A method is described whereby on-load values of crack tip crack opening displacement (COD) can be measured at the midsection of precracked three-point bend specimens by infiltration of silicone rubber. A calibration curve relating midsection COD to clip gage displacement was derived from measurements on the silicone rubber “castings.” This calibration curve can be used to calculate midsection COD from on-load clip gage displacement and specimen geometry only, without further infiltration measurements. These values of COD have been shown to be simply related to the stress intensity factor, independent of material, as theoretically predicted.
The central region of a Charpy specimen in three-point bend has been found to remain in plane strain until well after general yield. Thus, plane strain values of COD at fracture initiation, (COD)c, can be determined from small specimens. Two initiation detection methods are described whereby (COD)c can be determined. These values of (COD)c can be used to accurately predict KIc values which agree with data obtained on larger, more expensive valid ASTM KIc specimens.