PSI - Issue 68

Tuncay Yalçinkaya et al. / Procedia Structural Integrity 68 (2025) 325–331 Yalc¸inkaya et al. / Structural Integrity Procedia 00 (2024) 000–000

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quality (Karakas¸ et al., 2021; Gu¨nay et al., 2022). In particular, the local application of forces during forming enables the formation of materials with exceptional strength, expanding their uses into high-performance applications. During flow forming, the rollers cause complex plastic deformations in the tube, leading to possible defects such as flaring, pile-up, fish scaling, and waviness or ductile fracture. For this reason, accurate fracture predictions are essential for improving forming limits and achieving successful manufacturing. Computational methods, mainly finite element (FE) analysis with ductile failure criteria (DFC), represent cost-e ff ective methods in formability limits prediction and process optimization. Ductile failure models for predicting fractures are broadly categorized into uncoupled and coupled frameworks. The uncoupled models, including the Cockroft-Latham (CL, Cockroft and Latham (1968)), Johnson-Cook (JC, Johnson and Cook (1985)), and Modified Mohr-Coulomb (MMC, Bai and Wierzbicki (2010)), separate damage evolution from the stress-strain response and are preferable because their implementation is relatively more straightforward. Coupled models, like those by Gurson (Gurson, 1977) and Lemaitre (Lemaitre, 1985), in which stress-strain a ff ects the damage evolution are expected to provide more realistic predictions. In the literature there are various studies on the failure prediction during flow forming using FE simulations and uncoupled damage criteria. In the authors’ previous studies (Vural et al., 2022; Erdog˘an et al., 2022), FE simulations of flow-forming for calibrated models with multiple damage parameters (JC and MMC) are analyzed. Additionally, Depriester and Massoni (2014), Ma et al. (2015), and Xu et al. (2018) utilize models with a single damage param eter, and which are easier to calibrate, such as Cockroft-Latham (CL) and Rice-Tracey (Rice and Tracey, 1969), to investigate the capability of failure prediction of several criteria for titanium alloy components. Mocellin et al. (2022) determine formability limits for AA6016-T6 material with several basic damage criteria and report that among the di ff erent criteria, the CL criterion is the most accurate model to predict experimental data. In our most recent work (Erdogan et al., 2023), damage location and fracture limit predictions for IN718 alloy are investigated using JC, CL and MMC criteria, and the successful predictions of the CL model inspired the possibility that other basic models may similarly find applicability in the flow forming process for IN718. In this context, the aim of this study is to extend our previous research and examine the performance of several uncoupled damage models to predict fracture in the flow-forming process. Specifically, the models; Ayada (Ayada, 1987), Ayada-m (Ma et al., 2015), Ko-Huh (KH, Ko et al. (2007)), Brozzo (Brozzo et al., 1972), Le-Roy (LR, Le Roy et al. (1981)), McClintock (MC, McClintock (1968)), Oh (Oh et al., 1979), Rice-Trace (RC, Rice and Tracey (1969)), CL and Freudenthal (Freudenthal, 1950) are assessed and calibrated them with smooth tension specimens. Both tensile tests and the flow forming process are numerically modeled in the Abaqus / Explicit, incorporating all damage criteria into a user-defined subroutine (VUSDFLD). The performance of these models is evaluated through tensile test simulations employing three di ff erent specimen geometries, with failure predictions compared to experimental data. Subsequently, these models are employed to examine their e ff ects on formability limits at di ff erent thickness reduction ratios (37.5%, 50% and 70%). The results are compared to experimental observations to determine each model’s predictive capabilities regarding formability limits and fracture initiation locations.

2. Material and Methods

In this section, the material and the experiments are discussed. The calibration of plasticity and damage parameters is presented and the FE models of tensile tests and flow forming process are explained.

2.1. Materials and Experimental Tests

The nickel-based superalloy material Inconel 718 (IN718) has high strength and low ductility, making it favored in high-performance applications in the aerospace, automotive and defense industries. Plasticity and damage behavior is studied through tensile tests of IN718 materials with specimens of four di ff erent geometries with di ff erent stress states, namely uniform stress (ST), notch stress (NT), plane-strain stress (PST), and in-plane shear (ISS), with their dimensions shown in Fig. 1. Displacement control tensile tests are performed using the MTS 100 kN Tension–Torsion Fatigue / Static testing machine and a high-speed camera, with force data collected from the testing machine and dis placement data from the digital image correlation (DIC) analysis with NCORR open source 2D DIC software. Addi tionally, backward flow forming experiments are also conducted at thickness reduction ratios of 37.5%, 50% and 70% using the Repkon flow forming machine. During these experiments, a 120° angular setting is between three rollers,