Abstract
Optical non-destructive testing (NDT) has gained more and more attention in recent years, mainly because of its non-destructive imaging characteristics with high precision and sensitivity. This paper provides a review of the main optical NDT technologies, including fibre optics, electronic speckle, infrared thermography, endoscopic and terahertz technology. Among them, fibre optics features easy integration and embedding, electronic speckle focuses on whole-field high precision detection, infrared thermography has unique advantages for tests of combined materials, endoscopic technology provides images of the internal surface of the object directly, and terahertz technology opens a new direction of internal NDT because of its excellent penetration capability to most of non-metallic materials. Typical engineering applications of these technologies are illustrated, with a brief introduction of the history and discussion of recent progress.
1. Introduction
NDT (non-destructive testing) techniques, used over 30 years, are analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage to the samples [1]. The term non-destructive evaluation (NDE) is also commonly used to describe this technology. Because NDT refers to the detection of faults without affecting the operation of the equipment, it has been used in a variety of fields [2,3]. Currently, widely used NDT methods include ultrasonic, eddy current, microwave and acoustic emission, etc. However, these methods have limitations on the type of objects that can be inspected or the type of defect to be detected. Recent developments in optical NDT technology give higher detection accuracy and sensitivity, plus ease of signal multiplexing and resistance to electromagnetic interference. Main types of optical NDT are surface measurements, such as infrared thermal imaging, endoscopic and speckle imaging for component surfaces and subsequent image analysis to determine the presence of a defect. Optical fibre sensing is used for dynamic parameter measurements.
3. Discussion and Conclusions
This article outlines the various optical sensing and detection methods applied in NDT. This investigation attempted to establish the probability of detecting barely visible damage with optical NDT methods such as digital shearography, ESPI and infrared thermography. Compared to traditional NDT methods, these optical NDT techniques provide a wide range of advantages such as resistance to electromagnetic interference, non-contact, whole-field (except for ESPSI, which is a hole-field technique), not limited to particular material types, real time results (depending on some certain material types) and have been proven reliable in a large number of applications in the laboratory/factory/field environment [99]. In addition, optical NDT methods provide higher sensitivity and resolution for high precision applications. Despite the high cost of optical NDT methods, the advantages over conventional NDT methods mean its expansion into various applications.
