Failure as a Design      Criterion

   Fracture Mechanics

   Failure Analaysis

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Wire Rope Failure

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Undercarriage Leg Failure

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Aircraft Towbar Failure

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Hail Damage

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Insulator Caps


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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 FAST FRACTURE 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) Micro-appearance

This resource will cover only simple examples of micro-features of cleavage, intergranular fracture and microvoid coalescence.

Cleavage

Cleavage is the mechanism of brittle transgranular fracture and occurs through cleaving of the crystals along crystallographic planes. The cleavage planes can be identified by x-ray crystallography and, sometimes through the geometric shape of etch pits. Cleavage facets are best defined in relatively large grained structures, fractured at low temperatures. Fine grain sizes and higher temperatures can lead to the occurrence of quasi-cleavage, which blends cleavage facets with areas of dimple (MVC) rupture, such that the cleavage steps become tear ridges. The appearance of typical cleavage facets is shown in Figure 1. So-called 'river patterns' are formed when the cleavage fracture is forced to re-initiate at the boundary of a grain in a different orientation (via a step-wise process). These tear ridges tend to merge in the direction of crack growth, and can be used to identify local crack initiation and growth events. Many small regions of river patterns are apparent on Figure 1. Figure 2 shows nice river patterns (twist misorientation) at a higher magnification and also shows tilt boundaries, where the grains are merely tilted with respect to each other. The formation of river patters is schematically shown in Figure 3, which also illustrates twist and tilt boundaries.

Figure 4 shows an interesting example of cleavage fracture in chromium hard plating on a steel shaft. Chromium has a body-centred cubic (bcc) structure and the cleavage planes are {1 00}, i.e. the sides of a unit cell. In the case shown in Figure 4, fracture has occurred under a Hertzian stress system which has induced triaxial tension. This is reflected in the blocky cubes which have resulted from simultaneous fracture on intersecting sets of {1 0 0} planes.

Cleavage 500x.jpg (106085 bytes)

Figure 1 1040 carbon steel
Cleavage Tilt.jpg (69605 bytes)
Figure 2 1040 carbon steel
River_patterns.JPG (37437 bytes)
Figure 3 Formation of river patterns
Cr_Plate_Cleavage.jpg (129087 bytes)
Figure 4 Cleavage on intersecting {1 0 0} planes in bcc Cr

Microvoid coalescence (MVC) is the mechanism of ductile transgranular fracture. In many structural steels, both ductile and brittle fracture may occur together on a single fracture surface, as the regime of temperature and strain rate in many fractures lies in the ductile-to-brittle transition range. Ferritic materials (body centred cubic crystal structure) exhibit a fairly sharp ductile-to-brittle transition temperature (DBTT), which is influenced by strain rate, temperature and stress state. Tri-axial stress states (high plastic constraint) tend to promote brittle fracture, while bi-axial stress states tend to promote ductile behaviour. The reason for this transition is related to the strong temperature dependent component of yield stress in ferritic alloys. Austenitic steels and alloys (face centred cubic crystal structure) tend not to show this transition. Figure 5 illustrates the DBTT in ferritic steels, and shows the type of fracture surface observed on the lower energy shelf (cleavage) and on the upper energy shelf (MVC). Figure 6 shows MVC fracture in a 1040 steel alloy. Figure 7 illustrates a case of mixed ductile (MVC) and brittle (inter-pearlitic cleavage) fracture in a normalised steel alloy. Figure 8 indicates that the slope of microvoids (dimples) on the two fracture surfaces can provide information about the type of applied load - this can be particularly useful in the case of shear voids (see Case Study 2 - Failure of an Undercarriage Leg). Shear dimples are illustrated in Figure 9. Cleavage_MVC.JPG (52325 bytes)
Figure 5 DBTT effects on fracture in steel

MVC 500x.jpg (107969 bytes)
Figure 6 MVC in 1040 carbon steel
MVC plus Pearlite 30Deg2.5kx.jpg (66088 bytes)
Figure 7 Mixed MVC and inter-pearlitic cleavage
MVC_model.JPG (37388 bytes)


Figure 8
Shear Voids 1.5kx.jpg (57617 bytes)
Figure 9 Shear microvoids in 1040 steel

Go to Part 5 of the Fractography Resource

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