The rubber material is one matter whose applications are mostly exposed to huge amount of heat, cold weather, and humidity and temperature changes. Then rain moisture dust and other contaminants. As a result of prolonging facing these environmental factors what happens is a crack develops on the surface and these crack just not stop there they grow and grow.
To stop this you must conduct flex testing. This is mostly applied in shoe soles, as the rubber sole which is daily used on roads. It comes in contact with a maximum level of flexing and friction they must not crack easily or the customer list will down by bringing down the brand too