Mechanical And Acoustic Properties Of Planar Auxetic Phononic Crystal

The progress of human civilization is associated with the development of materials.Advanced materials and structures with high performance have always been the unremitting pursuit of human beings.Due to the special configurations of the microstructures,the auxetic metamaterials have negative Poisson’s ratio and can produce acoustic band gap effect.Therefore,auxetic metamaterials have attracted much attention of many researchers,and are widely used in the fields of aerospace,transportation and medicine.However,there are still some challenges in the existing studies of auxetic metamaterials,such as the structures with high stress level easily caused by conventional re-entrant design,the lack of comprehensive discussion of mechanical and acoustic properties,monotonous research methods,which to a certain extent limit the development of the auxetic metamaterials.In this thesis,a novel low stress level auxetic metamaterial with oval-shaped holes is designed based on the Cassini oval.Through tensile test,numerical simulation,machine learning model and computational homogenized method,this thesis systematically investigates the mechanical performance of the auxetic metamaterial with oval-shaped holes,and provides guidance for the structural design of auxetic metamaterial.What’s more,this thesis also investigates the acoustic band gap effect of the auxetic metamaterial,which can expand its applications in engineering fields such as filtering,damping and noise reduction.The main contents of this thesis are as follows:(1)The perforated auxetic metamaterial with oval-shaped holes is designed based on the Cassini oval to reduce the stress level and the number of geometric design parameters.The specimens are fabricated by using 3D printing technology and tensile tests are carried out to investigate its mechanical properties.Meanwhile,the numerical model,machine learning model and computational homogenized model are established and also used to characterize the mechanical properties of the auxetic metamaterial with oval-shaped holes.All results derived from these models are in good agreement with the experimental results.Furthermore,parametric analyses are conducted and the zero Poisson’s ration design is discussed.(2)The band gap effect and the propagation characteristics of elastic waves of the auxetic metamaterial with oval-shaped holes are studied numerically.It can be found that the auxetic metamaterial can form band gaps,which effectively prohibit the propagation of elastic waves within specific range of frequency in it.Through the parametric analysis,we also found that the width and frequency range of the band gap can be flexibly modulated by adjusting the porosity and the base material.(3)A novel hierarchical auxetic metamaterial is designed based on the idea of geometric fractal.The mechanical properties of the hierarchical structure are studied by tensile test,numerical simulation and computational homogenized method.The results show that fractal design can effectively improve the auxetic behavior of the structure.However,such enhancement is directional,and significantly influenced by the geometric parameter and fractal order.Additionally,the band gap effect of the hierarchical structure is also studied.The results show that with the increase of porosity or fractal order,the band gaps gradually shift from high frequency region to low frequency region,and the width and number of band gaps also increase.(4)Considering the application in practical engineering,the effect of random geometric perturbations on the mechanical performance of the auxetic metamaterial is studied.Three types of disordered perforated auxetic systems including orientated disordered system,dimensional disordered system and complete-disordered system are designed.The mechanical properties of these disordered systems are tested and simulated.The results show that all disordered systems are auxetic,which means the auxetic behavior of auxetic metamaterials is robust to random geometric perturbations.In summary,this thesis designs a novel auxetic metamaterial with oval-shaped holes,then systematically studies its mechanical and acoustic band gap properties,which provide a basis for the application of the auxetic metamaterial in practical engineering.Additionally,the research methods and ideas applied in this thesis are also valuable for the design and development of other metamaterials.