The Effect of the Laboratory Specimen on Fatigue Crack Growth Rate

One of the responses of a material to extreme forces, such as stress, temperature, etc., is to crack. A crack appears when the material reaches a limit in its capability to absorb damage and fails. Sometimes, a crack will grow under a periodically applied condition, such as cyclic loading, that are well below the stresses required to fail the material, denoted fatigue crack growth. Over the past thirty years, laboratory experiments have been devised to develop fatigue crack growth rate data that is representative of the material response. The crack growth rate data generated in the laboratory is then used to predict the safe operating envelope of a structure. The ability to interrelate laboratory data and structural response is called similitude. In essence, a nondimensional term, called the stress intensity factor, was developed that includes the applied stresses, crack size and geometric configuration. The stress intensity factor is then directly related to the rate at which cracks propagate in a material, resulting in the material property of fatigue crack growth response.