Study On The Behavior Of Guided Wave Propagation And Detection Of Defect In Rock Bolt

Rock bolt is widely used in many large construction projects,such as,coal mining industry,highway,railway,subway,tunnels,as well as deep excavation and high slope maintenance.Detection is an indispensable process in anchorage project.The quality and safety of bolt anchoring project can be ensured by improving the detection quality of bolt anchoring and the reliability of detection results assessment.Real-time,fast and non-destructive detection of rock bolt becomes an important issue in underground engineering and tunnel construction.The influence factors of rock bolt detection,such as,wave dispersion,attenuation and defect identification quantification are studied systematically by combining theoretical analysis,experiments and numerical simulation methods.In this article,firstly by solving the dispersion equation of longitudinal guided waves in free bolt,according to guided wave dispersion property and multimodal features obtained from frequency dispersion curve,the guided wave excitation signal and center frequency selection issue were preliminarily analyzed.In order to minimize the dispersion of guided waves and excite or select a single mode guided wave frequency.And then,through indoor tests,the low frequency of 100 kHz and high frequency of 1 MHz incentive guided waves were applied to test the three types of typical defective steel bar,including corrosion defect,grout defect,and debonding defect.Through the analyses of group velocity,frequency,waveform dispersion,and attenuation of echoes,the excitation coefficient of instrument,attenuation coefficient of guided wave,the reflection coefficient and transmission coefficient of each typical defect were introduced to quantitatively analyze the influence mechanism of different typical defects on wave attenuation.The main reason of energy loss of guided wave in anchor bolt was obtained.According to the comparison of reflection coefficient and transmission coefficient between assumed value and theoretical value,the radial three dimensional effect of guided wave along anchor bolt was verified.The research shows that,in the process of anchor bolt detection,the severe attenuation of guided wave directly limits the detection range of guided waves.The main energy loss of anchor bolt detection includes efficiency loss of instrument excitation(Ks,0.42)and spreading attenuation of guided wave(0.63).Coupling effect of steel bar and transducer is the important factor which influences the excitation efficiency of guided wave.Besides,due to the three dimensional effect of guided waves along the radial direction,the group velocity of defect echo in rock bolt with corrosion is 3535m/s,and the group velocity of defect echo in rock bolt with grout defect is 2785 m/s.The difference of velocity can be applied to the identification of two types of defects in actual guided wave detection of rock bolt.The propagation of the ultrasonic longitudinal guide waves in free anchor and anchoring anchor was simulated by finite element model Abaqus.Parametrical analysis of the interaction between geometrical dimensions,main material parameters and the guided wave were carried out.The changing rules of attenuation of the guided wave,the group velocity,the dispersion and the radial three-dimensional effect of the anchoring anchor were studied by changing the geometric dimension and main material parameters of anchors,which makes up the deficiencty of theoretical analysis and experiment conditions.Finally,sensitivity of different incident wave frequencies extented from 100kHz to the range from 30kHz to 200kHz to to the defect in rock bolt was analysed based on the experimental study in chapter 3 and the study on related influence factors in non-destructive tesing of rock bolt.Quantitative relationship between cross sectional area of typical defect and amplitude ratio of echoes were investigated by numerical simulation method.The relationship between the behavior of wave echoes and the geometrical parameters of the defect was established in order to improve the accuracy of guided wave non-destructive testing of rock bolt through accuratly assessing of the size of defect.