Fatigue Crack Paths 2003

The modes of crack tip surface displacement may also be used to characterise crack

paths. It is well known [1, 11] that on a macroscopic scale, and under essentially elastic

conditions, fatigue cracks tend to grow in ModeI, so attention is often confined to this

mode. This applies to macrocracks (Stage II in Forsyth’s notation [9, 18]). A ModeI

crack path is not necessarily flat. On a microscopic scale fatigue crack surfaces in

metallic materials are generally very irregular, and modes of crack tip surface

displacement will usually differ from those observed on a macroscopic scale.

It was pointed out in 1953 [7], and recently re-emphasised [19], that on a

macroscopic scale there are two fundamentally distinct classes of fatigue crack growth;

maximumprincipal stress dominated, and shear dominated. Mode I fatigue crack

growth is maximumprincipal stress dominated. Shear dominated fatigue crack growth

is an important exception to the tendency to ModeI fatigue crack growth. It is often

observed when crack tip plasticity becomes significant, and this applies to microcracks

(Stage I in Forsyth’s notation [9, 19]. A shear dominated fatigue crack grows in ModeII

or ModeIII or in a combination of the two.

Tubular Welded Joints

Fatigue crack path data were collected [20] during a test on a tubular Y-joint of the type

shown in Fig. 4. The chord thickness was 16 mm, and the brace thickness 12.5mm. The

joint was madefrom BS4360: 50Dsteel, and it was tested as welded. During the test the

joint was immersed in artificial sea water, with cathodic protection applied. The brace

was loaded perpendicularly at its end, in the out-of-plane direction, using a variable

amplitude fatigue loading. One side was in tension only, and the other in compression

only. The chord was built in at its ends. Crack growth was in the chord at the toe of the

weld.

Figure 4. Tubular welded Y-joint. Dimensions in mm.

Figure 5 shows the crack front family obtained [21]. Crack surface lengths are

measured along the toe of the weld. Crack depths were recorded automatically using