Research On Interlaminar Toughening And Low Velocity Impact Of Carbon Fiber Reinforced Polymer

In recent years, composite materials are widely used in a variety of fields, especially in aviation and aerospace fields. Among them carbon fiber reinforced polymer (CFRP) composites have attained a certain position as structural materials because of their high specific strength and stiffness and other special advantages, which are superior to those of conventional engineering materials and improve the fuel economic of aerospace vehicle. Meanwhile CFRPs have been used in civilian areas including transportation, construction, sports equipment, etc. However the plate-shell structure of fiber reinforced composites is easy to cause interlaminar delamination due to the weak mechanical performance of interlayer and further reduces the mechanical properties of whole structure. On the other hand, when in service the probability of suffering from low velocity impact is very high, it easily results in matrix cracking and delamination but can hardly be detected by the naked eye, and then causes a severe drop in residual strength of laminates, especially the transverse in-plane compressive strength. The above mentioned damages will bring great threat to the safety, thus it has important theoretical significance and application value to research the typical damage and interlaminar toughening technology of laminated composites.In this paper a method which is referred to as "Ex-situ" toughening technology is adopted by using the highly porous polyamide non-woven fabric as toughening layer. By this way, it realizes the increasing of mode I and mode II fracture toughness and prevents the evolution of delamination. The bilinear cohesive zone model (CZM) is used for the analysis of interlaminar mechanical behavior and a user-written constitutive model based on continuum damage mechanics (CDM) is proposed to characterize the lamina. Then the finite element models about double cantilever beam (DCB) and end notched flexure (ENF) tests are established by ABAQUS software to carry out the numerical analysis and research the effect of key parameters on mechanical properties of the whole structure. The results reveal that the peak load and corresponding displacement all increase with the increasing interlayer strength. Meanwhile with the increasing laminate thickness, the initial slope, peak load and delamination propagation distance all increase except for the reduced displacement corresponding to peak load. Moreover both the initial slope and peak load of mode â…  and â…¡ fracture decrease with the increasing initial crack length. The difference is that, with the increasing initial crack length the displacement corresponding to peak load of mode â…  increases while for mode â…¡ it decreases firstly, and then increases.On the basis of above mentioned quasi-static mechanics study, the dynamic mechanics research on low velocity impact is conducted. Firstly, the low velocity impact test and damage detection for example C-scan and CAI are carried out to analyse the C1500 material, especially the effects of thickness and stacking sequence of specimen on impact response and damage. The conclusions are that when the thickness of specimens are the same, the impact loads of [0/90]4S and [45/-45]4S are lower than [45/0/-45/90]2S, but the former two have longer time history. Under the impact energy of 6.67 J/mm, the damage area of [45/-45]4S is more close to [45/0/-45/90]2S compared with [0/90]4S which is the smallest. And the damage area of specimen decreases with the decreasing quantity of lamina. Meanwhile the CAI of C1500 is about 160-180 MPa.Finally by using the experimental and numerical analysis methods the research on low velocity impact damage of laminated composites toughened by polyamide non-woven fabric is conducted to illustrate the mechanical behavior and damage mechanism of CFRP. Then the effects of the thickness and strength of PNF/3266 interlayer on the damage of composites are numerically analyzed which is helpful for the optimization of interlayer toughened composites. The results reveal that the dominant damage modes in the laminate under impact are matrix cracks and delamination. With the increasing interlayer thickness, the initial stiffness and peak load are both reduced. And the effect of interlayer shear strength on the load bearing capacity of the specimen is more obvious than the normal strength. With the decreasing interlayer thickness or the increasing interlayer strength, the damage area is reduced effectively.