Toughness and Brittleness

An otherwise well-made pipe will sustain steady pressures up to 2.19 MPa (318 psi) without failing by brittle fracture if it contains initial flaws as large as 1 mm. Similar calculations show that initial flaws or inclusions smaller than 4.5 mm permit steady operation at 150 psi (1.03 MPa). 

LEFM discus.sed to this point refers to the resistance of bodies to crack growth under static loads. Crack growth under cyclic loading is faster than under static loads at the same stress amplitudes, because the rate of loading and the damage both increase with higher frequencies. 

Polymers which yield extensively under stress exhibit nonlinear stress-strain behavior. This invalidates the application of linear elastic fracture mechanics. It is usually assumed that the LEFM approach can be used if the size of the plastic zone is small compared to the dimensions of the object. Alternative concepts have been proposed for rating the fracture resistance of tougher polymers, like polyolelins, but empirical pendulum impact or dart drop tests are deeply entrenched forjudging such behavior. 

Ductile-brittle transitions are more accurately located by variable temperature tests than by altering impact speed in an experiment at a fixed temperature. This is because a linear fall in temperature is equivalent to a logarithmic increase in straining rate. The ductile-brittle transition concept can be clarified by sketches such as that in Fig. 11-25. In the brittle region, the impact resistance of a material is related to its LEFM properties, as described above. In the mixed mode failure 

The relative importance of the yield zone can be estimated, for a given product, by comparing its yield stress and fracture toughness. The parameter proposed for this purpose is where Oy is the yield stress [21]. This ratio has units 

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Institute of Metallurgists (1960) Toughness and Brittleness of Metals (Iliffe). 

Hard steel 99% iron, 1 % carbon Tough and brittle Cutting tools, chisels, razor blades... 

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