Iniciativas concretas y vigentes de telecentros en Quito/ Ecuador

The protection capability of the ionomer samples is compared to that of aluminium alloy bumpers with similar areal density. The aluminium used is Al7075-T6. The protection capability was assessed by looking at two complementary data sets regarding the witness plate, namely, the extent on the witness plate of craters produced by the impact of the debris cloud and the amount of momentum transferred to the witness plate by the debris cloud. Both aspects provide indirect information on the debris cloud potential to damage structures on its flight path. Moreover, they should decrease when the target protection capability increases.

63 The witness plate craters area was measured from high-resolution (600dpi) images of the witness plates by means of an image analysis code developed in MATLAB® [52], while the momentum transfer was assessed by using the ballistic pendulum.

The test matrix is reported in Table 4-1. As reported in the table, incomplete self-healing occurred only in one sample. Figure 4.11 shows test’s 8813 target sample and witness plate. The crater on the witness plate indicates that the bumper was perforated, but no hole was present on the ionomer.

Table 4-1 Test summary for hypervelocity impacts on ionomer and aluminium targets.

Shot No. Target Projectile Perforation (Yes/No) Hole closure (Yes/No) Material Thickness (mm) Diameter (mm) Velocity (km/s)

8813 Surlyn® 2.0 1.5 1.93 Yes Yes

8829 Surlyn® 3.0 1.5 1.80 Yes Yes

8833 Surlyn® 5.0 1.5 1.64 No Yes

8836 Surlyn® 5.0 1.5 4.10 Yes Yes

8838 Surlyn® 3.0 1.5 4.00 Yes Yes

8839 Surlyn® 2.0 1.5 3.90 Yes No 8841 Al7075-T6/3 1.5 1.5 1.34 No No 8842 Al7075-T6/3 0.8 1.5 2.64 Yes No 8843 Al7075-T6/3 0.8 1.5 1.37 Yes No 8844 Al7075-T6/3 1.0 1.5 1.28 Yes No 8845 Al7075-T6/3 1.5 1.5 3.70 Yes No 8846 Al7075-T6/3 1.0 1.5 4.05 Yes No 8847 Al7075-T6/3 0.8 1.5 4.05 Yes No 8848 Al7075-T6/3 1.0 1.5 2.64 Yes No

In Figure 4.12 and Figure 4.13 scanning electron microscope (SEM) micrographs of the front and rear side of shots 8813 and 8839 are shown, respectively. From Figure 4.12 it can be observed that a molten circular zone with a diameter approximately equal to that of the projectile is present in the impact zone. In the damaged area around the visible hole in Figure 4.13 spallation occurred, thus damaging the material, and removing a bigger amount of material from the impact area, which presumably hindered the self-reparation of the hole.

In Figure 4.14 the ratio of the total crater area (Atot) and specific area (Asp) is plotted. The specific

area is the bumper surface corresponding to a mass of 1 kg. The damage area Atot is in fact

proportional to Asp, since the specific area is the inverse of the surface density and the lower the

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the total crater area increases more than linearly with the impact speed for the aluminium and the ionomer, but the increasing slope is steeper for the aluminium. It can be also observed that at highest impact velocities the ionomer bumper seems to produce less debris. This is probably due to the fact that with increasing speed increases also the amount of the target material that goes in the debris cloud, but such material is less dangerous if it has a low density, as it is with ionomer compared to aluminium.

Figure 4.11 Test no. 8813: perforated and rehealed ionomer bumper (left) and witness plate craters (right).

In Figure 4.15 the momentum transferred to the pendulum (Qpend) is plotted against the projectile

momentum (Qp). A tentative trend for the aluminium samples is also plotted, showing large data

scattering due to the limited number of tests. The uncertainty in the momentum estimation is reported by error bars, while the horizontal line refers to the light-gas gun noise. It can be seen from the figure that, for both the ionomer and aluminium, the momentum transferred to the ballistic pendulum by the debris cloud increases more than linearly with the impact speed. The increasing trend is again steeper for aluminium than for the ionomer.

Figure 4.12 Test no. 8813: SEM micrographs of the impact zone on the target front (left) and rear (right) face [Courtesy of Politecnico di Milano].

65 Figure 4.13 Test no.8839: SEM micrographs of the impact zone on the target front (left) and rear (right) face [Courtesy

of Politecnico di Milano].

Figure 4.14 Witness plate total crater area divided by the bumper specific area [54].

Even if Figure 4.14 and Figure 4.15 indicate higher protection capability of the ionomer, it has to be considered that the damaging potential of the debris cloud is highly dependent from the size and speed of its biggest fragment. This means that a debris cloud made of one big fragment is much more dangerous than a cloud with the same total mass consisting of many tiny dispersed particles. Therefore, the area of the largest crater on the witness plate (Amax) divided by the specific area of

the bumper is plotted in Figure 4.16. We can see that Amax/Asp for the ionomer is increasing more

than for the aluminium, with speed increase. Furthermore, for aluminium bumpers the protection capability begins to increases after approximately 3 km/s. For ionomer bumpers values of Amax/Asp

exceed those of aluminium bumpers even when the total crater area (Atot/Asp) is lower. This means

that fragments in the debris cloud are better fragmented for aluminium bumper than for ionomer bumpers. In fact, by observing the witness plates after the tests (see Figure 4.17), it is clearly visible

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that the debris in the cloud are better fragmented and dispersed after impacts on aluminium targets compared to ionomer bumpers, as indicated previously also by Figure 4.16.

Figure 4.15 Momentum transfer to the witness plate mounted on the ballistic pendulum and located behind the target [54].

67 Figure 4.17 Witness plate damage comparison after tests on ionomer (3 samples on the left) and tests on aluminium (3

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