*3.4. Delamination*

As mentioned in the Introduction, in the case of multilayer composite materials, the energy is absorbed not only as a result of the destruction of reinforcing fibers, but also as a result of the destruction of connections between successive reinforcing layers (delamination). The inclusion of this process in numerical simulations has an effect on the obtained results. Figures 13–15 show maps of the delamination areas between selected layers as an effect of the striker's impact. The areas marked in red (for which the parameter value was 1) were the areas where the bonded connection between layers was broken (delamination occurs).

**Figure 13.** Colored maps of delamination areas after impact of the conical striker between layers: (**a**) 1 and 2, (**b**) 5 and 6, (**c**) 7 and 8, (**d**) 13 and 14.

The areas of material delamination resulting from impact by a conical striker indicate an increase of the diameter of delamination area in layers with a smaller amount of destruction in the reinforcing material. In the area of the first reinforcing layers, where the damage in the material is the biggest, the delamination covered a narrow area around the damage (Figure 13a). The delamination area increases with successive layers of reinforcing material, which is accompanied by a reduction in the number of deleted elements in reinforcing layers. In the case of the layers that were not damaged, as a result of the striker impact, the diameter of the area where delamination occurred was slightly increased.

In the case of the composite hit by an ogival striker, the delamination area occurring between the first and second reinforcement layer (Figure 14a) was smaller than in the case of the conical striker. The delamination area increased with each next reinforcing layer, until reaching a diameter of 14 mm between layer 13 and 14 of the reinforcing material. Concentration of the delamination was observed in places located in the axis of impact of the striker. As in the case of the conical striker, if more damage in reinforced layer was observed, then the less delamination occurs between this and next reinforcing layer.

**Figure 14.** Colored maps of delamination areas after impact of the ogival striker between layers: (**a**) 1 and 2, (**b**) 5 and 6, (**c**) 7 and 8, (**d**) 13 and 14.

**Figure 15.** Colored maps of delamination areas after impact of the blunt striker between layers: (**a**) 1 and 2, (**b**) 2 and 3, (**c**) 9 and 10, (**d**) 13 and 14.

The colored maps presented in Figure 15 indicate that, in the composite material that was hit by a blunted striker, the internal structure was damaged—delamination between the reinforcement layers. The diameter of the delamination area is almost the same over the entire thickness of the composite.

The detection of areas in which the material structure is damaged, like a delamination, is particularly important because this damage changes the behavior of the entire composite material and a ffects its mechanical properties. In the case of the practical application of multilayered composite materials, the lack of detection of such material damage can lead to dangerous situations in which there may be a serious failure, loss of load capacity, or reduction of the strength of elements working under high pressure, which can directly lead to the health and life threat of people working near such elements. The delamination is particularly di fficult to detect because they are usually not visible and their detection requires specialized tests. An interesting solution for monitoring such damage is the triboelectric sensor. These sensors do not require external power supply and they are mounted directly on the composite material. The results presented in the work of [67] showed it can be observed that the voltage outputs of the sensor are proportional to the extension of the damage in the composite. The triboelectric sensor can be used to predict the damage state of the composite plates and the size of the delamination caused by impacts of the strikers [67].

Figure 16 showed the diameters of the measured delamination areas between first and second reinforced layers, as well as the thirteenth and fourteenth. It is characteristic for all analyzed cases that the delamination areas increase with each subsequent reinforced layer. It is also worth noting the relationship between diameter of delamination area and the damage in the reinforced layer. If the damaged area was bigger, than the delamination area was smaller. The impact with the conical striker caused the largest damage in the reinforcing layers, but the delamination area was the smallest among the analyzed cases. The impact with the blunt striker caused the smallest damage in the reinforcing layers, but the delamination area was the biggest among the analyzed cases.

**Figure 16.** Diameters of delamination areas between reinforcement layers 1 and 2 and 13 and 14.
