PSI - Issue 52

Minori Isozaki et al. / Procedia Structural Integrity 52 (2024) 176–186 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Carbon Fiber Reinforced Plastic (CFRP) has been widely used in the aerospace industry because of its superior specific strength and specific stiffness. Thermosetting resin composite materials (FRTS) have been used for many of them, but FRTS has various problems such as high cost and time required for processing. Then, recently, Europe and other countries have been converting to thermoplastic composite materials (FRTP). FRTP has advantages, such as superior impact resistance, short processing time, and the ability to be reformed. A particularly noteworthy advantage is the possibility of weld-joining. Currently, mechanical joining using bolts and rivets is the mainstream joining method for thermoplastic composites (CFRTP), but if welded joining is realized and the need for fastening parts is eliminated, advantages such as weight reduction, improved mechanical properties, and recyclability can be expected. Thus, weld joining of CFRTP is a noteworthy technology, and this paper discusses ultrasonic welding as one of efficient welding methods (Zhao et al. 2017). In the ultrasonic welding method, ultrasonic vibration is applied to the welding target through Horn. The propagation of ultrasonic vibration to the welding interface generates heat due to energy dissipation by friction and viscoelasticity, and this generated heat is used to melt the resin to achieve adhesion (Yaqiong et al. 2021). In this process, a protrusion made of resin called an energy director (ED) is inserted between the adherends to increase the density of vibration energy, enabling stable welding with less energy (Koyanagi et al. 2022, Takamura et al. 2021). Ultrasonic welding is one of the excellent bonding methods that can weld at room temperature and pressure, in a short time, and with low energy, but ultrasonic welding is not generally applied to CFRTP. This is due to the difficulty in predicting optimal welding conditions such as welding time, pressure, amplitude, ED type, etc., and the consequent unreliability of the welding process. Therefore, for the practical application of ultrasonic welding in CFRTP, a quantitative understanding of the mechanism and adhesive properties of ultrasonic welding is needed through numerical simulation. In order to optimize the welding conditions, we have numerically analysed the temperature rise during ultrasonic welding (Koyanagi et al. 2022, Takamura et al. 2021). In this paper, we discuss the evaluation method of adhesion, which is important for optimization of welding conditions. Particularly, the strength of the bonding interface was focused on, assuming interface fracture in ultrasonic welded CFRTP. Most evaluations of adhesion in ultrasonic welding for CFRTP have been performed by single lap shear tests (Villegas et al. 2010, Villegas et al. 2016, Yaqiong et al. 2021). However, it has been found that even when strength tests are conducted in which forces are applied only in the shear direction, the failure does not depend solely on pure shear stress, but rather on a mixed mode of shear and tensile stresses (Reis et al. 2020). Therefore, it is essential to consider the combined shear and tensile stresses and to perform a comprehensive evaluation to have a correct understanding of the bond strength of ultrasonic welds. In this study, we conducted shear and tensile strength tests on CFRTP bonded by ultrasonic welding. The test in the shear direction was the ‘ compression shear test ’ , which is a shear test in compression. The reason for this is to reduce the effect of bending on the specimen due to load loading, and to perform on a small specimen without fabrication. The ‘ flatwise test ’ was used as the tensile test. This is a method often used to evaluate the strength of honeycomb structures. After the tests were conducted, numerical simulation of these tests was performed to investigate the stress state at the adhesive interface at failure in shear and tensile tests, respectively, considering the stress distribution. The strength of the adhesive interface in a mixed state of shear and tensile (normal) stress can be considered in terms of a fracture envelope based on a parabolic criterion (Koyanagi et al. 2014, Koyanagi et al. 2010, Ogihara et al. 2010). Finally, a fracture envelope will be developed based on these two types of tests. Then, based on this parabolic criterion, we propose to evaluate the strength of the adhesive interface in ultrasonic welding for CFRTP.

2. Compression shear test

2.1. Experiment

CF/PEEK prepreg (IMS/PEEK (TEIJIN LIMITED), 1.5 mm thick, 30 mm square (16-ply)) was used as the adherend, and welding tests were performed using an ultrasonic welder (Ultrasonic Welder JP80s, SEIDENSHA