Location, location & size: defects close to surfaces dominate fatigue crack initiation

Itziar Serrano-Munoz,Yves Nadot,Rajmund Mokso,Jean-Yves Buffiere,Catherine Verdu

doi:10.1038/srep45239

Abstract

Metallic cast components inevitably contain defects such as shrinkage cavities which are inherent to the solidification process. Those defects are known to significantly alter the fatigue life of components. Yet very little is known, quantitatively, on the dangerosity of internal casting defects compared to surface ones. In this study, fatigue specimens containing controlled internal defects (shrinkage pores) are used to foster internal cracking. In situ fatigue tests monitored by X ray synchrotron tomography revealed that the internal nucleation and propagation of cracks was systematically overran by surface cracking initiated at castings defects up to ten times smaller than the internal ones. These findings indicate that the presence of internal defects in cast components can be tolerated to a larger extent than is allowed by nowadays standards

Highlights

Metallic cast components inevitably contain defects such as shrinkage cavities which are inherent to the solidification process
It has been observed since long that gases have a deleterious effect on fatigue life
Internal failure in cast Al alloys is rarely observed during high cycle fatigue (105

Summary

Introduction

Metallic cast components inevitably contain defects such as shrinkage cavities which are inherent to the solidification process. In situ fatigue tests monitored by X ray synchrotron tomography revealed that the internal nucleation and propagation of cracks was systematically overran by surface cracking initiated at castings defects up to ten times smaller than the internal ones These findings indicate that the presence of internal defects in cast components can be tolerated to a larger extent than is allowed by nowadays standards. The propagation period depends on the intensity of the driving force (induced by the remote stress field) at the crack-tip, as well as on the surrounding environment It has been observed since long (first studies date back to 1917) that gases have a deleterious effect on fatigue life. As for the environmental conditions of internal cracking, they are generally considered to be close to those of vacuum as their fracture surface features look similar[26,28,29]

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