Material properties of composite shell

The shell of the AGV-T2 helmet is made of a number of composite layers.

According to the information provided by the helmet manufacturer, the constituents of these layers are Kevlar 49 fibres, carbon (T700) fibres, glass fibres and an epoxy resin. However, these data were not enough to model the shell. To obtain more information, six samples were cut from chin bar, side, rear inferior, rear superior, crown and front regions of the shell. They were moulded in resin, polished and inspected under a microscope. The observations suggested that five different laminas were used in the shell: a Kevlar/carbon/epoxy hybrid unidirectional (UD) lamina, a glass/epoxy twill weave woven lamina, a glass/epoxy UD lamina, an unknown fibre/epoxy plain weave woven lamina and a carbon/epoxy UD lamina. The microscopy images were further processed using the microscope’s software. The outcomes were approximate thicknesses and fibre volume fractions of the laminas as well as lay-up of the shell at different regions.

A section of the sample cut from the rear superior region is shown in Figure 17 as an example. From the shell exterior, there is an approximately 0.11 mm thick paint layer. The paint was not considered in FE modelling. The second layer is the glass/epoxy twill weave woven lamina. Its thickness was measured at 0.2 mm and its fibre volume fraction (Vf) was approximated at 0.55. Its material properties, given in Table 18, were found in manufacturers’ databases.

The third part of the section is a [±60] sub-laminate made of the hybrid UD lamina.

Its thickness is nearly 0.230 mm. The sub-laminate forms a considerable portion of the shell thickness. Figure 18 shows an area of the shell where the hybrid lamina is visible and a plate made of this lamina. The hybrid lamina is made of strips of carbon (T700)/epoxy and Kevlar 49/epoxy UD composites. The width of each strip is about

10 mm, while average size of shell elements used for the shell was less than 10 mm.

To obtain material properties suitable for insertion in the FE model of the shell, it was assumed that the hybrid lamina is in-plane homogenous. Its mechanical properties were calculated from the properties of its constituents by using the rule of mixtures and making some assumptions about its failure modes. Then, its properties were validated by carrying out coupon tests and comparing the results with FE predictions.

This process is explained in the next section.

Figure 17 Section of a sample cut from the rear superior region of the shell.

As shown in Figure 17, there are some angle-ply laminates through the thickness of the shell made of glass/epoxy UD laminas. The thickness of each lamina and its fibre volume fraction were approximated at 0.085 mm and 0.6, respectively. The mechanical properties of this lamina were found in literature (Table 18).

Microscopy inspections revealed that in the rear inferior region of the helmet, several layers of a UD carbon/epoxy composite are used. The fibre volume fraction of this lamina was measured at 0.5, and its mechanical properties were obtained from the properties of its constituents as explained in the next section.

Table 18 Mechanical properties of two laminas of the shell glass/epoxy twill about 0.4 mm. The type of its fibres was not known. As can be seen in Figure 17, it has a relatively low fibre volume fraction (it was estimated at 0.4). This layer is probably used for moulding purposes and does not contribute significantly to the mechanical properties of the shell. Nonetheless, it was included in the FE model of the shell as a cross-ply laminate. The properties of the lamina were calculated from the properties of its constituents as explained in the next section. For the fibres, the properties of polymer fibres with an intermediate modulus and relatively low strength were used.

The shell of an AGV-T2 helmet was sliced with a band-saw and its thickness was measured with a micrometer at different points. In this way, the approximate boundaries of each region were determined. Figure 19 depicts these regions and the relevant lay-up, which are the best approximations found by using microscopy images and visual inspection. In the side patch region, where the chin strap and visor are

attached to the shell, several layers of the glass/epoxy UD lamina are used to reinforce the shell, which were included in the FE model.

Figure 18 The side region of the shell showing the hybrid UD lamina (left) and a plate made of it (right).

In spite of the fact that the thicknesses of laminas may vary over the shell in the actual helmet, it was assumed that they are constant and one value was used for each of them in the FE model of the shell. Comparisons between the final thickness of the shell at each region and the thicknesses found from microscopy images and measured by the micrometer in the same region showed that they are consistent and therefore justified the assumption. For instance, at the crown region the thickness was measured at 2.460±0.140 mm with the micrometer and at 2.370 mm with the microscope. Its value in the FE model was 2.460 mm.

1, 2 and 3) chin bar: [(0TW,G)2/±30H/(±30G)2] 4) rear inferior: [0TW,G/±30H/(0C /90C)4/(±30G)2] 5) rear superior: [0TW,G/(±30H/±30G)2/0PW,G/±30G] 6) vent: [0TW,G/(±30H)2/(±30G)5/0PW,G/±30G] 7) crown:

[0TW,G/(±30H)2/(±30G)2/(±30H)2/(±30G)2/0PW,G/±30G] 8) front: the same as 5.

9) side: [0TW,G/±60H/(±30G/±30H)2/±30G/±60G/±60H

/±60G/ /0PW,G/±60G]

10) side patch: [0TW,G/(±30G/±60G)2/±30G/±60H/±60H

/±30G/±30H/(±30G)2/±30H/(±30G)2/±60H/(±60G)2/±60H/

±60G/0PW,G/±60G]

Figure 19 Shell regions and relevant lay-up; white lines indicate the reference material direction; TW: Twill Weave, H: Hybrid, G: Glass fibre, C: Carbon fibre and PW: Plain Weave.

The matrix is epoxy.