Methodology for Mapping the Residual Stress Field in Serviced Rails Using LCR Waves

Non-destructive stress measurement by ultrasonic testing is based on calculating the acoustoelastic modulus obtained from the relationship between material stress and sound wave velocity. A critically refracted longitudinal (L_CR) wave, which is a bulk longitudinal wave penetrating below and parallel to the surface below an effective depth, is most suitable for ultrasonic stress measurement tests because it exhibits a relatively large change in travel time in response to a change in stress. In particular, the residual stress distribution through the thickness of the subject can be calculated if transducers of different frequencies are applied because of the characteristic of propagation to different depths of penetration depending on the frequency. The main purpose of this study was to visualize the internal or residual stress distribution through the thickness of rails using L_CR waves. To this end, L_CR probes with different center frequencies were designed and manufactured, and the residual stress values ​​of an unused railroad rail and two used railroad rails operated under different conditions were calculated. This was done using the ultrasonic signals received from each probe, of which the distributions were mapped. Through these mapping results, different residual stress values could be calculated according to the depth. The differences in residual stress generation and distribution according to the conditions surrounding the contact between train wheels and rails, and their characteristics, were visualized and analyzed. As a result, it could be concluded that the non-destructive evaluation technique using L_CR waves could detect differences in the residual stress of a rail, and thus can be used to measure the residual stress of the rail accurately.

Ultrasonics methodology for defect and stress detection of in-serviced T-shaped R-zone components

The T-shaped R-zone component widely exists in aerospace and satellite communication area, such as supporting structure under the wing skin and the mechanical supporting frame of the satellite large load-bearing high-precision test platform, which seriously affects the performance and the safety of the aircraft and the satellite. In this paper, the overall process of ultrasonics methodology of T-shaped R-zone components is proposed, systematically investigated and experimentally verified. The detection of the defect and the stress is respectively realized based on the synthetic aperture total focusing method and the Mohr's stress circle theory with the aid of the linear phased array probe and the longitudinal wave along the stress propagation direction. The theoretical simulations and the experimental measurements both demonstrate the effectiveness of the proposed methodology. The dimensional and the positioning errors are respectively around 0.67 mm and 1.69 mm for the 1 mm × 4 mm crack defect existed in the T-shaped R-zone component. The detecting error of the stress is less than 17.91 MPa in the entire inner of the T-shaped R-zone component, which is applied a pressure in the range of 0-400 MPa.

Measurement of fastening force using dry-coupled ultrasonic waves

The accurate measurement of assembly fastening force of the high-pressure compressor rotor is of great significance to improve the structural connection quality and comprehensive performance of aero-engine. To solve the problem that liquid couplant reduces the measurement accuracy and causes the surface of the bolt to rust and corrode, a method of measuring the fastening force with dry-coupled ultrasonic wave was proposed in this paper. The measurement model of the fastening force including the thickness of the protective film of the ultrasonic transducer and the couplant was established, and the influence of the thickness variation in the couplant on the measurement accuracy of the fastening force was analyzed. Based on the propagation model of the ultrasonic wave on the heterogeneous interface and the principle of ultrasonic dry coupling, a coupling device was designed. At last, the experiment of fastening force measurement based on dry coupling and liquid coupling ultrasonic wave was carried out and compared. The experimental results show that the average relative errors of the fastening force measurement based on dry coupling and liquid coupling ultrasonic wave are 2.13% ± 0.42% and 3.15% ± 0.80%, respectively. Therefore, the dry coupling method can be as good or better in measuring the accuracy of the fastening force. Furthermore, it also overcomes the limitations of the liquid coupling method, which should make it more suitable to the measurement of fastening force in the aerospace field.

Plane stress measurement of orthotropic materials using critically refracted longitudinal waves

Stress measurement in anisotropic materials attracted considerable interest from the field of engineering. This study presents an improved ultrasonic method for measuring plane stresses in orthotropic materials. This study considers the velocity variations of critically refracted longitudinal (LCR) waves induced by orthotropic anisotropy and internal stresses. The stress-acoustic relation in orthotropic materials is used as theoretical basis for the proposed ultrasonic method. To verify the accuracy and usability of these relations, measurements are performed on a 6061 aluminum alloy and a carbon fiber reinforced plastic (CFRP) composite are subjected to under tensile tests. As preconditions for testing, longitudinal wave velocity is measured and LCR wave wedges are designed for these two materials. Stress-acoustic coefficients are determined by a series of calibration tests. Finally, we confirm the accuracy of this method by comparing the acoustoelastic coefficients of the 6061 aluminum alloy with certain reference values. The acoustoelastic coefficients of CFRP composite are compared with the theoretically predicted curve to assess its usability for orthotropic materials.

On the validity of several previously published perturbation formulas for the acoustoelastic effect on Rayleigh waves

This article revisits the evaluation by a perturbation theory of the modification of the Rayleigh wave velocity under a static loading varying with depth. Two derivations, that have been exposed in the past and presented as comparable, are questioned. A new derivation of the perturbation formula is given by adapting Auld's approach. Validation with exact calculations is provided. The examples cover depth-varying static stress as well as depth-varying third order elastic properties.