The International Space Station Flow Enclosure Accommodating Novel Investigations in Combustion of Solids (FEANICS) module is designed to perform various combustion experiments in zero gravity. The module fits into a chamber known as the Combustion Integrated Rack and is observed via eight pressurized sapphire windows as shown in the following figure. The FEANICS project intends to replace one of the sapphire windows with a 63-mm-diameter zinc selenide (ZnSe) window that will allow a laser beam of 10.6-μm wavelength to pass. The laser will be used to ignite materials in a zero-gravity environment and thereby study combustion and improve safety in space vehicles.
Combustion Integrated Rack and ZnSe window assembly for use with the FEANICS module.
Long description of figure 1.
ZnSe is a soft, weak ceramic that exhibits crack growth in the presence of water. Furthermore, it has a large average grain size. This results in single grains dominating behavior by fracturing at energies lower than expected from data generated with macrocracks. Thus, the design of hardware, such as a window fabricated from polycrystalline ZnSe, requires not only the usual life analysis performed for fracture-critical components on the International Space Station, but consideration of the potential for failure from single grains. Unfortunately, much of the data available on ZnSe were generated by using large cracks spanning many grains.
In order to design a window sufficient to sustain the required mission pressure cycle and humidity, researchers at the NASA Glenn Research Center reviewed the literature on chemically-vapor-deposited ZnSe (ref. 1) and reinterpreted it for failure from small, realistic flaws. The reinterpretation of the existing data resulted in a substantial change in the crack-growth parameters and the predicted component life. For ceramics and glasses, slow-crack-growth can be described by the equation
where A and n are slow-crack-growth parameters and KI is the mode I applied stress intensity. For 100-percent humidity, the slow-crack-growth parameters were estimated as n ≈ 40 and Amacro ≈ 1000 m/sec (MPa√m)-n for macrocrack failure as shown on the right side of the graph.
Crack-growth velocity v as a function of stress intensity factor KI for ZnSe in water. Two types of crack growth are shown: that within a single grain and that from a large crack encompassing many grains. DCB, double-cantilever beam; DT, double torsion; B-O-3B, point loading of circular plates supported by three balls; KIc, fracture toughness; A and n, slow-crack-growth parameters; r, Pearson coefficient.
Two approaches were used to determine parameters for small-crack failure: (1) strength-based data were analyzed using small-crack-fracture energies, and (2) macrocrack curves were shifted to the small-crack region by
where Amacro and KICmacro are the macroscopic slow-crack-growth coefficient and fracture toughness, respectively, and Asingle and KICsingle are the corresponding single-crystal values.
For 100-percent humidity, the slow-crack-growth parameters for small-crack or single-crystal failure were estimated to be n ≤ 40 and Asingle = 1020 m/sec (MPa√m)-n as shown on the left side of the graph. Reasonable agreement between the strength-based data and the shifted macrocrack data was exhibited for both 45-percent relative humidity air and water, implying that equation (2) gives a reasonable estimate of crack-growth parameters for small cracks.
In addition to reinterpreting the available literature, Glenn researchers measured properties such as strength, hardness, and grain size for currently available ZnSe by testing windows similar to the actual hardware. The measured properties were found to be in good agreement with literature on well-polished test specimens. Fracture toughness was noted to vary by a factor of 3: the measurements ranged from 0.33 to 0.9 MPa√m, with the lower values representing failure from small flaws within grains and the larger values representing macroscopic cracks in dry environments. Future work may include testing of witness coupons manufactured concurrently with the flight window.
Find out more about the research of Glennís Life Prediction Branch: http://www.grc.nasa.gov/WWW/LPB/research/acl/
Dr. Jonathan A. Salem, 216-433-3313, Jonathan.A.Salem@grc.nasa.gov; and Charles L. Denniston, 216-433-8534, Charles.L.Denniston@nasa.gov
Author: Dr. Jonathan A. Salem
Headquarters program office: HSRT
Programs/Projects: Life Support & Habitation, Microgravity Science
Last updated: October 16, 2006
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