Creep Behavior Of Continuous Glass Fiber Reinforced Polypropylene Composites And Numerical Prediction

Continuous glass fiber reinforced polypropylene composites(CGFRPP)has the advantages of high toughness,high damage tolerance,corrosion resistance and low cost,which could be recycled and reused.Hence,CGFRPP has been rapidly developed in recent years.Despite the rapid development of CGFRPP,its fatigue resistance and viscoelastic behavior have not been systematic theoretical researched.The viscoelastic behavior of composite materials is the main reason ofaging.When the temperature or external force is increased,the internal friction of the material causes the viscoelastic deformation remarkable.In this paper,a theoretical model reflecting creep behavior of CGFRPP and a microcosmic finite element model of CGFRPP composites are used to predict the creep deformation trend of CGFRPP under different environmental conditions,which were based on the long-term/short-term creep test data of polypropylene(PP)and CGFRPP laminate.At the same time,the creep mechanism and influence factors on CGFRPP composites were studied.This thesis provides reliable reference value for CGFRPP composite material research and application.First of all,the creep behavior of PP resin was studied.The creep test results showed that,increasing temperature or stress levels would cause a significant improvement in PP creep behavior.In the creep process,the crystallinity of PP increases with the increase of creep strain significantly.The radius of free volume reaches the maximum at the fracture strain of Ecr,and the activity of PP molecular chain reaches the maximum simultaneously.The four-element Burgers viscoelastic model was used to fit the creep test results.It can be found that the model parameters have obvious stress dependence.Based on the fitting results,the Burgers theoretical model of stress dependence is established,and the theoretical model is compiled into UMAT subroutine code to predict the creep strain of PP specimens under different stress levels.Then,the influence of temperature,stress level and fiber angle on the creep behavior of CGFRPP monolayer were studied.The creep test results showed that the creep strain of CGFRPP monolayer along the fiber direction was mainly controlled by resin matrix at low stress level.The shear stress at the interface of fiber and resin increased significantly with the increase of off-axis tension angle.The creep phenomenon intensifies with the increase of tensile stress on PP matrix.The coupling effect of tension and shear were obviously when the off-axis tension angle in 45 degrees.At the same time,the four-element Burgers viscoelastic model is used to fit the creep test results.It can be found that the model parameters have obvious stress and angle dependence.A mathematical expressions of stress dependence and angle dependence of model parameters were established,and the tensile creep behavior at different stress level or off-axis angle is predicted.The predicted values of the model are in good agreement with the experimental results,which verifies the applicability of the theoretical model.Finally,the creep behavior of CGFRPP monolayer was established by finite element method.It was assumed that the fiber was linear elastomer,the PP matrix was viscoelastomer,and the viscoelastic constitutive relationship was substituted by the UMAT subroutine above.The effective elastic constants of CGFRPP monolayer,10 h tensile creep behavior along the fiber direction and 15 min biaxial tensile creep behavior perpendicular to the fiber direction were predicted respectively by establishing the finite element cell model.Compared with the experimental results,showing that the CGFRPP cell model could accurately predict the effective elastic constants;the creep predictions along the fiber direction are in good agreement with the experimental values;the biaxial tensile creep predictions perpendicular to the fiber direction are smaller than the experimental values,and the errors are more significantly affected by the interface between the fiber and the matrix.The above results show that the creep prediction value of the finite element model have a excellent guiding significance for the practical application of CFRPP composites.