Particulate impact damage predictions

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Rain and solid particle impact damage may produce significant levels of optical degradation in forward-looking IR-transmitting windows on aerospace vehicles. While a significant level of effort is devoted to characterizing the relative erosion resistance of one material compared with another, considerably less effort is devoted to establishing how the particulate impact damage affects a window or dome for in-flight environments. Short of actual flight tests this determination requires an understanding of the relation between laboratory-scale erosion testing and the in-flight conditions to which the window may be subjected. The actual flight environment is complicated by the probability that it may consist of a combination of rain and solid particles. Therefore it is necessary to establish the effects of prior particle impact damage in the sequence of exposures to the range of particulate environments that may be encountered. Particulate erosion is frequently not the damage mode that has to be considered, but catastrophic failure of the window is often a possible outcome of the particulate impacts depending on the configuration of the window and the material from which it is made. In the latter case detailed structural analyses of the damaged window are required to determine if it will survive. The factors influencing the mechanical behavior of IR-transmitting windows in realistic flight environments are outlined. The results from recent particulate impact tests representative of the flight environment are described.