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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]

In this paper, electrical resistance measurement (ERM) was used to detect the direction and amplitude of the bending force on fiber-reinforced polymer composite (FRPC) tubes. The method was based on research into the influence of position on the sensing behaviors of carbon fibers (CFs) as separated resistive elements under flexural tests. CFs were inserted into the fabric reinforcement fabricated by a circular braiding machine as axial tows that are separated by bias tows (Kevlar fibers) to serve as sensors. In experiments, samples of FRPC tubes with four axial tows were fabricated and bent at two different angles relative to the distribution of CFs while deformation was maintained in the elastic zone. The resistance change ratios of the CFs varies from −1.5% to 0.3% and the zero-crossing point on the tube is at 105°. The results showed that CF resistances would change in a particular pattern as the tube was being bent, and the bending direction and force amplitude could be derived from this pattern. [ABSTRACT FROM AUTHOR]

Non-crimp fabric reinforced polymer (NCFRP) composite is a suitable choice as structure material in aerospace use. One of the most important issues related to adiabatic failure is the correlation among temperature elevation, adiabatic shear band and failures of the material. In this study, impulsive damage and failure modes with temperature rise in biaxial NCFRP composite specimens were investigated under increased strain rates from 200/s to 2500/s. Path and distribution of temperature rise were experimentally characterized when the composite was subjected to in-plane and out-plane impulsive compression. It is found that (i) localized temperature can rise above a hundred degrees Celsius, reaching or even exceeding the T g of the resin matrix and polyester tricot in specimen, which causes stress concentration and subsequent failure in warp/weft fiber tows in localized positions. (ii) For the out-of-plane compression, the localized temperature rise indicates dispersed inter-layer shear deformation in the biaxial NCFRP composite by stress wave mismatch effect. With the increase of strain rate, intensive temperature rise is a good announcement for an adiabatic shear band quickly and thoroughly splitting the specimen. (iii) For the in-plane compression, slight impact loading triggers small debonding and cracking along warp/weft fiber tows or at the fiber-matrix interface by fiber tows microbuckling effect. With the increase of strain rate, the localized temperature rise reveals that single shear band is developed and further evolved into zigzag shear bands, while catastrophic delamination failure is the dominant mode at higher strain rate. Higher temperature values at kinking positions are caused by fiber tows buckling-fold effect. Thermal degradation effect imperils the polymeric matrix and tricot around the kinking positions where bridging cracking, obstructing the spread of adiabatic shear band, and leading to the pull-out/breakage/splitting damages of warp/weft fiber tows. (iv) This work indicates that high-speed thermographic technology is an important method for investigating the dynamic behavior of composite materials. Localized temperature rise is an effective indicator in complex correlation with the multiple physical interaction including the adiabatic shear band, matrix/tricot softening, fiber tows kinking, localized damages and failures. [ABSTRACT FROM AUTHOR]

• The impact response of carbon/glass intralaminar hybrid braided pipes was studied. • Impact behaviors of hybrid pipes with and without internal pressure were compared. • The damage extension mechanisms were analyzed by micro-CT 3D damage reconstruction. • Ductile and brittle hybrid fibers restrict each other and promote impact resistance. • The hybrid fiber layout and the internal pressure had a synergistic effect. This study investigated the low-velocity impact behaviors of intralaminar hybrid carbon/glass braided composite pipes under internal pressure. The effects of intralaminar fiber layout and internal pressure on the mechanical responses, damage extension, and material cost-effectiveness were analyzed. The internal damage distribution of composite pipes with different hybrid fiber alternating numbers but identical mix ratios was evaluated through Micro-CT three-dimensional damage reconstructions. The yarn splitting of brittle carbon fiber materials was mutually prevented by the induction of intra-yarn debonding and delamination damages by hybridization with ductile glass fiber materials, which were prone to produce out-of-plane displacement and shear stresses under impact loading. A smaller hybrid fiber alternating unit was conducive to exploiting the hybrid impact toughening effect by avoiding pure reinforced fiber materials aggregated under the impactor. Internal pressure had a supporting effect on the composite pipe and was effectively synergistic with the hybridization effect. The change in the dominant damage mode resulted in significant reductions in impact deformation and damage severity compared to the unpressurized case. [ABSTRACT FROM AUTHOR]

Ultrasonic testing (UT) is commonly used to inspect the geometric shape of internal damage in composite materials and the test results need to be interpreted by trained experts. In this work, an automatic signal classification method based on deep learning is proposed for depth estimation of the detects introduced by low-velocity impact (LVI) in carbon fiber reinforced plastics (CFRPs). Three kinds of neural networks, LSTM, CNN, and CNN-LSTM are used to analyze the attributes with different depths. Then, trained models are applied to identify the depth information of impact damage. The results show that the CNN-LSTM model is a more accurate in-depth classification for LVI defects in CFRP based on A-scan signals than the other two structures. [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]

Accurate 3D representations of multiphase layered composite fabric, including distinguish and label the pores, matrices, warp and weft yarns, can assist in understanding and ultimately improving composite material design performance. In this work, a new data enhancement algorithm is proposed to generate realistic-looking artificial learning datasets for expanding the datasets. Then, we demonstrate how supervised, the Swin Transformer approach can help to realize accurate segmentation of low-resolution and poor-contrast fabric datasets by populating databases with real and artificial learning datasets. Most of the pixels were correctly identified and the boundaries of the yarn could be identified clearly, except for some spots and a few merged boundaries. The work presented here provides an effective tool that can be widely used in semantic segmentation for various composite materials (woven composites, fiber-reinforced composites, short-fiber materials, etc.), which enables higher-precision reconstruction of fiber-composites and significant reduction in dataset processing time, and provides a route to generating artificial learning dataset. [ABSTRACT FROM AUTHOR]

• Hybridization of glass fiber into carbon composite presents toughening effect. • Infrared thermography accurately evaluates the progressive damage. • Off-axial tensile properties are more sensitive than on-axial for hybrid composites. • The quick crack propagation in hybrid composites is prevented. • Damage of interior carbon fiber layers can be restrained through hybridization. Hybrid composites are increasingly attractive for advanced engineering applications. In this study, the open-hole tensile (OHT) properties and failure mechanism of plain woven carbon fiber, glass fiber, and carbon/glass hybrid fiber composites under on-axis (0°) and off-axis (45°) loads were investigated through experimental and numerical approaches. The progressive damage was characterized by high-speed infrared thermography. The numerical simulation presented a good explanation on the damage development. It indicates that the carbon/glass hybrid composites exhibited significant improvement in failure strain and toughness compared to pure carbon fiber composite. With the insertion of glass fiber layers, the failure modes of hybrid structures were modified and quick propagation of cracks was prevented. Once the cracking on their carbon layers had initiated there was an increase in damage to glass fiber layers showing the transfer of stress from carbon layers to glass layers. Similar mechanical properties were reached among sandwich-like hybrid samples in on-axis tension, but much greater properties can be achieved when the carbon fiber at the interior layers of the hybrid specimens under off-axis load. Since glass fiber with great ductility was laminated as the surface layers in hybridization, resulting in less delamination and higher ultimate strength in 45○ off-axis tension. [Display omitted] [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]