# Characterization and Finite Element Modeling of Microperforated Titanium Grade 2
## 1 Introduction  
A new generation of large HLFC structures are being developed to reduce complexity by using microperforation along the outer skin to control suction without the need of internal chambering. Outer skins made of titanium alloys and variable microperforation patterns entail a new challenge in terms of forming. This must be investigated before deciding on the most suitable skin forming technology and alloy for these new HLFC structures, which will contribute to reduce 10% of both fuel consumption and pollutant emissions in future aircraft [1]. The process chain to manufacture HLFC components starts with the microperforation of metallic sheets, followed by a sheet metal forming process and, finally, cleaning tasks. The perforation is carried out by laser techniques that fulfill quality and high-rate production requirements [2,3]. On the other hand, several metal forming manufacturing processes are being considered to perform HLFC structures. Stretch Forming (SF) is one of the most promising technologies to manufacture HLFC components. SF consists of a rigid punch, called die, and a flat sheet clamped by gripping jaws at both edges. The sheet is forced to get in contact with the die while the tensile load is kept constant, which causes the sheet to stretch and bend over the die simultaneously [4]. Parts with various curve radii and thatare wrinkle-free can be formed by this process at lower tooling costs in comparison with drawn stamping. The main disadvantages of SF are the limited ability to form sharp or re-entrant contours and the springback problem. To avoid these problems, different hot forming processes are nowadays commonly used. In the hot forming (HF) process, the metallic sheet is heated above recrystallization temperature to induce a soft and malleable state. Then, the sheet is positioned on a hot die while a hot punch goes down and shapes the component. The sheet is held under pressure and temperature for a time period and, finally, is released and cooled by natural convection. A soft and malleable state during large deformations reduces or avoids springback and cracking in small bend radii areas. On the other hand, there is elevated risk of oxidation in HF, and optimization of temperature exposure time, so using shielding gas and/or lubricants is needed. Other technologies such as laser forming or laser been forming [5; 6] can be considered, but they are currently in a lower TRL level. This paper focuses on the mechanical properties of micro-drilled and non-drilled titanium grade 2 sheets at room temperature. Titanium is the ninth most abundant element in the earth's crust (about 0.6%), and despite its relative scarcity, many industries have benefited from its favorable properties. High strength, low density, and excellent corrosion resistance are some of the main properties that make titanium an interesting material for a wide variety of applications, such as aerospace, energy, and chemical industries, or even uses in the biomedical field. Among all of them, titanium plays a key role in the aerospace industry, where weight plays a critical role in the design of more fuel-efficient aircraft. For instance, titanium alloys are employed for aero-engine applications or in the airframe systems where they can represent a moderate proportion of these components. Pure titanium sheets, in their four distinct grades (1, 2, 3, and 4), show highly anisotropic mechanical behavior at room temperature due to the inherent crystallographic texture and the manufacturing process. It is crucial to use yield criteria that capture the fundamental characteristics of the plastic behavior during forming operations and under complex loading conditions [7]. This anisotropic behavior imposes severe restrictions on the form of yield conditions. The aim of this study is to characterize the planar anisotropic behavior of titanium sheets and to use a realistic yield criterion that can faithfully describe the material's response and reproduce the experimental values.