Date on Master's Thesis/Doctoral Dissertation

5-2018

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Mechanical Engineering

Degree Program

Mechanical Engineering, PhD

Committee Chair

Lian, Yongsheng

Committee Co-Chair (if applicable)

Cobourn, Geoffrey

Committee Member

Cobourn, Geoffrey

Committee Member

Williams, Stuart

Committee Member

Willing, Gerold

Author's Keywords

ice; adhesion; icing; measurement

Abstract

Icing is a lethal and costly aviation hazard affecting aircraft of all sizes ranging from small UAVs to large commercial craft such as the Boeing 787, resulting in hundreds of deaths over the last century and resulting in billions of dollars of economic impact. Three predominant types of icing, namely, airframe icing, engine icing, and rotorcraft icing, dominate aircraft icing research. Each type poses unique challenges. In the case of rotor icing, attention needs to give to the rotating environment with large oscillatory loads and icing conditions. In the case of airframe icing, special attention needs to be paid to a variety of loading cases from the available options for de-icing and anti-icing equipment. In the case of engine icing, the formation mechanism is poorly understood and the structure of the ice needs to be studied in greater detail. Airframe icing requires However, all three share the same fundamental problem that ice sticks to surfaces. How strongly ice adheres to a surface dictates how hard it is to remove. The adhesion strength then regulates the primary threat from icing - the maximum aerodynamic penalty that accretion will have. It also dictates the threat of impingement from shed ice elsewhere on the aircraft, such that a piece of ice from the main rotor could strike the tail rotor on a helicopter, destroying it. There exist many methods to evaluate the adhesion of ice to a given substrate, the most common being pusher tests and centrifuge tests. These and other methods are problematic in evaluating the adhesive properties of ice to a substrate in aircraft icing conditions; no data in the literature exists that accounts for stress concentrations at the interface and the strain rate at the interface, and no data was found in the literature on the grain structure of impact ice at speeds relevant to aircraft icing. A new method to measure the adhesion of impact ice has been developed based on a lap joint shear test. Lap joint tests are common in adhesion measurements since they produce nearly uniform stress at the interface of interest. In support of this end, a new shear rig and a new wind tunnel model for the Icing Research Tunnel were designed and fabricated. Six nights of testing in the Icing Research Tunnel were conducted to obtain samples, which were later tested in a laboratory environment. The tests were displacement controlled and samples were tested at four crosshead speed rates. The grain structure of the ice was documented using a cross-polarized optical microscope for the first two nights of testing, showing significant differences in the grain structure dependent on velocity and whether the cloud was in the SLD range or not. Correlations with temperature and test section velocity are demonstrated. It was also demonstrated that residual stresses, which are unaccounted for in the literature, play a significant role in the adhesion of impact ice. Options to improve the test methodology further are discussed. Finally, a shedding model predicting the trajectory of ice at a shed event has been developed and validated against test data. This model successfully predicted the front of a multi-break shed event in the Icing Research Tunnel.

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