FATIGUE CRACK GROWTH PROBLEM 3 © M N James

This problem illustrates the case where a cracked structure will not meet the fatigue life requirement, and explores the possibilities of increasing the cyclic life.  It is straightforward and should take around 20 minutes to complete.

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A structure contains a critical component made from A514 steel.  After fabrication of the structure, a welding defect 7.6 mm deep is discovered in this steel plate.  The flaw is essentially an edge crack under tension loading, and the required cyclic life of the structure is 100 000 cycles.  The component is subject to a fluctuating load which causes a stress variation from 172 MPa to 310 MPa.

Material properties for the A514 steel are: yield stress = 689 MPa, K1C = 165 MPa m½ geometry correction factor Y = 1.12, and the Paris law is:

where da/dN is in m/cycle, and:

i)    Calculate the fatigue life of this component based on attaining a critical defect size for fast fracture.

ii)    Accurately construct the curve showing crack length against number of applied load cycles.

iii)    Discuss the various measures that could be adopted to extend the life of the structure to 100 000 cycles.

iv)    What is the effect of reducing the initial defect size to 5 mm (by weld repair with better control of process parameters)?  Explain this result in terms of the shape of the curve of crack length versus cyclic life.

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Answer:   Nf = 87 992 cycles

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