Influence of Fatigue Damage on the Hugoniot Elastic Limit, Phase Transformation, and Spall Behavior of alpha-Iron and 4340 Steel

Strengthening defects such as particle inclusions and dislocation clusters have a significant influence on the Hugoniot elastic limit (HEL) and spall response of materials. The influence of such defects from material processing routes has been thoroughly, though not completely, studied. However, high cycle fatigue loading which can impart extensive and detrimental damage, has received much less attention to date. Here, alpha-Iron (Fe) and 4340 steel samples fatigued to different stages of the anticipated fatigue life are subjected to shock loading from which the HEL and spall strength are extracted. For most experiments conducted in this study, the applied shock stresses remain below the phase transformation stress with the exception of the 4340 steel, where an additional experiment for each fatigue condition probes the influence of fatigue damage when the phase transformation stress is exceeded. Results are discussed with respect to the influence of the degree of fatigue damage on the dynamic mechanical response as well as the influence of alloying on the post-fatigued shock response within Fe-based alloys.