The Largest Open Access Portal of Nondestructive Testing (NDT)

Time of Flight Diffraction (TOFD) relies on a beam with relatively high divergence to provide suitable volume coverage. Refracting wedges provide a means of directing the beam towards the inspected volume. Most wedges are made of common polymers with acoustic velocities between about 2400m/s and 2800m/s which are 40-47% the average compression velocity in steel. This provides useful positive refraction in steels with relatively small probe centre spacing (PCS). However, when testing lower velocity materials and where access from the weld centreline is limited, it can be useful to have wedges that have lower velocities. Some applications can use water encased in a reservoir, but preventing the water from leaking and avoiding air bubbles can be problematic. A viable alternative is to fabricate the wedge from a low attenuation low velocity polymer. Examples of the quality of signals obtained using low velocity polymer are given in this paper

Determining acoustic velocity is a critical aspect for the correct calculation of Snell’s Law to determine refracted angle and to position the location of flaws along the soundpath of an ultrasonic beam. This has been seen as a critical part of preparations in girth weld inspections using zonal discrimination where it is mandated by codes. The typical steels used in pipeline construction are slightly anisotropic, making it necessary to determine the shear velocities at different angles in the plane of inspection. Anisotropy occurs due to the alignment of the acicular crystals in the direction of rolling. When the pipe is made with a longitudinal seam the velocities change in a relatively simple fashion because the changes occur in predominantly one plane of the crystal axes. However, when the pipe is made by the spiral welding process, the girth weld inspection requires the beam to cross the plane of the crystal axes at an angle. Refraction calculations for the longitudinal seam pipe have been demonstrated to use the fast SH shear mode. However, for pipe made using the spiral welding process, it has been found that the fast SH shear mode need not provide the best velocity for calculating the refracted angle in a zonal discrimination technique.

Coal is a complex resource and usually it varies within the same deposit. As far as applications are concerned the properties of coal are required to be checked. Acceptable and well-known methods are ingrained and these are widely used in laboratories worldwide. Amongst the techniques used the most commonly use technique is non-destructive technique (NDT) for detecting component and flaw characterization. Other than the flaw characteristics, parameter which is equally important to assess the structural integrity of engineering components is the material property. Researchers all over the world are working on extensive study to characterize, both microstructural and mechanical properties of materials by ultrasonic testing. This paper highlights the ultrasonic testing parameters useful for material characterization studies. In this paper we have presented our electronic system that has been developed for the measurement of velocity and attenuation of ultrasonic signal through coal sample which helps in characterization of coal.

The report presents an algorithm called the total focusing method for the circular phased array. It shows an improvement in the focusing for the main lobe beam and eliminates the effects of the side lobe. The report visualizes the effects for different ring sizes of the circular array and the effect of different numbers of array elements. The results of this modelling indicate that, for a given number of elements, placing them around the circular ring allow them to perform best and that the element spacing should be less than half a wavelength to avoid grating lobes and to eliminate grating lobes. In addition to that it describes how the TFM images have been built by computing half time traces of the transmitter and receiver array.

This paper presents a methodology and a prototype developed in our NDT group, for the application of nondestructive Impact-Echo technique, which has been implemented and introduced in Venezuela. This technique has shown excellent results to evaluate and characterize different types of concrete structures with experiences in other countries, well-documented and accessible. It is worth noting that this technique can be applied to a wide range of viscoelastic materials and the emphasis in this paper is made in the inspection of concrete structures as an alternative methodology to existing destructive methods. The study is based on the fact that concrete is one of the most used materials in infrastructure construction and therefore the results would be of immediate use. In addition to the possibility of thickness measurement, the technique can detect and evaluate a wide range of "defects" in the concrete structures, such as cracks open to the surface, the presence of laminations, cavities, etc. Any imaging application of this technique would help to determine the internal state in a wide range of basic structures such as floors, beams, slabs, walls, columns, tunnels, etc.

In the early 1980’s, the ultrasonic polar scan (UPS) technique was developed to assess the fiber direction of composites in a nondestructive way. In spite of the recognition by several researchers as being a sophisticated and promising methodology for nondestructive testing (NDT) and materials science, little advance was made during the following 30 years. Recently however, the UPS technique experienced a strong revival and various modifications to the original UPS setup have been successfully implemented. This revival has exposed several powerful capabilities and interesting applications of the UPS technique for material characterization and damage assessment. This paper gives a short historical overview of the UPS technique for characterizing and inspecting (damaged) fiber-reinforced plastics. In addition, a few future research lines are given, which will further expand the applicability and potential of the UPS method to a broader range of (damaged) materials, bringing the UPS technique to the next level of maturity.

There are many advantages to adhesively bonding stiffeners onto aircraft structures rather than using traditional mechanical fastening methods. However there is a lack of confidence of the structural integrity of adhesively bonded joints over time. Acousto-ultrasonic Lamb waves have shown great potential in structural health monitoring applications in both metallic and composite structures. This paper presents an experimental investigation of the use of acousto-ultrasonic Lamb waves for the monitoring of adhesively bonded joints in metallic structures using 3D scanning laser vibrometry. Two stiffened panels were manufactured, one with an intentional disbonded region. Lamb wave interaction with the healthy and disbonded stiffeners was investigated at three excitation frequencies. A windowed root-mean-squared technique was applied to quantify where Lamb wave energy was reflected, attenuated and transmitted across the structure enabling the size and shape of the defect to be visualised which was verified by traditional ultrasonic inspection techniques.

Towards developing a method capable to assess the efficiency of rejuvenators to restore embrittlement temperatures of oxidized asphalt binders towards their original, i.e., unaged values, three gyratory compacted specimens were manufactured with mixtures oven-aged for 36 hours at 135oC. In addition, one gyratory compacted specimen manufactured using a short-term ovenaged mixture for two hours at 155oC was used for control to simulate aging during plant production. Each of these four gyratory compacted specimens was then cut into two cylindrical specimen 5 cm thick for a total of six 36-hour oven-aged specimens and two short-term aging specimens. Two specimens aged for 36 hours and the two short-term specimens were tested using an acoustic emission approach to obtain base acoustic emission response of short-term and severelyaged specimens. The remaining four specimens oven-aged for 36 hours were then treated by spreading their top surface with rejuvenator in the amount of 10% of the binder by weight. These four specimens were then tested using the same acoustic emission approach after two, four, six, and eight weeks of dwell time. It was observed that the embrittlement temperatures of the short-term aged and severely oven-aged specimens were -25oC and -15oC, respectively. It was also observed that after four weeks of dwell time, the rejuvenator-treated samples had recuperated the original embrittlement temperatures. In addition, it was also observed that the rejuvenator kept acting upon the binder after four weeks of dwell time; at eight weeks of dwell time, the specimens had an embrittlement temperature about one grade cooler than the embrittlement temperature corresponding to the short-term aged specimen.

Internal stress in structural steel members is an important parameter for steel structures in their design, construction, and service stages. However, it is hard to measure via traditional approaches. Among the existing non-destructive testing (NDT) methods, the ultrasonic method has received the most research attention. Longitudinal critically refracted (Lcr) waves, which propagate parallel to the surface of the material within an effective depth, have shown great potential as an effective stress measurement approach. This paper presents a systematic non-destructive evaluation method to determine the internal stress in in-service structural steel members using Lcr waves. Based on theory of acoustoelasticity, a stress evaluation formula is derived. Factor of stress to acoustic time difference is used to describe the relationship between stress and measurable acoustic results. A testing facility is developed and used to demonstrate the performance of the proposed method. Two steel members are measured by using the proposed method and the traditional strain gauge method for verification. Parametric studies are performed on three steel members and the aluminum plate to investigate the factors that influence the testing results. The results show that the proposed method is effective and accurate for determining stress in in-service structural steel members.

Guided Wave Testing (GWT) using novel Electromagnetic Acoustic Transducers (EMATs) is proposed for the inspection of large structures operating at high temperatures. To date, high temperature EMATs have been developed only for thickness measurements and they are not suitable for GWT. A pair of water-cooled EMATs capable of exciting and receiving Shear Horizontal (SH0) waves for GWT with optimal high temperature properties (up to 500 C) has been developed. Thermal and Computational Fluid Dynamic (CFD) simulations of the EMAT design have been performed and experimentally validated. The optimal thermal EMAT design, material selection and operating conditions were calculated. The EMAT was successfully tested regarding its thermal and GWT performance from ambient temperature to 500 C.

A practical compact solid-state terahertz imaging system is presented. Various beam guiding architectures were explored and hardware performance assessed to improve its compactness, robustness, multi-functionality and simplicity of operation. The system performance in terms of image resolution, signal-to-noise ratio, the electronic signal modulation versus optical chopper, is evaluated and discussed. The system can be conveniently switched between transmission and reflection mode according to the application. A range of imaging application scenarios was explored and images of high visual quality were obtained in both transmission and reflection mode.

Wheels are very important for the safety of a train. The diameter of the wheel is a significant parameter that needs regular inspection. Traditional methods only use the contact points of the wheel tread to fit the rolling round. However, the wheel tread is easily influenced by peeling or scraping. Meanwhile, the circle fitting algorithm is sensitive to noise when only three points are used. This paper proposes a dynamic measurement method based on structured-light vision. The axle of the wheelset and the tread are both employed. The center of the rolling round is determined by the axle rather than the tread only. Then, the diameter is calculated using the center and the contact points together. Simulations are performed to help design the layout of the sensors, and the influences of different noise sources are also analyzed. Static and field experiments are both performed, and the results show it to be quite stable and accurate.

Nondestructive Testing (NDT) assessment of materials’ health condition is useful for classifying healthy from unhealthy structures or detecting flaws in metallic or dielectric structures. Performing structural health testing for coated/uncoated metallic or dielectric materials with the same testing equipment requires a testing method that can work on metallics and dielectrics such as microwave testing. Reducing complexity and expenses associated with current diagnostic practices of microwave NDT of structural health requires an effective and intelligent approach based on feature selection and classification techniques of machine learning. Current microwave NDT methods in general based on measuring variation in the S-matrix over the entire operating frequency ranges of the sensors. For instance, assessing the health of metallic structures using a microwave sensor depends on the reflection or/and transmission coefficient measurements as a function of the sweeping frequencies of the operating band. The aim of this work is reducing sweeping frequencies using machine learning feature selection techniques. By treating sweeping frequencies as features, the number of top important features can be identified, then only the most influential features (frequencies) are considered when building the microwave NDT equipment. The proposed method of reducing sweeping frequencies was validated experimentally using a waveguide sensor and a metallic plate with different cracks. Among the investigated feature selection techniques are information gain, gain ratio, relief, chi-squared. The effectiveness of the selected features were validated through performance evaluations of various classification models; namely, Nearest Neighbor, Neural Networks, Random Forest, and Support Vector Machine. Results showed good crack classification accuracy rates after employing feature selection algorithms.

The impact-echo (IE) method is a popular non-destructive testing (NDT) technique widely used for measuring the thickness of plate-like structures and for detecting certain defects inside concrete elements or structures. However, the IE method is not effective for full condition assessment (i.e., defect detection, defect diagnosis, defect sizing and location), because the simple frequency spectrum analysis involved in the existing IE method is not sufficient to capture the IE signal patterns associated with different conditions. In this paper, we attempt to enhance the IE technique and enable it for full condition assessment of concrete elements by introducing advanced machine learning techniques for performing comprehensive analysis and pattern recognition of IE signals. Specifically, we use wavelet decomposition for extracting signatures or features out of the raw IE signals and apply extreme learning machine, one of the recently developed machine learning techniques, as classification models for full condition assessment. To validate the capabilities of the proposed method, we build a number of specimens with various types, sizes, and locations of defects and perform IE testing on these specimens in a lab environment. Based on analysis of the collected IE signals using the proposed machine learning based IE method, we demonstrate that the proposed method is effective in performing full condition assessment of concrete elements or structures.

During the last decades, structural health monitoring (SHM) systems are used in order to detect damage in structures. We have developed a novel structural health monitoring approach, the so-called “effective structural health monitoring” (eSHM) system. The current SHM system is incorporated into a metallic structure by means of additive manufacturing (AM) and has the possibility to advance life safety and reduce direct operative costs. It operates based on a network of capillaries that are integrated into an AM structure. The internal pressure of the capillaries is continuously monitored by a pressure sensor. When a crack nucleates and reaches the capillary, the internal pressure changes signifying the existence of the flaw. The main objective of this paper is to evaluate the crack detection capacity of the eSHM system and crack location accuracy by means of various non-destructive testing (NDT) techniques. During this study, detailed acoustic emission (AE) analysis was applied in AM materials for the first time in order to investigate if phenomena like the Kaiser effect and waveform parameters used in conventional metals can offer valuable insight into the damage accumulation of the AM structure as well. Liquid penetrant inspection, eddy current and radiography were also used OPEN ACCESS Sensors 2015, 15 26710 in order to confirm the fatigue damage and indicate the damage location on un-notched four-point bending AM metallic specimens with an integrated eSHM system. It is shown that the eSHM system in combination with NDT can provide correct information on the damage condition of additive manufactured metals.

Quality of embedment of optical fibre sensors in carbon fibre-reinforced polymers plays an important role in the resultant properties of the composite, as well as for the correct monitoring of the structure. Therefore, availability of a tool able to check the optical fibre sensor-composite interaction becomes essential. High-resolution 3D X-ray Micro-Computed Tomography, or Micro-CT, is a relatively new non-destructive inspection technique which enables investigations of the internal structure of a sample without actually compromising its integrity. In this work the feasibility of inspecting the position, the orientation and, more generally, the quality of the embedment of an optical fibre sensor in a carbon fibre reinforced laminate at unit cell level have been proven.