Fatigue in pearlite structures.

The basic mechanisms of fatigue failure were studied in a eutectoidcarbon steel. Three different structures were produced in the steel,these being coarse pearlite,;fine pearlite and spheroidised carbide.Pearlite structures were selected because of their practical importanceand also because multi-phase alloys have, until recently, receivedlittle attention from research workers.Specimens were fatigued either in reverse plane bending or uniaxialtension-compression conditions. In the latter case, constant amplitudestrain controlled tests were performed using a closed loop servo-hydraulic machine. Much of the testing was conducted under conditionsgiving failure beyond 10[5] cycles of stress.Cyclic properties and changes of deformation resistance during testing were determined. The major emphasis was on the observation of the development of fatigue damage during the life of the pre-polished specimens. A number of optical and electron microscopical techniques were used to examine damage.Fatigue damage appeared early in the life in all three structures.It existed as slip bands in the ferrite. The ferrite-cementite interface was shown to be the principal source of mobile dislocations.Damage was more localised in coarse than fine pearlite and all damage was associated with a very shallow surface topography.Cracks appeared at slip bands and ferrite-cementite interfaces.In coarse pearlite and spheroidised carbide they were detected at up to 25% of the life whereas in fine pearlite they could not be detected until about 75% of the life. Surface crack propagation rates were greater in lamellar than spheroidised carbide structures. The proportion of thefull fracture zone occupied by the fatigue crack area was greatest in the spheroidised structure and least in the coarse pearlite.