Crack propagation analysis and rehabilitation by HFMI of pre-fatigued welded structures

Abstract

This paper deals with a crack propagation analysis of welded structures and rehabilitation after pre-fatigue loading by the high-frequency mechanical impact (HFMI) posttreatment technique. The investigated specimen type is a thinwalled longitudinal stiffener made of mild steel S355. Fracture mechanical calculations are primarily performed on the basis of the weight-function approach. Thereby, the local residual stress condition at the weld toe is considered by the aid of a structural weld simulation, whereas the numerically evaluated residual stress distribution in depth agrees well to X-ray measurement results. The fracture mechanical analysis illustrates that by incorporating the residual stress state, the calculated lifetime is in good accordance to the conducted fatigue test results. By application of the HFMI-treatment as rehabilitation method, it is found that the beneficial post-treatment effect increases especially by a reduction of the applied load-level. Hence, particularly for minor nominal stress ranges near the high-cycle fatigue region, the mechanical post-treatment as repair method is utmost effective leading to almost equal fatigue strength as for the HFMI-treated specimens without pre-cycling. Finally, proposals for the crack growth assessment of welded structures and a conservative application of HFMI as rehabilitation method for mild steel joints are provided.

Effects of High Frequency Mechanical Impact on Fatigue Life of Semi-Automated Gas Metal Arc Welding (GTAW) of HSLA Butt Weld

“…All fatigue test data points are above the IIW recommendation, even if a bonus factor of 1.5 for HFMI treatment and a benign thinness bonus factor are applied [10]. Leitner et al [11] found that the influence and effective of stress ratio may be related to the fundamental change in the local residual stress state. HFMI-treated high-strength steel joints it is observed that the effect of the HFMI-induced compressive residual stress state acts as significant benefit factor to increase the fatigue resistance due to this post-treatment technique and leads to a reduction of the local residual stress condition.…”

Crack Initiation and Propagation Fatigue Life of Ultra High-Strength Steel Butt Joints

“…A number of different methods exist for monitoring crack propagation. For welded structures, local strain measurement at hot spots [21,22] is widely used due to its flexibility and easy handling. This procedure is expedient if the specimen has a low number of distinct hot spots, such as a longitudinal stiffener, where crack initiation will occur.…”

Life Extension Technique for Welded Structure Using Hfmi/Pit: A Review on Past and Current Researches With Applications

“…They concluded that the butt joint treated with HFP achieved fatigue life almost similar to the fatigue strength of base metal, the T-joint Transition knee point Nk shifts to a lower number of load cycles implying a significant increase in endurable strength limit, while the longitudinal attachment HFP treated shows an increase of the slope k in the low-cycle fatigue regime. The data of these investigations are shown in Martin Leitner et al [17] also investigated the crack propagation of the welded structure of the carbon steel S355 and rehabilitate its using post treatment HFMI. Around 1 mm pre-crack was occurred on the welded T joint of the carbon steel S355 before post treated with HFMI/PIT.…”