Issue 52

A. Drai et alii, Frattura ed Integrità Strutturale, 52 (2020) 181-196; DOI: 10.3221/IGF-ESIS.52.15 182 proclaimed at the end of this work according to the obtained numerical results. K EYWORDS . HPT; Finite Elements; PMMA; Behavior; Plastic strain. I NTRODUCTION here have been extensive investigations over the last two decades into the production of bulk ultrafine-grained (UFG) materials through the application of severe plastic deformation (SPD) [1, 2]. Recently, SPD techniques have been used in the modification of microstructure. They made possible to produce nano-crystalline (NC) microstructures out of metallic materials [3, 4]. Such SPD methods include High Pressure Torsion (HPT), Equal Channel Angular Pressing (ECAP), Accumulative Roll Bonding (ARB), Multiple Forging, Twist Extrusion (TE) and some others. Among the various SPD techniques, Equal-Channel Angular Pressing (ECAP) and High Pressure Torsion (HPT) are the most common [5]. To date, processing by HPT has proven to be the most effective of all the SPD methods in producing bulk nanostructured materials [6]. Its principle consists in putting a disk between two massive anvils under a simultaneously or successively action of high compression and torsion as shown in Fig. 1. The occurrence of superplasticity was reported not only after HPT processing of thin disc specimens [7,8], but also using the ring samples [9,10], bulk samples [11, 12] as well as after continuous high-pressure torsion extrusion [13]. Different materials have been tested experimentally [14] and theoretically [15] through the application of HPT process such as aluminum alloys [8, 16], nickel [17], copper [15], titanium alloys [3, 18], polymers [19], zirconium [20, 21], Magnesium alloys [22], aluminum-zinc alloys [23], intermetallics [24, 25] and others (see [26, 27]). Figure 1: Schematic illustration of HPT process. In the past, FEM simulations of the SPD processes were performed with the purpose of analysis and optimization of the SPD processing variables and prediction of the mechanical and microstructural characteristics of the SPD processed samples [15]. For example, for HPT, the effects of processing parameters, such as geometries of the disc [28], applied pressure [29], temperature distribution [30, 31], rotation rate [31], friction conditions [32], material porosity and properties [32, 33] and slope of the inclined anvil's surface [34] on the plastic strain behavior were reported in the literature extensively. This work is devoted to study the behavior of polymethyl-methacrylate (PMMA) during HPT process. It is important to note that the behavior of this amorphos polymer is sensitive to the strain rate, temperature, hydrostatic pressure and torsion angle [35]. A three-dimensional finite element analysis has been conducted using hexahedral element with 8-nodes. Our particular attention has been focused on the distribution of the plastic strain in the deformed material as a function of the different parameters of the process, namely, the sequence of the different phases, the hydrostatic pressure, the torsion angle and the temperature of the material during the process. T