Structural Tailoring and Actuation Studies for Low Power Ultrasonic De-icing of Aluminum and Composite Plates

Open Access
Author:
Zhu, Yun
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 03, 2010
Committee Members:
  • Joseph Lawrence Rose, Dissertation Advisor
  • Joseph Lawrence Rose, Committee Chair
  • Edward C Smith, Committee Chair
  • Bernhard R Tittmann, Committee Member
  • Clifford Jesse Lissenden Iii, Committee Member
Keywords:
  • aluminum
  • composite
  • de-icing
  • ultrasonic
  • Actuation
  • Tailoring
Abstract:
ABSTRACT Ice accretion during flight of a rotorcraft affects the performance and control of the rotorcraft. Current electro-thermal de-icing systems require large input power (>20 kw for Bell 412, 4 blades 12000 lbs model). Ultrasonic guided wave de-icing systems are introduced as a possible substitute for the current de-icing systems. These systems generate sufficient shear stresses at the interface between the accreted ice and the substrate to exceed the adhesion strength of ice, causing delamination of the ice layer via fracture and/or fatigue. Compared to the electro-thermal de-icing systems, ultrasonic de-icing systems have non-thermal in nature and have a potential to be low-power. In ultrasonic de-icing systems, the interface shear stresses were generated at the ultrasonic vibration state. In order to gain insight of the ultrasonic de-icing systems based upon the ultrasonic waves and vibration, ultrasonically generated displacement wave structures for ultrasonic wave modes (mode 1 and mode 2) at the transient state were studied and compared to the displacement distributions at the steady state. For modes 1 and 2, the wave structures in the transient state retain their properties as the ultrasonic guided waves transition to a vibration state. A new structural design feature, termed called Tailored Waveguides (TWG) was introduced to maximize delaminating stresses via the introduction of discontinuities in the rotorcraft plate like structure. These discontinuities allowed the shear stresses at the ice/substrate interface to be focused on specific regions, hence facilitating low power ultrasonic de-icing. Finite element analysis results for TWG structure demonstrated that the introduction of tailored waveguides can generate more than 300% larger interface stresses compared to those of equivalent mass structures without tailored waveguides (i.e. uniform plates). Furthermore, parametric studies indicated that the interfacial shear stresses (ice/aluminum) can be doubled by reducing the spacing between the tailored waveguides. Several different waveguide and ice accretion configurations were examined. Experimental trends and power measurements correlated very well (within 15%) with numerical predictions over a wide range of test conditions. A shear mode piezoelectric actuator (exhibiting d15 coupling) was studied and compared to an in plane mode piezoelectric actuator (exhibiting d31 coupling). According to the finite element analysis results, the stresses generated by the shear mode actuator are 28% higher than the ones generated by the in plane mode actuator. Shear stresses at the ice/substrate interface for different loading functions at different frequencies were explored. Normal mode actuation at 10 kHz generates the highest interface shear stresses, while the shear mode actuation at 20 kHz generates the highest values. At 30 kHz, the in plane actuation generates the highest interface shear stresses. Filled TWG structures were studied as optional approaches to a TWG structure. The filled TWG structure can increase the magnitude by 15%-20% in shear stress at the ice/aluminum interface without changing the shape of the geometry. Depending on the material properties of the filler inside the TWG structure, the interface shear stress values may vary. The material filled inside the TWG with a high Young’s modulus and low density generates high interface shear stress values. Interface stress concentration coefficients (ISCC) and wave structure were studied for multilayer composite plates, thus the optimal mode (mode 1) and exciting frequency of 300 kHz- 450 kHz were chosen for de-icing on composite plates from the numerical analysis. Shear stresses at the inner ply interfaces were also calculated and compared to the failure stresses. It turned out that the shear stresses at the inner plies of the composite plate were ~10% of the failure stresses. De-icing experiments on the composite plate was conducted in an icing tunnel. The top and bottom edges of the composite plate remain clean as predicted by the finite element analysis at an input power of 60 watts.