Hybrid nano/microcomposites for enhanced fracture toughness and fatigue life

The objective of this investigation was to develop, process, and test hybrid nano/microcomposites with nano-reinforced matrix and demonstrate an enhancement in mechanical properties, with emphasis on damage tolerance measured in terms of fracture toughness and fatigue life. The material investigated was carbon fabric/epoxy with the matrix reinforced with multi-walled carbon nanotubes (CNTs). A solventbased method with a dispersion enhancing block copolymer was used. The mixture was infused into the carbon fiber preform using a wet layup process. The neat and CNT-modified matrices were characterized by measuring their fracture toughness in both Modes I and II. The measured strain energy release rate for plane strain conditions, taking into account the increase in Young's modulus, showed a 33 % increase for the nanocomposite over the neat resin. The most critical property of the composite with a great impact on fatigue life is the interlaminar fracture toughness. The Mode I strain energy release rate of the reference and hybrid composites was measured by means of the double cantilever beam (DCB) test. It was found that that the interlaminar strain energy release rate for the hybrid composite was approximately 300% higher than that of the reference composite. Similar tests were conducted with end-notched flexure (ENF) beam specimens under three-point bending to determine the Mode II delamination fracture toughness. The strain energy release rates measured by these tests showed over 35% increase in Mode II strain energy release rate for the hybrid composite. Two types of fatigue tests were conducted for the reference and hybrid composites. In the first type, short beam specimens were tested under cyclic threepoint bending aimed at producing a cyclic Mode II interlaminar shear stress. Stresslife curves obtained from the interlaminar shear tests showed that, at a given cyclic load amplitude, there is a significant difference of more than an order of magnitude in lifetimes between the reference and nano-reinforced composite materials. In the second group, fatigue tests were conducted by cyclic loading of double cantilever beams producing cyclic interlaminar Mode I stress. In these tests, the crack growth rate was monitored as a function of fatigue cycles for various cyclic load or stress intensity amplitudes. The results showed a much lower crack growth rate for the hybrid composite than for the reference one. A Paris law fit showed that the exponent parameter for the hybrid composite was noticeably lower than that for the reference composite.