Cyclical stress loading

Fatigne/cyclic stresses Load- and non-load-bearing paths Stress concentrations Vibration/noise... 

The cyclic stress intensity AK increases with time (at constant load) because the crack grows in tension. It is found that the crack growth per cycle, da/dN, increases with AK in the way shown in Fig. 15.8. 

Basically, anything that can be done to reduce the temperature of the product by removal of heat generated by the cyclical stress will improve the possibilities of surviving the cyclical stress. If the heat transfer capability is limited, then the only alternative is to use stiff materials and low stress levels on the product compared with the strength capability of the material. The heavier products that result will be relatively inefficient in the use of material. In some cases when the load applied is an inertial load (such as an impeller on a pump) it may be that only a trade-off of weight for low stress level can cause failure. 

Stress factors Sustained stress, cyclic stress, compression set (in rubbers) under continuous loading... 

Cyclic stress developing from on-off and peak-load boiler operations... 

Certain types of corrosion are selective. Thus, corrosion cracking is observed primarily in the case of alloys and only when these are in contact with particular media. Corrosion is often enhanced by various extraneous effects. Stress corrosion cracking can occur under appreciable mechanical loads or internal stresses corrosion fatigue develops under prolonged cyclic mechanical loads (i.e., loads alternating in sign). 

Bills (7) has applied an adaptation of this law to solid propellants and propellant-liner bonds for discrete, constantly imposed stress levels considering U to be the time at the ith stress level and tfi the mean time to failure at the ith stress level. A probability distribution function P was included to account for the statistical distribution of failures. For cyclic stress tests the time is the number of cycles divided by the frequency, and the ith loading is the amplitude. The empirical relationship... 

Three basic problems may cause a total joint replacement to fail or to have a limited service life. The first problem, arises, because the elastic modulus of the stem greatly exceeds that of the bone. Flexural loading caused by walking creates local cyclic stress concentrations due to the non-compliance of the stem. These stresses can be intense and even severe enough to cause death of local bone cells. If this... 

A material that is subjected to cyclic application of stresses may fail after a large number of load cycles without nearly reaching the maximum failure stress of direct loading. The effect of such cyclic stresses is to initiate microscopic cracks at centres of stress concentration within the material or on the surface, and subsequently to enable these cracks to propagate, leading to eventual failure. For high stress amplitudes less cycles are needed for failure than for low stress amplitudes. For high frequencies less cycles are needed for... 

Stresses. The main mechanical properties to consider are maximum stress or stress intensity factor, <7m ix or Kmax, cyclic stress or stress-intensity range, Act or AK, stress ratio R, cyclic loading frequency, cyclic load waveform (constant-amplitude loading), load interactions in variable-amplitude loading, state of stress, residual stress, and crack size and shape, and their relation to component size geometry.31... 

Fig. 6.5 Cyclic stress-strain behavior observed during the room temperature fatigue of unidirectional SiQ/LAS-II at a maximum stress below the 105 fatigue limit (loading frequency = 10 Hz, crmin/crmax = 0.1). The cyclic stress-strain curves show very limited, if any, hysteresis. Note that the strength and strain capability is retained after cyclic loading below the proportional limit stress. After Prewo.42...

Fig. 8.16 Variation of static and cyclic fatigue crack velocity, daldt, with the applied (maximum) stress intensity factor, KIy for fatigue tests on A CVSiCw composites conducted at 1400°C. The inset shows a schematic of the change in crack velocity for a change from static- cyclic-> static loading at fixed Kt.51...

Such approaches have been applied to the contact cracking problems under sliding conditions in order to assess the values of the stress intensity factors K and Ku under cyclic tangential loading. As an example, Fig. 18 shows the numerical simulations corresponding to a cracked epoxy substrate. 

Fig. 18 Calculated values of the mode I (Ki) and mode II (Kn) stress intensity factors within a cracked epoxy at various stages of a cyclic contact loading. Two cracks 350 nm in length and oriented at 10° with respect to the normal to the epoxy surface have been considered. For symmetry reasons, only the results corresponding to one crack (denoted A) have been represented. The tangential cyclic loading has been divided into eight successive steps...

There is a cyclic stress-strain hysteresis with loading and unloading, with zero residual strain at the end of the cycle, which accounts for the superelasticity. 

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