Finite element modelling considering the bending behavior of uncured unidirectional prepregs
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摘要: 热固性树脂基预浸料沿纤维方向具有极高的拉伸模量和较小的弯曲刚度，准确描述预浸料的这种力学特性对于成型过程中褶皱等缺陷的预测与抑制、提升成型过程的有限元仿真精度具有重要意义。本文建立了基于纤维方向准确跟踪并考虑非线性剪切行为的单向预浸料本构模型，并通过共节点壳膜混合单元实现了预浸料高拉伸模量和低弯曲刚度的解耦。同时，以国产AC531/CCF800H单向预浸料为对象，系统测量了未固化预浸料的拉伸模量、剪切模量和弯曲刚度。最后，通过单向预浸料的偏轴拉伸试验和轴向压缩试验分别验证了本文所建立的有限元模型在膜单元和壳单元主导的受力条件下的有效性。Abstract: Thermoset resin based prepregs exhibit very high tensile modulus and relatively low bending stiffness, and the accurate characterization of such special properties of prepregs is critical to predict and avoid the wrinkle defect during forming process, and improve the accuracy of finite element simulation of composite forming process. In the present paper, a finite element model that accurately tracks the fiber directions was built considering the nonlinear in-plane shear behavior of unidirectional prepregs, and the high tensile stiffness and significantly lower bending stiffness of the prepreg were decoupled using superimposed membrane-shell elements. Besides, the tensile stiffness, no-linear shear stiffness, and bending stiffness of the domestic prepreg system AC531/CCF800H were characterized experimentally. Finally, the 30° off-axis tensile test and axial compression test were used to prove the validity of the proposed model, respectively.
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图 1 单向预浸料本构模型中的不同坐标系
Figure 1. Coordinate systems used in the constitutive model of unidirectional prepregs
图 2 壳膜共节点混合单元示意图
Figure 2. Schematics of the superimposed membrane-shell elements sharing the same nodes
图 5 拉伸试验中AC531/CCF800H单向预浸料的应力-应变曲线
Figure 5. Stress-strain curves of the AC531/CCF800H unidirectional prepregs in tensile tests
图 6 偏轴拉伸试样示意图
Figure 6. Schematics of the off axis tensile specimen
F—Global applied load; Ft, Fτ—Normal and tangential components of the applied force along the fiber direction; θ—Off-axis angle
图 7 面内剪切试验中AC531/CCF800H单向预浸料剪切应力与剪切应变的变化关系及其多项式拟合结果
Figure 7. Experimental in-plane shear response of the AC531/CCF800H unidirectional prepregs and its polynomial fitting results
图 9 偏轴拉伸过程中的试样形貌
Figure 9. Shape of the specimen during off axis tensile tests
FEM—Finite element method; U, U3—Millimeter displacement in Z direction; W—Out-of-plane displacement obtained by experimental observation
图 10 预浸料轴向压缩试验装置
Figure 10. Set up for the axial compression test of unidirectional prepregs
图 11 不同压缩位移下AC531/CCF800H单向预浸料形貌的仿真结果
Figure 11. Simulated shapes of the AC531/CCF800H unidirectional prepreg under different compressive displacements
图 12 不同压缩位移下AC531/CCF800H单向预浸料形貌的有限元仿真与实验结果对比
Figure 12. Comparisons of prepreg shapes obtained by simulations and experimental observations under different compressive displacements for AC531/CCF800H unidirectional prepreg
表 1 仿真模型中壳膜混合单元属性
Table 1. Properties of membrane-shell elements used in FEM
Element
typeThickness/
mmE1/
MPaE2/
MPaG12/MPa Membrane 0.16 73017 31.1 37400000γ4−
4440000γ3+181635γ2−
3140γ+26.76Shell 0.16 32 0.008 0 Notes: E1 and E2—Longitudinal and transverse elastic moduli; G12 —In-plane shear modulus; γ—In-plane shear strain. 
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