Structural design of optical components launched into space requires fracture mechanics properties. To perform component design of a calcium fluoride (CaF2) prism, the fracture toughness and slow crack growth parameters were measured on the {100}, {110} and {111} low index planes. The fracture toughness is lowest on the {111} plane at 0.35 ± 0.01 MPam with a very flat cleavage surface exhibited during both fracture toughness and strength testing. Slow crack growth was significant on the {111} plane with a power law exponent of n = 30 ± 8. For engineering purposes, slow crack growth was insignificant on the {100} and {110} planes with n > 75. The facture surfaces have distinct patterns that are indicative of the cleavage plane. Biaxial testing with disks implies that design for general multiaxial states should be based on {111} strength and crack growth properties.
Structural design of optical components launched into space requires fracture mechanics properties. To perform component design of a calcium fluoride (CaF2) prism, the fracture toughness and slow crack growth (SCG) parameters were measured on the {100}, {110}, and {111} low index planes. The fracture toughness is lowest on the {111} plane at 0.35 ± 0.01 MPa√m with a very flat cleavage surface exhibited during both fracture toughness and strength testing. SCG was significant on the {111} plane with a power law exponent of n = 30 ± 8. For engineering purposes, SCG was insignificant on the {100} and {110} planes with n > 75. The facture surfaces have distinct patterns that are indicative of the cleavage plane. Biaxial testing with disks implies that design for general multiaxial states should be based on {111} strength and crack growth properties.
We report on the cause and corrective actions of three amplifier crystal fractures in the space-qualified laser systems used in NASA Goddard Space Flight Center’s (GSFC) Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2). The ICESat-2 lasers each contain three end-pumped Nd:YVOO4 amplifier stages. The crystals are clamped between two gold plated copper heat spreaders with an indium foil thermal interface material, and the crystal fractures occurred after multiple years of storage and over a year of operational run-time. The primary contributors are high compressive loading of the NdYVO4 crystals at the beginning of life, a time dependent crystal stress caused by an intermetallic reaction of the gold plating and indium, and slow crack growth resulting in a reduction in crystal strength over time. An updated crystal mounting scheme was designed, analyzed, fabricated and tested. Thee fracture slab failure analysis, finite-element modeling and corrective actions are presented.
Mechanical strength measurements of transparent ceramic window material coupons are customarily fit to a Weibull equation that describes the strength and its distribution. Predictions of window lifetime under stress are commonly based on slow crack growth parameters obtained by measuring the mechanical strength of coupons over a range of constant stress rates. This tutorial paper describes how to derive Weibull and slow crack growth parameters from strength measurements and how to use those parameters to predict window lifetime under stress. Proof testing is employed to ensure that a window begins its life with a known, minimum strength.
The mechanical properties of several transparent ceramics were investigated to determine if their use might lighten next generation spacecraft windows. The measured fracture toughness and slow crack growth parameters were used as inputs to functions describing the required mass for a desired window life. Transparent magnesium aluminate (spinel, MgAlO4) and AlON exhibit superior slow crack resistance relative to fused silica, which is the historical material of choice. For spinel, slow crack growth, strength and fracture toughness are significantly influenced by the grain size, and alumina rich phases and porosity at the grain boundaries lead to intergranular fracture in coarse grain spinel. The results imply that transparent ceramics can lighten window panes from a slow crack growth perspective.
The slow crack growth parameters, fracture toughness, and inert strength of the r-and a-planes of sapphire were measured in water in order to perform a life prediction on a pressurized sapphire window. The window is being considered for use in a combustion chamber on the International Space Station. Sapphire is relatively susceptible to stress corrosion in water despite a large strength in the absence of humidity. Two life prediction approaches were considered: a deterministic fracture mechanics approach and a Weibull based reliability approach. Preliminary results indicate that the window is feasible if a short finish is avoided. Fractography and additional predictions are being performed.
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