Actually, to be precise, the
O-ring didn't rip. Unbeknowst to the designers and
engineers at
Morton Thiokol (producers of the
SRBs) the material used in the O-rings, while elastic by design to allow it to seal a flexible joint, stiffened and became
brittle at extremely
low temperatures. The Shuttle had been on the pad during an extremely cold rainstorm and cold temperatures before
launch. The 'best guess' from the evidence is that an O-ring failed after it was subjected to the shrinkage and
embrittlement caused by the cold when enough pressure was placed on it by the
rocket exhaust.
Richard P. Feynman advanced this theory to the investigating committee by dunking an O-ring into a pitcher of ice water, and then when the subject came up he removed it and snapped it in his hands.