1. FATIGUE TEST EXPERIMENT # 5 Instructor: M.Yaqub

2. FATIGUE

3. 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.

4. 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.

6. 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.

7. 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.

8. 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.

9. FACTORS EFFECTING FATIGUE LIFE FACTORS EFFECTING FATIGUE LIFE  Stress  Temperature  Surface Finish  Corrosion  Stress Concentration