Multi-Timescale Simulations of Temperature Elevation for Ultrasonic Welding of CFRP with Energy Director

Ultrasonic welding is an energy-efficient technology that enables quick bonding of thermoplastic composite materials under normal temperature and pressure conditions. Here, numerical multi-timescale simulation is proposed to understand the welding principle, using numerical simulations of ultrasonic welding. The simulation results are validated by comparing with temperature measurements in welding tests. In the multi-timescale simulations, microsecond-scale simulations are performed first. The ultrasonic wave is modeled as a vibration load, and the energy dissipation per vibration at 25, 75, 125, 175, 225, and 275 ∘ C is analyzed. Then, the time derivative of the temperature rise is obtained. In the normal scale simulations, the ultrasonic wave and holding pressure are replaced by a constant load, and the entire process of ultrasonic welding is simulated. The slope of the temperature rise is fitted to the time derivative of the temperature rise obtained from the microsecond-scale simulations, using the material constant as a parameter. Explicit multi-timescale simulations were performed to investigate the relationship between stress concentration and temperature rise due to ED geometry. The result reveals similar temperature behavior to the experimental one, indicating the validity of the multi-timescale method. It suggests that viscoelastic energy dissipation and stress concentration are responsible for the temperature spike.