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This work addresses the influence of Bauschinger effect upon residual stresses and associated fatigue lifetimes for pressurized, autofrettaged thick cylinders.The model employed allows for the variation with radius of Bauschinger Effect Factor (BEF) thoughout the autofrettaged tube since the percentage plastic strain, which determines BEF, will vary from a maximum value at the bore to zero at the elastic-plastic interface.
Accounting for BEF variability it is demonstrated that the residual compressive hoop stress at the inner radius of the tube reaches a maximum value at the percentage overstrain level below which reversed yielding does not occur. Existing experimental residual stress measurements from a variety of sources are shown to support this thesis. This value of overstrain may serve to maximize crack initiation lifetime in autofrettaged thick cylinders.
For a tube with significant heat-checking and associated initial crack-like defects it is necessary to consider fatigue crack growth rates governed by a crack growth law such as Paris' Law. For a tube of radius ratio 2.0 and at a value of approximately 40% overstrain, slightly in excess of that for the onset of reversed yielding, the fatigue lifetime exhibits a maximum value. In general fatigue lifetimes achieve a maximum value at overstrain levels in which yielding reaches 1.4 times bore radius and are almost constant thereafter. Furthermore such extended overstrain leads to a small increase in residual stress at the outside diameter (OD), thus increasing R ratio at that location and reducing fatigue lifetime for crack growth originating at the OD. Existing experimental lifetime measurements are shown to require the inclusion of BEF to properly account for these observed lifetimes.
Bauschinger effect, crack growth, fatigue cracks, fatigue lifetimes, cylinders, fracture (materials), fracture mechanics, residual stress, stress intensity factor
Visiting professor, Royal Military College of Science, Cranfield, Swindon
Research engineer, Army Armament R D & E Center, Watervliet, NY