Detection Mechanism And Method Of GFRP Anchor Damage By Piezoelectric Wave Method

The GFRP（Glass Fiber Reinforced Polymer）bolt anchor is one of the important means of reinforcement in the geotechnical engineering.The security and reliability of the anchor structure is influenced by environment and construction quality.And this structure is prone to anchor solid damage and GFRP bolt damage.As one of the key issues to ensure the safety of engineering structure,real-time nondestructive monitoring to GFRP bolt anchor is the focus and bottleneck in nondestructive detection of the GFRP bolt anchor structure.Smart piezoelectric materials have characteristics,such as high sensitivity,wide frequency response,dual function of actuating and sensing and excellent stability.Piezoelectric active sensing technology is an effective mean for structural safety inspection.The detection mechanism and method of piezoelectric wave for damage of GFRP bolt anchor structure were systematically studied by theoretical analysis,numerical simulation method,experimental study and signal processing.（1）Based on the bi-directional electromechanical coupling of smart piezoelectric materials,a combined system of piezoelectric patches-GFRP bolt anchor structure was constructed.The lumped mass method was adopted to develop the equivalent mechanical model.Using the simple variable method,the author analyzed the influence rule of physical parameters on the vibration status of the combined system.The result shows that the maximum displacement of the piezoelectric patches and the dynamic response amplitude will increase with the increases in the angular frequency,and decrease with the increases in the bolt stiffness coefficient and damping coefficient.Both of them form the dynamics principle of the piezoelectric wave method for detecting the damage of the GFRP bolt anchor structure.（2）A finite element numerical model of the combined system of piezoelectric patches-GFRP bolt anchor structure was developed using the principle of multi-field coupling.And the influence rule of physical parameters and structure damage on the response signal characteristics was analyzed.The maximum value of the response signal voltage in the anchoring combined system increases gradually with decreasing anchor elasticity and contact damping coefficients;The influence of the contact damping coefficient on the maximum value of the response signal voltage has a certain threshold.When the damping coefficient is less than the threshold value,the maximum value of the response signal voltage decreases sharply so that the criticality of anchor structure damage failure can be determined;Damage to the anchor structure causes a changeable voltage amplitude of the response signal.With the defect depth of bolt increasing,the maximum value of the response signal voltage decreases sharply and then slowly;The maximum value of the response signal voltage doesn’t change much with increasing defect width of bolt.The numerical simulation study reveals the mechanism of physical parameters and structure damage on the response signal characteristics.（3）Combined with the dynamics principle of the piezoelectric wave method for detecting the damage of the GFRP bolt anchor structure,physical parameters and the influence mechanism of structural damage on the response signal characteristics,the author established the functional relationship between reflected wave,transmitted wave and incident wave.The expressions of reflection coefficient and transmission coefficient has been obtained.And the author analyzed the correlation between different structural damage and stress wave characteristics,finding that anchor solid damage will enhance the dynamic response of the receiving end of the bolt,and the bolt body damage weakens the dynamic response at the receiving end of the bolt.Under variable working conditions,damage detection on GFRP bolt anchor structure were carried out.The correlation between different structural damage and stress wave characteristics was established:the maximum value of detection signal amplitude,root mean square value and energy value increase with the increase of anchor solid damage length and decrease with the increase of anchor defect depth.According to the theoretical analysis and the experimental study,the detection mechanism for the GFRP bolt anchor structure damage can be characterized through detecting signal amplitude maxima,root mean square values and energy values.（4）Wavelet packet was used to decompose the detection signal.The features of the decomposed signal are extracted by the fractal box dimension.Using the fractal box dimension of the decomposed signal and the root mean square value of the original detection signal as the input feature vectors,the model output parameter was the damage factor of the GFRP bolt anchorage body.Based on the Relevance Vector Machine（RVM）theory,an RVM prediction model for the damage degree of GFRP bolt anchorage body was established.The optimal value of the width factor of the RVM kernel function was obtained as 3.6483 by using the Particle Swarm optimization（PSO）,which constituted the PSO-RVM prediction model and a method for rapid prediction of bolt anchor quality in engineering applications.Examples verification shows the average relative error of the PSO-RVM prediction model is 3.04%.And the accuracy and reliability of the model is high,achieving intelligent identification of damage in GFRP bolt anchorage body.（5）The CEEMDAN theory was adopted to decompose the original detection signal.Then the power spectrum density and energy value of each component were compared.The efficient components were selected to reconstruct the signal in order to remove the noise of the original detection signal.The wavelet packet energy method and RMSD were used to define the damage factor of the GFRP bolt,forming a quantitative characterization method for the time-variant damage of the GFRP bolt based on CEEMDAN and the wavelet packet energy method.The shear damage detection of the GFRP bolt anchor structure was used to obtain the detection signals and calculate the trend of the damage factor with time.The GFRP bolt was damaged at the 10^th to 13^th minute when the energy loss was high.This was consistent with the test results.The theoretical analysis and test results verifies the feasibility and accuracy for quantitative characterization of the GFRP bolt time-variant damage.This study reveals the mechanism of the piezoelectric wave method for detecting damage of the GFRP bolt anchor structure,and develops a detection method,which provides a theoretical basis and a real-time detection method for the practical engineering application in damage detection of the GFRP bolt anchor structure.