In the present study, a high-velocity sheet metal forming experiment has been performed using a hemispherical end nylon striker inside the shock tube. The striker moves at a high velocity and impacts the sheet mounted at the end of the shock tube. Three different velocity conditions are attained during the experiment, and it helps to investigate the forming behavior of the material at different ranges of velocity conditions. Various forming parameters such as dome height, effective strain distribution, limiting strain, hardness, and grain structure distribution are analysed. The dome height of the material increases monotonically with the high velocity. The effective-strain also follows the similar variation and a bi-axial stretching phenomenon is observed. The comparative analysis with the quasi-static punch stretching process illustrates that the strain limit is increased by 40%-50% after the high-velocity forming. It is because of the inertial effect generated on the material during the high-velocity experiment, which stretches the sheet further without strain localization.
TopIntroduction
Austenitic stainless steel has been more desirable for different structural applications, particularly in the field of automotive industries due to its relatively high strength, high formability and increased resistance to corrosion (Campos, Butuc, Gracio, Rocha, & Duarte, 2006), (Chen, Gau, & Lee, 2009). The application of the SS 304L grade steel sheet is vital in many industries. However, its plastic deformation behavior is affected mainly by its various material properties such as strain hardening properties, strain rate sensitivity, and anisotropic ratio (Makkouk et al. 2008), (Talyan, Wagoner, & Lee, 1998). The existing forming properties of the material can be enhanced either by conducting the forming process at higher-temperature or at a higher-strain rate (Stachowicz, Trzepieciński, & Pieja, 2010), (Lichtenfeld, Van Tyne, & Mataya, 2006). To avoid the martensitic transformation and omit the annealing process, the worm forming is useful but, it adds the cost of the forming process and makes the environment unpleasant for the people work on it (Kim, Huh, Bok, & Moon, 2011). Thus, the plastic deformation at a high strain rate becomes more popular for the last two decades (Bronkhorst et al. 2006), (Kim et al. 2011).
In order to characterize the material properties, a quasi-static tensile test and split-Hopkinson pressure bar test are used for so long (Lee, & Lin, 2001), (Gilat, Kuokkala, Seidt, & Smith, 2017). Though the engineering stress and strain data are quite accurate to predict the material properties at the different regimes of strain rate, the uni-axially obtained results restrict their usage during real-time application. It may not be wise to interpolate the uni-axial results for the calculation of the bi-axial properties of the material. Therefore, several types of research have been employed in the field of quasi-static as well as dynamic loading conditions to characterize the bi-axial material properties (Acharya et al. 2019).
Many researchers have used quasi-static bulge testing technology since 1940, and it has become an established experimental technique for the determination of forming limit diagrams of the sheet material (Koç, Billur, & Cora, 2011), (Kaya, 2016). The forming limit diagram (FLD) proposed by Keeler (Keeler, 1989) is one of the major characterization techniques for the prediction of failure of the sheet material and it has been used widely by the sheet metal forming industries to evaluate the formability of the material by minimizing the shop floor trials. There are several standard experimental techniques are also available for the material forming behavior analysis such as Erichsen cup test, limiting dome height (LDH) test and Fukui conical cup test, etc (Logesh, Raja, & Velu, 2015), (Meuleman, Siles, & Zoldak, 1985), (Gerdeen, & Daudi, 1983). These all hydraulic operated experimental facilities enable us to perform the test only in the lower magnitude of strain rate. However, various modification on the simple bulge test has been performed to study the material properties at a higher range of strain rate. Broomhead and Grieve (Broomhead, & Grieve, 1982) used a drop hammer rig to apply pressure loading which helped to determine the FLD of low carbon steel with a strain rate up to 70 s-1. In order to obtain the multi-axial material properties under higher strain rate conditions, Grolleau et al. (Grolleau, Gary, & Mohr, 2008) modified the conventional Split Hopkinson Pressure Bar (SHPB) apparatus as a dynamic bulging device to perform biaxial tests on the material at higher strain rate but, in a limitation, several complexities have been observed in the experimental set up during the experiment (Ramezani, & Ripin, 2010).