Non-Destructive Testing Technology
Year:2021 | Volume:2 | Issue:8|Papers Count:9

Detecting structural defects such as cracking and corrosion is largely achieved by techniques such as eddy current and ultrasonic testing. For this purpose, these methods require point-by-point scanning over the area of interest. Digital image correlation provides a cheaper and quicker testing method. In this method, imaging of the surface of the structure is performed during loading and unloading by using a suitable camera. In the traditional digital image correlation, to tracking the surface, an artificial pattern should be created on the surface. Creating a suitable pattern on large surfaces is so difficult. With the use of the laser source, a random laser speckle can be created on the sample instead of an artificial pattern. In this paper, the detection of a crack in a polycarbonate plate is investigated. The amount of strain around the crack is more than in other areas, and this issue can be completely detected by the method of digital image correlation with laser speckle. If the structure can be loaded easily, this method is a quick test to identify cracks.

In this research, a method based on the classification and prediction of the neural network to determine the location and orientation of cracks in pipes is presented. For this purpose, first, the finite element method is used to model wave propagation and crack modeling in different locations and orientations. In this regard, two types of longitudinal and torsional guided excitation waves have been used. The obtained signals are processed to calculate the appropriate characteristics. In the present study, reflection echoes were measured، and five features were extracted at six levels from discrete wavelet decomposition of raw signals. Selected properties of the signals are processed to limit the size of the neural network model without losing information. For this purpose, the firefly algorithm method was used and fed to an artificial neural network that predicts the location and orientation of the crack. In this study, conventional multilayer perceptron diffusion networks have been used. According to the obtained results, it is observed that the proposed method shows good accuracy in predicting the location and orientation of the crack, and the percentage of neural network errors is less than 7%.

Nowadays, it is of special importance to know the precision casting parts, especially gas turbine parts, and to investigate the reasons for their destruction, due to their special role in power supply and oil and gas transmission. Therefore, the use of alloys capable of withstanding thermal and mechanical stresses and operating at temperatures above 800 ˚ C is essential in the manufacture of such parts. Non-destructive testing plays an important role in identifying defective components. Therefore, the use of corrective or preventive actions will be an effective step towards eradicating defects. In this study, we decided to control the behavior of parts by introducing common defects in precision casting and identifying them by X-ray industrial radiography. In this regard, we can mention the types of shrinkage and porosity defects as common defects, and melting temperature, feeding system, number of covering layers, regional temperature, and cooling rate of the base metal as effective factors in causing defects. Identifying and taking action to prevent or reduce defects will decrease production risk, save time, reduce initial and current costs in the application industry and increase the reliability of parts produced and efficiency.

In order to utilize NDT methods to evaluate manufactured parts via Additive Manufacturing (AM), scales are needed to represent probable defects in the produced part. In light of common defects in AM produced parts (gas porosity, powder porosity lack if fusion, etc. . . ) and difference in their forming mechanism with probable defects in parts manufactured via traditional means, using present calibration blocks are not very effective. In the presented paper herein, an innovative reference block with designated internal characteristics for the calibration of ultrasonic equipment has been designed and manufactured by SLM method. Then, it was tested by pulse-echo ultrasonic test method. To attain confidence in the precision and accuracy of designated characteristics, cutting of the part took place and their dimensions was measured by light microscope and their distance from free surface by micrometer. To obtain results indicated affirmation in dimensional accuracy and precision and hence their utilization as a reference block in BDT measurement was confirmed.

This paper introduces and reviews digital shearography applications in the detection and measurement of crack type defects in materials. Digital shearography is a laser light-based NDT method for fullfield and non-contact measurement of an object deformation (surface displacement derivatives). This method is equivalent to a full-area strain gauge that allows you to observe the strain distribution over a large area. Shearography is used to inspect and non-destructive evaluation of different materials. However, because of some advantages of shearography compared to traditional methods in the evaluation of some materials like composites, and on the other hand, the popularity of these structures in important industries such as aerospace and military, shearography is increasingly being considered. Regarding understanding the significance and influences of the shape and geometry of defects in shearography results, this article attempts to prove and introduce digital shearography as a superior method in the field of both quantitative and qualitative evaluation of sub-cracks in various materials by reviewing previous studies and research. As concluded form previous research the loading, shear distance and direction, crack length, depth, and angle have the most influence on the results and fringe patterns formation. Shearography ability to estimate crack geometry has also been demonstrated and proven.

Spot welding is a type of resistance welding that is used to weld sheets. In this method, the pressure of the electrodes and the heat generated by electrical current lead to welding two or more sheets. Although the use of ultrasonic waves is one of the most common methods for assessing the accuracy of spot welding, in this study, the mechanical properties of the weld, such as the tensile strength of the spot weld, is studied with ultrasonic waves attenuation. For this purpose, spot welding is created in sheets of DP590 by changing the electric current parameter in the range of 11 to 14 kA. Then the ultrasonic waves with the frequency of 20 MHz passed through the spot weld and then the attenuation of the ultrasonic waves is measured. After that, by using the tensile-shear test, the strength of the spot welding is measured. The microstructure of the welds has also been studied by electron microscopy. The results show that there is a direct relationship between ultrasonic wave attenuation and tensile-shear strength. Soby increasing ultrasonic wave attenuation, spot welding strength decreases, which can be attributed to the change in spot welding microstructure.

In this paper, resonance ultrasonic spectroscopy is employed to evaluate a linear grating of fibrous composite materials. The ultrasonic longitudinal wave scattering from the embedded linear grating in an elastic matrix is simulated using finite element method. To involve the frequency effects of the measurement system, the modified short-pulse method of isolation and identification of resonances (MIIR) is employed to calculate the resonance modes and frequencies of the elastic fibers. The conformity conditions of the obtained results from analytical methods and the short pulse MIIR method are then investigated, including various orders of scattering. The FE-based resonance ultrasonic spectroscopy method is employed to investigate the behavior of the backscattered resonance spectrum for a linear grating of two fibers embedded in an epoxy matrix and evaluate the effects of grating structure and measurement system. The obtained results show that, due to the second order of scattering, variation of the transducer position with respect to the linear grating of fibers can shift the detected resonance frequencies.

Carbon fiber reinforced polymers, or so-called CFRPs, are widely used in different industries due to their unique properties, such as high strength-to-weight ratio. A common example of application of this material are composite planels that are used in the construction of composite panels curved and flats surfaces. Despite the excellent properties of these materials, a variety of defects such as fiber disbonding or matrix cracks, affect their final performance. Therefore, the development of non-destructive inspection methods to identify defects in the parts made of this material is very important. In this paper, samples of carbon fiber composites have been tested by X-ray radiography method. The results have been compared with two new NDT methods of digital shearography and infrared thermography. The results show that thermography is more successful in detecting many internal defects than the traditional X-ray radiography and shearography. The results also show that in the curved geometry, which is not possible to complete the radiography test due to a limitation of some X-ray sources, the thermography method presents very acceptable results. It is also possible in many cases to use shearography method in flat samples to identify some kinds of defects, more successfully.

In recent decades, the pavement health monitoring issue has attracted the attention of many researchers and has led to new advances and technologies. In this study, while examining the monitoring concept, the existing methods for monitoring the structural health of pavements have been categorized. To date, two general monitoring approaches from surface and inside the pavement have been used to assess their structural health. The new monitoring method using embedded equipment inside the pavement seeks to provide solutions aimed at continuous monitoring of structural health. These solutions rely on pavement instrumentation. This means that sensors are embedded inside the road during the road construction process. Utilizing monitoring methods inside the pavement can bring many technical and economic benefits to road networks. Compared to the pavement monitoring method from its surface, it can facilitate monitoring all pavement layers separately, continuously, and remotely without interfering with traffic flow. This approach is in line with the realization of smart roads and, unlike most of the old methods, it is a non-destructive method. For this purpose, in this study, a review of recent research on monitoring methods inside the pavement has been conducted. To this end, electromagnetic, fiber optic and wireless monitoring sensors have been evaluated. Finally, some suggestions have been proposed to utilize these monitoring techniques for Iranian road infrastructures.