# Withstanding Capacity of Machine Guards: Evaluation and Validation by 3D Scanners
## 4 Discussion  
Years of research and practice have demonstrated that the standardized ballistic tests for machine guards, which entail a single shot withstanding evaluation, do not provide a statistical approach to the problem of guards' safety. In fact, multiple single shots with the same test conditions lead to different results in terms of penetration resistance. Furthermore, with the current methods, the minimum number of shots needed to evaluate the uncertainty of the results is difficult to evaluate because the test is considered \"passed\" when the panel presents permanent deformations without cracks or an incipient fracture visible only on one surface of the panel. References [15; 16] are focused on this topic and all the uncertainties are explained in full detail. According to these articles, the authors conclude that the standardized tests are hardly reproducible and repeatable because they are affected by local impact conditions, and that three shots without any through cracking of the sheet can be considered as the minimum set to be tested for safety requirements. This achievement is not reported yet in any standard regulation and it is under discussion in different working groups of ISO TC 39/SC10. Therefore, the current standardized tests do not allow to make comparisons between different panel design solutions. This paper, together with other previous works such as [14; 15; 16; 18; 29], was based on a regression approach, implying the execution of multiple tests with the procedure presented in Section 2.2. This approach allows to overcome the aforementioned limitations of the standardized tests, lowering the uncertainty of results due to their dispersion. In fact, performing six or seven impact tests starting from an impact velocity that will surely imply the full penetration of the tested panel (unsafe condition) and decreasing this  
Figure 11: Deflection and reduced thickness of test panels: (**A**) test 6, 300 \(\times\) 300 mm; (**B**) test 3, 300 \(\times\) 300 mm; (**C**) test 5, 500 \(\times\) 500 mm. velocity in the subsequent impacts to reach a non-penetrating condition, the R&I regression curve can be calculated [27], together with the ballistic limit, \(V_{bl}\). With this method, the ballistic limit can be predicted with a very good approximation (\(R^{2}\) > 0.99) and the knowledge of this value allows to set a safe test velocity (\(V_{safe}\)), which in turn can be used to perform a single standardized impact test, obtaining a reliable result. Nevertheless, according to the conditions of suitability provided by the aforementioned standards, \(V_{bl}\) cannot be considered equal to the safe speed (\(V_{safe}\)). The evaluation of the withstanding capacity of machine guards based on \(V_{bl}\) allows to investigate how different test parameters affect the test results, such as the panel size. This would be impossible using the current standardized tests. In this paper, the influence of the panel size on the withstanding capacity to ballistic penetration was investigated with a new method based on the use of a metrology grade 3D portable laser scanner and 3D inspection software. Two panel sizes were investigated: the size of 500 \(\times\) 500 mm, which was the standardized one, and 300 \(\times\) 300 mm, which was the minimum panel size allowing the visual inspection of the working zone, when mounted on a real machine. The new presented technique, reported in Section 2.3, allowed to reconstruct the 3D digital models of the panels after the impact test. Then, the plastic deformation was evaluated by 3D maps representing the distribution of deviations between the deformed and undeformed panels. Results obtained with the regression curve demonstrated that the curve for the two different panel sizes must be considered as the regression of the same experiment; in fact, as Figure 8 shows, the confidence interval of \(V_{bl}\) (level 90%) related to the size 300 \(\times\) 300 mm contains the interval corresponding to the size 500 \(\times\) 500 mm.