【正文】 但是安裝損耗在每次不同的實(shí)驗(yàn)中都不盡。對(duì)兩個(gè)自由錨桿的能量損失不隨頻率和錨桿長度而變化，而是接近一個(gè)常數(shù)。由于傳播距離很短，所以錨桿中的色散衰減可以忽略。由圖表可知:自由錨桿中的總衰減不隨頻率而變化。由數(shù)據(jù)分析可看出，隨著輸入頻率的增加，首次到達(dá)的時(shí)間和回聲到達(dá)接收端的時(shí)間緩慢增加，而由首次到達(dá)產(chǎn)生的回聲的降幅在任何輸入頻率下都相同。對(duì)部分錨固錨桿，自由部分的群速度跟在自由錨桿中的群速度相同。根據(jù)錨桿長度和用這種方法確定的傳播時(shí)間，可以計(jì)算出超聲導(dǎo)波的群速度。5khz的窄頻帶。在matlab中通過一個(gè)濾波程序就可以實(shí)現(xiàn)。色散隨著頻率增加而增加。 估算群速度從記錄錨桿輸入端的勵(lì)磁信號(hào)到錨桿另一端的首次到達(dá)之間的延時(shí)定義為波在錨桿中的傳播時(shí)間。盡管這個(gè)方法沒有考慮整個(gè)波形的一小部分，但是計(jì)算出的振幅比仍能反映錨桿中的能量損失。下面的實(shí)驗(yàn)都用t1=t2=100us的時(shí)間間隔來計(jì)算。從圖中可清楚的看出：當(dāng)時(shí)間間隔很小時(shí)(通常小于整個(gè)波形的25%)，由方程5得到的振幅比隨間隔時(shí)間的長短而變化。為了說明時(shí)間間隔的長度對(duì)結(jié)果準(zhǔn)確度的影響，用不同時(shí)間間隔計(jì)算自由錨桿中的振幅比(首次到達(dá)和回聲的界限很明顯)，用百分比表示。參數(shù)、和它們的含義如圖3所示。因此，建立一種有效的分析方法來分析超聲波的衰減和獲得有意義的結(jié)果非常關(guān)鍵。收到的波形有時(shí)會(huì)很不清楚，使首次到達(dá)和回聲的界線難以確定。據(jù)了解，好的錨固質(zhì)量由于能量泄漏及色散導(dǎo)致沿錨桿更高的能量損失。衰減越高，能量損失越大，振幅比越低。衰減估計(jì)是比較首次到達(dá)波和回聲的振幅比。在每次試驗(yàn)中，輸入頻率的范圍介于25至100千赫。兩端測(cè)試錨桿均是平滑的，真空潤滑脂被用來提供傳感器間的良好接觸。電腦被用來記錄，顯示和處理信號(hào)。超聲波正弦輸入信號(hào)被用來激發(fā)在錨桿非錨固末端的發(fā)射機(jī)。設(shè)備安裝的說明圖，見圖2。3個(gè)具有不同錨固長度的錨桿（試件3~5）被用來觀察頻率和錨固長度對(duì)導(dǎo)波衰減的影響。樣本的大小及其他說明見表1。錨固錨桿在現(xiàn)場(chǎng)是用圓柱混凝土在鋼筋周圍砌成塊狀來模擬該錨固錨桿的(圖1)。 試驗(yàn)樣本試驗(yàn)樣本包括2個(gè)自由錨桿和3個(gè)長度不同的錨固錨桿。2 超聲導(dǎo)波測(cè)試的實(shí)驗(yàn)了解超聲波在自由錨桿（非錨固）中的特點(diǎn) ，是研究超聲導(dǎo)波在錨固錨桿中行為所必不可少的。對(duì)波頻率和錨固長度對(duì)超聲導(dǎo)波在自由及錨固錨桿中傳播的影響進(jìn)行了研究。結(jié)果表明，記錄的此類振幅衰減和能量損失在錨桿測(cè)試中將大于真正由色散衰減所致。正是在這些接觸面上，有些能源不可避免地折射，造成能量損失。理論上說，當(dāng)一個(gè)波到達(dá)毗鄰另一不傳送機(jī)械波的界面（例如真空或空氣），沒有折射發(fā)生，所有能量都被反射回去。需要指出的是：正如我們?cè)趯?shí)驗(yàn)中觀察到的，在測(cè)試錨桿實(shí)驗(yàn)室中記錄的導(dǎo)波的振幅衰減和能量損失并不僅僅來源于色散衰減。因此，可以合理地假設(shè)在錨固錨桿中衰減由兩部分組成:色散衰減和傳播衰減，這兩者都是跟頻率密切相關(guān)的。作為一個(gè)導(dǎo)波的到達(dá)界面，并不是所有的波能都可以在界面上反射的。這種波變形的現(xiàn)象稱為能量色散。(b) 色散衰減：一種由于傳播過程中波形變形導(dǎo)致的能量損耗。它隨波的傳播距離的增加而增加，并可能根據(jù)物質(zhì)的性質(zhì)在長距離傳播時(shí)成為很顯著的原因?？偹p是所有因素影響的結(jié)果，這也適用體波和導(dǎo)波： (2)其中是受第i個(gè)因素影響的第i個(gè)分量的衰減系數(shù)；是受第i個(gè)因素影響的距離；Ri是第i個(gè)分量衰減后振幅的比；如果對(duì)所有的因素都相同，則有 或 (3)其中是所以衰減系數(shù)的和。這些因素都可能造成一些衰減。然而,很少有研究導(dǎo)波的衰減，特別是在錨桿錨固方面。舉例來說，縱波振幅衰減可以表示為一個(gè)距離函數(shù)。在體波的衰減方面已經(jīng)有廣泛的研究和試驗(yàn)。一般來說，衰減是指信號(hào)強(qiáng)度的減弱。本文將進(jìn)一步討論這個(gè)實(shí)驗(yàn)結(jié)果。頻率越低，速度越低。我們最近研究測(cè)試錨桿中波的頻率和固化時(shí)間對(duì)超聲導(dǎo)波群速度的影響。群速度決定其傳播速度，波整體以該速度傳播。導(dǎo)波的一個(gè)重要的特征是其速度不僅取決于材料性能，而且取決于材料的厚度及波的頻率。雖然超聲導(dǎo)波是一個(gè)很有前景的監(jiān)測(cè)錨桿的方法，但是在這一領(lǐng)域的研究仍尚處于初期階段，許多技術(shù)問題仍待解決。近年來，這一領(lǐng)域的研究已經(jīng)非?；钴S。用拉拔實(shí)驗(yàn)結(jié)果來衡量錨固質(zhì)量，受錨桿初次破壞后關(guān)鍵錨固長度的限制。通常用拉拔實(shí)驗(yàn)和應(yīng)力解除法來測(cè)試錨固質(zhì)量。在許多應(yīng)用中，錨桿用水泥或樹脂錨固。同時(shí)還發(fā)現(xiàn)設(shè)置能量損失引起衰減的兩個(gè)主要來源，一是對(duì)某一特定類型的測(cè)試體系的一個(gè)固定量，二是色散和傳播能量損耗隨波的頻率和錨桿長度的變化而變化。在自由錨桿中，波的衰減不受錨桿長度和波頻的影響。從波的頻率和錨固長度上對(duì)群速度和衰減超聲導(dǎo)波的影響進(jìn)行評(píng)估。 at frequencies higher than 75 kHz the velocity increase in the grouted bolts slowed down, and at the highfrequency end (., 100 kHz), the velocity was approaching that of the free bolts. In fact, at high frequencies, it was more dif?cult to separate the grouted length and the free length from the recorded signals. Therefore, frequencies higher than 75 kHz are not remended for the test.6. Discussions and conclusionsThis research examined the attenuation and group velocity of the guided ultrasonic waves in rock bolts. The test results showed variations with frequency and grouted length. It was determined that due to the short length of rock bolts used in the ?eld, the dissipative attenuation can be ignored.In free bolts, the dispersive and spreading attenuation along the bolt is negligible and the main source of attenuation is from the setup loss of energy, which reduced the amplitude by 20% in one round trip for the equipment setup in this research. The setup loss is considered to be independent from frequency and bolt length, but dependent upon the speci?c equipment setup. The group velocityin the free bolts decreased by about 10% as the frequency increased from 25 to 100 kHz.In grouted bolts, the setup loss is assumed to be the same as that in the free bolts because the test setup was the same. However, the dispersive and spreading (DISP) attenuation increased with frequency and grouted length, and it was moresevere than that from the setup loss. The amplitude ratio due to the DISP attenuation decreased as the frequency and grouted length increased. The group wave velocity in the grouted length of the test bolts increased steadily as the frequency increased to 75 kHz while the increase slowed down at a higher frequency. However, at 25 kHz, the group velocity wasnearly 50% lower in the grouted length than that in the free bolts. As the frequency approached 100 kHz, the velocity difference between the free bolts and the grouted length was reduced to less than 10%.As indicated earlier, the experiments in this study were conducted using a transmissionthrough setup (., with sensors on both ends of the tested bolts). This type of setup is not applicable to the ?eld where only one end of a rock bolt is accessible. The next step of this research will be to conduct similar tests using a transmissionecho setup (., with a sensor at one end only). This will require a different testing device, which is being custombuilt for the speci?c testing requirements. During the next stage of research, tension will also be applied to the bolt samples to study the tension effects. The ultimate goal of this research will be to develop a nondestructive testing device using guided ultrasonic waves for ?eld monitoring of grouted rock bolts, particularly the grout quality, grouted length, bolt failure, and bolt tension.AcknowledgmentsThis research was supported by a research grant from the Natural Sciences and Engineering Research Council of Canada.中文譯文頻率和錨固長度對(duì)超聲波在錨桿中傳播行為的影響. Zoua, Y. Cui, V. Madengaa, C. Zhang摘要以頻率從25至100千赫的超聲波作為勵(lì)磁輸入，研究超聲波在自由和錨固錨桿中傳播的特性。 the group velocity dropped by about 10%. The group velocity was apparently not affected by the bolt length.5. Effects of frequency and grouted length on behavior of guided wavesExperiments were also conducted on the grouted rock bolts using frequencies from 25 to 100 kHz. The typical raw waveform for sample 4 at an input frequency of 35 kHz is displayed in Fig. 3. It was observed from the recorded data that the waveforms in grouted bolts showed dispersion, apparently more serious at higher frequency ranges. At the same time, as the input frequency increased the lengths of time for the ?rst arrival and the echo to reach the receiving end decreased signi?cantly, following an opposite trend from that observed in the free bolts. The wave amplitude reduction of the echo from the ?rst arrival also becamemore severe.. Attenuation in grouted rock boltsThe results o