Fatigue Crack Paths 2003

schematic section through a twist crack at a scale of observation of around 0.1 mm.The

crack growth direction is perpendicular to the paper. The twist crack consists of narrow

ModeI facets connected by irregular, predominantly ModeIII cliffs. It is produced by a

combination of maximumprincipal stress dominated crack growth and shear dominated

crack growth [31].

Figure 15. Three point bend slant notch specimen. Dimensions in mm.

Figure 16. Twist crack in a mediumstrength structural steel.

Some laboratory scale fatigue tests on a medium strength structural steel [32]

illustrate the development of twist cracks. The tests were carried out on three point bend

slant notch specimens (Fig. 15). Figure 16 shows a typical crack surface (β = 60°). The

expected tendency to ModeI fatigue crack growth was observed on two distinct scales.

On a scale of 1 m mcrack fronts were approximately straight, and initially fatigue crack

growth was mixed mode. As a crack grew the crack front rotated until it was

perpendicular to the specimen surfaces, and fatigue crack growth was in ModeI. Onthis

scale the crack follows a curved path which tends towards a plane of symmetry as

shown schematically in Fig. 17. This is in accordance with the well knownobservation

[1], that the tendency to Mode I crack growth means that cracks tend to grow

perpendicular to the maximumprincipal tensile stress. More generally, it has been

observed [33] that fatigue crack paths often tend towards a particular plane. Attractor is

an appropriate modern nonlinear dynamics term for such a plane [34] in a physical

rather than a state space sense.