Thermal cycling stress-crack

Fatigue corrosion occurring as Thermal fatigue cracking (thermal effect corrosion Corrosion fatigue Cycles of thermally induced stress leads to metal failure. Results from a combination of thermal cycling stress and SCC or other corrosion process. 

Leaks due to weld cracks resulting from thermal cycling or high stress may occur at any of the weld connections. 

The Primary Reformer is a steam-hydrocarbon reforming tubular furnace that is typically externally fired at 25 to 35 bar and 780°C to 820°C on the process side. The reformer tubes function under an external heat flux of 75,000 W/m2 and are subject to carburization, oxidation, over-heating, stress-corrosion cracking (SCC), sulfidation and thermal cycling. Previously SS 304, SS 310 and SS 347 were used as tube materials. However these materials developed cracks that very frequently led to premature tube failures (see Table 5.10)88. 

Thermal Stress Resistance. Epoxy-glass printed circuit boards, containing various electronic components were dip-coated and cured as described above and then cycled from -65°C to +125°C in forced air chambers according to the following schedule 1 hr at -65°C, 2 min at room temperature, 1 hr at +125°C, 2 min at room temperature, 1 hr at -65°C, etc. Such boards survived 20 to >50 cycles without cracking and/or delamination of the coating. 

Gases and liquids permeate fluoropol5miers to different extents depending on variables such as temperature, pressure, and the composition of the processing fluid. An increase in temperature accelerates the rate of permeation into the polymers. Thermal cycling can cause a part to stress-crack or form blisters because of successive evaporation and condensation of permeated chemicals. Steam is a well-known permeant of polytetrafluoroethylene and can create blisters upon cycling. 

Pottant. The central core of an encapsulation system Is the pottant, a transparent, polymeric material which Is the actual encapsulation media In a module. As there Is a significant difference between the thermal-expansion coefficients of polymeric materials and the silicon cells and metallic Interconnects stresses developed from the thousands of dally thermal cycles can result In fractured cells, broken Interconnects, or cracks and separations In the pottant material. To avoid these problems, the pottant material must not overstress the cell and Interconnects, and must Itself be resistant to fracture. From the results of a theoretical analysis ( ), experimental efforts O), and observations of the materials of choice used for pottants In commercial modules, the pottant must be a low-modulus, elastomeric material. 

---