Low-Cycle Fatigue of HT80 High-Strength Steel Welds

Abstract

Low-cycle fatigue process and failure at high stresses beyond elastic range are investigated for individual weld zones, namely base metal, heat-affected zones and weld metal, of an 80kg/mm² class heat-treated highstrength steel weld.
The specimens, shape of which is transverse-weld cantilever beam of uniform strength in bending, are divided into three types, that is, a plain plate as cut out from a weldment and similar plates with transverse small circular holes acting as stress raisers in each weld zone or with a reinforcement. In the fatigue processes at alternating stresses applied by displacement-controlled repeated loading, the stress-strain behaviours for each weld zone and propagation of macroscopic cracks initiated from stress raisers are observed at different numbers of cycles of stress. The main results are summarized as follows:
As fatigue proceeds, stress-strain hysteresis loops, areas of which are equivalent to absorbed energy per unit cycle of stress, become progressively wider for all the weld zones with a decreasing elastic limit and an increasing plastic strain except that in the first stages of fatigue both the elastic limits and plastic strains of over-heated zone and weld metal vary reversely to the above. The magnitudes of absorbed energy in heataffected softened zones are so great that main macroscopic cracks which led to fracture take place in these zones of all the specimens regardless of presence of stress raisers.
In notched specimens macroscopic cracks appear at every stress raiser early in the cycles of loading, and propagate similarly with approximately linear relation to cycles of stress but at different growth rates in the weld zones such as the greatest of the softened zones until the crack growth rates in the softened zones begin to increase rapidly at the final stages of fatigue.