From SKYbrary Wiki
This curve describes the typical equipment failure rate against time since fit and takes the shape of a bath tub cross section. The initially very high failure rates due to ‘infant mortality’ are followed by very low failure rates typical within the useful or design life of a component. With increasing time, this is followed by a period of progressively increasing failure rates as ‘wear-out’ takes over. This finding is the basis of maintenance Reliability work.