Vol 55, No 12 (2019)

Systems for nondestructive testing (NDT) of rails are multichannel and concurrently use several methods, including ultrasonic, magnetic flux leakage (MFL), visual, and others. Periodic inspection of rails does not allow easily monitoring the condition of rails and tracking the trend in the development of defects simultaneously through multiple (up to 14) channels based on several parameters of ultrasonic- and magnetic-testing signals. We propose approaches to developing a generalized (integral) parameter that takes into account the main characteristics of signals from defects concurrently through all testing channels (methods). Using this parameter, we have analyzed the development of various defects in the rail head during repeated (up to 17 times) inspections. As expected, longitudinal defects in the head develop over a rather long (up to 4 years) period of time. At the same time, transverse cracks, most dangerous due to the likelihood of a sudden break in the rail, develop quite rapidly (0.5–4 months). Attention is drawn to the significant instability of ultrasonic flaw-detection signals during repeated monitoring and to regular changes in magnetic-testing signals. It is shown that it is possible to monitor the development of rail defects and take proactive measures for the timely removal of dangerous defects. The proposed approaches can be used in monitoring the development of defects in other fields of NDT.

A procedure is proposed for rejecting girth welds based on an analysis of the main parameters of acoustic emission (AE) signals recorded during multiple-pass welding. The accuracy of locating AE signals recorded from defects and their filtering from noise and interference were ensured by the use of a modified tabular method and rejection grid. Normalized coefficients were calculated from the energy and amplitude of localized AE signals and used for in-process assessment of weld quality. Threshold values were determined for the normalized coefficients above which the weld is rejected. Defects in girth welds were identified using this procedure, and the possibility of determining defect hazardousness in real time was demonstrated.

Longitudinal and shear ultrasound pulse velocity (UPV) were measured in concrete under tensile stresses. 17 beams made of 7 mixtures were tested under flexural tensile loading until failure while UPVs were recorded. At low stresses, before 30% of the maximum tensile stress, mixtures showed acoustoelastic response, with linear UPV variation as the stress increased. Beyond 30% of the maximum load UPVs started to decay, indicating the degradation of the material due to the onset and propagation of microcracks. Close to failure, mixtures presented an even steeper UPV decay, indicating the degradation of the elastic properties. Measured third-order elastic constants indicate a high acoustoelastic sensitivity of concrete under tensile stresses. The different magnitudes of the third-order parameters of the mixtures demonstrate the potential of acoustoelasticity in ultrasonic investigation of concrete. Results also proved that mechanical damage due to tensile stresses, beyond the acoustoelastic region, induce a decrease in both Lame constants.

Hydrogen saturated samples of technically pure titanium have been studied by the electron-positron annihilation method (EPA), coupled with the thermoelectric power measurements performed in these samples saturated by different amount of hydrogen. The structure of the hydrogenated samples was additionally investigated by X-ray diffraction. The complete coincidence of the moment of occurrence of a change in the structure of hydrogenated titanium depending on the amount of introduced hydrogen has been established. The intensity of positron annihilation drops with increasing hydrogen concentration in α-titanium to 0.04 wt % and then remains unchanged up to values of 0.05 wt % (\(\alpha + \delta \)) -titanium to increases afterwards. At the same time, a sharp change in the values of the thermoelectric power occurs in this range. In the region of 0.05%, the annihilation rate stabilizes and begins to increase, while the thermoelectric power begins to decrease slowly. The inflection point on the dependence of thermoelectric power on hydrogen concentration corresponds to the onset of the formation of titanium \(\delta \)-hydrides. An increase in the positron lifetime is observed in the concentration range of 0.05–0.08 wt %, then the lifetime stays stable up to concentrations of 0.08–0.12 wt %. A transition from (\(\alpha + \beta \)) to (\(\alpha + \delta \)) phase is formed in this range. Next, the positron lifetime increases, as does the number of defects, while the thermoelectric power gradually drops (to a concentration of 0.24 wt %). This is followed by a stabilization mode of all the above parameters to 0.35 wt %.

A procedure has been developed to detect surface defects in the head and foot of a rail using a magnetic method involving thin-film magnetoresistive field sensors. The setup consists of a system for moving the rail in the longitudinal direction, a magnetizing solenoid, cartridges with field sensors for inspecting the rail head and foot, a DC electromagnet with attached pole pieces, and a “robotic” transverse movement unit with an additional cartridge for revealing long-sized defects (over the entire length of the rail). The electrical signals of the sensors are transmitted to an analog-to-digital converter and processed digitally. The possibility of detecting all surface defects stipulated by the regulatory and technical documentation of JSC Russian Railways is demonstrated.

Friction stir welding of pipes is gaining research interest as it delivers high performance weld joints when compared to fusion welding techniques. Mechanical testing of such welded pipes will yield only results about mechanical behaviour but cannot detect the defects. An attempt has been made in employing non-destructive techniques in analysing the defects in Aluminium Alloy 6063 pipe joint obtained using friction stir welding. Three different tool pin profiles have been used in this study. The non-destructive testing methods employed helped to show the variation in the quality of the welds achieved through the three different tool pin profiles. Dye penetrant testing has been used in assessing the surface defects where solvent removable penetrant SKL-SP1 has shown evident results. Radiographic testing has been used in assessing root side defects. Tool with taper cylindrical pin along with tool rotational speed of 2000 rpm and weld speed of 0.6 rpm has produced weld with minor defects based on liquid penetrant and radiographic test results. Also the capability of non-destructive techniques in analysing the defects produced through friction stir welding has also been evaluated and presented.