In the paper the fracture mechanisms in ultra-high-strength steel are examined. However, the emphasis is on cleavage fracture, which was observed in the whole temperature range tested. The extent of cleavage depends on the specimen thickness and temperature. The experimental program consisted of tensile and fracture tests and was followed by scanning microscope observations of the fracture surfaces. Then, a full 3D elastic-plastic finite element analysis was carried out assuming finite strains. The numerical results support the postulate that the onset of cleavage jump is observed when the maximum opening stresses become higher than the critical value (the material property at a given temperature) over a distance greater than the critical value (material property independent of temperature). A discussion of so-called 3D stress parameters is presented.

AnAlysIs of stress And strAIn fIelds In And Around InclusIons of vArIous shApes In A cylIndrIcAl specImen loAded In tensIon.A numerical analysis is performed of the stress field in and around inclusions of various shapes. Inclusions both stiffer and more compliant than the metal matrix are analysed. The critical stresses required for inclusion fracture are estimated after observation of cavities and inclusions by scanning electron microscopy. Real inclusions were observed after performing uniaxial loading to different amounts of overall strain. The material tested was Hardox-400 steel.

The values of JIC measured in laboratories, when used in practical applications, may lead to conservative results in assessing the critical state of a structural element. When the thickness of the structural element is less than required by standards the fracture toughness can be much higher than measured in the laboratory. In this paper the fracture toughness of brittle or semi-brittle materials is discussed. The out-of-plane constraint is characterized in this paper by other quantity then the specimen thickness. This quantity can be related to the thickness if the thickness is uniquely defined.

An analysis is presented of the stress field in and around inclusions of various shapes. Results were obtained by the finite element method. Inclusions were located within elementary cells located at the centre of the specimen next to the crack front. The influence of an in-plane constraint on the stress distribution was tested.

In the presentation the cellular automata theory is applied to model the evolution of cleavage and ductile fracture mechanisms in front of the crack in three-point-bent specimens containing crack, made of Hardox-400 steel. The experimental and numerical results were used to define: dead elements, weakened elements and healthy elements. One element in the cellular automata model represents one grain. In some of elements large inclusions/interstitials are present. They are randomly distributed; however, to some degree the distribution of these inclusions depend on the strain level. Each element of the cellular automata can change the actual state due to the primary stress distribution, orientation of the cleavage planes {100} and the neighborhood of the fractured grain. The computer program allows for reproduction of the fractured surface as well as to simulate fracture mechanisms in various geometrical and temperature conditions.