Cumulative fatigue damage and life prediction theories: a survey of the state of the art for homogeneous materials

Prediction of non propagating cracks

Effects of defects, inclusions and inhomogeneities on fatigue strength

The paper presents an energetic approach useful to predict of the static and fatigue behavior of components weakened by sharp re-entrant corners. Despite the fact that stresses and strain energy density tend toward infinity at the point of singularity, the energy in a small volume of material surrounding the notch tip has obviously a finite value and such a value is thought of as the entity that controls the failure. The energy, averaged in a volume of radius R (which depends on the material properties), is a precise function of the Notch Stress Intensity Factors and is given in closed form for plane stress and plane strain conditions, the material being thought of as isotropic and linear elastic. The method is validated taking into account experimental data already reported in the literature, concerning both static tests carried out on polymethyl metacrylate (PMMA)and Duraluminium specimens and fatigue tests on welded joints and notched components in structural steels. As a matter of fact, the method proposed here is the re-formulation, on one hand, of some recent area/volume criteria (in which averaged values of the maximum principal stress are used to predict component fatigue limits) and, on the other, of N-SIF-based criteria, where the Notch Stress Intensity Factors are thought of as the parameters that control static and fatigue failures.

A finite-volume-energy based approach to predict the static and fatigue behavior of components with sharp V-shaped notches

Geometrical effects in fatigue: a unifying theoretical model

Previous studies have shown that both threshold stress intensity factors and fatigue crack growth rates are dependent on crack size The average growth rates for very short cracks considerably exceed those given by conventional stress intensity-crack growth laws fitted to long crack data Elastic and elastic plastic fracture mechanics solutions are modified to predict this behavior of short cracks by introducing an effective crack length U into the solutions for intensity factors and the J integral method of analysis The threshold stress at a very short crack length approaches the fatigue limit of the ma­terial, and therefore the value of k can be obtained once the threshold stress intensity factor and the fatigue limit are known The accuracy of the term k in predicting crack growth rates for short cracks is found to be independent of the applied strain level It varies linearly with the grain size of material and can be considered at surface as a measure of the reduced flow resistance of surface grains due to their lack of con­straint

Fatigue Crack Propagation of Short Cracks

Elastic and elastic-plastic fracture mechanics solutions are modified to predict the behaviour of short cracks. An effective crack length, l
0 is introduced into the solutions for both the linear elastic stress intensity factor and the J integral. Crack growth results for short cracks, in both elastic and plastic strain fields of unnotched specimens, when interpreted in terms of the modified solutions, show excellent agreement with elastic long crack data. The modified J integral solutions are extended to plastically strained notches, and the solutions obtained are tested in the correlation of data for growth of sort cracks near notches of varying severity with data for long crack under elastic loading. Although constant stress amplitude tests of these notches gave crack growth rate versus crack length curves which varied from monotonically increasing for blunt notches, to an initial decrease followed by an increase of sharp notches, all the data fell within the long crack data when correlated by the J integral solutions. Conversely, these solutions can be used to predict elastic and inelastic short crack growth curves for notches of various severities.

J integral applications for short fatigue cracks at notches

A Strain Based Intensity Factor Solution for Short Fatigue Cracks Initiating from Notches

This study deals with the fatigue of welded components emphasizing the use of small specimen data, a general stress-strain expression, the fatigue notch factor, and the effect of residual stresses. A computer-based fatigue damage analysis for welded structures is investigated; although accurate fatigue predictions have been made for notched structures, this type of analysis has not previously been applied to welded components. Fatigue crack propagation data are presented emphasizing the mean stress effect on the rate of cracking. Crack measurements have been made for both butt and fillet welded specimens in which the cracks commence from the toe of the weld. For fillet welded attachments and butt welds with substantial reinforcement, a residual stress adjustment was made based on the assumption that the maximum stress at the notch root is equal to yield stress. A fatigue damage analysis for welded specimens subjected to block loading using a computer-based damage model gave results close to experimental data. With a proposed stress function as the controlling stress parameter for crack growth, crack propagation data were successfully correlated.

K. N. Smith

Papers

Prediction of non propagating cracks

Fatigue Crack Propagation of Short Cracks

J integral applications for short fatigue cracks at notches

A Strain Based Intensity Factor Solution for Short Fatigue Cracks Initiating from Notches

Fatigue Life and Crack Propagation Analyses of Welded Components Containing Residual Stresses