Hi
Apple cables (including the Lightning to USB cable included with iPhone) are covered against defects in materials and workmanship under Apple's One-Year Limited Warranty.
If a cable fails under normal use, contact Apple Support (mail-in service may be available) or make a Genius Bar reservation with a view to having it checked and, if agreed by Apple, replaced under warranty:
Note that the warranty does not cover accidental damage, nor does it cover damage caused by misuse or using the cable outside Apple's guidelines.
Apple's handling instructions for the Lightning to USB Cable include:
"Certain usage patterns can contribute to the fraying or breaking of cables. The Lightning to USB Cable, like any other metal wire or cable, 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 Lightning to USB Cable."
http://help.apple.com/iphone/9/#/iphbbe12ba1
"When you unplug a cable from your product, a power source, or an AC adapter, hold on to the connector at the end of the cable."
Using Apple power adapters, cables, and duckheads with Apple products - Apple Support