Study On The Mechanical Behavior Of Shear-Stiffening Composites

Shear stiffening gel(STG),which is a kind of low cross-linking polyborosiloxanes(PBs)whose mechanical properties can adapt to the environmental stress field and sensitively response to the external excitation.STG is a kind of shear-thickening intelligent materials that combines sensing,functionality,self-healing capabilities,and adaptive capabilities.In the natural state,STG is in a viscous flow state with a low elastic modulus.When subjected to stress with higher strain rate,the storage modulus and viscosity of STG are sharply increased,and STG is transformed into the high elastic rubbery state and the glassy state.As soon as the stress is unloaded,STG returns to the initial viscous flow state.Due to this unique mechanical property,shear-stiffening composites basing on STG show great application prospects in the fields of human body protection,sensitive sensing,and vibration isolation.However,the performance optimization of shear-stiffening composites,the dynamic mechanical behavior under different strain rate loads,and the analysis of mechanical mechanisms based on microstructure evolution are still lacking,and many problems need to be solved.In this work,the magnetic responsive shear stiffening composites were prepared by vulcanization of the mixture of STG,the methyl vinyl silicone rubber(VMQ),and the carbonyl iron particles(CIPs).The mechanical properties of the magnetic responsive shear stiffening composites could be controlled by changing the volume fractions of VMQ and CIPs.Then,the mechanical behavior of the magnetic responsive shear stiffening composites was studied at different strain rates with and without the effect of the magnetic field.The constitutive model of the composites was established.Based on the fracture and reformation of dynamic B-O bonds,the mechanical mechanism of shear stiffening composites was proposed.Finally,using STG as the core materials,sandwich composites was designed.The mechanical response of the sandwich panels under low-speed impact load was studied.The stress distribution and energy dissipation of the sandwich panel were analyzed by finite element analysis.The specific contents are as follows:1.Control of mechanical properties of shear stiffening composites.Firstly,a kind of magnetic responsive shear stiffening composite was prepared by vulcanizing the mixture of STG,the methyl vinyl silicone rubber(VMQ),and the carbonyl iron particles(CIPs).The storage modulus of the composite increases remarkably with the increase of the oscillation shear frequency,showing excellent shear stiffening performance.By adjusting the volume fraction of CIPs and VMQ,the mechanical properties of shear stiffening composites can be controlled.Besides,the magnetic response shear hardening elastomer(MSTE)overcame the cold flow problem of STG and had excellent self-healing properties.Based on the standard linear solid model,the constitutive model of shear stiffening composites was established,and the static and dynamic mechanical behaviors of the composites were accurately described.Finally,the viscoelastic properties of STE were further studied by steel ball bounce experiments.On the 8mm STE piece,the rebound rate of the free-falling ball is as high as 95.6%.In high-speed photography,the propagation of the elastic wave in the STE was observed.2.Force-magnetic coupling properties of shear stiffening composites.The low strain rate rheological properties and high strain rate dynamic mechanical behaviors of magnetic responsive shear stiffening composites were investigated using a rheometer and a modified Hopkinson pressure bar.The rheological properties of MSTG and MSTE at low strain rates were all synergistically affected by shear rate and magnetic field strength.As the shear rate and the magnetic field increased,the storage modulus increased significantly.The volume fraction of CIPs and the volume ratio of STG to VMQ had a significant effect on the performance of the composite.When the volume fraction of CIPs increased,the initial storage modulus of MSTG and MSTE increased,which resulted in the reduction of the shear stiffening performance and the promotion of the magnetorheological effect.With larger STG volume fraction,the crosslink density of MSTE matrix was smaller,and the shear stiffening performance and the magnetorheological effect was better.At high strain rates,the mechanical behavior of shear stiffening composites exhibits outstanding rate dependence.Taking MSTG with CIPs content of 10.2 vol%as an example,when the strain rate was 7236 s-1,the elastic modulus of MSTG reached 126.6 MPa,while in the natural state,the modulus was only 160 Pa.Finally,based on the breaking and reformation of dynamic B-O bonds,possible mechanical mechanisms of shear stiffening composites were proposed.Under high strain rate stimulation,large anoumt of B-O bonds took action to provide obstruction force to the movement of the molecular chains,and the elastic modulus of the shear stiffening composite increased.Under the effect of the magnetic field,CIPs formed magnetic dipoles.In the magnetic field,the magnetic dipoles attracted each other,moved slowly and orient along the direction of the magnetic field to form chain-like microstructures,which provided support force to the composite.3.Study on mechanical properties of shear stiffening composites with sandwich structures.In this chapter,STG was used as the core material,and aluminum alloy and neoprene rubber were used as the panel materials to prepare two kinds of sandwich structures.The low-speed impact mechanical response of the sandwich panels was studied.Both cylindrical and plate supports were used to provide two different experimental conditions to match different applications.The energy absorption and impact resistance of STG sandwich,CR sandwich and EPDM sandwich panels were studied and compared.In the impact tests,the contact force of the STG sandwich panels was significantly less than the contact force of the sandwich panel with the CR core and the EPDM core,and the STG sandwich panel absorbed more energy.The flexible sandwich panel with CR plates showed better impact resistance than sandwich panels with Al plates and could be used in more complex working environments.Finally,the finite element analysis of the impact process of Al-STG-Al sandwich panels was carried out,and the stress distribution of the panel was studied.Due to the action of the STG core,the concentrated stress on the front plate was distributed to a large area on the back plate,and the peak stress was greatly reduced.Based on the above results,it can be concluded that the STG core sandwich structure shows great application potential in applications of impact resistance and cushioning protection.