Overcoming the Dependence of the Yield Condition on the Absence of Macroscopic Structures

A growing demand for cheap, strong, and lightweight structures has resulted in an increasing need for materials incorporating macroscopic structures such as surface textures in the form of dimples or internal non-uniformities such as porosity. This has highlighted the potential for misleading results when applying the current standards for the determination of yield strength to materials incorporating such non-uniformities. In the present study, discontinuous, continuous, and severely continuous (departure from linearity significantly prior to macroscopic yield) yield behaviours during tensile loading have been explored with respect to the determination of yield strength. This has clearly shown the limitations of standard measures as well as the incompatibility of different measures, making the comparison of yield strengths deceptive. Therefore, a novel derivative yield method has been proposed. Based on the second derivative of stress with respect to strain, this method is independent of both yield type and the presence or lack of macroscopic structures, and it correctly identifies the upper yield for discontinuous yield as well as the first significant departure from linearity for continuous yield. Furthermore, the derivative yield method is shown to more accurately characterise the behaviour of dimpled steel, non-arbitrarily quantifying the yield strength for severely continuous yielding. This wide applicability, in many cases, eliminates the need for the comparison of results produced by mutually incompatible yield criteria, e.g., when quantifying strengthening due to dimpling.