Analysis of Fatigue Growth from Cold-expanded/interference Fitted Stop Drilled Holes
DEFENCE SCIENCE AND TECHNOLOGY ORGANISATION MELBOURNE (AUSTRALIA)
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When fatigue cracks are found in aircraft structures the safe life of the structure is of concern. If the fatigue life can be safely extended then aircraft operating costs can be lowered. Previous work has found that plastic expansion of a hole resulting in residual compression upon elastic recovery can result in a decrease of the mean stress under cyclic loads resulting both in an increase of life and an increased critical crack length. In an experimental program recently carried out at AMRL a procedure was used to stop drill the crack, cold work the stop hole and use interference fit plugs. This resulted in an increase of stress level by a factor of 2 for fatigue life of 10,000 Falstaff blocks. Overall it has been found that the combination of interference fit plugs with cold expansion of the stop drilled holes significantly reduce the crack driving force and hence extends the fatigue life of a cracked structure. Results achieved in this report indicate that this is a promising procedure for the life extension of RAAF aircraft structures.