Some Practical Implications of Exponential Crack Growth
A review of experimental data show that for many lead fatigue cracks in service components loaded with service spectra, exponential growth (i.e. log crack depth versus cycles or hours) applies for the majority of the life. This behaviour is shown to extend from the micro to macro range of crack sizes in a variety of metals. As a consequence of this, it will also be shown that the crack growth rate is directly proportional to the crack depth. By combining these observations with traditional fracture mechanics approaches to crack growth modelling, a model that is a function of the stress intensity factor (K) with a fixed crack depth influence (non-similitude for the K parameter alone) is proposed. It will then be shown that this model allows for Region I to be smoothly integrated with Region II of the constant amplitude da/dN data. Further, it will be shown that for variable amplitude crack growth data, crack growth ranging from microns to many millimetres can be modelled using this single model.
This modelling approach is of particular importance in structural integrity analysis where fatigue cracking cannot always be avoided and the majority of the fatigue life of highly stressed, nominally gross defect free structure is spent growing physically small cracks from initiating discontinuities (i.e. loads in Region I for constant amplitude loading growth rates) up to the point of loss in acceptable strength.
KeywordsFatigue Crack Fatigue Life Crack Length Crack Growth Rate Fatigue Crack Growth
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