The effect of serrated yielding on the determination of r-values in aluminium alloys and yield locus calibration

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 527; no. 29; pp. 7557 - 7564
• Main Authors: Kilpatrick, W., Brown, D., McMurray, R.J., Leacock, A.G.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 15.11.2010; Elsevier
• Subjects: Age hardening; Aging; Aluminium alloys; Aluminum base alloys; Applied sciences; Calibration; Dislocations; Elasticity. Plasticity; Exact sciences and technology; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Plane stress; Serrated yielding; Sheet metal forming; Strain; Stresses; Yield strength; Aluminium alloys; Sheet metal forming; Age hardening; Dislocations; Stress analysis; Aluminium base alloys; Uniaxial tension stress; Ageing; Tension test; Solution heat treatment; Strain gradient; Plasticity; Yield strength; Serrated yielding; Portevin Le Chatelier effect
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2010.07.082

▶ The r-value is affected by serrated yielding as shown by the collinear 0–0.05 strain range plots. ▶ The irregular variation caused by serrations affects the gradient of the strain plot. ▶ Discontinuous yielding occurred within a critical strain range of 0.07 ≤ plastic strain ≤ 0.13. ▶ The Hill 1948 yield criterion is affected by calibration from r-values subject to serrated yielding. This study investigates the effect serrated yielding (Portevin-Le Châtelier effect) has on the determination of the plastic strain ratio ( r-value) for the 2024 aluminium alloy. The aluminium alloy specimens were initially in the fully annealed O-condition. The specimens experienced a softening mechanism through a solution heat treatment (SHT) process, followed by an oil quench. The specimens were permitted to recover for periods ranging between 0.5 and 12 h by room temperature (RT) natural ageing before undergoing a uniaxial tensile test along the longitudinal axis to determine the r-value as a function of natural ageing time. The current study reveals the irregular variation in the transverse strain affects the gradient of the longitudinal–transverse strain plot and consequently the r-value. It is demonstrated that the serrated yielding directly influences the Hill 1948 yield locus when calibrated using two r-values and one yield strength for the 2024 aluminium alloy. Serrated yielding results in an underestimation of the plane stress (rolling) of 0.5 ≤ plane stress rolling (MPa) ≤ 5.6, plane stress (transverse) of 1.0 ≤ plane stress transverse (MPa) ≤ 9.2, and biaxial stress of 1.4 ≤ biaxial stress (MPa) ≤ 7.4. It is also shown that serrated yielding occurs after a critical strain of 0.07 ≤ plastic strain ≤ 0.13 and is reduced by natural age hardening.