About endurance limit of ductile inhomogeneous materials

The theory describing the fatigue mechanism in elasto-plastic material containing pores or inclusions has been developed. An attempt at quantitative determination of the effect of endurance limit reduction by analysis of sizes of plastic zones formed near the inclusions, and their cracking has been done. The geometrical configuration, consisting of a round inclusion from which a nucleating crack emerged, was considered, and the stress intensity factor of such configuration was analysed. Based on a threshold value of K below which crack propagation ceases, the critical value of loading stress was determined. Theoretical results were compared with results from experiments, showing quite good agreement.Glossary of Symbols a rack length (mm) - A plastic zone range (mm) - B width of specimen (mm) - D pore diameter (mm) - H materials hardening coefficient - K stress intensity coefficient (N mm^–3/2) - K _I,K _II stress intensity coefficient for first and second mode of fracture - KIZ equivalentK _I coefficient for zigzag crack. (N mm^–3/2) - KTH threshold value ofK (N mm^–3/2) - KTHM KTH in microscale (N mm^–3/2) - L length of flat crack (mm) - Na real length of zigzag (mm) - N _f fatigue life in cycles - P loading force variation (N) - RA reduction of area of sample - S loading stress (MPa) - W height of specimen (mm) - Y yield stress of matrix material (MPa) - , , coefficients inA/D=f(S/Y) formula - K stress concentration coefficient - , , coefficients inK=f(S, A/D) formula - _f, ₁ coefficients - _p plastic strain components - , parallel and perpendicular to crack front surface development correction coefficients - surface development coefficient