Ice Adhesion Strength Mitigation Via Low Surface Roughness Erosion Resistant Coatings PDF Download

In-flight ice accretion on fixed-wing aircraft and rotorcraft can be catastrophic if not mitigated. Most modern ice protection systems are active systems, which require electrical or mechanical power to remove accreted ice. Despite their proven capability to protect aircraft from ice accretion, these methods can reduce the aerodynamic efficiency of the vehicle and increase its weight, cost, and complexity. Scientists and engineers now seek passive, erosion-resistant materials and coatings with low ice adhesion strength. Ideally, such materials, when applied to vulnerable components of an aircraft, would cause any ice to shed off the surface under normal aerodynamic loading. To aid in the development of low-ice-adhesion-strength materials, the growth and structural behavior of impact ice in a wide range of atmospheric conditions must be characterized. Facilities such as the NASA Icing Research Tunnel (IRT), the Anti-Icing Materials International Laboratory (AMIL), and the Penn State Adverse Environment Research Testing Systems (AERTS) laboratory, to name a few, have spent decades investigating the relationship between ice adhesion strength, temperature, surface roughness, airspeed, and other parameters. The structural behavior of ice has been examined under pure shear, tension, and compression, and mixed-mode loading. However, one important loading consideration that has not been widely investigated on atmospheric ice is strain rate. Very few published ice adhesion studies report the strain rate applied to the ice samples. Several previous studies of laboratory-prepared ice in compression revealed that ice undergoes a ductile-to-brittle transition under high strain rate conditions, and that the adhesion strength is a power function of the strain rate. Other studies, in which lab-prepared ice was loaded in pure shear, reported similar trends. It is unclear whether the same behavior can be expected of dynamically-accreted atmospheric impact ice. Knowledge of the relationship between impact ice adhesion strength and strain rate is important because it can be used to design future ice protection systems, and it may dictate the appropriate course of action for a pilot flying through icing conditionsfor instance, whether a helicopter pilot should increase the rotor speed rapidly or slowly to induce shedding of the ice. NASA Glenn Research Center funded the design and construction of a new centrifuge-style ice adhesion test rig (AJ2) by the Penn State AERTS lab. The ice is accreted dynamically by spinning flat metal test coupons at high speed inside a simulated icing cloud environment, so the water droplet sizes and impact speed are representative of in-flight icing, without the need for a wind tunnel. The rig motor allows for user-defined acceleration rates, so the strain rate on the ice can be controlled. The adhesion strength of the ice is calculated from the voltage output of strain gauges mounted on the cantilever beams holding the test coupons. Unlike other small-scale adhesion test methods, AJ2 allows researchers to collect real-time adhesion data and control the testing environment without any direct interaction with the ice, thus preserving the fidelity of the data. As per NASA requirements, ballistic and structural analysis was performed on the rig to verify its safety. The design and analysis of the AJ2 rig is described in detail in this paper. Many experiments were performed at Penn State to investigate how the adhesion strength of impact ice related to the strain rate applied to it. Stainless steel test coupons of known surface roughness were tested in a range of environmental temperatures. The strain rates applied to the ice ranged between 5x10-7 and 5x10-5 s-1. It was discovered that a similar power function exists between strain rate and adhesion strength as found in the freezer-ice studies described in the literature. Despite scatter in the data, regression analysis determined the trends to be statistically significant. The data suggests that strain rate has a stronger effect on adhesion strength for smoother surfaces as opposed to rougher surfaces. The power 1/n for a coupon roughness of 64 nm (Sa) was double that of the 80-nm coupon; this was the case for both tested temperatures. Similarly, lower temperatures caused a higher power 1/n and coefficient c in the power function. The variation of the coefficient with temperature is consistent with Glens power law for the creep of glacier ice in compression. However, Glen did not observe a variation of the power with temperature. The value of n in the current study ranged from 2.5 for the smoothest sample at the coldest temperature, to 9.7 for the roughest sample at the warmest temperature. In most cases, n was within the range of previously-reported values in literature (1.5 to 6). These findings suggest that the creep behavior of atmospheric impact ice in shear is similarbut not identicalto freezer ice in compression. The proven strain rate testing capabilities of the AJ2 rig will aid icing research efforts by yielding baseline prediction data for future design of ice-resistant materials.