Metal strength characteristics at two stages of fatigue failure and different test bases

Abstract

1. The greatest differences in the strength limits exhibited over the test basis range 104<Nb<106 cycles when the fatigue failure process is separated into two stages is in absolute values shown more for the cut-out testpieces of various shapes and less for the smooth testpieces. When the test basis is increased to 107−2·108 cycles this difference is reduced. 2. When the stress concentration is varied, as the stress levels and their concentrations are raised there is an overall increase in the working life (in cycles) of testpieces with developing fatigue macrocracks which is independent of their shapes and absolute dimensions. 3. The final fractures (breaks) of beams of width 24 and height 90 mm of type 45 steel with a hardness HB=217 kgf/mm2 which are in a brittle condition as a result of cyclic damage accumulation occur with a macrocrack area a quarter of that for the same beam of the same steel with a hardness NB=187 kgf/mm2. 4. The nature of the distribution for the stress intensity factors for beams of dimensions 24×90×700 mm of type 45 steel with a hardness HB=217 kgf/mm2 is similar to the type of distribution of the macrocrack critical lengths for these beams and close to a normal distribution. 5. The coefficients of variation for the mean stress intensity factors (with no account taken of plastic strain at the crack tip)\(V_{K_c }\), are three to four times greater than the coefficients of variation of the mean strength limit stress amplitudes\(V_{(\sigma _a )s.1.}\) for beams of dimensions 24×90×700 mm made from type 45 steel with a hardness HB=217 kgf/mm2.