Carbonization with Misfusion: Fundamental Limits of Carbon‐Fiber Strength Revisited

D-loops, a new type of structural defect in carbon fibers, are presented, which have highly detrimental effect on their mechanical properties and can define a new fundamental upper limit to their strength. These defects form exclusively during polyacrylonitrile carbonization, act as stress concentrators in the graphitic basal plane, and cannot be removed by local annealing.