The end goal of this work is to couple the experimental and optical characterization of ultrasonic spot welds in a chopped fiber-thermoplastic matrix composite with computational efforts to produce a physics-based model to guide the design of composite joints. The experimental work discussed here will guide the computational efforts, which will first focus on the development of a mathematical model that accurately describes the multi-physics of the welding process. This model will be based on principles of continuum mechanics and therefore will address both the mechanics and thermodynamics of the welding process, as well as the coupling between these. The development of this model will help guide the design of composite joints created with USSW. This predictive model will be used to derive optimal welding parameters based upon weld application and material properties. This will shorten the time necessary to apply USSW in different applications proving useful for many industries concerned with the manufacturing of novel, light-weight materials. 2.2 Mechanical Characterization of Composite Material The use of USSW on polymer matrix composites has been studied and characterized previously [6]. In this work, initial weld characterizations were performed on pure polypropylene matrix material, followed by further characterization of polypropylene matrix composite materials welds. The composite used in this work was ESP 105 CC, a polypropylene matrix TPMC, produced by RTP Company (Winona, MN, USA). The ESP 105 CC composite is composed of 30 wt% chopped glass fibers and was provided in 1.52 mm thick extruded sheets [7]. The mechanical properties of this material provided by the manufacturer can be found in Table 2.1. The ultrasonic spot welding process varies based upon the input parameter settings used during the weld. As such, a study was necessary to investigate the effects of weld parameters. The parameters that were varied in this study were amplitude, time and weld pressure. In the interest of limiting the use of the composite material, the study was performed on pure polypropylene material. Since the composite material is composed of 70 wt% pure polypropylene, the response to the welding parameters was assumed to be similar to that of plain polypropylene sheeting. The quality of the welds resulting was characterized both optically, by observing bond area, and mechanically, by measuring peak load during lap shear testing. The criteria used to characterize the welds were total bonding area (shear area), and the peak load under lap shear. The bonding area was measured optically prior to lap shear testing using the open source image analysis software ImageJ. In the preparation of the lap shear samples, tabs were adhered to the ends of each test specimen to avoid the production of a bending moment during testing and the samples were tested at a rate of 1.3 mm/min as outlined in ASTM 3164-03 [8]. Samples with a broad range of weld parameters were investigated in order to isolate the parameters with the greatest impact on the quality of the weld. It was found that these parameters were the weld time and weld amplitude. The failure mode exhibited by the samples was almost immediate fracture, at the base of the weld, when the maximum load was reached. The location of the failure in the samples is most likely due to bending stresses, and subsequent stress concentrations that are inherent in lap shear testing, which are also highlighted by Degen et al. [6]. The results of the optical and mechanical characterization of plain polypropylene sheeting are shown in Table 2.2. From this study, the following welding parameters were found to create welds with the largest bond area and highest peak load during lap shear testing in pure polypropylene sheeting: an amplitude of 24μm, a weld time of 2 s and a weld pressure of 483 kPa. The chosen parameters are not immediately apparent upon inspection of Table 2.2, but these values were found to create the largest shear weld areas while exhibiting the least standard deviation and therefore creating the most repeatable welds. Upon finding suitable welding parameters for the pure polypropylene sheeting, the same parameters were used to weld the polypropylene matrix composite, ESP 105 CC. As a reference, a representative 25.4 mm 25.4 mm USSW sample of the composite material is shown in Fig. 2.2. Five lap shear samples of both the plain polypropylene and composite materials were welded using the above parameters and evaluated in terms of bond area, maximum load during lap shear, and maximum shear stress. From the results shown in Fig. 2.3, it is apparent that the pure polypropylene samples exhibited better performance in terms of both average shear area and average failure load in lap shear than their composite material counterparts. Table2.1 Mechanical properties of ESP 105 CC [7] Property Value (Metric units) Young’s modulus 6205MPa Ultimate tensile strength 55MPa Elongation at break 1.5–2.5% Impact strength (IZOD, unnotched) 0.32 J/mm 2 Characterization of Thermoplastic Matrix Composite Joints for the Development of a Computational Framework 13
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