A B S T R A C T An in situ optical microscopy fatigue testing is proposed in this paper to investigate the forward and reversed plastic zone size under cyclic loadings for Al-7075-T6. This experimental study is used to verify the hypotheses in a recently developed small time scale formulation of fatigue crack growth. During the testing, the entire cyclic loading cycle is divided into a certain number of steps. Images of the crack tip are taken at each step. The full strain field around the crack tip is determined using the digital image correlation (DIC) technique. The plastic zone size is obtained by combining the DIC results and the material constitutive relationship. Experimental measurements from the proposed study are compared with theoretical predictions. It is observed that the crack closure has a large effect on the reversed plastic zone size. The plastic zone size remains almost constant when the unloading path is below a certain stress level, which is one of the hypotheses used in a previous crack growth model. Discussions are given for the modelling of plastic zone size variation under cyclic loadings and several conclusions are drawn based on the current investigation. N O M E N C L A T U R Eσ = Applied stress K = Applied stress intensity factor K closure = Crack closure stress intensity factor σ closure = Crack closure stress a = Crack length K open = Crack opening stress intensity factor θ = Crack tip opening angle δ = CTOD K eff = Effective stress intensity factor range F = Geometry correction factor ρ f = Forward plastic zone size ρ r = reversed plastic zone size ε y = Tensile yield strain σ y = Tensile yield strength D = Thickness of the specimen W = Width of the specimen E = Young's modulus I N T R O D U C T I O NVarious approaches have been proposed in the past to correlate the fatigue damage with applied driving forces. EarCorrespondence: Y. Liu.

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