Critical stress

Advances in powder metallurgy have brought the technology to a point where it could be applied advantageously to the production cf highly stressed, critical components. Specifically, PM forging now can replace the conventional forging of certain low-alloy steel components. 

Table 11.14 gives critical stress, critical strain, and critical stored energy of NR-EPDM blends for initiation of ozone cracking. All the properties show an increase on addition of TOR, especially the critical stored energy. Stored energy is a strong indicator of ozone resistance and shows an increase by about 80% on addition of 20 parts of TOR to the compound. This was confirmed by SEM pictures of surface ozone cracks and the results of dynamic mechanical moduli and tan d measurement. 

Critical Stress, Critical Strain, and Critical Stored Energy... 

For existing equipment a vibration measure survey should be undertaken or examination of piping suspected of being under stress. Critical examination of connection points should be made where the induced piping vibration stress is the greatest. For new designs a stress analysis can be prepared on the pipework by specialized consultants. 

SAE Standards. Society of Automotive Engineers standards cover specifications for highway and off-road vehicles Some SAE specifications may be used in chemical plant and refinery applications, e.g., bearing materials. SAE also administers the Aerospace Materials Specifications (AMS), which include procurement requirements for extreme quality aerospace materials. AMS products may be applicable for land based, highly stressed critical components such as reciprocating compressor shafts. 

Critical Shear Rate - Fluoropolymers, and generally thermoplastic materials, must be processed below the velocity at which melt fracture occurs, referred to as the critical shear rate. Melt fracture in molten plastics takes place when the velocity of the resin in flow exceeds the critical velocity, the point where the melt strength of the polymer is surpassed by internal stresses. Critical velocity of most fluoropolymers is usually much lower than most thermoplastics. 

Valproic add, a common anticonvulsant, can accumulate and induce a hyperammonemic encephalopathy, presenting as acutely impaired consciousness, focal neurologic symptoms, and increased seizure frequency. Moreover, valproate-induced hyperammonemic encephalopathy can occur more readily in the child (or adult) with carnitine deficiency or with congenital urea cycle enzymatic defects. In fact, the occult presence of a urea cycle enzymatic defect is occasionally uncovered by the development of valproate-induced encephalopathy an occult enzyme deficiency can even make its first presentation, in the absence of valproate, in the generically stressed critical care patient, young or old (Verrotti et al., 2002 Thakur et al., 2006 Summar et al., 2005). 

Equation 5.21 holds for plane-stress conditions at the crack tip, which are most likely to occur if the sheet is thin (we discuss later what thin implies in this context). For plane-stress, then, the operative fracture parameter is G which is known as the firacture energy. A related (and more useful) parameter is the plane-stress critical stress intensity factor K, (5.N.6), which is defined, in the case of a wide sheet, by... 

HSCC can occur only if hydrogen is absorbed and tensile stresses are present. Both hydrogen activity and stress critical values must be exceeded, Fig. 1-15 (Pirchner, 1986 Popperling, 1986). 

More specific values of the fracture toughness are obtained by the application of the experimental procedures derived from the fracture mechanics theories. In the case of no plastic deformation in front of the crack tip, the LEFM is employed, obtaining the values of the stress critical intensity factor (fQ) or critical stress energy release rate (G,.). On the other hand, when there is plasticity in front of the crack tip, we must use the principles of the elastoplastic fracture mechanics in this sense, two experimental procedures are widely employed the J-integral analysis and the EWF. 

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