ABSTRACT
Flax fibers are widely used as the strongest natural fibers in composite parts with advanced structures. Due to their excellent mechanical properties and recyclability, they have attracted more attention from the aviation and automotive industries, etc. These composite parts are usually obtained by preforming reinforcements or prepregs at high temperatures, and their mechanical behaviors are greatly affected by temperature variations. To improve the understanding of the mechanical properties of flax fiber materials, especially for the braided fabric with non-orthogonal structures, uniaxial tensile tests at different temperatures and tensile speeds were conducted on hollow tubular braided fabrics. The thermomechanical properties of Flax/Polyamide12 (PA12) prepregs were analyzed. The results show that temperature and tensile speed have obvious effects on the strength and shear stiffness of tubular fabrics. The strength and shear stiffness of the fabric decreases as the ambient temperature increases. Meanwhile, the strength of the fabric can also be improved by appropriately increasing the tensile speed. In addition, according to the experimental results, a theoretical model is established to describe the shear angle on the smallest circumference of the fabric, which provides a theoretical basis for the subsequent simulation process. The test results can provide a reference for the manufacture of flax fiber-reinforced composites with tubular structures.
ABSTRACT
A new combination of sulfonium salts has been investigated to cure opaque and thick carbon composite materials through photoinduced thermal frontal polymerization reaction. The photopolymerization occurs at the surface of the cycloaliphatic epoxide through the excitation of a triarylsulfonium salt and releases enough heat to decompose an alkyl-based sulfonium salt acting as a latent thermal initiator. Thus, a thermal front propagates into the medium leading to the polymerization of the whole sample. Thermal properties and optimal parameters are investigated to obtain frontal polymerization in the depth of the material. Front velocities were as high as 12.9 cm min-1 and were found to increase with an increasing concentration of thermal sulfonium salt. The effect of an addition of carbon filler is investigated with a concentration of up to 50 wt%, which allows the formation of a composite material with a high content of carbon without the need for thermal post curing.
ABSTRACT
The use of shape memory polymer composites is growing rapidly in smart structure applications. In this work, an active asymmetric composite called "controlled behavior composite material (CBCM)" is used as shape memory polymer composite. The programming and the corresponding initial fixity of the composite structure is obtained during a bending test, by heating CBCM above thermal glass transition temperature of the used Epoxy polymer. The shape memory properties of these composites are investigated by a bending test. Three types of recoveries are conducted, two classical recovery tests: unconstrained recovery and constrained recovery, and a new test of partial recovery under load. During recovery, high recovery displacement and force are produced that enables the composite to perform strong two-way actuations along with multi-shape memory effect. The recovery force confirms full recovery with two-way actuation even under a high load. This unique property of CBCM is characterized by the recovered mechanical work.
