<title>Non-normal angle of incidence rain impact testing with whirling arm apparatus</title>

Abstract

Quantitative measurements of high speed rain drop impact damage to FLIR grade ZnS are reported for oblique angles of incidence. Specimens of ZnS and PMMA calibration witness pieces were tested at normal incidence and a 10 degree(s), 20 degree(s), 30 degree(s), and 45 degree(s) angle of incidence (AOI) as measured from the normal. The number of drop impacts on the PMMA increase slightly at the small angles over the value observed at 0 degrees. The density of impact events observed on the ZnS specimens increases more dramatically with small angles of incidence before dropping markedly at 30 degree(s) and 45 degree(s). Geometric considerations suggest a cos (theta) dependence. However, differences in air flow around the specimens may be responsible for the anomalous observations. The enhanced effects observed for the damage to the ZnS are not unique and support earlier observations by Gorham and Fields (Wear 41 (1977) 213 - 222). Measurements of the fracture stress also correlate with the angle of incidence. There is a reduction in strength at 10 degree(s) over normal incidence but thereafter the failure stress increases dramatically with increasing angle, which is indicative of a reduction in the magnitude of raindrop induced flaws. Specimens were tested at two velocities, 168 m/s and 210 m/s.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.