Ice Adhesion Strength Modeling Based on Surface Morphology Variations

Open Access
Author:
Knuth, Taylor Dayton
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 14, 2015
Committee Members:
  • Jose Palacios, Thesis Advisor
  • Douglas Edward Wolfe, Thesis Advisor
  • George A Lesieutre, Thesis Advisor
Keywords:
  • adhesion strength
  • surface morphology
  • predictive
  • topography
  • surface geometry
  • Newtonian mechanics
  • two-dimensional loading
Abstract:
A physics-based analytical model to predict the adhesion shear strength of impact ice on varying surface morphologies was developed and validated experimentally. The model focuses on the surface morphology effects on ice adhesion strength. As super-cooled water droplets, having a typical median volume diameter ranging from 10 to 80 μm, impact and freeze on the leading edges of aircraft, it is hypothesized that the small drops expand and clamp to surface discontinuities, contributing to the ice adhesion strength of the material. The derivation of a Newtonian mechanics model to calculate the forces required for the removal of ice that has expanded and clamped inside macro surface structures is presented. The model requires knowledge of the macro-scale (10-6 m) surface geometry. Newtonian mechanics accounted for the expansion and clamping of freezing ice including temperature dependent ice properties. The model is dependent on Young’s modulus, the thermal coefficient of expansion of ice, and the coefficient of static friction between ice and the adhering substrate. All of these properties are dependent on the variation of temperature. The research validated the developed model experimentally. Firstly, the individual parameters as functions of temperature were obtained from literature review and experimental measurements. Previous research revealed the correlation with temperature of the Young’s modulus and the thermal coefficient of expansion for ice. The relationship for the thermal coefficient of expansion found is valid for temperatures ranging between -193.15 and 6.85 °C (-315.67 and 44.33 °F). The Young’s modulus temperature relationship was obtained from tests presented in the literature that used sea ice. Secondly, the static coefficient of friction is dependent on the surface interaction between the accreted ice and the surface material. Through bench top testing, it was determined that the coefficient of friction of ice is also dependent on temperature. The coefficient of friction was experimentally acquired for a mercaptan and amine blended epoxy (Great Planes 30 Minute Pro Two-Part) applied to an aluminum substrate. The coefficient of friction varied from 0.046 with a standard deviation of 0.015 at -5.8 °C (21.6 °F) to 0.190 with a standard deviation of 0.019 at -15.7 °C (3.7 °F), a change of 420%, and is dependent on loading conditions and the test environment. The final phase of the research was the experimental validation of the ice adhesion model through adhesion strength testing on the Adverse Environment Rotor Test Stand (AERTS). To conduct validation testing, controlled surfaces were created. The surfaces were coated with the same mercaptan and amine epoxy blend to create a surface that approached a Ra of zero. The actual surface roughness measured was a Ra of 0.01 μm (3.94 x 10-7 in.). This pristine coating provided a baseline against other surface of the same coating that had controlled surface roughness. The epoxy surfaces were ablated using a laser at differing intensities to create surfaces with varying roughness depths. The laser etched the coatings at 0.35, 0.6, and 1.2 W, each with a respective surface roughness of 1.13, 1.95, and 5.11 Ra (4.45 x 10-5, 7.68 x 10-5, and 2.01 x 10-4 in.). All of these coatings were tested within the Federal Aviation Regulation Part 25 and Part 29 Appendix C icing envelope to recreate realistic environmental icing conditions. The pristine surface was had an adhesion strength of 4.11 psi (28.3 kPa) with a standard deviation of 0.75 psi (5.17 kPa) at -8 °C (17.6 °F) and 7.99 psi (55.1 kPa) with a standard deviation of 0.94 psi (6.48 kPa) at -16 °C (3.2 °F). While, for example, the coating with the most severe ablation (Ra of 5.11 μm) was had an adhesion strength of 22.7 psi (156.8 kPa) with a standard deviation of 2.70 psi (18.62 kPa) at -8 °C and 42.4 psi (292.5 kPa) with a standard deviation of 3.45 psi (23.79 kPa) at -16 °C. These measured values were then compared to the model predictions. The maximum discrepancy between prediction and experimental results was 9% for the 25 experimental tests conducted using the 1.2 W ablation surface.