AviChina Industry & Technology (China)

ABSTRACT This study investigated the high‐cycle fatigue (HCF) behavior of 300M steel with shot peening (SP) followed by surface coatings (Cd/Ti plating, Cr plating, HVOF spraying) under axial stress control. Axial fatigue tests revealed that Untreated specimens exhibited the highest fatigue limit (845 MPa). SP alone slightly reduced the limit (−0.84%) due to the competing effects of surface roughness and compressive residual stress. SP + Cd/Ti minimally impacted HCF, while SP + Cr and SP + HVOF reduced fatigue limits by 7.12% and 6.73%, respectively. Post‐HVOF grinding restored fatigue strength by 20 MPa. The S‐N curves further highlighted that SP and Cd/Ti plating enhanced medium‐to‐high‐cycle fatigue life, but coatings like Cr plating and HVOF accelerated crack initiation. Fracture analysis showed SP shifted crack initiation to internal inclusions, while coatings induced interface cracks. A multifactor life prediction framework integrating surface treatment coefficients, roughness, and residual stress was developed, achieving predictions within ± 2.5 scatter bands.

Glycidyl azide polymer (GAP)-based polyurethane, a kind of energetic thermoplastic elastomer (ETPE), is a promising binder for advanced solid propellants, but its thermal decomposition involves overlapping competitive reactions that conventional single-step kinetic models cannot characterize accurately, limiting its engineering applications. To address this limitation, a constrained asymmetric Gaussian deconvolution strategy with fixed peak area ratios and shape constraints was developed in this work. This strategy was applied to resolve overlapping reaction rate curves converted from derivative thermogravimetric data of GAP-based ETPEs with 50 wt% GAP content at four heating rates of 5, 10, 15 and 20 K·min-1. The complex decomposition process was successfully split into five stages, assigned to azide cleavage, polyether backbone scission, carbamate cleavage, hydrocarbon product degradation and residue decomposition, with a goodness of fit of R2 > 0.998. Apparent activation energies of the five stages were determined through cross-validation by the Friedman and Flynn-Wall-Ozawa methods without prior assumption of reaction mechanisms, following the order of residue decomposition (181.4 ± 1.0 kJ·mol-1) > hydrocarbon product degradation (159.9 ± 1.0 kJ·mol-1) ≈ azide cleavage (156.5 ± 0.6 kJ·mol-1) > backbone scission (135.1 ± 0.7 kJ·mol-1) > carbamate cleavage (111.9 ± 1.1 kJ·mol-1). Pre-exponential factors with lnA0 values ranging from 22.2 to 34.0 were derived via the kinetic compensation effect. Finally, generalized master plots were employed to compare with classic solid-state reaction models for mechanistic insight, and the Šesták-Berggren model fit three major stages excellently (R2 > 0.996) by accounting for synergistic nucleation-growth and phase boundary mechanisms, enabling high-precision kinetic equations. It should be noted that the constrained deconvolution method proposed in this work has general applicability for kinetic analysis of GAP-based ETPEs with different formulations and other complex energetic polymer systems, while the obtained kinetic parameters are composition-specific and only applicable to the corresponding ETPE formulation studied herein.

Described in this paper is an investigation on the aerodynamic characteristics of a full-scale realistic supersonic rocket-sled in operating conditions. The scale adaptive simulation (SAS), coupled with the layer dynamic mesh method, as well as a rocket-sled test, is employed here to present the detailed aerodynamic characteristics. The method used, overcoming the disadvantages of the wind tunnel test and the numerical blowing method, presents the realistic aerodynamic characteristics of an operating rocket-sled. Considering the realistic geometry, the effects of the sled-beam, supports, rails, fasteners, and sink on the aerodynamic characteristics of a supersonic rocket-sled are systematically studied. The aerodynamic characteristics, in terms of the transient flow structures, momentum transport process, lift and drag force, the aerodynamic characteristic frequency is considered at Mach numbers ( Ma ) of 1.5, 2, 2.5 and 3, while the corresponding test results are obtained at Ma of 1.5, 2, and 2.5. The results found show that the bow shock and compression waves of the rocket-sled are reflected at the multi-facilities, causing the complex wave structures, vortex shedding and aerodynamic characteristics. Compared with the simplified flat ground condition, the increase of 818% in the time-averaged lift of the rocket-sled is induced by the full-scale realistic multi-facilities at Ma = 2. The fasteners on the ground contribute to the increase of the lift, while the rail is not helpful. The sink on the ground produces a dramatically asymmetrical flow field which is revealed by the fraction coefficient and limiting streamlines of the rocket sled surface. In addition, the reflected compression waves are significantly enhanced by the periodically fixed fasteners, resulting in the periodic characteristics of the lift and drag force. The PSD (Power Spectral Density) analysis of the pressure, lift and drag force is employed to illustrate the characteristic frequency. The dominant frequencies of the rocket-sled at Ma = 1.5, 2, 2.5 and 3 are 407.6 Hz, 544.6 Hz, 679.9 Hz and 817.2 Hz, respectively. The results provide a reference for avoiding the resonance between the rocket-sled in operation and the rail.

It has long been desired to achieve mechanical enhancement and structural health monitoring by introducing carbon nanotubes (CNTs) into traditional carbon fiber (CF) composites. Herein, the initiation of micro-damage and crack propagation has been investigated by utilizing in situ electrical resistance changes in interlaminar hybrid CNT network/CF composites during the shear loading process. The results show a clear relationship between the crack propagation and the electrical resistance response particularly when approaching the failure of the single-layer CNT network hybrid composites. Furthermore, the chemically modified CNT network exhibits evident enhancement on main mechanical properties of the CF composites, superior to the thermoplastic toughening method. The characterizations manifest that the multiscale interlayered CNT/CF structure can simultaneously resist the crack propagation along both the in-plane direction and the cross-plane direction, which consequently enhances the flexural and compressive strengths of the composite material. This discovery provides a novel idea for the potential application of CNT network/CF hybrid composites in the integration of mechanical reinforcement and structural health monitoring, namely, that the CNT network acts not only as a reinforcing phase but also as a sensor for the structural health monitoring of the composites.