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The work is an initial effort on adopting a statistical approach to correlate the fracture behavior between a notched and a fracture mechanics specimen. The random nature of cleavage fracture process determines that both the microscopic fracture stress and the macroscopic properties including fracture load, fracture toughness, and the ductile to brittle transition temperature are all stochastic parameters. This understanding leads to the proposal of statistical assessment of cleavage induced notch brittleness of ferritic steels according to a recently proposed local approach model of cleavage fracture. The temperature independence of the 2 Weibull parameters in the new model induces a master curve to correlate the fracture load at different temperatures. A normalized stress combining the 2 Weibull parameters and the yield stress is proposed as the deterministic index to measure notch toughness. This proposed index is applied to compare the notch toughness of a ferritic steel with 2 different microstructures.

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A B S T R A C T During cyclic loading of austenitic stainless steel, microstructural changes occurred, which affected both mechanical and physical properties. For certain steels, a strain-induced martensitic phase transformation was observed. The investigations showed that for the given material and loading conditions the volume fraction of martensite depended on the cycle number, temperature and initial material state. It was found that the martensite content continuously increased with the cycle number. Therefore, the volume fraction of martensite was used for indication of the fatigue usage. The temperature dependence of the martensite formation was described by a Boltzmann function. The martensite content decreased with increasing temperature. Two different heats of the austenitic stainless steel X6CrNiTi18-10 (AISI 321, DIN 1.4541) were investigated. The martensite formation rate was much higher for the cold-worked material than for the solution-annealed one. All applied techniques, neutron diffraction and advanced magnetic methods allowed the detection of martensite in the differently fatigued specimens.Keywords austenitic stainless steel; lifetime extension; low-cycle fatigue; non-destructive testing; nuclear safety; strain-induced martensite. N O M E N C L A T U R Ea = lattice constant A, B, t o = parameters of the Boltzmann function for data analysis bcc = body centred cubic lattice B x , B z = axial and radial components of the magnetic induction c α , (c α ) o = volume fraction of martensite for the fatigued (strained) and initial material state D = usage factor, ratio of the applied cycle number to an averaged cycle number representing the technical crack initiation DMC = neutron diffractometer for cold neutrons used for determination of the volume fraction of martensite EC = eddy current fcc = face centred cubic lattice FERROMASTER ® = device for measurement of the magnetic susceptibility or permeability FLUXGATE = high sensitive magnetometer for measuring the remaining magnetic induction (remanence) GMR = Giant Magneto-Resistance sensor used for eddy current impedance measurement with high local resolution ICP-OES = inductive coupled plasma emission photometry used for the determination of the chemical composition of steels LCF = low-cycle fatigue M d = starting temperature for the strain-induced martensite formation

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