Abradable Linings are usually made of a composite material which will be abraded away should the tip of a rotating blade touch the material. In flight the casings of an engine are subject to large changes in ambient temperature, so they will expand or contract. As we know the air temperature at 30,000ft is close to -50°C this would cause the casings to contract onto the rotor and the blades will then rub. To overcome this problem abrasive materials where used on early engines to wear down the tip of the blades, but this may cause balance problems. So most engines now use abradable linings that maintain minimum tip clearance but do not affect balance. They are usually found on the fan as this is the cold area of the rotating assemblies.