1、英文原文Effects of frequency and grouted length on the behavior of guided ultrasonic waves in rock boltsD.H. Zoua, Y. Cui, V. Madengaa, C. ZhangAbstractExperiments were conducted to study the behavior of guided waves in free and grouted rock bolts. Ultrasonic waves with frequencies from 25 to 100 kHz we
2、re used as excitation inputs. Tests were rst conducted on free bolts to help understand the behavior of guided waves in non-grouted bolts. The effects of wave frequency and grouted length on the group velocity and attenuation of the guided ultrasonic waves were then evaluated. The test results indic
3、ated clear but different trends for the group velocity in the free and the grouted bolts. The attenuation in free bolts was not affected by bolt length and frequency. However, in grouted bolts it increased with frequency and grouted length. It was also found that the two main sources of attenuation
4、are the setup energy loss, which has a xed quantity for a specic type of test setup, and the dispersive and spreading energy loss which varies with frequency and bolt length.2007 Elsevier Ltd. All rights reserved.Keywords: Rock bolts; Guided waves; Attenuation; Amplitude; Group velocity1. Introducti
5、onRock bolts are widely used in underground and surface excavations in mining and civil engineering for ground reinforcement and stabilization. In many applications, rock bolts are grouted in the ground with cement or resin. Testing of the grout quality and monitoring of the bolt tension of rock bol
6、ts has long been a challenge in the eld. Conventionally, grout quality is assessed by pull-out test and over-coring. Both methods are destructive and time consuming. The usefulness of pull-out test results as a measure of the grout quality can be limited by the critical length of grout beyond which
7、the steel bolt will fail rst. Therefore, other methods, such as non-destructive testing methods using ultrasonic waves have become attractive. In recent years, research in this area has been very active. It is noticed that properties of guided waves, such as velocity and attenuation, are functions o
8、f the input wave frequency. Although the guided ultrasonic wave seems to be a promising method for monitoring rock bolts, research in this area is still in the early stage and many technical problems remain to be solved. In a grouted bolt, wave behavior is not only related to the grout quality but a
9、lso to the wave frequency. The grouted length and the properties of materials surrounding the bolt may all play an important role.One of the important characteristics of a guided wave is that its velocity not only depends on the material properties but also on the thickness of the material and the w
10、ave frequency. Unlike a bulk wave, the guided wave propagates as a packet, which is made up of a band of superimposed components with different frequencies. It is the group velocity that denes the speed at which the envelope of the packet moves along. It has been shown that in a rock bolt, the rate
11、of energy transfer is identical to the group velocity. Our recent research examined the effects of wave frequency and the curing time of grout on the group velocity of guided ultrasonic waves in rock bolts.We found that the wave group velocity is much lower in grouted bolts than in free bolts. The l
12、ower the frequency, the lower the velocity. Our test results indicated that the input frequency for rock bolt testing below 100 kHz would provide better resolution and clearer signals. This observa-tion is supported by the results discussed further on in this paper.Attenuation is another important c
13、haracteristic of a guided wave. In general, attenuation refers to the total reduction in the signal strength. Attenuation occurs as a natural consequence of signal transmission over a distance due to wave energy loss. There have been extensive research and experiments on attenuation of bulk waves. W
14、ave attenuation is dened by an attenuation coefcient. For example, the p-wave amplitude decay can be expressed as a function of travel distance. (1)where Aa is the amplitude at location a, Ab is the amplitude at location b, is the attenuation coefcient, constant, L is the distance from locations a t
15、o b, R is the amplitude ratio, R=Ab/Aa.However, there has been little research on attenuation of guided waves, especially in grouted rock bolts. Wave attenuation in grouted rock bolts is very complicated and is often affected by many factors including the grouting material and the grout quality. Eac
16、h of these factors may cause some attenuation.In general, the observed wave attenuation may have several components, some of which may be frequency-dependent and some frequency-independent. The total attenuation is the sum of the contributions of all inuencing factors 14, and this relationship applies to both bulk waves and guided waves: (2)where is the attenuation
