Staff

Abstract:
In this work, the very-high-cycle fatigue (VHCF) response of 10H2M steel in the as-received state and after 280,000 hours of service at 540 °C under pressure was analysed. Although after prolonged service, only a moderate decrease in yield strength was observed, fatigue performance was significantly reduced in the stress range applied. Fractographic analysis revealed that in both material states, fatigue cracks initiated near the surface, with carbides acting as preferential nucleation sites. In the as-received material, cracks propagated in a relatively uniform, transgranular manner with ductile overload fracture at final failure. In contrast, the prolonged-service material exhibited rougher fracture surfaces with more frequent crack deflections and secondary cracking, linked to carbide coarsening due to service. The results demonstrate that prolonged exposure does not fundamentally change the surface-dominated initiation mechanism but intensifies the role of carbides and boundary-related defects, thereby reducing fatigue resistance. EBSD-informed finite-element representative volume element (RVE) simulations further revealed that prolonged service exposure increased the local carbide/matrix stress concentration factor, providing micromechanical evidence for the experimentally observed deterioration in fatigue resistance. Additionally, X-ray diffraction (XRD) analysis confirmed thermodynamically driven carbide precipitation and crystallographic evolution, characterised by reduced carbon supersaturation, crystallite refinement, residual micro-stress relaxation and texture evolution following long-term service exposure.

Keywords:
Very high cycle fatigue, Power plant steels , Failure mechanisms , Ultrasonic testing

Abstract:
Anisotropic yield criteria and kinematic hardening laws have been independently validated for additively manufactured alloys. However, their specific combination has not been implemented in a finite element user subroutine and validated against 360° sequential biaxial probing data for LENS-deposited nickel-base alloys. Present work delivers the first validated implementation of this coupling for LENS-deposited Inconel 625, combining the Szczepiński quadratic yield criterion with Armstrong–Frederick kinematic hardening. The implemented return-mapping algorithm accumulates the plastic multiplier across Newton–Raphson iterations before updating the plastic and equivalent plastic strain state variables. This distinction is significant for the non-radially-symmetric Szczepiński surface where the flow direction rotates at each iteration unlike the fixed radial direction of isotropic return mapping. A two-step iterative boundary-condition correction scheme ensures each probing direction converges within ±1.5° of the experimental stress angle, achieved for 16 of 17 sequential paths. Path P1, the virgin specimen opening probe retains a residual angular deviation of −2.97° due to absent accumulated back-stress at the start of the sequential cycle. The validated UMAT reproduces all 17 directional yield points with a mean absolute angular error of 0.86°, mean absolute axial and shear stress deviations of 11.4 MPa and 12.5 MPa, respectively. Path-level deviations reach a maximum of 32 MPa in axial stress at P10 and 41.4 MPa in shear stress at P5, attributed to angular misalignment between back-stress and probing direction in the compressive–shear and near-pure-torsion sectors, respectively. This establishes the framework as a computational tool for characterising multiaxial yield surface under complex loading.

Keywords:
Inconel 625, Anisotropic yield Surface, Szczepiński yield criterion, User subroutine, Kinematic hardening, Sequential multiaxial loading

Abstract:
Present investigation systematically quantifies the role of interior defect morphology on tensile fracture behavior in Binder Jetting Additive Manufactured (BJAM) 17-4 PH stainless steel. Unlike prior investigations relying on stochastic natural defects, BJAM is uniquely employed to fabricate tensile specimens with five precisely controlled interior defect geometries such as spherical, disc-shaped, ellipsoidal, inclined ellipsoidal, and two-spherical at the mid-gauge location of round and square cross-sectional configurations. These artificial defects, occupying 16–35% of the gross cross-sectional area, serve as morphologically defined analogues of shrinkage porosities typical of conventional steel castings. A novel shape-independent empirical net section yielding method is developed that directly correlates projected defect area to fracture stress across all five defect geometries and both cross-sectional configurations. Results demonstrate that tensile strength reduction is governed by projected defect area independent of defect shape, with predictions falling within ±10% for the majority of configurations, providing a practically applicable fracture stress prediction tool for defect containing BJAM components. 3D finite element simulations coupled with a ductile damage model are implemented to accurately predict crack initiation sites and experimental load–displacement responses, achieving excellent agreement with experimental findings and providing independent computational validation of the empirical framework.

Keywords:
Binder jetting additive manufacturing, Ductile damage model, Interior defects, Shrinkage porosity, 17-4PH steel

Abstract:
This study advances beyond qualitative strength reduction trends in additive manufacturing fracture studies by establishing quantitative, configuration-based fracture assessment for Binder Jetting Additive Manufactured (BJ-AM) 17–4PH stainless steel with engineered surface cracks. Multiple crack geometries such as edge, inclined, single-corner, and double-corner configurations were precisely implanted in square (7 × 7 mm) and rectangular (3.5 × 14 mm) specimens with identical cross-sectional areas. Experimental investigations demonstrate that fracture resistance is governed by crack configuration and geometry-induced constraint, not crack area alone. Double-corner cracks retained 79–86% strength despite 25% crack area, while single-edge cracks exhibited 46–54% retention with only 20% crack area. Thickness-dependent constraint effects unique to BJ-AM geometries were quantified through comparative testing, revealing rectangular specimens (3.5 mm thickness) exhibit reduced constraint and lower fracture resistance than square specimens (7 mm thickness). J-integral governed elastic-plastic fracture assessment, validated through Extended Finite Element Method simulations predicting failure within ±10%, establishing predictive frameworks for defect-tolerant BJ-AM structural design.

Keywords:
Binder jetting additive manufacturing, Fracture toughness, J-integral, Extended finite element method, Surface cracks, 17–4PH stainless steel