The effect of aging on the impact resistance of heat tempered lenses.
Stock sperocylindrical lenses (125) with approximately 2.2-mm center thickness were bevel-edged to 48-mm size in the S-10 standard military lens shape. One time each, 25 lenses were drop balled directly after heat treatment; 25 after 3 months;25 after 6 months; 25 after 9 months; and the final 25 after 1 year. Comparing these samples against each other showed no statistical difference in breakage. All the surviving lenses were then drop balled repeatedly until broken or 25 impacts had been made. After 25 hits, 76.8% of the lenses did not break. There was no statistical difference in breakage when these results were compared with 125 heat-treated lenses of similar specifications that were drop balled for 25 trials, if not broken sooner, just after routine heat-treatment processing. Lenses of approximately 3.0-mm center thickness, with other aspects the same, were also treated. The effect of drop balling nonheat-tempered lenses of dimensions similar to the aged lenses was studied; the first drop had a casualty rate of 75%, and no lens survived more than 8 impacts.
D005139:Eyeglasses; D006358:Hot Temperature; D013314:Stress, Mechanical; D013995:Time; D013997:Time Factors