Read Consulting was asked to examine a failed sight glass and perform a glass failure analysis. The is a compression sealed soda lime glass window in a 1215 steel bushing. Thus the glass window is in compression.
Several representative failed sight glasses were examined, and all had the same crack pattern. The cracking is very shallow (as indicated by an interference pattern in the glass), and it occurs near the top surface of the glass. The crack geometry appears to be related to the “screwdriver” notches in the steel bushing. This indicates the presence of a high spot on the bushing interior associated with machining the screw driver slot. It is recommended that the bore of the bushing be Honed after machining.
