J. C. Krapez

Thermographic Nondestructive Evaluation: Data Inversion Procedures Part II: 2-D Analysis and Experimental Results

Abstract In Part I of this paper, a one-dimensional procedure was described for nondestructive thermographic evaluation. In Part II, a two-dimensional approach is presented to retrieve the subsurface defect depth, lateral size, and thermal resistance from the time-variable surface temperature distribution as recorded by a thermographic camera. The algorithm performance is tested on experimental data obtained under transient lateral-scanning surface heating conditions on a variety of artificial defects including bottom-open cavities of different geometries as well as TeflonTM implants in CFRP plates. In general, the defect depth and lateral size are inferred with satisfactory accuracy. The thermal resistance inferred through our inversion routine in the artificial delamination case was always higher than the incremental resistance due to the implants, suggesting the presence of resistive interfaces between the inclusions and the host material. Assuming a constant relative contrast error, the precision on t...

Thermographic Nondestructive Evaluation: Data Inversion Procedures Part I: 1-D Analysis

Abstract A data inversion procedure is presented, for the one-dimensional case, to retrieve the subsurface defect depth and thermal resistance from the time evolution of the surface temperature distribution as recorded by a thermographic camera (thermal “defectometry”). The inversion algorithm is based on the global minimization of a specific functional (nonlinear least squares problem) through an iterational routine taking into account simultaneously all of the defect variables and their effects on the values of the observable parameters. Surface losses are included as an unknown Biot number in the inversion routine. This paper deals with one-dimensional thermal flow. Part II, in this same issue, covers the two-dimensional theory and the experimental results.

Optics-Based Techniques for the Characterization of Composites and Ceramics

The needs for on-line and unsupervised industrial inspection for quality control and process monitoring are rapidly increasing with the accelerated trend toward factory automation. Optical techniques are particularly well suited for industrial inspection needs because of their noncontact nature and imaging capabilities.

Subsurface flaw detection in reflective materials by thermal transfer imaging

In this paper a thermal imaging apparatus is described for the nondestructive detection of subsurface defects in materials that would not usually lend themselves to thermal imaging because of their low emissivity and high susceptibility to background reflection noise. This is accomplished by transferring the thermal image produced by surface temperature perturbation of the workpiece material to a high emissivity material with which it is continuously brought in contact. The transferred thermal image may be observed by a suitable infrared device, resulting in a high radiance image with minimum reflectivity or variable emissivity noise. Numerical simulations, as well as experimental results, are presented.

Temperature Recovery and Contrast Computations in NDE Thermographic Imaging Systems

Thermographic imaging systems find application in many nondestructive evaluation problems. However, because of the inherent nature of the image-formation process, several degradations arise which preclude a more generalized use of thermographic NDE. In this paper, we analyze radiometric and noise degradations and propose a calibration process for the imaging systems which allows recovery of object surface temperature evolution. Moreover, a constrast computation technique which combines both the time and spatial reference techniques is shown to increase the defect visibility and make possible a more quantitative (sizing) defect evaluation.

Performance modeling and assessment of infrared-sensors applicable for TALOS project UGV as a function of target/background and environmental conditions

TALOS (Transportable and Autonomous Land bOrder Surveillance system - www.talos-border.eu) is an international research project co-funded from EU 7th Framework Program funds in Security priority. The main objective of TALOS project is to develop and field test the innovative concept of a mobile, autonomous system for protecting European land borders. Unmanned Ground Vehicles (UGVs) are major components of TALOS project. The UGVs will be equipped with long range radar for detection of moving vehicle and people, as well as long focal length EO/IR sensors allowing the operator to recognize and identify the detected objects of interest. Furthermore medium focal length IR sensors are used to allow the operator to drive the UGV. Those sensors must fulfill mission requirements for extremely various environmental conditions (backgrounds, topographic characteristics, climatic conditions, weather conditions) existing from Finland in the North and Bulgaria / Turkey in the South of Europe. An infrared sensor performance model was developed at ONERA in order to evaluate target detection, recognition and identification range for several simulations cases representative of the whole environmental variability domain. Results analysis allows assessing the operability domain of the infrared sensors. This paper presents the infrared sensor performance evaluation methodology and the synthesis of a large number of simulation results applied to two infrared sensors of interest: a medium / long range cooled MWIR sensor for observation and a short / medium uncooled LWIR sensor for navigation.

Conical-cavity fiber optic sensor for temperature measurement in a steel furnace

An integrating-cavity sensor for temperature measurement of the steel sheet in a continuous annealing furnace is described. The sensor includes a fiber optic cable for transmission of the infrared radiation from the sensor head within the furnace to the double-wavelength detection unit located outside the furnace. The sensor head includes a reflecting conical cavity positioned above the steel sheet to increase, and thus stabilize, the effective infrared emissivity of the sheet surface as well as to reduce spurious reflections from other sources within the furnace. The fiber optic cable is both decentered and tilted with respect to the cavity axis to minimize straight-reflection losses through the fiber cable gate. Preliminary results after in-furnace installation are described.

Processing of thermal images for the detection and enhancement of subsurface flaws in composite materials

Subsurface imaging using thermal wave generated either by line heating with lateral displacement of the part to be inspected or full-field heating with no displacement is an attractive approach for the NDE of composite materials. Proper detection of flaws, such as delaminations or unbonds require dedicated thermographic image processing.

Reflecting-cavity IR temperature sensors: an analysis of spherical, conical, and double-wedge geometries

Reflective cavity pyrometers are increasingly used for on-line temperature sensing of metal sheets. The presence of the reflector increases the effective emissivity of the sheet, thus reducing the measurement error. Hemispherical geometries have been mostly used in the past for the reflector. This paper presents a Monte-Carlo analysis for a number of different cavity configurations. The value of the effective emissivity is evaluated for different cavity parameters and standoff range over the sheet as well as for different degrees of the metal sheet real emissivity. It will be shown, in particular, that a trucated hemisphere with centre of curvature belowthe sheet is preferable to the more conventional full hemisphere to reduce losses due to sheet fluttering. Applications to infurnace temperature monitoring of steel sheets will be indicated.

INFRARED THERMOGRAPHIC INSPECTION BY INTERNAL TEMPERATURE PERTURBATION TECHNIQUES

SUMMARY A thermographic technique to inspect structures using an internal perturbation source such as a sudden change of the liquid temperature flow is presented. After a brief survey of the apparatus the technique is applied to detect wall thickness loss due to cavitation corrosion in liquid or gas carrying pipes. The same technique was used to reveal the internal structure of turbine blades after proper image processing.