A body subjected to strain a number of times does not obey Hooke's law due to 

According to Hookes law stress is directly proportional to strain. But when a material is subjected to strains alternatively there is a loss of strength of the material called Elastic Fatigue. Due to elastic fatigue the behavior of the body shows that of a less elastic body. When fatigue is removed from an elastic body it regains the original degree of elasticity when allowed to rest for a while.