Low Cycle Fatigue Failure of AA7020 Aluminum Alloy at different heat treatments


The present work encompasses Low Cycle Fatigue (LCF) ofthe Al-alloy AA7020 with three conditions; annealing, naturalaging and artificial aging. The LCF tests carried out using standardspecimens cantilever beam types. Optical Light Microscope (OLM)and Scanning Electron Microscope (SEM) were employed toexamine the fracture features .The results confirmed that AA7020-O sustained cyclic ha rdening, while 7020-T4 & 7020-T6undergoing cyclic softening, therefore make the annealingconditions more resistance to LCF. The values of fa tigue str engthexponent (b) is varying from (-0.064) to (-0.14) and fatigueductility exponent (c) from (-0.554) to (-0.60), whereas thesevalues within the general limitation of the metals. The number oftransition cycle (NT) for annealing condition is more comparing tothe other conditions which emphasis that the annealing alloy willwithstand more cycles before introducing the plastic zone. Theinformation extracted from Engineering Stress-Strain curve; (σu/σy)as well as strain harden exponent (n) can be need to estimate thebehavior of annealing and artificial alloy, while the natural agingalloy will need LCF testing to definite the conducting because ofits "n" less than (1.2) and (σu/σy ) more than (1.4) .The SEMexamination districted many point of cracks initiation for the threealloys at stresses more than Yield point. OLM investigation of thecross-section of fracture surface indicated the dominating ofapplied stress when it is more than Yield point of artificial agingalloy. Where the stress concentration is the most important role forannealing specimens because of companion of cycle strainhardening