Optical Testing Techniques Research Articles

Non-destructive techniques for corrosion detection: A review

Corrosion detection (CD) has become a high priority in chemical industries, defense and transportation sectors to extend the life of existing or new systems while ensuring the safety of the existing components and reducing downtime to minimise economic losses. Here, this article is aimed to review and discuss non-destructive techniques used widely in the industry to detect, monitor and repair corrosion problems early on. The comprehensive review provides a detailed discussion, a functional mechanism, advantages and disadvantages of crucial non-destructive CD techniques widely used in the industry, helping the reader choose the type of corrosion monitoring methods effectively. An extensive literature review of visual and optical testing techniques, acoustic emissions, eddy current, guided wave and equipment, infrared thermography, radiographic, microwave and millimeter wave, and terahertz imaging is discussed. The underlying mechanism, its merits and limitations, along with the usage scenario, is explained that can be related across different areas making it interdisciplinary research for corrosion monitoring techniques.

Investigation of frequency-dependent fatigue crack growth behavior in perfluorosulfonic-acid membrane: In-situ experiment, constitutive modeling and crack growth prediction

Fatigue crack growth behavior of perfluorosulfonic-acid membrane is investigated under distinct loading frequencies. The evolution of cyclic plastic zone (CPZ) is traced based on in-situ optical microscopy testing and digital image correlation (DIC) technique. The crack tip deformation process is directly observed by in-situ scanning electron microscope (SEM). The experimental results show that fatigue cracks experiencing a higher loading frequency display a comparatively reduced propagation rate. The CPZ can be solely used to interpret the fatigue crack growth rate variation under different loading frequencies. The crack-front microspores are easily formed under lower loading frequency due to the enhancement of near-tip plasticity deformation. Then, a cyclic elastic-viscoplastic constitutive model is developed, which successfully capture the mechanical behavior of the membrane. Finally, based on the plastically dissipated energy, a numerical method incorporated with the developed material’s constitutive relationship is established, which gives reasonable prediction of fatigue crack growth under various loading frequencies.

Research on the Dynamic Behaviors in the Sliding Friction of Silicone

With the promotion of soft viscoelastic materials in the field of flexible bionic robots, the behavior of sliding friction between soft viscoelastic materials and rigid materials such as metals has attracted great attention. In this study, the research object was silicone, and a research method combining a dynamic force testing technique and a non-contact optical testing technique for deformation and strain was proposed to study the frictional behavior of silicone and the effect of the tooth structure surface on the sliding friction. We analyzed the temporal changes in the friction force for different loads, the dynamic characteristics of large deformation and micro-strain on the front side (define the contact surface as the bottom surface) of the silicone. The results showed that within a certain range, the sliding friction coefficient of the silicone increased monotonically with increasing load, and there were stick-slip and deformation waves during the relative motion. The larger the load was, the faster the deformation wave propagated, and the tooth structure surface had sliding friction anisotropy.

Comprehensive Optical Strain Sensing Through the Use of Colloidal Quantum Dots.

The adaptation of colloidal quantum dots loaded within a polymer for use in nondestructive testing can be used as an optical strain gauge due to the nanomaterial's strain sensing properties. In this paper, we utilized InP/ZnS colloidal quantum dots loaded within a polymer matrix applied onto the surface of a dog-bone foil precoated with an epoxy. By employing an empirical formula and a calibration factor, there is a propinquity between both the calculated optical strain and mechanical stress-strain reference data. Fluctuations are observed, which may be due to both additional strain responses not seen by the mechanical data and quantum dot blinking. These results and methods show the applied use of this novel optical nondestructive testing technique for a variety of structures, especially for structures that operate in harsh environments.

Guest Editorial: Non‐Destructive Testing

Guest Editorial: Non‐Destructive Testing

Deformation Monitoring System Based on 2D-DIC for Cultural Relics Protection in Museum Environment with Low and Varying Illumination

Quantifying the condition of large cultural relics, such as marine archaeological shipwreck, is important to verify stability and reliability. Deformation monitoring system plays a key role in the preservation and long-term conservation of cultural relics. Two-dimensional digital image correlation (2D-DIC) method has proven its efficiency in being able to provide accurate quantitative information of structural deformations. In this study, a deformation monitoring system with four cameras based on 2D-DIC is developed to perform noncontact, optically based measurement to monitor the deformation of shipwreck in museum environment with low and varying illumination. Because the consistency of the accuracy of 2D-DIC measurements for different locations is the most basic requirement in the application of structural deformation monitoring, selecting the appropriate exposure time and quantifying the bias errors on 2D-DIC measurements should be helpful to the optimal use of this optical nondestructive testing technique. A theoretical criterion is deduced to quantitatively characterize the dependence of interpolation bias upon natural patterns and illuminations. Then, an exposure adjustment scheme is built based on the aforementioned criterion. Numerical experiments reveal that the exposure adjustment scheme is able to provide consistency interpolation error for different natural patterns even though the environmental illumination is different as well. The deformation monitoring system with the proposed exposure adjustment scheme is promising for developing flexible and robust in situ structural health monitoring for use in museum environment with low and varying illumination, making 2D-DIC technique a really useful tool for in situ long-term monitoring of large cultural relics.

Acoustic shearography for crack detection in metallic plates

Shearography is an optical non-destructive testing (NDT) technique which is suitable for large area and fast inspections. However, it is difficult to detect cracks in metals by using conventional stress loading methods for shearography, including stress loadings by vacuum, thermal and vibrational excitations, due to the high rigidity and/or the high thermal conductivity of metals. In this work, we propose and demonstrate an acoustic shearography method for detection of cracks in metallic plates. Piezoelectric transducers bonded on the metallic structures are used to generate acoustic waves, which interact with crack defects and act as the stress loading for shearography testing. Systematic investigations on the wave-defect interaction based shearography indicate that the optimal testing frequencies are determined by the resonant frequencies of the piezoelectric transducers, which are independent of defect type and sample dimensions of the metallic plates. By using the optimal testing frequency, we successfully detected surface fatigue cracks as short as 3 mm and subsurface cracks at 10 mm depth in aluminum plates. We also demonstrate the feasibility of simultaneous detection of multiple notches in multiple fastener holes with one piezoelectric transducer. This wave-defect interaction based shearography developed here allows fast and effective detection of deep subsurface fatigue cracks, which is promising for practical NDT of defects in metallic structures.

Sparse ensemble matrix factorization for debond detection in CFRP composites using optical thermography

With the increasing use of CFRP in aircraft and automotive industry, it has become necessary to monitor the health of the composite structure during the manufacture and service process from flaws such as debond or delamination by using optical pulsed thermography non-destructive testing (OPTNDT) techniques. However, current OPTNDT methods cannot efficiently tackle the influence from the strong noise, and the resolution enhancement of the defects detection remains as a critical challenge. To alleviate this problem, this paper proposes the sparse ensemble matrix factorization approach to remove the noise and enhance the resolution for the defects detection. Specifically, the algorithm is based on the sparse representation and noise is modeled as a mixture of Gaussian (MoG) distribution. It provides a projection from the raw data onto the sparse and low dimensional sub-space while the defects information is significantly enhanced with the layer decomposed approach in the low dimensional space. Not withstand above, a Gaussian low pass filtering and non-linear enhancement is conducted for further enhancement. The proposed method is coupled with several comparison studies to verify the efficacy.

An Optical Surface Inspection and Automatic Classification Technique Using the Rotated Wavelet Transform

This paper discusses a novel optical nondestructive testing and automatic classification technique using customized image processing techniques. In contrast to conventional spatial analyses, which are highly susceptible to noise and human perception, our proposed transform domain approach provides a high degree of robustness and flexibility in feature selection, and hence a better classification efficiency. The presented algorithm classifies the part-under-test (PUT) into two bins of either acceptable or faulty using transform-domain techniques in conjunction with a classifier. Since the classification is critically dependent on the features extracted from stored images, a sophisticated scalable database was created. The initial database contains 100 parts with various degrees of reflectivity and surface conditions. The transformation algorithm relies on the discrete wavelet transform (DWT) and rotated wavelet transform (RWT) for feature extraction, while a K-nearest neighbor (KNN) classifier is employed for PUT classification. The maximum accurate classification efficiency achieved is 80% and more than 93% by DWT and RWT, respectively.

Microstructure and mechanical properties of Al-nano/micro SiC composites produced by stir casting technique

In this research, composites of A356- nano SiC (0.5 and 1.5wt%) and A356-5wt% of micro SiC were produced by stir casting technique. After applying T6 heat treatment on a part of cast ingots, all samples were studied by optical and SEM microscopy and mechanical testing techniques. It was found that adding SiC particles decreases dendrite lengths because the particles act as the preferential nucleation sites for the solidification of matrix. The best modifying effect was observed in the composite containing micro SiC particles. The T6 heat treatment on A356 resulted in the dissociation and globularization of Si platelikes and formation of complex AlMg SiFe intermetallics and nano-sized Mg2Si particles. The mechanical strength of A356 increased considerably by combining T6 heat treatment and adding micro/nano SiC particles. The highest strength was obtained in the composite containing 1.5wt% nano SiC. The SEM observations showed that fracture in cast A356 occurs along the dendrite boundaries. However, the T6 treated alloy showed a combined interdendritic and cleavage fracture. The same combined mode of fracture was observed for the alloy with 0.5wt% nano SiC. The brittle fracture in the composites was attributed to the existence of SiC and/or intermetallic compounds in the matrix. The fracture mode for other composites was fully cleavage.

Effect of Plasma Nitriding Parameters on the Wear Behaviour of AISI D6 Tool Steel

Abstract In the present investigation, the effects of plasma nitriding pressure and time on the structural and tribological characterization of AISI D6 grade cold work tool steel were studied. DC glow discharge plasma nitriding process under NH3 gas was performed at temperatures of 500 °C and at pressures of 150 Pa, 450 Pa and 800 Pa for durations of 2 h, 7 h and 10 h. The mass loss and the coefficient of friction were measured using a ball-on-disc wear test with 1.668 N, 4.143 N, and 9.113 N test loads. Cross-section and surface characterizations were evaluated using optical microscopy, SEM, and microhardness test techniques. The surface microhardness was found to increase with the nitriding time and the pressure. The wear resistance of the specimen strongly depends on the nitriding parameters and the applied test load in determining whether a plastic deformation of the surface layer has occurred or not.

How to reveal subsurface defects in Kevlar® composite materials after an impact loading using infrared vision and optical NDT techniques?

An integrated system between infrared vision and optical non-destructive testing techniques can be considered a viable, robust and reliable approach for both aerospace manufacturing and in-service inspections. In this paper, infrared vision is applied in different spectral bands on two impacted panels made of aramid–phenolic composite by applying two different methods, respectively: (1) near and short-wave infrared reflectography and transmittography, and (2) mid-wave active infrared thermography. Furthermore, optical methods, namely digital speckle photography and holographic interferometry, are used as well to highlight the damages due to the impacts on the samples. Some techniques provide more straightforward detection capabilities than others for different defect types.

Theoretical and experimental ultrasonic investigation of a thin plate with regions of different periodic double-corrugations

In order to provide a qualitative measurement of the depth of periodic double-corrugations existing on both sides of a thin plate, theoretical and experimental ultrasonic techniques have been applied. The aesthetic galvanized steel sheets under investigation contain many regions of periodic double-corrugations, each region with a different corrugation depth. An ultrasonic technique is investigated as a means to improve existing optical testing techniques in quality and speed. Theoretical simulations using the theory of diffraction from periodically corrugated surfaces have been employed to obtain reflection spectra to facilitate qualitative observations regarding the depth of the corrugation in the different regions. Experiments to obtain reflection spectra correlate well with the theoretical simulations, and show the application of ultrasound to the qualitative measurement of the corrugation depth. Because the wavelengths necessary to make the investigations produce Lamb waves, some difficulty lies in the generation of diffracted bulk waves and Lamb waves. The research shows that correlations between theory and experiments exist, but they can only be obtained after thorough simulations that do not produce easy to apply rules of thumb.

Optical contactless probing: an all-silicon, fully optical approach

With decreasing feature size and increasing chip densities, the classical mechanical probe approach for internal fault detection and functional testing faces increasing challenges. Contactless testing might resolve many of the challenges associated with conventional mechanical wafer testing. This article reviews and explains existing optical contactless technologies and introduces a new test methodology - a fully optical contactless testing technique. The proposed method's uniqueness lies in the fact that it is a fully optical technique that uses visible light and is completely compatible with standard silicon IC processing. The technique produces results that demonstrate its feasibility and show its advantages over other optical contactless testing methods.

Least-squares fitting of wavefront using rational function

This paper describes a mathematical method for approximation of the measurement data in optical testing techniques. Presented method uses a least-squares fit of two-dimensional rational function model to interferometric data. It is suitable for an analytical expression of wavefront, and one can obtain sufficient accuracy with a relatively small number of approximation coefficients. Properties of the proposed method are analyzed in this work. It is shown that the technique can be used for analyzing wavefront deformation in optical testing methods.

Optical testing techniques for new semiconductor processes and new materials

AbstractIndustrial semiconductor manufacturing—and therefore industrial semiconductor metrology—is being faced with a new generation of processes and materials coming out of development laboratories worldwide. These need careful monitoring, particularly in the early stages of manufacture. Many metrology challenges are well met by optical techniques, and the article discusses three such techniques that have transferred to industrial production process control. Laser ellipsometry brings high precision to ultrathin 25 Å gate oxide measurements and can analyse many types of (transparent) new materials. Impulsive stimulated thermal scattering is a new opto‐acoustic technique and therefore is covered more fully here—it complements laser ellipsometry by analysing single and bilayer metal (opaque) films after these have been exposed to a variety of semiconductor processes. Photoluminescence is invaluable for analysing compound semiconductors, where it measures directly the quality of epitaxial layers and so helps to understand and reduce defect mechanisms and improve yields. The article concludes by discussing industrial requirements for integrating the measurement system into a deposition system for in situ process control. Copyright © 2001 John Wiley & Sons, Ltd.

Optical testing techniques for new semiconductor processes and new materials

Industrial semiconductor manufacturing—and therefore industrial semiconductor metrology—is being faced with a new generation of processes and materials coming out of development laboratories worldwide. These need careful monitoring, particularly in the early stages of manufacture. Many metrology challenges are well met by optical techniques, and the article discusses three such techniques that have transferred to industrial production process control. Laser ellipsometry brings high precision to ultrathin 25 Å gate oxide measurements and can analyse many types of (transparent) new materials. Impulsive stimulated thermal scattering is a new opto-acoustic technique and therefore is covered more fully here—it complements laser ellipsometry by analysing single and bilayer metal (opaque) films after these have been exposed to a variety of semiconductor processes. Photoluminescence is invaluable for analysing compound semiconductors, where it measures directly the quality of epitaxial layers and so helps to understand and reduce defect mechanisms and improve yields. The article concludes by discussing industrial requirements for integrating the measurement system into a deposition system for in situ process control. Copyright © 2001 John Wiley & Sons, Ltd.

Experimental analysis of vibration modes of plates using ESPI

In the world, there are several types of vibration, and these vibrations especially affect the mechanical industry. In this paper, experimental analysis of vibration modes of plates is discussed. Electronic speckle pattern interferometry (ESPI) is one of the optical nondestructive testing techniques. By using the ESPI, vibration modes of plates with various excitation points, ratio of longitudinal and lateral length, and materials are measured and qualitatively compared with the results of theoretical analysis proposed by Warburton. Finally, these vibration modes are quantitatively compared with the result of FEM analysis. The results of this study are as follows: (1) By comparing the theoretical and experimental frequencies, we confirmed qualitatively that deviations of frequencies are within 10% in accuracy. (2) By comparing the experimental vibration mode shapes with the numerical ones of the FEM analysis, we can conclude quantitatively that measuring vibration modes by using the ESPI has high accuracy.

Optical testing of condoms

Condoms provide one of the most important means of preventing pregnancy and the spread of human immunodeficiency virus (HIV) and other sexually transmitted diseases. Pinhole defects may lead to the passage of sperm or viruses through the condom wall. Embedded particles, which may become dislodged in handling or use, may represent latent pinhole defects. Thin regions in the condom wall may lead to breakage in use. Testing for such defects in regulatory laboratories, or in the factories as part of production screening or quality assurance efforts, is a major tool for ensuring condom reliability. A new optical method for testing condoms is presented, sensitive to pinholes, thin regions, and embedded particles. The method could supplement or replace the existing, less powerful, electrical test techniques in either regulatory testing laboratories or in the factory. The optical techniques presented provide quantitative data on the condoms tested, rather than just accept/reject decisions. They thus support detailed monitoring of production or lot characteristics that is generally not possible with existing electrical techniques. The ability of the optical techniques to separately test for and identify not only pinholes, but also thin regions and embedded particles, which are not separately tested for with the electrical technologies, is an especially important new capability. Further, the new optical techniques allow the locations of defects found to be precisely marked, making possible a microscopic examination of defects useful in studying their structure and possible causes. The optical technique is also totally nondestructive, in that it does not alter the defects whose structures are of interest. Initial results are reported on the testing of latex condoms purchased in retail stores. Condoms identified optically as being potentially defective were subjected to industry standard leak and burst testing. Results of the various tests are summarized. The data suggest that use of the new optical test techniques could have prevented an unexpectedly large number of condoms with significant pinhole and thin region defects from reaching the public. Optical test systems could be used to support either regulatory, quality assurance, or clinical field testing of latex or nonlatex condoms. Due to their high rate capability and level of automation, they could also be applied to 100% screening testing in the factory. The new test techniques could also be used to detect defects in a wide range of other thin sheets and membranes.

Two-dimensional electronic-speckle-pattern interferometry and concrete-fracture processes

Electronic-speckle-pattern interferometry (ESPI) is an optical nondestructive testing technique. A two-dimensional in-plane displacement sensitive interferometer was designed to observe progressive development of microcracking of carbon-fiber-reinforced concrete specimens subjected to tension. By using the two-dimensional ESPI system, the fracture processes, crack initiation and crack propagation of concrete specimens were monitored qualitatively and in real time. The displacement and strain fields were analyzed quantitatively by using digital image processing techniques.