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Surface Damage and Fatigue Strength of Ultrafine Grained Copper Treated by Mild Annealing
Teshima, N.,Goto, M.,Han, Seung Zeon,Euh, Kwang Jun,Yamauchi, K.,Kawagoishi, Norio Trans Tech Publications, Ltd. 2013 Key Engineering Materials Vol.577 No.-
<P>Fatigue tests of ultrafine grained (UFG) copper produced by equal channel angular pressing (ECAP) showed negligible enhancement of fatigue strength in high-cycle fatigue regime. This was attributed to the thermal instability of UFG microstructure; a grain coarsening occurred during a large number of stress repetitions. Aiming at an enhancement of fatigue strength in the long-life field in excess of 10<SUP>7</SUP>cycles, post-ECAP mild-annealing for improving thermal stability was conducted. Grains with sizes up to a few tens of micrometers surrounded by fine grains were formed after the annealing as a result of discontinuous recrystallization. The improved stability of post-ECAP annealed micrstrucvture was related to the 9% increased fatigue strength in long life fields. The effect of bimodal microstructure on surface damage formation and fatigue strength was discussed.</P>
Fatigue life enhancement of defective structures by bonded repairs
Wang, Q.Y.,Kawagoishi, N.,Chen, Q.,Pidaparti, R.M. Techno-Press 2004 Structural Engineering and Mechanics, An Int'l Jou Vol.18 No.3
Defective metallic components and structures are being repaired with bonded composite patches to improve overall mechanical and fatigue properties. In this study, fatigue crack growth tests were conducted on pre-cracked 7075/T6 Aluminum substrates with and without bonded Boron/epoxy patches. A considerable increase in the fatigue life and a decrease in the stress intensity factor (SIF) were observed as the number of patch plies increased. The experimental results demonstrate that the patch configurations and patch thickness can enhance fatigue life by order of magnitude. Quantitative comparisons between analytical and experimental data were made, and the analytical model based on a modified Rose's analytical solution appears to best estimate the fatigue life.
Goto, M.,Ando, Y.,Han, S.Z.,Kim, S.S.,Kawagoishi, N.,Euh, K. Pergamon Press ; Elsevier Science Ltd 2010 Engineering fracture mechanics Vol.77 No.11
The growth behavior of cracks is monitored on specimens of ultrafine grained copper produced by equal channel angular pressing. Temporary retardation of crack growth under low stress amplitudes occurs when the crack length reaches about 0.1mm, but there is no similar retardation at high stress amplitudes. Dependent on stress amplitude, different crack growth path morphologies develop. Analysis of the fracture surfaces is conducted by scanning electron microscopy, showing planer, granular and striated surfaces. The physical background of growth path and fracture surface formation is discussed by considering crack growth mechanism and microstructural inhomogeneity.
Goto, M.,Han, S.Z.,Euh, K.,Kang, J.H.,Kim, S.S.,Kawagoishi, N. Elsevier Science 2010 Acta materialia Vol.58 No.19
Fatigue tests were conducted on smooth specimens of ultrafine-grained copper produced by 4 and 12 passes of equal channel angular pressing (henceforth referred to as UFG4 and UFG12, respectively). A major crack was initiated from shear bands at an early stage of stressing. The UFG4 and UFG12 samples exhibited different growth behavior tendencies at a low crack growth rate (CGR). For UFG12, the CGR initially increased as the crack was extended with continued fatigue cycling, but then abruptly decreased before CGR reaching 10<SUP>-6</SUP>mm/c. This drop was temporary and was gradually recovered with subsequent cycling. The drop and recovery in CGR corresponded to the transitions from planar to granular fracture surface and from granular to striated fracture surface, respectively. For UFG4, there was no temporary CGR reduction, which corresponded to the change in the fracture surfaces from a planar to striated surface without any granulated surface formation. To understand the changes in growth rate and fracture surface morphologies, a quantitative model describing the crack growth mechanism is developed in this study by considering the reversible plastic zone size and the microstructural factors. The relationship between the crack growth behavior and the formation of the fracture surface is discussed based on the model.