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Numerical analysis of crack propagation based on phase field method in welded steel structures
MSc. Ehsan Farahani
Land (Sachsen-Anhalt) ;
Welding is considered as one of the most indispensable processes in many industrial sections for joining. In many structures, welds are known as a critical sections led to mechanical failures. There are a variety of physical defects such as undercut, insufficient fusion, excessive deformation, porosity, and cracks that can affect weld quality. Of those defects, cracks are considered to be the worst since even a small crack can grow and lead to failure. All welding standards show zero tolerance for cracks whereas the other defects are tolerated within certain limits. There are three requirements for cracks to form and grow: a stress-raising defect, tensile stress, and material with low fracture toughness. Microscopic defect locations are available in practically all welds including geometric features and weld chemistry that can raise the local stress enough to induce a crack. That leaves the engineer to work with the stress environment and toughness: if either of the two can be effectively controlled then cracks can be prevented from initiating and growing. Toughness is a measure of resistance to crack growth; resistance can be provided by blunting of the crack tip in ductile materials. However, if applied strain rate is very high (as would be the case when a spot weld cools at the end of the pulse) and the stress field is multi-axial, even ductile materials exhibit poor toughness and produce rapid crack growth. Hard materials, such as martensite formed during cooling of steels, are brittle and have poor toughness. Having a deep understanding of the residual stresses in welding, micro structure and mechanical behavior of HAZ, multi axial fatigue strength, crack progress behavior and the effect of improvement techniques on welded structures will result in manufacturing more reliable and minimizing weight and increasing structural strength.
The following objectives of this project are:
- Modeling welding process by considering the phase transformation changes occurred in base and weld metal during the heating and cooling process.
- Effect of weld material strength and number of weld passes on the fatigue strength.
- Influence of heat treatment process like stress releasing, annealing hardening on fatigue behavior.
- Development of damage mechanics rules based on numerical analysis for predicting the ductile failure, fatigue life crack initiation.
- Numerical modelling of fatigue crack initiation and propagation based on phase field theory.
- Achieving experimental data by carrying out on universal servo hydraulic machine to investigate the influence of multi axial stresses on fatigue strength and fatigue life.
- The effect of residual stresses caused by welding on the fatigue life.
- Investigating HFMI process on residual stresses and fatigue strength by means of numerical and experimental work.

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