Failure in amorphous plastics brittle and ductile fracture

7 Failure in amorphous plastics brittle and ductile fracture 

The neck cross-sectional area remains unaltered during drawing until the neck has grown in length to reach the clamps holding the sample. Then a complex failure mechanism can occur by either of two processes brittle fracture and ductile fracture. 

The impact has propagated in a very narrow zone and chemical bonds have been broken. If we examine a diagram of applied stress against resulting strain, the area under the curve, which measures the energy dissipated or work done in the process (work is force times distance), is relatively small. Furthermore, this figure shows that failure occurs after only a small extension of the material. 

At first sight, it is puzzUng why the onset of such restricted motions should influence failure properties that are normally considered to involve the large scale movement of polymer chains. The explanation Ues in the way in which 

The strength of a plastic can be related, first, to the number average molecular weight of the polymer using a simple formula presented by Flory more than 60 years ago  

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