IMPLEMENTATION OF BOND MODEL

According to Equation 14 the tensile bond strength can be predicted depending on the process parameters and material properties, quantities that have been determined in the previous section. In this theoretical study, the focus is on the investigation of the interface temperature influence on the bond strength of sandwich specimens.

5.6.1 Parameter definition

The interface temperature can be influenced by heating the parts to be joined, namely the skins as well as the core. As presented in chapter 5.1, the process window for joining CF/PEEK skins and the PEI core is narrow since extensive heating (TSkin > 340 °C) leads to skin de-consolidation and to core collapse. Lower temperatures (TSkin < 230 °C) do not enable fusion bonding based on the healing theory (refer to Figure 51). Therefore, in consideration of heat loss during transfer, the bond strength is to be predicted for skin temperatures (TSkin) between 260 °C and 320 °C in steps of 10 °C and core temperatures (TCore) of room temperature (TR = 23 °C), 100 °C and 200 °C. A core temperature of 200 °C is considered the maximum possible core pre-heat temperature (in consideration of heat loss during transfer) since core collapsing was observed at core temperatures above Tg of the core. The pressure for manufacturing is kept constant at a low level of 0.2 MPa to avoid extensive core compaction.

In the following section the results are presented and discussed. Here, the predicted tensile strength is normalised to the strength of a fully healed interface 𝜎̅ = ^𝜎

𝜎_∞ ∗ 100 % =

𝜎

𝜎_T(PEI)∗ 100 %.

Furthermore, the results are compared to the tensile strength of the core, which was characterised according to DIN53292 (chapter 5.7.2.2) and determined to be 2.64 % (also normalised to the bulk material strength). As a result, sandwiches featuring a predicted tensile strength below 2.64 % of the reference tensile strength 𝜎_∞ are expected to fail adhesively (Adh) within the interface, while sandwiches with a predicted normalised tensile strength above 2.64 % will probably fail cohesively (Coh) within the core, based on the assumption of the weakest link.

5.6.2 Bond strength prediction depending on skin temperature

The temperature of the skins influences the bond strength to a large extent because the skins supply most of the heat energy to soften the polymer and to enable molecular mobility to achieve intimate contact and to allow polymer interdiffusion.

As an example, Figure 54 shows the calculated temperature with time for a skin laminate temperature of 260 °C and the interface temperature with time, as well as the predicted normalised tensile bond strength. The core is thereby kept at room temperature. Since the

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temperature of the interface stays relatively low (~230 °C), where the motion of the molecules is slow and the reptation time is high, the predicted tensile bond strength is low. The model predicts a normalised tensile bond strength, which is 0.56 % of the bulk PEI tensile strength.

The prediction of the normalised strength of 0.56 % means that the sandwich will feature an insufficient (lower than the core strength) bond quality and that the sandwich will probably fail within the interface, since the core itself features a normalised tensile strength of 2.64 % in.

In comparison, Figure 55 exemplarily displays the predicted normalised tensile bond strength based on the temperature evolution for a skin temperature of 310 °C. Due to the higher temperature of the skins, the interface temperature is increased up to approximately 260 °C, where interdiffusion of the polymers occurs to a larger extent (Tr ~ 639 sec). Therefore, the model predicts that the bond quality is improved and the sandwiches feature a normalised tensile bond strength of approximately 7.85 % in comparison to the reference tensile strength.

Since the normalised tensile strength of the core is around 2.64 %, the model predicts that the bond quality is sufficient (not the weakest link), albeit low in comparison to the bulk material strength, and failure of the sandwich will occur within the core.

Table 18 summarises the predicted normalised bond strength for skin temperatures in the range 260 °C – 320 °C. For all predictions, the core temperature is kept at room temperature.

In addition, a prognostication about the expected failure mechanisms is given.

Table 18: Predicted normalised bond strength and failure mechanisms for different skin temperatures

TSkin [°C] 260 270 280 290 300 310 320

Predicted 𝜎̅ = ^𝜎

𝜎_∞ [%] 0.56 1.34 2.2 3.35 5.05 7.85 13.13 Expected failure

mechanism [Adh/Coh]

Adh Adh Adh Coh Coh Coh Coh

From Table 18 it becomes clear that the model predicts that sandwiches manufactured with skin temperatures above 290 °C and a core kept at room temperature will feature sufficient bond strength, since a cohesive failure within the core is prognosticated.

Figure 54: Normalised tensile bond strength prediction for a skin temperature of 260 °C

Figure 55: Normalised tensile bond strength prediction for a skin temperature of 310 °C 0

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5.6.3 Bond strength prediction depending on core temperature

The model shows that heating the core leads to an improvement of the bond strength since the temperature level of the core or core surface is already elevated before the heat from the skins is transferred into the core. In combination with heated skins, high temperatures at the interface can be achieved, which improves bond development. Table 19 gives a summary of the predicted normalised bond strengths for several skin temperatures and elevated core temperatures as well as expected failure mechanisms. For a sandwich manufactured with 260 °C heated skins, the sandwich will probably fail adhesively within the interface for core temperatures of 23 °C and 100 °C. By heating the core to 200 °C, the model predicts a normalised tensile bond strength of 5.65 % and failure of the sandwich is expected to be cohesively within the core. In the case of sandwiches manufactured with skins heated to a temperature of 280 °C, a core temperature of 100 °C is probably sufficient to achieve a cohesive failure within the core, since the normalised tensile strength is increased from 2.2 % for a core at room temperature to 7.04 %. An increase of core temperature to 200 °C will lead to a further increase of the predicted tensile bond strength. If the skin is heated up to 300 °C, core heating will improve the predicted bond strength, however it is expected that even sandwiches with the core kept a room temperature feature a sufficient bond quality and will fail within the core. Therefore, heating of the core seems to be unnecessary in this case.

Table 19: Predicted normalised bond strength and failure mechanisms for varying core temperatures and different skin temperatures

The modelling results are valid provided that the core does not collapse due to the increased temperature during processing. A statement about core collapse cannot be provided by the model. In addition, the model predicts that a skin temperature of more than 370 °C is necessary to achieve full healing (𝜎̅ = 100 % = 105 MPa). However, since it is sufficient to achieve a tensile bond strength which is superior to the tensile strength of the core itself (weakest link) and the fact that 370 °C leads to skin de-consolidation and core collapse, the aim of achieving full healing is not further pursued.