Failure as a Design      Criterion

   Fracture Mechanics

   Failure Analaysis

 :: 

Wire Rope Failure

 :: 

Undercarriage Leg Failure

 :: 

Aircraft Towbar Failure

 :: 

Hail Damage

 :: 

Insulator Caps


 :: 


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.

Intergranular Fracture

Intergranular fracture (also known as 'rock candy' fracture in structures with a coarse grain size, e.g. castings) is another low energy form of fracture that usually indicates either an embrittlement problem or a processing problem (quench cracking). It is important to remember that most forms of embrittlement (stress corrosion cracking, hydrogen, liquid metal) can occur by either cleavage or intergranular mechanisms (or even MVC in some cases), depending on the local stresses and the alloy and microstructure.  In this respect see, for instance, reference 1 below. Figures 1 and 2 illustrate this for HY 80 pressure vessel steel (2½Ni 1½Cr ½Mo) which has been charged with hydrogen under an applied tensile stress to induce small cracks. Quasi-cleavage occurs where the crack initiates (the stresses are highest), and the fracture changes to intergranular as the crack grows and relieves the hydrogen pressure.  The classic form of intergranular fracture is shown in Figure 3.

Qc_ig.jpg (79130 bytes)

Figure 1 Quasi-cleavage and IG fracture
Qc-ig2.jpg (77825 bytes)
Figure 2 Quasi-cleavage and IG fracture - hydrogen charged HY 80 steel
IG_Fracture.JPG (60514 bytes)
Figure 3 Intergranular fracture in steel


Interestingly, IG fracture can also occur in fatigue by at least two mechanisms. The first occurs by an environmental mechanism during fatigue crack growth, when the crack tip plastic zone size becomes of the same order as the grain dimension. When cracks in steels are growing in a moist air environment isolated IG facets are observed on the fatigue fracture surface in this range of growth growths (which correspond to the 'knee' in the crack growth rate curve, at the transition from the Paris law regime to the threshold regime of growth). Figure 4 illustrates such IG facetting. The second mechanism occurs at low growth rates in low carbon interstitial-free (IF) grades of steel used, for example, in car door panels (Figure 5). In both cases the local plasticity is limited in extent to less than the grain size. In such cases (and in cases of brittle fracture involving segregation of precipitates to grain boundaries) fine-scale MVC may be observed on the IG facets (Figure 6).
IG_Fatigue.JPG (87015 bytes)
Figure 4 Isolated IG facets of fatigue fracture surface
IF_facets.JPG (80143 bytes)
Figure 5 IF facets near crack initiation site in IF steel
IF_facets2.JPG (80819 bytes)
Figure 6 Fine MVC on IG facets (fatigue of IF steel)

Other forms of intergranular fracture, e.g. interdentritic (see Figure 4, Part 3) and inter-pearlitic (see Figure 7, Part 4), are also found and have been illustrated in other parts of this resource.

Try our your expertise in identifying the various mechanism of fracture in the example given in the Activity.

References

1.    CD Beachem (1972), A new model for hydrogen-assisted cracking (hydrogen "embrittlement"), Metallurgical Transactions, Vol. 3 pp.437-451.

TOP

Failure Analysis  -  Fracture Mechanics  -  Failure As A Design Criterion