Fatigue estimation

In the case of a pressure vessel subjected to cyclic loading (as here) cracks can grow by fatigue and a vessel initially passed as safe may subsequently become unsafe due to this crack growth. The probable extent of crack growth can be determined by making fatigue tests on pre-cracked pieces of steel of the same type as that used in the pressure vessel, and the safe vessel lifetime can be estimated by the method illustrated in Case Studv 3. 

Fatigue Life. The formulas for estimating fatigue life are more complex and usually require several assumptions. Without attempting to evaluate the merit of such formulas, it is suggested here that alloy selection or metallurgical variables operating within an alloy may... 

Example 2.21 A rod of plastic is subjected to a steady axial pull of 50 N and superimposed on this is an alternating axial load of 100 N. If the fatigue limit for the material is 13 MN/m and the creep rupture strength at the equivalent time is 40 MN/m, estimate a suitable diameter for the rod. Thermal effects may be ignored and a fatigue strength reduction factor of 1.5 with a safety factor of 2.5 should be used. 

Example 2.22 A certain grade of acrylic has a Kc value of 1.6 MN and the fatigue crack growth data as shown in Fig. 2.77. If a moulding in this material is subjected to a stress cycle which varies from 0 to 15 MN/m, estimate the maximum internal flaw size which can be tolerated if the fatigue endurance is to be at least 1(P cycles. 

If the material has a critical stress intensity factor of 1.8 MN m and it is known that the moulding process produces defects 40 m long, estimate the maximum repeated tensile stress which could be applied to this material for at least 10 cycles without causing fatigue failure. 

A series of utuaxial fatigue tests on unnotched plastic sheets show that the fatigue limit for the material is 10 MN/m. If a pressure vessel with a diameter of 120 mm and a wall thickness of 4 mm is to be made from this material, estimate the maximum value of fluctuating internal pressure which would be recommended. The stress intensity factor for the pressure vessel is given by K = 2hoop stress and a is the half length of an internal defect. 

Endo, K., Komai, K. and Nakamuro, N., Estimation of corrosion fatigue strength from corrosion resistance and notch sensitivity of the materials . Bull. Jap. Soc. Mech. Eng., 13, 837-46 (1970)... 

Fiber tension development is measured at different Ca concentrations. The maximum Ca activated tension is denoted Pq and the Ca concentration needed to produce 50% of Pq is estimated and denoted Caso. These two measurements can be used to study Ca related fatigue mechanisms. A pH decrease from 7.0 to 6.5 produced a 30% decrease in Pq and a decrease in Ca sensitivity (2-fold increase in Caso) (Fabiato and Fabiato, 1978 Godt and Nosek, 1989). Similar changes in Po and Ca5o are observed when Pj is increased from 0.9 to 17 mM. 

In addition, the use of field fortification samples measures the carefulness factor of the Field Scientist during the field research and allows a Study Director/Manager or distant observer to obtain a quality control estimate on the field portion of the study. For this reason, the field fortification samples are usually meant to be different from laboratory procedural fortifications and are meant to be prepared under field conditions, which are considerably more rigorous than are controlled laboratory conditions. For example, environmental factors such as heat, humidity, wind, human stress, and other human factors such as fatigue to the Field Scientist are an integral part of any field worker exposure/re-entry study. Field fortifications made to matrices under these conditions will test and readily demonstrate the ability of the Field Scientist to perform such a difficult study under trying circumstances. 

If anything, the equal area method tends to over-estimate the compensation required and in some instances the additional material can reduce the fatigue life of the vessel. More sophisticated methods for determining the compensation required have been introduced into the latest editions of the codes and standards. 

The most common symptom of chronic HCV infection is persistent fatigue. An estimated 20% of patients with chronic HCV infection will develop cirrhosis and half of those patients will progress to decompensated cirrhosis or hepatocellular carcinoma. 

Workers exposed to average levels of TCE estimated to be 100-2 00 ppm have reported increased incidence of fatigue, vertigo, dizziness, headaches, memory loss, and impaired ability to concentrate. Other effects noted at about 100 ppm and above include paresthesia, muscular pains, and gastrointestinal disturbances. 

Racemic f/zreo-methylphenidate was approved for the treatment of fatigue, nausea, and depression in 1955 under the brand name Ritalin , and was first used in children in 1958 with an approval for hyperkinetic disorder (ADHD) in 1960. Methylphenidate has an even shorter half-life than amphetamine, and its use in treating ADHD was limited by the fact that schoolchildren would need to visit a nurse during the day to take a second dose of this scheduled drug (in order to maintain efficacy throughout the entire school day). In humans, injecting methylphenidate produces effects similar to intravenous cocaine, but oral methylphenidate is adsorbed very slowly from the gut into the blood and takes an unusually long time (estimated 2.5 h) to reach a peak concentration... 

As mentioned earlier, fatigue is the result of crack initiation and propagation. Crack growth under fatigue is estimated by the Paris equation ... 

A phase I trial was conducted with monophosphoryl lipid A (MPLA) prepared from S. typhimurium or S. minnesota injected i.v. in patients with disseminated cancer. Fever, chills and fatigue were Ihe most common side effects and the dose of 250 pgfai2 i..e. (250x1.7) 65=6 pglcg was estimated acceptable [189]. 

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