The evolution of short fatigue crack lengths and crack density: two approaches

The density and size of short cracks on the surface of 1Cr18Ni9Ti stainless steel smooth specimens during low cycle fatigue are investigated using a r eplica technique. The density and size data are analysed from two different observation policies, i.e. Policy I pays attention to the whole specimen t est piece and Policy II is related to an 'effective short fatigue crack cri terion', which pays attention to the dominant crack (DC) initiation zone an d the zones ahead of the DC tips. The results reveal that both the crack de nsity and crack size evolution exhibit a specific character during the micr ostructural short crack (MSC) and physical short crack (PSC) stages. The Po licy I-based observations exhibit an increasing density and little scatter of the density data. The increasing density violates the general test obser vation of decreasing collective crack effects in the PSC stage. The little scatter is too small to reflect the intrinsic scatter of fatigue properties . Both the crack density and crack size evolution from this policy show lit tle relationship with the intrinsic localization of fatigue damage. However , Policy II-based observations show an increasing crack density and an incr easing density scatter in the MSC stage. The density and scatter reach thei r maximum values at the transition point between the MSC and PSC stages. Th en, they decrease with fatigue cycling in the PSC stage and tend to their s aturation values when the DC size is above about 500 mum. This behaviour sh ows a good agreement with, the general test observations of decreasing coll ective crack effects and growth rate scatter in the PSC stage. Further, bot h approaches exhibit an evolutionary positively skewed crack size distribut ion, and an increasing difference between the average crack length and the DC length in the PSC stage, indicative of decreasing collective crack effec ts. A three-parameter Weibull distribution (3-PWD) is appropriately used to describe the crack sizes and a 6.5 to 7.6 mum value of location parameter of the distribution is obtained to reflect a minimum value for the initial cracks. It is worth noting that Policy I-based observations show an increas ing positively skewed crack size distribution, an increasing scatter of the size data and a decreasing shape parameter of the 3-PWD. This represents a n increasing collective crack effect and an increasing irregularity of inte ractive cracks, which violates the general test observations. In contrast, Policy II-based observations exhibit a decreasing positively skewed size di stribution shape and an increasing (from <1 gradually to > 1) shape paramet er of the 3-PWD that is in agreement with the general test observations. Th e increasing shape parameter indicates that the collective crack effects ac t as an evolutionary process from an initial non-ordered (chaotic) random s tate gradually to an independent random state at the transition point betwe en the MSC and PSC stages and then, to a loading history-dependent random s tate. This behaviour is in accordance with, the evolutionary DC growth beha viour. Therefore, the evolutionary short crack behaviour associated with th e intrinsic localization of fatigue damage should be appropriately revealed from the 'effective short fatigue crack criterion'-based observations.