# Withstanding Capacity of Machine Guards: Evaluation and Validation by 3D Scanners
###### Abstract  
Ballistic penetration of machine guards is a topic of great significance for ensuring safety and avoiding projection of objects out of the working space of machines. Although standardized tests are performed according to EU Directive 2006/42/EC, they have some limitations because they are carried out using a given penetrator that perpendicularly impacts a given surface of about 500 \(\times\) 500 mm. Nevertheless, the withstanding capacity of those guards depends on a lot of different design parameters and physical quantities that have not been fully investigated. This paper is focused on the study of the influence of the machine guard size on the withstanding capacity to ballistic penetration throughout theoretical models and experimental tests based on an innovative method involving the use of 3D optical scanners for inspection. The experimental analyses described in this work, compared to theoretical results, demonstrate that a maximum plastic deformation area can be defined, independently of the plate size, given the same material, thickness, and penetrator. This result allows to set proper new ranges for standardized tests, overcoming the limitation of using specific sample sizes. ballistic penetration; 3D scanner; deformation analysis; machine guards; ejection risk +
Footnote †: journal: Applied sciences  
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close to the maximum predictable velocity of the machine tool or at a specified fixed velocity. A single shot is considered valid to predict the guard safety and the test results are read and analyzed based on the visualization of the damage reported by the test panel after the impact: the presence of through-hole cracks implies that the test has failed, and the machine guard cannot be installed. Only test panels presenting plastic deformation and no through-hole cracks are considered safe [6]. Many test parameters can influence the withstanding capacity of the guards and primarily the shape of the impacting projectile. The influence of the projectile shape was investigated by [PERSON] [14] and [PERSON] [15] using an experimental approach and performing tests with three different penetrators on polycarbonate panels of 4 mm thickness. Results showed that penetrators with a cylindrical shape and flat head were better-retained by the panels than prismatic [6] and truncated-cone projectiles [11]. Other important specifications can be defined during the design of machine guards and must be considered during the tests, such as the free open window and the constraint overlapping size of the panel subjected to the ballistic impact. This overlapping parameter is very important for the design of polycarbonate vision panels on metallic guards, which is fixed at 25 mm for a standardized panel of 500 \(\times\) 500 mm in size. Many other test parameters affect the withstanding capacity of panels to ballistic penetration, but some of them are difficult to set and fully control during the tests. Thus, due to a certain number of parameters which cannot been controlled, an inevitable uncertainty affects the current standard impact tests, and a big effort should be made to reduce the unreliability of results. The uncertainties of the test results are related to the material delivery conditions, impact angle, shape of the projectile head, size of the sheet, and stiffness of the frame. Some of these parameters are difficult to control, such as material delivery conditions, impact angle, and velocity. For instance, during its flight, the projectile may be subjected to rotation effects, which affect the impact angle. Furthermore, the impact velocity can be controlled for just 5% of the value. [PERSON] et al. [15] investigated the dispersion of results of multiple tests at the same impact speed. The results showed that, considering three identical shots, one completely penetrated the machine guard and the other two were retained by the test panel.