Final answer:
Under severe cyclic conditions, backing rings made of galvanized or unpainted steel should not be used due to their susceptibility to corrosion and fatigue. Stronger materials like stainless steel or high strength alloys are recommended for durability.
Step-by-step explanation:
Backer rings or backing rings are often used in welding to provide a backing with which to perform a full penetration weld. However, under severe cyclic conditions, certain types of backing rings, like those made of galvanized or unpainted steel, should not be used. These materials may be susceptible to corrosion, and the cyclic loading can lead to premature failure through fatigue. Consequently, it is recommended to employ backing materials that are more robust and can withstand the stresses caused by repetitive loading. Instead, using materials like stainless steel, high strength alloys, or composites can help enhance the durability and longevity of welds under such extreme conditions.