Influence of bending on low cycle fatigue life of cylindrical test pieces at elevated temperature

Abstract

In fatigue testing, normally cyclic-through-zero-testing, reversed-bending misalignment caused by load-strain offset from the frame centre line in a non-rigid system can have the most critical influence on low cycle fatigue (LCF) endurances. Hence, a unique systematic experimental study was established to investigate the influence of reversed bending on fatigue properties at high temperature. A special test programme provides a total of 18 tension-compression fatigue tests on alloy Nimonic 101 at 850 C. Prior to start of the fatigue test, bending was adjusted, whereby three different bending levels were of interest: a first level at bending rate of less than 2% addressed to good alignment, a second level at a medium bending rate of 20% and finally, a third level at a high bending rate of 40%. Bending measurements follow a code of practice. Standard strain controlled fatigue tests were conducted at two typical load levels. A low level of total strain range represents the elastic regime and a high level represents the elasto-plastic regime, which is expected to be the most important level.The advanced features of this experimental work deals with the development of a special composite test piece and the application of a commercial alignment fixture to adjust bending due to lateral grip offset. The test results show that in the elasto-plastic regime, a systematic reduction of LCF endurance of up to factor two with increasing superimposed bending rate of up to 40%. In the elastic regime, no systematic effect was observed. The within-laboratory repeatability was found to be small, demonstrating good testing practice with regard to strain control and temperature control as well.