Influence of skin temperature (T Skin )

The investigations on the influence of the skin temperature show that the selected skin temperatures of 300 °C, 310 °C and 320 °C enable the successful production of sandwich with a strong fusion bond between skins and core. Figure 69 summarises the mechanical properties of the TPC sandwiches manufactured with different skin temperatures. All results are normalised to the performance of the reference sandwich specimens 300-125-2. The results show that heating the skins to higher temperatures (TSkin = 310 °C, 320 °C) in comparison to the reference skin temperature of 300 °C significantly improves the tensile and the peel strength, while properties such as compression, bending or edgewise compression are marginally influenced by a variation of the skin temperature. To highlight the influence on tensile strength, Figure 70 shows the normalised tensile strength results depending on varying skin temperatures in more detail, including failure mechanisms and sandwich thicknesses. The results show that all three skin temperatures ensure a bond strength that is higher than the core strength itself, leading to cohesive failure of the core in the area adjacent to the skins, which is referred to as the boundary layer, see for example Figure 71. However, on a closer examination, different thicknesses of the boundary layer can be observed. For specimens manufactured with 300 °C heated skins, failure occurs ~400 µm into the core, see Figure 71a.

In comparison, Figure 71b shows that for specimens manufactured with a skin temperature of 320 °C, failure occurs approximately 950 µm away from the core surface. Furthermore, differences in the tensile strength can be noticed. The tensile strength of sandwich specimens manufactured with TSkin = 300 °C is significantly below the normalised tensile strength of the initial foam (𝜎 ̅_{𝐶𝑜𝑟𝑒} = 1.24), which was characterised as well, while the tensile strength of the

specimens manufactured with 310 °C and 320 °C is almost equal to this value. This indicates that the core structure and its performance are influenced by joining process.

Figure 69: Influence of the skin temperature on the mechanical properties

In the previous chapter (chapter 5.7), it was already observed that the process temperatures influence the sandwich thickness and thereby the core cell structure. Skin temperatures of 300 °C in combination with a core kept at room temperature led to core cell compaction in the boundary layer, while skin temperatures of 300 °C in combination with a heated core of 100 °C led to stretched cells in the boundary layer. Stretching of the cells is assumed to result from inner core collapse towards the centre. During processing the core collapses due to extensive heating, though skin and core surface, which is strongly bonded to the skins, cannot follow the core collapsing movement towards the centre due to the limited mobility of the skins regulated by mould stops. The relative movement of core centre and core surface area leads to stretching of the cells in the boundary layer. Collapse of the core hindered the fusion bonding process due to the missing interfacial contact. However, it could not be observed that the change of the core influenced the core performance itself. A similar phenomenon can be observed in this complementary study, though in contrast to the previous results the performance of the core is thereby influenced. Therefore, this phenomenon is described in detail in the following section.

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6 Mechanical characterisation 97 __________________________________________________________________________

Figure 70: Influence of the skin temperature on the tensile strength (bars) and sandwich thickness (line)

Figure 70 illustrates the decrease of sandwich thickness with the increase of skin temperature.

For skin temperatures of 300 °C, the specimens reach approximately the aimed thickness of 16.3 mm. For skin temperatures of 310 °C and 320 °C the sandwich thicknesses are significantly below Saimed = 16.3 mm. Since the mould design hinders extensive compaction of the core under load, sandwich thicknesses below 16.3 mm indicate core collapse during processing. Figure 72 shows micrographs of the cell structure of the core close to the skins of specimens manufactured with skin temperatures of 300 °C (a) and 320 °C (b), one untreated (upper picture) and one potted in epoxy resin and sanded (lower picture). The affected cells in the boundary layer are clearly visible in the upper pictures of Figure 72. A visual comparison between the cell structures in the core centre or beyond the boundary layer and original core cell structures does not reveal a noticeable effect on the cells. As observed in the investigated specimens in chapter 5.7, the potting resin flows again into the affected cells in the boundary layer, see lower pictures of Figure 72.

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a) b)

Figure 71: Thickness of the boundary layer of a) specimens 300-125-2 and b) specimens 320-125-2

By comparing the affected cell structures in the boundary layer of the specimens 300-125-2 and 320-125-2 various effects of the temperature on the cells can be detected. Specimens manufactured with skin temperatures of 300 °C feature compacted cells in the boundary layer (Figure 72a), while those with skin temperatures of 320 °C show compacted and stretched cells in a larger boundary layer, see Figure 72b.

ohesive failure in core boundary layer ohesive failure in

core boundary layer

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ore boundary layer

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6 Mechanical characterisation 99 __________________________________________________________________________

a) TSkin = 300 °C b) TSkin = 320 °C

Figure 72: Cell structure in the boundary of a) specimens 300-125-2 and b) specimens 320-125-2 (without potting resin in the upper picture and potting resin in the lower picture)

For better illustration, Figure 73 highlights the stretched in the boundary layer of the specimens 320-125-2.

516 µm

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ompressed cells

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Potting resin

riginal cells

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Figure 73: Highlighted boundary layer with stretched cells (without potting resin) 7 9 m

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By comparing the cell structures presented in Figure 72 to the location where the failure in the boundary layer occurred (Figure 71) it can be observed that the specimens fail at the interface between affected and unaffected cells. A similar effect was observed by other researchers [39,66]. McGarva et al. [66] describe failure of Polymethylarylimide (PMI) foams parallel to the interface between compressed and original cells after joining to heated Polyamide based skins during double cantilever beam testing. In contrast PMI foams without compressed cells (joined to thermoset based skins) failed with a kinking crack propagation in the centre of the core.

Akermo et al. [39] report on smooth failure near the core surface of Polypropylene (PP) foam cores featuring densified cells in the core surfaces after joining to heated PP based skins.

Despite this, neither researchers specifically mention a lower strength value for the core in comparison to an original core.

Failure of the core in the boundary layer could be explained by the stiffness discontinuity between affected and original cells, which represents a sharp transition inducing stress concentration under load. While the stiffness discontinuity between compressed cells and original cells may depict a stronger transition than stretched cells to original cell, the performance of foam cores with compressed stress cells might be further reduced than the performance of cores with stretched cells. This could explain the higher tensile value of the specimens 310-125-2 and 320-125-2 where stretched cells are observed opposed to the tensile strength of the specimens 300-125-2 where compressed cells in the boundary layer are seen. Furthermore, weakening of the core of specimens having a core density of 110 kg/m³ could not be observed in chapter 5.7. This might be explained by the softer transition between affected and unaffected cells since the original core already features a high cell density. Raster electron microscopy of the failure areas did not reveal any additional explanation for failure in the interface between affected and original cells. Detailed studies on failure paralleling an interface in general are reported by Hutchinson et al. [192].

A similar phenomenon in the boundary layer structure can also be observed for the climbing drum-peel specimens, leading to increased peel strengths with increasing TSkin, see Figure 69.

Although the tensile and drum-peel performance can be partially increased by increasing the skin temperature, it must be considered that the sandwich thicknesses lay significantly below the aimed thickness, which indicates that temperatures of 310 °C and 320 °C cause uncontrolled core collapse and lead to unreproducible results. Furthermore, several properties such as shear and compression strengths are not significantly improved by increasing TSkin. Therefore, a skin temperature of 300 °C seems most reasonable for the manufacturing process.