Characterization of Thermoplastic Matrix Composite Joints for the Development of a Computational Framework

Abstract

Many industries, notably the automotive and aerospace industries, are now utilizing thermoplastic matrix composites (TPMCs) for their improved strength and stiffness properties compared to pure thermoplastic polymers, as well as their manufacturability compared to traditional thermoset matrix composites. The increase in the utilization of TPMCs ushers in the need for the development and characterization of joining methods for these materials. A widely used technique for joining thermoplastics is ultrasonic spot welding (USSW). During USSW, high frequency, low amplitude vibrations are applied through an ultrasonic horn resting on the polymer surface. The vibrations induce frictional heat, producing a solid state joint between polymer sheets. Advantages such as short weld cycle time, fewer moving components and reproducibility make this technique attractive for automation and industrial use. Prior work showed USSW as a feasible, repeatable joining method for a polycarbonate matrix filled with chopped glass fibers. The mechanical properties required for full characterization of the TPMC used in this work were not provided by the manufacturer. As such, the constitutive behavior of both as-received and USSW thermoplastic composite material (polypropylene matrix filled with 30 wt% chopped glass fibers) was characterized. The fiber orientation and distribution in TPMCs has a direct impact on constitutive behavior. To characterize these qualities, optical techniques such as scanning electron microscopy (SEM) and micro computed tomography (micro-CT) were employed. Digital image correlation (DIC) was used to acquire full field strain measurements from the composite material under different loading scenarios. Because the constitutive behavior of polymers is greatly dependent on temperature, temperature measurements during the USSW process and measurement of mechanical properties as a function of temperature will be conducted through infrared (IR) imaging and dynamic mechanical analysis (DMA), respectively. Following the calibration of the constitutive model for the polypropylene matrix TPMC, the mechanical and thermal properties will be used to develop a computational framework for the purpose of predicting the structural response of a composite joint under various loadings.