Fatigue conditions

This optimum condition is designed to ensure that the botes of all components yield at the same time. If the cylinder is subjected to fatigue conditions, it has been suggested (39) that a better design criterion would arrange for the maximum normal stress, which controls fatigue crack propagation, to be the same in each component. 

Environments. Among the environmental factors that can shorten life under thermal fatigue conditions are surface decarburization, oxidation, and carburization. The last can be detrimental because it is likely to reduce both hot strength and ductility at the same time. The usual failure mechanism of heat-resistant alloy fixtures in carburizing furnaces is by thermal fatigue damage, evidenced by a prominent network of deep cracks. 

Crack extension is often observed to vary significantly at the same nominal value of AK (= Y AOpos Tta) for different values of R-ratio. Elber [26] was the first to explain this observation for metals in terms of the crack closure phenomenon. He determined, by measuring specimen compliance, that fatigue cracks open and close at the crack tip at positive values of stress due to contact between crack surfaces behind the crack tip. For elastic fatigue conditions it is generally found that P p = P, and Kop = K, where P is the applied load. 

Vosikovsky, O., Macecek, M., and Ross, D.J. Allowable defect sizes in a sour crude oil pipeline for corrosion fatigue conditions, International Journal of Pressure Vessels and Piping, 13,197-226,1983. 

The following conditions should be satisfied stress must not exceed the tensile yield strength at any point fatigue conditions should be considered and brittle fractures avoided. 

Damping is responsible for heat dissipation under fatigue conditions the temperature increase may cause drastic changes in the material properties. 

SCC often occurs in hot gaseous atmospheres on materials under creep or fatigue conditions. A few cases of susceptibility have been reported for certain titanium alloys in gases, including moist chlorine, dry HC1, and dry hydrogen. In moist chlorine gas at 288°C Ti-8Al-lMo-lV alloy exhibits cracking, for example. Various binary titanium alloys are also known to experience cracking in this gas at 427°C. (Schutz)24... 

Endurance Prediction of Thickwalled High Pressure Components under Corrosion Fatigue Conditions... 

On the other hand, very high temperatures and low cyclic loading frequencies (high cycle times) promote near-tip creep-fatigue conditions where... 

Fig. 22 Contact cracking processes within random copolymers of methylmethacrylate under fatigue conditions (from [93]). a In situ observation of the contact areas, b Changes in the contact stiffness as a function of the number of cycles for the cyclohexyl maleimide system (see Table 2). Crack propagation is associated with a drop in contact stiffness as indicated by the dotted line. In the case of the glutarimide system, no significant change in contact stiffness was detected after contact cracking...

The same critical potential is observed for intergranular fracture under fatigue conditions as for monotonic loading (SCC), provided that the loading frequency in fatigue is sufficiently low (/ < 0.1 Hz for R = 0.5, AK = 20 MPam / ) [28]. 

The latter pathway is very vague in the 1,4-PD series. No nitroxide corresponding to N,N -diphenyl-l,4-PD (DPPD) was detected in the oxidized substrate by Adamic and co-workers (2. It has been explained as a consequence of the quick oxidation of the initially formed monoaminyl into BQDI (4) without nitroxide formation. An ESR signal characteristic of nitroxide corresponding to N-isopropyl-N -phenyl-1,4-phenylenediamine (IPPD) was detected in an oxygen deficient system (9), i.e. under rubber fatiguing conditions. It is therefore possible that the mononitroxide is an intermediate formed... 

SCC is often encountered in hot gaseous atmospheres on materials under creep or fatigue conditions. SCC susceptibility of titanium alloys in moist chlorine, dry HCl, and dry hydrogen has been observed. The titanium alloy (Ti-8Al-lMo-l V) exhibits cracking in moist chlorine gas at 288 C. Binary titanium alloys also exhibit cracking in moist chlorine at 427 °C (96). 

Peak stress, F. Peak stresses are the additional stresses due to stress intensification in highly localized areas. They apply to both sustained loads and self-limiting loads. Tliere are no significant distortions associated with peak stresses. Peak stresses are additive to primary and secondary stresses present at the point of the stress conc-entration. Peak stresses are only significant in fatigue conditions or brittle materials. Peak stresses are sources of fatigue cracks and apply to membrane, bending, and shear stresses. Examples are ... 

Chen AS, Almond DP, Harris B. Impact damage growth in composites under fatigue conditions monitored by acoustography. Int J Fat 2002 24 257-61. 

FIGURE 20.45 Wohler curves in air after precorrosion and under corrosion fatigue conditions. 

Since a frequently repeated minimum stress amplitude is needed to initiate corrosion fatigue in stainless steels in passivating solutions, it must be assumed that this load limit is connected with the mechanical stress capacity of passive layers. Only when particularly marked slip starts at one point on the surface will the layer be (racked. At that point, a constantly repetitive process begins, in which a new activation process at the same spot always follows repassivation. Each new passivation process consumes metal, deepens the corrosion, and increases the stress peak until, because of the constantly rising stress, repassivation is no longer possible. The resistance of passive metallic materials under fatigue conditions in electrolytes is therefore largely dependent on three factors ... 

Fig. 2.6 Crack propagation rate as a function of cyclical component of the stress intensity factor at fatigue conditions for thermoplastic polymers, A o - onset value of crack propagation, - fracture toughness at quasi-static loading conditions C and m - material parameters.

Shot-peening is used to alter the surface condition of stainless steel subjected to fatigue conditions. Essentially the initial imposition of extra surface stresses causes local deformation that re-adjusts the stress state within the region. This process relaxes high local stresses. The subsequent creation of a thin layer of compacted steel introduces a residual compressive surface stress. Under usage conditions, externally applied tensile stresses are offset by this inherent compressive stress, as a... 

Fatigue—condition of stress from repeated flexing or impact force upon the adhesive-adherend interface weakening of material caused by repetitive loading and unloading. 

The forces are transmitted to the tooth by a wire that is coupled to a bracket bonded to the tooth surface. The most common brackets are made of stainless steel and have either a groove cast into the base or a metal mesh brazed onto the bracket base to increase the available surface area for bonding. The challenge of orthodontic bonding is that the bracket/enamel bond must be sufficient to withstand the forces applied by the wire and the fatigue conditions produced by thermal and occlusal load cycling in the mouth, yet must be easily debonded without damaging the enamel surface at the end of treatment (typically two years). 

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