# Steel Cruciform Sample with Nitrided Arms Achieves Higher Plastic Strain in the Gauge Region
## 1 Introduction  
Currently, the static tensile test is the primary method for determining the properties of materials. Many years of research has led to the standardization of this method (ISO 6892-1). However, in the case of sheet material properties it does not provide sufficient results for the analysis of the biaxial stress state, which is the most common stress state for elements that are produced by the process of stamping. This is not only due to the fact that sheet metal parts have different properties depending on the sheet rolling direction, but it appears that the formability of the sheet depends on the strain path which they are subjected to. Products of the sheet metal forming process can be found in many branches of the industry. Parts manufacturing by extrusion is one of the cheapest methods for mass production. In order to reduce the costs of production manufacturers aim to reach the limit of material durability. Expensive test rigs have been replaced by cheaper numerical simulations, however, the optimization of the element design and the process of its forming, using numerical simulation, requires knowledge of the material behavior when subjected to a specific load. There are a few tests to determine the Forming Limit Curve (FLC) of a material. The two most popular methods use a piston to deform the sample in a direction perpendicular to its plane [1,2]. In [PERSON]'s method, the deformation takes the shape of a spherical dome due to the curvature of the piston. The solution proposed by [PERSON] [3] preserves the flatness of the sample in the measuring area. The main disadvantages of these methods are the multitude of sample shapes to be tested and friction between the pistonand the sample, which influences the results and makes the numerical simulations more complicated due to the strong nonlinearity of the analysis. A new idea of introducing plane stress state to the material was proposed by [PERSON] [4]. His solution was based on a cruciform specimen, stretched by its arms in two perpendicular directions (Figure 1). However, it turned out that the proposed cruciform specimen subjected to biaxial stretching was not suitable for a material formability test, since the achievable plastic strains in the gauge area reached only a single percent and were far from the necking and fracture strains. [PERSON] and [PERSON] [5] stated, that there was no standard sample geometry specified for biaxial stretching of the sheet, and its shape is currently the subject of research [6; 7; 8; 9; 10; 11]. Although, research in this area has been conducted for many years, it was only recently (2014) when [PERSON] designed a sample shape which was adopted as a standard for carrying out biaxial tensile tests in accordance with the ISO16842 standard [12]. Unfortunately, in this version it is still not possible to study the full characteristics including plastic deformation to failure, commonly known as the Forming Limit Diagram (FLD), as the achieved plastic deformations are too low [13]. Therefore, researchers all over the world are carrying out experiments on the new cross shaped samples trying to achieve the highest and most uniform values of deformation in the centre of the sample, while eliminating the phenomenon of stress concentration in the remaining parts and preventing damage in the region of the arms [14]. The main approaches in the literature include:  
* Applying rounds or undercuts in the middle of the sample (Figure 2a,b) [15];
* Use of different types of cuts and slits in the arms (Figure 2c) [16];
* Reduction of the thickness of the central part (Figure 2d) [17]; and
* Strengthening the sample arms by their plastic processing (Figure 2e) [18]. Figure 1: Schematic description of theoretical loading of a cruciform sample subjected to biaxial stretching. The idea of this work was to increase the strength of the sample arms, which normally are the weakest part of the specimen.