Study On Mechanical Properties And Mechanism Of Shear Thickening Material

Shear thickening fluid (STF) is a heterogeneous suspension of nanometer or micrometer particles in a non-Newtonian fluid. Its effective viscosity increases with shear rate. It changes from liquid-like to solid-like while bearing a sudden force. When the load is removed, it returns back to liquid-like state. Due to the nonlinear mechanics properties, the STF has been used in many practical applications such as vibration control, shock resistance, body armor, etc. Over the past decades, many efforts have been done on investigating the characterization of mechanical properties and microscopic mechanism of the STF. However, many problems are still presented. To this end, this dissertation is focused on studying the characterization of mechanical properties and their shear thickening mechanism. Firstly, the rheological properties of the STF under low strain rate and small loads were systematically investigated. Then, its dynamic properties under the high strain rate and large load were studied. After that, a new model named particles-relaxation was put forward by studying the role of shear in the transition of the flow state. Finally, a novel shear thickening gel was prepared and its mechanical properties were studied. The details are as follows.1. The rheological properties under the low strain rate and small loads were studied by using Anton Paar MCR301rheometer. The effect of the shear acceleration on the rheological properties of the STF was discovered. The mechanical properties of squeezing flow were characterized. The complete rheological properties of the discontinuous shear thickening were characterized. These results provided experimental basis for designing the STF-based device. Firstly, the effect of the shear acceleration on the rheological properties of the STF was studied. The critical shear rate increased with the increasing of the shear acceleration and the slope of the relationship between viscosity-shear rate curves demonstrated a decreased nature, all of which were due to the significant momentum effect of the particles under the high shear acceleration load. Then, the mechanical properties of STF under squeezing flow were analyzed. It was found that the critical squeezing velocity increased with increasing of the gap, and decreased with the increasing of the volume fraction. The occurrence of the jamming depended on the squeezing rate. The larger squeezing rate often led to the more stable force chain, thus, the normal force increased with the increasing the squeezing rate. Finally, the intrinsic rheological characteristics of the discontinuous shear thickening were analyzed by using the date acquisition card. It was found that the viscosity did not increase monotonically, but could be governed by an S-shaped flow curve. The start points and the maximum points of the torque and normal force were different.2. The dynamic properties of the STFs under the high strain rate and large loads were studied by using the modified split Hopkinson pressure bar (SHPB) technique. The liquid to solid transiting phenomenon, the strain rate effect and energy dissipation were systematically studied, which provided much valuable data for their practical applications. In the squeezing flow, the STF transformed from liquid-like to solid-like, and began to yield when the stress reached a critical value. The above phenomenon could be explained by the formation of the permanent stable cluster. In the case of lateral confinement, the bulk modulus increased with the increase of the strain rate, which was due to the formation of the force chain structure. At last, by comparing the reference wave, the energy dissipation in the STF was proved by using the modified SHPB technology.3. By introducing the shear condition, a novel particle-relaxation model was proposed to analyze the ST effects. This model explained the solid-liquid-solid transition phenomena of the dense particle suspension very well. By increasing the strain rate, the stress-induced transition from hydrodynamic state to boundary lubrication state. In boundary lubrication, the mechanical behavior of the suspensions was similar to that of the dry granular flow. Take the cornstarch-water suspension as an example, it was found that the DST mechanism was due to external shear field activated the time characteristics of particle rearrangement. In other words, the adjustment process of the interior of the particles could not match the outside load in time thus led to the DST. This model explained the solid-liquid-solid transition of the DST suspension. Based on the test data, the phase diagram in the t-φ plane was plotted, and it was useful to understand the basic mechanism of the particle suspension.4. A shear thickening gel with excellent protective performance was prepared. The impact resistance, the mechanical properties and the mechanism of stiffening behavior were evaluated, which provided much valuable information for it practical application. Here, the protection performance was characterized by the falling weight tests through investigation the absorption energy and the deformation resistance. The STG transformed from viscous fluid state to rubbery state, when the strain rate was at the low, medium, high strain rate, respectively. The rubbery state transition strain rate was about2.5s-1, the phase transitions led to the deformation resistance and the energy absorption. Finally, a jamming model for the phase transition was proposed and this method would be useful for studying other kind of polymer materials.