Termisk utmattning : Hur termiska belastningar avgör livslängden på en Pressure seal-dysa och en bultad dysa

Thermal fatigue is a major problem in various mechanical constructions. Details with strict boundary conditions subjected to cyclic thermal loadings experience significant internal stresses. Due to the capability of materials to expand or contract differently, components in separate materials, assembled, behave in their own ways. The cyclic thermal loads result in strains that cause stresses in the exposed materials. The magnitude of the resulted stresses is commonly dependent on the thermal loads and the material properties, and even if the magnitude of the stresses is too low to cause an immediate failure of the material, these stresses applied repeatedly will eventually result in localized plastic deformation. In the heavily deformed locations, stresses might reach the ultimate tensile strength of the material and thus initiate a fatigue crack. Repeated exposure to stresses will lead to crack growth and failure of components. In the present work a stress computational model in Abaqus was built up to calculate the stresses generated when components are subjected to thermal loads. The stress analysis was performed on two different spray nozzles attached to a steam pipe. The nozzles work under cyclic conditions, they start at a rather high temperature which decreases while cold water is passed through. This occurrence is repeated since the water flow runs sporadically. Cold water runs only when the temperature is too high, it is applied to regulate the temperature inside the steam pipe. The nozzle covers are the main differences between them, one is called a Pressured seal nozzle while the other nozzle is sealed with bolts. The results of the stress analysis showed that the highest stresses were found in the welding between the nozzle and the steam pipe. The Pressured sealnozzle’s stresses were more uniformly distributed around the welding compared to the nozzle with the bolts, which experienced stresses a little more varying in its welding. Worth to remark, the nozzle with bolts had one smaller area in the welding where the stresses were significantly higher than what appeared to be the average value of the stress in the welding of the Pressured seal nozzle. These results indicate that the Pressured seal nozzle’s welding should experience a more uniform fatigue while the other nozzle might get larger fatigue symptoms in a smaller area, while the rest of the part is not so heavily stressed