Constant load fatigue tests with and without a single tensile overload were performed using the center cracked sheet specimens of two different thickness (2mm and 8mm) of a high tensile steel. The effect of specimen thickness on the crack propagation behaviour and the difference between the behaviours at the surface and the center of sheet were discussed. The conclusions obtained are summarized as follows:
(1) The dl/dn under constant load was independent of the specimen thickness in lower ΔK level, but the thick specimen shows higher dl/dn than the thin specimen in higher ΔK level.
(2) The effective stress intensity factor range ratio U measured on the surface under a constant load was not affected by the specimen thickness and U was constant (U=0.7) over a wide range of ΔK.
(3) The dl/dn at the surface of the thick specimen did not differ from that at the center.
(4) The both specimens showed the retardation behaviour following overload and the intensity of retardation was about the same, but the affected crack length l* was longer in the thin specimen.
(5) The retardation cycles Nd increased with the magnitude of overload and was larger in the thin specimen at a given stress range.
(6) The retardation behaviour at the center differed from the surface. The maximum intensity of retardation at the center was attained immediately after overload but with an interval at the surface, and the l* is longer at the center.
(7) By using ΔKeff, the range from the maximum intensity to the end of retardation was able to be arranged on dl/dn-ΔKeff relation under a constant load for both specimens. But, the range from the application of overload to the maximum intensity of retardation was not able to be arranged. This is because the effect of blunting due to overload exists in that range.