Type Ⅳ high-pressure hydrogen storage vessels have become a key development direction for the global low-carbon transition due to their high hydrogen storage density and light weight. However, the complex stress states and continuously varying layup angles in the dome region present significant design challenges for composite material layups. Traditional grid theory, which primarily focuses on cylindrical body stress analysis, makes it difficult to ensure dome region strength and a safe burst mode. To address this issue, this study proposes an improved grid theory that accounts for dome stresses and establishes fundamental layup arrangement rules through combined finite element analysis.

This study integrates theoretical derivation with finite element simulation. Based on traditional grid theory, spiral-direction and hoop-direction correction coefficients were introduced to ensure dome region strength and to regulate the burst mode. The minimum winding angle of the cylindrical body is calculated using the geodesic winding principle. A reaming winding strategy is employed to prevent fiber accumulation at the polar opening and to enhance the dome transition region strength. A total of 18 layup schemes are designed, which include three experimental groups with different angle combinations and one control group based on traditional grid theory. A 1/36-axisymmetric finite element model of the type Ⅳ hydrogen storage vessel is established in Abaqus, utilizing the WoundSim plugin. Periodic boundary conditions, fixed-end constraints, and internal pressure loads are applied. The Endcap factor in the plugin accurately simulates fiber turning points in the dome region and fiber accumulation at the polar opening, thereby constructing a high-precision finite element model of the composite layup. The peak stress, stress distribution, and failure mode of the composite layup are analyzed under an internal pressure load equal to the minimum guaranteed burst pressure.

Simulation results from the 18 design schemes indicate that in the control group, designed using traditional grid theory, the peak stresses in both the dome and cylindrical body regions are similar and significantly exceed the material’s ultimate strength, confirming the necessity of revising traditional grid theory. Among all designs, four qualified schemes are identified, revealing a critical spiral-direction correction coefficient of 1.667 and a critical hoop-direction correction coefficient of 1.153. Comparative analysis of the schemes reveals the following: 1) Increasing the proportion of high-angle spiral layers reduces fiber accumulation and slippage at the polar opening while improving fiber stress distribution uniformity in the dome’s non-polar opening region, thereby reducing design redundancy. 2) The arrangement sequence of layup angles influences the location and severity of stress concentration. 3) Placing the hoop layer adjacent to the liner reduces hoop stress in the cylindrical body’s middle section but increases stress levels in the winding initiation zone. 4) The ratio of the spiral-direction correction coefficient to the hoop-direction correction coefficient critically regulates the vessel’s burst mode.

By introducing hoop- and spiral-direction correction coefficients, an enhanced design of the composite vessel dome region can be achieved. The layup angle configuration and arrangement rules derived from these finite element results provide a theoretical basis and engineering guidance for the rapid design and verification of fiber layup schemes for type Ⅳ hydrogen storage vessels.

Continuous silicon carbide (SiC) fiber-reinforced titanium matrix composites (TMCs) have become critical structural materials in aerospace because of their exceptional specific stiffness and strength. However, their anisotropic mechanical properties and complex interfacial failure modes pose notable challenges for damage prediction and structural reliability. This study addresses the critical knowledge gap regarding the multiscale fracture mechanisms of practical SiC fiber-reinforced TMCs containing hierarchical C/TiC/Ti interfacial architectures formed by hot isostatic pressing (HIP). Existing research predominantly focuses on idealized Ti/TiC systems; the crucial influence of pyrolytic carbon layers with turbostratic structures is neglected. Our work pioneers a comprehensive investigation into the mixed-mode fracture behaviors of carbon-rich (pyrolytic carbon/amorphous carbon) and TiC-dominated interfaces through atomic-scale modeling, providing essential parameters for the optimization of interfacial design against multiaxial failures.

We developed a multiscale simulation framework combining molecular dynamics and interfacial mechanics analysis. Atomic models of SiC/C/Ti multilayer interfaces were constructed to replicate realistic HIP-generated microstructures. For pyrolytic carbon/amorphous carbon interfaces, the analytical bond-order potential (ABOP) was employed to simulate liquid quenching (8000 K) and annealing (4000 K). Turbostratic carbon configurations matching chemical vapor deposition (CVD) characteristics were generated. The Ti/TiC interfaces were modeled using the 2-nearest-neighbor modified embedded atom method (2NN-MEAM) potential to capture lattice mismatch (4%) and interdiffusion between α-Ti (0 0 0 1) and TiC (1 1 1) planes. The following two critical loading scenarios were simulated: (1) tensile separation (Mode Ⅰ) with 0.5 Å/ps displacement rate and (2) shear deformation (Mode Ⅱ) at 5 Å/ps sliding velocity. The NVT ensemble with Nose-Hoover thermostat was used to maintain a 300 K operating temperature. Fracture energy release rates were calculated through the J-integral analysis of traction-separation curves. Atomic bond evolution was quantified through polyhedral template matching and common neighbor analysis in OVITO software.

The pyrolytic carbon/amorphous carbon interfaces demonstrated distinct anisotropic fracture mechanisms as follows: (1) tensile loading caused the sequential fracturing of graphene-like layers (max traction: 7.36 GPa; Type Ⅰ energy release rate: 7.87 J/m²), and (2) shear deformation induced 45° delamination through interlayer sliding (max shear: 4.53 GPa; Type Ⅱ energy release rate: 15.50 J/m²). By contrast, the Ti/TiC interface exhibited superior tensile strength (12.8 GPa; Type Ⅰ energy release rate: 8.71 J/m²) but unexpected shear-induced failure: (1) shear stress was concentrated at the Ti lattice defects rather than at the interface, and (2) 45° cleavage fracture in Ti matrix (Type Ⅱ energy release rate: 12.14 J/m²) revealed matrix failure. The crack propagation paths fundamentally differed between the interfaces. The pyrolytic carbon interfaces showed self-similar crack growth along weak Van Der Waals gaps, and the Ti/TiC interfaces displayed crystallography-dependent branching along (0 0 0 1) planes.

This study establishes the quantitative correlation between HIP-processed interfacial architectures and fracture resistance anisotropy in SiC fiber-reinforced TMCs, bridging atomic-scale mechanisms to macroscopic composite performance. The following three key advances are achieved: (1) identification of pyrolytic carbon interfaces as the tensile weak link (10.6% lower critical energy release rate compared with Ti/TiC interfaces), (2) discovery of shear-induced matrix failure mechanisms through dislocation pileup and shear band formation overriding interfacial strength, and (3) development of process-informed traction-separation laws incorporating HIP temperature for multiscale modeling. The results fundamentally revise the conventional "weak interface" paradigm by demonstrating the load-dependent dominance of different interfacial layers—pyrolytic carbon governs tensile failure, whereas Ti matrix plasticity dictates the shear response. This study provides fundamental data and mechanistic insights for the subsequent development of multiscale simulation models for predicting spontaneous crack initiation in fiber-reinforced Ti matrix composites and addresses interfacial delamination under combined thermomechanical loading in aeroengine components.

The primary objective of this research is to meticulously examine how pulse magnetic field assisted deep cryogenic (MDC) treatment affects the transformation and stabilization of retained austenite in Cr4Mo4V bearing steel. This study aims to elucidate the underlying mechanisms by which the pulse magnetic field influences the microstructural changes in bearing steel, particularly focusing on the stabilization of retained austenite, which plays a crucial role in determining the mechanical properties and overall performance of the steel.

To achieve a comprehensive understanding of how retained austenite transformed under various treatment conditions, this study utilized several material characterization techniques, including X-ray diffraction (XRD), vibrating sample magnetometry (VSM), and electron backscatter diffraction (EBSD). The use of EBSD analysis allows for a detailed comparison of variations in the dislocation density among samples processed under different conditions. For comparative analysis, the experimental set-up was divided into two distinct treatment processes: the conventional deep cryogenic (DC) treatment and the MDC treatment. Following these treatments, the samples were subjected to high-temperature tempering to evaluate the thermal stability of the retained austenite.

The XRD analysis revealed a reduction in the volume fraction of retained austenite from (23.8%±0.6)% to (21.5%±0.9)% following the DC process. A relatively smaller reduction to (22.5%±0.5)% was observed with the MDC process. These results, supported by VSM and EBSD analyses, highlight the capacity of the pulse magnetic field to partially inhibit the transformation of retained austenite. Further examination of the high-temperature stability of austenite in samples treated with DC and MDC revealed that MDC samples demonstrated improved retention, maintaining 7.1% of retained austenite after high-temperature tempering, compared to 4.9% in DC-treated samples. This indicates that the retained austenite in Cr4Mo4V bearing steel exhibits improved high-temperature stability following treatment with the MDC process. Furthermore, the dislocation density analysis revealed that the DC process led to a 9.8% increase in the dislocation density, whereas the MDC process moderated this increase to only 6.5%. This difference suggests the magnetic field's role in inhibiting dislocation diffusion, which in turn reduces martensite nucleation sites, thereby stabilizing retained austenite. The dislocation density change of the samples treated with DC and MDC after a high-temperature tempering validates this point. The dislocation density in DC-treated samples was approximately 1.23×10¹⁵ m^-2, while it decreased to 1.13×10¹⁵ m^-2 in MDC-treated samples. The dislocation density change reflects the extent of phase transformation.

This study provides a thorough analysis that clearly demonstrates the significant impact of applying a pulse magnetic field during deep cryogenic treatment on the microstructural evolution of Cr4Mo4V bearing steel. The magnetic field not only moderates the increase in the dislocation density but also enhances the mobility of dislocations. This contributes to the stabilization of retained austenite, which is crucial for improving the mechanical properties and performance of bearing steel. The findings of this research lay a solid foundation for optimizing heat treatment processes using the magnetic field assisted deep cryogenic treatment.