Impact damage from ice is a potential problem in many aerospace and industrial applications, including jet engines and airframes, ground-based turbomachinery, and spacecraft launch systems. In particular, ice impact damage is considered a serious threat to the space shuttles because of the potential damage that can result when ice formed on the external fuel tank sheds during launch. As part of NASA’s return-to-flight effort, a test program was conducted at NASA Glenn Research Center’s Ballistic Impact Facility to study the impact behavior of ice projectiles. The objectives of the study were to measure the forces generated by different types of ice, both to quantify the damage potential and to generate data to aid in developing computational models to simulate ice impact.
Ice, in its solid form, is a crystalline material that can be made up of single or multiple grains. In practical applications, ice can be solid (fully dense) or be partially filled with air in the form of bubbles. The effect of the granular makeup and density on the strength of ice was one of the phenomena to be quantified in this study.
Forces due to ice impact were measured by firing ice projectiles at a single piezoelectric load cell assembly oriented at incidence angles of both 90° and 45°. The ice projectiles were cylinders of various sizes, ranging in diameter from 0.44 (7/16) to 1.25 in. and in length from 1.5 to 3.3 in. These projectiles were accelerated by a 12-ft-long, 2-in.-diameter gas gun using a polycarbonate sabot. Impact velocities were in the range of 300 to 800 ft/sec. All tests were conducted in a vacuum.
A series of five images taken at the moment of impact, showing the progression of a fracture wave in ice. The fracture wave traveled approximately 1.92 in. in 15.38 μsec, or 10,400 ft/sec.
Initial tests were conducted to observe the fracture behavior of ice. Using a digital high-speed imaging system, running at 260,010 frames/sec, Glenn researchers studied the impact of ice on a solid aluminum target (see the photographs). We found that, in the range of impact velocities of interest, the speed of a fracture wave is much greater than the impact velocity. The practical implication of this is that the projectile completely fractures as soon as the impact occurs, and in the velocity range of this study, the structural properties of the ice have little effect on the impact force. This was supported by experimental results, which demonstrated that the measured impact forces were quite repeatable from test to test, and that the response was not dependent on the granular makeup (see the graph). The measured force, however, was directly dependent on the density of ice, as was expected.
Forces measured from impacts involving three types of ice: single crystal (one grain), columnar (multiple grains elongated over the whole length), and polycrystalline (randomly oriented equiaxed grains). Impact velocity was approximately 700 ft/sec. There was no significant difference in the response from the three types of ice.
Results of these experiments were used to develop computational models that accurately predict ice impact damage in structures. The techniques developed also were used under a reimbursable space act agreement to successfully evaluate design changes in industrial airfoils that significantly reduce the damage due to ice impact.
Glenn contacts: Dr. J. Michael Pereira, 216-433-6738, J.M.Pereira@nasa.gov; Dr. Santo A. Padula II, 216-433-9375, Santo.A.Padula@nasa.gov; Duane M. Revilock Jr., 216-433-3186, Duane.M.Revilock@nasa.gov; and Matthew E. Melis, 216-433-3322, Matthew.E.Melis@nasa.govLast updated: October 11, 2006
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