Fatigue Crack Paths 2003

Aluminium Alloy Specimen

Under microscopic examination the most striking feature of many of the fracture

surfaces created by a growing fatigue crack is the presence of distinct line markings,

parallel to each other, and normal to the local direction of crack growth. Figure 11 is an

example on a fatigue crack surface of an Al-7.5 Zn-2.5 M g specimen tested under

constant amplitude loading [1]. The figure was derived from Fig. 2.4 of Ref. [26]. These

lines are called striations, and each striation corresponds to one load cycle. Hence the

distance between striations is the amount the crack front has movedforward during one

cycle, and striations are examples of crack front families. The scale of observation in

Fig. 11 is about 10-3 mm.In effect, the crack surface has been projected onto a plane, so

some detail has been lost [15].

Figure 11. Crack front family on a fatigue crack surface of an Al-7.5 Zn-2.5 M g

specimen.

Slant Crack Growth in Thin Sheets

The slant crack growth which sometimes takes place in thin sheets is an important

exception [1, 27], to the tendency to ModeI crack growth. It is a special case in that it

takes place in isotropic, homogeneous materials under essentially elastic conditions.

Slant fatigue crack growth is sometimes stated to be mixed Modes I and III, but this is

only true for the sheet centre line. Awayfrom the centre line it is mixed Modes I, II and

III [28].

Slant fatigue crack growth is usually observed following a transition from an initial

maximumprincipal stress dominated crack. The main features of the transition are

shown [29] in Fig. 12. The transition starts with the appearance of shear lips. Shear lips

increase in size as crack growth proceeds until they either reach a maximumsize or in

'thin' sheets they meet completing the transition. After the transition the crack growth

surface is flat, and its inclination to the sheet surface is approximately 45°. A large