The objective of the fatigue test is to illustrate the behavior of A-36 steel and it’s response to a repetitive or cyclic loading. The technique of measuring a material’s fatigue life will also be examined, as well as how to present and calculate the relevant data. This lab will also show the effects of surface condition on a material’s fatigue life.
Introduction/ Experimental Setup:
The experimental method of the fatigue test is as follows: two specimens of similarly proportioned A-36 steel will be provided, one having a polished surface and the other having a shot-peened finish. A specimen is placed into the machine (called a rotating-beam fatigue test machine) that has a counter that counts once for each 100 revolutions and is turned on. While rotating a constant moment is applied to the specimen, the stress at any point makes a complete cycle from tension at a given point on the specimen to compression and back again. The data recorded for the specimens includes the bending moment applied and the number of cycles to failure (assuming the specimen failed). The fatigue test can often run longer than 2 hours and therefore our lab group used fatigue test data from a previous experiment.
Typically, an S-N (Maximum Stress versus Number of Cycles) diagram is used to show the relationship between the number of cycles N for fracture and the maximum value of the applied cyclic stress. Generally, the fatigue decreases rapidly between 10^3 to 10^6 cycle range. The curve then flattens out and the stress corresponding to failure at an infinite number of cycles is referred to as the “endurance limit, Se”. Oftentimes, an S-N curve is approximated before a fatigue test is run. This is done by first determining the ultimate tensile strength Su, of the specimen, marking 0.9 Su at 10^3 cycles and 0.5 Su at 10^6 cycles. These two points are joined together and then a horizontal line is continued from 0.5Su. According to the Goodman line, the maximum alternations stress decreases from the endurance limit (Se) to zero