In this study we examine the fiber orientation distribution, fiber length and Young's modulus of extruded short‐fiber reinforced thermoplastics such as polypropylene. Axial orientation distributions are presented to illustrate the influence of extrusion ratio on the orientation state of the fibrous phase. Fibers are markedly aligned parallel to the extrusion direction with increasing extrusion ratio. The orientation state of extruded fiber‐reinforced thermoplastics (FRTP) is almost uniform throughout the section. The control of fiber orientation can be easily achieved by means of ram extrusion. Experimental results are also presented for Young's modulus of extruded FRTP in the extrusion direction. Young's modulus follows a linear trend with increasing extrusion ratio because the degree of the molecular orientation and the fiber orientation increases. The model proposed by Cox, and Fukuda and Kawada describes the effect of fiber length and orientation on Young's modulus. The value of the orientation coefficient is calculated by assuming a rectangular orientation distribution and calculating the fiber distribution limit angle given by orientation parameters. By comparing the predicted Young's modulus with experimental results, the validity of the model is elucidated. The mean fiber length linearly decreases with increasing extrusion ratio because of fiber breakage due to plastic deformation. There is a small effect on Young's modulus due to fiber breakage by ram extrusion.

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The in-plane angle determined using an optical reflection microscope with an image analyzer is ambiguous, because the same elliptical image would be obtained for a fiber. A method of determining the three-dimensional (3D) fiber orientation distribution using the scanning acoustic microscopy (SAM) image of an elliptical mark and interference fringe is proposed. The interference fringe appearing in the composite in the direction of a fiber is observed by SAM. One can easily determine the in-plane angle without ambiguity by adopting this information of interference fringes. It is shown that this technique is effective for the determination of the 3D fiber orientation distribution in plaque and cylindrical composite parts.