Low-Cycle Fatigue Testing Using a Small Specimen of Sn-58Bi Solder at 313 K and 353 K

Low-cycle fatigue tests using small specimens of Sn-58Bi solder have been performed at 313 K and 353 K. The samples had a gage diameter and gage length of 3 mm and 6 mm, respectively. The tests were conducted in strain-control mode, using triangular waveforms with a strain rate of 0.5%/s over a total strain range of 0.5% to 1.5%. The failure life was longer at 353 K than at 313 K, and the slope of the total strain range against failure life at 353 K was steeper than at 313 K. Stress amplitude was retained at 313 K even beyond the middle of the sample fatigue life, but decreased dramatically just before failure due to the brittleness of the specimen. However, at 353 K, the number of cycles to failure was greater after the initial decrease in stress amplitude because of the increase in crack resistance (ductility) of the specimen when hot. The morphology of cracks was affected by the temperature of the test. The main crack that appeared at 313 K was smooth and oriented nearly perpendicular to the specimen axial direction. However, the crack that formed at 353 K had a zigzag shape. The increased ductility and crack resistance at 353 K contributed to the increased failure life at higher temperatures.