Abstract
The novel destructive method is implemented for quantitative assessment of fatigue damage accumulation in the stress concentration zone accompanied by residual stress due to cold expansion of the through-thickness hole. Damage accumulation is reached by preliminary cyclic loading of plane specimens with cold-expanded holes. Narrow notches, emanating from the hole edge at different stages of high-cycle fatigue, serve to manifest a damage level. These notches are inserted without applying external load. Deformation response to local material removing, caused by pure residual stress influence, is measured by electronic speckle pattern interferometry (ESPI) in terms of in-plane displacement components. Normalized values of the notch mouth open displacement (NMOD), in-plane displacement component at the initial point of the notch acting in the notch direction (U0), in-plane displacement component at the final point of the notch acting in the notch direction (U1) and the stress intensity factor (SIF) are used as current damage indicators. Numerical integration of curves, describing an evolution of each fracture mechanics parameter over lifetime, produces the damage accumulation function in an explicit form. It is established that all four fracture mechanics parameters give very close results.
Highlights
A process of cold hole expansion widely serves to enhance the fatigue life of aerospace structures [1]
Inserted notches serve to manifest the level of fatigue damage accumulation as well as a probe hole is used for residual stress energy release in the hole-drilling method
It has been established that normalized notch mouth open displacement (NMOD), U0, U1 and stress intensity factor (SIF) values, which are obtained for the narrow notch emanating from the cold-expanded hole at different stages of high-cycle fatigue, can be reliably involved into the analysis as representative damage indicators
Summary
A process of cold hole expansion widely serves to enhance the fatigue life of aerospace structures [1]. The essence of the developed approach is that the evolution of each of four fracture mechanics parameters, which are referred to the artificial narrow notch inserted without applying external load, can be effectively used for quantification of damage accumulation The performance of this methodology has been earlier demonstrated by using fracture mechanics parameters evolution, namely, NMOD, SIF and T-stress for notches emanating from throughthickness open hole in plane specimens at different stages of low-cycle fatigue [16]. This paper concerns an evolution of both non-singular (NMOD, U0, U1) and singular (SIF) parameters relevant to the narrow artificial notch This approach can be implemented for quantifying damage accumulation in the vicinity of the cold-expanded hole under high-cycle fatigue conditions. Where Nm is current number of loading cycles; Nm NF corresponds to full failure of the specimen; FMPk defines fracture mechanics parameters used for the analysis; FMP 1 is NMOD v0 ; is SIF
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