Yu Jianbo

Focusing of Time Reversal Lamb Waves and Its Applications in Structural Health Monitoring

The focusing of time reversal acoustic fields for dispersive and multimodal Lamb waves is theoretically investigated and experimentally verified. It is demonstrated that the time reversal Lamb wave signal will reach the maximum amplitude when the observation point is located at the damage location. Based on the time reversal focusing theory, a damage imaging method is proposed for structural health monitoring using Lamb waves. The experiments employ a transducer network consisting of four piezoelectric transducers as actuators and as well sensors for excitation and measurement of Lamb waves. The results show that this method is able to accurately predict damage location and provides an estimation of the possible area even for damage close to one of the transducers or even slightly outside the transducer network.

A time reversal damage imaging method for structure health monitoring using Lamb waves

This paper investigates the Lamb wave imaging method combining time reversal for health monitoring of a metallic plate structure. The temporal focusing effect of the time reversal Lamb waves is investigated theoretically. It demonstrates that the focusing effect is related to the frequency dependency of the time reversal operation. Numerical simulations are conducted to study the time reversal behaviour of Lamb wave modes under broadband and narrowband excitations. The results show that the reconstructed time reversed wave exhibits close similarity to the reversed narrowband tone burst signal validating the theoretical model. To enhance the similarity, the cycle number of the excited signal should be increased. Experiments combining finite element model are then conducted to study the imaging method in the presence of damage like hole in the plate structure. In this work, the time reversal technique is used for the recompression of Lamb wave signals. Damage imaging results with time reversal using broadband and narrowband excitations are compared to those without time reversal. It suggests that the narrowband excitation combined time reversal can locate and determine the size of structural damage more precisely, but the cycle number of the excited signal should be chosen reasonably.

Effects of electromagnetic vibration on the structure and mechanical properties of Al-6%Si alloy during directional solidification

The effects of electromagnetic vibration on the grain refinement in directional solidification were investigated. It was found that the electromagnetic vibration applied in the melt not only can refine grains remarkably but also can enhance both tensile strength and ductility values of Al-6%Si alloy. SEM graphs show that coarse dendrite structure was broken up into a somewhat globular structure, and the morphology of eutectic silicon was changed from flaky to fibrous under electromagnetic vibration treatment. The refine mechanism under electromagnetic vibration was discussed.

Solidification structure of eutectic Al-Si alloy under a high magnetic field-aid-electromagnetic vibration

An electromagnetic vibration was generated by simultaneously imposing a strong static magnetic field (up to 10 T) and an alternative electricity current to the metal. Its effects on the solidification structure of eutectic Al-Si alloy have been investigated experimentally. It is found that the eutectic structure has been refined by solely imposing high magnetic field while it is coarsened under the electromagnetic vibration. Furthermore, polyhedral Si grains and non-dendritic α-Al appeared when the electromagnetic vibration strength was strong enough. The refining of eutectic structure is attributed to the decrease of diffusion coefficient caused by the strong magnetic field. The coarseness of eutectic structure may be attributed to the convection caused by electromagnetic vibration. Strong convection may break co-operative growth of eutectic phases to form polyhedral Si grains and non-dendritic α-Al.

Ultrasonic guided wave tomography of pipes

Ultrasonic guided wave tomography has been shown to be an effective nondestructive evaluation technique for pipe-like structures. Guided waves propagating in pipe with damage are simulated using the finite element and the first arrival times of the received signals are extracted as input data of tomography. Alterative reconstruction technique (ART) is used to modify the slowness value of each grid unit in ray tracing. Tomographic reconstructions are performed for damages of different positions and shapes in pipes. The results show that guided wave tomography can depict accurately the locations and sizes of the damages.

Grain Refinement During Directionally Solidifying GCr18Mo Steel at Low Pulling Speeds Under an Axial Static Magnetic Field

The present work investigates how axial static magnetic field affects the solidification structure and the solute distribution in directionally solidified GCr18Mo steel. Experimental results show that grain refinement and the columnar to equiaxed transition is enhanced with the increases in the magnetic field intensity (B) and temperature gradient (G) and the decrease in the growth speed. This phenomenon is simultaneously accompanied by more uniformly distributed alloying elements. The corresponding numerical simulations verify a thermoelectric (TE) magnetic convection pattern in the mushy zone due to the interaction between the magnetic field and TE current. The TE magnetic convection in the liquid should be responsible for the motion of dendrite fragments. The TE magnetic force acting on the dendrite is one of the driving forces trigging fragmentation.

Time reversal and cross-correlation analysis for damage detection in plates using Lamb waves

The paper mainly presents the time reversal theory and cross-correlation algorithm to focalize the Lamb wave scattered signals caused by the damage. Time reversal theory has been developed to improve the detection ability of Lamb waves. Cross-correlation of scattered signal envelope with excitation signal envelope is used for damage imaging. Experiments are carried out on an aluminium plate using a transducer network consisting of four identical piezoelectric transducers as actuators and as well sensors for excitation and measurement of Lamb waves. The results show that this method is able to accurately predict damage location and provides an estimation of the possible area even for damage close to one of the transducers or even slightly outside the transducer network.

Effect of Sintering Conditions on Texture Formation of Si3N4 Ceramic Shaped up in a Strong Magnetic Field

Si3N4ceramics with preferential texture along a or b axis were shaped up by gel-casting in a magnetic field of 6T and then pressureless sintered. Effect of the sintering temperature and time on the texture formation of the Si3N4 ceramics was investigated. It was found that the phase transformation of αto β was completed for the Si3N4 ceramics prepared by the present process. With the increasing sintering temperature the texture formation of Si3N4 ceramics may be enhanced, i.e. of which the degree of texture increased from 0.40 at 1700℃ to 0.76 at 1800℃. The extension of sintering time at 1750℃ had few effect on texture development of Si3N4 ceramics.