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Bio-inspired helicoidal carbon fiber reinforced polymer composite (CFRPC) laminates stacked with Bouligand configurations are considered to own great potential in engineering fields. The optimization design of helicoidal layups with single-form and combination-form of linear and nonlinear helicoidal arrangements is carried out using ABAQUS and Isight. Two single-objective optimizations and one multi-objective optimization of maximum fundamental frequency and the difference among the first four adjacent vibration frequencies of helicoidal CFRPC laminated plates are performed and the nonlinear free vibration characteristics of optimal ones are investigated for the first time. Numerical studies are carried out for 16-ply symmetrical rectangular helicoidal laminated plates under two different boundary conditions compared with quasi-isotropic counterparts. The results show that the combination-form helicoidal layup sequences can improve the linear vibration characteristics of CFRPC laminated plates. The plates stacked with the combination form of nonlinear Fibonacci and linear patterns are expected to provide better vibration characteristics. [ABSTRACT FROM AUTHOR]

[Display omitted] The Bouligand structure has been observed in a variety of biological materials, such as lamellar bone and exoskeleton of lobsters. It is a hierarchical and non-homogeneous architecture that exhibits excellent damage-resistant performance. This paper presents a multiscale fracture model considering the material inhomogeneity, the multiscale property, and the anisotropy to reveal the toughening mechanisms in the Bouligand structure. Firstly, the macro and micro constitutive properties of this composite are derived. Then, a multiscale fracture model is developed to characterize the local stress intensity factors and the energy release rates at the crack front of twisted cracks. Our results demonstrate that the decrease in the local energy release rate can be attributed to two-step mechanisms. The first mechanism is that the multiscale structure and the material inhomogeneity cause a release of stress near the initial crack tip. The second mechanism is that the twisted crack leads to the transformation from single-mode loading to mixed-mode loading, which enhances the fracture toughness. These results can not only reveal the toughening mechanism of the Bouligand structure but also provide guidelines for the design of high-performance composites. Biological materials in nature often possess excellent mechanical properties that have not been achieved by synthetic materials. Bioinspired Bouligand structures provide prototypes for designing high-performance materials. In this study, we propose a multiscale theoretical fracture model to investigate the fracture properties of Bouligand structures with twisted cracks. We systematically consider the roles of material inhomogeneity, anisotropy, and multiscale properties. Our analysis demonstrates that the remarkable toughness of Bouligand structures results from the combined effects of material inhomogeneity and twisted cracks. This research contributes to unveiling the secret behind the outstanding toughness of Bouligand structures and provides inspiration for the development of novel designs for man-made composites. [ABSTRACT FROM AUTHOR]

Bioinspired nanocomposites have attracted considerable attention as robust, lightweight, and functional materials for various applications. These bioinspired materials contain high fractions of reinforcements and form highly ordered hard/soft nanocomposite structures with balanced molecular interactions. Herein, we fabricate bioinspired ternary nanocomposites of sodium carboxymethylcellulose (CMC) and polydopamine (PDA)-modified cellulose nanocrystals (DCNCs) with bouligand-type architectures via water-borne evaporation-induced self-assembly (EISA). The surfaces of cellulose nanocrystals (CNCs) were coated with PDA polymer to improve the interfacial adhesion between CNCs and CMC. We tune the structure formation and material properties by using different feed ratios of polymer-to-CNCs. Material properties were tailored further in ternary nanocomposites by balancing CMC and PDA amounts within the total polymer content at a given feed ratio. CMC–DCNC ternary nanocomposites with higher CNC content exhibit better-ordered structures, synergistic mechanical performances, and improved functional properties than CMC–CNC binary nanocomposites. CMC–DCNC nanocomposites with 70 wt % CNC loading show excellent mechanical strength (183 ± 36 MPa) and stiffness (15 ± 0.5 GPa) at 20% relative humidity (RH), placing these materials among the top-end of CNC-based bioinspired nanocomposites reported so far. Due to the stable and efficient interfacial interactions between DCNCs and CMC, ternary nanocomposites demonstrate comparatively much higher mechanical properties than CMC–CNC and pristine CMC films, even at higher RH (50 and 80% RH). Crack propagation studies using field-emission scanning electron microscopy display different crack growth and toughening mechanisms like crack deflection, layered delamination, and microcrack generation, resulting in synergistic mechanical improvement in the ternary nanocomposites. Due to the presence of PDA, CMC–DCNC nanocomposites also exhibit higher thermal stability, UV-shielding properties, free-radical scavenging ability, and water vapor barrier properties, which are essential characteristics for packaging materials. Thus, CMC–DCNC nanocomposites prepared from sustainable building blocks using a bioinspired design strategy show potential as robust, green, UV-protective films for food packaging. [ABSTRACT FROM AUTHOR]

Inspired by the bionic Bouligand structure, helicoidal carbon fiber-reinforced polymer composite (CFRPC) laminates have been proven to own outstanding out-of-plane mechanical properties. This work aims to further explore the excellent bending characteristics of helicoidal CFRPC laminated plates and find out the optimal helicoidal layup patterns. The optimization design of laminated plates stacked with single-form and combination-form helicoidal layup sequences are carried out by using the finite element method (FEM) and adaptive simulated annealing (ASA) optimization algorithm on the Isight platform. Then, the nonlinear bending responses of optimal helicoidal CFRPC laminated plates are investigated via the FEM for the first time. The helicoidal CFRPC laminated plates under three different types of boundary conditions subjected to transverse uniformly distributed load are considered. The numerical results reveal that the combination-form helicoidal layup sequences can decrease the dimensionless bending deflection of laminated plates by more than 5% compared with the quasi-isotropic plate and enhance the out-of-plane bending characteristics of CFRPC laminated plates effectively. The boundary conditions can significantly influence the nonlinear bending responses of helicoidal CFRPC laminated plates. [ABSTRACT FROM AUTHOR]

Herein, biomimetic C f /ZrB 2 -SiC ceramic composites with bouligand structures are fabricated by combining precursor impregnation, coating, helical assembly and hot-pressing sintering. First, C f /ZrB 2 -SiC ceramic films are achieved through a precursor impregnation method using polycarbosilane (PCS). Second, the PCS-C f /ZrB 2 -SiC ceramic films are coated with ZrB 2 and SiC ceramic layers. Finally, hot-pressing sintering is employed to densify helical assembly C f /ceramic films with a fixed angle of 30°. The microstructures and carbon fiber content on the mechanical properties of biomimetic C f /ZrB 2 -SiC ceramic composites are analyzed in detail. The results show that the coated ceramic layer on PCS-C f /ZrB 2 -SiC films can heal the cracks formed by pyrolysis of PCS, and the mechanical properties are obviously improved. Meanwhile, the mechanical properties could be tuned by the contents of the carbon fiber. The toughening mechanisms of C f /ZrB 2 -SiC ceramic composites with bouligand structures are mainly zigzag cracks, crack deflection, multiple cracks, carbon fiber pulling out and bridging. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) printing allows for the creation of complex, layered structures with precise micro and macro architectures that are not achievable through traditional methods. By designing 3D structures with geometric precision, it is possible to achieve selective regulation of mechanical properties, enabling efficient dissipation of mechanical energy. In this study, a series of modular samples inspired by the Bouligand structure were designed and produced using a direct ink writing system, along with a classical printable polydimethylsiloxane ink. By altering the angles of filaments in adjacent layers (from 30° to 90°) and the filament spacing during printing (from 0.8 mm to 2.4 mm), the mechanical properties of these modular samples can be adjusted. Compression mechanical testing revealed that the 3D printed modular Bouligand structures exhibit stress-strain responses that enable multiple adjustments of the elastic modulus from 0.06 MPa to over 0.8 MPa. The mechanical properties were adjusted more than 10 times in printed samples prepared using uniform materials. The gradient control mechanism of mechanical properties during this process was analyzed using finite element analysis. Finally, 3D printed customized modular Bouligand structures can be assembled to create an array with Bouligand structures displaying various orientations and interlayer details tailored to specific requirements. By decomposing the original Bouligand structure and then assembling the modular samples into a specialized array, this research aims to provide parameters for achieving gradient energy absorption structures through modular 3D printing.

The mechanically robust, high-performance, and functional nanocomposites have recently attracted considerable attention in both scientific and industrial research. Herein, we prepare the high-performance ternary nanocomposites using naturally occurring building blocks via a water-borne evaporation-induced self-assembly (EISA) process. Cellulose nanocrystals (CNCs) have been used as reinforcing fillers to be incorporated into the sodium carboxymethyl-cellulose (CMC) polymer matrix. The surface of CNCs was coated with polydopamine (PDA) to further improve the inherent properties of the CMC–CNC binary nanocomposites. The PDA acts as a bridge to enhance the interfacial interactions between CNCs and CMC. We optimized the PDA coating thickness on CNCs to tune the interfacial adhesion by varying the self-polymerization time of dopamine. The results showed that the self-polymerization of dopamine on the CNC surfaces for 24 h (DCNC-24) led to better structure formation and improved mechanical and thermal properties of the CMC–DCNC-24 ternary nanocomposites. At 50% RH, the CMC–DCNC-24 ternary nanocomposites exhibit an excellent tensile strength of 154 ± 12 MPa, Young's modulus of 10 ± 1 GPa, and toughness of 5 ± 1 MJ/m3. Due to their enhanced and stable interfacial interactions, the ternary nanocomposites demonstrate higher mechanical performances even at high relative humidity conditions compared to the pure CMC and CMC–CNC films. At 80% RH, CMC–DCNC-24 films have a strength of 36 ± 4 MPa, stiffness as high as 4.0 ± 0.5 GPa, and toughness of 4.0 ± 1.0 MJ/m3. The CMC–DCNC ternary nanocomposites have also shown enhanced thermal stability. In addition, the existence of PDA with UV-absorption ability induces UV-shielding properties to CMC–DCNC ternary films. Thus, the self-assembled CMC–DCNC ternary nanocomposites prepared in this work have implications to be applied as high-performance UV-shielding films for packaging. [ABSTRACT FROM AUTHOR]

The impact of soil particles on the soil-engaging components can lead to wear and tear, even in the failure of agricultural machinery systems. The bionic Bouligand-type (twisted plywood) arrangement structure can be expected to provide new strategies in this case. This study aims to explore the wear-resistance performance of the bioinspired Bouligand structure for the soil-engaging components. A series of computational simulation experiments were also carried out on the abrasive wear using the EDEM platform. Three parameters of the geometric feature were first selected as the experimental independent variables, including the beam diameter, twist angle, and overlap distance of the Bouligand-type structure. By contrast, the abrasion loss was used as the response value. Multivariate quadratic polynomial regression models were then established for the optimization. The geometric feature parameters of the Bouligand-type structure were also optimized, according to the relationship between the independent variable and the response value. The optimization results showed that the favorable wear-resistance performance was achieved under the optimal combination of geometrical feature parameters, with a beam diameter of 1.0 mm, a twist angle of 16°, and an overlap distance of 0.13 mm. With the optimal parameters, the wear-resisting properties of the Bouligand-type structure were compared with the conventional solid ribbed surface and smooth surface. The computational results show that the abrasion losses were 2.13×10-6 g for the Bouligand-type structured surface, 2.26×10-5 g for the conventional ribbed surface, and 2.73×10-5 g for the conventional smooth surface, respectively. The bouligand-type structured surface reduced the abrasion losses by 90.6% and 92.2%, respectively, compared with the conventional ribbed surface and smooth surface, respectively. Correspondingly, the Bouligand-type structure substantially reduced the abrasion loss from the abrasive wear, particularly for better wear-resistance performance. Furthermore, the EDM-FEM coupled simulation was used to evaluate the internal deformation and strain behavior of the samples, in order to further investigate the wear-resisting enhancement from the Bouligand-type structure. In addition, the averaged deformation of the Bouligand-type structured, conventional ribbed, and smooth surface were 1.82×10-8, 7.97×10-9, and 1.62×10-9mm, respectively, where the averaged equivalent stresses were 1.16×10-6, 6.36×10-6, and 1.01×10-5 MPa, respectively. The results show that the Bouligand-type structure presented relatively higher internal deformation and strain, compared with the rest. The reason was that the Bouligand-type structure shared the better capability to absorb the impact energy from the abrasive particles for reduced abrasion loss. The rotary abrasive test bench was used to further validate the simulation. The minimum wear amount of Bouligand structural parts was 0.12 g, and the minimum standard deviation was 0.012, the wear resistance was stable compared with the conventional ribbed and smooth surface. Consequently, there were relatively stable variations in the abrasion loss of the Bouligand-type structure over the wear time. This research can also provide a new theoretical reference and technical basis for the development of promising wear-resistant materials. [ABSTRACT FROM AUTHOR]

Natural Bouligand structure is a special class of hierarchical architectures, which have proven to be effective in enhancing its mechanical properties. In this work, we demonstrate that the thermal buckling capacity of Carbon Fibre Reinforced Plastics can also be improved by mimicking bio-inspired Bouligand architectures. A finite element approach using commercial software ABAQUS is developed to predict the critical buckling temperature of the Bouligand inspired laminated plates under thermal loading. Numerical predictions show a good agreement with analytical results obtained from the literature. The effectiveness of Bouligand-type composite laminates in enhancing thermal buckling behavior is first confirmed by comparing with unidirectional, cross-ply and quasi-isotropic laminates. We then present the effects of pitch angle and ply thickness on the thermal buckling temperature of bio-inspired Bouligand-type laminates. Finally, we conclude that the improved mechanical and thermal properties can be simultaneously achieved by the Bouligand structure of laminates as the pitch angle is increased. The present study highlights the high thermal buckling behavior of biomimetic Bouligand structure, offering a possibility in developing biomimetic composites with good trade-off between mechanical properties and thermal performance. [ABSTRACT FROM AUTHOR]

The exoskeleton structure of mantis shrimp's dactyl clubs presents an extraordinary helicoidal reinforcement pattern, holding immense potential for revolutionizing fiber-reinforced composite materials. While extensive investigations have shed light on its unique advantages, the inherent nature of curved Bouligand structures remains incompletely comprehended. To elucidate these unexplored characteristics, the present study introduces a novel analytical model to efficiently calculate the interlaminar shear and normal stresses in helicoidal laminated curved beams under transverse loading. The analytical model systematically determines the optimal uniform fiber interply (pitch) angle for these bio-inspired structures with an arbitrary number of layers. The computational analysis revealed that achieving minimum interlaminar stresses always requires the precise orientation of π or 2π helicoids along the laminate thickness when the number of layers exceeds two. This theoretical discovery is remarkably identical to the study of stacked chitin fibrils in arthropod clubs reported in the literature. For a 37-ply composite, the flat and shallow-curved beam configurations primarily resulted in a 5° optimal pitch angle, while the deep-curved configuration yielded a 10° optimal angle. To validate the model, three-point bending testing were performed. Four fiber pitch angles were thoroughly examined, including the optimal and quad angles. The digital image correlation (DIC) technique was employed to analyze the structural responses and detect the onset of delamination. The numerical and experimental results demonstrated good consistency, exhibiting significant enhancement in the delamination resistance of up to 30 percent with the optimal angle. [ABSTRACT FROM AUTHOR]

Natural polymeric materials are an attractive potential replacement for non‐biodegradable plastics. However, one of the main challenges for high‐end applications of these materials, is to maintain their high mechanical properties in a watery environment. Here, inspired by the reinforcement principle of multiscale architecture in natural crab exoskeletons, an ultra‐strong and hydrostable film is developed through a top‐down method directly from the crab shell. The resulting chitosan film exhibits an outstanding isotropic tensile strength of ≈220 MPa and a superior wet strength of ≈70 MPa, which is significantly higher than that of conventional polysaccharide‐based materials and many commercially used plastics. Its excellent mechanical properties are due to the realization of strong interactions between chitosan fibrils even in wet conditions through interdigitation and densification processes without other additives. Additionally, it also possesses outstanding transparency, flexibility, antimicrobial ability, and biodegradability, making it a promising high‐performance and eco‐friendly material for many potential applications. [ABSTRACT FROM AUTHOR]

Inspired by the bionic Bouligand structure, helicoidal carbon fiber-reinforced polymer composite (CFRPC) laminates have been proven to own outstanding out-of-plane mechanical properties. This work aims to further explore the excellent bending characteristics of helicoidal CFRPC laminated plates and find out the optimal helicoidal layup patterns. The optimization design of laminated plates stacked with single-form and combination-form helicoidal layup sequences are carried out by using the finite element method (FEM) and adaptive simulated annealing (ASA) optimization algorithm on the Isight platform. Then, the nonlinear bending responses of optimal helicoidal CFRPC laminated plates are investigated via the FEM for the first time. The helicoidal CFRPC laminated plates under three different types of boundary conditions subjected to transverse uniformly distributed load are considered. The numerical results reveal that the combination-form helicoidal layup sequences can decrease the dimensionless bending deflection of laminated plates by more than 5% compared with the quasi-isotropic plate and enhance the out-of-plane bending characteristics of CFRPC laminated plates effectively. The boundary conditions can significantly influence the nonlinear bending responses of helicoidal CFRPC laminated plates.

The interest in soft elastomers has been revived because they are needed in the emerging soft robotics and wearable devices. It is imperative to adjust the mechanical properties of elastomeric materials for real‐life applications as well as to develop an approach to construct more complex and customized architectures. In this study, an embedded extrusion‐based three‐dimensional (3D) printing method was used to prepare silicone elastomeric composites. Cellulose nanocrystals were introduced to tailor the rheology of the ink and serve as reinforcements after solidification. The printing paths were carefully designed to write Bouligand structures, which have been widely found in arthropod cuticles and fish scales because of their exceptional damage resistance. Compared with the pure elastomer and randomly distributed composites, the 3D‐printed elastomeric composites with bioinspired structures exhibited simultaneously enhanced stiffness, extensibility, and toughness. This strategy may be extended to the realization of other biomimetic designs and paves the way for soft material manufacturing. [ABSTRACT FROM AUTHOR]

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This paper concerns the effects of hygrothermal ageing on the tensile behaviour of assymetric helicoidally stacked carbon fibre reinforced plastic (CFRP) composites. MR70 12P carbon fibre epoxy prepreg sheets were manufactured into laminated composites comprising constant inter-ply pitch angles ranging from 0 ° to 30 ° . The composites were tested in tension (according to BS ISO 527-5:2009) as either dry unaged specimens or following hygrothermal ageing in seawater at the constant temperatures of 40 ° C and 60 ° C for 2000 h. Both tensile modulus and tensile strength are found to be detrimentally affected by hygrothermal ageing, and the extent to which ageing affects these properties is a function of the inter-ply pitch angle. Higher hygrothermal ageing temperatures are found to decrease the tensile modulus and strength ratios of asymmetric helicoidally stacked composites when compared against UD composites subjected to the same conditions and the strength and stiffness ratios of all composites when compared against unaged equivalents. Significantly, therefore, we show that the degradation of helicoidal composite properties under hygrothermal conditions, in general, occurs more rapidly than it does in UD composites, and thus the long-term use of helicoidal composites in immersed environments should take into account these differences. A second order relationship is observed for the mechanical properties of the composites when plotted against their inter-ply helicoidal pitch angles. As such, a mixtures model was modified to incorporate the observed effects of laminate inter-ply pitch angle and used to predict the tensile modulus of unaged composites. The predictions are within one standard deviation of the experimental arithmetic mean; however, the model can only be used for dry helicoidal composites, as ageing alters the microstructures in an irregular manner between the different sample sets. The development of this mixture model is useful as it provides a justifiably simple route to predicting the properties of dry helicoidal structures, albeit within the bounds of specific interply-pitch angles. Finite element analyses (Hashin failure) elucidate the plies that are most likely responsible for composite failure. The validity of these numerical predictions is evidenced by observing primary fracture paths in the composites. Finally, hygrothermal ageing is found to enable greater in-plane (mode III) twisting of individual laminates under loading, with certain laminate angles being more prone to twisting than others. [ABSTRACT FROM AUTHOR]

Inspired by nature, this study employed the Bouligand structures, as observed in the dactyl club of mantis shrimp, to enhance the performance of 3D-printed Strain-Hardening Cementitious Composites (3DP-SHCC). Twenty sets of printed and mold-cast specimens were prepared to investigate the effects of Bouligand structures on the flexural performance of 3DP-SHCC. Three printing patterns (parallel, cross, and Bouligand printing) and five pitch angles (15°, 30°, 45°, 60°, 75°) were used for the Bouligand printing pattern. The results showed that the flexural performance of parallel-printed SHCC exhibited significant directionality. The flexural strength and toughness of parallel-printed SHCC in the X direction were 1.28 times and 3.27 times greater than the mold-cast SHCC. However, the flexural strength and toughness in the Y direction were 0.58 times and 0.25 times those of the mold-cast SHCC. The cross-printed and Bouligand-printed SHCC alleviated the directionality. The flexural performance of cross-printed SHCC was lower than that of the mold-cast SHCC. Compared to parallel-printed SHCC, the flexural strength and toughness of Bouligand-printed SHCC increased by 1.25 to 1.73 times and 3.48 to 5.10 times, respectively, in the Y direction. Bouligand-printing twisted the fiber angles, enabling effective crack bridging across multiple directions and endowing the SHCC with uniform mechanical properties in various directions. The Bouligand structures provided valuable inspiration for optimizing 3DP-SHCC structures. [ABSTRACT FROM AUTHOR]

This research aims to further the understanding of the structure and mechanical properties of the dermal armor of the boxfish (Lactoria cornuta). Structural differences between collagen regions underlying the hexagonal scutes were observed with confocal microscopy and microcomputed tomography (μ-CT). μ-CT revealed a tapering of the mineral plate from the center of the scute to the interface between scutes, suggesting the structure allows for more flexibility at the interface. High-resolution μ-CT revealed, for the first time, a 3D image of the dermal armor’s complex collagen structure. Helical interfibrillar gaps in the collagen base were found that are similar to the Bouligand-type structure of the lobster, Homarus americanus, thereby suggesting that the collagen in the boxfish is also of a Bouligand-type structure. In situ scanning electron microscopy tests were performed in shear and tension between two connected scutes and suggest that the interfacial collagen is structurally designed to preferentially absorb energy during deformation to protect the internal collagen. Similarly, in situ small-angle x-ray scattering was performed in shear and tension and further corroborated the complex collagen structure. Lastly, these experimental results are coupled with finite element simulations that characterize the interfacial collagen and corroborate the non-linear deformation response seen during in situ testing. Overall, these findings further the understanding of the structure and mechanics of the dermal armor of the boxfish which may help provide a basis to synthesize bioinspired composites for impact-resistant materials, specifically with bioinspired Bouligand-type structures to create novel fiber-reinforced composites.

The Bouligand structure, characterized by a helicoidal arrangement of fibers, is prevalent in arthropods. Leveraging the exceptional impact resistance inherent in this structure, mantis shrimp could effortlessly crack the shells of shellfish using their dactyl club. Drawing inspiration from this natural phenomenon, we propose a novel Bouligand-like structured carbon nanotube film (BCNF) that combines the Bouligand structure with super-strong one-dimensional carbon nanotubes (CNTs). Through systematic investigation via molecular dynamics simulations, we explore the impact resistance and energy dissipation properties of BCNF. Our results reveal that the impact behaviors under different velocities can be tuned by adjusting the pitch angle of BCNFs. At an impact velocity of 1 km/s, the projectile exhibits three typical phenomena after impact, including embedding in the film, bouncing back, and penetration. At an impact velocity of 2 km/s, all films are penetrated. Furthermore, to enhance the impact resistance of BCNF, we introduce intertube crosslinking bonds, forming a crosslinked Bouligand-like structured CNT film (CBCNF). The study demonstrates that crosslinking effectively modulates the energy dissipation mode of the film. With increasing crosslinking density, the predominant energy dissipation mode shifts from interface sliding and bending of the CNTs to the fracture and collapse of CNTs. This research provides valuable insights into the microstructure design of CNT-based films and offers theoretical guidance for their applications in impact protection. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper concerns the effects of hygrothermal ageing on the tensile behaviour of assymetric helicoidally stacked carbon fibre reinforced plastic (CFRP) composites. MR70 12P carbon fibre epoxy prepreg sheets were manufactured into laminated composites comprising constant inter-ply pitch angles ranging from 0° to 30°. The composites were tested in tension (according to BS ISO 527-5:2009) as either dry unaged specimens or following hygrothermal ageing in seawater at the constant temperatures of 40 °C and 60 °C for 2000 h. Both tensile modulus and tensile strength are found to be detrimentally affected by hygrothermal ageing, and the extent to which ageing affects these properties is a function of the inter-ply pitch angle. Higher hygrothermal ageing temperatures are found to decrease the tensile modulus and strength ratios of asymmetric helicoidally stacked composites when compared against UD composites subjected to the same conditions and the strength and stiffness ratios of all composites when compared against unaged equivalents. Significantly, therefore, we show that the degradation of helicoidal composite properties under hygrothermal conditions, in general, occurs more rapidly than it does in UD composites, and thus the long-term use of helicoidal composites in immersed environments should take into account these differences. A second order relationship is observed for the mechanical properties of the composites when plotted against their inter-ply helicoidal pitch angles. As such, a mixtures model was modified to incorporate the observed effects of laminate inter-ply pitch angle and used to predict the tensile modulus of unaged composites. The predictions are within one standard deviation of the experimental arithmetic mean; however, the model can only be used for dry helicoidal composites, as ageing alters the microstructures in an irregular manner between the different sample sets. The development of this mixture model is useful as it provides a justifiably simple route to predicting the properties of dry helicoidal structures, albeit within the bounds of specific interply-pitch angles. Finite element analyses (Hashin failure) elucidate the plies that are most likely responsible for composite failure. The validity of these numerical predictions is evidenced by observing primary fracture paths in the composites. Finally, hygrothermal ageing is found to enable greater in-plane (mode III) twisting of individual laminates under loading, with certain laminate angles being more prone to twisting than others.

Summary: Exoskeletons of Odontodactylus japonicas, the “smasher-type” mantis shrimp, feature a raptorial appendage comprising a Bouligand architecture of chitin nanofibrils with newly observed voids or defects between the polysaccharide α-chitin and protein interfaces. Here, we use a continuous-fiber 3D printing technology to simulate such materials in carbon fiber-reinforced (helicoidal) composites, complete with the presence of voids due to imperfect printing. The specific impact energies of the 3D printed helicoidal composites are clearly superior and further enhanced by the presence of the voids. To explain the role of the Bouligand architecture, interlaminar stresses are computed and found to yield anti-delamination characteristics, and a theoretical model is derived to evaluate the optimal helicoidal architecture. Finite element modeling indicates that the voids tend to deform and coalesce on loading and appear to guide the fracture into the formation of an ideally twisted crack in the printed helicoidal composites, thereby contributing to the impact toughness.

Carbon fiber reinforced polymer (CFRP) composites are strong and lightweight materials extensively used across aerospace, automotive, and construction industries. Bioinspired designs such as Bouligand structures have the potential to improve the mechanical properties of CFRP composites. In this study, we examine the effects of the single and double Bouligand meso-structures with varying twist angles on the flexural and fracture properties of additively manufactured continuous CFRP composites. 3-point flexural tests are conducted to characterize the flexural properties such as flexural modulus, strength, and energy absorption. Single edge notch bend (SENB) tests are performed to quantitively characterize the mode I fracture toughness and effective fracture energy. The elasto-plastic fracture theory is used to quantify the contributions of elastic and plastic energies to the effective fracture energy. Experimental results indicate that twist angles significantly affect the flexural behavior of CFRP composites. The Bouligand meso-structures with a twist angle of 10° increase the flexural strain energy absorption by 2-3 times. The single and double Bouligand layup configurations with the same twist angle result in different flexural properties. In addition, the twist angle affects the magnitude of fracture toughness. A twist angle of 10° results in an improvement of up to 60% in fracture energy dissipation compared to the quasi-isotropic meso-structure. The single and double Bouligand meso-structures with the same twist angle exhibit almost the same fracture toughness and absorbed fracture energy. The Bouligand meso-structures do not increase fracture toughness and elastic fracture energy for crack initiation compared to the quasi-isotropic meso-structure. [ABSTRACT FROM AUTHOR]

Many biological architectures are Bouligand structures, which comprise uniaxial fiber layers stacked in a periodic helical arrangement and are characterized by high damage resistance. As an effective flexible protective structure, fish skin is a Bouligand structure that protects the body while ensure flexibility during swimming and predation. In this paper, an analytical model inspired by fish skin is established based on previous studies, and the parameters for describing crack growth are determined. Then, mathematical expressions for the local stress intensity factors and plastic zone are used to predict how the helical stacking angle α influences the crack propagation. The results show that crack deflection and twisting improve the fracture toughness of the composite structure greatly, with the optimal fracture toughness being that for α = 60° – 70°. Moreover, biomimetic flexible composite structures inspired by fish skin are produced using silicone and Kevlar fibers. Scanning electron microscopy is used to observe the cross-sectional morphology of the composite structures, showing that the interfaces between the silicone and Kevlar fibers are highly compact. Results from experimental impact tests agree well with the predicted results.

A novel bioinspired strategy was proposed by coupling 3D printing with surface modification, which was employed to build the programmable helicoidal catalytic ceramics and improve the catalytic performance. Specially, the prepared helicoidal catalytic ceramic was able to transform the airflow direction, thereby prolonging the contact time between the gas and the catalyst for improving the catalytic efficiency. Thus, 3D printing integrating with biomimetic Bouligand structure offer promising capabilities to design creatively helicoidal catalytic ceramics, which is of great significance for the development of biomimetic catalytic materials in the future. [Display omitted] • 3D printed helicoidal ceramic devices with Bouligand structure were realized. • The surface modification endows the devices with excellent catalytic properties. • Excellent reusability and long-term stability were obtained for the ceramic device. Bouligand structure composed of twist-aligned nanofiber lamina as one typical biological skeleton is widely presented in various organism. Inspired by this, a strategy was proposed by coupling a novel method of 3D printing with surface modification, attempting to build the programmable helicoidal catalytic ceramics and then improving the catalytic performance. In this work, the biomimetic Bouligand architectures with different pitch angles (α), where the pitch angle is defined as the rotational angle of two adjacent twisted layer filaments, were built. Typically, the construction of bioinspired helicoidal catalytic ceramics with Bouligand structure can be divided into two steps, including the 3D printing of unidirectionally aligning ceramic filaments helical stacked helicoidal Al 2 O 3 ceramics, and the surface functionalization of platinum (Pt) catalysts. To this end, we designed and fabricated several helicoidal catalytic ceramics with pitch angles (α) of 30°, 45°, 60°, and 90° for exploring the catalytic performances of toluene vapor, and the acquired corresponding toluene conversions at 200 °C were 92.51%, 92.46%, 90.67%, and 78.51%, respectively. Furthermore, as an optimal twisted helicoidal catalytic ceramic device rotated from 0° to 180° with a pitch angle of 30°, the conversion of toluene at 200 °C can reach 95.52%, and exhibited excellent catalytic activity, favorable repeatability, and superior water resistance in the process of toluene oxidation. Thus, the prepared helicoidal catalytic ceramic was employed to transform the airflow direction, thereby prolonging the contact time between the gas and the catalyst for improving the catalytic efficiency. Overall, the simple yet efficient method for achieving tailored catalytic performance by controlling the pitch angles, dimensions, as well as the structural motifs, that can provide valuable ideas and revelations for developing light-structure catalytic materials in the future. [ABSTRACT FROM AUTHOR]

Bio-inspired helicoidal carbon fiber reinforced polymer composite (CFRPC) laminates are one of the novel composite structures that have potential applications in many engineering fields. The optimization design of helicoidal layups with linear, nonlinear Recursive, Fibonacci, Exponential and their combination-form arrangements are carried out using ABAQUS and ASA algorithm on Isight platform. Two cases of the compressive postbuckling under uniaxial compression and the thermal postbuckling under uniform temperature rise of helicoidal CFRPC laminated plates stacked with optimal layups are performed and the secondary buckling and mode jumping in the postbuckling region are examined for the first time. Numerical investigations are carried out for 16-ply symmetrical helicoidal laminated plates with various width-to-thickness ratios under different boundary conditions and are compared with quasi-isotropic counterparts. The results show that the bio-inspired helicoidal CFRPC laminated plates can enhance the buckling load and the buckling temperature as well as the compressive and the thermal postbuckling strength. In some cases, the secondary buckling and mode jumping may occur for rectangular helicoidal laminated plate in the postbuckling region. [ABSTRACT FROM AUTHOR]

The Bouligand structure is a fibrous laminate with uniaxial layers assembled periodically into a helicoidal pattern. Compared to other types of laminates, its resistance to in-plane damage still lacks clarification. The failure behavior of Bouligand laminates under off-axis open-hole tensile (OHT) loading was investigated compared to cross-ply and quasi-isotropic structures using experiment and numerical simulation. After verifying the progressive damage model based on Hashin criteria, the numerical predictions from various off-axis loadings were compared between these three structures. The results show that Bouligand laminates exhibit low but nearly stable OHT strength. In comparison, the cross-ply structure exhibits high OHT strength variation when loaded in different directions. With the change of off-axis angle, the OHT strength of the Bouligand structure changes by 10.1 % from 405.2 MPa to 364.1 MPa, while it changes by 69.1 % from 533.8 MPa to 165.1 MPa for the cross-ply structure. Besides, the Bouligand structure exhibits higher OHT strength when the off-axis angle exceeds 5°. It also exhibits a more stable OHT strength than the quasi-isotropic structure. It is shown that the failure of fiber and matrix is the primary factor affecting the OHT strength of the laminate, while the delamination has little effect on the OHT strength. [ABSTRACT FROM AUTHOR]

• Computational simulation framework to portray the anisotropic characteristics of hardened 3DCP structures reinforced with steel fibers. • The fiber-reinforced concrete specimen extracted from 3D printed Bouligand structures for extensive experiments. • Anticipating well the material anisotropy of hardened fiber-reinforced 3DCP structures. • Predicting complex crack propagations in hardened 3D printed fiber-reinforced concrete beams. Latencies and limitations exist in the simulation of bio-inspired structures or highly complex geometries manufactured by digital concrete fabrication. This study develops a computational framework to portray the anisotropic material attributes and predict the crack patterns of hardened 3D printed fiber-reinforced concrete beams. Bio-inspired Bouligand structures printed with four pitch angles of 0˚, 15˚, 30˚, and 45˚ are adopted to validate the numerical model. Extensive experiments, including uniaxial compressive tests, splitting tensile tests, and three-point bending tests, are conducted. The material model is attained from the samples extracted from the printed concrete blocks with and without steel fibres after a 28-day curing age for an accurate representation of the design and process. The micromechanical modelling approach is employed to obtain the homogeneous materials for the macro-scale model. The finite element (FE) analysis provides sound agreement with the experiment results of both stress–strain curves and crack patterns. In addition, under bending loading, the 3D printed beams with a unidirectional printing pattern or a small pitch angle perform better than those with larger pitch angles. [ABSTRACT FROM AUTHOR]

Developing advanced lightweight structures with both high strength and high toughness remains challenging. Herein, we provide a combined experimental and simulation method to fabricate 3D-printed geopolymer complex structures with lightweight, high strength and superior toughness for the first time. In this research, short carbon fiber reinforced geopolymer (C sf GP) composites were printed using 3D printing technology, and the rheological properties of C sf GP inks and the mechanical properties of hardened geopolymer composite structures were systematically studied. The C sf GP inks exhibited obvious shear-thinning behavior, which facilitated ink-extrusion from a micro-nozzle, conserving the filamentary shape and supporting the subsequent printed layers. In C sf GP composites, the consistent orientation distribution of short carbon fibers primarily enhanced their mechanical properties. When fiber content was 3 wt %, flexural strength and compressive strength of the C sf GP composite were 309.2% and 375.8% higher than those of nonreinforced geopolymer, respectively. Subsequently, C sf GP composites of the Bouligand architectures were successfully 3D-printed, and they showed superior load-bearing capacity and non-brittle failure mode due to their hierarchically ordered architectures and sophisticated interfaces. 3D printing together with Bouligand-structure designing provide a novel approach for the C sf GP composites of lightweight, high strength and superior toughness, which would lead to a resurgence of interest in new lightweight structure design and manufacturing strategies. [Display omitted] • Short carbon fiber reinforced geopolymer (C sf GP) inks were 3D printed by direct ink writing. • Consistent fiber orientation distribution was achieved in the printed C sf GP composites. • C sf GP of Bouligand structures were designed and printed, and the fracture behavior of these structures was investigated. • The obtained C sf GP of Bouligand structures showed light-weight, high strength and superior toughness, and non-brittle failure behavior. [ABSTRACT FROM AUTHOR]

Abstract: The structure and mechanical properties of the exoskeleton (cuticle) of the sheep crab (Loxorhynchus grandis) were investigated. The crab exoskeleton is a natural composite consisting of highly mineralized chitin–protein fibers arranged in a twisted plywood or Bouligand pattern. There is a high density of pore canal tubules in the direction normal to the surface. These tubules have a dual function: to transport ions and nutrition and to stitch the structure together. Tensile tests in the longitudinal and normal to the surface directions were carried out on wet and dry specimens. Samples tested in the longitudinal direction showed a convex shape and no evidence of permanent deformation prior to failure, whereas samples tested in the normal orientation exhibited a concave shape. The results show that the composite is anisotropic in mechanical properties. Microindentation was performed to measure the hardness through the thickness. It was found that the exocuticle (outer layer) is two times harder than the endocuticle (inner layer). Fracture surfaces after testing were observed using scanning electron microscopy; the fracture mechanism is discussed. [Copyright &y& Elsevier]

Abstract: This is an experimental study on the mechanical and structural gradients through the cuticle of Homarus americanus (American lobster). The exocuticle (outer layer) is characterized by a very fine woven structure of the fibrous chitin–protein matrix (‘twisted plywood’ structure) and by a high stiffness (8.5–9.5GPa). The hardness increases within the exocuticle between the surface region (130MPa) and the region close to the interface to the endocuticle (270MPa). In the endocuticle, which is characterized by a much coarser twisted plywood structure, both the stiffness (3–4.5GPa) and hardness (30–55MPa) are much smaller than in the exocuticle. The transition in mechanical properties and structure between the exocuticle and endocuticle is abrupt. The differences underline the important role of the internal structure of the twisted plywood structure and of the interface between the two cuticle layers for the overall mechanical behavior of the exoskeleton. The excellent mechanical stability of the interface (irrespective of the change in the mechanical properties) is attributed to the fact that the structural change of the twisted plywood pattern across the interface consists only of a change of the stacking density of the chitin–protein layers. The observed gradients in stiffness and hardness through the cuticle thickness are interpreted in terms of honeycomb mechanics of the twisted plywood structure. The possible role of gradients in protein cross-linking and in the mineral content is also discussed. [Copyright &y& Elsevier]

Here, experiments and analysis revealing the unique defense mechanisms of the coelacanth, a lobe‐finned fish termed a "living fossil", are reported, as it was considered to be extinct until 1938, but extant for 400 million years. This defense is provided by primitive elasmoid scales having a rare double‐twisted Bouligand structure of lamellae which provides extraordinary resistance to deformation without failure. Distinct from other elasmoid scales, the collagen fibrils in the coelacanth fish scales form bundles which are embedded in a matrix comprising fibers arranged perpendicular to the layered (double‐twisted Bouligand) structure that provide added rigidity and resistance to deformation. Using in situ synchrotron small‐angle X‐ray scattering during uniaxial tensile testing, the deformation mechanisms of the collagen are identified in terms of fibril stretching, reorientation, sliding, bending, and delamination. The unique structure of the coelacanth scales, distinct from modern elasmoid scales such as the carp, provides a variety of mechanisms to arrest cracking, making it an outstanding damage‐tolerant material to resist predator attacks. The unique double‐twisted Bouligand structure of the scales of the "living fossil" coelacanth fish is examined. Combining ex situ and in situ scanning electron microscopy experiments, coupled with small‐angle X‐ray scattering analysis, the mechanical behavior and in particular the toughening mechanisms are uncovered for this ancient armor. The findings may potentially provide insight for improving the penetration resistance of new engineering structural materials. [ABSTRACT FROM AUTHOR]