Methodology for the Experimental Assessment of True Stress-Strain Curves After Necking Employing Cylindrical Tensile Specimens: Experiments and Parameters Calibration

Abstract

Simulations and structural integrity evaluations including severe plasticity have undergone significant expansion during recent years (e.g. fracture mechanics FE models including ductile tearing and/or generalized yielding), which demand accurate true stress-strain data until fracture. This is a consequence of the use of high toughness ductile materials subjected to severe loadings and high levels of operational efficiency and optimization. However, tensile tests present one inconvenience when providing such data, since the occurrence of plastic instability (necking) complicates the direct assessment of true stress-strain curves until final fracture. Two main difficulties can be pointed out: i) the nonuniform geometry assumed by the cross sections along its length and; ii) the imposition of a complex triaxial stress state. The first occurrence can only be overcome by real-time physical measurements. The second occurrence demands a correction model to provide an equivalent stress including triaxial effects. Current authors recently demonstrated that even the well-known Bridgman’s correction presents limitations, particularly for strains greater than ∼ 0.50–0.60, which motivated proposals to better describe the geometrical evolution of necking minimizing the need for real-time physical measurements [1]. As a new step in this direction, this work presents three key contributions: i) first, experiments regarding the geometrical evolution of necking were largely extended incorporating 10 materials to corroborate the validity of the recently proposed model (including Carbon, stainless steels and copper); ii) second, and for the same materials, the necking region was investigated in more details to verify to which extent an osculating circle well describes the high deformation region. A new model could be proposed to better support future solid mechanics analyses regarding equilibrium and stress/strain fields; iii) finally, a modified Bridgman’s model is proposed, followed by recommended practices for testing. The results provide further support to σ-ε assessment considering severe plasticity and demanding less physical measurements.

A Review of Flow Characterization of Metallic Materials in the Cold Forming Temperature Range and Its Major Issues

“…They could obtain the flow curve at the strain of 0.85 in the case of a carbon steel SM490A with no pre-strain. Ganharul et al [111] and Donato et al [112] defined the problem clearly and suggested an engineering measure to give further support to the determination of true material properties considering severe plastic deformation after necking. It is noteworthy that they emphasized properly the importance of ENM especially for experimental techniques including DIC.…”

A Novel Flow Model of Strain Hardening and Softening for Use in Tensile Testing of a Cylindrical Specimen at Room Temperature

“…Literature survey on post-necking strain hardening in tensile test of standard cylindrical specimens shows that this trend has still been growing. In the early 2010 s, Gonharul et al [30] and Donato et al [31] defined the problem clearly and suggested an engineering measure to give further support to the determination of true material properties considering severe plasticity after necking. It is noteworthy that they emphasized properly the importance of ENM especially for experimental techniques including DIC.…”

Caracterización del comportamiento mecánico durante la fase de estricción de los aceros corrugados Tempcore 500C: experimentación y simulación

“…Other authors (Mirone, 2004;Ganharul et al, 2012;Donato and Ganharul, 2013) analyse the stress distribution in the neck independently of its profile, and propose an empirical relationship between true and equivalent stress depending solely on instantaneous true strain values. Finally, additional relationships are proposed by Ling (1996) and Mirone et al (2019) in order to obtain true stress values from several parameters such as the strain and stress ones on the onset of necking and the increase in specimen gauge length.…”