Thirtieth Index Card

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• EVER since the Titanic was discovered in the depths of the North Atlantic a dozen years ago, her steel plates melting into rivers of rust, expeditions have repeatedly probed the hulk. And investigators armed with a growing body of evidence have been working to solve riddles posed by the opulent liner's sinking.
• The ship, of course, was moving too fast through a sea of towering ice when it struck a large floe on its inaugural voyage in 1912. But much uncertainty has surrounded the exact nature of the damage and whether it might have been avoided in whole or part if the ship's design or construction had been different, perhaps preventing the loss of more than 1,500 lives.
• Now, after years of analysis and any number of false leads, experts say they have preliminary evidence suggesting that the Titanic, the biggest ship of her day, a dream of luxury come to life, may have been done in by structural weaknesses in some of her smallest and least glamorous parts: the rivets.
• Two wrought-iron rivets from the Titanic's hull were recently hauled up from the depths for scientific analysis and were found to be riddled with unusually high concentrations of slag, making them brittle and prone to fracture.
• ''We think they popped and allowed the plates to separate and let in the water,'' said William H. Garzke Jr., a naval architect who heads a team of marine forensic experts investigating the disaster.
• The rivet analysis, which Mr. Garzke and other experts said must be considered tentative because of the small number of rivets sampled, sheds light on findings made public last year. Experts, diving down nearly two and a half miles to peer through thick mud with sound waves, discovered that the Titanic's bow had been pierced by six thin wounds, the damage apparently done as hull seams were forced open. The finding laid to rest the myth that the iceberg had sliced open a 300-foot gash in the ship's side and strengthened interest in the possibility of rivet failure.
• The new analysis was done by Dr. Timothy Foecke, a metallurgist at the National Institute of Standards and Technology, a Federal agency in Gaithersburg, Md. It helps set industry standards and employs some of the Government's top metallurgists.
• Working with Mr. Garzke, who is chairman of the marine forensics panel of the Society of Naval Architects and Marine Engineers, a professional group based in Jersey City, Dr. Foecke analyzed the two Titanic hull rivets, cutting them in half and probing their composition with tools like microscopes and image analyzers. His work revealed an overabundance of slag, the glassy residue left over from the smelting of metallic ores.
• ''The microstructure of the rivets is the most likely candidate for becoming a quantifiable metallurgical factor in the loss of Titanic,'' Dr. Foecke concludes in a report, ''Metallurgy of the R.M.S. Titanic,'' to be formally released early next month.
• In the report and interviews, Dr. Foecke said the slag content of the rivets was more than three times as high as is normally found in modern wrought iron, making it less ductile and more brittle. While it is not clear whether a better grade of rivets would have saved the ship, he said, the developing evidence points in that direction.