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

Abstract

We develop a new flow model based on the Swift method, which is both versatile and accurate when used to describe flow stress in terms of strain hardening and damage softening. A practical issue associated with flow stress at room temperature is discussed in terms of tensile testing of a cylindrical specimen; we deal with both material identification and finite element predictions. The flow model has four major components, namely the stress before, at, and after the necking point and around fracture point. The Swift model has the drawback that not all major points of stress can be covered simultaneously. A term of strain to the third or fourth power (the “second strain hardening exponent”), multiplied and thus controlled by a second strain hardening parameter, can be neglected at small strains. Any effect of the second strain hardening exponent on the identification of the necking point is thus negligible. We use this term to enhance the flexibility and accuracy of our new flow model, which naturally couples flow stress with damage using the same hardening constant as a function of damage. The hardening constant becomes negative when damage exceeds a critical value that causes a drastic drop in flow stress.

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

“…In addition, the method can be used to reveal the material deformation behaviors even between fracture start point and fracture point [113], which is important to understand the fracture phenomena occurring in tensile test. Joun et al's approach also has a unique and important strong point, in that it can also be used to predict the flow behaviors during softening just before fracture [35]. However, the results are composed of a discrete flow information at the sample points of strain.…”

“…However, V 2 's of their results, i.e., 47.3 for Al6061-T4 and 855 for CP Ti, say that it is not an improved Voce model but an improved Swift model if we focused strictly on their motivation. Recently, Razali et al [35] proposed a modified Swift model to cope with the limitations of the traditional fundamental flow models.…”

“…There are various test methods for flow characterization such as the cylindrical tensile test, sheet specimen tensile test, tension/compression test [20], sheet bulging test [20,21], layer compression test [22], multi-layer plane strain compression test [23], in-plane torsion test [24,25], torsion test [26], shear test [27], forward extrusion test [28], cruciform-biaxial test [29], rolling test [30], hydraulic bulge test [31], etc. However, many researchers are still relying on the traditional tensile test-based extrapolation method, especially in bulk metal forming engineering, which may be quite erroneous at high strains in most cases [33][34][35]. This is owing to complexity, cost, and lower reliability of the developed methods.…”

General Blending Approach of Fundamental Cold Flow Models for an Almost Complete Cold Flow Model in Terms of Tensile Test

“…Additional flow constants should be used to solve this matter, which inevitably increases the complexity of finding the flow constants. Razali et al [ 34 ] added the second strain hardening parameter to the Swift model to cope with this matter. Its mathematical flexibility, however, is not acceptable and a smoothing function was thus assisted.…”

“…In addition, Ludwik, Voce, and Swift models have another degree of freedom for satisfying the flow stress at a selected strain. In the previous study, [ 34 ] it was shown that it is very difficult to satisfy both pre‐ and postnecking strain hardenings at once using the only remained degree of freedom that the Ludwik, Voce, and Swift models have after satisfying the necking conditions. It is thus indispensable to use additional flow constants to satisfy their corresponding points additionally selected on the target flow curve, called the reference flow curve (RFC).…”