Characters of Fatigue Damage of Smooth-surface Samples by the Effective Short Fatigue Crack Criterion

Abstract

Characters of fatigue damage of smooth-surface samples are experimentally drawn through investigation on evolutionary crack sizes, density values, and growth rates of 1Cr18Ni9Ti welded metal. Previous effective short fatigue crack (ESFC) criterion, which consists of three interrelated concepts, i.e., ESFCs, dominant ESFC (DESFC), and density of ESFCs, is employed with a viewpoint of damage localization. Results reveal that the damage is subject to microstructural short crack (MSC), physical short crack (PSC), and long crack (LC) stages. ESFCs and DESFC contribute to fatigue damage directly. The ESFCs sizes increase randomly and competitively, and exhibit a sudden increase when the ESFCs coalesce with each other to form the DESFC. But the DESFC size always increases stably. With the DESFC size increasing and the crack tips transferring, the non-ESFCs, in the form of the density of ESFCs, contribute to fatigue damage indirectly at the initial district of DESFC and then the crack tip districts. The density always increases in MSC stage. After reaching the maximum value at the transition point between MSC and PSC stages, it decreases rapidly in PSC stage, and then, tends to a saturation value in LC stage. The growth rates of ESFCs decrease randomly in MSC stage and then wave around the dominant growth rate in PSC stage. While the growth rate of DESFC shows a stable decrease in MSC stage and a constant increase in PSC stage. The behavior of DESFC is the result of the interactive and evolutionary collective action of short cracks, and therefore it is a reasonable description and quantification for the fatigue damage of smooth-surface samples.