A Model for the Inclusion of Notch Plasticity Effects in Fatigue Crack Growth Analysis

Abstract

Notches or other stress concentrations are by far the most common sites for the initiation and growth of fatigue cracks in aircraft structures. The growth of these cracks is directly influenced by the material stress-strain response in the vicinity of the notch. Specifically, when the applied (remote) stress is sufficient to cause local plastic deformation at the notch, the response (local) stresses can no longer be found using elastic stress concentration factors, and they become dependent on the prior loading history. This is to say that the response stresses can no longer be treated as state variables. The occurrence of fatigue crack growth at notches which experience local yielding one or more times during their design lifetime is, in fact, quite common in many cyclically loaded structures. Some of the assumptions inherent in “traditional” Linear Elastic Fracture Mechanics (LEFM) based fatigue crack growth analysis may be inappropriate for such problems. In particular, the assumption that the stress distribution on a critical plane remains proportional to the elastic distribution throughout the loading history becomes incorrect when one or more of the applied loads causes plastic deformation and introduces or alters a residual stress field in this region. This paper first describes an elastic-plastic stress-strain response algorithm which may be used to estimate response stress distributions on a critical plane on a cycle-by-cycle basis. This is followed by a discussion of the manner by which stress intensity factors may be calculated based on these response stress distributions using Green's functions. Finally, the use of these stress intensity factors for the calculation of crack growth rate and, ultimately, crack growth life, is demonstrated.

The Variability of Fatigue in Notched Bars of IN100

“…It is clear that the stress-strain response of the notch drives the crack initiation and growth and this is often treated in an empirical manner [5]. After a small crack is initiated, the growth of the crack is affected by the notch plasticity and changes in the crack closure [6][7][8]. Recent work has considered the importance of small crack growth during the critical period between crack initiation and detection at an engineering size [8][9][10].…”

A Detailed Evaluation of the Effects of Bulk Residual Stress on Fatigue in Aluminum

“…The specific Green's functions that were used in the current study are described in refs. [12] and [13]. The total SIF is used to calculate fatigue crack growth rate which in turn is integrated to obtain the crack growth life.…”