Effects of strain rate variation on the shear adhesion strength of impact ice

Abstract

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, thereby increasing 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. The structural behavior of ice has been examined under pure shear, tension, compression, and mixed-mode loading. However, one important loading consideration that has not been widely investigated on atmospheric ice is strain rate.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 conditions—for 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. The design and analysis of the AJ2 rig is described in detail in this paper.Experiments were performed using AJ2 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 5E−8 and 5E-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 relationship between strain rate, temperature, and adhesion strength to be statistically significant. The power “1/n” for a coupon roughness of 64 nm (Sa) was greater than that of the 80-nm coupon; this was the case for both tested temperatures. However, for the relatively smooth surfaces tested, regression analysis suggested that the surface roughness had negligible effect on adhesion strength. 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 Glen's 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 2.6 for the smoothest sample at the coldest temperature, to 8.8 for the roughest sample at the warmest temperature. In most cases of the current study, “n” was within the range of previously-reported values in literature (1.5 to 6).