Fatigue performance requirements

Fracture mechanic The fracture mechanics theory developed for metals is also adaptable for use with plastics. The basic concepts remain the same, but since metals and plastics are different they require different techniques to describe their fatigue-failure behaviors. Some of the comments made about crack and fracture influences on fatigue performance relate to the theory of fracture mechanics. The fracture mechanics theory method, along with readily... 

Thermo-oxidative stability is primarily a function of the vulcanization system. Peroxide vulcanization or cure systems tend to perform best for reversion resistance as a result of the absence of sulfur and use of carbon-carbon crosslinks. Efficient vulcanization (EV) systems that feature a low sulfur level (0.0-0.3 phr), a high acceleration level, and a sulfur donor similarly show good heat stability and oxidation resistance. Such systems do, however, have poor resistance to fatigue because of the presence of predominantly monosulfidic crosslinks. Conventional cure systems that feature a high sulfur level and low accelerator concentration show poor heat and oxidation resistance because the polysulfidic crosslinks are thermally unstable and readily oxidized. Such vulcanization systems do, however, have better fatigue resistance. Semi-EV cure systems, which are intermediate between EV and conventional systems, are a compromise between resistance to oxidation and required product fatigue performance. 

Recognize that with the many varying properties of the different RPs, there are those that meet high performance requirements such as long time creep resistance, fatigue endurance, toughness, and so on. Conversely, there are RPs that is volume and low cost driven in their use. As explained in this book, each of the different materials reqviires their specific RP processing procedures. 

Cyclic fatigue is another long-term durability requirement for ceramic matrix composites. As part of the LC project, a limited assessment was made of the isothermal fatigue performance of the SiOC Nextel 312 BN 2-D composites. 

The crucial role of the heart in serving as the pump to maintain a continuous blood supply to all the organs of the body depends on flexible heart valve leaflets that need to cycle back and forth with each beat of the heart. This occurs about 100,000 times each day and places exceptionally high demand on the bending fatigue performance of a prosthetic heart valve material, which should yet be required to last some minimum number of years. The development of a percutaneously delivered textile based heart valve with flexible and fatigue resistant leaflets is described in Chapter 16. This prototype under development shows promise based on the in vitro fatigue work and animal trials aheady conducted. 

These materials have higher stiffness than PAEK between Tg of the PAEK and Tg of the sulfone or etherimide. The performance of the PAEK can be improved by adding reinforcing fibres but there remain some applications in which this performance window is critical. Since they are amorphous they undergo less shrinkage on cooling and this leads to better dimensional tolerance and stability. They are also relatively transparent. However, amorphous resins tend to have much lower chemical resistance, inferior fatigue performance and will lose useful properties immediately above Tg. These limitations of amorphous resins mean that it is usually obvious when a PAEK is required. 

The foregoing analysis of the skin-doubler specimen shows that it is essential to know the stiffness characteristics of the adhesive. Since good design practice places bond lines in shear, it was decided that the shear modulus is the primary stiffness parameter. Furthermore, it is recognized that more than the initial portion of the shear stress-strain curve was required. It was clear that the total curve was not linear. It was anticipated that the nonlinearity portion would bear heavily on creep and fatigue performance. Accordingly, the primary requirements for the strain measuring device were set as follows ... 

Mechanical properties of copper strength,. creep resistance, and fatigue performance are improved by alloying (but conductivity is impaired) The gasing reaction of copper with oxygen requires precautions when temperatures exceed 700°C. 

Adhesives are used extensively in nature. Their diversity is a source of inspiration for developing new man-made adhesives different from those currently available. Bioadhesives exhibit a certain number of characteristics that make them differ greatly from artificial adhesives sensitivity to - and critical dependence on - the presence of humidity, recurrent use of molecular constituents, ability to form sound attachments with a wide diversity of substrates, variation in properties in response to performance requirements, improved fatigue resistance and resilience, and conferring to the biological hybrid materials a damage-tolerant design. 

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