Analysis of the compressive behavior in fiber direction of 3D printed continuous fiber reinforced thermoplastic using pure compression and four-point bending tests
Topic(s) : Manufacturing
Co-authors :
Matteo CHIMIENTI (FRANCE), Guilherme MACHADO (FRANCE), Olivier MONTAGNIER (FRANCE), Christian HOCHARD (FRANCE)Abstract :
Continuous carbon fiber reinforced polymer (CFRP) composites are widely used in the transportation, marine and aerospace industries due to their high specific mechanical properties compared to metal alloys. The application of 3D printing to CFRP has opened a new era for the design and production of complex composite structures, for which the cost and environmental impact can be customized based on the quantity of material. 3D printing of composite materials is a recent additive manufacturing process that allows the production of structural parts through coextrusion of thermoplastic material and continuous fiber (carbon, glass, Kevlar….) [1]. Several material characterization studies have analyzed the mechanical behavior of CFRPs. The addition of continuous carbon fiber effectively increases the mechanical properties in terms of stiffness and deformation at failure of the material subjected to traction. The tensile behavior of the material has been widely studied [2,3] but not much is yet known about its compressive behavior.
This study proposes the analysis of the compressive behavior of CFRP specimens. First, ASTM-D695 pure compression tests were conducted on hourglass specimens. The figure 1 shows the complete tensile and compressive behavior of unreinforced Polyamide-12 specimens in green and Polyamide-12 specimens reinforced with eight layers of continuous carbon fiber (CFRP) in blue. It can be seen that the reinforced specimens have greater stiffness than the plastic specimens. Furthermore, the unreinforced specimens maintain a symmetrical behavior in the two cases of traction and compression while the specimens reinforced with continuous carbon fiber present a quasi-linear behavior up to failure (reached at 1% deformation) in the case of traction, while in the case of compression, although finding the same stiffness that we have in traction, failures are achieved for low levels of deformation (0.1%) well away from 1%. Pure compression tests show premature failure probably due to non-consolidation for this process and therefore to the presence of defects between the layers.
In order to analyze this sensitivity to defects, four points bending tests were carried out on sandwich structures featuring a polyamide 12 core and two skins (on the top and bottom) made of continuous carbon fiber. The study aims to vary the thickness of the plastic core in the sandwich or the thickness of the carbon fibers skins. The maximum strain at failure decreases with increasing thickness of the sandwich for the same thickness of the skins, as shown in figure 2 and as previously studied by Wisnom [4]
In parallel with these tests, numerical simulations are carried out in order to study the physical origin of the compression failure (buckling….).
This study proposes the analysis of the compressive behavior of CFRP specimens. First, ASTM-D695 pure compression tests were conducted on hourglass specimens. The figure 1 shows the complete tensile and compressive behavior of unreinforced Polyamide-12 specimens in green and Polyamide-12 specimens reinforced with eight layers of continuous carbon fiber (CFRP) in blue. It can be seen that the reinforced specimens have greater stiffness than the plastic specimens. Furthermore, the unreinforced specimens maintain a symmetrical behavior in the two cases of traction and compression while the specimens reinforced with continuous carbon fiber present a quasi-linear behavior up to failure (reached at 1% deformation) in the case of traction, while in the case of compression, although finding the same stiffness that we have in traction, failures are achieved for low levels of deformation (0.1%) well away from 1%. Pure compression tests show premature failure probably due to non-consolidation for this process and therefore to the presence of defects between the layers.
In order to analyze this sensitivity to defects, four points bending tests were carried out on sandwich structures featuring a polyamide 12 core and two skins (on the top and bottom) made of continuous carbon fiber. The study aims to vary the thickness of the plastic core in the sandwich or the thickness of the carbon fibers skins. The maximum strain at failure decreases with increasing thickness of the sandwich for the same thickness of the skins, as shown in figure 2 and as previously studied by Wisnom [4]
In parallel with these tests, numerical simulations are carried out in order to study the physical origin of the compression failure (buckling….).