The primary objective of this study is to reveal the role of shear-thickening fluids on the low-speed impact behavior and quasi-static puncture resistance conditions of impregnated p-aramid fabric. This was achieved by progressively increasing the silica ratio methodically. Rheological experiments indicated that 60% represents a crucial threshold for silica content, over which the rheological performance of fluids markedly improves. The impregnated targets exhibited a substantial increase in both dynamic impact and quasi-static puncture resistance in comparison to the neat fabric. Results from tests using the same amount of impact energy showed that impregnated targets had much better impact resistance (ranging from 30.5% to 119.2%) and better energy absorption (ranging from 22.9% to 61.3%) than the untreated targets. In quasi-static tests, impregnated targets exhibited significantly higher puncture resistance, ranging from 42.3% to 90.46%, compared to the neat fabric. The enhanced performance of impregnated targets was ascribed to the presence of interfiber friction, the thickening mechanism of the fluid, and the hardness of the particles. Compared to the neat fabric, the performance enhancement achieved in dynamic impact tests is greater than that observed in quasistatic tests. The variation in performance was associated with the contact area of the threat with the target. Due to the intense force exerted by the knife tip, its contact area with the target is smaller in comparison to that of the impactor. This caused the particle hardness and thickening mechanism to play a lesser role in quasi-static tests compared to impact tests. In addition, to reveal the effect of impact energy, tests were carried out at three different impact energy levels: 20J, 40J, and 60J. The impact resistance of both neat and impregnated textiles improved as the impact energy went up. Nevertheless, the neat fabric exhibited a greater augmentation in resistance in contrast to the impregnated one.

In this study, a multi-linear constitutive relation taking into account temperature and fiber bridging is proposed for characterizing delamination behavior in composite laminates under various temperature conditions. An approach combining analytical solution and J-integral is also established for determining the cohesive parameters in the multi-linear constitutive relation. To validate the proposed constitutive relation, mode I quasi-static delamination experiments of unidirectional (UD) and multidirectional (MD) carbon/bismaleimide laminates are carried out at 25 ℃ (room temperature), 80 °C and 130 ℃. The experimental results show that the increasing temperature resulted in a monotonic increase in the fracture toughness of the UD laminates while affect the fracture toughness of MD laminates slightly. A FE model is established with the implementation of the proposed multi-linear constitutive relation using UMAT subroutine. Good agreements between the experimental and simulated results demonstrate the validity of the proposed constitutive relation, with the relative difference of peak load between predicted and experimental values less than 8.2 % and the relative difference of initial and steady-state fracture toughness between predicted and tested results less than 15 %. This study provides the possibility to numerically study the temperature effect on the delamination behavior of laminates and has promising applications in the damage tolerance design of composite structures.

Due to the parameter differences and the existence of random factors, the performance of composite repair structure shows a large dispersion. In order to obtain stronger, lighter and more reliable repair structure, multi-objective optimization design of carbon fiber reinforced polymers (CFRP) laminate repair structure is investigated by combining the reliability theory, artificial neural networks and the genetic algorithm. First, a 3D simulation model of the composite laminate patch repair structures is established using the 3D Hashin criterion and the cohesive region model. Then, the Latin Hypercube sampling (LHS) method is used to realize the random sampling, and the strength proxy model of the repair structure is established by using Back-propagation artificial neural network, further a multi-objective optimization model with tensile strength, reliability and weight as objective functions is built considering the design parameters and the random parameters. Finally, NSGAⅡalgorithm is used to solve the multi-objective optimization problem, and a set of solutions on the Pareto front surface are obtained, also the optimal design parameters of the composite repair structure meets the requirements is obtained.

Auxetic metamaterials have garnered extensive attention over the past few decades due to their exceptional and superior mechanical properties. However, owing to their unique porous structure, it is challenging to ensure that structures possess strong energy absorption capabilities while exhibiting excellent auxetic characteristics. This study introduces a rotating triangular auxetic metamaterial (RTAM) by perforating traditional rigid rotating triangles. Quasi-static compression tests and numerical simulations are conducted on the new structure to investigate the effects of wall thickness and re-entrant angle of the triangular perforated plate on mechanical properties and Poisson’s ratio. The plateau stress and specific energy absorption (SEA) of RTAM are 4 and 10 times higher than that of traditional trichiral auxetic metamaterials (TCAM), respectively. With an increase in wall thickness, both plateau stress and SEA of the structure are improved significantly. As the re-entrant angle increases, the SEA of the structure initially decreases and then increases. RTAM achieves both lightweight structure and ideal mechanical performance, providing an approach for manufacturing lightweight and high-strength auxetic metamaterials, with significant potential applications in the field of energy absorption.

Numerical analysis of the post-impact damage propagation process during CAI tests on titanium-based fibre metal laminates was conducted, and the model was validated by collation of mechanical parameters predicted by it with experimental results. Delaminations induced by the impact were claimed as the hotspot for other modes propagation in most of studied cases and intensively propagated shortly before reaching peak compressive force. Metal – composite interfaces away from impact surface were locations of most intensive delamination propagation during CAI, constituting 63.5 % of overall damage area for titanium-carbon laminate and 81.6 % for titanium-glass laminate. Similarly to delaminations, damage due to fibre and matrix compression propagated from impact point and lead to significant load-carrying ability loss even in low volumes. Propagation of fibre and matrix tension was connected more to buckling progression, than post-impact damage and had the highest volumetric share in composite layers damage modes (max 13.5 % for titanium-carbon laminate and 8.3 % for titanium-glass laminate).

Thermoplastic based composites (TPC) have emerged as a new generation of eco-friendly materials with tougher matrices capable of overcoming the major weaknesses of the thermoset counterparts related with poor resistance to delamination and recycling difficulties. Although TPC materials present low surface energy, adhesive bonding is still effective when the requirements for fusion bonding procedures are not met. Recent advances in adhesive technology have unveiled two-part acrylic adhesive specifically designed for low energy surfaces, characteristic of TPC materials. In this work, unidirectional carbon-reinforced polyamide 6 (CF/PA6) bonded joint was characterized under pure mode II loading using end-notched flexure (ENF) test. The experimental fracture tests revealed unstable crack propagation and a data reduction scheme based on the equivalent crack concept was developed to obtain the strain energy release rate distribution along the crack front for the specimen’s length beyond the actuator central loading point. The proposed procedure was successfully validated using a finite element analysis including a cohesive zone modelling and applied to the experimental results. The obtained Resistance-curves showed that this adhesive is capable to provide a significant bonding resistance in pure mode II loading even in low energy surfaces characteristic of TPC materials.