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A B S T R A C T In this paper, the warm pre-stress (WPS) effect on the cleavage fracture of an 18MND5 (A533B) RPV steel is investigated. This effect, which describes the effective enhancement of the cleavage fracture toughness at low temperature following a prior loading at high temperature, has received great interest in light of its significance in the integrity assessment of structures, such as nuclear pressure vessels, subjected to thermal transients. Several loading cycles between room temperature (RT) and −150 • C are considered: Load-Unload-Cool-Fracture (LUCF), Load-Cool-Fracture (LCF) and Load-Cool with Increasing K-Fracture (LCIKF). All experiments complied with the conservative principle, which states that no fracture will occur if the applied stress intensity factor (SIF) decreases (or is held constant) while the temperature at the crack-tip decreases, even if the fracture toughness of the virgin material is exceeded. The experimental results indicate that an effective WPS effect is present even at small pre-load (K wps = 40 MPa √ m), and that a minimum critical slope (− K / T) in the LCIKF cycle has to be exceeded to induce cleavage fracture between RT and −150 • C. Numerical modelling was performed using mixed isotropic and kinematic hardening laws identified on notched tensile (NT) specimens, tested in tension to large strains (up to 40%), followed by large compressive strains. Detailed microstructural investigations on compact tensile (CT) and NT fracture test specimens were performed so as to determine the nature of the cleavage initiation sites, as well as the local mechanical conditions at fracture. Based on this local information, a new cleavage model was calibrated and applied to predict the probability of cleavage fracture after WPS: it is shown that the predictions are in good agreement with the experimental results.a 0 = crack length in CT toughness specimen (mm) d = distance (mm) K = SIF increment between RT and −150 • C (MPa √ m) (− K/ T) = transient slope of the LICKF cycle between RT and −150 • C (MPa √ m· • C −1 ) K, K el = stress intensity factor (MPa √ m) K frac = stress intensity factor at fracture (MPa √ m) K Ic , K Jc = mode I fracture toughness for the virgin material (MPa √ m)K wps = stress intensity factor after warm pre-stressing (MPa √ m)

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