Ductile Fracture Analysis in Nakazima vs. SPIF Tests

IF 1 Q3 ENGINEERING, MULTIDISCIPLINARY
Marcos Borrego Puche, David Palomo, Andrés J. Martínez-Donaire, Domingo Morales-Palma, Carpoforo Vallellano
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Abstract

The Forming Limit Curve (FLC) shows the limit combinations of principal strains on the sheet surface that can be successfully achieved before necking appears. Above the FLC, Atkins in 1996 proposed the existence of an unstable region where localized necking develops before reaching at the Fracture Forming Limit (FFL). Only the methodology for the evaluation of the FLC is covered in an international standard ISO 12004-2, where the basis of the tests consists of stretching of a previously clamped sheet blank over a Marciniak or Nakazima punch, providing an almost linear strain path in the sheet surface of the specimen. On the contrary, in single-point incremental forming (SPIF) processes, the hemispherical-shaped tools usually employed are relatively small compared to the general dimension of the specimen, producing a highly nonlinear strain path derived from both the incremental nature of the process and the severe curvature imposed by the small radii of the punches used in the forming process.Many authors have observed fracture strains in SPIFed samples well above the FFL obtained with Nakazima tests under the ISO 12004-2 standard. At the macroscopic level, the reason for this behaviour has been explained mainly based on the effect of bending and the difference in the stress triaxiality level, among others. This research analyzes the initiation of ductile fracture in Nakazima and SPIF specimens under a scanning electron microscope to elucidate the reasons of those differences at the microscopic level.
Nakazima与SPIF试验的韧性断裂分析
成形极限曲线(FLC)显示了在缩颈出现之前,板料表面主应变能够成功达到的极限组合。在FLC之上,Atkins(1996)提出了一个不稳定区域的存在,在到达断裂形成极限(FFL)之前,局部颈缩会发展。国际标准ISO 12004-2中只涵盖了FLC评估的方法,其中测试的基础包括在Marciniak或Nakazima冲床上拉伸先前夹紧的板材坯料,在试样的板材表面提供几乎线性的应变路径。相反,在单点增量成形(SPIF)过程中,与样品的一般尺寸相比,通常使用的半球形工具相对较小,从而产生高度非线性的应变路径,这源于该过程的增量性质和成形过程中使用的冲头的小半径所施加的严重曲率。许多作者观察到SPIFed样品的断裂应变远高于ISO 12004-2标准下Nakazima试验获得的FFL。在宏观层面上,这种行为的原因主要是根据弯曲的影响和应力三轴性水平的差异等来解释的。本研究在扫描电镜下对Nakazima和SPIF试样的韧性断裂起裂过程进行了分析,从微观层面阐明了造成这些差异的原因。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Advances in Science and Technology-Research Journal
Advances in Science and Technology-Research Journal ENGINEERING, MULTIDISCIPLINARY-
CiteScore
1.60
自引率
27.30%
发文量
152
审稿时长
8 weeks
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