Fatigue Crack Paths 2003

Figure 2. Crack surfaces of an L65 aluminium alloy lug subjected to programme fatigue

loading.

Figure 3. Modesof crack tip surface displacement.

C R A CGKR O W TM OH D E

A fundamental fracture mechanics concept is that of crack surface displacement [2, 11].

If a load is applied to a cracked body, the crack surfaces move relative to each other,

and there are three possible modes of crack surface displacement (Fig. 3). These are:

Mode I where opposing crack surfaces move directly apart; Mode II where crack

surfaces move over each other perpendicular to the crack front; and Mode III, where

crack surfaces move over each other parallel to the crack front. In fracture mechanics

the interest is in what happens in the vicinity of the crack front, and it is more correct to

refer to the crack tip surface displacement. By superimposing the three modes, it is

always possible to describe a general case of crack tip surface displacement.