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By inducing periodic gaps, conventional automated fiber placement avoids overlap defects, which are generally prohibited in composite component manufacturing. However, bistable laminates are highly sensitive to imperfections, and it is critical to explore the effect of periodic gaps on the performance of bistable laminates. The principal curvatures and load–displacement curves of thermoplastic composite laminates with manually embedded periodic gaps was studied using experimental and numerical methods in this paper. SEM microscopy was used to observe the gap areas of the cross section of laminates to assess the microstructure of gaps with width variations after curing. In addition, a numerical method was proposed based on the experimentally determined evolution of gap widths and thicknesses of laminate. Subsequently, the effects of gap width, single‐layer thickness, and total thickness on the principal curvature were investigated. Gaps embedded in the 0° and 90° layers of bistable laminates demonstrated a substantial decrease in the snap load of the laminates. Interestingly, by embedding different gap widths in the 0° layers of the laminates, the snap load discrepancy between the snap‐through and snap‐back processes increased as the gap width widened, and the same trend was also found in the curvatures in the two stable states. Highlights: Bistable characteristics of laminates with periodic gap defects.Utilizing gaps embedding in 0° layer to tailor the mechanical properties. [ABSTRACT FROM AUTHOR]

Highlights Processing parameters during the laser‐radiated in‐situ manufacturing process change the thermal history of the thermoplastic composite, which affects porosity and fiber‐resin interfacial bonding quality, and hence the wedge peel strength of the laminate. The effect of different laser powers and consolidation forces on the wedge peel strength of the specimens was investigated. Due to the fiber‐rich area on the tape surface during the laser heating phase as well as the insufficient consolidation force and consolidation length during the consolidation phase, the wedge peel strength decreased due to increased porosity and weak fiber‐resin bonding at the interlayer bonding interface. A conformable consolidation roller of lower hardness was used to improve the wedge peel strength of the laminates, which reduced the initial temperature of the cooling phase, thus inhibiting the void rebound and increasing the bonding strength at the fiber‐resin interface. The cross‐section and peeling surface were characterized by optical microscope and scanning electron microscope. The wedge peel strength of the laminates, with reduced voids and increased interfacial bonding strength between the fibers and the resin, is improved. Mechanism of void formation and evolution in different phases of laser‐radiated in‐situ consolidated laminate. Effect of consolidation roller hardness and deformation on wedge peel strength. [ABSTRACT FROM AUTHOR]

Laser-assisted thermoplastic composite automated placement method has been applied to the molding of composites due to its advantages of fast molding speed and high heating efficiency. This study focuses on designing a robot for laser-assisted thermoplastic composite automated fiber placement, employing a modular approach to create a versatile fiber placement head. Control functions, such as electrical circuits, a computer interface, and tension control, were developed. Hybrid laminates, combining GF/PP unidirectional and braided composites, were manufactured by independently adjusting laser power (275 W–400 W) and compression force (150 N–400 N). Bonding properties were analyzed using wedge peel tests and microscopic detection, revealing superior toughness in laser-assisted laminates compared to autoclave counterparts. Altering laser power and compression force impacted polypropylene resin distribution, influencing hybrid interlayer peel strength. The inclusion of braided materials and elevated mold temperature proved effective in minimizing lamination warpage in GF/PP laminates. • The modular laser-assisted head for automated fiber placement is designed for rapid bonding of thermoplastic composites. • The peel strength of hybrid laminates made by laser-assisted placement was superior to those made by autoclave, which had better toughness. • Adding braided material increases laminate stiffness, reducing warpage during automated thermoplastic composite placement. [ABSTRACT FROM AUTHOR]

• A magnetorheological elastomer actuated multi-stable gripper reinforced stiffness with twisted and coiled polymer. • The multi-stable gripper can conform objects with different sizes by means of its stable and adaptive characteristics. • The multi-stable gripper is composed of a multi-stable laminated composite shell as the grab unit for supporting, have multiple steady states. This study proposes a magnetorheological elastomer actuated multi-stable gripper reinforced stiffness with twisted and coiled polymer. The multi-stable gripper can conform objects with different sizes by means of its stable and adaptive characteristics. The multi-stable gripper is composed of a multi-stable laminated composite shell as the grab unit for supporting, a twisted and coiled polymer as the deformation element for variable stiffness and a magnetorheological elastomer as the smart material for actuation. The magnetorheological elastomer was activated by external magnetic field inducing the gripper change from the first stable state to the second or the third stable state to complete the gripping action. The driving force of the multi-stable laminated composite shell was discussed by the experimental and numerical methods, then the width of deformation and transition element was designed to reduce the driving force. Furthermore, the stiffness of the multi-stable gripper is analysed with respect to the compression, extraction, and gripping performance of the gripper through experiments, and then, the gripping performance was compared with current soft grippers. The experimental results show that the multi-stable gripper exhibits sufficient compliance and adaptability. That is, the multi-stable gripper has a faster response time and larger grasping weight compared with soft grippers. [ABSTRACT FROM AUTHOR]

The morphing characteristics and damage analysis of variable stiffness bistable laminate fabricated by continuous fiber 3D printing technology are systematically studied. Conventionally, bistable structures used in morphing applications are achieved with straight fibers. With the application of fiber placement technology, it is now possible to prepare variable stiffness laminates with different directions and layup angles of fibers. Moreover, the structure will be damaged with the increase in deformation times. How to effectively suppress the failure of laminates is also a highlight for this composite structure. In this paper, three curvilinear fiber paths are designed and compared. And a semi-analytical model based on the Rayleigh-Ritz method is used to predict the curvature of stable configuration. In particular, continuous fiber thermosetting resin matrix composites 3D printing technology is proposed for preparing variable stiffness laminates, which has several advantages compared to other traditional preparation methods of variable stiffness laminates. The corresponding finite element model is also developed to investigate the bistable behavior and damage analysis. In addition, the influence of different angle parameters and fiber paths on the stable configuration and the snap-load are also discussed. Finally, the damage evolution and mechanical properties of straight-fiber laminates and variable stiffness laminates in tension and compression are carried out based on the Hashin failure criteria. [ABSTRACT FROM AUTHOR]

• The mechanical and buffering characteristics of laminate can be improved by adjusting the initial curing curvature. In this study, the influence of initial curing curvature on the negative stiffness and buffering characteristics of bistable laminates was analyzed. The geometric parameters and mechanical characteristics of bistable laminates may be affected by the initial curing curvature. Initially, a geometric nonlinear finite element model is developed, and its reliability is verified by comparing with experimental. The finite element model was used to investigate the influence of initial curing curvature on mechanical characteristics, including snap load, stiffness, and strain energy. On this basis, the influence of initial curing curvature on the buffering characteristics was studied by implicit dynamic analysis. The results of the study indicate that bistable laminates show pronounced negative stiffness characteristics during the snap process, which is not impacted by the stable state number. As a result of adjusting the initial curing curvature, the snap load and negative stiffness extremum can be designed, the strain energy difference can be adjusted, and the buffering range and efficiency of the laminate can be enhanced. [ABSTRACT FROM AUTHOR]

• This paper proposes a continuous fiber thermosetting resin matrix composite 3D printing technology, which aims to combine continuous fiber, thermosetting resin and 3D printing technology to realize the manufacture of multistable curvilinear-fiber laminates. • Continuous fiber 3D printing technology solves the limitation of interface separation or stress concentration caused by lap joints or mechanical connection in traditional multistable structures. • Continuous fiber 3D printing technology provide the possibility for free variation of fiber angle. Different from the traditional preparation process of curvilinear-fiber laminates, it shows unique advantages. A novel manufacturing method for curvilinear-fiber multistable laminates using continuous fiber 3D printing technology is proposed, and a numerical and experimental investigation on the morphing behavior is performed. Traditional multistable laminates are developed from multiple straight-fiber laminates jointed or bolted together, but the designability of straight-fiber laminates is limited. More importantly, the fibers at the lap joints or bolted connection are discontinuous or fractured, which cause interfacial separation or stress concentration. Different from the traditional preparation process, continuous fiber 3D printing technology ensures the continuity of adjacent deformable elements and integrity of the structure, thereby reducing the interfacial separation or stress concentration of the laminates caused by fiber discontinuity or fracture, and providing the possibility for free variation of fiber angle. Continuous fiber 3D printing technology further improves the design space for composites instead of traditional straight-fiber laminate. In this paper, two different curvilinear-fiber multistable laminates are designed and prepared, and a finite element model was developed to predict the possible cured configurations and analysis the snap behavior. The numerical and experimental results show good agreement in stable configurations and snap process. Subsequently, a parametric study is carried out to explore the effect of different angle parameters on the snap process and stable configurations. [ABSTRACT FROM AUTHOR]

In this study, a novel orthogonal bistable laminated composite shell driven by magnetorheological elastomer was developed. The magnetorheological elastomer served as the actuator of the stable transition between the two stable states of the bistable structure is rapid, reversible, non-contact and continuously controllable. The experimental results revealed that the trigger time of the stable transition can reach millisecond level. First, the magnetorheological elastomer and orthogonal bistable laminated composite shell were fabricated. Then, experiments and finite element simulations were conducted to measure the trigger force required to actuate the transition of the bistable structure. The experimentally measured trigger force agreed well with that estimated by the finite element simulation. The shape characteristics of the orthogonal bistable laminated composite shell were studied both in the presence and absence of the magnetorheological elastomer. The effect of the NdFeB content in the magnetorheological elastomer on the magnet field strength required to drive the transformation of the bistable structure was studied systematically, and a suitable magnetorheological elastomer was chosen to investigate the effects of the geometric dimensions of the magnetorheological elastomer actuator on the trigger magnetic field. The response time for the triggering of the transition of the bistable structure was measured, and the results indicated that the proposed magnetorheological elastomer actuation could be used to flexibly drive orthogonal bistable laminated composite shells. [ABSTRACT FROM AUTHOR]

A novel bistable hybrid composite laminate embedded with bimetallic strips was designed by combining the stable-state characteristics of the bistable laminate with the temperature-sensing characteristics of the bimetallic strip described in this paper. A contactless method that deforms the bimetallic strip through the temperature field, driving the bistable laminate, is proposed. Firstly, an analytical method based on Dano's theory was developed to determine the principal curvature of the laminates, and qualitative and quantitative comparisons between theoretical, experimental, and numerical results were conducted and showed good agreement. Then, the influence of bimetallic strip characteristics such as the thickness, width, and material properties, along with the aspect ratio of the laminates, on the principal curvature were examined. In addition, the reciprocating drive of the laminates was realized by changing the temperature generated by a thermal field, and the effect of the bimetallic strip arrangement was also studied. Compared with the conventional square bistable cross-ply laminates, it was found that the new laminates have different curvatures in two stable states. In particular, they can be easily driven by changing the ambient temperature (contactless drive), without requiring complicated external driving devices. This study provides theoretical support and a new driving method for the design of adaptive and deployable structures. A novel bistable hybrid composite laminate embedded with bimetallic strips was designed in this paper, and the temperature field was applied as a contactless driving method. Firstly, an analytical method based on Dano's theory was developed to determine the principal curvature of the laminates, and the qualitative and quantitative comparisons between theoretical, experimental and numerical results were conducted and showed a good agreement. Then, the influence of characteristics, such as the thickness, width and material properties of bimetallic strips, and the aspect ratio of the laminates on the principal curvature were examined. Besides, the reciprocating drive of laminates is realized by changing the temperature caused by a thermal field, and the effect of arrangement of bimetallic strips was also discussed. Comparing with the conventional square bistable cross-ply laminates, it is found that the new laminates have different curvatures in two stable states. Especially, it can be easily driven by applying a temperature field, without needing external complicated driving devices. It will provide theoretical support and a new driving method for the design of adaptive structures and deployable structures. [Display omitted] [ABSTRACT FROM AUTHOR]

• A cross-shaped unsymmetric laminates has three stable configurations. • The width will change the concave-convex boundary of the laminates. • The sidelength and width determine whether the laminate has multistability. This paper presents cross-shaped unsymmetric fibre-reinforced cross-ply laminates with concave–convex boundaries, which have three stable configurations. The third stable equilibrium configuration is a transition state which occurs in the snap process from the first to second equilibrium configurations. The snap processes, including the first–third–second and second–third–first, between the different stable equilibrium configurations of tristable cross-shaped unsymmetric laminates were investigated by experimental and numerical methods, and the parameters that characterise the tristable performance of the laminates, including the principal curvature, snap load, and displacement, were measured. The principal curvatures in different stable configurations were determined with analytical, experimental, and numerical methods, and the load–displacement curves and buckling phenomena in the snapping process were successfully recorded. Moreover, the influences of the reinforcement material, thickness of the single layer, and geometric parameters on the tristable performance of the cross-shaped unsymmetric fibre-reinforced cross-ply laminates were discussed. According to the results, the analytical, experimental, and numerical results agree well. [ABSTRACT FROM AUTHOR]