Ultrasonic Nondestructive Signals Processing Based on Matching Pursuit with Gabor Dictionary

The success of ultrasonic nondestructive testing technology depends not only on the generation and measurement of the desired waveform, but also on the signal processing of the measured waves. The traditional time-domain methods have been partly successful in identifying small cracks, but not so successful in estimating crack size, especially in strong backscattering noise. Sparse signal representation can provide sparse information that represents the signal time-frequency signature, which can also be used in processing ultrasonic nondestructive signals. A novel ultrasonic nondestructive signal processing algorithm based on signal sparse representation is proposed. In order to suppress noise, matching pursuit algorithm with Gabor dictionary is selected as the signal decomposition method. Precise echoes information, such as crack location and size, can be estimated by quantitative analysis with Gabor atom. To verify the performance, the proposed algorithm is applied to computer simulation signal and experimental ultrasonic signals which represent multiple backscattered echoes from a thin metal plate with artificial holes. The results show that this algorithm not only has an excellent performance even when dealing with signals in the presence of strong noise, but also is successful in estimating crack location and size. Moreover, the algorithm can be applied to data compression of ultrasonic nondestructive signal.

Influence of Ultrasonic Surface Rolling Process and Shot Peening on Fretting Fatigue Performance of Ti-6Al-4V

At present,there are many studies on the residual stress field and plastic strain field introduced by surface strengthening,which can well hinder the initiation of early fatigue cracks and delay the propagation of fatigue cracks.However,there are few studies on the effects of these key factors on fretting wear.In the paper,shot-peening(SP)and ultrasonic surface rolling process(USRP)were performed on Ti-6Al-4V plate specimens.The surface hardness and residual stresses of the material were tested by vickers indenter and X-ray diffraction residual stress analyzer.Microhardness were measured by HXD-1000MC/CD micro Vickers hardness tester.The effects of different surface strengthening on its fretting fatigue properties were verified by fretting fatigue experiments.The fretting fatigue fracture surface and wear morphology of the specimens were studied and analyzed by means of microscopic observation,and the mechanism of improving fretting fatigue life by surface strengthening process was further explained.After USRP treatment,the surface roughness of Ti-6Al-4V is significantly improved.In addition,the microhardness of the specimen after SP reaches the maximum at 80μm from the surface,which is about 123%higher than that of the AsR specimen.After USRP,it reaches the maximum at 150μm from the surface,which is about 128%higher than that of AsR specimen.It is also found that the residual compressive stress of the specimens treated by USRP and SP increases first and then decreases with the depth direction,and the residual stress reaches the maximum on the sub surface.The USRP specimen reaches the maximum value at 0.18 mm,about−550 MPa,while the SP specimen reaches the maximum value at 0.1 mm,about−380 MPa.The fretting fatigue life of Ti-6Al-4V effectively improved after USRP and SP.The surface integrity of specimens after USRP is the best,which has deeper residual compressive stress layer and more refined grain.In this paper,a fretting wear device is designed to carry out fretting fatigue experiments on specimens with different surface strengthening.

Characteristics and Microstructure of a Hypereutectic Al-Si Alloy Powder by Ultrasonic Gas Atomization Process

A hypereutectic Al-Si alloy powder was prepared by ultrasonic gas atomization process. The morphologies, microstructure and phase constituent of the alloy powder were studied. The results showed that powder of the alloy was very fine and its microstructure was mainly consisted of Si crystals plus intermetallic compound A19FeSi3, which were.very fine and uniformly distributed.

Simple Ultrasonic-Assisted Clean Graphene Transfer

Clean graphene transfer has received widespread research attention, where most methods are focused on cleaning the upper surface of graphene to improve the transfer technique. However, the residue formation on the bottom surface of graphene is also inevitable;therefore, cleaning the bottom surface is crucial. In this study, we proposed an improved graphene wet transfer method using an ultrasonic processing(UP) step for etching copper(Cu). Using this method, the bottom surface can be cleaned efficiently. The results of atomic force microscopy(AFM)and Raman spectroscopy mapping revealed that the graphene films transferred with UP had smoother and cleaner surfaces, less contamination, and higher quality than those transferred without UP.

Feature extraction of wood-hole defects using wavelet-based ultrasonic testing

The primary bottleneck to extracting wood defects during ultrasonic testing is the accuracy of identifying the wood defects. The wavelet energy moment was used to extract defect features of artificial wood holes drilled into 120 elm samples that differed in the number of holes to verify the validity of the method. Wavelet energy moment can reflect the distribution of energy along the time axis and the amount of energy in each frequency band,which can effectively extract the energy distribution characteristics of signals in each frequency band; therefore,wavelet energy moment can replace the wavelet frequency band energy and constitute wood defect feature vectors. A principal component analysis was used to normalize and reduce the dimension of the feature vectors. A total of 16 principal component features were then obtained, which can effectively extract the defect features of the different number of holes in the elm samples.

Effects of sonication amplitude on the microstructure and mechanical properties of AZ91E magnesium alloy

Light metals are gaining increased attention due to ecological sustainability concerns and strict emission regulations. Magnesium(Mg)is one such metal that has the potential to replace high density components, which can reduce emissions through lightweighting. However,the mechanical properties of Mg alloys must be improved for them to become viable candidates for structural applications. To this end, the current study examines the effect of sonication vibrational amplitude on the microstructure and mechanical properties of AZ91E Mg alloy.The molten alloys were subjected to ultrasonic treatment at a frequency of 20 kHz, 180 s of processing time and vibrational amplitudes ranging from 1.25 to 15 μm. The resultant castings were characterized using optical microscopy, scanning electron microscopy and tensile testing. It was found that sonication with amplitudes up to 7.5 μm was able to effectively refine the secondary phases of the alloy. Similar trends were observed for grain size and yield strength. The refinement in microstructure was likely caused by the finer grain size and cavitation induced undercooling of the liquid metal. In addition, it was also noted that even the lowest level of amplitude(1.25 μm) was able to increase the density, improve the ultimate tensile strength and ductility of the castings. The tensile strength and ductility were thought to have been enhanced by ultrasonic degassing and refinement in microstructure, while the yield strength was improved through the Hall-Petch effect. The results from this study provided a basis for optimizing the sonication process and promoting its use in industry. As a result, Mg alloys improved through ultrasonic processing have the potential to replace higher density components, with consequent energy efficiency and environmental and ecological benefits.

Contribution of ultrasonic surface rolling process to the fatigue properties of TB8 alloy with body-centered cubic structure

The effect of ultrasonic surface rolling process(USRP) as a severe plastic deformation technology was investigated on the evolution of microstructure, residual stress and surface morphology of TB8 alloys with body-centered cubic structure. Stress-controlled rotating-bending fatigue tests indicated increased fatigue strength in USRP samples prepared using different number of passes compared to the base material, which was attributed to the presence of gradient structure surface layers. Five subsequent USRP passes resulted in the highest fatigue strength, due to the optimal surface properties including higher extent of grain refinement, larger compressive residual stresses, "smoother" surface morphology and increased micro-hardness. However, the effect of USRP technology on improving fatigue strength of TB8 alloy was not significant in comparison with that of other titanium alloys(for example, Ti6 Al4 V), which was attributed to the notable surface residual stresses relaxation revealed from measurements on postfatigued USRP samples. Electron backscatter diffraction analysis confirmed that fatigue crack initiation occurred in the larger grains on the surface with high Schmid factor. Small cracks were found to propagate into the core material in a mixed transgranular and intergranular mode. Further analysis indicated that grain growth existed in post-fatigued USRP-treated TB8 samples and that the average geometrically necessary dislocations value reduced after fatigue loading.

Enhanced Surface Mechanical Properties and Microstructure Evolution of Commercial Pure Titanium Under Electropulsing-Assisted Ultrasonic Surface Rolling Process

The effects of electropulsing-assisted ultrasonic surface rolling process on surface mechanical properties andmicrostructure evolution of commercial pure titanium were investigated. It was found that the surface mechanical prop-erties were significantly enhanced compared to traditional ultrasonic surface rolling process （USRP）, leading to smallersurface roughness and smoother morphology with fewer cracks and defects. Moreover, surface strengthened layer wasremarkably enhanced with deeper severe plastic deformation layer and higher surface hardness. Remarkable enhancementsof surface mechanical properties may be related to the gradient refined microstructure, the enhanced severe plasticdeformation layer and the accelerated formation of sub-boundaries and twins induced by coupling effects of USRP andelectropulsing. The primary intrinsic reasons for these improvements may be attributed to the thermal and athermal effectscaused by electropulsing treatment, which would accelerate dislocation mobility and atom diffusion.

Active and passive compliant force control of ultrasonic surface rolling process on a curved surface

Ultrasonic surface rolling process(USRP)is one of the effective mechanical surface enhancement techniques.During the USRP,unstable static force will easily do harm to the surface quality.In order to achieve a higher surface quality on the part with a curved surface,an active and passive compliant USRP system has been developed.The compliant USRP tool can produce the natural obedience deformation along the part surface.Force control based on the fuzzy Proportional-integral-derivative(PID)method is then designed to maintain the static force during the USRP.Experiments have been performed on a real aero-engine blade with curved surface.It is proved that the deigned active and passive compliant USRP system can significantly reduce the force variation from 42.2 N to 4.2 N,and achieve a uniform surface quality after processing.

Ultrasonic attenuation spectroscopy for multivariate statistical process control in nanomaterial processing

Ultrasonic attenuation spectroscopy （UAS） is an attractive process analytical technology （PAT） for on-line real-time characterisation of slurries for particle size distribution （PSD） estimation. It is however only applicable to relatively low solid concentrations since existing instrument process models still cannot fully take into account the phenomena of particle-particle interaction and multiple scattering, leading to errors in PSD estimation. This paper investigates an alternative use of the raw attenuation spectra for direct multivariate statistical process control （MSPC）. The UAS raw spectra were processed using principal component analysis. The selected principal components were used to derive two MSPC statistics, the Hotelling＇s T2 and square prediction error （SPE）. The method is illustrated and demonstrated by reference to a wet milling process for processinR nanoparticles.

Laser ultrasonic generation and optical detection and processing

History, present situation and importancy of the laser-generated ultrasonic technique are presented. Basic principles and some experimental results of laser ultrasonic generation and optical detection and processing are discussed. Several problems about applying this technique to NDT are also discussed in this paper.

Deformation mechanism and in-situ TEM compression behavior of TB8βtitanium alloy with gradient structure

Severe plastic deformation is known to induce grain refinement and gradient structure on metals’surfaces and improve their mechanical properties.However,the fundamental mechanisms behind the grain refinement and micromechanical properties of materials subjected to severe plastic deformation are not still well studied.Here,ultrasonic surface rolling process(USRP)was used to create a gradient microstructure,consisting of amorphous,equiaxed nano-grained,nano-laminated,ultrafine laminated and ultrafine grained structure on the surface of TB8βtitanium alloy.High energy and strain drove element co-segregation on sample surface leading to an amorphous structure during USRP processing.In situ transmission electron microscope compression tests were performed in the submicron sized pillar extracted from gradient structure and coarse grain,in order to reveal the micromechanics behavior of different grain morphologies.The ultrafine grained layer exhibited the lowest yield stress in comparison with single crystal and amorphous-nanocrystalline layers;the ultrafine grained layer and single crystal had an excellent strain hardening rate.The discrepancy among the grain sizes and activated dislocation sources led to the above mentioned different properties.Dislocation activities were observed in both compression test and microstructure evolution of USRP-treated TB8 alloy.An evolution of dislocation tangles and dislocation walls into low angle grain boundaries and subsequent high angle grain boundaries caused the grain refinement,where twinning could not be found and no phase transformation occurred.