The plates in the central part of "Titanic" hull (covering approximately 60 percent of the total) were held together with triple rows of mild steel rivets, but the plates in the bow and stern were held together with double rows of wrought iron rivets which were – according to materials scientists Tim Foecke and Jennifer McCarty – near their stress limits even before the collision. These "Best" or No. 3 iron rivets had a high level of slag inclusions, making them more brittle than the more usual "Best-Best" No. 4 iron rivets, and more prone to snapping when put under stress, particularly in extreme cold. Tom McCluskie, a retired archivist of Harland & Wolff, pointed out that , "Titanic"s sister ship, was riveted with the same iron and served without incident for nearly 25 years, surviving several major collisions, including being rammed by a British cruiser. When "Olympic" rammed and sank the U-boat with her bow, the stem was twisted and hull plates on the starboard side were buckled without impairing the hull's integrity.