Critical Evaluation of Identified Flow Curves Using Homogeneous and Heterogeneous Solutions for Compression Test

Abstract

Background The cylindrical profile model (CPM) is commonly used to convert the load–displacement measurement from the axis-symmetric compression test (ACT) to the flow curve. The model ignores the barrelling of the sample which results in serious flow curve distortion and underestimation of the effective stress and strain. To minimize these, a new solution of ACT with heterogeneous deformation is proposed in this work as an alternative to CPM. Method Representative point is introduced in this new approach for better utilization of the new solution. Some key attributes of a typical flow curve are also considered to explore the sensitivity of the identified flow curve to the heterogeneous deformation. Sample flow curves are identified based on the new solution and compared with that of CPM. Results It was found that CPM underestimates the maximum effective stress, strain and strain rate. Significant deviations were found between the reference flow curves and those of CPM. The model is unable to appropriately account for the rate-dependent behaviour of material and work hardening variations in the samples. Therefore, given its non-realistic homogeneous effective strain and strain rate in the sample, the CPM based flow curves are questionable. The new ACT solution provided more reliable flow curves than those of the CPM. Conclusions Considering the heterogeneous deformation at the centre of the sample, the new ACT solution provided flow curves that for strains bellow 0.4 closely resemble the reference curves obtained from the finite element model. Thus, the new ACT solution reduces the serious limitations of CPM and provides less error in the study of the hot deformation phenomena (e.g. recovery and recrystallization). Further recommendations were also given to limit the deviations in the identified flow curve.

Enhancing Axisymmetric Compression Testing: Modeling and Addressing Flow Curve Errors

“…In the work by Khoddam et al [ 38 ] material flow curves were discerned at various nominated points within the sample to facilitate result comparison and to determine the optimal tracing point for constitutive characterization using ACT. In the present study, our focus has been on the sample center for verification purposes.…”

A comprehensive experimental and numerical study of foldover onset and growth during the barrelling compression test

“…the time integration scheme and element formulation) elsewhere. 3,5 A test scenario was considered with the initial sample diameter and height of D 0 = 10 mm , H 0 = 16 mm and H f = 8 mm with 3200 deformation steps which were recorded every 10 steps. The deformation step is related to the sample height H according to H = 16 − 8 / 3190 ( step − 10 ) .…”

“…1,2 However, only a few indirect measurement methods exist with serious limitations. 3,4 The erroneous BCT based flow curves have been perceived in numerous published works as measured values rather than computed values. To find more reliable and detailed solutions to the test, much more theoretical-analytical developments on the 1 unwanted effects, for example, foldover and friction, are needed.…”

“…These effects complicate the interpretation and transformation of the test results to the real forming processes and thus eventually invalidate the physical simulation. Practically, the test cannot be performed in a uniaxial mode to simulate high strains over a wide range of test conditions 3 ; thus, it becomes impossible to avoid unwanted effects. Foldover increases with friction and deformation and is practically inevitable in many forming scenarios.…”