Fatigue Crack Paths 2003

The purpose of this introductory paper on fatigue crack paths in metals 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.

C R A CPKA T HD E S C R I P T I O N

In describing the path taken by a growing fatigue crack it is necessary to describe a

crack growth surface which contains successive positions of the crack front, and also a

family of lines on this surface which describes successive positions of the crack front.

As a mathematical concept, a crack in an unloaded body is a cut which has zero

width. It is therefore possible, without ambiguity, to speak of a single ‘crack surface’,

and to describe its shape [11]. However, if a load is applied to the body the two

opposing crack surfaces move relative to each other, and it may be necessary to

distinguish between these ‘upper’ and ‘lower’ crack surfaces. It may also be necessary

to make this distinction for a physical crack, which is not necessarily of zero width,

even in a stress free body. The distinction is unnecessary if the scale of observation is

such that it is impossible to distinguish between the two opposing crack surfaces.

The two opposing crack surfaces meet at a crack front. There is usually no ambiguity

in describing its shape, except at a very small scale where the positions of individual

atoms have to be taken into account. Describing a crack growth surface in terms of

successive crack front positions avoids the need to distinguish between the two crack

surfaces.

In practice, experimental crack growth surfaces may be determined by examining

one of the crack surfaces. Irregular surfaces, such as crack surfaces, cannot be

adequately characterised unless the scale at which measurements are made is first

specified in some way [5]. In theory, there are difficulties in describing a crack growth

surface at a particular scale of observation [15], and subjective judgements are needed.

In practice, it is usually reasonably straightforward, if somewhat tedious. Modern image

processing techniques, in conjunction with a stereo pair of scanning electron

micrographs [16] make an automated approach possible. Recent developments in digital

camera technology [17] now permit the extraction of three dimensional information

using a monomicroscope.

Aluminium Alloy Lug

Figure 2 shows the crack surfaces of an 84 m mwide L65 aluminium alloy lug, part of

an aircraft structure, which has been subjected to programme fatigue loading. Both

halves of the lug are shown. The programme included both ground to air and gust loads.

The crack surfaces have the banded appearance characteristic of programme loaded

specimens, with each band corresponding to one load programme. Different features in

the bands correspond to different load levels, and the bands provide a crack front

family.