Abstract
The present work encompasses Low Cycle Fatigue (LCF) of
the Al-alloy AA7020 with three conditions; annealing, natural
aging and artificial aging. The LCF tests carried out using standard
specimens cantilever beam types. Optical Light Microscope (OLM)
and Scanning Electron Microscope (SEM) were employed to
examine the fracture features .The results confirmed that AA7020-
O sustained cyclic ha rdening, while 7020-T4 & 7020-T6
undergoing cyclic softening, therefore make the annealing
conditions more resistance to LCF. The values of fa tigue str ength
exponent (b) is varying from (-0.064) to (-0.14) and fatigue
ductility exponent (c) from (-0.554) to (-0.60), whereas these
values within the general limitation of the metals. The number of
transition cycle (NT) for annealing condition is more comparing to
the other conditions which emphasis that the annealing alloy will
withstand more cycles before introducing the plastic zone. The
information extracted from Engineering Stress-Strain curve; (σu/σy)
as well as strain harden exponent (n) can be need to estimate the
behavior of annealing and artificial alloy, while the natural aging
alloy will need LCF testing to definite the conducting because of
its "n" less than (1.2) and (σu/σy ) more than (1.4) .The SEM
examination districted many point of cracks initiation for the three
alloys at stresses more than Yield point. OLM investigation of the
cross-section of fracture surface indicated the dominating of
applied stress when it is more than Yield point of artificial aging
alloy. Where the stress concentration is the most important role for
annealing specimens because of companion of cycle strain
hardening
Keywords
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