Fatigue Crack Paths 2003

alternating current potential drop (acpd) equipment. From the nature of the acpd

technique the scale of observation is about 0.1 mm.Individual data points are connected

by straight lines. The crack growth surface is curved so some detail has been lost in

plotting the data on Cartesian coordinates. Figure 6 shows two sections through the

crack growth surface. The crack paths are approximately perpendicular to the chord

surface. The average deviation from the perpendicular, on a number of similar sections,

was about 10°. Crack growth surfaces in tubular welded joints are some times tortuous

and this complicates the interpretation of crack path data. Figure 7 shows an example

[22].

Figure 5. Crack front family for tubular welded joint.

Oscillating Crack Front Families

The crack surfaces produced on many metallic alloys by a fatigue crack growing

continuously under a constant amplitude loading have, in general, a smooth matt

appearance, free from markings visible to the naked eye [23]. However, it has been

known for a long time that some high strength aluminium alloys, exhibit quite distinct

crescent shaped markings in tests on thin sheets at high mean stress values [8]. This is

shown schematically in Fig. 8. The markings become larger and more pronounced as

crack growth proceeds.

Each dark area is an element of brittle fracture occurring in one stress cycle [8]

whereas each light area corresponds to a period of fatigue crack growth. Confirmation

of this composite crack growth behaviour is provided [23] by the fact that ‘clicks’ may

be heard during a crack growth test. The bursts of brittle fracture correspond to the pop

in phenomenon sometimes observed during fracture toughness testing [2]. After each

pop-in fatigue crack growth rates are higher near the surface than in the interior and the

crack front gradually becomes straighter until another pop-in takes place. The result is