Mechanical behavior of high dosage short carbon fiber reinforced epoxy composites

Short fibers are effective reinforcements in strengthening and toughening polymer materials, even small amounts of fibers. However, for high fiber dosage the achieved dispersion and interface adhesion is quite poor reaching to lower stiffness and strength efficiency. In high dosage composites, the effects of the fiber length and volume fraction on mechanical properties is not entirely consensual; nonetheless this paper intends to contibute for a better understanding of this occurrence. Composite plates were manufactured by compression moulding, using short carbon fibers reinforcements (2, 4 and 6 mm in length) with 45-75 % wt fiber fraction. Tensile and dynamic mechanical analysis (DMA) specimens were machined from molded plates for the desired dimensions. DMA was performed with bi-supported specimens in temperature ranged from 20 to 180 °C. High dosage composites exhibit very low efficiency parameters both in stiffness and particularly in tensile strength. However, the stiffness increases in order of 25 % when fiber length increases from 2 mm to 4 mm, but afterwards tends to decrease for 6 mm fiber length composites. The same tendency was observed for the tensile strength. For very high fiber dosage composites, tensile strength decreases and Young’s modulus tends to remain at constant values. However, the results of DMA indicate that bending modulus storage increases when fiber length increases from 2 mm to 6 mm.