Propagation of fatigue cracks from notches

Abstract

The traditional design approaches to fatigue at notches, based on stress level–endurance relationships, are briefly reviewed. It is shown, by considering crack propagation from notches and invoking a change in control mode from notch plasticity to crack-tip plasticity, that a critical stress condition can be obtained which must be exceeded if the crack is to propagate to failure. The traditional techniques are then reinterpreted and explained by this propagation method. An example is given of crack growth from a sharp defect at a weld toe. It is shown that the integration of an elastic fracture mechanics growth law can reproduce stress range–cycles to failure data for this situation. There are, however, complexities of stress analysis and crack shape. A simple treatment of residual stresses affecting the threshold and slow–growth regimes, shows some promise as a technique for accounting for residual stresses.

MST/70     

Propagation behaviour of microstructural short fatigue cracks in the high-cycle fatigue regime

In the high-cycle fatigue regime, it is assumed that crack initiation mechanisms and short fatigue crack propagation processes govern fatigue life of a component. Moreover, it is now becoming accepted that the conventional fatigue limit does not imply complete reversibility of plastic strain and is connected to crack initiation. However, interaction of the crack tip with microstructural barriers, such as, e.g. grain boundaries or second phases, leads to a decrease and eventually to a stop in the crack propagation. In the present contribution, examples for propagating and non-propagating conditions of short fatigue cracks in the microstructure of a duplex steel are given, quantified by means of automated EBSD. To classify the results within the scope of predicting the service life for HCF- and VHCF-loading conditions, a numerical model based on the boundary element method has been developed, describing crack propagation by means of partially irreversible dislocation glide on crystallographic slip planes in a polycrystalline model microstructure (Voronoi cells). This concept is capable to account for the strong scattering in fatigue life for very small strain amplitudes and to contribute to the concept of tailored microstructures for improved cyclic-loading behaviour.     

Atomic simulation of cracks under mixed mode loading

A discrete atomic model of a crack tip in iron under mixed mode loads is examined. The results indicate that the behavior of the crack at the atomic scale as a function of the ratio of mode I to mode II component of load is quite complex. In general, crack tip plasticity appears to increase as the mode II component of load increases.

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Résumé On examine un modèle atomique discret de l'extrémité d'une fissure dans du fer soumis à des charges de mode mixte. Les résultats indiquent que le comportement de la fissure à l'échelle atomique en fonction du rapport des composantes de charge du mode I sur celles du mode II est totalement complexe. En général, la plasticité à l'extrémité d'une fissure parait s'accroitre lorsque la composante de mode II de la contrainte s'accroit.

     

Dugdale crack closure analysis of fatigue cracks under constant amplitude loading

Fatigue fracture mechanics which means structural analysis including crack propagation and crack closure in conjunction with a local failure criterion provides detailed insight into mechanisms of cyclic loaded crack sheets. Using the Dugdale method and rigid-perfectly plastic strip material law the infinite sheet with colinear cracks was parametrically investigated in respect of: SSY limits, crack closure occurrence, formation of contact stresses and displacements, influences of material parameters, $\text{R-}\text{ratio}$ and maximum load on effective stress ratio. Rationales for the load ratio, mean load and maximum load dependence and the form of $\text{da/dN-}\text{curves}$ are given based on crack closure analysis.     

Short surface fatigue cracks growth under constant and variable amplitude loading

This paper deals with the fatigue strength of S355NL steel, of a common use within the shipbuilding industry, under uniaxial constant and variable loading. Indeed, ship structures are subjected to variable loading due to various sea states, wind and waves. As a consequence, a better knowledge of fatigue behavior under real loading conditions is needed. This study aims at analyzing the influence of loading conditions (load ratio and variable amplitude loading) on the short crack behavior and last, with a proposed model to assess the fatigue crack life. The tools used to prepare inspections in critical areas only take into account the long crack behavior. The results from the proposed model were compared to the assessments these tools are providing with.     

The growth of fatigue cracks at high temperatures under predominantly elastic loading

The extent of applicability of linear elastic fracture mechanics (LEFM) to high temperature fatigue-crack growth in $\text{1}\text{2}$ and 1% Cr-Mo-V turbine-casing steels has been established using a range of specimen geometry and thickness. Crack growth rates exceeded those at room temperature and generally increased with declining frequency. The relation for crack growth was: dL/dN = CΔK², where L was cracuength, N cycle number, C a frequency-dependent term and ΔK the stress intensity amplitude. There was little effect of increasing the maximum stress intensity of the cycle (or mean stress level) provided widespread creep deformation was avoided. A limiting crack tip displacement (C.O.D.) for LEFM analysis was established for the cast Cr-Mo-V steels examined.The relative contributions of creep and oxidation to fatigue-crack growth were examined. Oxidation was found to predominate at high frequencies and low amplitudes, whilst creep effects were significant at low (10^?3 Hz) frequencies.     

Propagation of fatigue cracks from surface flaws in austenitic Type4 316 butt welds

Fatigue crack propagation from surface flaws in butt-welded Type 316 joints has been studied at 550°C. The effect of position of the flaw, mean stress and post-weld heat treatment have been included together with an assessment of the validity of using small compact tension specimens to predict rates of growth in large joints.

It is implied that the residual stress arising from welding is not necessarily additive to the applied stress during fatigue crack propagation in austentic weldment. However, appropriate care should be exercised over simulating the degree of constraint existing in a given situation.     

Nucleation and growth of small fatigue cracks in filled natural rubber under multiaxial loading

This paper presents and discusses observations of crack nucleation and small crack growth in a filled natural rubber compound subjected to multiaxial loading. A hollow cylindrical specimen was used in which simultaneous axial and shear strains are produced. The loading path types investigated include axial, torsion, proportional axial-torsion, and non-proportional axial-torsion loadings. It is shown that cracks appear and grow in a particular orientation during a typical fatigue test. The nature and evolution of these cracks with applied cycles were studied by direct observation. The observed failure plane behavior is compared to predictions based on the calculated cracking energy density. Effects of crack closure, load phase angle, crack density, and crack face shearing (mode II), as well as aspects that distinguish between “nucleation” and “growth” processes in rubber are discussed.     

Propagation of short fatigue cracks from notches in a Ni base superalloy: experiments and modelling

A notched specimen containing a semicircular slot (0.1 mm deep) was designed to simulate the growth of three-dimensional short cracks under a stress concentration. Fatigue tests were performed on N18 superalloy at 650 °C with trapezoidal loading cycles. A high-resolution optical measurement technique proved to be capable of detecting half-surface crack increments as small as 10 μm, and the potential drop method was found to be inappropriate for very small crack lengths. The stress intensity factor, Δ K , was calculated using a weight functions method. Non-uniform stress fields were determined by FEM modelling using elasto-viscoplastic constitutive equations. The plasticity-induced crack closure effect was calculated within the specimen using viscoplastic FEM modelling. The prediction of crack aspect ratio was used to investigate differences of closure along the crack front. The role of notch plasticity on these differences is discussed. Using these calculations, it is shown that the apparent differences between the growth behaviour of short and long cracks can be largely accounted for.     

Propagation of physically short cracks in a bainitic high strength bearing steel due to fatigue load

Physically short cracks in a bainitic high strength bearing steel were fatigue loaded. The rapid propagation rate of early open short cracks agreed with that of long closure free cracks. After some rapid growth, the short cracks entered a transition period to the rate of growth limited long cracks. Potential drop showed that the short cracks were open to the tip throughout the growth sequence, which excluded crack face closure in the wake as the growth limiting mechanism in this material. Instead the short crack effect was related to residual stresses and other mechanisms at the crack tip. Crack manufacturing procedures were determined for straight long and short start cracks in the present material. LEFM with effective material parameters and limit compensation predicted the short crack lives.     

Propagation of physically short fatigue cracks and the life of structural elements. Report 1

A relationship for describing the kinetics of short fatigue cracks is proposed and experimenteally confirmed on the basis of the energy two-parameter criterion of fracture. It is established that with use of this relationship it is possible to quantitatively describe the influence of the concentration and asymmetry of the stress cycle and the role of the compressive portion of the cycle and the surface layer on the kinetics of a crack and the threshold values K_th.Translated from Problemy Prochnosti, No. 9, pp. 3–8, September, 1990.     

Propagation of physically short fatigue cracks and the life of structural elements. Report 2

Analytical relationships between the characteristics of fatigue and the crack resistance of a material are obtained on the basis of the relationship for describing the kinetics of short fatigue cracks. In the limiting case assuming the size of the original crack to be equal to a structural element expressions were determined for the Kitagavi-Takahashi diagram, the Coffin-Manson type relationship (in an energy treatment), and the limiting amplitude diagram (Goodman type). A relationship is proposed between the threshold stress intensity factor and the structure of the material.Translated from Problemy Prochnosti, No. 9, pp. 8–11, September, 1990.