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This paper focuses on the in-plane compression mechanism of two-dimensional biaxial braided composite laminates under quasi-static compression and dynamic compression. Three types of specimens with different off-axial angles in respect of OA00, OA60, and OA90 were employed for compression experiments. The surface damage process of the specimen was recorded using optical camera and infrared thermal imaging technology. The internal damage morphology of the tested specimens was analyzed by X-ray computed tomography. The progressive damage behavior of braiding yarns and matrix resin was simulated by a three-dimensional meso-level finite element model. Experimental and simulation results revealed that the off-axial angle significantly affects mechanical properties and damage mechanisms. The stress distribution and compression deformation of braiding yarns are affected by the boundary conditions of load-bearing yarn, which relies on the off-axial angle. The stress of OA00 specimen is concentrated at the interface of yarn and matrix resin. The failure mode of OA00 specimen is dominated by the shear band, which is induced by the interface microcracks between yarn and matrix resin. The stress of OA60 and OA90 specimens are concentrated at the undulation position of the braiding yarn. The critical failure mode of OA60 and OA90 specimens is delamination induced by interlaminar microcracks. The OA60 specimen achieves a better compression response with respect to bearing the load and absorbing the fracture energy because the sub-laminate that is formed after delamination can continue to bear the load until its intralaminar fracture occurs. [ABSTRACT FROM AUTHOR]

I-section carbon fiber reinforced polymer (I-CFRP) laminates are widely used as hatch and cargo doors of wide-body airliners. However, estimating the loading capacity of I-CFRP laminates has always been empirically dependent since there is no uniform failure criterion for composite materials. This study attempts to reveal the failure characteristic points of I-CFRP laminates from a thermodynamic perspective. First, the test strain data can be modeled as state variables, thus equating the loaded engineering structure to a thermodynamic system. Based on the relation of state variables, matrices (Modes) and Hamiltonians (Characteristic parameters) that characterize the overall stressing state evolution of the specimen can be established. Integrating the Hamiltonians of each part into the whole is similar to the group representation and renormalization of Wilson's phase transition theory. Applying the clustering analysis (CA) criterion in combination with the bifurcation and transition of the mode and characteristic parameter curves reveals the phase transformation loads of the specimen. The accuracy and stability can be verified for the phase transition loads before and after the renormalization transformation. In conclusion, this study reveals the deformation-failure law of I-CFRP laminates from the thermodynamic perspective, which provides a new reference and method for the design of composite laminates. [Display omitted] • Eleven I-CFRP laminates axial compression tests were conducted. • Modeling the behavior of I-CFRP laminates from a thermodynamic perspective. • Propose a matrix representation (Mode) and Hamiltonian (Characteristic parameter) of the I-CFRP laminates' stressing state. • Reveal the phase transition point of I-CFRP laminates: EPB and FS points. [ABSTRACT FROM AUTHOR]

• Experimental study of the repeated low-velocity impacts of various fiber structures. • 3D woven composites mainly dissipate impact energy through fiber breakage. • 3D orthogonal woven composites mainly show vertical crack paths. • The durability of 3D woven composites under high-energy impact is improved. • The structural integrity of the 3D orthogonal woven composites is high. This paper presents an experimental investigation into the response of carbon fiber reinforced composites with various fiber architectures to repeated low-velocity impacts. Mechanical response diagrams, including repetition numbers and damage morphologies of the composite samples, are provided. Varied energy levels (20 J, 35 J, 50 J) and impact numbers (up to 30th) are considered to induce perforated damage in the composite samples. It is found that the mechanical responses of the composite samples under repeated impact events exhibit distinct characteristics in terms of impact threshold and stiffness degradation. The 2D unidirectional (2DUD) laminated composite sample exhibits a high impact threshold during impact events but stiffness rapidly decays after an initial substantial decline. The 2D plain woven (2DPW) laminated composite sample accumulates impact damage at both high and low levels, resulting in a gradual stiffness decrease during impact events. In addition, the 3D orthogonal woven (3DOW) composite sample demonstrates a higher impact threshold with its stiffness gradually decreasing under high-level impacts. In contrast, both 2DUD and 2DPW composite samples exhibit extensive plastic deformation of the resin matrix with minimal fiber fracture, enabling efficient dissipation of impact energy. Conversely, the 3DOW composite sample primarily dissipates impact energy through fiber fracture. Additionally, it is noteworthy that while both 2DUD and 2DPW composites demonstrate enhanced robustness against repeated impacts at low-energy levels, a significant improvement in durability is observed specifically for the 3DOW composite under high-energy level impacts. [ABSTRACT FROM AUTHOR]

• Advantages of asymmetric hybridization on impact resistance and resilience. • Rules of external damage area and internal crack volume distribution. • Damage mechanism of carbon/Kevlar hybridization laminates. The ply structure has a significant effect on the impact resistance and failure behavior of composite laminates. In this paper, carbon/Kevlar hybrid fabric reinforcements were prepared with different Kevlar ratio (0%, 25%, 50%, 75%, 100%) by replacing carbon fabric plies on the non-impact side by Kevlar plies. The low-velocity impact tests for all configurations were carried out under 15 J and 25 J energy respectively. The results show that the maximum deflection, impact duration, impact absorption energy and absorption energy efficiency of hybrid laminates increased with the Kevlar hybrid ratio, whereas the peak force presented a decreasing trend, which indicates that the adding of Kevlar layer makes the laminates more ductile than non-hybrid carbon structure. The damage mechanism of the carbon fiber ply was brittle fiber fracture, resin cracking and resin shedding, while the fibers pull-out and tows splitting was found in Kevlar ply. The critical structural damages start from the non-impact side and evolve to the impact side. And the rules of internal crack volume distribution with Kevlar plies are consistent with the trend of external damage area on the non-impact side. It is suggested that in order to take into account the peak strength and excellent structural resilience, Kevlar asymmetric-hybrid ratio of 25% is the most appropriate. The reason is that an appropriate small ratio of Kevlar cushion ply can not only effectively avoid the tensile fracture at non-impact side happened in the non-hybrid carbon structure, but also withstand enough impact load by maintaining a high carbon fiber content. [ABSTRACT FROM AUTHOR]

• Advantages of symmetric intercalation hybridization on impact strength and resilience. • Correlation of apparent damage area index and internal damage volume data. • Damage mechanism of symmetric and asymmetric carbon/Kevlar structures. Hybrid fiber composite laminates with the same materials, manufacturing methods and molding processes may present different mechanical behaviors. In this paper, the effect of intercalation sequence on the mechanical performance of carbon/Kevlar plain laminates were studied. According to different intercalation methods, the hybrid layout was divided into symmetric and asymmetric configurations. It is found that the symmetric intercalation structures (KCCK and CKKC) possess better load-bearing and resilience capacity compared with asymmetric structures (CCKK and CKCK), even better than pure carbon structure (CCCC) under high energy impact. The impact damage volume of symmetric specimens was significantly smaller than that of asymmetric specimens by quantitative micro-CT analysis. The apparent damage index (d f and d r) can be used to estimate the internal damage data (d CT , H and V). KCCK, a typical symmetric sandwich-like structure, presented the most slight damage among all symmetric and asymmetric configurations, with shear cracks observed at the brittle carbon fiber core layers, while the delamination cracks between Kevlar-carbon interlayers were away from the central damaged region. In CKCK, interlaminar cracks were formed at each carbon-Kevlar interlayers, which shared and relieved the primary interlaminar crack at the core layers compared with CCKK. [ABSTRACT FROM AUTHOR]