弯曲元剪切波速试验中圆柱状试样的尺寸效应研究

Size effect of column sample in the shear wave velocity test by bender element

在弯曲元剪切波速测试中,试样尺寸是一个重要的影响因素。在以往的研究中,试样长度与激发波波长之比常被认为与消除近场效应有关,然而与试样长度相关的边界反射目前鲜有试验研究。由于近场效应与边界反射都与试样尺寸相关,文中将这种与试样尺寸相关的效应称为尺寸效应,且为在试验中尽量避免近场效应对试验的干扰,根据以往消除近场效应的研究成果,将试样长度与波长之比大多控制在2以上。采用弯曲元法在无侧限条件下测试了6个不同长细比的圆柱状试样,给出了各试样的接收波波形以及不同数据处理方法得到的剪切波波速。结果表明随着试样长细比的增大,测试波形出现复杂的形态,接收波起跳点提前出现导致测试误差,且该现象受测试频率的影响不明显。分析表明这些现象难以通过近场效应来解释,复杂的数据处理方法也无助于提高测试的精度,其中起跳点法造成的误差最大。基于P波的侧向边界反射分析,给出了临界长细比的理论表达式。该理论总体上能够解释试验中表现出的尺寸效应,但由于接收波形受多种因素的影响表现得更为复杂而有待进一步深入研究。

Sample size is an important factor in the shear wave velocity test by bender element.In previous studies,the ratio of sample length to excitation wavelength is often considered to be related to the elimination of near-field effect,while there are few experimental studies on the boundary reflection related to the sample length.Because the near-field effect and boundary reflection are related to the sample size,this effect related to the sample size is called size effect in this paper.In order to avoid the influence of near-field effect as much as possible,according to the previous research on eliminating near-field effect,the ratio of sample length to wavelength is mostly controlled above 2.Six cylindrical samples with different aspect ratios are tested by bender element under unconfined conditions.The received waveform of each sample and the shear wave velocity obtained by different data processing methods are given.The results show that the received waveforms become complex as the increase of the aspect ratio of the sample.In particular,the take-off point of the received wave appears in advance,which causes the test error,and this phenomenon is not obviously affected by the test frequency.The analysis shows that these phenomena cannot explain by near-field effect.And complex data processing methods are helpless to improve the accuracy of the test,among which the take-off point method causes the largest error.Based on the P-wave lateral boundary reflection analysis,the theoretical expressions of critical aspect ratio are given.The theory can basically explain the size effect in the test,but due to the multiple influencing factors,the received waveforms appear more complex and needs to be further studied.