Materials Science Forum Vols. 490-491

Abstract: Residual stress has been mapped in the heads of three normal grade and one heat treated (350HT) ex-service railway rails. Transverse, vertical and in-plane shear stresses were deduced using energy dispersive synchrotron X-ray, magnetic permeability and laboratory X-ray measurements. Differences were found between samples both in the distribution of compressive stress introduced at the running surface, and in the locations of the balancing tensile stress below this. The heat treated rail was found to have a thinner layer of surface compression than the normal grade rails, with tensile stress being closer to the active gauge corner. The thinner layer of surface compression in the heat treated rail may reduce the protective benefits for rail integrity that surface compression is thought to confer.

Abstract: The truncation of diffraction patterns in residual stress determination is often observed for broadened peaks when the 2θ acquisition range is not wide enough. The loss of information effects induced can either be traduced by a bad estimation of the background line, for the methods including a background subtraction, or a restriction of the analysis area for the others. In that borderline case, the results obtained by all methods with theirs specific parameters, developed to estimate the peak localisation are rather distributed in a wide range of stress values. In this paper we propose to review and to test some of the most common methods for stress evaluation (parabola, middle of chord, centred centroïd, asymmetrical pseudo-Voigt fitting). A separate study is made concerning error introduced on the 2θ peak position and on the final stress value estimated. For the parabola method, an analytical expression including some approximations such as the peak shape and its full width at half maximum is then given for the prediction and the correction of these errors. This study is sponsored by PSA PEUGEOT-CITROËN, RENAULT and SNECMA.

Abstract: The issue of nondestructive testing in aeronautical structures is of considerable importance in the aviation industry today. And a high sensitivity magnetic field sensor, which has recently been developed is designed for non-destructive stress testing. It is based on idea of the magnetic field produced by pulsed currents and perturbed by the presence of stress. The sensor can be effectively utilized for the detection of defects and stress concentration in conducting materials using eddy current testing measurements. The principle of the measuring technique is based on the unbalance of the magnetic field where the stress or cracks exist. Also, the excited input-signal is special designed. A pulsed current was inputted and changed into a self-attenuation signal which does the effect in the probe.

Abstract: The results presented in this paper are part of a process to analyse systematically the sources of uncertainty in X-ray stress determination. They concern one part of the effects of temperature variations which could intervene either as random fluctuations or as a monotonic drift during the acquisition. The proposed formulation is in agreement with the recommendations of the ISO guide on the expression of uncertainty (GUM). It was found that the effect is usually negligible for laboratory experiments which are often temperature controlled and for most materials. However the uncertainty can reach 20 MPa for austenitic steels and a temperature drift of 2 K.

Abstract: Shot peening is an effective method of improving fatigue performance of machine parts in the industry by producing a thin surface layer of compressive residual stresses that prevents crack initiation and retards crack growth during service. Nondestructive evaluation of the prevailing compressive residual stresses in the shallow subsurface layer is realized by the critically refracted longitudinal (Lcr) waves. This paper presents experimental data obtained on SMAT (surface mechanical attrition treatment) steel alloy S355 sample. Comparative travel-time shows that there are statistically significant differences in treated and untreated specimen. With knowledge of the acoustoelastic constants which are obtained by a test calibration, the experimental data indicates that compressive residual stresses are distributed near subsurface (hundreds of micron). These stress results show that the Lcr technique is efficient for evaluation of residual stresses after the surface treatment.

Abstract: The impact-indentation stress measurement method makes use of the principle of superposition stress fields resulting from the residual stresses and the surface plastic deformation. The main laws measuring stress by impact-indentation method are that, in the direction of major stress, the strain increment is in direct proportion to major residual strain in same indentation sizes and the strain increment in the fixed distance from center of indentation is in direct proportion to indentation size in the same residual stress fields. The stress measuring equipment has been developed and has more advantages than the hole-drilling method, such as higher precision, nondestructive, more convenient and rapid in the measuring process. Generally, its measurement error is less than 15 MPa. It has successfully applied in measuring residual stresses of various welded structures in the field.

Abstract: X-ray computer tomography for non-destructively measuring residual stress with depth resolution was suggested to aim at improving of traditional X-ray stress analysis in this paper. Based on higher penetrating capacity of X-ray in low atomic order materials it could non-destructively measure three-dimensional residual stresses and calculate the stress free lattice spacing, d0, by current X-ray stress analyzer with taper slits. The evaluation of reliable results were controlled by three criteria, linear correlation coefficient, R, Poisson’ ratio, ν, and the stress free lattice spacing, d0. Finally, the residual stresses in the axial direction around the weld of arc Beryllium weldment could be calculated by this method.

Abstract: The work of nanoindentation approach is developed and applied for measure of residual stress relaxation during indenter penetrates in thin films. The residual stress at given penetration depth is measured from the different integral area which under load-depth curve of different stress state. This method can measure residual stress relaxation without the need for estimating yield strength, strain hardening index and hardness.