Study On Dynamic And Static Mechanical Properties And Deformation Mechanism Of Domestic PMI Foams

Polymethacrylimide（PMI）foam,as a typical type of polymer foams,owns the highest specific stiffness and specific strength among all foam materials.Due to its excellent mechanical properties,PMI foam has a wide range of application value and prospects in the aerospace fields.The study of mechanical properties and microscopic deformation mechanism of PMI foam is of great significance to the safety evaluation of equipment in service and the subsequent optimization design of materials or structures.In service,PMI foam is usually subjected to different rates of loading,and its mechanical properties under dynamic loading have been rarely touched.The strain rate effect of the PMI foam is also controversial.In addition,the micro deformation mechanism of the PMI foam under dynamic loading remains to be explored.In this paper,the macroscopic mechanical properties and microscopic deformation mechanisms of the PMI foam with different initial densities are studied under dynamic and quasi-static loading conditions.The cell morphology of the PMI foam is characterized by scanning electron microscopy and computed tomography（CT）.The mechanical responses of the PMI foam under a wide range of strain rates ares tested by a material test system and a split Hopkinson pressure bar specially designed for soft materials,respectively.The dynamic compression process of the PMI foam is captured and analyzed using high-speed photography and digital image correlation techniques.The PMI foam samples frozen at certain strains after high strain rate loading are recovered and characterized by ex-situ CT,and the deformation mechanisms under dynamic loading are then studied.The macroscopic stress-strain curves of the PMI foam with different initial densities at a wide range of strain rates show that PMI foam has considerable strain rate effect,and the strain rate indices of PMI foam with different initial densities are similar.The strain rate effect of the PMI foam with 220 kg m^-3is more prounced.Based on the macroscopic mechanical properties of the PMI foam,a strength-density model is proposed and fitted to the experimental data,which can predict the strength of PMI foam at different initial densities.The results of high-speed photography,digital image correlation and ex-situ CT experiments show that the deformation mode of PMI foam is mainly deformation banding under dynamic loading,which has no essential difference with that under quasi-static loading.Cell walls first show elastic buckling,and then cell walls bend to cause cell collapse to form deformation bands.The deformation bands generally start from the end of the foam in the loading direction,gradually expand and spread to the middle.High-density PMI foam shows brittle fracture under high strain rate loading.