The effect of sheet thickness on the fatigue crack propagation behaviour was studied by using the specimens of low carbon steel with different thickness (2mm and 8mm) under the condition of stress ratio R=0 and frequency f=10Hz.
Under the constant load-amplitude-tests, the fatigue crack propagation rate was not affected by sheet thickness and the effective stress intensity factor range ratio, U, measured on the specimen surface increased with increasing stress intensity factor range, ΔK.
Both of the specimens indicated the delayed retardation behaviour following a single tensile overload application, and the affected crack length, l*, in the thin specimen was larger than that in the thick specimen. Retardation behaviour could be classified by the effective, stress intensity factor range, ΔKeff.
The midsection of sheet thickness in the thick specimen as well as its surface indicated the same delayed retardation behaviour. It seems that this behaviour depends strongly on the stress state at the crack tip.
The value of l* was markedly larger than the calculated plastic zone size. However, the hardened zone ahead of the crack tip measured by a micro Vickers hardness tester (25g) showed a good agreement with l*.
