| 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. FOD is a prime cause for maintenance and repair in aircraft engines. In this thesis, a framework for analyzing FOD and its effect on fatigue cracking is established. The 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 thesis focuses on fatigue properties of cracks emerging from different locations of shallow and deep indentations, and the comparison between theoretical and experimental results on the residual stress fields caused by FOD.; Relatively simple dimensionless formulas for the relation between the residual stress intensity factors of semi-circular surface cracks emerging from specific locations at the indentation, 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: When and to what extent do the residual stresses caused by FOD reduce the critical crack size associated with threshold fatigue crack growth? Formulas for the critical crack sizes are obtained. Specific results are presented for the blade alloy, Ti-6Al-4V, revealing that FOD can reduce the critical crack size by as much as 60% for shallow indents and more for deep impacts. For deep indents, it is found that elastic stress concentration is the dominant factor in reducing critical crack threshold when the applied load ratio is large, otherwise the residual stresses are important.; In the last chapter, dynamic finite element results of residual stress fields are presented and compared with parallel experimental studies. Size, rate and other dynamic factors are factored into the approach. The cases of stress relaxation, laser shock peened sample and inclined impacts are also studied. The comparison shows that by using the dynamic analysis and incorporating rate sensitivity, the numerical approach yields good agreements with experimental results and can account for various phenomena observed in practice. |
