Annihilation and Healing of Small Inner Cracks and Extension of Fatigue Life

In this chapter, a damage healingDamage healing due to compressive creep was examined by applying pc-typePc-type fatigue loading to Cr–Mo–V forged steel specimens in which cavities and small cracksSmall crack were introduced to the inside by cp-typeCp-type creep-fatigue tests. In the slow-tensile fast-compression (cp-type) creep-fatigue tests at high temperature, many small cracksSmall crack and cavities initiate inside the specimen, and at the end of the life, they coalesce and grow, leading to rapid fracture. In the pc-typePc-type fatigue tests, small cracks and cavities do not initiate inside the specimen, and the cracks on the specimen surface grow stably and lead to fracture. After introducing damage to the specimen by loading it to 3/4 of the life of cp-typeCp-type creep-fatigue, applying pc-typePc-type fatigue loading causes the small inner cracks and cavities to shrink and disappear. In other words, pc-typePc-type fatigue loading has the effect of healing the damage caused by cp-type fatigue. This healing effect appears even with a small number of cycles (5 cycles) of pc-type fatigue loading. Materials in which small inner cracks and cavities are annihilated and healed by pc-typePc-type fatigue loading exhibit a creep-fatigue life equivalent to that of virgin materials. The above-mentioned healing effect is thought to be due to the coalescence of the crack surfaces during compressive creep and the shrinkage of the cavity due to grain boundary diffusionGrain boundary diffusion under compressive stress.