THE EFFECT OF STRESS RATIO AND FREQUENCY ON FATIGUE CRACK GROWTH

Abstract The influence of stress ratio and the loading frequency on fatigue crack growth rates in BS 4360–50C steel was investigated in laboratory air.
Fatigue crack growth tests were performed on compact tension specimens (CTS) made in two thicknesses 12 and 24 mm. Tests were conducted at two frequencies of 0·25 and 30 Hz, applying a stress ratio R varying from – 0·7 to 0·7. The results were analysed using the linear elastic fracture mechanics approach. They showed that the increase in both positive and negative R caused increased fatigue crack growth rates. Also an empirical effective stress intensity factor range, Δ K _eff, was found more appropriate to correlate the fatigue crack growth data than the Δ K factor frequently used in crack growth studies.
The loading frequency had only a little influence on crack growth rates at low R . However, at high R , growth rates were significantly higher at lower frequencies. It is suggested that this frequency influence may be associated with environmental effects, due to the embrittlement caused by hydrogen from the moist air, while the crack was fully open.