A verified solution of friction factor in compression test based on its sample's shape changes

Abstract

The friction factor is a key input for "barrel compression test " and for a meaningful interpretation of the test data. Despite its widespread use, due to the complexity of the problem, there are very few solutions available for the test, let alone for the friction factor. Extended-Avitzur (EA) model has serious known limitations to calculate the friction factor. To estimate the friction factor more reliably, a closed-form solution of the friction factor has been proposed here. The solution is based on the "Exponential Profile Model " (EPM) and provides an instantaneous value for the friction factor. It simply relies on the sample's initial and deformed dimensions. Unlike existing experimental procedures, the proposed solution integrates the test results and friction factor identification based on a single set of experimental load-displacement-barreling data. Merits of the model and its solution were highlighted and compared to those of the conventional Cylindrical Profile Models (CPMs). A finite element model was developed as the reference to produce pseudo-experimental test data and to verify the presented solution. The deformation data were used in the EPM and the EA model to calculate the friction factors by each model and to compare them with the reference data as the benchmark. Contrary to EA's estimated friction factors, those identified by the EPM were in good agreement with the reference values. Recommendations were provided to identify a deformation zone at which the EPM's friction factor can be estimated meaningfully.

State of the art methods to post-process mechanical test data to characterize the hot deformation behavior of metals

“…A recently developed model for ACT (details could be found in Khoddam, 35 Khoddam et al 36 ) allows the estimation of the test's heterogeneous deformation and the friction factor via its closed-form solution. Based on a single set of the test's barrelling-load-deformation data and the model's closed-form solution, one can estimate the strain and strain rate changes in the sample, instantaneous friction to eventually identify the material's flow response.…”

“…86,88 Another approach is to indirectly characterize/measure friction using the barrelled sample geometry, for example, Alexandrov et al, 89 Yao et al, 90 Fan et al, 91 and Solhjoo and Khoddam. 88 Methods to account for friction, excluding the barrelling, with either a fixed Coulomb friction coefficient and mixed sticking-sliding friction, as well as a recently developed method by Khoddam et al 36 are reviewed and discussed in section 6.…”

“…Therefore, virtual finite element generated test data are used in studies of such solutions. 36 The next section provides a conceptual experimental setup to measure the required barrelling data for the EPM method, namely H À D À B. This will allow the implementation of advanced techniques based on real test data.…”

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

“…As the BCT sample deforms, its boundary condition changes via partial migration of the sample’s free surface to the platen-material contact area, that is, foldover . Several unwanted effects, such as friction induced barrelling, 5 heterogeneous deformation and foldover, drive the test into a multiaxial mode. These effects complicate the interpretation and transformation of the test results to the real forming processes and thus eventually invalidate the physical simulation.…”

“…Khoddam et al 21 proposed an energy-based approach to evaluate closed-form solutions of the test. Khoddam et al 5 utilised a proposed kinematic solution 22 in an energy conservation framework and developed a closed-form solution for friction factor during the test. They found that their identified friction factor is valid before the onset of foldover.…”