Analysis of the Effect of Residual Stresses on Fatigue Life of Welded Elements

There is a Supporting Software (SS) that is an integral part of UCC Measurement Unit. The SS allows controlling the measurement process, storing the measured and other data and calculating and plotting the distribution of residual stresses. The software also allows an easy connection with standard PC’s.

**Figure 1. **Example of the graphic presentation of the residual stress measurement data in a low carbon steel plate after local heating.

The software **UltraMARS** is a separate optional product. This software consists of Advance Database (AD) and Expert System (ES) for Analysis of the Effect of Residual Stresses on Fatigue Life of Welded Elements. An example of presentation of the residual stress measurement data, using the developed AD, is shown in Figure 1. The software allows comparing different data on residual stress measurement and transferring selected data for further fatigue analysis. In Figure 1, the left side of the screen displays information on the measured ultrasonic wave velocities as well as other pertinent technical information on the sample. The right side of the screen displays the distribution of calculated residual stresses.

In the example presented below, a plate made from low carbon steel, with a yield strength of 296 MPa, was heated locally, with the focal point of heating located approximately 50 mm from the left side of the plate. The distribution of both components of residual stresses in the specimen, as a result of this local heating are shown in the right side of Figure 3. As can be seen, in the heating zone, both residual stress components are tensile and reach the yield strength of the considered material. In the compression zone, located between the edge of the plate and the centre of the heating zone, the longitudinal component of residual stresses reaches – 140 MPa.

**Figure 2.** Fatigue curves of transverse loaded butt weld:1 – with high tensile residual stresses for all levels of R;2,3,4 & 5 – without residual stresses at R=-1, -0.5, 0 and 0.5

Using the Expert System (ES) it is possible to assess, through calculations, the redistribution of the residual stresses, under the effect of various improvement treatments and cyclic loading, and their influence on the service life of welded elements without having to perform time- and labor-consuming fatigue tests. The program package enables to calculate the fatigue life of welded components after application of improvement treatments directed at the redistribution of residual stresses, such as heat-treatment, vibration treatment, overloading, ultrasonic peening, etc. As an example of using the ES for residual stress analysis, the results of computation of the residual stress effect on the fatigue life of a transverse loaded butt weld (made in shop in flat position, toe angle ≤ 30°, NDT), depending on the stress ratio R, are presented in Figure 2. The presence of tensile residual stresses causes a decrease in the fatigue strength of the welded joints: the lower are the R and the stress range – the higher is the influence of the welding residual stresses. For instance, the decrease of the limit stress range at N=2×10^{6} cycles in the case of R=0 is 42% and for R=-1 is 70%. The effect of the welding residual stresses is relatively low at R ≥ 0,5.