Initiation and Growth of Cracks Near Fatigue Threshold in Plain Carbon Steels

Fatigue crack initiation, short crack growth behaviour and fatigue thresholds of short and long cracks of a few plain carbon steels (0.003%C, 0.14%C, 0.25%C, 0.47%C and 0.62%C steels) have been studied. The phenomena of the formation of
small cracks in 0.003%C (single phase) and 0.14%C (multiphase) steels have been studied on dumble-shaped plate type specimens under varied cyclic stress amplitudes
at the load ratio of R = 0. Simple experimental techniques have been developed with new specimen configurations to examine short crack growth behaviour vis-à-vis
microstructural features of materials and to determine the fatigue thresholds using a rotating bending machine. The fatigue thresholds have been estimated by load drop
procedure and each crack profile is recorded together with the associated microstructures during these studies. The microstructures of the investigated steels have been characterized using optical and scanning electron microscopes. The influence of the microstructural features of these steels on the near threshold crack
growth has been examined. Conventional mechanical properties like hardness and tensile parameters have been determined following standard procedures.
Studies on crack initiation revealed that ferrite-pearlite interfaces and/or
ferrite-ferrite grain boundaries are preferred crack initiation sites in the investigated
steels. The lengths of the initiated cracks at ferrite-pearlite interfaces or at grain
boundaries are usually larger in size compared to those of grain body cracks. The
formation of slip band inside the grain body, slip band impingement at grain boundary
and elastic-plastic incompatibility are attributed as the main causes for crack
initiation. The formation of irregular voids inside slip bands, initiation and growth of
small voids at grain boundaries and subsequent joining of these are some interesting
observations for fatigue crack initiation in 0.003%C steel. It is observed here for the
first time that the angle between the direction of banding and the loading axis, has
pronounced effect on the orientation and on the size of initiated cracks.
Studies on short crack growth have demonstrated that the developed technique
(with new specimen configuration) can be used satisfactorily to understand short
crack growth behaviour and to determine the transition length from short to long
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cracks. The estimated critical crack lengths can be considered as function of the
nature of the short cracks. Short crack growth rate decreases or gets arrested with
increasing crack length under the same applied crack driving force. The fatigue
threshold values obtained from short crack growth experiments are found to lie in a
narrow range and are closer to long crack thresholds. The cracks were found to have
affinity to grow through interfaces, but the length of a crack passing through a
specific phase was found to be approximately proportional to its amount.
The developed technique to determine fatigue thresholds using a rotating
bending machine yielded reliable results. The fatigue thresholds of four steels with
carbon contents 0.003, 0.25, 0.47 and 0.62% were found to be 3.8, 4.2, 5.8 and 6.0
MPam1/2 respectively. The developed procedure is based on the measurement of
da/dN with high sensitivity of the order of 10-12 m/cycle and the obtained threshold
values appear to represent effective fatigue thresholds determined by conventional
technique. Analysis of the crack growth behaviour near the notch indicates occurrence
of non-propagating cracks only in 0.25%C and 0.47%C steels. The occurrence of nonpropagating
cracks has been attributed to the presence of some secondary stress field
at the crack tip. Fatigue threshold at R = -1 for plain carbon steels increases with
increasing carbon content or the harder pearlitic constituent.
A new model for short fatigue crack growth has been proposed to overcome
some of the limitations of the earlier models. The model accounts the effect of mixed
mode loading and the surface characteristics of a specimen, and describes both short
and long crack growth. A series of computations have been carried out to examine the
influence of different parameters considered in the model to describe short crack
growth rate. The generated experimental results as well as some reported results have
been satisfactorily described using the proposed model.