PSI - Issue 72

H.G.E. da Silva et al. / Procedia Structural Integrity 72 (2025) 26–33

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matrices of both materials, REM and ET445, are epoxy resins. The prepreg properties are given in Table 1. Two configurations were created with different prepreg layups (Table 2).

Table 1. Prepreg properties. UD Prepreg carbon fiber

0˚/90˚ Prepreg glass fiber

Fiber volume fraction

64% 126 7.22 0.264 3.80

57% 29.8 29.8 0.294 3.07

Young’s modulus ( E 1 ) [GPa] Young’s modulus ( E 2 ) [GPa]

Poisson’s ratio ( ν 12 )

Shear modulus ( G 12 ) [GPa]

Table 2. Skin layup for the two configurations.

Ply material Carbon fiber Glass fiber Carbon fiber Carbon fiber Glass fiber Carbon fiber

Configuration 1

Configuration 2

0˚

0˚

±45˚

0˚/90˚

90˚ 90˚

90˚ 90˚

±45˚

0˚/90˚

0˚

0˚

The polyurethane-based adhesive SikaForce ® -7710 L100, which is made up of two components, was used to bond the laminates to the core. Adhesive’s properties are as follows: tensile strength of 13 MPa, tear strength of 9 MPa and an elongation at break of 8%. The core used was a rigid, closed-cell, polyimide-based foam (PMI) from ROHACELL ® labelled 71WF, with the properties presented in Table 3.

Table 3. Core properties.

Property

Value

Property

Value

Density [kg/m 3 ]

75

Compression strength [MPa] 1.7

Young’s modulus ( E ) [MPa]

105 0.25

Tensile strength [MPa] Shear strength [MPa]

2.2 1.3

Poisson ratio (ν)

2.2. Fabrication and testing The sandwich skins were produced from prepregs supplied by CIT on a roll, from which layers with an area of 300×300 mm 2 were cut. After cutting the layers, they were manually stacked on the desired order and orientation. A spatula was used to compress the layers to prevent bubbles. Once the layup was complete, two layers of release film and one layer of peel ply had to be cut for each side of the laminates. These layers ease demolding, absorb excess resin, and provide a surface finish, after curing. The laminates were cured on a hot plate press at 140ºC for 28 minutes. Subsequently, the temperature was reduced to 65ºC by air cooling. A PMI block with 300×300 mm 2 was disc cut to match the dimensions of the laminates. To evenly spread the adhesive, a spatula was used. After, the laminates were stacked on the core in the desired directions. After stacking, a steel plate was placed over each structure, consolidating it and removing any excess adhesive, as it was left to cure for a week at room temperature. Then, it went back to the hot plate press for 6 hours at 60ºC to consolidate the cure. Finally, the specimens were cut to the desired width. The core compression tests and flexure tests to the sandwich structures were carried out on a Shimadzu Autograph AGX 100 kN universal testing machine. Three core specimens were tested in the compression test, after individual measurements of length, width, and thickness, using the setup shown in Fig. 2 (a). A flat plate press was used for the core compression test at a speed of 0.4 mm/min. In the 3PB tests, shown in Fig. 2 (b), four specimens were used, two for each configuration. Deflection at mid-span was measured using a dial comparator. A speed of 5 mm/min was used with a distance between supports of 250 mm for both configurations (ASTM C393 standard).