Fatigue Crack Growth in 7050T7451 Aluminium Alloy Thick Section ...

DSTO-TR-14773.6.1 Crack morphologyAll the main fatigue cracks produced during this phase of the testing were examined by QF andthe crack growth rates measured (reported in Section 3.7.2). As part of this examination severalgeneral observations about the appearance of the crack surfaces, and the secondary cracks, can bemade. The most obvious difference between the etched specimens examined in Barter, 2003, andthe peened specimens was the number of larger cracks. While numerous cracks had initiated inthe etched specimens, and many of these cracks were not far behind the growth of the crack/s thatcaused failure, this was not the case in the peened specimens. The peened specimens had very fewsignificant cracks, although a large number of much smaller cracks did exist. In general only a fewsignificant cracks intersected the fracture surface, and in some cases only a single significant crack(Figure 10A). This led to some very jagged fracture surfaces (Figure 10B) due to the fast fracturerunning along the grain orientation rather than linking together significant cracks during failure.This can be compared with the large number of cracks usually found on the fracture plane of oneof the etched specimens, Figure 11. The distribution of cracks over the surface of the specimenswas reasonably even although the size of the cracking compared to the main crack/s was usuallyvery small. Some of these small secondary cracks could usually be seen in the stretch zones(surface regions that yielded during failure of the specimen) either side of the failure plane.Examples are shown in Figure 12 .ABFigure 10 The fracture in one of the peened specimens ‘A’ (390MPa loading) showing the single crack fromwhich failure occurred. Note that since the cracking was across the rolling direction the fracturewas heavily distorted along the rolling direction as shown in ‘B’.11