Influence of core compaction distance (d compaction )

The investigations reveal that the core compaction distance has a considerable influence on the mechanical properties of the sandwiches, as displayed in Figure 74. Again, all results are normalised to reference specimens 300-125-2.

The results clearly show the influence of the core compaction distance on all characterised mechanical properties of the sandwich structures. In the case of sandwiches manufactured with 1 mm core compaction distance, the skin-to-core bond strength is low and weaker than the core tensile strength itself. The low bond quality can be explained by insufficient contact of foam and core which is required for interdiffusion of the molecules, see chapter 5.5.1.

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Figure 74: Influence of the core compaction distance on the mechanical properties

Due to heating the core slightly collapse (which occurs for all cases), but the skin cannot follow the core and keep contact due to the limited core compaction distance of 1 mm controlled by the mould stops. The sandwich thickness is measured to be 1.5 mm below Saimed = 16.3 mm, which proves core collapse. In contrast, by applying dcompaction of the 2 mm, the skin can further move into the core and keep contact. Therefore, dcompaction of 2 mm leads to the aimed thickness. Figure 75 shows partial adhesive failure of the specimens manufactured with 1 mm compaction distance in comparison to pure boundary layer failure of specimens manufactured with 2 mm compaction distance, where the skin can follow the core collapsing, see Figure 71.

Adhesive failure leads to a reduced measured tensile strength of specimens 300-125-1.

Figure 75: Adhesive and boundary layer cohesive failure of the specimens 300-125-1 0

Even though the peel strength of the specimens manufactured with 1 mm core compaction distance is higher than of the specimen with 2 mm and 3 mm core compaction distance, analysis of the failure mechanisms (Figure 76) shows the insufficient bond quality of the specimens 300-125-1 leading to partial adhesive failure. A reason for the higher peel strength, although adhesive failure occurred, might be that it takes more energy for the crack to propagate through interface and foam than for propagating along the pre-defined path in the boundary layer at the interface between compressed and original cells as observed for the specimens 300-125-2 and 300-125-3.

a) b)

Figure 76: Failure mechanisms as occurred during drum peel testing: a) Partial adhesive failure in series 300-125-1 and b) Cohesive boundary layer failure in series 300-125-2

The weak skin-to-core bond for specimens manufactured with 1 mm core compaction distance can also be observed during edgewise compression testing. While specimens manufactured with 2 mm and 3 mm core compaction distance buckle under load, skin separation is the main failure mechanism of the specimens manufactured with 1 mm, see Figure 77.

a) b)

Figure 77: a) Buckling of the structure during the edgewise compression test as occurred in the specimen series 300-125-2 and 300-125-3; b) Skin separation during the edgewise compression test as occurred in series 300-125-1

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Specimens manufactured with 3 mm core compaction distance show also deviating properties regarding tensile, shear or edgewise compression performance (Figure 74), albeit similar failure modes as for specimens manufactured with dcompaction = 2 mm occur. By increasing the core compaction distance to 3 mm, the performance regarding tensile strength is significantly reduced in comparison to the tensile strength of specimens with 2 mm core compaction distance. An explanation might be again the stiffness discontinuity between affected and original cells which causes stress concentrations and thereby core failure. Figure 78 shows that the boundary layer cells of specimens 300-125-3 are compressed within a layer of approximately 400 µm, which is a comparable affected boundary layer to the specimens manufactured with 2 mm compaction distance, see Figure 72. This means that the cells of the specimens 300-125-3 are densified to a higher extent in the boundary layer than the cells of the specimens 300-125-2, since the core is compacted for 3 mm instead of 2 mm. This leads to the assumption that the higher densification of cells in the boundary layer of specimens 300-125-3 leads to a sharper stiffness discontinuity between affected and originals cells in comparison to the stiffness discontinuity resulting from 2 mm core compaction. Following, higher stress concentrations lead to a decrease of the tensile performance of the core.

Figure 78: Cell structure of the specimens 300-125-3 (without potting resin)

Furthermore, the testing results show that a higher core compaction distance additionally leads to a reduction of shear strength, see Figure 74.

In order to eliminate doubts that the decrease of the sandwich performance or core performance in the case of the specimen 300-125-3 is affected by an insufficient fusion bond between skins and core, but rather caused by a higher cell compression, additional trials that aim to compact the core cells locally, are conducted. Therefore, additional trials that compress the core surface cells without fusion bonding the core to a skin are performed. By doing so,

411 m

Skin

Compressed cells

Original cells

1000 µm

PEI foam cores (60 kg/m³) are compacted 2 mm and 3 mm in the boundary by means of a 300 °C heated aluminium plate (so no fusion bonding to CF/PEEK skins takes place) and characterised by tensile and shear testing. Figure 79 exemplarily shows the compressed cells (without fusion bonding) at the core surface for a core compaction of 2 mm.

Figure 79: Compacted cells in the boundary layer of specimens without fusion bonding (without potting resin)

The comparison of the normalised tensile strength (normalised to the tensile strength of the initial untreated core) of the compacted foams to the initial foam shows that compacting the foam 2 mm (specimens 300-2), 3 mm (specimens 300-3) respectively, leads to a significant decrease of the tensile strength, see Figure 80a. Furthermore, the compacted foams fail cohesively in the boundary layer, while the initial foam fails cohesively within the core centre.

These results confirm the assumption that foam compaction leads to a weakening of the foam caused by an interface between affected and original cells. The comparison of the normalised shear strength of the compacted foams to the initial foam shows that foam compaction (2 mm and 3 mm) leads to a slight decrease of the shear strength, see Figure 80b. However, a difference in the failure modes, which is cohesive failure, cannot be observed. A reason for the decreased shear strength of the compacted foams could also be stress concentration at the interface between affected and original cells, which leads to crack initiation under lower loads.

a) b)

Figure 80: a) Tensile strength results of initial and compressed PEI foam cores (60 kg/m³) and b) Shear strength results of initial and compressed PEI foam cores (60 kg/m³)

475 m Compressed

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Finally, it has to be mentioned that core compaction additionally increases the areal density of the sandwiches and thereby negatively influences the performance-to-weight ratio. A core compaction of 3 mm in comparison to a core compaction of 2 mm increases the weight by approximately 5 %. Summing-up, 2 mm compaction distance seem to be the optimal compaction distance in this study.