Compression and Bending Properties of Short Carbon Fiber Reinforced Polymers Sandwich Structures Produced via Fused Filament Fabrication Process

Abstract

Additive manufacturing, through the process of thermoplastic extrusion of filament, allows the manufacture of complex composite sandwich structures in a short time with low costs. This paper presents the design and fabrication by Fused Filament Fabrication (FFF) of composite sandwich structures with short fibers, having three core types C, Z, and H, followed by mechanical performance testing of the structures for compression and bending in three points. Flatwise compression tests and three-point bending have clearly indicated the superior performance of H-core sandwich structures due to dense core structures. The main modes of failure of composite sandwich structures were analyzed microscopically, highlighting core shear buckling in compression tests and face indentation in three-point bending tests. The strength-mass ratio allowed the identification of the structures with the best performances considering the desire to reduce the mass, so: the H-core sandwich structures showed the best results in compression tests and the C-core sandwich structures in three-point bending tests. The feasibility of the FFF process and the three-point bending test of composite wing sections, which will be used on an unmanned aircraft, have also been demonstrated. The finite element analysis showed the distribution of equivalent stresses and reaction forces for the composite wing sections tested for bending, proving to validate the experimental results.

Keywords: failure analysis; fused filament fabrication; mechanical testing; sandwich structures; short carbon fiber; wing structure.

Figure 1

Design and dimensional description of composite sandwich structures (mm): (a) C profile dimensions; (b) C-core composite sandwich structure subjected to compression test; (c) the C-core composite sandwich structure subjected to bending test; (d) the dimensions of the Z profile; (e) Z-core composite sandwich structure subjected to compression test; (f) Z-core composite sandwich structure subjected to bending test; (g) Hat profile dimensions; (h) Hat core composite sandwich structure subjected to compression test; (i) Hat core composite sandwich structure subjected to bending test.

Figure 2

Wing sections: (a) front view of the wing section (mm); (b) internal composite structure of the wing segment subjected to three-point bending.

Figure 3

Microscopic analysis of the filament before 3D printing (100× magnification): (a) Longitudinal section of the filament; (b) Cross section of the filament.

Figure 4

Establishing the 3D printing position of structures: (a) Composite sandwich specimens subjected to compression test; (b) Composite sandwich specimens subjected to three-point bending test; (c) Wing sections tested for three-point bending.

Figure 5

Testing of composite structures: (a) Flatwise compression of composite sandwich structures; (b) Three-point bending of composite sandwich structures; (c) Three-point bending of the wing sections.

Figure 6

Compression test results of composite sandwich structures: (a) Mean values of compressive strength and modulus of elasticity; (b) Load–Displacement plot.

Figure 7

Microscopic analysis of sandwich structures subjected to flatwise compression (20× magnification): (a) C core; (b) Z core; (c) Hat core.

Figure 8

Results of three-point bending tests for sandwich structures: (a) Mean values of compressive strength and modulus of elasticity; (b) Load–Displacement curves.

Figure 9

Microscopic analysis of sandwich structures tested for bending at three points (20X magnification): (a) C core; (b) Z core; (c) Hat core.

Figure 10

Results of three-point bending tests of the wing sections: (a) Mean values of bending strength; (b) Load Curve—Displacement.

Figure 11

Microscopic analysis of three-point bending wing sections (magnification 20×): (a) Fracture of the wing skin; (b) Failure of the wing skin—top view.

Figure 12

Microscopic analysis of wing sections tested for three-point bending (magnification 100×): (a–c) Longitudinal section; (d) Cross-section.

Figure 13

Analysis of the strength-to-mass ratio of composite sandwich specimens: (a) Mean mass of specimens; (b) The specific ratio for the six types of specimens tested.

Figure 14

Finite element analysis of wing sections: (a,b) Determination of boundary conditions; (c) Model discretization; (d) Failure of the wing section following three-point bending tests; (e) Comparative analysis of reaction force; (f) Equivalent stress distribution [MPa].