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The results of investigations into fatigue crack behaviour of laser-hardened, medium-carbon steel specimens under reversed bending ( R = -1) are presented. Characteristic features of crack behaviour, such as crack source location, temporary crack arrest at the border between laser tracks and the matrix material, and discontinuous crack growth in the laser tracks are discussed. In order to explain the sites of primary crack sources, results of residual stress measurements are included as well as analytical descriptions of these stress distributions as a function of sample depth. A mechanism of cracking has been established on the basis of fractographic analyses of fracture surfaces. NOMENCLATURE c =carbon content in a steel E = Young's modulus f = martensite volume fraction K = stress intensity factor I = crack length N = number of cycles Tp = field of peak temperature in a sample V k , V t = specific volume fraction of martensite and austenite, respectively x = specimen longitudinal direction y = specimen transverse direction z = depth a = linear thermal expansion coefficient v = Poisson's ratio C T~, ub,, CTG = principal residual stresses uth = thermal residual stresses utr = phase transformation residual stresses u; =yield stress of austenite

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