Experimental Study On Negative Thermal Expansion Properties Of Hoberman Metamaterials

In engineering construction,use at most is the most common form of concrete,but the concrete expansion deformation occurring in the process of temperature change is internal and external constraints,will produce a lot of stress,once more than self-bearing limit will be cracks,concrete threat to engineering,and maintenance repair cost is extremely high and process trival,so finding is not sensitive to the temperature change of the structure of the material or be able to realize the negative thermal expansion is particularly important.At present the research of new type of negative thermal expansion materials mainly by improving their predecessors' structure or design of the structure of the new unit,by periodically to negative thermal expansion,this article was inspired from Hoberman toy ball designed Hoberman metamaterial unit cell structure,through periodic arrangement to realize the negative thermal expansion properties of negative thermal expansion materials database added new research direction,combined with the increase of material manufacturing this structure can be applied to the 3 d printing in concrete construction,not only can reduce the thermal expansion of concrete,Moreover,it is of great significance to realize the goal of rapid,economical,safe and environmental protection in the construction of the project by saving the formwork in the production of the overflow weir surface,the inlet and outlet section of diversion tunnel,and the tailpipe section of hydropower station.It is of great significance in the fields of aerospace,civil use,biomedicine,optics and semiconductor applications.Moreover,the research group has verified that Hoberman structure can also generate low-frequency phonon band gap,which has a great application prospect in the aspects of vibration reduction,noise reduction and sound control.Hoberman metamaterials basic structure is made of double materials,triangle bottom made of high thermal expansion coefficient materials,two waist is made of low thermal expansion coefficient materials,two triangle share the same bottom to form a diamond,when the structure is heated,triangle bottom edge of the high,lead to the diamond unit come to a point so that the entire ring unit cell overall contracting inward,the smaller the ring,and to achieve the effect of negative thermal expansion.In this paper,the negativethermal expansion properties of Hoberman metamaterial cell were studied by experiment,theoretical analysis and numerical verification.(1)in the experiment,the linear expansion coefficient of zirconium ulurate/photosensitive resin composites with different volume fractions was first measured.The results showed that the linear expansion coefficient gradually decreased with the increase of the volume fraction of zirconium ulurate until the volume fraction was 20%.Then pure photosensitive resin and the volume fraction of 20%zirconium ursolic acid/photosensitive resin composites as raw material to make Hoberman unit cell structure,based on the measurement of thermal expansion coefficient we found Hoberman structure greatly reduces the thermal expansion coefficient,but do not show the effect of negative thermal expansion,which may be because of handmade specimen inevitable uneven brush,specimen in some places in curing between layer and layer has a small hole,when the specimen when manual parting,how much the damage caused by the minor.(2)in theoretical analysis,we simplified the structure into beam elements and triangular rigid frame.Based on the knowledge of structural mechanics,the element stiffness matrix was calculated by displacement method,which was then converted into global coordinates,and the constitutive model was calculated by applying the boundary conditions of thermal load.Finally,the coefficient of thermal expansion was calculated to be-3.2 ×10-5 ?-1.(3)in the numerical verification,the finite element simulation was carried out by ANSYS,and the negative thermal expansion phenomenon was obvious in the numerical simulation results,which was consistent with the theoretical analysis results.Through the mutual verification of theoretical model,experiment and numerical analysis,the experimental results show that the thermal expansion coefficient of ring cell structure is much lower than that of the two materials themselves.Both theoretical calculation and numerical analysis prove that Hoberman structure has negative thermal expansion performance.