Brittle Breaks

Tenacity The cohesiveness of a material or how it behaves under tension and compression. Common forms of tenacity include brittle (breaks and powders easily), malleable (material can be hammered into sheets), sectile (material can be cut into thin layers with a knife), and ductile (the material can be drawn into a wire). 

Tensile Yield Stresses of Cast Films. At room temperature all of the BPFC-DMS polymers investigated (with one exception) reached their yield stresses before fracturing. BPF polycarbonate on the other hand is brittle, breaking at about 11,000 psi. Traces of residual chloroform make the homopolymer ductile however the yield stress decreased linearly with chloroform content. Extrapolation of these results to a dry polymer gives a yield stress of 14,000 psi. 

In principle, the force necessary to cause a brittle break F = E jL can be calculated from the energy E required to separate chemical and physical bonds by an interbond-partner distance L. To break extended-chain poly-(ethylene) crystals perpendicular to the chain direction (i.e., breaking covalent bonds), a force of about 20 000 MPa is necessary, whereas to cause a break parallel to the chain direction (i.e., working only against dispersion forces), only 200 MPa is required. Experimentally, however, a maximum tensile strength of 20 MPa is observed (the so-called crystal paradox). Consequently, the break must occur at inhomogeneities, since these lead to an inhomogeneous distribution of the tensile stress onto disruption points and thus lead to stress concentrations. 

After the bitumen specimen is placed in the apparatus and the testing temperature is obtained, they are stretched to an elongation of 1333% (400 mm). If the specimen breaks prior to the desired elongation, the test is repeated. In case the second specimen also breaks prior to the desired elongation, the test is repeated by increasing the temperature in steps of 5°C, until the test is complete without brittle break. 

Colony consistency is cartilaginous (rubbery). Colony consistency is brittle (breaks up into granules). 

Amorphous silica occurs in sponges and many plants, especially in grasses, where it is found in the cell walls and contributes to the strength of the plant. The tips of the needles of stinging nettles are made of hollow tubes of silica, which is why they are so brittle, breaking oif after puncturing the skin (Figure 10.21). In this way silica acts as a defence mechanism for the plant. 

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