For AA 6061-T4, In uniaxial tensile testing the "uniform elongation" or the strain at which the necking starts to occur is around 0.15. If I understand the forming limit curve (FLC) correctly, the leftmost major strain in FLC (i.e, the major strain at which necking starts to appear under uniaxial loading) must be 0.15 too. But that is not the case, It is always higher than the uniaxial fracture strain as its shown in attached images (i.e 0.3), What is causing this? Ain't the uniaxial loading path in FLC the same thing as uniaxial tensile testing? The same pattern is also observed in TRIP 600 steel, for which the image is attached.
I am referring to Article Aluminum Alloy Sheet-Forming Limit Curve Prediction Based on...
The failure strain obtained from an uniaxial tensile test is typically lower than that obtained from a formability limit curve due to differences in testing conditions, material response, strain paths, strain rates, anisotropy, and sample geometry. Uniaxial tensile tests subject the material to simple uniaxial loading until failure, whereas forming tests involve complex multiaxial loading conditions. Materials may exhibit different deformation behaviors under these different loading conditions, leading to variations in failure strain. Additionally, strain rate effects, anisotropic behavior, and differences in sample preparation and geometry further contribute to the discrepancies observed between the failure strains obtained from these two testing methods.
This difference can be attribute to different in testing conditions ( strain rate, anisotropy, tool parameters in FLC, friction)
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