Mean Value-Amplitude Method for the Determination of Anisotropic Mechanical Properties of Short Fiber Reinforced Thermoplastics

Abstract

Short fiber reinforced thermoplastics show distinct anisotropic behaviors due to their microstructure. The mechanical testing of specimens cut from injection molded plates at different angles to the injection molding direction reveals direction-dependent properties. However, these results are an average value for the tested cross section, which in more detail has a core-shell microstructure. When analyzing the stresses and deformation of a structural component, the local anisotropy will be very different compared to these tensile specimens. Therefore, a methodology is needed to transfer the properties obtained by mechanical testing to the local properties of an injection molded component. The core-shell microstructure and tests with different specimen thicknesses enable the determination of microstructure-dependent material properties. This paper presents a method using a mean value representing isotropy and an amplitude applied to the mean value to determine orientation-dependent mechanical properties. The amplitude in turn depends on the degree of anisotropy. The method is applied for extracting the anisotropic Young’s modulus of the core and shell layer of short glass fiber reinforced polyamide 46. The results obtained by this method and their reliability are discussed.

Improved Mean Value‐Amplitude Method for Determination of Orientation‐Dependent Modulus of Short Fiber‐Reinforced Thermoplastics

“…The fitting factor k a was introduced to avoid unrealistically high amplitudes. First assumptions of k a = 0.5 led to plausible results, [ 15 ] which, however, can neither be generalized nor are physically justified. Therefore, a more well‐founded determination of the fitting factor k a is presented in the following.…”

“…The derivation of the equations was presented recently. [ 14 ] In total, μ‐CT investigations and tensile tests must, thus, be carried out for four specimen configurations (two directions and two plate thicknesses). By averaging the Young's moduli of all tensile tests, the average Young's modulus is first determined, which is assumed to be the isotropic Young's modulus E iso with a completely random fiber orientation distribution (eigenvalues of the orientation tensor = 1/3).…”

“…The basics of the mean value-amplitude method have already been presented in refs. [14,15]. In the following, the focus is on the determination of a microstructure-based correction factor, which brings the anisotropy closer to real conditions.…”