Part of the fracture surface of the lower left hand saddle clamp is shown in Figure 6
. Multiple crack initiation sites can be observed,together with three distinct regions; a brighter fatigue region at the upper surface, a middle darker region, and a second brighter region at the bottom with the appearance of shear. After degreasing
and cleaning with a de-rusting agent, the fracture surface was inspected in a scanning electron microscope. This showed beachmarks in the fatigue region, together with significant corrosion (Figure 7
). These regions were duplicated on the right side saddle camp bracket. Figure 8
shows the appearance of the fatigue region near the initiation sites at higher magnification, with evidence of remnant striations and periodic higher loads (presumably from variable landing loads). The corrosion pitting is clear.
Near the end of the fatigue crack (which varied in depth between 1.5 - 2.0 mm), there is much less corrosion pitting, and the appearance is typical of fatigue crack growth in a high strength quenched and tempered steel (Figure 9
). The degree of pitting on the fracture surface, and its variation with depth of crack, is evidence that the crack had been in existence and growing steadily for some considerable time prior to the failure. The mechanism of fracture in the dark middle band of Figure 5
is shown in Figure 10
at a similar magnification to that used for the fatigue area. The appearance of the fracture mechanism in the third shiny,lower region is seen in Figure 11.
Any differences in microscopic appearance between Figures 10 and 11
should be noted carefully, because optically they appear very different, yet microscopically they seem rather similar. It should be noted that all three regions lie in the same perpendicular plane. The information to be gleaned from these fractographs is considered in Activity 2
Figure 6 Optical fractograph
Figure 7 Low magnification SEM fractograph
Figure 8 Fatigue initiation region
Figure 9 Fatigue crack near tip
Figure 10 Dark middle band in Figure 5
Figure 11 Shiny lower band in Figure 5
Proceed to third part of case study.