SHPB试验中合理加载波形比选分析

针对SHPB试验中如何选择合理加载波形问题,将矩形波、坡形波、三角波和常规梯形波等入射波形统一表达为不同升时的梯形波,并给出其加载应力路径方程。首先,计算分析了加载持续时间和最大应力幅值相同情况下,采用不同升时梯形波和半正弦波加载时,试件应力平衡时间t′_u随波阻抗比β变化情况,以及不同β的试件应力均匀性变化特征。其次,从试件应力平衡时间和应力均匀性角度分析两种加载波形的优劣性。升时t′_r=2、4的梯形波优于半正弦波,半正弦波优于t′_r≥7的梯形波;而t′_r=0、1、3、5、6的梯形波与半正弦波加载时t′_u曲线变化过程中出现了交叉现象,交叉前半正弦波优于此升时的梯形波,交叉后两者优劣性相反。最后,提出试验中合理加载波形的选择与确定。即无论β为何值,t′_r=2或4的梯形波加载时,试件均能获得较短应力平衡时间和较好应力均匀性,试验中应将t′_r=2的梯形波作为最佳加载波形优先选用,其次可考虑选用t′_r=4的梯形波;此外,计算给出不同β值对应的其他合理加载波形以便试验中选用。

Comparison and Analyses on Reasonable Loading Waveform in SHPB Experiment

Aiming at the selection of reasonable loading waveform for incident waves in SHPB experiment,rectangular wave,sloping wave,triangular wave and traditional trapezoidal wave were uniformly expressed as trapezoidal wave with different rising times.Moreover,the equations for loading stress path were also given.Firstly,when the loading duration and maximum stress amplitude of trapezoidal wave and half-sine wave are the same,the changes of time for stress equilibrium tu'over wave impedance ratio β have been analyzed,and the changing characteristics of stress uniformity for different wave impedance ratios β have also been investigated.Then,advantages and disadvantages for trapezoidal wave and half-sine wave are studied according to time for stress equihbrium and stress uniformity.Trapezoidal wave with rising time tr'of 2 or 4 is superior to half-sine wave,while half-sine wave is superior to trapezoidal wave with rising time bigger than or equal to 7.When rising time of trapezoidal wave is 0,1,3,5 or 6,there is a crossover phenomenon in time for stress equilibrium tu'between trapezoidal wave and half-sine wave.Before crossover,half-sine wave is superior to trapezoidal wave.While after crossover,trapezoidal wave is superior to half-sine wave.Finally,reasonable loading waveform for SHPB experiment is put forward.Trapezoidal wave with rising time tr'of 2 or 4 presents short time for stress equilibrium and fine stress uniformity,whatever wave impedance ratios β is.In SHPB experiment,the best and first choice of loading waveform is trapezoidal wave with rising time tr'of 2.Second choice is trapezoidal wave with rising time tr'of 4.Furthermore,other reasonable loading waveforms for different wave impedance ratios β are also given,which canbe easily choose in experiment.