基于非线性超声调制频谱识别铝合金板材的疲劳裂纹

基于非线性超声调制频谱法,对航空铝合金板材中的疲劳裂纹识别进行研究;以两个不同频率的超声兰姆波为激励信号.依靠超声换能器、波形发生器和激光测振仪等,对含有疲劳裂纹和无损伤的2024-T351铝合金薄板试样进行对比实验;分别采用时域、频域和时频域联合法分析非线性超声波在铝合金薄板试样中传播的响应信号.结果表明:非线性声学特征即调制频谱及三阶谐波可作为识别2024-T351铝合金板材介质中疲劳裂纹的判据,通过对试样表面进行扫描,建立调制频谱的峰值幅度与位移的关系,据此可确定样板中疲劳裂纹的位置和轮廓,这为航空铝合金板材疲劳裂纹的识别提供更多技术支撑.     

In-situ monitoring of ultrasonic attenuation during rotating bending fatigue of carbon steel with electromagnetic acoustic resonance

Using electromagnetic acoustic resonance (EMAR), we studied the evolution of the surface-shear-wave attenuation and phase velocity during rotating bending fatigue of a 0.45% C steel. In the EMAR method, we used a magnetostrictively-coupled electromagnetic acoustic transducer (EMAT) for the contactless measurements of the axial shear wave that is a surface-shear wave, propagating in a cylinder-specimen circumference direction with the axial polarization. The attenuation coefficient always showed sharp peaks around 90% of the fatigue life, independent of the fatigue-stress amplitude. In addition to the ultrasonic measurements, we made crack-growth observations using replicas and measured recovery of attenuation and velocity after stopping the cyclic loading just before and after the peak. From these results, we concluded that the evolution of the ultrasonic properties is caused by a drastic change in dislocation mobility being accompanied by the crack growth.     

Nondestructive sizing and localization of internal microcracks in fatigue samples

A large number of fatigue experiments on standardized samples is required for the development of databases of the fatigue properties of specific material systems. To facilitate such studies, different visual monitoring methods for surface fatigue cracks have been used; however, the problem of monitoring internal fatigue crack initiating during cold dwell fatigue of Ti is much more complicated. This paper describes the development and integration of several nondestructive evaluation methods for monitoring and sizing microcracks in titanium fatigue samples. For in situ monitoring of crack initiation and evolution ultrasonic Lamb wave signals are excited and acquired in the sample continuously during fatigue tests at different levels of fatigue load using a high-speed data acquisition system. Localization of the secondary cracks is done by both the in situ ultrasonic method and an ultrasonic immersion scanning method here referred to as “vertical C-scan” (VC-scan). The VC-scan is developed for imaging small cracks aligned normal to the fatigue sample axis. Microradiography has been performed on fatigue samples to confirm the localization and sizing of the detected cracks with other ultrasonic NDE techniques. The fusion of data from different NDE techniques provides useful information on the initiation, location, shape, size and growth history of fatigue cracks.     

Noncontact fatigue crack visualization using nonlinear ultrasonic modulation

This paper presents a complete noncontact fatigue crack visualization technique based on nonlinear ultrasonic wave modulation and investigates the main source of nonlinear modulation generation. Two distinctive frequency input signals are created by two air-coupled transducers and the corresponding ultrasonic responses are scanned using a 3D laser Doppler vibrometer. The effectiveness of the proposed technique is tested using aluminum plates with different stages of fatigue crack formation such as micro and macro-cracks. Furthermore, the main source of nonlinear modulation is discussed based on the visualization results and the microscopic images.     

New samples with artificial voids for ultrasonic investigation of material damage due to creep

This paper describes the application of acoustic birefringence measurement for evaluation of material degradation due to creep. The problem with ultrasonic investigation of the influence of creep-induced voids on ultrasonic wave velocities is the lack of samples containing well-defined voids. The only way to “produce” damaged material with numerous voids is accelerated creep laboratory test or long-term service of material in high temperature and stress. To describe voids it is necessary to perform laborious metallographic examination of damaged material. Paper presents glass samples with artificial semi-flat voids created with laser engraving technique. Results of ultrasonic wave velocities and acoustic birefringence measurements in glass samples proved that laser engraving can be helpful for the experimental investigation of voids influence on solid acoustic properties.     

Ultrasonic wave intensity reflected from fretting fatigue cracks at bolt joints of aluminum alloy plates

The in-process ultrasonic measurement was carried out during fatigue testing of the bolted aluminum alloy 2024-T3 plates by using a water bag to obtain the surface acoustic wave (SAW) reflected from a fretting fatigue crack. The echo reflected from the fretting fatigue crack was detected at a position 1.5–2.1 mm ahead of the bolt hole under fastened conditions. In the fatigue process, the intensity of the scattering wave gradually increased in the fretted region ahead of the bolt hole with the number of fatigue cycles, and then a steep increase in the scattering wave intensity was observed. Relationships between the crack length and the average and the peak intensities were also obtained, and the average and peak intensities increased with the crack length. A relationship between the average intensity of the scattering wave and the surface roughness was also obtained. Experimental results suggested that appearance of a fretting fatigue crack in the fretted region caused a steep increase in the scattered wave intensity.     

Quantitative evaluation of micro-cracks using nonlinear ultrasonic modulation method

Nonlinear acoustic coefficients have a close relationship with structural cracks. A nonlinear ultrasonic modulation method for micro-crack quantitative evaluation is developed. The influence of phase threshold on the crack evaluation is discussed. Ultrasonic modulation using a unilateral incentive model is applied to the quantitative evaluation of micro-cracks for different specimens. The experimental results indicate that the method can be beneficially applied for micro-cracks detection. A proper phase threshold can improve the reliability of the method based on nonlinear ultrasonic modulation. The presented modulation factor can be used to quantitatively evaluate the structural cracks, and it is suitable for small-crack quantitative evaluation.     

在役转轴表面疲劳裂纹的爬波检测

采用常规超声检测方法对在役转轴表面疲劳裂纹检测的效果很差。针对此类在役转轴疲劳裂纹产生的机理,结合超声爬波传播的特性,选用超声爬波对在役转轴产生的表面疲劳裂纹进行检测。对某规格的阶梯转轴进行的检测结果表明,超声爬波检测方法简便可行,能够有效地检测出转轴在役运行状态中出现的疲劳裂纹,因此可用于在役转轴表面疲劳裂纹的日常检测。     

Development of a Non-collinear Nonlinear Ultrasonic-Based Technique for the Assessment of Crack Tip Deformation

Non-collinear nonlinear ultrasonic (NCNLU) wave mixing technique has been established to study the localized plastic deformation at the crack tip during fatigue. A pair of ultrasonic shear wave was mixed non-collinearly to obtain a longitudinal wave of frequency equal to the sum of the two shear wave frequencies under a resonant condition. Experiments were carried out on notched 9Cr-1Mo 3-point bend specimen during high-cycle fatigue. The variation of the NCNLU parameter with the stress accumulation at the crack tip during the fatigue crack initiation and propagation and mapping of the deformation zone around the crack tip are described in this paper.     

不锈钢增材制造件的激光超声表面波声速研究

利用激光超声技术对增材制造件进行无损检测具有重要的理论和应用价值.因此,针对不同激光功率下制备的316L不锈钢增材制造样品进行试验.利用激光超声在样品不同成型方向上测试,对表面波声速进行了表征.结果表明:不同激光功率下制备的316L不锈钢增材制造件的成型质量与表面波传播速度有着密切的关系,而且同一增材制造件在不同成型方...     

Fatigue Life Prediction of Metallic Materials Based on the Combined Nonlinear Ultrasonic Parameter

The fatigue life prediction of metallic materials is always a tough problem that needs to be solved in the mechanical engineering field because it is very important for the secure service of mechanical components. In this paper, a combined nonlinear ultrasonic parameter based on the collinear wave mixing technique is applied for fatigue life prediction of a metallic material. Sweep experiments are first conducted to explore the influence of driving frequency on the interaction of two driving signals and the fatigue damage of specimens, and the amplitudes of sidebands at the difference frequency and sum frequency are tracked when the driving frequency changes. Then, collinear wave mixing tests are carried out on a pair of cylindrically notched specimens with different fatigue damage to explore the relationship between the fatigue damage and the relative nonlinear parameters. The experimental results show when the fatigue degree is below 65% the relative nonlinear parameter increases quickly, and the growth rate is approximately 130%. If the fatigue degree is above 65%, the increase in the relative nonlinear parameter is slow, which has a close relationship with the microstructure evolution of specimens. A combined nonlinear ultrasonic parameter is proposed to highlight the relationship of the relative nonlinear parameter and fatigue degree of specimens; the fatigue life prediction model is built based on the relationship, and the prediction error is below 3%, which is below the prediction error based on the relative nonlinear parameters at the difference and sum frequencies. Therefore, the combined nonlinear ultrasonic parameter using the collinear wave mixing method can effectively estimate the fatigue degree of specimens, which provides a fast and convenient method for fatigue life prediction.     

Research on short circuiting transfer mode of ultrasonic assisted GMAW method

Abstract

Ultrasonic assisted gas metal arc welding (U-GMAW) has been recently developed to improve the metal transfer characteristics. The ultrasonic wave is applied on the metal transfer process by means of an acoustic field. Welding electrical signal measurement and high speed camera are employed to study the differences of short circuiting metal transfer between conventional GMAW and U-GMAW. Compared with the conventional GMAW, the short circuit frequencies of U-GMAW are obviously increased under the same welding parameters. Moreover, the voltage range of the stable short circuiting transfer is enlarged, and the weld appearances become more uniform with the action of the ultrasonic wave. The high speed video images indicate that the U-GMAW arc is compressed and the electrical field intensity is increased. The decrease in the arc length is the main reason for the increase in the short circuit frequency.     

Ultrasonic surface wave propagation and interaction with surface defects on rail track head

Electromagnetic acoustic transducers (EMATs) are non-contact ultrasonic transducers capable of generating wide band ultrasonic surface waves on metallic samples. A lab-based laser-EMAT system has been developed to observe the ultrasonic surface wave propagation and interaction with surface breaking defects on the sample rail head surface. A wide band EMAT generating surface waves with a frequency content between approximately 50 and 500 kHz is used to propagate ultrasonic waves on the surface of a rail head down the length of the sample. A stabilised Michelson interferometer is used to measure the out-of-plane displacement of the surface wave. A complete picture of the ultrasonic surface wave on the sample surface over time is reconstructed using this technique, with exceptionally high spatial and temporal resolution. Despite the curvature of the rail head, the ultrasonic surface wave propagating down the rail is found to have similar properties to Rayleigh waves by direct comparison to those observed on flat samples using the same technique.     

超声细化处理AZ80镁合金过程中的声场数值模拟

采用不同强度超声对AZ80镁合金熔体进行处理以改善合金的凝固组织。当施加的超声强度为30.48W/cm2时,合金的平均晶粒尺寸由未经超声处理时的303μm降低为148μm。为了进一步了解超声改善镁合金微观组织的机理,采用数值模拟的方法研究超声声压对空化泡行为的影响,并且对熔体中的超声场分布情况进行分析。结果表明,熔体内不同位置所受的声压是不同的,因此不同位置上的铸锭试样的晶粒细化程度也不同。随着超声强度的增加,声压值增加,而合金的晶粒尺寸则随之降低。     

Degradation behavior of moduli in extruded pure magnesium during low- to giga-scale cyclic tension fatigue

The mechanical properties of extruded pure magnesium during cyclic tension fatigue in the low- to giga-scale regime at room temperature have been investigated using ultrasonic reflection methods with longitudinal and shear waves. The acoustic velocities and calculated Young’s and shear moduli decreased by a large percentage with an increase in the number of cycles in all cycle modes due to growth of grain boundary voids. The eventual degradation of the properties was largest during giga-cycle fatigue, in which the moduli decreased by ～9%. The elastic behavior depended on the drive stress and the number of cycles rather than on fatigue time. Longitudinal and shear wave propagation characteristics and investigations of a grain boundary before and after fatigue using electron backscatter diffraction based on field-emission scanning electron microscopy and focused ion beam transmission electron microscopy revealed that the largest boundary void gap width was less than several nanometers (almost closed). The Poisson’s ratio and bulk modulus were affected notably by the void gap, in which the threshold corresponds to the longitudinal wave amplitude. Other damage phase data were determined using scanning electron microscopy, Vickers hardness, and surface roughness tests under progressive fatigue; these results also indicated slight grain boundary degradation.     

基于波动理论的管结构损伤检测方法

针对在役海洋平台的无损检测大都采用超声等主动检测手段,这会受到工况的影响。而声发射等被动检测手段避免了这一问题,实现了对现役海洋平台的在线监测。综述了基于瞬态波、应力波、导波和模态声发射等波动理论进行的管结构声发射检测研究成果。对基于波动理论进行复杂结构检测的前景做了展望。     

Impact wave and damage detections using a strain-free fiber Bragg grating ultrasonic receiver

A strain-free mobile fiber Bragg grating (FBG) ultrasonic receiver is applied for the impact-related experiments of carbon fiber reinforced plastic laminates. The strain-free FBG sensor detects an impact-induced acousto-ultrasonic wave and its responses are compared with those of a piezoelectric sensor. Ultrasonic mode wavelength-related averaging effect in FBG ultrasonic sensors is also reported. The mobile FBG sensor can be useful for the acoustic characterization and the sensor placement optimization being required before construction of a built-in FBG network. Finally, the mobility of the strain-free FBG sensor head is extended to ultrasonic scanning application. Based on its high scanning spatial resolution, impact damage sizing is conducted more precisely.     

基于小波包的奥氏体焊缝缺陷超声信号识别

在使用超声反射波法对奥氏体不锈钢焊缝进行缺陷检测时,由于粗大晶粒组织的声散射作用,致使缺陷信号不易于识别. 针对这一问题,提出一种基于小波包技术的改进噪声抑制方法. 阐明了改进方法的技术原理,采集了奥氏体不锈钢焊缝中含人工缺陷信号的超声回波数据. 利用所提方法对超声回波进行了噪声抑制处理,并对人工缺陷进行了定量化测量. 结果表明,和传统小波包法相比较,所提方法能更为有效地滤除超声反射回波中的噪声成分,可识别直径1.5 mm的平底孔.     

Performance of non-destructive evaluation by diffracted SH ultrasonic waves in predicting degree of fatigue in cyclic bending of ferritic steel

The degree of fatigue in ferritic wrought steel during cyclic bending was determined by analysis of diffracted SH ultrasonic waves with the aid of multiple regression analysis. As the degree of fatigue increases, accompanied by residual stress, the incident waves curve to the interior region with positive stress field owing to the acoustoelastic effect. As a result, the propagation time of the launched waves lengthens, leading to a modulation of the received waveform. Multiple regression analysis for the waveform modulation produces a reliable estimation, with correlation coefficient of 0.948, for the degree of fatigue.     

Experimental study of nonlinear Rayleigh wave propagation in shot-peened aluminum plates—Feasibility of measuring residual stress

Shot-peening is widely used in the aerospace industry to enhance the resistance of structural components to fatigue damage and stress corrosion by putting the outside layer of a component under an initial, residual compressive stress. The ability to measure these near-surface residual stresses is useful from a quality control and certification perspective, and can help predict the fatigue life of shot-peened components. This paper presents experimental results to examine the feasibility of measuring near-surface residual stresses using nonlinear Rayleigh surface waves. Experiments are conducted on aluminum alloy (AA 7075) samples shot-peened at different peening intensities and thus with different levels of residual stresses. The surface roughness of these samples is also measured. The nonlinear ultrasonic results show a large increase in the acoustic nonlinearity parameter, indicating the potential of nonlinear ultrasonics for the in situ measurement of near-surface residual stresses. The effects of surface roughness and the driving frequency on the measured acoustic nonlinearity parameter are briefly discussed. Finally, a preliminary model is used to interpret some experimental results. Future work to evaluate the separate contributions of cold work, residual stress and surface roughness to the total measured nonlinearity is also discussed.