The Effect of Welding Joints Design on the Fatigue Crack Growth Rate

Abstract

This project is the first in a series of studies that deal with the evaluation of welded joints. Each one type characteristic toughness and strength for constant amplitude and variable amplitude fatigue loads is more than other. It is intended to know the influence of the difference in welded joints design for types (V, VV, U, UU) of fatigue crack's behaviour and to compare it with base metal which is considered Mild Steel (St-52-3) and to show the influence of fatigue amplitude on the fatigue crack growth rate in this case. The testing is run by using Reverse Bending Method and fatigue test machine at stress ratio ( $\mathrm{R}=-1$ ) for constant Amplitude load. Fatigue results showed that the fatigue limit of the base metal is higher than the fatigue limit for all the welding groups, so it has been also noticed that the fatigue limit for the specimens welded by a welding joint type (U) is bigger than the fatigue limit at the specimens welded by a welding joint type (V) and this is bigger than (VV) and this is bigger that (UU). At the same time the base metal showed relatively higher resistance to fatigue crack initiation. This has been explained in terms of microstructures changes and the set-up of residual stresses due to welding microstructures examination has revealed that most of fatigue cracks occurred in the heat-affected zone (HAZ), especially in the overheated zone. This may be due to change in crystal structure and grain growth. Still the appearance of the fracture surface shows the fatigue characteristic.