The fatigue experiments were carried out on a MTS-810 machine. The specimens used had the dimensions; length = 150 mm, width = 20 mm and thickness = 10 mm. A 3 mm deep V notch was created in the centre of the edge of the specimen. The specimen surface was ground and polished. To facilitate measurement of crack extension on surface during fatigue loading a number of lines were scribbled in the notch region.

The specimens were subjected to bending loading under sinusoidally varying tension-tension load cycle. A stress ratio of 0.1 was maintained and loading frequency of 15 Hz was used. A load control mode was used to achieve constant load amplitude throughout the experiment. Acoustic emission sensor was fixed over the specimen, close to the notch and signal was picked up right since the beginning of each test. The various parameters of acoustic emission such as ring down count, event duration, peak amplitude, energy, rise time and slope were displayed and stored simultaneously.

Ring down count, peak amplitude and energy were plotted against number of loading cycles and also as a function of crack growth. Similarly the crack growth was also plotted as a function of the stress intensity factor range of the fatigue loading. The Paris equation, giving a relationship between the fatigue crack growth rate and the stress intensity factor range was derived for the steel as follows:

da/dN = 1.7 x10^-7K^{1.2 8}

Where da/dN is the crack growth in mm per cycle and
K is the stress intencity factor in kg mm ^-3/2

In addition, the acoustic emission parameters namely energy, peak amplitude and ring down count were also correlated with the stress intensity factor range as given below:

dE / dN = 8.74x10-6 K^1.6

dPA / dN = 3.5x10-5 K^1.35

dRDC / dN=9.6x10-5 K^0.95

where dE/dN is energy per cycle, dPA/dN is peak amplitude per cycle and dRDC/dN is ring down count per cycle from this it has been possible to develop the relationships between the crack growth rate and the rate of emission parameters; energy and peak amplitude as follows:

The present study has revealed that the energy is the best parameter to monitor fatigue crack initiation and its propagation behaviour in plain carbon steels. Out of the three acoustic emission parameters investigated, the ring down count is considered to be of least significance for the monitoring of fatigue crack initiation and propagation in these steels.