Article

Numerical predictions of fiber orientation and mechanical properties for injection-molded long-carbon-fiber thermoplastic composites

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Abstract

The use of long carbon fiber-reinforced thermoplastic composites as lightweight automotive materials offers superior mechanical performance compared to glass fibers. However, the anisotropic fiber orientation distribution with shell–core structure is critical to determine mechanical properties. Recently, both theoretical models of fiber orientation—Anisotropic Rotary Diffusion and Reduced Strain Closure and Improved Anisotropic Rotary Diffusion and Retarding Principal Rate—have been significantly developed in the field of fiber suspension rheology. From injection molding simulations, numerical predictions of fiber orientation are derived with related experimental validations, while dramatic changes in orientation distribution are discussed at various fiber concentrations and different polymer matrices. More importantly, the reinforcing ability is discussed in regard to the tensile modulus and the stress–strain response for a variety of long carbon fiber composites. POLYM. COMPOS., 2017.

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The shear-thinning behaviour of a liquid is represented in terms of a relaxation timeλ, defined by the ratioη 0/G0 of initial viscous and elastic constants. The relationship provides a very simple basis for the evaluation ofλ andG 0 from viscosity/shear data. Results are compared with relaxation times and moduli from primary normal-stress measurement, from stress relaxation and from direct measurement of recoverable shear strain. Good agreement is found but there is experimental evidence the recoverable shear strainγ e is related to normal stressN 1 and shear stressσ byγ e = N1/3σ, which does not agree with the theoretical prediction of eitherWeissenberg orLodge.
Article
The rheology of polyolefines, polyamide 6, and polystyrene-acrylonitrile filled with glass fibers of different concentrations and aspect ratios have been investigated in simple shear flow, capillary rheometry, and uniaxial elongation. A comparison is made with unfilled and glass bead-filled melts. Fiber orientation was investigated by X-ray microradiography. Steady-state viscosities are obtained on fibers aligned parallel to the direction of flow. Entrance pressure losses, the shape of the viscosity function, and the appearance of a yield stress are depending on the fiber aspect ratio. The temperature dependence of the viscosity function is not significantly different from that of the unfilled melt. Transient shear stresses were measured on samples of different initial orientations of the fibers. The change of fiber orientation during shear flow gives rise to a pronounced overshoot of shear stress and normal stress difference. Elastic strains in shear are increased by the fibers but elastic elongational strains are reduced. Mechanisms are proposed to explain the experimental observations.
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