Fiber Break Prediction And Experimental Study Of Injection Molded Long Fiber Reinforced Thermoplastic Composites

Compared with short fibers,long fiber reinforced thermoplastic composites(LFRT)not only retain the ability to form articles using traditional injection molding,but also significantly improve the strength,rigidity,creep resistance and low-temperature impact resistance of products.It is an important alternative material for lightweight and structured products,also one of the most rapidly developing high-performance composite materials.However,in injection molding,long fibers will be damaged and shortened under the effect of complex multiple fields.When its length is less than the critical value,the fiber will be easily pulled out from the matrix and the strength will not be fully developed,resulting in the failure to meet the product design requirements.Therefore,investigating the mechanism of fiber breakage during the forming process,so that the fiber retains a sufficient length to transfer the load,is a major research goal in academia and industry.In this context,the thesis carried out a numerical prediction method and experimental research of fiber length breakage during LFRT injection molding filling stage.Committed to building a mechanical model that characterizes the relationship between fiber breakage and injection flow field stress,to achieve a more accurate prediction of fiber length breakage under the effect of injection flow field stress.The main research work is as follows:(1)Starting from the Oseen disturbance equation,the perturbation of the melt motion by a single external force was studied.The fiber immersed in the melt was described as a rigid long straight cylindrical particle,and the force equation of the fiber under arbitrary orientation in the laminar shear field was derived.(2)Based on the fiber stress equation and Euler buckling criterion,the fiber deflection equations for small and large deformations were derived,respectively.Using the tensile strength of the fiber as a criterion for whether the fiber breaks,a mechanical model was established to characterize the relationship between the degree of fiber buckling and breaking and the shear stress.The calculation results show that the fiber break occurs almost simultaneously with the buckling.With the change of the fiber orientation angle,the degree of buckling and breakage of the fiber changes periodically.When the orientation of the fiber is 45° with the shear direction,the fiber has the greatest stress and is most likely to break.The degree of fiber buckling and fracture is positively related to the aspect ratio of the fiber and the shear rate.(3)Based on the Jeffery's equation,the dynamics of the fiber in the three dimensional flow field and two dimensional flow field were studied,and the orientation characteristics of the fiber in the plane steady-state shear flow field during the entire rotation period were analyzed and calculated.It was found that regardless of the initial orientation state of the fiber,its rotation period in the shear flow field increases with the increase of the aspect ratio of the fiber,and decreases with the increase of the shear rate of the flow field.For the most part of each rotation cycle,the fiber is always moving in the flow plane.(4)According to the orientation characteristics of the fibers in the plane shear flow field,the shear experiment of long glass fiber reinforced polypropylene with initial fiber lengths of 3 mm and 6 mm at a specific shear rate was carried out using a rotary rheometer.FASEP 3Eco-type high-precision fiber length analyzer counts the remaining length and number of fibers after shearing,and compares with the model prediction results.The results show that the fiber breakage ratio increases with the increase of the shear rate of the flow field and the length of the initial fiber,and the prediction results agree well with the experimental results at different shear rates,which verifies the reliability of the model constructed in this paper.The thesis constructs a mechanical model that characterizes fiber breakage and injection molding flow field stress,which makes the prediction of long fiber breakage in the shear flow field rise from qualitative to quantitative level.This is of great significance to guide mold design,molding process settings and fiber length regulation in products.