Fatigue relaxation

As austenitic stainless steels are widely used, their fatigue and fatigue-relaxation behavior has been studied for more than three decades [106—112]. Their stress—strain behavior differs from that of martensitic steels. As mentioned concerning creep, the initial microstructures differ strongly and therefore hardening is observed in austenitic stainless steels, whereas softening is observed in martensitic steels. The evolution of the stress amplitude with the number of cycles is shown in Fig. 6.34(a) for various strain... 

Such predictions may provide inputs for fatigue-relaxation damage modeling, which should he based on the synergy between oxidation and oxide layer fracture in tempered martensite-ferritic steels but creep cavitation in austenitic stainless steels. 

Long-term creep and fatigue-relaxation tests are essential for providing macroscopic data and aUowrng the observation of the long-term dislocation microstnicture and damage evolutions. 

M. Mottot, P. Petrequin, C. AmzaUag, P. Rabbe, J. Grattier, S. Masson, Behavior in fatigue-relaxation of a high-creep resistant t)fpe 316L stainless steel, in C. Amzallag,... 

The behavior of materials under dynamic load is of considerable importance and interest in most mechanical analyses of design problems where these loads exist. The complex workings of the dynamic behavior problem can best be appreciated by summarizing the range of interactions of dynamic loads that exist for all the different types of materials. Dynamic loads involve the interactions of creep and relaxation loads, vibratory and transient fatigue loads, low-velocity impacts measurable sometimes in milliseconds, high-velocity impacts measurable in microseconds, and hypervelocity impacts as summarized in Fig. 2-4. 

Material behavior have many classifications. Examples are (1) creep, and relaxation behavior with a primary load environment of high or moderate temperatures (2) fatigue, viscoelastic, and elastic range vibration or impact (3) fluidlike flow, as a solid to a gas, which is a very high velocity or hypervelocity impact and (4) crack propagation and environmental embrittlement, as well as ductile and brittle fractures. 

Long time dynamic load involves behaviors such as creep, fatigue, and impact. T vo of the most important types of long-term material behavior are more specifically viscoelastic creep and stress relaxation. Whereas stress-strain behavior usually occurs in less than one or two hours, creep and stress relaxation may continue over the entire life of the structure such as 100,000 hours or more. 

Dawson, M.J., Gadian, D.G., Wilkie, D.R. (1980). Mechanical relaxation rate and metabolism studied in fatiguing muscle by phosphorus nuclear magnetic resonance. J. Physiol. 299,465-484. 

Edwards, R.H.T., Hill, D.K., Jones, D.A. (1975). Metabolic changes associated with the slowing of relaxation in fatigued mouse muscle. J. Physiol. 251,287-301. 

Resilin has a remarkably high fatigue lifetime (probably >500 million cycles) and our aim is to reproduce this desirable mechanical property in synthetic materials derived from our studies of resilin structure and function. We believe that recombinant resilin-like materials may be used, in the future, in the medical device field as components of prosthetic implants, including spinal disks and synthetic arteries. Spinal disks, for example, must survive for at least 100 million cycles of contraction and relaxation [30]. 

Resin curing of SBR and BR imparts excellent cut growth and abrasion resistance. Resin-cured NBR shows high fatigue life and high relaxation, while resin-cured HR shows outstanding ozone and age resistance [55]. 

Mechanical stress Fatigue, creep, stress-relaxation, set, abrasion... 

The service life of a product can be governed by many factors. These include fatigue failure under repeated stressing, excessive creep or stress relaxation, excessive change in stiffness due to thermal ageing, and excessive change in a physical property due to the action of chemicals. 

MPRs are handicapped by a lower elasticity than conventional rubbers, the more so the higher the hardness certain risks of creep, relaxation and permanent set, the more so the higher the temperature sometimes limited flex fatigue resistance ecological problems involved with halogens tool corrosion risks lack of sources cost. 

Signing the visitor sheet, I entered the offiee of the Chief of the Medieal Researeh Laboratories, Colonel Douglas Lindsey. He was a slim, wiry man in his early forties, and emanated an aura of relaxed authcrily. Unlike other senior offieers, he was dressed unpretentiously in Army fatigues. The other offieers dressed normally, with eolorful serviee ribbons on their blouses and shiny rank insignias on their shoulders. Doug Lindsey s imiform had evidently been eut from a different bolt of eloth. 

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