Study On Mechanical Behavior And Reinforcement Mechanisms Of Epoxy-matrix Nanocomposites

These years have wittnessed the extensive applications of nanocomposites in astronautic and aeronautic, national defense, transport and physics fields due to their excellent properties in mechanical aspect et al.. The particles own outstanding multi-characters such as large specific area, high area energy, namely nano effect, small scale effect and tunnel effect, when they was in nanoscale. Those advantages make nanocomposites draw more and more popularization for deeper and more extensive application in mechanics, thermotics, photonics and electronics engineering. However the dynamic mechanical behaviors of nanocomposites were investigated less enough. The reinforcement effects of different partciles, like rigid or soft particle, were necessary to well know before their further applications. Thus, it is meaningful for the design and application of composites in scientific and industrial committee, to investigate into the reinforcement behavior and mechanism of nanoparticle under dynamic loading.In this present disertation, the split Hopkinson pressure bar technique was utilized to study the mechanical behaviors of the matrix epoxy and related nanocomposites with nanosilica and nanorubber reinforced. Firstly the related factors were systmatically studied which may influence the measurement accurancy of SHPB experiments, especially in the small strain region. And the strain rate sensitivity of materials mechanical properties and the particles’ fraction effect were investigated. Then the reinforcement effects of nanoparticles were analyzed, and the unit cell model was established to probe into the mechanism of reinforced characteristics. Finally a constitutive model with eight parameters was also established to describe the three major characters of epoxy-like materials for potential engineering applications. The main conclusions were drawn, and listed as follows:(1) Theoretical formulations are obtained by re-conducting the actual process of the stress wave loading specimen. The analysis on the re-conducted stress-strain curves indicates that the stress inequilibrium has little negative influence on the accuracy level of the obtained curves. Even though the stress in specimen has not achieved a full stress equilibrium state, the error is lower than 5% in term of elastic modulus of specimen after 2 characteristic times, by using three wave method formula. For the specimen with linear elastic deformation under small strain deformation, an optimal formula of the incident wave is deduced which could achieve both stress equilibrium and constant strain rate loading on the specimen after 2 characteristic times. The related calculation case and the numerical simulation based on Abaqus demonstrate the validity of the proposed formula, and the experiment on thr brittle sandstone specimen is also used to confirm its feasibility.(2) A series of experiments and numerical simulations are conducted to investigate into the factors which influence the measurement accuracy of the dynamical elastic modulus using split Hopkinson pressure bar technique, and it is found that the indentation effect due to the specimen to bars end and the nonperfect condition between the sandwiched section are the main factors. For metal materials with higher elastic modulus, the error is highly beyond 5% due to the severe indentation effect for the specimen with common diameter used. This can be remedied by using adapters and longer specimen. Related simulations confirm the feasibility of these metholodogies. But for the epoxy specimen with rather lower elastic modulus than that of metal bar, the indentation effect can be reduced to be negligible once the specimen diameter was optimized. The nonperfect contacting condition is the major factor for polymer materials, fortunately the developed vertical SHPB can improve the contacting condition by using placing the vertical incident bar on the specimen face with the aid of the bar gravity. Experimental results of the epoxy specimen demonstrate the improvement in contacting. Thus for epoxy materials, their elastic modulus under high strain rates could be measured using vertical SHPB with enough accuracy.(3) A series experiments are conducted to investigate the potential mechanisms of epoxy materials while deforming at peak stress/strain around. The results indicate that the critical loading of totally recoverable deformation for epoxy material is approaching closely to its peak stress at least, but once the load is beyond the peak stress, some resident deformation exists after recovering partly. The mechanism is proposed that in 3-D molecular chain structures of epoxy materials, the crosslinking points start to rupture at the peak stress loading, since then the motion of the chain segments initiate, and strain softening starts. This mechanism can reasonably further explain the related deformation behaviors of epoxy materials involved.(4) Study on the peak stress and strain rate sensitivity of nanocomposites reinforced by nanosilica indicates that the reinforcement effect is little. The simulation based on Abaqus 6.11 is adopted to establish a unit cell model, with aim to probe into the intrinsic mechanisms. The results demonstrate the dependence of the reinforcement on the mechanical behavior of matrix epoxy. The strain softening and plateau stress deformation of matrix along the central axis of silica inclusion in unit cell model contributes a negative role in composites properties. Even the rigid particle can weaken the peak stress of composites if the strain softening ratio is enough low, which is confirmed by related unit cell simulations.Though the presence of soft rubber particle adding in epoxy weakens the peak stress of nanocomposites, it can reinforce the strain hardening behavior at large deformation. The intrinsic mechanism is confirmed by the simulation based on unit cell model that the rubber particle behaves like a rigid inclusion due to its incompression when it is loaded to a large deformation.(5) A constitutive model is established, which consists of three parallel parts: Maxwell model, Weibull distribution model and exponent function. This model can describe well the three main characteristics of epoxy-like materials: peak stress and strain softening, plateau stress and strain hardening deformation using only eight parameters. Additionally, this model can be extended easily to discrible the mechanic behavior of the epoxy-matrix composites reinforced with nanosilica particle.