Toughness comparing tests

Thermoplastic polyolefin (TPO) blends of metallocene-based polyolefin elastomers (POE) with polypropylenes have gained commercial significance because of the improved melt flow and toughness compared to the conventional TPOs based oti EPR or EPDM blends made with high melt-flow PP (Toensmeier 1994). in comparative tests with 70/30 PP/elastomer blends, the blends with POE maintained ductile behavior at —29 °C even with high melt flow index PP (MFl = 35), while the corresponding EPR-based blends were brittle with PP of MFl = 20. In addition, they showed improved knit-line strengths. 

The stiffness of the loading equipment is another factor that influences the toughness of tested elements. In a stiff testing machine, that is, with sufficient stiffness compared to the tested specimen, the elastic energy accumulated in the machine is not released after the first crack of the specimen. The test may be continued and a descending branch of the load-deflection (or stress-strain) curve may be established. In contrast, when the same element is tested on a less rigid machine, then after the first crack all the elastic energy... 

One of the first attempts to produce polyurethane was from the reaction of an intermediate polyol of 1,3- and l,4-bis(hydroxyhexa uoroisopropyl)benzene m- and -12F-diols) by reaction with epichlorohydrin. This polyol was subsequentiy allowed to react with a commercial triisocyanate, resulting in a tough, cross-linked polyurethane (129,135,139). ASTM and military specification tests on these polyurethanes for weather resistance, corrosion prevention, bUster resistance, and ease of cleaning showed them to compare quite favorably with standard resin formulations. 

In metals, inelastic deformation occurs at the crack tip, yielding a plastic zone. Smith [34] has argued that the elastic stress intensity factor is adequate to describe the crack tip field condition if the inelastic zone is limited in size compared with the near crack tip field, which is then assumed to dominate the crack tip inelastic response. He suggested that the inelastic zone be 1/5 of the size of the near crack tip elastic field (a/10). This restriction is in accordance with the generally accepted limitation on the maximum size of the plastic zone allowed in a valid fracture toughness test [35,36]. For the case of crack propagation, the minimum crack size for which continuum considerations hold should be at least 50 x (r ,J. 

Fig. 6.3. A theoretical plot of fracture toughness, R, with variation of frictional shear stress, Tf, compared with experimental fracture toughness values (A) Charpy impact and (A) slow bend tests for carbon-epoxy composites ( ) Charpy impact and (O) slow bend tests for carbon polyester composites. After...

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