Serration, deformation band propagation and strain marking in a 5083 aluminum alloy at low temperature

Abstract

Relations among load fluctuation, geometrical shape and propagation behavior of a deformation band were examined for an inhomogeneous plastic deformation generated at relatively low temperature. The geometrical shape and propagation behavior of the deformation band were evaluated by strain gauges and surface roughness of specimen. Quantitative analysis of the load fluctuation was also carried out by fast Fourier transform method. The following results were obtained. A wavy load fluctuation was observed in two-deformation regions; i.e., one of which was a yield deformation region and the other was a serration one. The geometrical shape and the tilt of the deformation band in each deformation region were similar, while the number of cycles, the velocity of propagation and the amount of increasing strain were different each other. However, it was clearly shown that the wavy load fluctuation generated in each deformation region resulted from a continuous propagation of the deformation band.

Initiation and Propagation Behaviors of Fatigue Cracks in 5056 Aluminum Alloy Studied by Rotating-Bending Tests with Smooth Specimen

“…where l represents the crack length, N is the number of cycles, E is Young's modulus, and the inverse of C 5 is the resistance against the fatigue crack propagation (18) . The relation of σ 0.2 /E(σ a /σ 0.2 ) 6 l -dl/dN shows a linear relation ( a logarithmic approximation was used) on the log-log diagram (Figure 9), and the result implies that the small-crack growth law is useful for 5056-H112. The value of C 5 was plotted versus the σ 0.2 of 5056-H112 along with the values of 2014-T4 (2) , 5052-O (3) and 6061-T6 (4) for comparison.…”

“…However, a line-like contrast from a second phase particle was observed, and the growth behavior was observed with the stress repetition. A line-like contrast was nucleated at a comparatively early stage during the fatigue testing, i.e., N＝10 6 . After line-like contrast shows small growth, the growth was stopped at N=8-10×10 6 ．Although the fatigue test continued by N＝3.5×10 7 , no growth was recognized．The line-like contrast in the SEM photos of Figures 12 (a) and (b) was a 0.5 µm width and 5 µm length．Figure 12 (c) also shows a photo of a fatigue crack for comparison.…”

“…A line-like contrast was nucleated at a comparatively early stage during the fatigue testing, i.e., N＝10 6 . After line-like contrast shows small growth, the growth was stopped at N=8-10×10 6 ．Although the fatigue test continued by N＝3.5×10 7 , no growth was recognized．The line-like contrast in the SEM photos of Figures 12 (a) and (b) was a 0.5 µm width and 5 µm length．Figure 12 (c) also shows a photo of a fatigue crack for comparison. The convex part and concave part of the plastic-replica correspond to the concave part and convex part of the material surface, thus a crack becomes a convex part in the plastic replica.…”

“…These phenomena are called dynamic strain aging. This is due to the interaction between the Mg solute atoms and moving dislocations (6) . The presence of non-propagating fatigue crack is a manifestation of strain aging.…”