The new evaluation method of hardness for plastics at cryogenic temperatures

It had been found that the indentation scar made at cryogenic temperatures disappeared at room temperature in plastics such as epoxy and glass fiber reinforced plastic (GFRP); thus we could not measure Vickers hardness numbers at cryogenic temperatures. We then measured the indentation depth with load. The specimens used were GFRP, polyethylene fiber reinforced plastic (DFRP), epoxy and SUS304 steel. The results showed that the elastic recovery ratio, defined as a ratio of the decrement in depth during unloading to the maximum depth at the loaded condition, was up to 90% for epoxy and up to 30% for SUS304 steel in liquid helium temperature. This paper suggested a new hardness evaluation method obtained from displacement-load curve during indentation. The new hardness HVd was defined as HVd = P/(24.5δ2), where P was load and δ was the indentation depth. Consequently, we could get the hardness number for plastics from 4 to 293 K, and it monotonically increased with decreasing temperature. (author)