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Holes are crucial features in mechanical components due to the stress concentration factor arising in the proximity of hole boundary. However, details with holes are widely used in engineering to guarantee some function, such as bolt or rivet joints. Anyway, the local stress concentration introduced by holes strongly limits the fatigue strength and is still a subject able to attract the interest of researchers and engineers. Through the years, many studies were proposed aimed at estimating the stress concentration factor both for isotropic [1], [2], [3] and orthotropic plates [4], [5], [6], [7]. Holed components are interesting even today, due to the increased manufacturing and machining possibilities offered by the new technologies that allow to define and put in practice original solutions for the local stress reduction. Meguid [8] introduced the so-called defense hole system in a uniaxially loaded plate with a reduction of 7.5-11 % of the stress concentration factor. In addition, other authors employed the defense hole systems for reducing the stress concentration factor [9], [10]. Jindal [11] investigated the influence of auxiliary holes on the static stress concentration around a central hole numerically by the finite element method (FEM) and experimentally by a two-dimensional photo-elastic method. Providakis et al. [12] presented a parametric study for the reduction of the dynamic stress concentration around a circular hole in an elastic finite plate subjected to a suddenly applied loading acting in its plane by using the BEM. Alternatively, shaped porous structures have been recently studied and applied to metal sheet in order to achieve a specific material behavior, e.g. auxetic behavior, and manufactured by using additive manufacturing techniques. Grima et al. [13] found that conventional materials containing diamond or star shaped holes, arranged in a periodic structure, exhibits various Poisson’s ratio values that can reach even negative values (auxetic behavior). Such evidence was found by Taylor et al. [14] by applying a pattern of elliptical voids, investigating different aspect ratios. As a consequence of the cited studies, Javid et al. [15] moved their interest to the stress behavior rather than on the material deformation and proposed a method for enhancing the fatigue life of turbine blades by replacing the commonly used circular holes with a novel orthogonal S-shaped void pattern, realized by means of the laser drilling. Such a novel pores design induces a soft mode of deformation based on rotation of the material around said voids, redistributing the stress field and modifying even the crack propagation phenomenon within the structure. The literature lacks in studies about employing the void patter for enhancing component’s mechanical performances. As a consequence, there is a wide range of fields in which the void patterns can be employed for mechanical performance improvement. The present study considers the case of a circular hole in an infinite plate under remote uniaxial load and proposes a C-shaped void pattern to redistribute the local stress field around the hole; in such a way the stress concentration factor can be consequently reduced. Moreover, the shape hereafter introduced is designed to be realized by means of the laser cutting technology. Furthermore, the introduced pattern is defined passing through the following steps: • Introduction of the analytical description of the case study in order to comprehend which is the critical region; • In defining the said pattern, it is necessary to define a suitable feature shape that is able to reduce the induced stress concentration factor as much as possible; • Then, the distance between the two contiguous pattern rows is defined as the one able to better redistribute the stress field in the critical region; • The same as before is done for defining the most suitable distribution of the features, considering a three-row pattern. Stefano Monti*

Reduction of the Local Stress Field around Holes through Porous Shaped Structures