Crack Paths 2009

Figure 7: Specimen tested under sea water flow, σa=160 MPa, Nf=1. 8×108 cycles.

Assessment of the crack initiation and propagation duration

Fatigue crack growth tests gave the da/dN=f(∆K) curve illustrated Figure 8a. This

shows that in air the mode I (R=–1) stress intensity threshold for R5 steel is around 3.3

MPa√m.The fatigue crack propagation duration was assessed according to the work of

Paris et al. [6, 7], based on the Paris-Hertzberg-Mc

1 Clintock crack growth rate

3

∆=bEKbdNdaeff



and

∆bEKeff

1=

at the corner, where E is the Elastic modulus and b the

Burger’s vector. Figure 8a shows the agreement of this equation with our experimental

data for E=211 GPaand b=0.258 nm. In our experiments at 20 kHz the measurement of

Kop is not possible, thus in first approximation, ∆Keff~Kmax and no water interaction with

the stress intensity factor was considered.

Figure 8. a) Experimental crack growth curve at 20 kHz, b) model of the fatigue growth

behaviour for short, small and long cracks according to [7].

To assess the crack propagation phase, a corrosion pit was modelled by an

hemispherical surface defect with radius R. A fatigue crack of depth a from the surface

of the hemisphere and perpendicular to the loading direction was assumed due to inter

crystalline corrosion cracking. According to the asymptotic approximation proposed by

Paris et al. [10] the crack tip stress intensity factor in mode I is: KI=σ√πa·Y(x,ν), where

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