Ultrasonic healing of structural thermoplastics
Francischetti, Vincent Steffan.
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This thesis is a preliminary investigation into the effectiveness of ultrasonic damage healing in thermoplastic materials. The work was based on the proposition that ultrasonics can form the basis of a damage detection and healing system that draws inspiration from the systems-level approach to wound healing seen in living organisms. Ultrasonic inspection has been well established as a method of nondestructive damage location. Testing includes tensile testing and investigation of the temperature distribution in specimens in response to ultrasonic energy input. Additionally, bending tests following the guidelines of ASTM 6272-02, to investigate healing were performed. Healing with ultrasonics is realized through focusing ultrasonic energy into the damage site, which results in a thermal process that occurs in the vicinity of the damaged area. Healing was defined as an increase in the mechanical performance of the healed specimens over the unhealed samples. Tensile specimens exhibited a 9% increase in displacement to failure. T-slot bending specimens used to investigate cross crack wave propagation showed that high power treated samples were able to withstand up to 14% more displacement than the unhealed samples. The preliminary "single notch bend" samples showed a minimum of 12% increase in displacement to failure but the bending apparatus geometry limited complete investigation. Further investigation into single notch bend samples using short specimens showed an increase in displacement of over 30%. Microscopic inspection revealed that ultrasonic treatment causes melting. The melted material bridges the crack opening which transfers stress away from the crack tip. Microscopic evidence does not suggest that crack blunting is a prevalent form of damage healing. Ultrasonic energy is shown to be a viable energy source for both damage detection and healing. The process still needs refinement, but even with limited material and equipment an approximate increase in displacement to failure of 30% and an increase in fracture energy of over 70% are demonstrated. Future testing can use the information about healing gleaned from this thesis in a time reversed acoustics system to heal remotely deployed structures on satellites or other equipment that cannot easily be repaired.