides thermal analyses, damage is also assessed by means of static tests, interrupted at different load levels, and followed by stiffness reduction measurements and microscopic analyses, allowing for a comparison of the obtained results.
Composite Structures
Impact induced combined damage in composite laminates attracts great attention due to its significant degradation of the structural integrity. However, the provision of the quantitative analysis of each damage portion is challenging due to its bare visibility and structural mixture complexity, so-called barely visible impact damage (BVID), which is referred to as inter-laminar delamination, and is inherently coupled with in-plane transverse and matrix damage also known as combined damage. Instead of focusing on one type of damage in most of the existing studies, this paper proposes a decomposition and targeted enhancement technique based on Stationary Wavelet Transform (SWT) for such coupled BVID in composite laminates using laser-line scanning thermography. Firstly, a combined damage model composed of in-plane damage and inter-laminar delamination is established by finite element numerical modelling to predict the thermal response pattern in the laser scanning thermography. Then, a feature separation and targeted enhancement strategy based on SWT in the frequency domain is proposed to improve the contrast of the matrix crack and delamination in combined damage scenarios induced by low-velocity rigid impact via drop-tower tests, meanwhile eliminating noise and suppressing the laser pattern background. The enhanced images of in-plane damage and delamination are furtherly processed by Random Sample Consensus (RANSAC) method and confidence map algorithms to calibrate the damage profile. The proposed technique is validated through inspecting a group of unidirectional carbon fibre-reinforced polymer composite samples, impacted by a variety of energy levels, in fibre-parallel (0°), 45°and orthogonal scanning modes. The results demonstrate that the proposed technique can pertinently isolate, enhance and characterise the inspected in-plane crack and inter-laminates delamination in a flexible manner. The proposed methodology paves the way towards automated infrared thermography data analysis for quantitative dissection of actual combined damage in composite laminates.
Damage detection on composite materials with active thermography and digital image processing
This research is focused on the use of active infrared thermography as a non-destructive testing technique for damage detection in carbon fiber reinforced plastics (CFRPs). The aim of this study is to examine the efficiency of various mathematical methods in thermographic data processing, with respect to the thermal excitation method and the type of artificial defect in the CFRP specimens. We applied two techniques of active infrared thermography to CFRP samples with artificial cracks and internal de-laminations at known locations. An infrared camera recorded the temperature field and generated a sequence of thermal images. To reveal the defects of the CFRP laminate, the thermograms were processed (a) as 2D images, and (b) as if each pixel was a 1D signal over time. We present representative experimental results, which illustrate that the depiction of the norm of the 1st spatial derivative of temperature and the 2D wavelet transforms proved to be most efficient for crack detection, whereas the 1D Fourier and 1D wavelet transforms did not yield clear results. In contrast, delamination damages could be identified through 1D techniques because the 1D Fourier transform as well as the 1D wavelet transform were very accurate.
Fiber optic thermal detection of composite delaminations
Thermosense: Thermal Infrared Applications XXXIII, 2011
A recently developed technique is presented for thermographic detection of delaminations in composites by performing temperature measurements with fiber optic Bragg gratings. A single optical fiber with multiple Bragg gratings employed as surface temperature sensors was bonded to the surface of a composite with subsurface defects. The investigated structure was a 10-ply composite specimen with prefabricated delaminations of various sizes and depths. Both during and following the application of a thermal heat flux to the surface, the individual Bragg grating sensors measured the temporal and spatial temperature variations. The data obtained from grating sensors were analyzed with thermal modeling techniques of conventional thermography to reveal particular characteristics of the interested areas. Results were compared and found to be consistent with the calculations using numerical simulation techniques. Also discussed are methods including various heating sources and patterns, and their limitations for performing in-situ structural health monitoring.
Thermographic Depth Profiling of Delaminations in Composites
AIP Conference Proceedings, 2003
A method for determining the depth of delaminations in composite specimens is presented. The method is based on a one-dimensional model for a composite with a delamination that is represented as a contact resistance between the upper and lower regions of the specimen. To estimate the depth and contact resistance of a delamination in an efficient method, a method is used that makes use of a eigenvector representation of the measured and theoretical data. The technique is shown to give estimates of manufactured delamination depths that are in good agreement with the specified depths.
Thermographic Inspection of Composite Materials
2006
Thermography is a non-contact NDT technique for inspection of materials in wide application areas, including corrosion detection in metals, and delamination, porosity and moisture detection in composite materials. Composites often are highly anisotropic in nature. This anisotropy coupled with low thermal diffusivity in thickness direction, severely restricts detection of deeper defects in composite materials. In the present work, a systematic methodology has been presented for detection of defects in composite materials. The material under consideration is carbon-epoxy, which has high thermal anisotropy because of carbon fibers. Carbon-epoxy laminates with Teflon inserts of varying depth were fabricated for experimental studies. Both one sided and two sided test methods were explored. Based on numerical solution of the heat conduction problem, the optimum combination of heat flux, heating time and observation time for ‘best defect detectibility’ was worked out in each case, assuming...
Proceedings
Mechanical stresses of materials are generally coupled with temperature variations and then, monitoring such variations can help gaining information about the material behavior under the applied loads. This can be accomplished with an infrared imaging device, which can be advantageously exploited to sense the thermal radiation associated with mechanical stresses and to obtain a legible explicative temperature map. In the present paper, glass/epoxy is used as material case study to show that thermal signatures visualized during the load application can be decoded into knowledge, which can contribute to the material characterization. In particular, glass/epoxy specimens are subjected to three types of tests: cantilever beam alternate bending, quasi-static bending and low velocity impact. Thermal images are acquired in time sequence during each test and after post-processed and analyzed. It is possible to get data about the damage initiation and its evolution under either quasi-static ...
Geometrical Limitations to Detection of Defects in Composites by Means of Infrared Thermography
Journal of Nondestructive Evaluation, 2004
The aim of the present experimental study is to gain information about limits in detection of defects in composites by infrared thermography. Specimens are manufactured of either carbon/epoxy, or glass/epoxy, and with inclusions of foreign materials to simulate defects of different size and positioned at different depths. Tests are carried out by using both pulse and lockin techniques. Results are presented in terms of difference of temperature (pulse), or difference of phase angle (lockin), between damaged zones and sound material. It seems that, apart from diameter and depth, the thickness is very crucial for defects visibility.