Nondestructive Characterization of Fiber Orientation in Long Fiber Thermoplastic Composites Using Vibration-Based Method

Long Fiber Thermoplastic (LFT) composites are formed by combining reinforcement fibers of 0.5 to 1.5 inch in length with a variety of thermoplastic polymer resin systems. LFTs enable higher mechanical performance than short-fiber equivalents; however, prediction of the effect of processing parameters on the performance of LFTs is less well understood. In discontinuous fiber reinforced composites, the fiber orientation is the most significant variable that determines its mechanical and physical properties. Stiffness and strength are maximized when the fibers are preferentially oriented in a single direction. The proportion and direction of fiber orientation depends strongly on the flow of the material. To characterize and optimize the effects of various processing parameters, an effective method to determine fiber orientation is required. In this paper, LFT glass fiber/polypropylene (PP) (20 wt. %, 14” x 14”) composite plates were processed using Extrusion-Compression Molding (ECM) technique. Samples of the plate were extracted at orientation angles of 0°, 45°, and 90° from the charge squeeze-flow direction. The samples were analyzed using continuous vibration excitation. The Frequency Response Function (FRF) was used to characterize the sample stiffness and damping capacity. The results showed that the 0° samples oriented parallel to the flow direction, exhibited the highest natural frequencies and the lowest damping capacity. The results were validated with destructive mechanical tests to construct an empirical model to predict the effective fiber orientation.

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