礫石受剪之音波與振波特性

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Title:	礫石受剪之音波與振波特性
Authors:	陳柏翰;Chen,Po-Han
Contributors:	土木工程學系
Keywords:	剪力試驗;加速度;音波;顯著頻率;acceleration;apparent frequency;shear test;sound wave
Date:	2013-03-07
Issue Date:	2013-06-19 15:19:08 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究以直接剪力試驗儀進行礫石材料之試驗剪動，將實驗分為兩種，一為在氣乾狀態下施加不同正向應力之剪動，另一則為施加相同的正向應力對部分飽和礫石進行之剪動，同時量測試驗時產生之音波與振波訊號。本實驗主要係量測礫石受剪時所產生之音波與振波，以探討礫石受剪時之力學特性。為找出礫石音波之主要頻率分佈，本研究確立了頻譜比對法，可藉以判斷音波與振波之變化並予以濾波。經比對後，發現大約為15 Hz以下範圍內受剪音波紅線之數值明顯增高，可視為主要頻率。利用此頻譜比對法可方便、迅速地找出主要頻率。另外亦發現此主要頻率不因含水量多寡而有所變動。至於振波加速度頻譜圖，大約在15 Hz以下有一顯著頻率，而音波頻譜圖中之相同位置亦有一次要頻率，說明振波加速度與音波間有密切之相關性。將試驗後之音波數據，濾除15 Hz以外之數值，以為後續分析之用。作用於氣乾礫石之正向應力愈大，受剪破壞時所產生之音壓逐漸增大。至於部分飽和礫石破壞之音壓則均小於氣乾礫石之音壓。根據降伏剪應力及破壞剪應力與正向應力關係，得知氣乾礫石之降伏值大約為破壞剪應力之76%，部分飽和礫石之降伏值則約為破壞剪應力之74%，因此可利用加速度訊號所對應之剪應力來瞭解降伏之徵兆。另外，礫石於降伏後加速度訊號開始有略為增加且有連續之數值，而音波訊號則在破壞時才有明顯之數值，其原因應為音波之較弱訊號容易消散，最後只剩能量較強之音波訊號。而加速度訊號不易衰減，可與音波訊號結合，做輔助判斷。This research performed a series of direct shear tests of gravel material under various conditions of normal stress and degree of saturation to study the properties of shear strength, sound waves and vibration waves. A spectrum comparison method of waves is proposed in this paper to distinguish and to filter the sound waves and vibration waves generated in the process of shearing. According to the results of experiments, it is found that within the area smaller than 15Hz, the magnitude of sound waves increased apparently compared to that of background noise and this can be seen as the apparent frequency or main frequency of this gravel material. The magnitude within this frequency area varied slightly in spite of the variation of water content. As to the spectrum of acceleration of vibration waves, similar apparent frequency was found before 15Hz by comparing the experimental data at the earth stage of shearing with that right before the period of failure. Furthermore, some other secondary frequencies were found at the same position of sound spectrum. This showed that there exists a close relationship between sound waves and vibration waves. To examine the shear strength properties of gravel in this study, the sound waves and the acceleration of vibration waves larger than 15Hz were filtered.From the experimental results, it is found that larger sound pressure of air dried gravel will be caused at failure for larger normal stress. But the sound pressure of partially saturated gravel is smaller than that of air dried gravel. According to the relationships between shear stresses at yielding or failure with normal stress, it is recognized that the yielding stress of air dried gravel is about 76% of failure shear stress, and this value decreased to 74% for partially saturated gravel. Therefore, the shear stresses related to the acceleration signals may be used to realize the yielding premonition. Furthermore, after yield, the acceleration signals increased slightly and last continuously. But the sound signals only show some apparent values at failure. Because the sound signal is weak and its energy dissipates easily, so only few strong signals at failure remained. Conversely, the acceleration signal does not decrease apparently, so it can be combined with the sound signal to distinguish the yielding and failure of gravel.
Appears in Collections:	[Graduate Institute of Civil Engineering] Electronic Thesis & Dissertation

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