Notch brittleness

The radius of the notch is quite important, particularly for plastics. For example, polyvinyl chloride (PVC) is a notch-sensitive material. If the notch is blunt (2-mm radius), the impact strength is higher than that for ABS. If the notch is sharp (0.25-mm radius), the impact strength of PVC is lower than that for ABS. Other polymers that are notch brittle are high-density polyethylene (HDPE), polypropylene (PP), polyethylene teraphthalate (PET), dry polyamides (PAs), and acetals. 

The brittle-ductile transition temperature depends on the characteristics of the sample such as thickness, surface defects, and the presence of flaws or notches. Increasing the thickness of the sample favors brittle fracture a typical example is polycarbonate at room temperature. The presence of surface defects (scratches) or the introduction of flaws and notches in the sample increases Tg. A polymer that displays ductile behavior at a particular temperature can break in the brittle mode if a notch is made in it examples are PVC and nylon. This type of behavior is explained by analyzing the distribution of stresses in the zone of the notch. When a sample is subjected to a uniaxial tension, a complex state of stresses is created at the tip of the notch and the yield stress brittle behavior known as notch brittleness. Brittle behavior is favored by sharp notches and thick samples where plane strain deformation prevails over plane stress deformation. 

A, tough (specimens do not break completely even when sharply notched) B, notch brittle C, brittle even when unnotched. 

BHP were also criticised for supplying material in quality which was sometimes difficult to weld and was notch brittle. Ihey had been involved in supplying steel for a pipeline a few years before, which had suffered brittle fractures but they had shown little interest and had carried out little research into the problem. 

Figure 2.8 shows the fragments of a 5000 psi monobloc Vessel purposely tested to destruction. The fragmentation lb typical of brittle rupture. Brittle rupture may result from the use of brittle materials, notch brittleness, ... 

B. Notch brittle specimens brittle when bluntly notched but do not break when unnotched. 

Orowan, E. 1945. Notch brittleness and the strength of metals. Trans. Inst. Eng. Shipbuild. Scot. 89 165. 

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