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Fractography Resource
- Introduction
- Fatigue - Macrofeatures
- Fatigue - Microfeatures
- Fracture - Macrofeatures
- Fracture - Microfeatures (Cleavage, MVC)
- Fracture - Microfeatures (IG)
- Compendium of Fractographs
- Activity 1
- Activity 2






CHARACTERISTICS OF A FATIGUE FAILURE IN METALS

The information that is presented here is generic, in the sense that fractographic features are greatly affected by microstructural variation, composition and environment but, usually, the following comments apply and the features illustrated will be apparent.

a) Overall appearance of fatigue cracks

This is illustrated in the three figures below.
Fatigue_macro1.JPG (42694 bytes)

Figure 1
Fatigue_macro2.JPG (38271 bytes)
Figure 2
Fatigue_micro2.JPG (43761 bytes)
Figure 3

Figure 1 shows an optical fractograph of a semi-elliptic fatigue crack. The fatigue region is smoother than the fast fracture zone, and may also be a lighter shade of grey, as seen in the second picture (Figure 2), where the bands at the top of the specimens are the fatigue cracks. The first figure also shows beachmarks, a common feature on fatigue fracture surfaces. These are the concentric rings in the fatigue region, which resemble tidemarks on a beach. In the present case, they are due to changes in environment as the crack grew (temperature changes, designed to mark the crack front), but can also be caused by changes in crack growth mechanism (e.g. bursts of fast fracture, interspersed with fatigue) as shown in Figure 4 for a large steel casting. Changes in load also cause beachmarks as shown in Figures 4, 5 and in the scanning electron microscope (SEM) fractograph of Figure 6. Figure 3 shows an SEM photograph of a small semi-elliptic fatigue crack surrounded by cleavage fracture in a high strength quenched and tempered (Q & T) steel. The SEM and optical fractographic features of similar fatigue cracks (Figures 1 and 3) can be compared and contrasted.
Fatigue_Beachmarks4.JPG (50384 bytes)
Figure 4
Fatigue_Beachmarks6.JPG (30913 bytes)
Figure 5
Fatigue_beachmarks5.JPG (42039 bytes)
Figure 6

Ratchet marks are usually present at the surface of components where a high local stress concentration is present. They indicate multiple crack initiation sites on different planes in the metal, which have then joined up via tear ridges, or shear, as the crack increases in size. If there is relative movement between the two halves of the fracture surface during fatigue cycling, the ratchet marks may be smeared or rubbed smooth (Figure 7). As shafts often contain shoulders, notches etc, ratchet marks are commonly observed on such components, even when the nominal applied stress level is low. As seen in Figure 8, however, ratchet marks are also common on plate components.

Ratchet_Smeared.JPG (63907 bytes)
Figure 7
Ratchet_marks1.JPG (27148 bytes)
Figure 8
Shaft_Chart.JPG (147957 bytes)
Figure 9
The chart given in Figure 9 demonstrates how fracture surface features on shafts can be readily interpreted to obtain information regarding type of load, level of nominal stress and degree of local surface stress concentration. Use of the chart is best illustrated through application to examples of failed shafts - Fractography Activity 1

Go to Part 2 of the Fractography Resource

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