Fatigue behaviour of welded joints

The strength of welded joints subjected to cyclic loading is much lower than the fatigue strength of the base metals involved. This poor fatigue performance can be explained by the inherent presence of geometric singularities at the weld, which result in stress concentrations, and by the high residual tensile stresses caused by welding shrinkage.

The various defects present in welds also contribute to reducing fatigue strength.

As a result, fatigue cracking is one of the main causes of failure in welded joints. Knowledge of this phenomenon is fairly recent and is not necessarily well understood by engineers.

When constructing a mechanically welded or boiler-made structure, welding engineers are most often concerned about metallurgical defects (cold cracking, hot cracking, etc.) or operational defects (lack of fusion, porosity), as these generally appear during the manufacturing stage. As for ductile fractures caused by exceeding the permissible load, these are generally well accounted for in calculation codes. Finite element analysis reveals areas that exceed the permissible criteria for the material, generally based on the elastic limit or the breaking point. On the other hand, fatigue failure is more difficult to understand: it is no longer a maximum stress that is sought, but a loading history (variation and number of cycles) which, in many cases, cannot be defined, given the variety of stresses that the structure or element will be subjected to.

However, fatigue cracking failures occur in nearly 60% of cases analyzed for in-service failures, far ahead of other causes, which may include corrosion or brittle fracture problems.

Among the main causes leading to fatigue failure, design flaws are often present, either due to a lack of understanding of the specific nature of fatigue cracking or due to difficulties in assessing the stresses that will be experienced. However, manufacturing defects (lack of fusion, lack of penetration, excessive thickness) appear in one-third of cases to have contributed to the failure.

Fatigue failure occurs in all sectors of activity: bridges, handling and lifting equipment, transportation (road, rail, air, sea), pumps, agitators, agricultural machinery, machine tools, bridges, metal furniture, whether urban or domestic, etc.

After reviewing the basics of welding and fatigue, the aim of this article is to present the various issues related to fatigue in welded joints with filler metal, particularly in arc welding with coated electrodes, submerged arc welding, TIG welding, MIG/MAG, and laser welding: characterizing behavior through testing and sizing these joints in service in order to improve their design and...