A TB that 100,000? cycles lifespan for a final failure must be way over designed for the task, in that with the increasing stress concentration of a continuously growing crack, combined with the decreasing load bearing cross section area in play, means the bar has to be way over designed to endure the expanding crack until the final failure. I am not sure how much safety factor is built in intentionally to delay final crack initiated failures, which once begun, must be the final outcome.