PSI - Issue 79

H.G.E. da Silva et al. / Procedia Structural Integrity 79 (2026) 97–104

99

a)

b)

Fig. 1. Specimen geometry (a) and 4PB testing setup (b); dimensions in mm.

Fig. 1 (b) presents the 4PB test configuration, following the guidelines of ASTM C393. The core material was characterized under compressive loading, as depicted in Fig. 2. The compression tests followed the ASTM C365 standard , applying a normal load to the specimen’s face to determine the mechanical response of the core material.

Fig. 2. Testing configuration under compression.

2.2. Skin, adhesive, and core properties

The sandwich laminates were fabricated using unidirectional (UD) carbon fibre prepreg plies (HS 160 REM, SEAL ® ) and balanced bidirectional (0˚/90˚) glass fibre prepregs (EE300 ET445, CIT - Composite Materials, Italy). Both prepregs feature epoxy resin matrices: REM for the carbon fiber (CF) layers and ET445 for the glass fibre (GF). The prepreg properties are summarized in Table 1 and the layups in Table 2.

Table 1. Carbon and glass-fibre prepreg properties.

UD carbon fibre prepreg

0˚/90˚ glass fibre prepreg

Fibre 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. Evaluated composite layups for the skins.

Layup1

Layup2

CF GF CF CF GF CF

0˚

0˚

±45˚

0˚/90˚

90˚ 90˚

90˚ 90˚

±45˚

0˚/90˚

0˚

0˚

To bond the skin to the core, a two-component adhesive system, SikaForce ® -7710 L100, was employed. The adhesive system consists of two parts: Component A, SikaForce ® -7710 L100, is a polyol-based material, while Component B, SikaForce ® 7010, contains isocyanate derivatives, resulting in a polyurethane adhesive once mixed. The properties of this adhesive are detailed in Table 3. The core material used in the sandwich structure was a rigid, closed-cell foam made from polyimide (PMI), specifically the 71WF grade from ROHACELL ® . The key mechanical and thermal properties of the core material are shown in Table 4.