Damage accumulation in stress concentration zone by data of non-destructive and destructive methods

The study decribes the novel developed and verified non-destructive method for damage accumulation quantifying in stress concentration area under low-cycle fatigue conditions. The created approach employs the evolution of plastic print diameter caused by a spherical ball indentation over lifetime. The ball indentation is performed near the hole in plane rectangular specimens with different levels of damage accumulation. The main scientific novelty of the approach resides in involving current damage indicators, which can be reliably derived on a base of the simplest measurements of two in-plane displacement components by speckle-pattern interferometry. Comparison of the results following from the proposed method with analogous data obtained by known destructive approach provides a way to estimate the reliability of non-destructive technique. There is a unique way for comparing two approaches by using the same specimens because plastic prints are applied to the single external face of each specimen. The opposite face is successively used for registration of fringe patterns, which are attributed by narrow notch inserting. Fracture mechanics parameters, which are related to artificial notche and obtained on different stages of low-cycle fatigue, provide damage accumulation function in an explicit form. Experimental dependencies of the notch mouth opening displacement and stress intensity factor from the loading cycle number are constructed by data of fringe patterns interpretation for seven specimens. Damage accumulation functions constructed proceeding from the ball indentation technique and artificial notch inserting method are in good agreement. Thus, high efficiency of the ball indentation approach with respect to quantitative analysis of damage accumulation is clearly substantiated.