Y.S. Chen

Review and comparison of shearography and pulsed thermography for adhesive bond evaluation

To reduce costs and facilitate automation in the automotive industry, adhesive bonding has gained popularity as a replacement for conventional mechanical fasteners such as bolts, screws, rivets, and welding. Adhesive bonding is particularly useful for bonding parts made of plastics and polymer composites, which are playing an increasing role in reducing vehicle weight. However, the adhesive bonding process is more susceptible to quality variations during manufacturing than tradi- tional joining and fastening methods. Shearography and pulsed ther- mography are full-field, noncontact, nondestructive testing methods that are widely used in the aerospace industry, offering significant potential as practical tools for in-process inspection of adhesive bond quality. The two techniques are often used to address a common set of aerospace applications, e.g., delaminations or skin-to-core disbonds in composite structures. However, they are fundamentally different, based on different flaw detection mechanisms: Shearography measures the samples me- chanical response to mechanical stresses, while pulsed thermography measures the samples thermal response to an instantaneous thermal excitation. For the convenience of potential users and readers, the au- thors review shearography and pulsed thermography. The potential of these techniques for inspecting adhesive bonding is demonstrated and compared. 2007 Society of Photo-Optical Instrumentation Engineers. DOI: 10.1117/1.2741277 Subject terms: adhesive bonding; shearography; pulse thermography; optical metrology; nondestructive testing; quality inspection.

Shearographic phase retrieval using one single specklegram: a clustering approach

In the field of optical measurement, phase always represents the physical quantity to be measured. Thus, phase retrieval from a fringe pattern is a key step for quantitative measurement and evaluation. Much research work has been conducted to develop phase evaluation methods such as fringe tracking and fringe skeletons in earlier, and the more precise methods of phase-shifting and Fourier transform more recently. For phase evaluation, the phase-shifting method requires three or more phase-shifted speckle patterns at each deformed stage; thus, it is not suitable for measurement of continuous deformation. The Fourier transform, on the other hand, requires a high-frequency carrier for phase separation in the spectral domain, which places an additional requirement on experimental arrangement. Thus, it would be desirable to develop a convenient method that can retrieve the modulated phase from a single fringe pattern. We propose an approach that utilizes the phase-clustering property to extract phase information from a single interference specklegram. To explore the ability and limitation for the proposed technique, typical shearographic fringe patterns are used for phase evaluation. Results obtained are similar to those from the standard four-step phase-shifting method. Nonrepeatable continuous movement is also measured by the proposed method, and the results confirm the robustness and accuracy of the clustering method.

Computerized tomography technique for reconstruction of obstructed phase data in shearography

Shearography is an interferometric method that overcomes several limitations of holography by eliminating the reference beam. It greatly simplifies the optical setup and has much higher tolerance to environmental disturbances. Consequently, the technique has received considerable industrial acceptance, particularly for nondestructive testing. Shearography, however, is generally not applicable to the measurement of an obstructed area, as the area to be measured must be accessible to both illumination and imaging. We present an algorithm based on the principle of tomography that permits the reconstruction of the unavailable phase distribution in an obstructed area from the measured boundary phase distribution. In the process, a set of imaginary rays is projected from many different directions across the area. For each ray, integration of the phase directional derivative along the ray is equal to the phase difference between the boundary points intercepted by the ray. Therefore, a set of linear equations can be established by considering the multiple rays. Each equation expresses the unknown phase derivatives in the obstructed area in terms of the measured boundary phase. Solution of the set of simultaneous equations yields the unknown phase distribution in the blind area. While its applications to shearography are demonstrated, the technique is potentially applicable to all full-field optical measurement techniques such as holography, speckle interferometry, classical interferometry, thermography, moiré, photoelasticity, and speckle correlation techniques.

Unified approach for rough phase measurement without phase unwrapping by changing the sensitivity factor

Phase evaluation is extraordinarily important in optical, acoustic and radar techniques where coherent signals are employed as information carriers. In most of the cases, phase information are obtained from an inverse trigonometric function and wrapped into −π to π. A phase unwrapping process is thus required to obtain the final unwrapped phase which represents the ultimate physical quantity to be measured. One-dimensional phase unwrapping is easily achieved by adding or subtracting an integer multiple of 2π to the wrapped phase to establish a smooth phase map. Two-dimensional phase unwrapping, however, is quite troublesome and an elegant unwrapping routing should be chosen in most of the cases to deal with phase residues caused by noise and other error sources. It would be valuable if two-dimensional phase unwrapping can be avoided and the physical quantity obtained directly. In the past, researchers have proposed other methods such as the multiple wavelengths approach which incorporates information from multiple wavelengths to eliminate the need for phase unwrapping. In this study, we extend the multiple wavelengths approach by varying the sensitivity factor, which is more convenient and cost-effective, to achieve the aim of requiring no phase unwrapping. Furthermore, an elegant phase derivative approach is used to solve the phase ambiguity problem in the multiple wavelengths method. Both simulation results and real experiment data of shadow moiré and shearography demonstrate the usefulness of this method, especially for discontinuous surface profile measurements such as steps. Advantages and disadvantages for the proposed method are also discussed in this paper.

Review and comparison of shearography and active thermography for nondestructive testing and evaluation (NDT&E)

Shearography and active thermography have received considerable industrial acceptance for nondestructive testing & evaluation (NDT&E). They are applicable to all materials: metal, non-metal, composites materials and even biological tissues. The principles and the methods of testing of these two techniques are reviewed, and their advantages and limitations are being compared. Both are optical techniques enjoying the advantages of full-field, non-contact and hence very high inspection speed. A fundamental difference between them is the mechanism of detecting flaws. Shearography is an interference optical technique which measures surface deformation and reveals flaws by looking for flaw-induced deformation anomalies. Active thermography is a surface thermal radiation measuring technique; it used thermal radiation properties to measure the distribution of surface temperature of the object. It detects flaws by the flaws anomalous heat transfer response. The methods of testing are also different. While shearography requires application of stresses to produce deformation, active thermography needs a controllable thermal radiation excitation to change the surface temperature. Both shearography and active thermography can detect surface and sub-surface flaws, unless the flaw is too remote from the surface. Different excitation methods, such as sonic, induction, flash heating, for the techniques are demonstrated together with some NDT&E applications such as detection of cracks, debonds and other type of flaws.

Electro-thermography technique for nondestructive testing (NDT) applications

In this paper, Electro-Thermography is introduced in nondestructive testing applications. Electro-Thermography is one of the novel active thermography techniques for nondestructive testing. It gains the advantages from the optical and electromagnetic properties in full-field, non-contact, high inspection speed, and sensitivity in geometry variation. It is mostly applicable to all kind of ferrous-metal, some composites materials. A fundamental difference among electro-thermography and other active thermography techniques are the excitation mechanism. Electro-Thermography is a combination of the electromagnetic induction and surface thermal radiation measuring technique; it used the induction method to excite the object, and then it used the radiation properties to measure the distribution of surface temperature of the object. It detects flaws by the flaws anomalous heating and heat transfer response. The method of excitation is also different from others irradiation excitation. Electro-Thermography needs an electromagnetic coil to generate eddy current through induction to change the surface and subsurface temperature. Electro-Thermography can detect surface and sub-surface flaws, unless the flaw is too remote and tiny from the surface. Some experiments in flaw detections and other types of inspections are demonstrated.

Dynamic phase measurement by clustering method

Phase measurement is a key step in quantitative optical metrology. While phase shifting technique is widely applied for accurate and reliable static or semi-static phase measurement, Fourier and wavelet transforms are often employed for high speed dynamic phase measurement. In our previous papers, the authors had proposed an alternative clustering method for dynamic phase measurement. The proposed method utilizes the phase clustering effect and the prior knowledge of the speckle field to extract the deformed phase map from one single deformed speckle pattern. The clustering method, however, may fail at area with abundant noise and large phase gradient. In this paper, we improve the clustering method by incorporating an advanced phase filtering methods for wrapped phase filtering. The reconstructed wrapped phase map is with very good quality and ready for phase unwrapping with any simple unwrapping algorithms. The basic ideas and the implementation approach will be described in details. Several examples based on shearography and holographic interferometry will be presented. Comparisons between the proposed method and phase shifting method will be made. The results demonstrate the accuracy and robustness of the integrated dynamic phase extraction method. The integrated phase retrieval method proposed here has great potential to simplify optical setup for dynamic phase measurement.

Clustering approach for phase extraction from one single fringe pattern in shearography

In the field of optical measurement, phase always represents the physical quantity to be measured. Thus phase extraction from fringe pattern is a key step for quantitative measurement and evaluation. Much research work has been conducted to develop effective phase evaluation methods such as fringe tracking and fringe skeleton in early years, and the more precise method of phase shifting and Fourier transform techniques in recent decades. For accurate phase evaluation, phase shifting method requires three or more phase-shifted fringe patterns at each deformed stage, thus it is not suitable for continuous deformation measurement. Fourier transform, on the other hand, requires a high frequency carrier fringe for phase separation in the spectrum domain, which places stringent requirement on experiment arrangement. Thus it would be desirable to develop a convenient method to retrieve the modulated phase from a single fringe pattern. In this paper, we propose a clustering approach which utilizes the phase clustering property to extract phase information from a single interference specklegram. To explore the workability and limitations of the proposed technique, typical shearographic fringe patterns are used for phase evaluation. Results obtained are similar to those from standard 4-step phase-shifting method with similar accuracy. Non-repeatable continuous movement is also measured by the proposed method, and the results confirm the robustness and accuracy of the proposed clustering phase extraction method.