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In the present study, stress (S) -number of cycles to failure (N) (S-N) fatigue and fatigue crack propagation behaviors of three thermomechanical control process steels with different microstructures but similar yield strengths of approximately 450 MPa were investigated. The P + F steel was predominately pearlite plus ferrite, whereas B1 and B2 steels were both bainitic steels with martensite-austenite and pearlitic islands. Despite the significant difference in microstructural features, the resulting fatigue crack propagation rates and near-threshold DK values were comparable with each other. The hard phases, such as pearlite colonies in the P + F specimen, tended to affect fatigue crack propagation behavior in a similar manner, and severe crack branching was observed in intermediate and high DK regimes. Despite similar fatigue crack propagation rates and near-threshold DK values, the resistance to S-N fatigue was substantially different for each steel specimen. Depending on fatigue crack initiators, such as the ferrite/pearlite phase boundaries for the P + F specimens and the cracked martensite-austenite and/or small pearlitic islands for the bainitic specimens, the cycles for crack initiation varied greatly.

In the present study, the hydrogen embrittlement (HE) and the hydrogen-assisted fracture (HAF) behaviors of electrochemically hydrogen-charged Al 8090 with different specimen orientations and aging practices were examined using smooth bar and single-edge notch (SEN) specimens. It was found that the cathodic hydrogen charging substantially reduced the tensile ductility and the resistance to fracture of Al 8090. The susceptibility to HE and HAF of Al 8090 was strongly dependent on specimen orientations and aging practices. Hydrogen attack was the most significant along the grain boundaries, and, consequently, T-S oriented SEN and T-oriented smooth-bar specimens showed the highest susceptibility among the orientations studied. The susceptibility to HE and HAF decreased from underaging (UA) temper to overaging (OA) temper. It is speculated that the formation and development of a precipitate-free zone (PFZ) along the grain boundaries, rather than the change in slip planarity with prolonged aging is responsible for the reduced susceptibility to HE and HAF. Further studies are, however, required to confirm this notion.

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