Fatigue Crack Paths 2003

Introduction to Fatigue CrackPaths in Metals

L. P. Pook1

1 University College London

ABSTRACT.As is well known many engineering structures and components, made

from metallic materials, contain cracks or crack-likeflaws. It is widely recognised that

fatigue crack growth must be considered both in design and in the analysis of failures.

The complete solution of a fatigue crack growth problem includes determination of the

crack path. Macroscopic aspects of fatigue crack paths have been of industrial interest

for a very long time. At the present state of the art the factors controlling the path taken

by a growing fatigue crack are not completely understood. In the last four decades there

have been substantial advances in the understanding and prediction of fatigue crack

paths, largely through developments in fracture mechanics and in the application of

modern computers and microscopes. The purpose of this introductory paper is to set the

scene for the more detailed papers which follow. To do this some of the more important

ideas are presented and illustrated by examples. These have been chosen to illustrate

some of the more important aspects of fatigue crack paths in metals.

I N T R O D U C T I O N

As is well known [1] many engineering structures and components, made from metallic

materials, contain cracks or crack-like flaws. It is widely recognised that fatigue crack

growth must be considered both in design and in the analysis of failures [2, 3]. The

complete solution of a fatigue crack growth problem includes determination of the crack

path. At the present state of the art the factors controlling the path taken by a growing

fatigue crack are not completely understood. In general, fatigue crack paths are difficult

to predict and, in practice, fatigue crack paths in structures are often determined by large

scale structural tests [2].

Crack tip features associated with the growth of a crack, may be viewed at different

scales [4], as shown in Table 1. All these scales are of interest in the consideration of

fatigue crack paths. To put these scales in perspective, reading this paper is at a scale of

observation of about 0.1 mm, viewing television at a scale of about 1 mm, and walking

about a room at a scale of about 10 mm. Present day fracture mechanics is largely

concerned with macroscopic aspects of crack growth, corresponding to the three largest

scales in the table [2]. This is why crack surfaces are often assumed to be smooth, even

though on a microscopic scale they are generally very irregular. The different results

obtained by viewing at different scales is typical of a wide range of physical phenomena

[5].