William P. Winfree

Real time computational algorithms for eddy-current-based damage detection

In the field of nondestructive evaluation, new and improved techniques are constantly being sought to facilitate the detection of hidden corrosion and flaws in structures such as aeroplanes and pipelines. In this paper, we explore the feasibility of detecting such damage by application of an eddy-current-based technique coupled with reduced order modelling. We begin by developing a model for a specific eddy current method in which we make some simplifying assumptions reducing the three-dimensional problem to a two-dimensional problem (we do this for proof of concept). Theoretical results are then presented which establish the existence and uniqueness of solutions as well as continuous dependence of the solutions on the parameters which represent the damage. We further discuss theoretical issues concerning the least squares parameter estimation problem used in identifying the geometry of the damage. To solve the identification problem, an optimization algorithm is employed which requires solving the forward problem numerous times. To implement these methods in a practical setting, the forward algorithm must be solved with extremely fast and accurate solution methods. In constructing these computational methods, we employ reduced order proper orthogonal decomposition (POD) techniques. This approach permits one to create a set of basis elements spanning a data set consisting of either numerical simulations or experimental data. We discuss two different algorithms for forming the POD approximations, a POD/Galerkin technique and a POD/interpolation technique. Finally, results of the inverse problem associated with damage detection are given using both simulated data with relative noise added as well as experimental data obtained using a giant magnetoresistive sensor. The experimental results are based on successfully using experimental data to form the POD basis elements (instead of numerical simulations), thus illustrating the effectiveness of this method on a wide range of applications. In both instances the methods are found to be efficient and robust. Moreover, the methods were fast; our findings demonstrate a significant reduction in computational time.

Technology and Applications of Terahertz Imaging Non‐Destructive Examination: Inspection of Space Shuttle Sprayed On Foam Insulation

The implementation of terahertz (THz) imaging for non‐destructive evaluation shows great promise in 2 and 3 dimensional non‐contact inspection of non‐conductive materials such as plastics, foam, composites, ceramics, paper, wood and glass. THz imaging employs safe low power non‐ionizing electromagnetic pulses, which produce images with lateral resolution <200 microns, and depth resolution <50 microns. We demonstrate the detection of voids and disbonds intentionally incorporated within the sprayed on foam insulation of a space shuttle external tank mock‐up segment using time domain THz imaging. Recently, highly integrated turn‐key THz imaging systems have been introduced commercially. An industrially hardened THz scanning system which has been deployed to scan the space shuttle tank with small remote THz transceiver on a 30 meter fiber optic umbilical, is described.

Remote measurement of in-plane diffusivity components in plates

A method of determining thermal diffusivity in thin plates is presented. The method, using infrared images of evolving thermal patterns previously injected with a laser, is noncontacting, one‐sided, and remote. It does not require independent estimates of either the emissivity of the sample or the sample thickness. With a line‐segment pattern for thermal input, it yields the in‐plane components of the diffusivity tensor in anisotropic materials and also the rate of heat loss to the environment of the plate. Two methods of data analysis are presented, one corresponding to a heating line of general cross section and the other considering a Gaussian cross section, thereby saving considerable computer time. Both methods produce a statistical evaluation of measurement quality as well as estimates of diffusivity and loss rate. Results are shown for plates of metals and graphite‐epoxy composite materials. Principal components and orientation for the diffusivity tensor are obtained in the anisotropic graphite‐epoxy sample.

A comparison of image processing algorithms for thermal nondestructive evaluation

Thermography involves the application of heat to a structure and observation of surface temperature anomalies to reveal subsurface defects. Detection of subsurface defects can be greatly enhanced by the real time capture of a series of thermal images in time and the subsequent analysis of these images using various image processing algorithms. By applying image-processing algorithms, defects not readily observable can be detected and quantitatively characterized. The focus of this work is to investigate several of the numerous data reduction algorithms for thermal nondestructive evaluation by comparing results on a set of test samples. Some new types of data reduction algorithms have been recently developed with advantages such as noise reduction, file size compression, and material property measurements. By comparing various algorithms on factors such as computational speed, simplicity of use, robustness to noise, quantitative information, and optimum defect detection the most efficient algorithm may be chosen based on the user’s needs.

Thermographic determination of delamination depth in composites

The determination of the depth and size of delaminations is important for assessing their impact on structural integrity. A common technique for depth determination from thermal responses relies on a calibration of the technique based on flat bottom hole in a NDE standard. This assumes a delamination will effectively block all the heat diffusion from the region above the delamination to the region below the delamination. For graphite fiber reinforced composites, where a thin delamination has a contact resistance, which is comparable with the thermal resistance of the layer above it, this assumption is inaccurate. This paper discusses thermographic depth profiling based on an analytical model for heat diffusion representing two layers connected by a contact resistance. The model is shown to accurately represent the thermal response obtained for flash heating of composite specimens with known delaminations. Using the model to fit the thermal responses enables an estimation of the depth of the delamination. The accuracy of the technique is determined from measurements on composite specimens with delaminations at known depths.

Thermographic imaging of cracks in thin metal sheets

The presence of cracks significantly decreases the structural integrity of thin metal sheets used in aerospace applications. Thermographic detection of surface temperature variations due to these cracks is possible after external heating. An approximate line source of heat is used to produce an inplane flow of heat in the sheet. A crack in the sheet perturbs the inplane flow of heat and can be seen in an image of the surface temperature of the sheet. An effective technique for locating these perturbations is presented which reduces the surface temperature image to an image of variations in the inplane heat flow. This technique is shown to greatly increase the detectability of the cracks. This thermographic method has advantages over other techniques in that it is able to remotely inspect a large area in a short period of time. The effectiveness of this technique depends on the shape, position and orientation of the heat source with respect to the cracks as well as the extent to which the crack perturbs the surface heat flow. The relationship between these parameters and the variation in the heat flow is determined both by experimental and computational techniques. Experimental data is presented for through-the-thickness, subsurface and surface EDM notches. Data for through-the-thickness fatigue cracks are also presented.

Status of Thermal NDT of Space Shuttle Materials at NASA

Since the Space Shuttle Columbia accident, NASA has focused on improving advanced NDE techniques for the Reinforced Carbon-Carbon (RCC) panels that comprise the orbiters wing leading edge and nose cap. Various nondestructive inspection techniques have been used in the examination of the RCC, but thermography has emerged as an effective inspection alternative to more traditional methods. Thermography is a non-contact inspection method as compared to ultrasonic techniques which typically require the use of a coupling medium between the transducer and material. Like radiographic techniques, thermography can inspect large areas, but has the advantage of minimal safety concerns and the ability for single-sided measurements. Details of the analysis technique that has been developed to allow in situ inspection of a majority of shuttle RCC components is discussed. Additionally, validation testing, performed to quantify the performance of the system, will be discussed. Finally, the results of applying this technology to the Space Shuttle Discovery after its return from the STS-114 mission in July 2005 are discussed.

Thermal Diffusivity of Diamond Films Using a Laser Pulse Technique

Polycrystalline diamond films were deposited using a microwave plasma-enhanced chemical vapor deposition process. A laser pulse technique was developed to measure the thermal diffusivity of diamond films deposited on a silicon substrate. The effective thermal diffusivity of a diamond film on silicon was measured by observing the phase and amplitude of the cyclic thermal waves generated by laser pulses. An analytical model is presented to calculate the effective inplane (face-parallel) diffusivity of a two-layer system. The model is used to reduce the effective thermal diffusivity of the diamond/silicon sample to a value for the thermal diffusivity and conductivity of the diamond film.

Thermal diffusivity imaging of aerospace materials and structures

Thermographic imaging is increasingly being utilized for inspection and characterization of materials and structures. Thermal diffusivity imaging provides a means for quantitative characterization of a materials which is independent of apparatus and measurement configuration. This enables more precise imaging of variations in the material or structure needed to track changes resulting from fatigue or aging processes.

Pulse Phase Thermography for Defect Detection and Visualization

Pulse Phase Thermography (PPT) has been reported as a novel powerful technique of the thermal NDE. It employs application of the Discrete Fourier Transform (DFT) to thermal images obtained following flash heating of the front surface of a specimen. The computed phasegrams are excellent for defect visualization in a wide range of materials. This is in part due to their low sensitivity to uneven heating. This work is an attempt to analyze advantages and limitations of PPT. Results of application of the DFT to simulated temperature decays are presented. The temperature evolution on a surface has been simulated based on an analytical solution of the 1D heat diffusion problem. A more sophisticated study has been done for different sizes of defects using numerical solution of the 3D mathematical model. Capabilities of PPT for in-depth scanning and for monitoring of the material loss are discussed. The recommendations for the practical application of the PPT are presented. Experimental results obtained following these recommendations are reported.