Improving Fatigue Life of Bridges, Pipelines and Offshore Structures
In structural steel components, cracks usually occur at locations containing high stress concentrations, high residual tensile stresses and initial flaws, or combinations of these factors. These cracks might be present after fabrication or may be initiated during the life of the structure. They grow in a stable manner under the application of cyclic loading until the crack size becomes large enough to cause unstable crack propagation. Once a crack has been detected, various actions can be taken. First, the crack can be monitored over time based on fracture control plans defining inspection intervals. Secondly, the crack can be repaired by adequate means. Lastly, the cracked component can be replaced.
European convention of constructional steel work ECCS (2005) listed the most important repair and strengthening methods to increase the remaining fatigue life of welded details. These methods are listed below:
Removal of cracks
Post-weld treatments (grinding, shot peening, air hammer peening, Tungsten inert gas TIG dressing, ultrasonic impact treatment UIT)
Adding plates or fibre reinforced plastic strips (FRP strips)
Drilling of stop holes
For steel bridges, pipelines and offshore structures in service, finding an efficient tool to improve the fatigue resistance of critical weld details is important.
Assessment of Improvements
Smith and Hirt (1985) stated that the fatigue strength improvement due to burr grinding at 2 x 106 cycles between 50 and 200% depending on the type of joint. Mohr et al. (1995) conducted statistical analysis on some improvement techniques as applied to welded specimens with transverse attachment plates, and found that the improvement on fatigue life produced by grinding was a factor of approximately 2.2.
Fatigue life improvement for various types of post-weld treatment methods other than grinding is dependent on the techniques used. The remelting method of the weld toe region using a TIG or plasma torch generally induces significant fatigue strength improvements due to the production of a smooth transition between the plate and the weld metal. The increase in fatigue strength in air at 2x106 cycles as compared with as-welded joints is approximately 100%. However, this fatigue life improvement does not increase with increasing material tensile strength unlike the case of grinding. Moreover, the seawater corrosive environment could have an adverse effect on the fatigue life improvement using TIG technique. Mohr et al. (1995) obtained a factor of 2.2 from a statistical analysis of many tests for fatigue life improvement for weld toes treated by TIG dressing.
In case of the peening methods (hammer, shot and ultrasonic impact), the fatigue life improvement results from inducing favourable compressive residual stresses to replace the tensile residual stresses produced by the welding process at the weld region. Accordingly, the fatigue crack initiation life of the welded components is substantially increased due to these compressive stresses fatigue life. The improvement in fatigue strength obtained by peening treatments are among the highest reported and are typically of the order of 50-200% for hammer peening, 30% for shot peening and 50-200% for ultrasonic impact peening (Kirkhope et al. 1999). In some conditions, the enhancement is so large that the weld is no longer critical and failure initiates in the base plate away from the weld (Josi and Grondin 2010) or in other cases for fillet welds the point of eventual failure moves from the weld toe to the weld root.
Comprising two post-weld treatments has a significant effect on the fatigue life improvement techniques. In general, these combinations should only take into account the weld geometry improvement method with a residual stress method (e.g., toe grinding and hammer peening, but not toe-grinding and TIG dressing) (Kirkhope et al. 1999). Although, this approach may lead to an expensive solution, it can be applied in cases where costs are of minor importance such as repair of a damaged structure or in other cases where extensive redesign of the structure to meet fatigue requirements is to be avoided.
Among the various post-weld treatment methods, weld toe grinding is considered to be cost effective. Weld toe grinding requires a burr grinding tool, which is considered to be relatively cheap compared to other tools used in post-weld treatments especially peening methods. It is also an easy method that does not require any further tests or checks as in peening and remelting methods. It does not need highly skilled and trained personnel like peening methods. Moreover, it has easier accessibility, when compared to disc grinding and water-jet eroding. It is also considered the best solution for fillet welds subjected to transverse loading (Kirkhope et al. 1999).
Post-weld Treatment Limitation
Kirkhope et al. (1999) illustrated that there could be initial weld defects and undercuts, which were not removed from the weld toe by the burr grinding. The limitation of the improvements obtained by TIG and plasma dressing are due to the uncertainty about the initiation site of the fatigue crack, caution should be used when considering applying TIG dressing improvement techniques to the longitudinally loaded joints and other weld details. Moreover, checks should be made to ensure that fatigue failure will not first occur at the weld root or some other site in the weld detail (Kirkhope et al. 1999). TIG dressing is sensitive to weld contaminants, much more so than other weld improvement methods. As a result, the weld and adjacent plate should be thoroughly de-slagged and wire brushed to remove all traces of mill scale, rust, oil and paint. Kirkhope et al. (1999) emphasized the limitations of fatigue life improvement regarding the hammer peening treatment, where excessive peening induce cracks, while the corrosion effect could reduce the effect of shot peening as its effect is applied for a very small thickness.
European Convention of Constructional Steelwork (ECCS) (2005). “Assessment of existing steel structures, remaining fatigue life”. Technical Committee 6 – Fatigue.
Kirkhope, K.J., Bell, R., Caron, L., Basu, R.I. and Ma, K.-T (1999). “Weld detail fatigue life improvement techniques. Part 1: review”. Marine Structures, 12, 447 -474.
Josi, G. and Grondin, G.Y. (2010). “Reliability-based management of fatigue failures”. Structural Engineering Report No. 285, Dept. of Civ. and Envir. Engrg., University of Alberta, Edmonton, Canada.
Mohr, W. C, Tsai, C., Tso. C-M. (1995). “Fatigue strength of welds with profile and post-weld improvements”. Proc. of 4th Int. Conf. Offshore Mech. and Arctic Engrg., Copenhagen, Denmark.
Smith, I.F.C. and Hirt, M.A. (1985). “A review of fatigue strength improvement methods”. Canadian J. Civil Engrg., 12, 166-183.
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