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TECHNICAL PAPERS

On Serrated Plastic Flow in an AA5052-H32 Sheet

[+] Author and Article Information
Wei Tong1

Department of Mechanical Engineering, Yale University, New Haven, CT 06520-8284wtong@smu.edu

Nian Zhang

Department of Mechanical Engineering, Yale University, New Haven, CT 06520-8284

1

Corresponding author.

J. Eng. Mater. Technol 129(2), 332-341 (Sep 01, 2006) (10 pages) doi:10.1115/1.2712466 History: Received May 24, 2004; Revised September 01, 2006

Temporal and spatial characteristics of serrated plastic flow or the so-called Portevin-Le Chatelier (PLC) deformation bands in a commercial Al-2.5%Mg sheet metal due to dynamic strain aging effects were investigated experimentally under quasi-static uniaxial tension. It was found that the thickness and width of test coupons, applied loading conditions (machine stiffness and crosshead speed), and mechanical polishing had an observable effect on the temporal characteristics of the serrated plastic flow while short and long interruptions during the tensile tests with stress relaxation and even completely unloading show minimal effects on the subsequent serrated flow behavior. Serrations in axial load in AA5052-H32 were found to associate with the appearance of discrete narrow deformation bands in the tensile test coupons with the bands aligned 59±1deg with respect to the tensile loading axis. The detailed strain distribution across the deformation bands showed a bell-like instead of steplike shape, indicating that the formation of these deformation bands was controlled by a local nucleation and growth process at a cross section in the polycrystalline sheet.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Uniaxial tensile serrated flow curves of AA5052-H32 compact test coupon CT2 (surface condition: mechanically polished): (a) originally recorded axial load versus time data and (b) serrations in terms of deviations ΔP from the average axial load P¯

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Figure 2

Uniaxial tensile serrated flow curves of AA5052-H32 standard test coupon ST1 (surface condition: as-received): (a) originally recorded axial load versus time data P(t); and (b) serrations in terms of deviations ΔP from the average axial load P¯

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Figure 3

Comparison of serrated flow curves of four test coupons (CT2, CT4, ST1, and ST2): (a) the normalized serrations ΔP∕P¯ and (b) an expanded view of selected serrations in flow stress Δσ=ΔP∕A0

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Figure 5

(a) Spatial morphology of PLC bands in test coupon ST2 captured by nine consecutive images Nos. 718–726. The eight images on the left show the enhanced differential optical contrast due to PLC bands while the eight axial strain contour maps show the incremental strains accumulated during the time interval of recording each adjacent image pair. (b) The corresponding temporal characteristics in terms of a sudden load drop per PLC band for the interval of each image pair. The numbers 718–726 designate the image frames and the corresponding filled circles mark the times and load levels at which the nine image frames were recorded in the experiment.

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Figure 6

Axial strain distribution profiles 1–4 (a) and 5–8 (b) along the tensile axis of test coupon ST2. The eight axial strain (Er1) distribution profiles are extracted from the horizontal centerline (x1) in each of the eight incremental strain maps shown in Fig. 5.

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Figure 8

Spatial morphology of ten PLC bands (in gray scale) in test coupon CT5. The ten PLC bands correspond to the ten single serrations in axial load shown in Fig. 7. A threshold of 0.2% axial strain was used in generating the gray-scale contour of axial strain distribution in each strain map.

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Figure 9

Axial strain distribution profiles along the tensile axis of test coupon CT5. The ten strain distribution profiles are extracted from the horizontal centerline in each of the ten incremental strain maps shown in Fig. 8.

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Figure 7

Uniaxial tensile serrated flow curve of AA5052-H32 compact test coupon CT5. The repeated relaxation in axial load (e.g., see points A, B, and C in the insert) is due to the imposed multiple complete stops of the step motor of the mini tensile tester. The numbers on the curve designate the ten single serrations that occurred within the gauge section and were captured by digital images.

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Figure 4

Images of test coupon ST2: (a) the initially undeformed coupon, (b) appearance of a diffuse neck prior to final failure, and (c) surface markings due to PLC bands (after fracture). Images (a) and (b) are the front surface decorated with sprayed black paint speckles, and image (c) is the back surface without paint speckles illuminated at a glancing angle. The orientation angle (θ) of the PLC bands was about −59deg (clockwise).

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