Stress Measurement In Plates Based On Higher-order Lamb Wave Technology

Stress status is important for the reliability evaluation of engineering structures.The residual stress will be commonly generated inside metal plates in the process of manufacturing.Furthermore,the plates will be affected by kinds of stress from environment during service.Excessive stress will cause deformation of the plates,and it is easy to produce cracks under corrosive environment,which will seriously affect the normal work of the whole system.Hence,timely and accurate stress measurement is necessary for the assurance of safety and stability of the plates in service.However,there are few researches on the stress measurement of metal plates,which mainly focus on the lower-order Lamb wave.And the lower-order Lamb are not sensitive to the stress and hardly to measure the local stress in plates.In order to solve these problems,the Lamb modes sensitive to stress were chosen to measure the stress in plate based on the study of acoustoelastic effect of higher-order Lamb.The main jobs of this paper include following aspects:(1)Building the Lamb acoustoelastic simulation model based on the Floquet-Bloch theory and Murnaghan hyper-elastic material model.And the correctness of model had been proven by the great agreement between the dispersion curves from acoustoelastic model and Rayleigh Lamb equation.Initial stress was applied to the hyper-elastic material model and the dispersion curves under uniform stress were obtained.Comparing the sensitivity to stress of different modes of Lamb wave,it had been found that the lower-order Lamb wave modes were not sensitive to stress while the higher-order Lamb wave modes near the cut-off frequency had higher sensitivity.(2)It was found that the frequency of S1 zero-group-velocity(ZGV)Lamb mode changed near-linearly with the applied stress based on the research of acoustoelastic theory.Hence,the S1 ZGV Lamb was proposed to measure the local stress in plate.A series stress fields with different gradients,peaks and numbers had been applied on plates.The S1 ZGV mode frequency at different positions of the aluminum plate were used for stress calculation.Comparing the stress measurement results with the actual stress,and a good agreement had been received which proved that ZGV Lamb wave method is an effective method to locally measure the complex stress field in thin plates.(3)Building the contact and non-contact Lamb stress measurement system.Firstly,a contact system composed of wedge,transducers,signal generator,pulse amplifier and oscilloscope was built according to the oblique incidence method,and Lamb wave signals with high signal-to-noise ratio were successfully excited in the plate via this system.Then another non-contact system based on air-coupled transducers was built on the basis of contact system,the motor control and signal receiving program was written by Lab VIEW software.The excitation and receiving of Lamb mode were achieved and the defect scanning imaging of plates with different thickness was completed based on the non-contact system.(4)The stress measurement in plates based on Lamb wave was completed.Firstly,the relationship between the propagation time of Lamb wave and stress was established.Then the stress of the variable section plate was calculated by measuring the propagation time difference of A1 and S0 mode respectively.The calculated stress was compared with the actual value,and the measurement errors were within 15 MPa which proved that Lamb wave can be used to measure the average stress in plates.