Endurance Limit

ENDURANCE LIMITIt is defined as the maximum amplitude of completely reversed stress that the standard specimen can sustain for an unlimited number of cycles without fatigue failure .

FATIGUE LIFE :- It is defined as the number of stress cycles that the standard specimen can complete during the test before the appearance of the first fatigue crack.

- Specimen is machined and polished .- Final polishing is done in axial direction in order to avoid circumferential stresses.- beam of circular cross section is subjected to bending moment Mb . - tensile stresses are induced in upper half - compresssive stresses are induced in lower half - max tensile stress in upper half is equal to max comp. stress in lower half .- 0 stress at the centre

- The graph appears to suggest that the endurance limit ranges from 40 to 60% of the tensile strength for steels up to about 1400MPa. -Beginning at about 1400MPa , the scatter appears to increase , but the trend seems to level off .- dispersion of the endurance limit is not due to a dispersion in the tensile strength , but rather than the spread occurs even when the tensile strengths of a large number of specimens remain exactly the same .- The ratio endurance limit to the ultimate tensile stress is called fatigue ratio.

Fatigue Strength :

Graph Observation-A region of low cycle fatigue extends from N=1 to about 1000 cycles . In this region fatigue strength Sf is only slightly smaller than the tensile strength Sut.-High cycle fatigue domain extends from 1000 cycles to 1000000 cycles .

Endurance Limit Modifying FactorsThe reasons behind the difference in actual endurance of a mechanical member and the values obtained in lab are :- Material : composition , basis of failure , variability.-Manufacturing :Method , heat treatment , fretting corrosion , surface condition , stress concentration.-Environment: corrosion , temperature .-Design : size , shape ,life , stress state, stress concentration etc.

Marin’s Equation :Se= Ka Kb Kc Kd Ke S’ewhere Ka = surface condition modification factor Kb= size modification factor Kc= load modification factor Kd= temperature modification factor Ke= miscellaneous –effects modification factor S’e= rotary beam endurance limit Se= endurance limit at the critical location

Surface Factor ka:- It depends on the quality of the finish of the part surface and on the tensile strength .Size Factor kb:- For axial loading there is no size effect so kb=1Temperature Factor kd :- When operating temperatures are below room temperature , brittle fracture is a strong possibility .When operating temperatures are higher than room temp. , yield strength drops off

Miscellaneous –Effects Factor ke:-ke is intended to account for the reduction in endurance limit due to all other effects viz.i) Residual Stressesii) Corrosion: - pitting iii) Electrolytic Platingiv) Metal Spraying v) Frettage Corrosion : It is the result of microscopic motions of tightly fitting parts or structures . E.g bolted joints , wheel hubs ,etc.