The Finite Element Analysis Of Tensile Fracture Process Of NF Modified C/C Composites

With the expanding applications of carbon/carbon composites (C/C composites), more requirements for the properties of C/C composites are brought forward in harsh application environment. It’s necessary and feasible to modify carbon fiber for improving the comprehensive performances or other special properties of C/C composites. Earlier research showed that after modified by Nano-fiber, the mechanical properties of c/c composites is significantly improved. But the strengthening and toughening mechanism are not clear. By means of fracture mechanics and interface mechanical this article study crack-tip stress intensity factor respectively before and after the modification, and discuss the strengthening and toughening mechanism of modified C/C composites. The main conclusions can be drawn as follows:Before modification, the analysis models of C/C composites which carbon substrates and carbon fiber with a transverse crack were built. The stress intensity factor for crack when it changes was studied. The influential factors of composites crack-tip stress intensity factor were studied by changing a series of parameters,which included the content of fiber, the length of fiber, the elastic modulus of matrix. Study results showed that carbon matrix crack stress intensity factor increases as the crack grows, but stress intensity factor is smaller when the crack tip extends to near the interface. Carbon fiber crack stress intensity factor also increases as the crack grows, but stress intensity factor will dramatically increase when the crack tip extends to near the interface. C/C composites has the best properties when the content and length of carbon fiber, elastic modulus of carbon substrates within a certain range. Then the analysis model of C/C composites contained multiple cracks was estabalished. Study results showed that there is exponential singularity at intersection between C/C composite through the interface crack and interface, and increases with the increase of elastic modulus of carbon fiber and carbon substrate ratio. Last, MATLAB software was used to calculate the index of crack tip Singularity under different conditions. The results corresponds to the finite element simulation.After modification, the parallel fiber axial tensile analysis model of NF-C/C composites which carbon substrates and carbon fiber with a transverse crack were built. The influential factors of composites crack-tip stress intensity factor were studied by changing a series of parameters, which included the content of Nano-fiber,the length of Nano-fiber, the growth angle of Nano-fiber, crack growth whether to touch Nano-fiber. Study results showed that when the crack is not within the range of Nano-fiber, the more content and the longer length on fracture toughness of C/C composites are beneficial. When crack in the context of Nano-fiber, the less content, the shorter length, and the growth angle of Nano-fiber around60°are good for fracture toughness of C/C composite materials; Similarly, under the above conditions analysis of variation of stress intensity factor of crack propagation in fiber. Study finds that more content, longer length are good for fracture toughness of C/C composites. Fiber stress intensity factors is not sensitive to the growth of Nano-fiber direction when crack touch interface.These also can be used for analysis of mechanism of tensile fracture of CNF-C/C composites.After modification, the vertical fiber axial tensile analysis model of NF-C/C composites which carbon substrates contained crack was built. The influential factors of composites crack-tip stress intensity factor were studied by changing a series of parameters, which included the content of Nano-fiber, the length of Nano-fiber, the growth angle of Nano-fiber. Study results showed that more content, longer length, the vertical growth is good for C/C fracture toughness of composite materials.