Cyclic loading crack growth under

LEFM discus.sed to this point refers to the resistance of bodies to crack growth under static loads. Crack growth under cyclic loading is faster than under static loads at the same stress amplitudes, because the rate of loading and the damage both increase with higher frequencies. 

In design analysis, G can be used to assess the onset of fracture under extreme loads and the onset of crack growth under static loads (creep failure), but more often the rate of crack growth under cyclic loads (fatigue failure). 

Fatigue damage is predominantly caused by crack growth under cyclic loading. Several factors affect the rate at which a crack propagates in a material ... 

A comparison of the crack velocities measured under static and cyclic loads is illustrated in Fig. 7.2. For this purpose, the crack velocity under cyclic loads, da/dt = da/dN x vc, plotted against the maximum stress intensity factor of the fatigue cycle, Kmax = AA7(1 — R), from the results shown in Fig. 7.1. The static crack velocity da/dt is also plotted against the stress intensity factor Kj corresponding to the applied load. In the intermediate range of crack growth, the static crack velocity generally follows the power-law relationship... 

Fig. 7. 13 Differences in crack wake contact and bridging mechanisms seen between static and cyclic fracture in AD 90 alumina at 1050°C. From Ref. 34. (a) Deflected crack path during static crack growth with no debris formation, (b) Deflected crack path with debris particles formed at a result of repeated rubbing between the crack faces under cyclic loading. Also seen are the debris and glass films which are squeezed out of the crack due to the pumping action of the crack walls.

The use of fatigue data and crack length measurements to predict the remaining service life of a stmcture under cyclic loading is possibly the most common application of fracture mechanics for performance prediction. In complex stmctures the growth of cracks is routinely monitored at intervals, and from data about crack growth rates and the applied loadings at that point in the stmcture, a decision is made about whether the stmcmre can continue to operate safely until the next scheduled inspection. 

A component is made of a steel for which K. = 54 MN m. Non-destructive testing by ultrasonic methods shows that the component contains cracks of up to 2a = 0.2 mm in length. Laboratory tests show that the crack-growth rate under cyclic loading is given by... 

The growth or extension of a fatigue crack under cyclic loading is principally controlled by the maximum load and stress ratio (minimum/maximum stress). However, as in crack initiation, there are a number of additional factors that may exert a strong influence, especially in the presence of an aggressive environment. 

High Temperature Crack Growth in Unreinforced and Whisker-Reinforced Ceramics under Cyclic Loads... 

Since the significant majority of the published literature on high temperature crack growth under static and cyclic loads is predicated upon experiments conducted on alumina and alumina matrix composites, the examples cited in the present review have centered around oxide ceramics and their composites. However, the implications of the results to other classes of ceramics, intermetallics, and brittle matrix composites are also described, wherever feasible, along with any available information in an attempt to illustrate the generality of the concepts developed here. 

The crack growth behavior of polymers under cyclic loading has been intensively studied and the state of knowledge is also well documented by excellent, recently published review articles mong which the comprehensive book on Fatigue... 

For a large number of polymers the crack growth behavior under cyclic loading has been studied and documented in review articles 165.171,207,208)... 

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