Foreign object damage and fatigue crack threshold : Cracking outside shadow indents


Research output: Journal PublicationsJournal Article (refereed)peer-review

44 Citations (Scopus)


Foreign Object Damage (FOD) usually happens when objects are ingested into jet engines powering military or civil aircraft. Under extreme conditions, FOD can lead to severe structural damage. More commonly it produces local impacted sites of the fan and compressor airfoils, lowering fatigue life of these components. FOD is a prime cause for maintenance and repair in aircraft engines. In this paper, a framework for analyzing FOD and its effect on fatigue cracking is established. A finite element analysis is used to identify three relevant regimes of FOD related to the depth of penetration into the substrate, and to determine the residual stresses. Most of the emphasis in this paper focuses on fatigue cracks emerging from shallow indentations, which are generally expected to be of most practical concern. Full three-dimensional finite element solutions are obtained for semi-circular surface cracks emerging from specific locations at the indentation revealing the influence of the residual stress on the stress intensity factor distribution. For shallow indents, a relatively simple dimensionless formula for the relation between the residual stress intensity factor the crack size, and the indentation width are developed. These results, together with results for the intensity factor variations due to cyclic loading, have been used to address the question: To what extent do the residual stresses caused by the FOD reduce the critical crack size associated with threshold fatigue crack growth? Formulas for the critical crack size are obtained. Specific results are presented for the blade alloy, Ti-6A1-4V, revealing that FOD can reduce the critical crack size by as much as 60%.
Original languageEnglish
Pages (from-to)31-51
Number of pages21
JournalInternational Journal of Fracture
Issue number1
Publication statusPublished - Jan 2001
Externally publishedYes

Bibliographical note

This work was supported in part by the Multi-University Research Initiative on ‘High Cycle Fatigue’, which is funded at Harvard by AFSOR under Grant No. SA1542-22500 PG, and in part by the Division of Engineering and Applied Sciences, Harvard University.


  • Contact mechanics
  • Crack mechanics
  • Critical crack size
  • Fatigue analysis
  • Foreign object damage
  • Indentation regimes


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