Tao Suo

Knee braces are widely used to protect against injuries during landing impact, but their effects on the mechanical responses of internal knee tissues remain unclear. In this study, landing impact models incorporating flexible and semirigid knee braces were developed using an established human body finite element model. Model evaluation was based on volunteer measurements of ground reaction force and lower limb kinematics. The effects of brace type and landing velocity on lower limb joint motion and the mechanical responses of the menisci and major knee ligaments were then examined. Knee braces had limited effects on peak ground reaction force, primarily altering joint motion and internal tissue responses during the unloading phase. Both brace types reduced meniscal compressive stress during the unloading phase and peak medial collateral ligament tensile strain, with comparatively small effects on peak strain in the other knee ligaments. As landing velocity increased, initial impact loading contributed more to the peak medial meniscal compressive stress and peak anterior cruciate ligament tensile strain. This limited the attenuation of these peaks by the braces. The biomechanical effects of knee braces therefore depended on loading phase, landing velocity, and tissue type.

This study comprehensively investigates the damage evolution and property degradation of SiC _f /Si ₃ N ₄ composites subjected to high‐speed waterjet impacts simulating rain erosion. The composites are fabricated by CVI using SiC fiber fabric preform with BN interface and Si ₃ N ₄ matrix. A single‐jet device generates stable waterjets (≈350 m/s, ≈4.9 mm diameter) to induce controlled erosion damage. Results demonstrate a staged progression of erosion damage: initial localized fiber bundle fracture and matrix cracking (0%–25% depth) is followed by pit widening via water‐wedging and delamination (25%–75% depth), culminating in penetration with a stabilized damage area (>75% depth). Tensile strength decreases by 47.5% after full penetration, with AE analysis indicating a transition from synergistic fiber‐matrix failure to a fiber‐dominated quasi‐brittle fracture mechanism due to interfacial weakening and matrix damage. Conversely, electromagnetic analysis shows that the erosion‐induced damage, including pits, cracks, and increased porosity, improved impedance matching. This leads to a significant enhancement in wave‐absorbing performance, with the minimum reflection loss decreasing from −5.96 dB for undamaged material to below −7 dB for fully eroded specimen across the X‐band. This work elucidates the dual influences of rain erosion on the mechanical and electromagnetic functionality of SiC _f /Si ₃ N ₄ composites, providing critical insights for aerospace applications.)

To address rain erosion damage of oriented polymethyl methacrylate (PMMA) under high-speed droplet impact, three types of tests (single pulsed jet impact, continuous single pulsed jet impact and continuous twin pulsed jet coupled impact) were conducted using a self-developed continuous jet rain erosion test system. High-speed photography, optical microscopy, optical profilometer and computed tomography (CT) were used to investigate the effects of impact velocity, number of impacts, and twin pulsed jet coupling on damage evolution. Single pulsed jet impact tests show that with increasing velocity, damage evolves from slight annular crazing to a central undamaged region surrounded by extensive annular crazing. Continuous single pulsed jet impact tests indicate that higher velocity shortens the incubation period and accelerates volume loss and damage area growth. The erosion crater profile exhibits a “two valleys and one peak” morphology: a central protrusion from repeated water-hammer compression, and side depressions from lateral outflow scouring. Continuous twin pulsed jet coupled impact tests reveal distinct features: due to mutual hindrance of lateral flows, ring cracks appear only on the side away from the adjacent impact; stress wave superposition may cause stress concentration between impact points, resulting in an asymmetric “single-peak” crater cross-section. This study achieves laboratory simulation of twin pulsed jet coupled impacts, providing an experimental basis for multi-jet interaction damage mechanisms.

Three-dimensional (3D) woven composites have good impact resistance and are expected to become the fan casing material for the next generation of turbofan engines. Conducting research on the performance of woven composite plates under high-velocity impact of flaky projectiles is of great significance for the containment design of the fan casing. Based on the principle of energy conservation, an analytical model for the high-velocity impact of flaky projectiles on carbon fiber woven composite plates was established for three typical failure modes: shear plugging, fiber failure, and momentum transfer. A segmented solution method combining analytical and numerical calculations was developed for the model. The critical penetration velocity of the plate obtained by the analytical method at different roll angles of the projectile is in good agreement with the experimental results, which verifies the accuracy of the analytical model. Moreover, the analytical results indicate that the critical penetration velocity of the plate increases first and then decreases with the roll angle of the projectile. Further energy conversion analysis points out that shear plugging is the main form of energy dissipation for woven composite plates, and the energy dissipation of shear plugging at a roll angle of 30° is higher than that at 0° and 60°. This elucidates the mechanism by which the roll angle of the projectile affects the critical penetration velocity of the plate from the perspective of energy dissipation.

This paper addresses challenges in speed tracking control for UAV systems equipped with variable-pitch propellers (VPPs), particularly those arising from communication delays and dynamic environmental disturbances. We propose a hybrid control strategy integrating an enhanced Smith predictor with fuzzy adaptive PID control. The Smith predictor compensates for time delays within the propulsion system, while the fuzzy PID controller dynamically adjusts for variations in UAV dynamics and external disturbances. Our approach incorporates real-time fluctuations in air-to-ground communication, encompassing both line-of-sight (LoS) and non-line-of-sight (NLoS) conditions. Theoretical analysis and numerical simulations validate the proposed architecture, demonstrating enhanced control performance, reduced delay effects, and improved robustness for real-time applications.