Investigation of Cyclic Deformation and Fatigue of Polycrystalline Cu under Pure Compression Cyclic Loading Conditions

It is commonly accepted that fatigue crack is initiated under tensile fatigue stresses. However, practical examples demonstrate that cracks may initiate under pure compressive fluctuating loads, e.g. the failures observed in aircraft landing gear frames. As the mechanism of such failures is rarely investigated, there is very limited or non-existent knowledge pool on cyclic deformation response under pure compressive fatigue condition. Our recent work verified that fatigue cracks may nucleate from stress concentration sites under pure compression fatigue, but whether or not a form of stress concentration is always needed to initiate a crack remains uncertain. In this study, compression fatigue tests under different peak stresses were carried out on smooth bars of fully annealed OFHC Copper. The purpose of these tests is to investigate not only the cyclic deformation response but also the possibility of crack nucleation without the stress concentrator. Results showed that overall the cyclic stress-strain response and microstructural evolution of OFHC Copper under pure compression fatigue exhibits rather dissimilar behaviour compared to those under symmetrical fatigue. The specimens hardened rapidly within 10 cycles under pure compression fatigue unlike the gradual cyclic hardening behaviour in symmetrical fatigue with the same peak stress amplitude. Compressive cyclic creep behaviour was also observed. Moreover, TEM observation showed that only moderate slip activity was detectable on the surface instead of typical PSB features. The surface observations revealed that surface slip bands did not increase in number nor height as cycling progressed. In addition, surface roughening by grain boundary extrusion was detected to become more severe with further cycling. Therefore, the plastic strain accommodated within the samples was not mainly related to dislocation activities. Instead, the mechanism of cyclic creep response for pure compression fatigue was correlated and caused primarily by grain boundary extrusions. Such phenomenon was found to eventually lead to crack nucleation. Furthermore, from symmetrical fatigue testing, it was noted that depending on the polarity of the loading spectrum in the first half cycle, i.e. compression or tension, regular or anti-Bauschinger effect was observed, respectively. Such observations further elucidate the role of compression loading spectrum versus tension loading in fatigue.Ph.D