Crack cyclic loading

A common feature of all experiments is that the test specimens are precracked. To achieve this, a notched sample is used and a crack is propagated from this notch by cyclic loading (see chapter 10) as shown in figure 5.13. Creating the initial crack in this way is necessary because the notch tip is usually not sharp enough to behave like a true crack. Cyclic loading allows to produce the initial crack at a load that is much smaller than that needed for static experiments (see chapter 10). 

The cyclic loading applied to hull structure will eventually lead to fatigue cracking. 

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. 

Data on fatigue crack propagation are gathered by cyclically loading specimens containing a sharp crack like that shown in Fig. 15.7. We define... 

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. 

The engine could still be run for demonstration purposes. Suppose that you are called in to assess its safety. We will suppose that a crack 2 cm deep has been found in the connecting rod - a cast-iron rod, 21 feet long, with a section of 0.04 m. Will the crack grow under the cyclic loads to which the connecting rod is subjected And what is the likely life of the structure ... 

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... 

During cyclic loading, any cracks in the material will propagate until they reach this critical size. If the article is to have an endurance of at least 10 cycles then equation (2.119) may be used to determine the size of the smallest flaw which can be present in the material before cycling commences. 

Under a repeated applied cyclic load, fatigue cracks begin somewhere in the product and extend during the cycling. Eventually the crack will expand to such an extent that... 

Several criticisms of these parameters have recently been pointed out. First, they have no specific association with a material plane (i.e., they are scalar parameters), despite the fact that cracks are known to nucleate on specific material planes. With traditional parameters it is difficult to account for the effects of crack closure under compressive loading. Traditional parameters have not been successful at unifying experimental results for simple tension and equibiaxial tension fatigue tests. Finally, a nonproportional loading history can always be constmcted for a given scalar equivalence parameter that holds constant the value of the scalar parameter, but which results in cyclic loading of material planes. For such histories, scalar parameters incorrectly predict infinite fatigue life. 

Benzeggagh, M.L., Prel, Y.J. and de Charentenay, F.X. (1985). Instrumentation of mode I and mode II tests for crack tip strain profile study under static and cyclic loading. In Proc. ECCM-l, p. 291. 

Figs. 4.44 and 4.45 show the increase in the debond length, f, and displacement, as a result of the reduction of p (from Po = 0.22 to p = 0.07) under cyclic loading. It is interesting to note that both I and 5 remain constant until the coefficient of friction, p, is reduced to a critical value p. (= 0.144 and 0.166, respectively for fiber pull-out and fiber push-out). The implication is that the debond crack does not grow... 

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