Here is a technical report on the engine damage on a DC-8 (CFM engines) operated by NASA after encountering a diffuse volcanic ash cloud. The encounter lasted for a period of seven minutes, was at night, and had it not been for the scientific instrumentation on board the aircraft no one would have been aware the event had taken place. Please note it was estimated the turbine blades may have had as little as 100 hours of life remaining as a result of damage incurred.
http://ntrs.nasa.gov/archive/nasa/ca...2003079762.pdf