Effects of pearlite morphology and specimen thickness on fatigue crack growth resistance in ferritic–pearlitic steels

Effect of pearlite morphology and specimen thickness on fatigue crack growth (FCG) behavior in ferritic–pearlitic structural steels was investigated. Two steels with different pearlite morphologies were employed: one had islands of pearlite colonies and the other had uniformly distributed pearlite particles. ΔK-decreasing/increasing FCG test was carried out by using a standard compact tension (CT) specimen with 6 mm thickness and a single edge cracked plate tension (SECT) specimen with 1.5 mm thickness. The results revealed that the uniformly distributed pearlite microstructure had a FCG resistance higher than the islands of pearlite colonies microstructure. To understand the effect of pearlite morphology on crack growth behavior in the Paris regime in detail, constant-ΔK FCG tests with in situ SEM observation were also carried out. From in situ observations, tortuous crack path was found in the uniformly distributed pearlite microstructure. These tortuous crack paths with large crack deflection angle would promote crack closure as well as crack tip stress shielding and then resulted in higher crack growth resistance. Thin specimen indicated lower FCG rate compared to thick specimen, which resulted from the difference of crack closure behavior.