PSI - Issue 28

Jan Seyda et al. / Procedia Structural Integrity 28 (2020) 1458–1466 Author name / Structural Integrity Procedia 00 (2019) 000–000

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4. Discussion The first trend that can be seen is that the density of the resulting small cracks increased with the level of loading, regardless of the loading case. The main crack is formed by the coalescence of small cracks. This phenomenon depends on the density of small cracks and the their growth rate. They both depend primarily on the loading level and material microstructure as reported by Ochi et al. (1985). The case of torsional loading is characterized by the longest fatigue life, compared to other cases. In this case, the cracks formed and grew only in two directions, 0° and 90° with respect to the specimen’s axis, on planes where only shear strain acted (Fig. 4 (b)). In turn, under axial loading, at which the fatigue life is a little shorter than for torsion, cracks formed and propagated in many directions from 0 ° to ± 45 ° (Fig. 4 (a)), more often coinciding with the planes of easy glide of the crystal lattice in individual metal grains, which has been described by Zhang and Miller (1996). On the other hand, normal strain accelerating the development of cracks also occur in the planes of maximum shear strain. The out-of-phase loading is characterized by the shortest fatigue life. The vector of maximum shear strain rotates in time in all directions (Fig. 4 (c)) activating many slip systems in the material. Small cracks initiated and propagated in all directions. However the longest and most opened cracks were those at which the maximum normal strain occurred. 5. Conclusions Replication technique and displacement-control, based on previously recorded history, were useful for tracking the small cracks on surfaces of specimens manufactured from PA38-T6 aluminum alloy. The small cracks were imprinted in replicas only after linking of at least 3 cracks. Only then, the crack is wide enough to be penetrated by the melted cellulose acetate sheet. Single cracks were visible only as shadows at higher magnifications. A different behavior was noticed under OP loading. Single small cracks of comparable length were more opened. This is most likely because in the plane of crack, the maximum shear strain act together with the highest normal strain opening the cracks. For the tested specimens, creating replicas of the specimens’ surfaces at the highest loading levels has become a difficult task, due to the extremely large surface irregularities resulting from high deformation which increase the number of defects on the replicas, such as air bubbles, wrinkles, and non-stick fragments. Shear damage mechanism was identified at the applied loading levels, regardless the loading case. Main crack formed by coalescence of small cracks. The exception was noted for out-of-phase loading at 0.002 equivalent strain, where propagation of cracks occurred. The main crack formed from 4 propagating cracks, and the total time of the main crack formation took less than 2% of fatigue life. In these preliminary tests, it was possible to observe differences in the fatigue process, not only depending on the loading level, but also on the loading case. The number of directions of small cracks as well as the level of shear and normal strain in these directions seem to be of particular importance. The research is still in progress. It will be extended, to some point, to the lower loading levels (high cycle regime), to see if there is a change in damage mechanism. Other cases of loadings than currently applied will be used, including asynchronous loadings. It is planned to perform the same research on austenitic stainless steel. References Shamsaei N., Fatemi A., 2014 .Small fatigue crack growth under multiaxial stresses. Int. J. Fatigue. 58, 126–135. McClaflin D., Fatemi A., 2004. Torsional deformation and fatigue of hardened steel including mean stress and stress gradient effects. Int. J. Fatigue. 26, 773–784. Foletti S., Corea F., Rabbolini S., Beretta S., 2018. Short cracks growth in low cycle fatigue under multiaxial in-phase loading. Int. J. Fatigue. 107, 49–59. Kurath P., Socie D.F., 1988. The Relationship Between Observed Fatigue Damage and Life Estimation Models. NASA Contractor Report 182191. Pejkowski Ł., Skibicki D., 2019. Stress-strain response and fatigue life of four metallic materials under asynchronous loadings: Experimental observations. Int. J. Fatigue. 128, 105202. Pejkowski Ł., Seyda J., Skibicki D., 2019. Short cracks observations on surfaces of specimens made of three materials, subjected to synchronous and asynchronous multiaxial loadings, MATEC Web Conf. 300, 15002. Main B., Molent L., Singh R., Barter S., 2020. Fatigue crack growth lessons from thirty-five years of the Royal Australian Air Force F/A-18 A/B Hornet Aircraft Structural Integrity Program. Int. J. Fatigue. 133, 105426.