Production sources tell TMZ ... the crew member -- a special effects coordinator -- died Thursday in Toronto where the DC Comics show is filmed.

The tragedy occurred at a test facility where production crew work out the kinks for stunts.  Once perfected, the scene is played out on location.

The stunt in question at the test facility was a car that was supposed to flip on its side. The car did indeed flip, but a piece of metal from some sort of welded contraption that was attached to the car flew off.

That object struck the special effects coordinator -- who was standing nearby -- in the head. EMT's put the man on a stretcher and took him to the hospital, but cops tell us he died en route.

Production will be shut down for 2 days. The Ministry of Labour is now investigating.

Executive Producers from "Titans" tell TMZ, "We are heartbroken and devastated by the passing of our treasured colleague, special effects coordinator Warren Appleby, after an accident which occurred at a special effects facility during the preparation and testing for an upcoming shoot.  Warren is beloved by all who worked with him during an impressive 25-year career in television and motion pictures. The executive producers, along with everyone, in the TITANS family, Warner Bros. Television Group and DC UNIVERSE, wish to express our deepest condolences, and heartfelt love and support, to Warren's family and friends at this most difficult time."