Not all cables have proper strain relief built to the connector ends. This is particularity true for some of the iPhone 6 USB to Lightning™ cables. Take this Anker branded iPhone cable for example, after several months’ normal use the cable towards the USB end started to show signs of excessive strain (see image below). The internal wiring would soon be damaged if the strain on this area continues.
Unfortunately, you cannot just add a commercially available strain relief to a preformed cable as the strain relief typically needs to be fitted on the the cable prior to the terminal being crimped on. So for these injection molded cables, we will need be more creative.
Here is where heat shrink tubing comes to rescue. Heat shrink tubing has been routinely used as strain relief. But due to the limited expansion ratio, you cannot use one with too large of a diameter (e.g. one that can fit through the USB connector). But not all hope is lost, the trick here is to first cut along the tube so that we can slide the cable in via the opening (see below):
Then we can carefully super-glue the opening shut. After the super glue dries, we can then use a hot air gun to shrink it down just as we would a normal piece of heat shrink tubing. And now we just beefed up the strain relief on the existing USB cable:
The same approach can be applied to pretty much any preformed cables.