The structural integrity of composite-metal bolted joints in fan casings is critical for preventing catastrophic failure under high-velocity blade-off conditions. This study investigates the impact response, energy absorption characteristics, and progressive damage evolution of these connections using high-speed air cannon experiments coupled with a three-dimensional continuum damage model (CDM). Implemented via a VUMAT subroutine, the numerical approach effectively captures progressive intralaminar and interlaminar failure modes, with accuracy validated against experimental residual velocity and damage morphology. Analyses reveal that the mounting edge radius acts as a primary stress concentration locus susceptible to severe delamination and fiber rupture. Structural impact resistance is maximized at the connection center due to superior load sharing, while being highly sensitive to hole-to-edge distance and laminate thickness. Using a surrogate-model-based multi-objective optimization framework combining RBF neural networks and genetic algorithms, the optimized configuration achieved a 45.3% reduction in residual kinetic energy and a 2.3% reduction in structural mass, demonstrating improved impact resistance while maintaining lightweight characteristics.

Turboprop engines are susceptible to cause severe damage to the core machine following bird strikes. An intake duct with a bypass structure can be used as an engine intake protection component to enhance the engine's resistance to bird strikes. Existing analysis methods cannot simultaneously consider internal airflow effects and the deformation process of the bird after impacting the inlet duct. This paper adopted the Arbitrary Lagrange-Euler (ALE) algorithm to simulate changes in the bird's trajectory under the influence of airflow. Experiments were conducted to investigate bird trajectories under different flow velocities, calibrating the influence of airflow on bird motion trajectories. Based on these experimental conditions, bird trajectory analysis was carried out, which validated the ALE algorithm's capability to accurately predict bird trajectories under airflow effects. Using this analytical model and typical turboprop engine inlet duct structure configurations, numerical analysis of bird ingestion in simulated inlet ducts was performed. Bird separation capability was evaluated by assessing the risk of bird entry into the main airflow channel. Analysis of inlet bird separation characteristics identified key design parameters and hazardous entry positions. Based on these findings, an improved design scheme for the simulated inlet was proposed. Numerical analysis of bird ingestion validated the enhanced bird separation capability of the improved inlet. This research provides an effective analytical tool and reference basis for bird excluding design in turboprop engine inlets.

In recent years, phase-field method (PFM) has attracted a lot of attention as an advanced numerical simulation technique in fracture mechanics. However, few PFM models adequately account for the effects of multi-axial stress states during crack initiation and propagation. This paper proposes a new phase-field model by integrating the critical plane method, multi-axial fatigue life models, and PFM, to simulate crack initiation and propagation under multi-axial stress states. We integrate the critical plane approach and multi-axial fatigue life models (SWT, MWHS, MGSE) and S-N curve into the phase-field framework, proposing modified models (E-MWHS, E-MGSE) to enhance prediction accuracy, six kinds of PFMs are thus obtained. The proposed PFM was applied to investigate how the multi-axial fatigue life models, critical plane search step size, cycle skipping increment, and strain energy density decomposition methods affect crack initiation and propagation, compact tension (CT) specimen was also used to quantify these parameters' influence on da/dN-Delta K and a-N curves. The fatigue life of Titanium alloy: TC4 and Nickel base superalloy: GH4169 predicted by the proposed PFM is mostly within the triple scatter band, and the multi-axial fatigue life models and strain energy decomposition method have a significant effect on crack growth rate and path. The proposed phase-field model can provides an analytical method for fatigue life prediction and reliability design of aero-engine components.

This study investigates the translaminar fracture of a thermoplastic T700/PEEK laminate under various quasi-static tensile and compressive loading rates. The critical energy release rate G degrees,plyIc was evaluated using Compact Tension (CT) and Compact Compression (CC) specimens. For tension, G degrees,plyIc was computed using four data reduction methods, revealing considerable variability, while compression employed a single method due to complex failure modes. Post-mortem analysis via 3D X-ray Computed Tomography (3DCT) and Scanning Electron Microscopy (SEM) characterized fracture morphology. Experimental results demonstrate that tensile translaminar fracture toughness G degrees,T Ic is rate-sensitive and consistently higher than its compressive counterpart G0;C Ic at all corresponding rates. A hierarchical quasi-fractal fracture theory explains the increasing G0;T Ic with loading rate. Microscopy reveals that tensile failure is hierarchical and self-similar, dominated by fiber/matrix debonding and fiber pull-out within plies. In contrast, compressive failure involves multiple, interactive modes across plies. Crucially, the tensile toughness of this thermoplastic laminate (342-420 KJ/m2) is significantly higher (nearly threefold) than typical thermosetting laminates (57-160 KJ/m2), and is at a similar magnitude level to some toughened thermosetting composites. This substantial difference must be accounted for in the design and analysis of thermoplastic composite structures.

This paper systematically investigates the influence of key bird-impact parameters—impact velocity, angle, laminate thickness, and projectile mass—on the damage evolution and vibration characteristics of T700/E1302 carbon fiber laminates through coupled bird-impact simulation and modal testing. The results reveal a distinct threshold effect in both damage evolution and the degradation of vibration properties. Critical thresholds are identified at an impact velocity exceeding 150 m/s, an angle greater than 80°, or a projectile mass of 160 g. Exceeding these thresholds leads to abrupt, non-linear escalation of delamination damage and significant reduction in natural frequencies. The third-order vibration mode demonstrates exceptional sensitivity to early-stage damage, establishing it as a robust early indicator of internal damage. Furthermore, whether the damage propagates to the structural boundary is identified as the decisive factor triggering abrupt changes in vibration characteristics. A parameter sensitivity analysis indicates that impact velocity is the most influential parameter. This research provides experimental evidence and theoretical guidance for the impact damage diagnosis and impact-resistant design of composite structures.

Voids, characterized by intra-filament and inter-filament voids, are critical defects in fused deposition modeling (FDM) 3D-printed continuous carbon fiber reinforced polymer (C-CFRP) composites, significantly influencing their mechanical properties and damage mechanisms. Although the existing studies have demonstrated that the intra-filament voids have completely opposite effects on the quasi-static and dynamic mechanical behavior of the fused filament, the mechanism by which the inter-filament voids formed during the deposition modeling affect the mechanical behavior has received limited attention. In particular, the link between layup-dependent void morphology, damage evolution, and strain-rate-dependent laminate behavior has not been clearly established. In this study, CT-based characterization was used to quantify inter-filament void content, size, orientation, aspect ratio, and flatness in unidirectional and angle-ply laminates. Quasi-static and dynamic mechanical tests, interrupted in-situ XCT/DVC observations, and fractographic analysis were then combined to reveal the corresponding damage mechanisms over a wide strain-rate range. The results show that inter-filament void morphology governs local deformation incompatibility, theoretical prediction errors, damage paths, and strain-rate sensitivity. This work not only elucidates the mechanistic role of inter-filament voids, but also provides benchmark data for void-informed multiscale modeling of 3D-printed C-CFRP laminates.

Hierarchical lattice structures offer a promising strategy to overcome the stiffness and load-bearing limitations of conventional lattice architectures. In this work, IN625 hierarchical body-centered-cubic (HBCC) lattice structures with different relative densities were fabricated by laser powder bed fusion (LPBF) and systematically investigated under quasi-static and dynamic compression. An analytical model based on Euler-Bernoulli beam theory and the displacement method was developed to predict the yield stress of HBCC lattices, and strain-rate effects were incorporated through the Johnson-Cook formulation. Finite element simulations and compression experiments validate the analytical predictions, with a maximum deviation of 16.1% across different relative density and less than 11.5% discrepancy under varying strain rates. The results demonstrate that the proposed analytical framework can reliably predict the yield stress of HBCC lattices over a wide range of densities and loading rates, providing an efficient theoretical tool for the design and dynamic performance prediction of hierarchical lattice structures. Quasi-static compression results reveal a distinct two-stage deformation mechanism across all densities, characterized by an initial bending-dominated stage followed by a stretching-dominated load-bearing stage. Under dynamic loading, pronounced strain-rate strengthening is observed at low strains; however, increasing strain rates progressively suppress the second deformation stage and lead to a marked reduction in high-strain load-carrying capacity due to premature instability of vertical struts. The analytical modeling proposed in this study can effectively support investigations of the dynamic response of lattice structures, providing a new methodological basis for characterizing and evaluating their dynamic behavior.

Voids, characterized by intra-filament and inter-filament voids, are critical defects in fused deposition modeling (FDM) 3D-printed continuous fiber composites, significantly influencing their mechanical behavior and damage mechanisms. While previous studies have mainly focused on fiber printing damage and inter-filament voids, the impact of voids formed within the filaments during deposition has received limited attention, especially in relation to dynamic mechanical properties vital for structural impact resistance. This research gap hampers accurate assessments of FDM structural components with both intra- and inter-filament voids. To address this, the present study innovatively investigates the impact of intra-filament voids by comparing two types of continuous carbon fiber (CCF) filaments— as-received and printed— in terms of microstructural characteristics, quasi-static and dynamic mechanical properties, and damage mechanisms. Through quasi-static tests, the effects of intra-filament voids and fiber damage caused by the printing process are preliminary decoupled. Dynamic tests further reveal that intra-filament voids positively influence the dynamic mechanical properties of the composites. In addition, the quantitative analysis of microstructure and mechanical performance provides essential data for developing microscopic and constitutive models that incorporate void defects, advancing the design and assessment of FDM composites.

This paper presents a mesoscopic unit cell structure that accounts for the differing properties of surface and inner cells in 3D braided composite materials. The structure is used to predict the dynamic response of a macroscopic model under high-speed ice impact, validated through experimental tests. Ice impact resistance tests were conducted on composite plates of varying thicknesses, analyzing displacement response and damage morphology under different impact speeds. The internal damage modes of the specimens were observed through CT scanning, revealing the ice impact resistance of plates with different thicknesses. A mesoscopic model, incorporating a viscoelastic resin constitutive model, was developed to simulate strain rate-dependent mechanical properties and applied to the macroscopic model. This approach accurately predicts the ice impact resistance of 3D braided composites, offering valuable insights for safety analysis and structural design of 3D braided composite structures in aviation.

The honeycomb structure, as one of the earliest two-dimensional metamaterials recognized and utilized by humans, has been extensively employed in various applications due to its ease of production, lightweight and high strength. However, with the ever-increasing demands for structural performance and environmental sustainability in modern engineering applications, the specific strength and specific stiffness of traditional hexagonal honeycombs can no longer meet the requirements. Inspired by the concept of twin-induced strengthening in metals, this work designed two types of modified honeycombs with exceptional mechanical properties. Experimental results demonstrated that our metamaterials exhibited outstanding relative stiffness and relative strength, even surpassing those of most three-dimensional metamaterials. Furthermore, one of the structures demonstrated a remarkable reusable capability of up to seven cycles before localized failure occurred. Additionally, by analyzing the anomalous phenomena observed during repeated quasi-static compression tests, this study revealed the critical role of self-contact effects in enabling reusability and enhancing mechanical performance. In summary, this research represents the first exploration of high-performance reusable metamaterials and suggests a promising direction for further studies in this field.

Bismaleimide (BMI) resin, a thermosetting material with superior heat resistance, mechanical strength, and environmental stability, is widely used in aerospace applications, especially in the military aircraft and turbofan engine characterized by high temperatures. While research has extensively covered the mechanical behavior of epoxy and thermoplastic composites, the effect of temperature on the BMI-based composites remains underexplored, especially within its service temperature range. Thus, a systematic study of the effects of temperature on mechanical properties and damage patterns of carbon fiber/bismaleimide resin composites was carried out through various experiments, ranging from quasi-static to dynamic and impact resistance. The test results of mechanical response curves, visual analysis, scanning electron microscopy, ultrasonic C-Scanning and X-ray tomography indicated that high temperatures reduce the interfacial properties, leading to diminished mechanical properties. The failure strain and modulus do not vary monotonically with temperature or strain rate, while the ballistic limit increases with temperature in general. These results provide valuable reference data and significant relevance of carbon fiber/bismaleimide resin composites in the aviation field.

Nickel-based superalloy lattice structures exhibit broad application prospects in the aerospace field. In this research, three lattice structures (BCC, 3ATC, and P-TPMS) were fabricated using IN718 and IN625 through SLM techniques, followed by quasi-static compression tests and numerical simulations. Among the three lattice structures made from the same material, the yield strength follows the order: P-TPMS > BCC > 3ATC. For the lattice structures made by IN718, the BCC and 3ATC structures featured limited load-bearing capacity and were subject to premature failure, while the P-TPMS structure exhibited the best overall performance, with a yield strength of 66.1 MPa and specific energy absorption of 22.1 J/g. However, this came at the cost of extensive structural damage. Benefiting from the excellent toughness of IN625, all lattice structures fabricated with this material were capable of achieving densification. Among them, the BCC and P-TPMS structures demonstrated no significant damage, making them ideal for applications involving large deformations. Meanwhile, the 3ATC structure showed damage and poorer stability. The matrix material and topology significantly influence the load-bearing capacity, deformation, and damage characteristics of lattice structures. Therefore, selecting appropriate materials and structures according to specific application requirements is crucial.

The unit cell configuration of lattice structures critically influences their load-bearing and energy absorption performance. In this study, three novel lattice structures were developed by modifying the conventional FBCCZ unit cell through reversing, combining, and turning strategies. The designed lattices were fabricated via laser powder bed fusion (LPBF) using Ti-6Al-4V powder, and the mechanical properties, energy absorption capacity, and deformation behaviors were systematically investigated through quasi-static compression tests and finite element simulations. The results demonstrate that the three modified lattices exhibit superior performance over the conventional FBCCZ structure in terms of fracture strain, specific yield strength, specific ultimate strength, specific energy absorption, and energy absorption efficiency, thereby validating the efficacy of unit cell modifications in enhancing lattice performance. Notably, the CFBCCZ and TFBCCZ lattices significantly outperform both the FBCCZ and RFBCCZ lattice structures in load-bearing and energy absorption. While TFBCCZ shows marginally higher specific elastic modulus and energy absorption efficiency than CFBCCZ, the latter achieves superior energy absorption due to its highest ultimate strength and densification strain. Finite element simulations further reveal that the modified lattices, through optimized redistribution and adjustment of internal nodes and struts, effectively alleviate stress concentration during loading. This structural modification enhances the structural integrity and deformation stability under external loads, enabling a synergistic enhancement of load-bearing capacity and energy absorption performance.

Low-velocity impact (LVI) can result in imperceptible damage to carbon fiber reinforced thermoplastic composites (CFRTP) laminates during service, leading to a reduction in structural strength. The thermal repair of damaged CFRTP laminates is conducted using the repairability of thermoplastic resin at high temperatures. However, the high-velocity impact characteristics of CFRTP laminates following thermal repair remain uncertain. This study examines CFRTP laminates made of two different materials (CF/PEEK and CF/PPS) with varying levels of low-velocity impact damage, and investigates the thermal repair process. A comparative experimental analysis examined the high-speed impact characteristics of CFRTP laminates under varying conditions. The results indicate that CF/PEEK laminates consistently exhibit superior compressive properties and impact resistance compared to CF/PPS laminates under similar conditions. Following damage from low-velocity impact, the compressive properties and high-velocity impact resistance of CFRTP laminates decrease, with CF/PPS laminates typically showing a lower performance retention rate. However, the thermal repair process proposed in this study significantly enhances the performance of CF/PPS laminates. Moreover, the degree of performance healing in CF/PPS laminates is consistently higher than that in CF/PEEK laminates, which is closely related to the semi-crystalline nature of PEEK resin.

A hierarchical body-centered cubic lattice structure is proposed to reconstruct load-transfer paths by applying a hierarchical design to the conventional BCC configuration. The influence of various structural parameters on the compressive behavior of the lattice was examined through compression tests and finite element analysis. The results demonstrate that the enhanced structure exhibits a two-stage deformation mode: an initial bendingdominated response followed by a mixed bending-tension mechanism, with a progressive increase in loadbearing capacity during compression. Notably, compared to conventional BCC and BCCZ lattices, the HBCC structure with different structural parameters exhibit increases in specific energy absorption of 207.1-773.2 % and 62.3-361.3 % compared to conventional BCC and BCCZ lattices, respectively. At 7 = 0.4 and 7 = 0.5, the HBCC achieves an optimal balance among load-bearing capacity, deformation stability, and energy absorption. This design strategy offers a novel approach to optimizing high-performance, lightweight energy-absorbing components.

3D woven composites excel in the thickness direction due to their fiber structures, overcoming delamination issues and showing superior impact resistance, ideal for aerospace. Aircraft operating in marine environments are chronically exposed to high humidity and salt spray, causing corrosion-driven deterioration of mechanical properties in structural components and materials, which significantly compromises flight safety. This study conducts salt spray aging tests on 3D angle-interlock woven composites, employing dynamic mechanical analysis, infrared spectroscopy, and scanning electron microscopy to assess hygroscopic characteristics and changes in physical and chemical properties pre-and post-exposure. Subsequently, quasi-static tensile and compression tests, along with dynamic compression tests, are conducted on specimens post-salt mist exposure to evaluate changes in mechanical properties. Finally, ballistic impact tests on the aged composites elucidate the mechanisms by which salt spray aging affects their impact resistance. This study delivers critical framework for structural design and durability maintenance of 3D woven composites operating under marine service conditions.

Bismaleimide (BMI) resin composite is a kind of thermoplastic composite that has attracted a lot of attention for its advantages in fracture toughness and damage tolerance. The composite structures are usually exposed to the threat of foreign object impact during service; static strength and fatigue resistance of composite structures with impact damage will dramatically degrade, which can lead to catastrophic damage of composite structures. In this paper, the hard foreign objects and air cannon were used to conduct a high-speed impact test on the BMI composite laminate. The effect of impactor diameter, impact velocity, and impact angle on impact damage size and microscopic damage mechanism was investigated; respectively, the results show that damage width and length increase with the growth of impact velocity and impactor size, the impact velocity has a greater impact on damage depth, and various impact angles will significantly change the damage morphology. After impact, the step-by-step method was used to analyze the fatigue strength of composite laminates with impact damage; it was found the depth of impact damage has the biggest impact on the fatigue strength. Based on conclusions of the fatigue test, a prediction model was proposed to predict the fatigue strength of composite laminate with the impact damages; the results show that prediction results are within 1.5 times of the error band. This paper reveals the damage behavior of composite laminate under high-speed impact; the proposed fatigue strength prediction method can provide a reference for the safety evaluation of composite structures.

Hybrid lattice structures have emerged as promising candidates for lightweight, energy-absorbing applications due to their superior mechanical properties compared to uniform lattice designs. However, the influence of unit cell arrangement on their performance remains further explored. This study investigates the mechanical and energy absorption (EA) behaviors of dual-phase hybrid lattices (DPHLs) with varying angular configurations (0 degrees, 45 degrees, and 90 degrees), fabricated via laser powder bed fusion using Ti-6Al-4V powder. By combining body-centered cubic with z-struts and modified face- and body-centered cubic with z-struts unit cells, it is demonstrated how structural anisotropy governs deformation modes and failure mechanisms. Quasistatic compression tests and numerical simulations reveal that DPHL-90 achieves an exceptional ultimate strength of approximate to 201 MPa and an EA capacity of 8.82 MJ m- 3, outperforming DPHL-0 (5.64 MJ m- 3) and DPHL-45 (5.55 MJ m- 3). Fractography analysis via scanning electron microscopy indicates that DPHL-0 fails via layer-by-layer collapse, while DPHL-45 and DPHL-90 exhibit shear-dominated fracture with 45 degrees-oriented bands. The junction regions between unit cells display mixed brittle-plastic failure, highlighting stress concentration effects. These findings provide critical insights for designing and optimizing hybrid lattice structures, particularly for impact-resistant aerospace applications.

The translaminar fracture of fiber-reinforced composites is vital in many applications, and can be influenced by hygrothermal environments in service. These effects are however not sufficiently well understood in the literature, especially the translaminar fracture mechanism of hygrothermal effects and its reversibility still need to be revealed. Thus, a systematic study of the hygrothermal and re-drying effects on the translaminar fracture toughness was carried out through compact tension tests, with the corresponding damage mechanisms investigated by the scanning electron microscope. The experimental results indicated a significant decrease in apparent stiffness with a minimal crack propagation after hygrothermal aging, while dried and re-dried specimens maintained stiffness and facilitated multiple stable crack propagations. In addition, the R-curves showed greater stability in dried and re-dried conditions compared to hygrothermal conditions, with normalized stable translaminar fracture toughness of 1.0, 1.68, and 1.5, respectively. Finally, the fracture surface analysis revealed distinctive toughening mechanisms under different conditions, highlighting significant fiber bundle pull-out under hygrothermal aging. In addition, the quantitative study of translaminar fracture toughness can serve as a basis for fiber failure criterion and provide model input for the simulation and evaluation of the effects of hygrothermal aging on CFRP performance.