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NASA engineers think they might have found the root cause of the cracks that have been appearing on the shuttle external tank. The cracks appear on structural units called stringers. Key quote:
“Some material used for the stringers was found to be ‘mottled,’ with a different surface appearance than the standard material. Testing revealed this mottled material had lower fracture toughness than the nominal material and exhibited unstable crack growth. All of the cracks found during tanking as well as cracks fixed during manufacturing were located on stringers made with this mottled material.”