Observations of Tension and Torsion Fatigue Cracking Behavior and the Effect on Multiaxial Damage Correlations
by
Julie Ann Bannantine
Abstract
A series of tension and torsion strain-controlled fatigue tests were conducted on several engineering materials. Detailed observations of nucleation and early crack growth were made. Cracking behavior is shown to depend upon loading mode (tension or torsion), strain amplitude and material type.
Three regions of cracking behavior were observed in torsion in AISI 304 and 316. In tests conducted at high shear strains, Region I, cracks initiated and remained on maximum shear strain (Stage I) planes. At intermediate strains, Region II, cracks initiated on shear planes but linked up to form growth on planes perpendicular to the maximum principal strain (Stage II). In Region III, at low strains, a few cracks initiated on shear planes but quickly branched and propagated on Stage II planes. Other materials exhibited characteristics of one or more of these regions when tested in torsion. Cracks in Inconel 718 remained on shear planes, Region I behavior, for a wide range of strain amplitudes while SAE 1045 exhibited Region II behavior only at very low strains. In gray cast iron, only tensile cracks, Region III behavior, were found.
Multiaxial models which were developed for shear strain dominated materials did not correlate multiaxial test results of a principal strain dominated material. Differences in cracking behavior for different materials and loading conditions need to be considered in successful life predictions for components subjected to multiaxial fatigue.
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