ABSTRACT The effect of hydrogen on the strain hardening or softening behavior of 316L stainless steel during the plastic deformation stage remains controversial. The influence of hydrogen on the plastic deformation and fracture behavior of 316L austenitic stainless steel was investigated using digital image correlation (DIC) and finite element method (FEM) in this study. The results show that hydrogen reduces the fracture strain of the material. After 48 h of hydrogen charging, the hydrogen embrittlement sensitivity index reaches 13.85%, whereas its effect on the hardening or softening behavior of the material can be considered negligible. DIC and FEM analyses indicate that hydrogen promotes strain localization and accelerates the onset of necking, leading to premature fracture before full macroscopic plastic deformation can develop. This further confirms that hydrogen primarily reduces the fracture strain of 316L stainless steel, without significantly influencing its hardening or softening behavior. Scanning electron microscope (SEM) fractography reveals that hydrogen‐induced damage transforms the fracture morphology from large and deep dimples to small and shallow dimples, indicating that the microvoid coalescence process is significantly accelerated, which is consistent with the premature fracture behavior observed by DIC/FEM.
ABSTRACT Both wear and fatigue are damage mechanisms of fretting phenomenon. Plasticity affects contact pressure of the fretting specimen. Hence, it is necessary to account for the contribution of plasticity in the study of fretting. First, the numerical model based on the finite element method is established and validated by the experiment. Then, the effect of fretting wear on fretting fatigue considering plasticity is investigated. It is shown that wear changes the crack initiation position. The crack initiation site gradually shifts toward the trailing edge with the increase of maximum bulk stress. Under the gross slip condition with significant plastic deformation, although wear can improve the fatigue crack initiation life, this beneficial effect is limited. Consequently, fretting fatigue rather than fretting wear is the primary fretting damage mechanism.
Abstract Ni-based superalloys produced via powder metallurgy (PM) undergo hot isostatic pressing (HIP) and subsequent forming operations, during which prior particle boundaries (PPBs) are formed and only partially eliminated. The residual PPB clusters strongly influence the mechanical and thermomechanical performance of these alloys. In this study, a polycrystalline discrete dislocation dynamics (DDD) framework is employed to investigate the role of PPBs in governing dislocation behavior, residual stress evolution, and macroscopic hardening in Ni-based superalloys. Realistic grain morphologies are incorporated, and PPBs are modeled as nontransmissive interfaces, representing a limiting case of strong dislocation blocking, which restricts dislocation motion across boundaries. The simulations reveal that PPBs induce localized dislocation pile-ups and back stresses, thereby enhancing strain hardening and yield strength. Larger and more densely distributed PPB clusters intensify these effects due to increased dislocation pinning. Residual stress analysis further shows that regions adjacent to PPBs retain higher compressive stresses after unloading, with relaxation strongly dependent on PPB density. These results highlight the critical role of PPB size, number, and spatial configuration in shaping the deformation and stress evolution behavior of PM-processed Ni-based superalloys. The study underscores the need to control PPB characteristics during HIP and post-HIP processing to achieve an optimal balance between strengthening and stress recovery.
Abstract This study presents the fracture mechanics based synergistic effect of load ratio (R = 0.1 to 0.7) and temperature (25 to 200°C) on fatigue crack growth in aerospace grade AA7075-T651. Quarter compact tension specimens under constant amplitude fatigue loading revealed that fatigue life decreased significantly (by 65%) for a temperature increase from 25 to 200°C at R = 0.1. However, increasing R from 0.1 to 0.7 at 200°C extended life several fold (26 times). A transition in the crack growth mechanism was observed from ΔK dominated behavior at ambient conditions to Kmax controlled propagation due to synergistic effect of elevated temperatures and high R values. Succinct and close-spaced striations were observed on fractographic examination of fractured surfaces for specimens tested at 25°C. However, the striations were blurred and widely spaced (a 25-fold increase in striation spacing compared to ambient temperature) for specimens tested at elevated temperatures. While ΔK was observed as the primary crack driving force for combinations of room temperature and low values of R, however, ΔKeff and Kmax dominated the crack growth behavior for high R and elevated temperatures. The use of a single-parameter model accurately predicted crack growth rates at low R (R = 0.5) across all temperature values used in this investigation. The two-parameter model improved the correlation of high R Paris-regime crack-growth, while deviations observed in the near-threshold region. This modeling approach provides with quantitative framework for predicting da/dN under complex thermomechanical conditions.
Abstract This study characterizes the microstructural evolution and coupled mechanical response along the build height of the SS316L wire arc additive manufacturing (WAAM). Correlation between elemental segregation, solidification mode, ferrite content, microhardness, and residual stress at top, middle, and bottom regions. A progressively Creq/Nieq ratio (1.31–1.75) toward the bottom region altered the solidification pathway from primarily austenitic in the top to an austenitic-ferrite mode at the middle and ferrite-stabilized solidification in the bottom region. This shift corresponds with ferrite number increases from ∼1 to ∼9 and a residual stress transition from tensile stress (80 ± 2 MPa) and lower hardness (216 ± 1.0 HV0.5) at the top to high compressive stress (220 ± 3 MPa) and high hardness (270 ± 2.5 HV0.5) near the substrate. Microstructural observations confirmed this gradient with finer cellular austenite evolving into a mixed dendritic columnar structure containing lathy, skeletal, and vermicular δ-ferrite. The results demonstrate that thermal history and solute redistribution inherently produce a functionally graded microstructure and property profile in SS316L WAAM.
