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

Abstract

Foldover is a common, yet complex, transient boundary condition in several ‘metal forming processes’ including the barrelling compression test (BCT). The onset of foldover during BCT is comparable to that of necking in the tensile test. However, its role in flow curve identification has been ignored in the current literature. Methods to detect and measure its onset and growth during the processes are also not available. We propose three methods to identify the onset of foldover and measure its progress with deformation. These are conducted together with case studies of BCT to have a fundamental understanding of the onset and growth of foldover in the test and more general cases. Experimental and numerical methods are developed to accomplish this based on (1) direct measurement using tracing points, (2) energy-based and (3) geometry-based indirect measurements. These outline the required supplementary data and allow proper detection of the onset. In the first method, the migration of tracing points from the free surface to the platen-sample interface, simulated numerically, is monitored and measured visually. The second method employs the load-stroke data to calculate the total power input to the deforming system. In the third method, a criterion has been derived to detect the onset of foldover by monitoring the geometry of the sample profile during the test and comparing it to a reference quadratic profile. The observations and outputs of each method are compared and discussed.

Enhancing Axisymmetric Compression Testing: Modeling and Addressing Flow Curve Errors

“…For further information on this impact and an approach to detecting the onset of foldover, refer to ref. [16]. However, even if foldover is successfully detected, it does not alleviate the complexity associated with data reduction and interpretation of the results.…”

“…The choice of the specimen center as the representative point was deliberate, considering points along the flow path that offer merits in terms of achievable total deformation and minimal variation of strain rate, as comprehensively explained elsewhere. [ 16 ] This rationale has been further clarified to ensure a nuanced understanding of the selection in the article.…”

“…More recently, these models have been employed to monitor the deformation path of five different tracing points within the sample during the test, revealing the heterogeneous nature of deformation in a barrelled sample. [ 16 ] Various techniques, including a proposed multilayer method, were utilized in this study to determine the flow curve at the designated tracing points. However, none of the tracing points or techniques demonstrated satisfactory compliance with their corresponding reference flow curve.…”

“…Accounting for Friction: In addition to the primary focus on flow curve identification and associated errors, this review also encompasses a comprehensive examination of prior research related to practical aspects and friction minimization, as extensively documented in multiple works, including refs. [4,7,16]. The inclusion of friction in a CPM presents a paradox, as it signifies the presence of barreling-induced shear deformation in the sample.…”