L. Liu

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.

Mineral Characteristics Analysis of PM2.5 in Northern China

PM2.5 (sandstorm-related) was sampled at four sites near dust source areas in Northern China in spring 2012. Every site was away from obvious pollution source. After drying 24 hours in a dry dish, X-ray diffraction (XRD), x-ray fluorescence (XRF) and scanning electron microscope (SEM) of all the samples were examined. The main phases were quartz, albite, calcite and mica. SEM observations showed that the particles were mainly irregular shaped, but different sample the particle morphology is slightly different. And the surface was rough because of erosion mark or pores on them. The chemical composition of PM2.5 is characteristic of high SiO2 and CaO, while low K2O, Na2O. Main elements of the samples were Si, Al, Fe, Ca, which were familiar with crust elements and dust fall components [, but some samples also contained low-level heavy metals like As and Pb.

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.

An improved shearography technique for dynamic deformation measurement

Dynamic deformation measurement is a hot topic in optical interferometry research. Currently, most proposed solutions in this field cannot simultaneously meet the fundamental requirements of accuracy and robustness, because these methods assume that the speckle field is constant during deformation or utilize iteration algorithm that generates a great deal of computation. In this paper, an improved shearography technique is presented. The fringe pattern is generated by the product of the speckle images preprocessed by frequency filter, and then the phase related to the deformation can be extracted from fringe pattern. Since only one image in deformed stage is used, the proposed method can be well applied for dynamic deformation measurement. Moreover, the proposed method has much more immunity to the fluctuation of speckle field compared with conventional method.

Image Reconstruction from Incomplete Data and Its Applications in Experimental Mechanics

In the field of experimental mechanics, there exist some circumstances when only data at the boundary can be obtained while the internal data are unavailable, or when some data are missed due to shadow, illumination saturation and other reasons. Thus it would be helpful if a reasonable estimation of the unavailable or missed data can be obtained. In this study, an algorithm is developed to reconstruct the missed data from the existing ones by generating a series of equations about the missed data and solving for an optimal solution using least-squares approach. Results based on both simulation data and real incomplete experimental data obtained by shearography and fringe projection show the usefulness and potential of the algorithm for experimental mechanics applications.

Novel Washer as Force Transducer for Monitoring Clamping Force of Bolted Joints

Bolts are widely used in industry for joining members together. The behavior and service life of bolted joints depend on several factors like bolt material, dimensions, surface finish, surface coating and thread tolerances. However, the uppermost factor affecting the reliability and durability is the correctness of the clamping force exerted by the bolt. Thus it is very important to monitor the bolt clamping force during assembly process to ensure a proper preload. Each of the available monitoring techniques including torque control, torque-angle control, strain gaged bolt and ultrasound suffers one or more limitations rendering the methods impractical for application in a production environment. In this paper, we propose an alternative method for determining clamping force. The method utilizes a simple washer whose surface has a speckle pattern; the speckle pattern acts as a strain sensor. By measuring the deformation of the washer using an image processing technique called fast digital image correlation, the clamping force can be determined. This method, which enables real-time monitoring of clamping force during fastening, has a potential of being used for mass measurement of clamping force in a production environment. Experiments conducted on a bolted structure with washers of different sizes have demonstrated the reliability and usefulness of this new approach.Copyright