Damage development in short-fiber reinforced injection molded composites

The demand for injection molded products in industry increases because of the capability of high-volume-production, suitable properties and high geometrical freedom of design. To improve the mechanical properties such as stiffness and heat distortion temperature of thermoplastic materials, reinforcing fibers are embedded in the thermoplastic matrix. Due to the flow conditions during processing, the resulting components show a complex fiber orientation distribution which is exhibited by a layered skin-core structure. Depending on the local fiber orientation relative to the loading direction, various damage phenomena and damage evolution processes appear on the fiber–matrix interface. In the case of fibers loaded perpendicular to their fiber axis, the debonding of the matrix starts from the outer fiber ends. Fibers which are loaded in their longitudinal axis are cracked or pulled out. To consider local fiber orientations as well as the local damage behavior at the fiber–matrix interface, the fiber orientation is simulated applying injection molding simulations. The damage behavior is investigated by using a statistical approach together with a combined cell model. The damage development of the fiber–matrix interface is captured by considering two extreme cases which are a totally damaged fiber–matrix interface and a perfect or intact interface within the unit cell. The process of damage evolution of the fiber–matrix interface during loading of the specimen is realized by an equation on the Weibull statistic which describes the transition from the intact interface to the totally damaged interface. The parameters of this equation are determined using inverse modeling by comparing simulation and experiment. The results show the different extent of damage development depending on the local fiber orientation as well as the poor mechanical properties of weak areas of a component such as a weld line where two melt fronts coalesce.