Effect Of Strain Rate On The Mechanical Behavior Of PMMA Under Different Loading Conditions

PMMA is a typical amorphous polymer,which is widely used in various environments due to its good light transmission,easy processing,low cost,stable chemical properties and high strength.As a typical polymer material,PMMA has remarkable viscoelastic characteristics.In this paper,the effect of strain rate on mechanical behavior of PMMA was studied under different loading modes.The main research contents are as follows:Using PMMA cylinder as the test object,uniaxial compression experiment and compression-shear composite loading experiment were conducted at three different strain rates.The results show that the elastic modulus and yield strength of PMMA increase with the increase of strain rate.It is found that the second invariant of stress bias tensor decreases linearly with the increase of the first invariant of stress tensor,the results show that increasing hydrostatic pressure can improve the yield strength of PMMA under press-shear stress;The material parameters of the rate-dependent yield criterion were determined according to the data of the composite loading experiment,and the rate-dependent yield surface of the PMMA was obtained.The results show that the yield surface expands in every direction with the increase of strain rate,which verifies that PMMA is a typical strain rate sensitive material.Based on the cyclic loading and unloading compression experiments with three strain rates,the effects of strain rate and prestrain on the peak stress,unloading modulus and viscosity of PMMA were studied.Meanwhile,by analyzing the change trend of dissipated energy density with the increase of cycle number,a power exponential model is proposed.The variation trend and influence factors of the model parameters are discussed in detail.The sensitivity of the evolution law of dissipated energy to strain rate is verified.The monotonic compression and cyclic loading and unloading experiments were carried out on the PMMA cylinder with three groups of different strain rates under quasi-static conditions.The cumulative dissipated energy and elastic energy storage limit were used to measure the failure of the material.It is found that the energy of the failure of PMMA is not only affected by monotonic loading and cyclic loading and unloading,but also has obvious strain rate correlation.The specific conclusions are as follows: Before the PMMA specimen is monotonically compressed to compaction,the damage degree is positively correlated with the strain rate,while under cyclic load,the damage degree is negatively correlated with the strain rate;for single pressure loading,the elastic energy storage limit of the material under high strain rate is smaller,and before reaching the elastic energy storage limit,the dissipatedenergy is larger than that of the other two low strain rate;for the cyclic loading and unloading at three groups of strain rates,the cumulative dissipation energy corresponding to the low strain rate increases the fastest;cyclic load at low strain rate reduces elastic energy storage limit,it is easier for materials to be damaged and fail under cyclic loading than under single pressure;Cyclic loading at high strain rate increases elastic energy storage limit,and materials are more likely to be damaged and fail under single pressure than under cyclic loading.