Failure as a Design      Criterion

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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


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) Micro-appearance

The classic feature associated with fatigue crack growth is the formation of fatigue striations. The term 'striation' just means that the fracture surface is ridged, grooved, or channeled and, in principle, could be applied to any such markings including beachmarks. By convention, however, the term in the fatigue community is reserved for particular markings which represent the increment of growth which occurs in one load cycle, and which reflect the operation of slip planes at a crack tip causing plastic blunting and sharpening. Clearly, therefore, these markings can only be resolved is the growth rate lies in a reasonable range, i.e. perhaps 10-3 to 10-6 mm/cycle. At a magnification of 1000x 10-3 mm/cycle (1 Ám) becomes equivalent to 1 mm. Such checks are useful in ascertaining whether an observed feature is, in fact, a striation or a beachmark. Striations vary in appearance according to alloy type and environmental conditions. The typical appearance for steel and aluminium are shown below in Figures 1 and 2.

Fatigue_striations2.JPG (69317 bytes)

Figure 1 Interstitial-free low carbon steel
8090_fatigue_striations.jpg (45012 bytes)
Figure 2 8090 Al-Li alloy
Fatigue_HiMag.JPG (34926 bytes)

Figure 3 0.4% carbon-manganese steel

Particularly in steel, striations may be confined much more to single grains on the fracture surface and, often, are merely vestigial in appearance leading to a fracture surface that is best described as 'ductile transgranular with possible vestigial striations' (see Figure 3)

It is important to be aware that the appearance of 'striations' on a fracture surface does not conclusively prove fatigue - other factors (such as environment and presence of hard inclusions) must also be considered before a definitive conclusion is drawn. Thus it has been reported that stress corrosion cracking (SCC) can give rise to striation-like markings due to incremental forming and breaking of a passive layer at the crack tip - see Figure 4. SCC_striations.JPG (43631 bytes)
Figure 4 SCC striation-like markings

It is also true that striation-like markings are produced by relative movement between the two fracture surfaces during cyclic loading. Whilst such marks are due to crack extension, probably by fatigue crack growth, the mechanism of formation requires the presence of hard inclusions and combined Mode 1 (tension) and 2 (shear in direction of crack growth) or Mode 1 and 3 (shear perpendicular to direction of crack growth) loading. These marks are called tyre tracks (see reference 1) and are shown in Figure 5. In this example, the tyre tracks are perpendicular to the direction of crack propagation or growth (DCP), and reflect Mode 1 and 3. Tyre tracks are distinguished by a rather 'nobbly' appearance, reminiscent of the tread on tyres. Tyre tracks in a normalised mild steel formed in low cycle fatigue are shown in Figure 6.

The major interest in striations really arises from the possibility that they can used to assess the stress on the component during fatigue crack growth, using fracture mechanics principles. The application of fracture mechanics to characterise fatigue crack growth, and to make residual life calculations is dealt with on an associated web page.  To apply this technique requires one to have a crack growth rate for the material, expressed in fracture mechanics terms as da/dN versus range of stress intensity factor (delta K), and to know the geometry correction function in the stress intensity factor equation. Local crack growth rate at a particular crack length can be measured from a fractograph as striation spacing. Using the crack growth rate curve then allows the instantaneous value of delta K to be found which, using the K calibration equation, can be correlated with a value of applied stress corresponding to Kmax in the fatigue cycle. This is seldom as straight forward as it sounds but, potentially will enable a check to be made of whether the design stress and applied stress match.  A significant difference between the two figures may indicate inadvertent loading from an additional source of stress (which may have contributed to the failure).
Tyretrack_striations1.JPG (59169 bytes)
Figure 5

Tyre_tracks.JPG (46423 bytes)
Figure 6 Tyre tracks in normalised mild steel

Go to Part 3 of the Fractography Resource


Failure Analysis  -  Fracture Mechanics  -  Failure As A Design Criterion