The 8-foot-long crack discovered in January in one of four giant steel beams supporting the roof of the Filene Center at Wolf Trap was caused by a welding flaw and severe cold, according to a preliminary report issued yesterday by the Wolf Trap Foundation.

The findings -- the first formal indication of what might have caused the huge fissure in the roof -- were based on nearly a month's testing by John W. Fisher, an engineering professor at Lehigh University and an authority on material fatigue and fracture. Fisher was hired by the foundation to do the study.

Fisher's study does not explicitly assign blame for the fissure, but it does dwell on problems in the beam's fabrication. Officials of the beam fabricator, Globe Iron Construction of Norfolk, could not be reached for comment yesterday.

Wolf Trap Foundation officials still say they expect repair work on the sagging Filene Center roof will be complete before the scheduled opening of the 1985 season on June 8.

The welding flaw took the form of a tiny discrepancy, similar to a crack, embedded in a weld connecting two 20-foot-long backing bars inside the hollow, 130-foot-long girder that ultimately gave way, according to the study. In addition to the crack, the material used in the weld was not tough enough, the study said, rendering the weld too weak to withstand the cold weather of mid-January, which rendered the steel brittle. The combination of these factors caused the break, the study concluded.

The report said that similar flaws in welding may exist in structural beams throughout the structure, and recommended that repair work be carried out throughout the $18 million performing arts center.

Fisher declined to say who was at fault in the failure, but added: "You should not be fabricating cracks of that type into the structure. It was not an acceptable . . . fabrication. I'm certain they Globe Iron were not aware of it."

The Filene Center was reopened last summer after fire destroyed the original structure in April 1982. After the fissure was discovered by workers Jan. 24, workers erected scaffolding to buttress the roof.

The Fisher report could help determine who pays to repair the sagging roof of the Filene Center.