Constitutive modeling of injection‐molded short‐fiber composites: Characterization and model application

Abstract

A previously developed constitutive model for short‐fiber reinforced thermoplastics is applied to an injection‐molded component with a complex geometry and microstructure. This macro‐scale continuum‐based model is able to capture the anisotropic viscoelastic‐viscoplastic response of the material. In injection‐molded short‐fiber composites, the anisotropic mechanical properties depend strongly on the fiber orientation distribution, which generally displays a marked variation throughout the product. This makes the material characterization and model application challenging. In this article, two characterization and model application strategies are proposed. These techniques, together with the developed constitutive model provide a strong tool for reliable prediction of the mechanical response of an injection molded product, where inputs to the finite element analysis are obtained directly from a numerical simulation of the injection molding process. In this article, from the output provided by an injection molding process simulation software such as Moldflow, the distribution of anisotropic elastic and plastic properties throughout the component is found and the data is imported to the finite element mesh. Mechanical tests are performed on a validation product and results are compared with model predictions from finite element simulations. Through this comparison, the performance of the constitutive model and also proposed procedures for characterization and model application are investigated.

Micromechanical Modeling of Anisotropy and Strain Rate Dependence of Short-Fiber-Reinforced Thermoplastics

“…The observed trends appear in good agreement with experimental observations of glass fiber-reinforced polycarbonate [10,[12][13][14]. In these experimental studies, it was demonstrated that the influence of loading angle/fiber orientation and strain rate dependence are, to a good approximation, factorizable-implying that the influence of the strain rate and that of the loading angle are actually decoupled.…”

“…The result is presented in Figure 10a, for a strain rate of 10 −1 s −1 , and demonstrates that the yield stress is intimately related to the fiber orientation in loading direction p σσ . A similar observation was also experimentally made on a polycarbonate composite system with 20% weight fraction of short glass fibers, although in the experimental case, the range of fiber orientations was more limited (0.3 < p σσ < 0.75) [14]. The observed trend is described well by σ y ( εre f , p σσ ) = c p p q σσ + σ y ( εre f , p σσ,re f ),…”

“…In an extensive experimental effort, it was demonstrated that the rate-dependent, anisotropic (yield) strength of short and long fiber-reinforced thermoplastics generally displays a multiplicative decoupling of the effects of strain rate and loading angle [12,13]. Additionally, it was shown that, at a fixed strain rate, the yield strength appears to be uniquely coupled to the average fiber orientation in the direction of testing [14]. This observation, in combination with the decomposition of the influence of strain rate and that of fiber orientation, could lead to a novel, fast method to accurately predict the anisotropic rate-dependent mechanical response for an arbitrary fiber orientation distribution.…”

Micromechanical analysis of age‐induced strength reduction in a multiaxially loaded short‐fiber reinforced thermoplastic

“…For more information on the fiber orientation of the validation beam one is referred to. 30 Note that the fiber orientation predicted by Moldflow is only used as a graphical illustration. The fiber orientation, used as an input to the numerical model, is measured via microCT imaging.…”

“…28 Next to this broad experimental validation of the factorizability, it is also validated by employing RVE simulations on SFRTs where a viscoelastic-viscoplastic constitutive model is used to describe the matrix. 29 Inspired by the work of Fara and Pavan, 7 who showed that the modulus, maximum stress, and fracture toughness of SFRTs are directly coupled to fiber orientation in the form of the orientation factor in the main orientation direction, Amiri-Rad et al 30 utilized this observation between the mechanical properties and the fiber orientation, and the factorization with respect to strain rate in a threedimensional (3D) anisotropic viscoelastic-viscoplastic model. Knowing the spatial distribution of fiber orientations and the time-dependent behavior of the matrix, allows the ductile failure of a product to be predicted.…”