Fracture strength polymers

The fracture strength and mode of fracture of a material have been found to be related to a number of characteristics of the polymer molecules of which it is made up. These include, among others, constitution, molar mass, polydispersity, crystallinity, and degree of crosslinking. Other factors which also affect fracture strength and mode of fracture are temperature, strain rate, and geometry of the specimen, all of which are decided upon prior to testing the material. 

The fracture strengths of polymers are generally lower than those of metals and ceramics. The mode of failure for thermosets is generally referred to as the materials being brittle. Cracks, related to bond breakage, occur at points of excess stress. These create weak spots and may lead to fracture if the applied stress, appropriate to create bond breakage, continues. 

Engineering materials, including polymers, generally have low fracture strengths relative to their theoretical capacity. The lower-than-ideal fracture strengths of engineering materials are generally attributed to the presence of flaws such as cracks, scratches, and notches inherent in these materials. 

Tensile testing of single crystalline metallic microwhiskers can also be studied following the experimental tensile testing constructed by Brenner and (b) results of whisker fracture strength as a function of whisker size, showed the clear size dependence (Fig. 3.34). The chitin whiskers are usually incorporated into polymer matrix to prepare CWs reinforced polymer nanocomposite. Thus, the mechanical... 

FIGURE 3-4 Fast fracture strength at temperature for commercial fibers based on silicon compounds and alumina. PP = polymer pyrolysis, SG = sol gel, SS = slurry spinning, EDFG = edge defined film growth, CVD = chemical vapor deposition. Source DiCarlo and Dutta, 1995. 

The toughness of a polymer can manifest itself in a complementary manner, in several different forms. In uniaxial tension of a smooth bar, the toughness is the plastic work required in order to fracture, per unit volume, which is given by the area under the stress-strain curve as shown in Fig. 13.1, where deformation under various strain rates e is terminated when the flow stress reaches a fracture strength fff that is flaw-sensitive but otherwise relatively insensitive to the deformation rate. 

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