Research On Vibration Measurement Method Of Existing Stress Of Prestressed FRP Sheets

It is of great significance and value to work out a convenient,effective and reliable detection method for pre-stressed FRP sheet that meets the practical construction requirements for lacking of a mature method to detect the applied stress of FRP material.This paper proposes a vibration method to test the applied stress of pre-stressed FRP sheet.The vibration test is performed for the carbon fiber plates with different lengths in various tensioning tonnage conditions by the hinged wave anchor tensioning system so as to study the feasibility and the accuracy of this method.The following works are principally addressed in this paper:(1)Outlining a series of practical pre-stress testing methods with their principles and summarizing their advantages and disadvantages after extensive reading of the literature.From the perspective of vibration mechanics,the principle of testing pre-stress by vibration method is systematically elaborated and the relationship between the tensioning force and frequency of the string vibration principle is summarized,which provides the calculation and analysis with reference and basis.(2)From the point of view of test,the paper tested and studied the vibration frequencies of CFRP sheets with different lengths under different tension conditions.The frequency-difference method is adopted to calculate the order of the natural frequency,and the principle of using the average method to process the fundamental frequency is employed to verify the correctness of the frequency order calculation.The error of the tested frequency is computed so as to verify the accuracy of the frequency test.At the same time,the linear fitting between the tensioning force and(fn / n)2 are performed and the results show that there is a preferable first-order linear relationship of the corresponding order quotient squares between the tensioning force and frequency,which shows that the vibration of the carbon fiber plate accords with the string vibration theory.(3)The tested frequency is substituted into the tension-frequency formula to invert the calculated value of the tension force,comparing with the actual value.The relative errors are calculated and the causes are analyzed so as to verify the feasibility and applicability of the vibration method to test the applied stress of carbon fiber plate.Meanwhile,the test results of the five test schemes are compared and analyzed: when the tension force is great(N ≥ 50kN),the test results have high precision and theconstruction error requirement is tolerated,then the author proposes a applicable scope of the method.(4)The vibration test research is carried out based on the reinforcement project.The results show that the test error under the actual construction condition is larger than that in the laboratory: the construction error requirement is tolerated when the tension force is 70 kN.Generally,the test results are in line with the rule gained in laboratory so that the correctness of the above conclusions is further validated as well as the error causes under the actual construction condition is analyzed.(5)The principle of frequency sampling and spectrum analysis is explained and the causes of errors in the process of sampling and spectrum analysis is illustrated as well as the influence of frequency testing error on tension force.The relationship between the calculated value of tension force and sampling frequency,frequency resolution and sampling number are obtained through the formula derivation and also in the test these three parameters should be considered and selected comprehensively and reasonably.