| Variable stiffness multi-stable composite laminate structure is not fixed at each composite laminate,it gives designers greater freedom.Variable stiffness composite laminates offer several advantages over traditional straight composite laminates,such as avoiding stress concentrations,weight loss potential,and highly designability.With two or more combinations through effective combinations for more than one steady state,the transition between stable states can be performed by external driving method such as mechanical load,smart material or temperature field.The above advantages have broad application prospects for variable stiffness and multi-stable composite structures in aerospace deformable mechanisms and wind turbine blades.Affected by the variation of characteristic parameters,many new problems have appeared in the practical application of variable stiffness multi-stable composite laminate structure.The in-depth understanding of the influence of characteristic parameters on the structure of variable stiffness multi-stable composite laminate structure is crucial for the development of new composite structures.In this paper,variable stiffness multi-stable composite laminate structure is studied by theory,finite element simulation and experiment.The influences of laying angle,material properties,layer number and moisture parameters are obtained,and the human elbow and knee are carried out under specific circumstances.The process was analyzed to study the potential application of variable stiffness multi-stable composite structures in human exoskeleton.The main research contents are as follows:1.Based on the classical laminate theory,the theoretical stable configuration of the variable stiffness multi-stable composite laminate structure is established,and the different stable configurations and stable mid-plane curves are predicted.The theoretical model shows that the first stable of the anti-symmetry/orthogonal/antisymmetry variable stiffness multi-stable composite laminate structure exhibits intermediate bending and flat state on both sides;the second stable is curved in the middle,but the direction of curvature is opposite to the first stable,the end also exhibits a curved state with the direction of curvature opposite the middle portion.The orthogonal/anti-symmetry/orthogonal variable stiffness multi-stable composite laminate structure has a first twist phenomenon on the left and right sides,and the curvature directions of the left and right sides are opposite to the curvature direction of the middle convex portion;the second stable is around.The sides are straight and twisted in the middle.2.In order to study the influence of characteristic parameters on the stable transition process of variable stiffness multi-stable composite laminate structure,the effects of laying angle,material properties,layer number and moisture were studied by finite element method.The results show that the load-displacement of the antisymmetry/orthogonal/anti-symmetry specimen is different from that of the orthogonal/anti-symmetry/orthogonal specimens,and the maximum load is reached through two different trends.The maximum load displacement of antisymmetry/orthogonal/anti-symmetry specimens is slightly larger than that of orthogonal/anti-symmetry/orthogonal specimens,but the corresponding displacement value is smaller than that of orthogonal/anti-symmetry/orthogonal specimens.In the anti-symmetry/orthogonal/anti-symmetry specimens,the displacement value of the specimens with smaller layers is larger than that of the specimens with more layers when they reach the second stable.With the increase of layers,the maximum load increases.The displacement of orthogonal/anti-symmetry/orthogonal specimens to the second stable increases with the increase of layers.Moisture has a significant effect on the maximum load of the anti-symmetry/orthogonal/anti-symmetry specimens,and as the moisture becomes larger,the maximum displacement decreases.Moisture does not affect the load-displacement trend of the orthogonal/anti-symmetry/orthogonal specimen during steady-state transition,but as the moisture changes,the maximum load and maximum displacement of the specimen become larger.3.Stable transformation experiments were carried out on the experimental specimens,and load-displacement curve was obtained.The experimental specimens with smooth and complete surface and stable state were prepared by hot-pressing cooling method.Through the experimental method,the load-displacement of variable stiffness multi-stable composite laminate structure during the stable transformation is obtained by using the modified testing platform of the tensile testing machine.The changing rules of the laying angle,material properties,layers number and moisture on the variable stiffness multi-stable composite laminate structures are verified.The maximum load and displacement of the anti-symmetry/orthogonal/anti-symmetry specimen is larger than that of the orthogonal/anti-symmetry/orthogonal specimen.Material properties will not change the load-displacement curve of the same laying angle.The load-displacement curves of variable stiffness multi-stable composite laminate structures are different with the layers number,and the rules of antisymmetry/orthogonal/anti-symmetry and orthogonal/anti-symmetry/orthogonal specimens are opposite.Moisture can change both the maximum load and the displacement at the second stable state.4.An initial exploratory study on the application of variable stiffness multi-stable composite laminate structure in human exoskeleton.When the elbows and knees are kept in a bent state,the specimen is in a stable configuration;when the elbows and knees are rotated through a certain angle to maintain the straight state,the specimen is in another stable configuration.Due to the fixed material properties,laying angle,layer number and moisture,the variable stiffness multi-stable composite laminate structures have a fixed shape in each stable.By measuring the angle of each specimen in the stable configurations,the applicability of the specimens in human exoskeleton is determined which shows the extensive application prospect of composite materials in human exoskeleton. |
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