FATIGUE LOADING Fluctuating or Cyclic loading (as shown in previous slide) is referred as Fatigue loading. A metal component may fail below stress corresponding to tensile strength or even below yield strength if it is subjected to Fatigue loading. Most of the failure in engineering are because of fatigue loading.
FATIGUE TEST PROCEDURE A Standard sample is used for the metal for which fatigue test is required. Test procedure is explain in the handout. A number of sample is used for the test and data plotted as applied stress (S) on Y-Axis and fatigue life (N) on X-Axis. This curve is called S-N Curve. Log-Log scale is used for S-N curve. Few S-N curves are given in next slide.
ENDURANCE LIMIT OR FATIGUE LIMIT From S-N curve, it is clear that when applied stresses are reduces, the life of the part increases and vise versa. For steel, a horizontal line at the end of the curve shows that the below this stress level, the metal will not have a fatigue failure. This level of stress below which no fatigue failure is called ENDURANCE LIMIT OR FATIGUE LIMIT. This can be observed in steel only. What is endurance limit for steel in the previous slide.
FATIGUE STRENGTH When there is no fatigue limit for metal, we define another stress called fatigue strength Fatigue strength is defined as the stress corresponding to some definite life cycle usually taken as corresponding to 10 millions cycles. Note that from S-N Curve first try to fine fatigue limit. If not possible, then find fatigue strength. What is fatigue strength for Aluminum and Brass in previous slide.
DATA NOT COMPLETE When there is no horizontal line for any S- N curve at the end and data is not available up to 10 millions cycles, it means neither fatigue limit nor fatigue strength can be obtained from the curve. In such cases, the data is not complete and few more sample has to tested to complete the data.
FACTORS EFFECTING FATIGUE LIFE FACTORS EFFECTING FATIGUE LIFE Stress Temperature Surface Finish Corrosion Stress Concentration