According to Apple:
Certain usage patterns can contribute to the fraying or breaking of cables. Apple cables is subject to becoming weak or brittle if repeatedly bent in the same spot. Aim for gentle curves instead of angles in the cable. Regularly inspect the cable and connector for any kinks, breaks, bends, or other damage. Should you find any such damage, discontinue use of the cable.
Based on our experience, Apple does provide a cable replacement for free only if the device that included the cable or the cable itself (in case it was purchased separately) is under warranty period.