Post weld treatment - Implementation on bridges with special focus on HFMI (2014)

Post weld treatment is a more or less well established method for fatigue enhancement, depending on industry. In offshore industry, it has become a common method, while for the bridge industry, experiences have not come as far. Even though there are some examples of PWT implementations on bridges, the need of further research in this field is necessary and the potential benefits from it are substantial. This has been emphasized in most of the research found. Among all improvement techniques, HFMI, which is a peening method, seems to be the most promising. This, due to the high potential increase in fatigue strength it imposes and better qualities that can be achieved compared to other peening methods. A study of treatment quality variation concluded that UIT is a robust method, yielding significant improvements even if pronounced over/under-treatment is achieved. Although basically no international codes cover improvements with PWT, recommendations have been established and for HFMI, guidelines are being developed both regarding improvement assessment and procedure and quality assurance. The guidelines are based on experimental data, mainly coming from relatively thin plates (<30mm) and CAFL with stress ratios equal to R=0.1. Few samples of yield strengths less than 355MPa are present. The number of different thicknesses and yield strengths in the sample pool presented in Table 5 is illustrated in Figure 35. Thick plate sizes and material yield strengths equal to or less than 355MPa are common and of high importance when implementing HFMI on bridges.VAFL can either be simulated by its actual variations in the structural detail, or with constant blocks of loads with different amplitudes, derived from e.g. Rainflow or Reservoir methods. Whichever is used in testing, the fact that they can decrease the magnitude and benefits of the compressive residual stresses from HFMI is known, especially at overloads where stresses get close to the material yield strength. The steel grade has a very important effect with regard to relaxation of the stresses induced by HFMI. In the study presented for VAFL in 2.3.3, residual stresses were measured before and after loading [19]. Although obvious residual stress relaxation was observed, the improvement prediction gave correct results with substantial increase in fatigue strength anyway, however, quantification of the relaxation and possible differences between ways of modeling VAFL need more investigation. Since fatigue testing is a costly procedure, the idea of modeling fatigue behavior by FEM with respect to HFMI treatment has been raised. To do this, one need to account for the crack initiation and propagation phases, taking into consideration the internal residual stresses from welding and HFMI. With the help of FEA, the effects of the parameters of importance for bridges can then be evaluated; for instance, different thicknesses and different compressive residual stresses and geometries from HFMI, resembling variation of treatment quality. Two promising studies are presented in 3.3.1 and 3.3.2 that predict fatigue life in different ways and yield satisfactory results. However, the study performed in 3.3.1 was only made on plain steel with a discontinuity in form of a hole. Implementing this method on a welded detail improved by HFMI does imply some challenges. For instance, welding a detail dramatically changes the microstructural properties in the material affected by the heat. Usually, analysing the fatigue of a welded detail basically includes modeling crack propagation with LEFM. But when the weld toe is HFMI-treated, the initiation phase must now also be regarded, having to account for the microstructural change due to welding. Then also, the question arises whether if HFMI in itself also changes the microstructure of the steel significantly or not, aside from geometry and residual stresses, and how this can be accounted for. Due to the complexity described in connection with initiation phase modeling, it is considered that simpler fatigue life predictions, for instance by means of strain-based fracture mechanics are more feasible when analyzing HFMI-improved details. Since fatigue failure occurs with a relatively large variation, one cannot draw conclusions regarding the adequacy of the FE simulations in a manner of strict quantitative comparison to experiment results. The orders of magnitudes are of more interest and evaluation of data points in S-N curves give a better understanding of the adequacy of the FE models. Further research is needed to investigate special cases and cover circumstances for bridges. Examples of nine bridges, including railway, tram & pedestrian, cable car and roadway bridges could be found where HFMI treatment had been used and one railway bridge on which needle peening had been applied. The information available varied between the bridges, but was generally quite scarce.