| The stress corrosion cracking of structural materials in nuclear power plants is one of the critical problems affecting the long-term safe operation of nuclear power equipment.The monitoring test of crack growth rate in the autoclave environment that simulates the actual working conditions of structural materials in nuclear power plants is an essential part of the safety evaluation and equipment life prediction of nuclear power plants.Given the exceptional environment,the direct current potential drop(DCPD)method is one of the few methods that can be applied to monitor crack propagation in the autoclave environment.In practice,the potential drop signal representing the crack length is not only affected by crack propagation but also by various non-crack propagation factors,and the signal is weak and easily disturbed.To clarify the factors affecting the potential drop signal and to improve the accuracy of crack propagation signal monitoring,the mechanism of monitoring crack propagation by direct current potential drop method and the anti-interference measurement of weak potential drop signal were studied in this dissertation.The main research work and results are as follows:The current density distribution and potential field distribution of typical cracked specimens were studied.The main factors affecting the relationship between potential drop and crack length and their normalized relationship were analyzed.By solving the potential field distribution of the specimen with cracks,the influence of crack morphology on measurement was analyzed.The results have shown that crack front bending causes crack growth measurements to be lower than true,while crack deflection,zigzag cracks,and crack bifurcation cause measurements to be higher than true.The accuracy of crack propagation measurement can be improved by using the actual crack morphology feature modeling to correct the measured values.The influence of the wiring points layout on the sensitivity,the amplitude,and the measurement position error sensitivity of the main potential drop signal,as well as the influence on the measurability and the crack correlation of the reference potential drop signal,were studied.The selection basis of measurement points layout of reference potential drop and the optimization method based on the multi-objective genetic algorithm were proposed.The optimized Pareto solutions of the measurement points positions are all located within the specimen boundary.Compared with the case where the measurement points are at the boundary of specimens,the signal amplitude can be increased by order of magnitude while ensuring low crack correlation.According to the time-domain analysis,frequency-domain analysis and statistical characteristic parameter analysis of the experimental data of the direct current potential drop signal monitoring,the characteristics of the signal and the interference signal were studied,and the direct current potential drop signal model has been proposed accordingly.The results show that the time-varying thermal electromotive force,temperature-related interference,and random noise are the main interference factors affecting the weak direct current potential drop signal.The characteristics of the above interference signals are different.The thermal electromotive force and random noise appear additive,while the temperature-dependent interference signal appears multiplicative.The thermal electromotive force is related to the specimen material,wire material,and temperature.The thermal electromotive force in an autoclave environment is one order of magnitude larger than that in air and has evident periodicity.According to the proposed direct current potential drop signal model,signal anti-interference algorithms were designed for the three interference signals included in the model.A signal anti-interference processing algorithm combining wavelet threshold denoising and variable current signal synthesis technique was proposed.The thermal electromotive force signal and noise signal can be effectively suppressed while the multiplicative interference signal is retained so that the reference potential drop method can be further used to suppress it.The processing results of the experimental data of direct current potential drop signal monitoring in air and autoclave environment show that the proposed signal processing algorithms can effectively suppress the interference and increase the signal-to-noise ratio by about 30 dB.By monitoring the potential drop signal in real-time during the slow tensile test and low-frequency tensile fatigue test,as well as the specimen deformation and electric field simulation analysis,the influence of the loading deformation on the potential drop was studied.The fluctuation of the potential drop signal consistent with the load frequency was measured experimentally,and its variation law was analyzed.The reasons for the fluctuation are the crack surface contact near the crack tip and the resistance strain effect.The change of potential drop signal induced by the load causes errors in crack propagation measurements.For a certain magnitude of load,the measurement error will increase as the crack grows.The material and load parameters were introduced into the calibration function.A calibration method incorporating the influence of the load was proposed,which can correct the measurement error caused by the load change.A prototype of an integrated crack growth monitor was developed,and the signal measurement at the microvolt level was realized.The crack propagation signal monitoring experiment of the compact tensile specimen in an autoclave environment was carried out using the prototype.The optimized wiring points layout was adopted.The proposed signal anti-interference processing algorithm was used to suppress the interference signal.And the direct current potential drop signal was converted into crack propagation signal according to the calibration method incorporating the influence of load.The standard deviation of the crack propagation signal was reduced to 5 μm. |
PDF Full Text Request