Fracture mode transition in low-temperature low-cycle fatigue of commercial pure iron

Strain-controlled low-cycle fatigue tests were performed under push-pull loading conditions at -140°C, -90°C and room temperature (RT) using commercial pure iron having ferrite grain size of 400 μm. To clarify the fracture mechanisms, observation of the fracture surfaces was also carried out with special focus on the role of deformation twins in the generation of microcracks which lead to final fracture of the specimen. ( 1 ) The transition of the fracture modes occurs from surface to internal fracture modes with an increase in the plastic strain range Δε_p at testing temperatures of -90°C and -140°C. In a large Δε_p regime crack initiation occurs inside the material, and this internal crack leads to final fracture. (2) Deformation twins strongly affect the generation of internal cracks at -140°C and -90°C. Three internal crack initiation sites were observed: ( i ) the intersection of deformation twin and grain boundary, (ii) the intersection of several deformation twins and (iii) the intersection of deformation twin and inclusion. The applicability of the Manson-Coffin law to low-temperature low-cycle fatigue of commercial pure iron is also discussed in relation to the fracture mode transition behavior.