Optical Nondestructive Testing Research Articles

The Application of Optical Nondestructive Testing for Fresh Berry Fruits

The Application of Optical Nondestructive Testing for Fresh Berry Fruits

Damage detection in composites using non-destructive testing aided by ANN technique: A review

Damages are inevitable in structures and effective damage detection techniques are important for maintaining their health. Many weight-sensitive engineering applications use composite materials, especially fiber-reinforced laminates. Common damages of these materials include delamination, fiber breakage, fiber pull-out, etc. Various non-destructive testing (NDT) techniques are reported in the literature for damage detection in composites, such as ultrasonic testing, vibration-based techniques, acoustic emission technique, optical NDT and imagining techniques. However, due to the complex properties of composite materials, conventional techniques for analyzing NDT data are difficult to implement. In this context, artificial neural network (ANN) technique is a promising alternative for analyzing NDT data for damage detection. In this study, an attempt is made to explore the state-of-the-art of damage detection in composites using NDT aided by ANN. The work discusses the pros and cons of different methods and is expected to help in identifying the appropriate method for damage detection in composites.

Deformation measurement by digital holographic interferometry

Nondestructive testing (NDT) methods and techniques play a vital role in enhancing product quality across various industries. Among these methods, the optical approach stands out, relying on the analysis of how optical radiation interacts with the test object. The key information parameters for optical testing of objects encompass their spectral and integral photometric characteristics. These characteristics are influenced by factors such as the substance’s structure, temperature, physical state, microrelief, angle of incident radiation, polarization degree, and wavelength. By leveraging optical methods, defects within materials can be detected without causing damage to the object. These defects encompass voids (discontinuities), delaminations, pores, cracks, inclusions of foreign bodies, internal stresses, alterations in material structure, and variations in physical and chemical properties, as well as deviations from specified geometric shapes. It’s important to note that optical methods are limited to detecting internal defects only in products made of materials that are transparent within the optical spectrum’s region. By harnessing the power of nondestructive optical testing, industries can ensure the integrity and quality of their products, detect potential flaws, and maintain stringent quality standards without causing any harm to the tested objects. The method with use of transformation of Fourier over sequence of the holograms, which are written down in various time points, is described. Possibility of measurement of deformations of composite material when heating is shown by low-power laser radiation.

The load-bearing behaviour of a reinforced concrete beam investigated by optical measuring techniques

Bending beams and slabs are typical examples for structural elements used for reinforced concrete structures such as bridge girders, T-beams and bridge decks. Their strength related failure modes at maximum loading can be divided into bending and shear failure. The failure of beams loaded in bending can occur with or without indication. Therefore, conventional design concepts aim on failure modes with sufficient indication (e.g. large deflections or cracks), as it occurs in the case of secondary flexural compression failure. These indicating factors can also be used for Structural Health Monitoring (SHM) of civil infrastructure systems (e.g. bridges) to identify structural changes. In this context, non-destructive testing (NDT) methods offer different techniques for measuring deflections or crack formation and opening. However, profound knowledge on the determining failure modes of bending beams and their detection by NDT methods is required for the reliable application of SHM. Different NDT methods have been used in this study for analysing the load-bearing behaviour of a reinforced concrete beam in bending. The different measuring techniques are briefly described and their applicability is discussed by means of experimental results. For this purpose, the load-bearing behaviour of a reinforced concrete beam having a span of 2.75 m was investigated in a four-point bending flexural test at laboratory scale. The focus is on the characterization of determining failure modes by optical NDT and the comparison with classical measuring techniques (e.g. deformation measurements by displacement transducers). The bending beam was equipped with two single-mode (SM) sensor fibres. One fibre served as Distributed Optical Fibre Sensor (DOFS), whereas the other fibre contained Fibre Bragg Grating (FBG) sensors. In addition, optical deformation measurements using Digital Image Correlation (DIC) and Stereophotogrammetry (SP) were conducted.

Characterization of ceramics based on laser speckle photometry

Abstract. Advanced ceramic components are frequently used in industrial applications. As a brittle material, ceramic reacts very suddenly to excessively high stresses. Existing defects lead to rapid crack growth followed by spontaneous destruction. This leads to a functional failure of the entire component. It is therefore important to develop innovative techniques to ensure a good quality condition of ceramic products. Laser speckle photometry (LSP) is an optical nondestructive testing method. It is based on the dynamic analysis of time-resolved speckle patterns that are generated by an external excitation. In this paper, we will present two investigations on ceramic components using the LSP technique. One is the nondestructive stress characterization on ceramic surfaces, and the other is the defect detection on ceramics components. The aim is to improve the quality and safety control of ceramic production in the challenging industrial field. Preliminary results have shown the potential of the LSP sensor system for the nondestructive characterization of ceramics in terms of stress monitoring and surface defect detection.

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

A gamma correction method based on constant-intensity images in phase-measuring profilometry

Phase-measuring profilometry (PMP) is an important technique for image analysis in optical non-destructive testing (NDT). Its measurement accuracy is significantly affected by gamma distortion in the projector, which makes the projected sinusoidal fringe patterns non-sinusoidal. In order to address this issue, a generic gamma non-linearity model based on constant-intensity images is proposed in this paper. In the proposed model, system defocus and noise are considered and analysed. It is demonstrated through theoretical derivation that the constant-intensity images remain unchanged, with no additional frequency being produced. For this reason, system defocus is not modelled in the proposed gamma calibration model and the related defocus parameters need not be calculated, which reduces the complexity of the calculation. Any noise that exists in the optical measurement system is another main factor influencing the greyscale levels of the image. A mutual information (MI)-based denoising method is proposed to reduce the noise and improve the accuracy of the gamma calibration. Furthermore, with the defocus analysis and the noise reducing method, a robust pixel-wise gamma calibration method is introduced. The experimental results in this paper show that the proposed gamma calibration method is able to accurately calibrate the gamma non-linearity of the system. Moreover, the phase precision is significantly improved and a higher quality measurement is achieved for the measured surfaces.

Non-destructive optical testing of the products obtained using additive manufacturing

Developing of additive manufacturing and the use of additive technologies at all stages of the product life cycle entails the necessity of developing methods providing control of their characteristics. Here we consider application of the optical measurement procedures to the objects of complicated shape. Products manufactured by the methods of additive technologies may have defects such as delamination, starved joint, porosity, warp, stress-strain state, roughness, etc. We present the devices designed and developed on the principles of interference, structured light and holography at the «VNIIOFI» Federal State Unitary Enterprise to identify the defects in the products of additive technologies. The advantages and shortcomings of the interference microscope, scanner-profilometer, and shearograph are examined with regard to the control of the aforementioned types of geometric defects. The special feature of the contact-free methods is the possibility of control of different types of materials: metals and dielectrics, transparent, reflective, scattering and composite materials. The results of experimental study aimed at detection of the surface and subsurface defects are presented. Interferograms and results of phase reconstruction are presented both for reflective and transparent objects. Defects of the refractive index of the microcavity made of an optical fiber were detected. Metrological characteristics of optical devices for non-destructive testing of products are estimated. Standards for calibration and graduation of the optical devices considered in the study were manufactured and tested. The use of specially developed standards makes it possible to use these methods both for qualitative assessment of the objects of complicated shape and flaw detection and for quantitative estimation of their characteristics.

Quantitative Phase Imaging Camera With a Weak Diffuser

We introduce the quantitative phase imaging camera with a weak diffuser (QPICWD) as a practical realization of quantitative phase imaging (QPI) on standard microscope platforms. The QPICWD is a compact stand-alone camera which measures sample induced phase delay under low-coherence quasi-monochromatic illumination by examining the deformation of the speckle intensity pattern. By interpreting the speckle deformation with an ensemble average of the geometric flow, we can obtain the high-resolution distortion field by solving the transport of intensity equation (TIE). Since the phase measured by TIE is the generalized phase of the partially coherent image, instead of the phase of the measured object, we analyze the effect of illumination coherence and imaging numerical aperture (NA) on the accuracy of phase retrieval, revealing that the phase of the object can be reliably retrieved when the coherence parameter (the ratio of illumination NA to objective NA) of the Kohler illumination is between 0.3 and 0.5. We present several applications for the new design including nondestructive optical testing of microlens array with nanometric thickness and imaging of fixed and live HeLa cells. Since the designed QPI camera does not require any modification of the widely available bright-field microscope or additional accessories for its use, it is expected to be adopted by the broader biology and medical community.

Defect detection using windowed Fourier spectrum analysis in diffraction phase microscopy

This paper introduces a technique to identify defects from fringe patterns for optical non-destructive testing and metrology. The technique relies on computation of the windowed Fourier spectrum of the fringe pattern at a given spatial frequency, and subsequent application of automated spectrum thresholding to localize the defect. The technique offers the advantages of high robustness against noise, fast implementation, high throughput and minimal operator intervention. The performance of the proposed technique is demonstrated for identifying defects of different types and sizes under varying levels of noise using numerical simulations, and practical validity is tested using experimental interferograms obtained in diffraction phase microscopy.

THE DEVELOPMENT OF OPTICAL TESTING TECHNOLOGY OF PCM STRUCTURES BY FIBER-OPTIC SENSORS

The ways of development of optical control technology of polymer composite materials structures by fiber-optic sensors during production and operation are described and investigated. A mathematical model describing the process of PCM monitoring using fiber optic sensors based on fiber Bragg gratings, clarifying the parameters of a mathematical model by experimentally determining the sensitivity coefficient of fiber optic sensors integrated in PCM, makes it possible to reduce the error in measuring strain by 5 – 7 times. The interaction in the system fiber-optic sensors – PCM and found that the integration of fiber-optic sensors based on quartz fibers in PCM, there is a partial destruction of the protective acrylate shell, which leads to the effect of microslip, which does not significantly affect the quality of measurements and can be compensated for by calibration. The requirements for the placement of fiber-optic sensors in the PCM at the manufacturing stage, including the formation of the input / output zone are formulated. The technology of optical non-destructive testing of composite materials with fiber-optic sensors is described, taking into account the features of the interaction of fiber-optic sensors with composite structures.

Multispectral Imaging: A New Solution for Identification of Coal and Gangue

Accurate identification of coal and gangue is an important prerequisite for the effective separation of coal and gangue. The application of imaging technology combined with image processing steps (like enhancement, feature extraction, etc.) and classifier is used to identify coal and gangue, which effectively avoids the shortcomings of traditional methods (radiation, pollution, etc.). However, ordinary image detection is greatly influenced by environmental factors such as light, dust and so on. Multispectral imaging technology, as a new generation of optical non-destructive testing technology, is less affected by illumination, so we propose a new solution for the recognition of coal and gangue by using multispectral imaging. Firstly, we respectively tested the classification performance of different image feature extraction methods under GS-SVM, GA-SVM, and PSO-SVM classifiers, and selected the best feature extraction method is LBP. And then, we compared the classification effects under different wavelengths and found that the ninth wavelength works best. That is, the difference in imaging between coal and gangue at 773.776 nm is greatest. Finally, the performance of the proposed model for the identification of coal and gangue was carried out. And the highest classification accuracy can be obtained by using GS-SVM as the classifier, at which point, $C=8$ , $g=0.17678$ . The results show that multispectral imaging technology can be used for the identification of coal and gangue, and the prediction accuracy of the model combined with LBP feature extraction and GS-SVM can reach 96.25% (77/80). The conclusions could provide reference evidence for the intelligent dry selection in coal preparation plants and underground coal mine.

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.

Mechanical and fracture properties of concrete reinforced with recycled and industrial steel fibers using Digital Image Correlation technique and X-ray micro computed tomography

Paper presents investigation of fracture phenomenon in plain concrete and in concrete reinforced with both recycled steel fibers (RSF) and industrial steel fibers (ISF). The wedge splitting test (WST), which enables stable crack propagation for quasi-brittle materials, was carried out on 75 × 75 × 75 mm cube samples. Initially, fracture process zone development was investigated only on the surface of samples using Digital Image Correlation which is a non-destructive optical testing method. Furthermore, to analyse the 3D cracking phenomenon (formation, development, width, shape and curvature) X-ray micro computed tomography was used. Micro-CT images were taken during continuous deformation process - without unloading sample during scanning. X-ray micro-computed tomography was also used to visualise and characterise air voids and fibers (length, diameter and orientation) embedded in concrete. The mechanical properties of plain, RSF and ISF reinforced concrete in terms of compressive strength, tensile splitting strength, shrinkage, tensile and residual strength in 3-point bending were additionally described.

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.

Light scattering by porous oriented polypropylene films

Oriented porous polypropylene films with lamellar crystal structure are studied by the laser radiation scattering method. The dependence of the scattering pattern on the degree of film orientation is determined. It is shown that the sizes of the central maximum of the scattering pattern depend linearly on the degree of film extension. The results obtained can be used in developing methods of optical nondestructive testing of polymeric materials.

Research Based on Optical Non-Destructive Testing of Pigment Identification

Optical Non-Destructive Testing (ONDT) can be applied as penetrating elemental and structure analysis technology in the Pigments identification field. Three-dimensional video microscopy, Raman microscopy and energy dispersive X-ray fluorescence spectroscopy are employed to measure the materials based on a Qing Dynasty meticulous painting. The results revealed that the dark yellow area within the decorative patterns was presented due to the interaction of Emerald green and hematite, and the bright yellow edge area was delineated by Cu-Zn-Pb composition. The interesting thing is that an artificial synthetic ultramarine blue was checked in the painting. According to the first synthesized time of ultramarine blue and Paris green, the time limit of the painting completion can be identified. The principle of Pigment subtractive colorant and nitikaset method were employed to interpreting the results. Optical testing combined with the area of cultural relic identification can be a potential method to build an expert identification system successfully. This work also help lay the optical method groundwork for further cultural relic identification, sterilization, and preservation.

Residual stress measurement on the butt-welded area by electronic speckle pattern interferometry

Residual stress always exists on any kind of welded area. This residual stress can cause the welded material to crack or fracture. For many years, the hole-drilling method has been widely used for measuring residual stress. However, this method is destructive. Nowadays, electronic speckle pattern interferometry (ESPI) can be used to measure residual stress with or without the hole-drilling method. ESPI is an optical nondestructive testing methods that use the speckle effect. Mechanical properties can be measured by calculation of the phase difference by the variation of temperature, pressure, or loading force. In this paper, the residual stress on the butt-welded area is measured by using ESPI with a suggested numerical calculation. Two types of specimens are prepared. Type I is made of pure base metal part and type II has a welded part at the center. These specimens are tensile tested with a material test system. At the same time, the ESPI system was applied to this test. From the results of ESPI, the elastic modulus and the residual stress around the welded area can be calculated and estimated. With this result, it is confirmed that the residual stress on the welded area can be measured with high precision by ESPI.