应力平衡和接触状态在Hopkinson杆测试聚合物动态弹性模量中的影响研究

针对用Hopkinson杆试验能否准确测量聚合物动态弹性模量以及其中主要影响因素的问题,本文基于重构试样初始加载阶段的应力波反射透射过程,分别计算了6个特征时间内的前三次反射波和透射波,得到试样的应力平衡度和试样的应力应变曲线。对于所研究的聚合物材料,通过比较重构的应力应变曲线的弹性模量与输入的材料弹性模量,发现在4个特征时间后,误差仅在3%左右。因此试样变形过程中的应力平衡与否不是材料在Hopkinson杆试验中弹性模量测不准的原因。通过环氧树脂试样试验发现,根据Hopkinson杆理论计算的应变结果要大于试样上应变片实测的结果,误差在11%左右。相应的数值模拟研究发现:试样和杆子端面接触状态直接决定着试样弹性模量测量的精度。关于惯性效应和压痕效应的研究也证实它们的影响是可以忽略的。

Investigation into the Influences of Stress Equilibrium and Contact State on Determining Polymer Dynamic Elastic Modulus by Hopkinson Bar

Aiming at the availability of accuratley determining polymer dynamic elastic modulus through Hopkinson bar test and at its main influencing factors, a method based on reconstruction of sample's stress wave reflection and transmission process on the initial loading stage is proposed in this paper. The first three reflected and transmitted waves within 6 characteristic times were calculated to obatin specimen's stress balance and stress-strain curve. By comparing the elestic modulus from reconstructed stress-strain curve with the input elestic modulus of studied polymer material, it is found that the error is about 3% after 4 characteristic times. So, during deformation process of specimen, stress balance is not the reason for elestic modulus undeterminancy in Hopkinson bar test. However, experimental results of epoxy specimens show that the strain error from Hopkinson bar formula is larger than the results measured from strain gage on specimen up to about 11%. Corresponding numerical simulation reveals that the contact state between bar and specimen end face directly determines the measurement precision of sample elastic modulus. Study also shows that the inertial effect and indentation effect can be neglected in Hopkinson bar test.