Criterion For Crack Nucleation Research Articles

On design cycles counting in regulatory fatigue strength estimation

A problem of design-based cycles counting for fatigue assessments of structures without initial cracks is considered in terms of requirements of strength analysis regulations for nuclear engineering components according to crack nucleation criterion. It is shown that a higher conservatism of these requirements for design stage analysis does not permit to apply the widely distributed rain-flow method. An alternative algorithm of cycles counting is presented. It satisfies the regulations requirements and is easy to realize in design practice.

Stress and fatigue analysis of fretting on a stressed specimen under partial slip conditions

AbstractA new Fretting on a Stressed Specimen (FSS) device has been conceived to help evaluate the influence of static stress on cracking during fretting wear in a sphere‐on‐plane contact. The mechanical analysis of the stressing state induced by such a loading has been performed for elastic conditions. It is completed by the analytical determination of multiaxial Dang Van's fatigue crack nucleation criterion. The theoretical developments are confirmed by tests on a well‐known low alloy steel. Analysing the stressing state of the maximum loading condition at the critical surface contact borders, it is shown that a simple uniaxial approach can be used to define the crack nucleation risk in fretting wear and FSS. Comparing the classic fatigue descriptions that define the admitted alternated stress amplitude as a function of the mean stress value, the VDI approach is found to be the most relevant. Satisfying the linear relationships between stress components, specific FSS relationships are introduced to fit the experiments better. Because of this analytical description, all materials, surface, geometry, and contact parameters can be connected.

Theoretical analysis of fatigue cracking under dry friction for fretting loading conditions

The development of wear maps has enhanced a new approach to the wear behaviour of materials as it is a precise description of the mechanical loading (local stress and strain fields).This paper studies crack nucleation using fatigue criteria in the case of an elastically loaded dry contact. Fretting behaviour is analysed for partial slip conditions which have been identified experimentally and theoretically (transition criteria developed using Mindlin's analysis).A Dang Van crack nucleation criterion is introduced for a ball-flat contact. Typical analytical expressions of the criterion were obtained on the surface and in the volume depending on the loading conditions and the contact system. They are expressed as a function of the mechanical properties, the friction coefficient, the contact geometries and the fatigue limits under both shear and tensile loadings of the material.The results obtained permit the investigation of the position and the direction of the first crack created by fretting conditions.

Application of minimum energy formalism in a multiple slip band model for fatigue—II. Crack nucleation and derivation of a generalised Coffin-Manson law

A situation of multiple parallel slip bands is considered. Each slip band is modelled as an accumulation of edge dipoles. The dislocation dipole density within the slip bands continually increases with fatigue cycling. The strain energy density within the slip bands consequently builds up with cycling until a critical cycle number when it becomes energetically favorable to nucleate a microcrack within one of the slip bands. This is proposed to be the crack nucleation cycle number. The above criterion based on the strain energy density is used to derive an explicit expression for the number of cycles for crack nucleation. It is shown that there is a parallel between this criterion for crack nucleation and other criteria that are based on the accumulation of a certain critical amount of damage. A generalised Coffin-Manson law for crack initiation is also derived. The size and most likely site of the just-nucleated crack is discussed.

Effect of stress concentrations on crack nucleation in 3% silicon-iron single crystals

Single crystals of 3% silicon-iron were deformed in tension in the [110]direction at temperatures from 77° to 293°K under conditions equivalent to plane strain. The crystals behaved in a ductile manner, but the mode of fracture depended on the testing temperature. In unnotched crystals tested at 77° or 123°K, a shallow neck formed, and fracture occurred by longitudinal cleavage nucleated in the center of the necked region. The effect of stress concentrations on crack nucleation was studied at several temperatures using crystals with deep or shallow notches, or a central hole. Longitudinal cleavage cracks were found to nucleate a small distance below the notch root on each side of the specimen. Crystals containing a central hole deformed in a manner, similar to unnotched crystals. Cleavage was always initiated at a small, spherical non-metallic inclusion. The nucleation of cleavage cracks (longitudinal and transverse) in the center of the necked region in unnotched crystals, below notch roots in notched crystals, and in the vicinity of the hole in crystals containing a central hole, suggest the existence of a critical tensile stress criterion for crack nucleation in 3 % Si-Fe single crystals. Metallographic evidence to support such a criterion is presented.

On the criterion for crack nucleation from a dislocation pile up

Abstract It is shown that recent calculations on crack size indicate an inadequacy in the usual crack nucleation condition.