Thermal shock resistance of infrared transmitting windows and domes

Abstract

Sudden exposure to a supersonic flight environment subjects a missile window, or missile dome, to intense convective heat loads stemming from the rise in temperature of the boundary layer. The ther- mal response of the window then results in temperature gradients through the thickness, which generate transient stresses that may ex- ceed the tensile strength of the material, thus causing thermal-shock- induced fracture. Since most of the materials that possess favorable optical properties in the infrared (IR) are relatively weak brittle solids, the problem of selecting window or dome materials and assessing their per- formance on a fly-out trajectory requires a careful evaluation of the win- dow's ability to withstand thermally induced shocks. In this context, it is essential to keep in mind that the transient stress intensity depends on the nature of the heat flow as characterized by the Biot number (Bi). The allowable heat flux depends not only on intrinsic material properties but also on the heat transfer coefficient, if the condition Bi.1 holds, or the thickness of the window, if the condition Bi,1 applies. In a first approxi- mation, the thermal shock performance of a ''thick'' window is controlled by the figure of merit (FoM) Bi.15RH , i.e., the Hasselman parameter for strong shocks; in a thermally thin regime, however, if s f designates the flexural strength the appropriate figure of merit is (FoM)Bi,15s f R H with n51/2 for flat plates and n52/3 for hemispherical shells, and not the Hasselman parameter RH for mild shocks. Judging from the results of thermal shock testing performed elsewhere, we conclude that in a lami- nar flow environment the allowable heat flux on a thermally thin IR dome can be expressed as follows: Qlim52RH /L, where L is the dome thick- ness. This expression provides a direct means of obtaining the Mach- altitude failure line for a dome of given thickness and given radius, if the initial wall temperature is known. Furthermore, it then becomes straight- forward to assess the thermal shock resistance capability of a thickness- optimized IR dome in terms of either the allowable heat load or, more simply, the allowable stagnation temperature. © 1998 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(98)02010-8)