Longitudinal Critically Refracted Research Articles

Ultrasonic Study of Longitudinal Critically Refracted and Bulk Waves of the Heat-Affected Zone of a Low-Carbon Steel Welded Joint under Fatigue

Currently, ultrasonic methods for assessing the fatigue lifetime of various structural materials are being actively developed. Many steel constructions are made by welding. The weld heat-affected zone is the weak point of the construction, as it is most susceptible to destruction. Therefore, it is actually important to search for acoustic parameters that uniquely characterize the structural damage accumulation in the heat-affected zone of a welded joint in order to predict failure. In this work, the specimens were made from the base metal and the welded joint’s heat-affected zone. The specimens were subjected to uniaxial tension–compression under a symmetrical cycle in the region of low-cycle fatigue with control of the strain amplitude. The propagation bulk velocities of longitudinal, shear waves and subsurface longitudinal critically refracted (LCR) waves during cyclic loading were studied. The acoustic birefringence of shear waves was calculated, and a similar parameter was proposed for longitudinal and LCR waves. The dependence of the elastic modulus ratio on the cycle ratio was obtained. It was shown that the acoustic parameters change most intensively in the heat-affected zone. According to the data of the C33/C55 ratio changes measured through the ultrasonic method, a formula for calculating the remaining fatigue life in the heat-affected zone was proposed.

Effects of factors from practical workpieces on ultrasonic LCR method stress measurement

The non-destructive stress measurement method is the main trend in residual stress analysis. The ultrasonic method, which utilizes the acoustoelastic effect of the longitudinal critically refracted (LCR) wave, is one of the time-saving measurement techniques. During the practical stress measurement on a workpiece, various external factors may impact the transmission of acoustic waves and the resulting stress value. This study revealed and discussed the effects of four factors on the LCR wave: surface roughness of the examined material, temperature of the material, external mechanical vibration, and surface paint. The stress coefficient was determined by comparing the offset time of the acoustic wave with the stress measured by X-ray analyzer in the zero-stress specimens, which had undergone annealing and deep cryogenic treatment. The test results indicated that the surface roughness did not affect the transition time of the acoustic wave, but it did decrease the intensity of the signal. The increase in temperature and the transition time of the acoustic wave were in a linear relationship. Mechanical vibrations from the environment would not affect the transition time or signal intensity of the acoustic wave, whereas the application of surface paint increased the transition time. Although the effect of paint on the actual workpiece could not be easily modified during stress measurement, the ultrasonic method was still suitable for monitoring the stress of a specific position of the workpiece throughout its operational lifetime. The experiment data in this study were applied to measuring the residual stress of an aluminum ship component, and the result showed a good correspondence with X-ray stress analyzer results.

Ultrasonic Non-Destructive Detection Method for Residual Stress in Rotary Forging Aluminum Alloy Plates.

Aluminum alloy plates are widely used to manufacture large-scale integral structure parts in the field of aerospace. During the forming and processing of aluminum alloy plates, different degrees of residual stress are inevitably produced. Fast and accurate detection of residual stress is very essential to ensuring the quality of these plates. In this work, the longitudinal critically refracted (LCR) wave detection method based on a one-transmitter and double-receiver (OTDR) transducer and the finite element simulation were employed to obtain the residual stress. Aluminum alloy plates with different deformation amounts were fabricated by rotary forging to obtain different residual stress states. Results reveal that the plate formed by rotary forging is in a stress state of central tension and edge compression. As the deformation increases from 20% to 60%, the peak residual tensile stress increases from 156 MPa to 262 MPa, and there is no significant difference in the peak compressive stress. When the deformation reaches 60%, the difference in the residual stresses at different depths is less than 13%, which indicates that the plastic deformation zone basically penetrates the entire longitudinal cross-section of the plate. The maximum deviation between measurement and FE is 61 MPa, which means the experimental data are in good agreement with the FE results.

Nitriding layer depth detection based on mixing frequency nonlinear ultrasonic parameters

Nitriding treatment can improve the surface properties of workpieces, thus increasing the service life of the workpiece. The depth of nitriding layer is not only one of the important indexes for evaluating the nitriding effect, but also an important factor affecting the end-use performance of the workpiece. While the existing hardness and metallographic methods cannot meet the needs for non-destructive testing of nitriding layer depth in shaft parts. Therefore, a method using non-linear ultrasonic testing technology is proposed for non-destructive evaluation of nitriding layer depth. In this study, 1045 steel shaft specimens with different nitriding layer depths were prepared by a liquid salt bath nitriding method. The total depth of the nitriding layer was measured using a microhardness tester, and metallographic microscopy was applied to observe microstructure changes before and after nitriding treatment. With the proposed non-destructive method, the longitudinal critically refracted (LCR) wave mixing detection model was established and the ultrasonic nonlinear coefficients were used for characterizing the nitrided layer depths. Experimental results show that the LCR wave sum frequency (LCRWSF) detection model better characterizes the nitriding layer depth of 1045 steel and has higher sensitivity. As a result, the LCRWSF model is more suitable to efficiently estimate the nitrided layer depth.

Characterization on vermicular graphite rate and tensile strength in vermicular graphite cast iron based on nonlinear ultrasonic technique

Ultrasonic techniques have been widely used to detect the percentage of vermicular graphite, defects, etc. in vermicular graphite cast iron. The linear ultrasonic velocity method is the main ultrasonic method for characterizing vermicular graphite rate and tensile strength in the current study, however, it is often easy to misjudge the vermicular graphite rate due to its insignificant variation. This study explores the feasibility of using the nonlinear ultrasonic technique (NUT) to characterize the vermicular graphite rate and tensile strength. Based on the longitudinal critically refracted (LCR) wave detection model, an experimental study to detect the vermicular graphite rate and tensile strength of vermicular graphite cast iron is carried out using the nonlinear ultrasonic harmonic method, and compared with the ultrasonic velocity method. The experiment results show that the relative nonlinear parameter of the LCR wave decreases along with the increase of vermicular graphite rate and increases along with the increase of tensile strength, and it has higher detection sensitivity and resolution than ultrasonic velocity by analyzing the obtained data. The increase in the acoustic nonlinearity parameter (ANP) is related to the increase in the number of grain boundaries in the microstructure. Therefore, the relationship among microstructure, ANP, and mechanical properties of vermicular graphite cast iron can be established, and it’s promising that a new approach might be developed for quickly detecting the vermicular graphite rate and corresponding tensile strength with the NUT.

Depth profiling of residual stress distribution in surface treated metallic structures using nonlinear ultrasonics

Robotic hammer peening (RHP) is a cold-working technique to improve the fatigue life of metallic structures by inducing compressive residual stress in the near-surface region. Measuring the depth profiling of residual stress distribution plays an important role in process design towards the advanced manufacturing of metallic structures. This study investigates the use of the nonlinear ultrasonic method to measure the residual stress profiles through the frequency-dependent penetration depth approach. Stainless steel specimens were treated with varying RHP intensities, and their morphological evolutions were characterized with a step-profilometer. The amplitudes of the second and third harmonic components of the longitudinal critically refracted (LCR) and Rayleigh waves were measured and analyzed. The effect of the surface roughness on the acoustic nonlinearity parameters before and after polishing at various peening intensities was briefly discussed. The results show a large variation in the acoustic nonlinearity parameter at the surface layer, indicating the potential of the proposed nonlinear ultrasonics for the measurement of depth profiling of residual stress.

Using longitudinal critically refracted waves to detect absolute stress of steel members considering the influence of temperature

Longitudinal critically refracted (LCR) waves are widely used to detect the absolute stress of steel members in service. The influence of temperature leads to deviation in absolute stress detection. However, existing methods compensate the detected stress by measuring the variations in the acoustic time at different temperatures in zero-stress specimens, which neglects the influence of temperature under different stresses. This study proposes a new ultrasonic method for detecting the absolute stress of steel members considering the influence of temperature. The most important feature of this method is considering the coupling between temperature and stress. A temperature sensitivity experiment was designed to compare the effects of temperature using a one-transmitter-one-receiver (OTOR) transducer group and a one-transmitter-double-receiver (OTDR) transducer group. The absolute stresses of three steel specimens (Q235B, Q345B, and Q390B) were detected at different temperatures to verify the proposed method. The experiment results indicate that the OTDR transducer group has a lower temperature sensitivity of 2.07 ns/°C compared with 14.16 ns/°C for the OTOR transducer group. Using the OTDR group can effectively reduce the negative influence of temperature and improve detection accuracy. The stress coefficient decreases with increasing temperature, and the temperature coefficient increases with increasing stress, which reflects the coupling of temperature and stress. The deviations of the proposed method are within 10 MPa and far less than the deviation of the traditional method. The proposed method can be used in practical steel structures at different temperatures.

A study on the effect of plasticity on the measurement of residual stresses using ultrasound technique

Residual stresses can have detrimental effect on the life cycle of engineering components and can affect the accuracy of the fatigue life prediction models. Accurate measurement of these stresses are therefore crucial. . However, destructive and semi-destructive methods of measuring residual stresses often cause damage to the components. In contrast, the non-destructive ultrasonic method allows for intact measurement of residual stresses even, in situation where portability is not feasible. Various waves can be utilized in the ultrasonic method for measuring residual stresses. In this study, longitudinal critically refracted (LCR) waves were employed due to their higher sensitivity, larger scanning area, and lower sensitivity to texture effect, making them preferable over other waves. The purpose of this study was to investigate the influence of plasticity on the measurement of residual stresses using the ultrasonic method. To achieve this, two stainless steel disks were subjected to quenching at temperatures of 300 °C and 700 °C to induce different levels of residual stresses and plasticity. The material properties of the disks were determined through standard tensile tests to facilitate numerical simulations. Tensile test specimens were extracted from the initial sheet to measure the yield stress and modulus of elasticity. The coefficient of acoustoelasticity, was determined by subjecting the tensile test specimens to different stress levels and recording the wave flight time. The coefficient of acoustoelasticity for stainless steel 316Lwas obtained using the slope of and (E(t-t0))/t0,. Residual stresses were experimentally measured using both the ultrasonic method and the center-hole drilling method, and the results were compared with those obtained from numerical analyses. The findings from center-hole drilling method, ultrasonic method, and numerical simulation were then compared and discussed. Different levels of plasticity were considered, and the accuracy of measuring residual stresses was analyzed and evaluated.

Assessment of residual stresses in railway rails using ultrasonic and Barkhausen noise techniques

Residual stress, a factor affecting the fatigue and fracture characteristics of rails, is formed during the processes of fabrication and heat treatment, and is also generated by shrinkage/tension due to the temperature differences and vertical loads on wheels due to the weight of vehicles. Moreover, damage to rails tends to accelerate due to residual stress caused by the continuous increase in the number of passes and the high speed of passing vehicles. Because this can have a direct effect on safety accidents on railroad rails, having a technique to evaluate and analyse the residual stresses in rails accurately is very important. In this paper, firstly, residual stresses applied in rails were evaluated using longitudinal critically refracted (LCR) wave. Non-destructive stress measurement using ultrasonic techniques is based on calculation of the acoustoelastic coefficient obtained from the relationship between material stress and LCR wave velocity. This is because LCR waves exhibit a relatively large change in flight time in relation to a change in stress compared to other ultrasonic modes. Barkhausen noise technique is also very suitable for measuring residual stresses on railway rails, which are ferromagnetic materials. Tensile and compression tests were performed with the specimens extracted from the inspection object, and the difference between the ultrasonic speed and the magnetization amplitude according to the applied stress was measured. Then, the feasibility of calculating the internal stresses in railway rails using the reference data obtained from the extracted specimens was determined, and the reliability of each method was verified. Furthermore, based on these results, the difference in the results for the loads asymmetrically applied according to the wheel shape was analysed by measuring for each part of new and used rails.

Research on an Ultrasonic Longitudinal Critically Refracted Wave Detection Method for the Depth Distribution of Stress

Aluminum alloy components typically have structural characteristics such as large size and complex shape, making the in situ non-destructive detection of internal residual stress in these structures a challenge that the manufacturing sector has tried to solve. Ultrasonic longitudinal critically refracted (LCR) waves have shown good sensitivity to normal stress in the horizontal direction and could be used to detect the distribution of internal residual stress in components, offering an advantage not shared by other detection methods. In this study, we investigated the propagation mode of LCR waves in a 2A14 aluminum alloy component and established the characterization model of the average normal stress of LCR waves in different depth ranges. The blocking effect of LCR waves by a groove with a depth equal to twice the wavelength was analyzed and experimentally verified using a machined aluminum alloy test specimen. Then, the propagation depths of LCR waves in the aluminum alloy at different frequencies were determined. A load test on a cantilever beam based on the stress depth distribution model was designed, and the stress characterization model and LCR waves’ propagation depth were further verified by the self-developed LCR wave stress detection system. The test results showed that the LCR wave could accurately detect the depth distribution of stress and could serve as a useful tool for evaluating the depth distribution of normal stress inside aluminum alloy components.

Quality index for friction stir welds in 7050 aluminum plates

AA7050-T7451 is a heat-treating aluminum alloy with high mechanical strength, ideal for producing lightweight structural components. Common types of welding lead to structural changes in the material that could decrease their mechanical properties affecting the fatigue life of the joints; however, a solid-state process known as friction stir welding (FSW) have been increasingly used as an alternative solution to reduce these severe effects. This work attempts to study the FSW behavior over 7050-T7451 aluminum alloys through the characterization of the resulting residual stresses and propose a manufacturing quality index based on their magnitude and distribution. An ultrasonic method employing longitudinal critically refracted (LCR) waves was used to determine the longitudinal residual stresses at different distances from the centerline in plates welded with different parameters.The resulting stress distribution is consistent with the literature review, presenting maximum values around 200 MPa at the center of the weld. A quality index was calculated through the average of all stresses in that region. It was found that higher residual stresses are produced with higher traverse and rotational speed (400 mm/min and 908 RPM). Therefore, to improve the process regarding the level of residual stresses, it is recommended to use lower welding tool velocities.

Analysis of the directivity of longitudinal waves based on double-fold coil phased EMAT

Longitudinal critically refracted (LCR) waves have already been widely applied for residual stress characterization. Such waves are usually generated using mode-conversion at the first critical angle of the incident longitudinal wave, which gives waves that then propagate at a dip-angle, and this places energy close to the surface of the specimen. The dip-angle needs to be minimized to improve both velocity measurement and residual stress characterization sensitivity. This paper reports a novel double-fold coil phased EMAT that can decrease the dip-angle. The performance of this new EMAT was investigated using both a COMSOL model and experiments. Initial model validation was provided through a comparison with experimental data. The EMAT design also enables scanning of samples, and operation in harsh environments where use of a PZT based transducer and couplants can complicate and limit inspection. The use of the EMAT has the potential to give more accurate time of flight (TOF) data and enhances the reliability and accuracy for residual stress measurement.

Reliability verification of stress data from extracted specimens using LCR wave stress data from full-section rail specimens

Non-destructive stress measurement using ultrasonic techniques is based on calculation of the acoustoelastic coefficient obtained from the relationship between material stress and sound wave velocity. Longitudinal critically refracted (LCR) waves are the most suitable for ultrasonic stress measurement testing. This is because these waves exhibit a relatively large change in flight time in relation to a change in stress. In general, to calculate the acoustoelastic coefficient, tensile tests are performed with specimens extracted from the test objects to be inspected, and the difference in the speed of ultrasonic waves is measured in relation to the stress applied. However, train rails are usually subjected to compressive loads and no research has been conducted to confirm whether the results obtained using this method match the coefficients obtained from test objects in full cross-section. Therefore, results from compression tests on full cross-section specimens are needed as reference data to determine the reliability of the results obtained using extracted specimens. In this study, experiments were conducted to compare the result from applying compressive loads to full cross-section specimens to the experimental result obtained from specimens extracted from each rail section. For this purpose, ultrasonic probes capable of generating LCR waves were designed and manufactured to apply the technique to measure the applied stress. Based on the results obtained using this probe, the acoustoelastic coefficient values obtained from each specimen were compared. Then, the feasibility of calculating the internal stresses in railway rails using the acoustoelastic constants obtained from the extracted specimens was determined, and the reliability of the method was verified. It was concluded that there was no significant difference in the acoustoelastic coefficients calculated using the data from extracted specimens and those using full cross-section specimens.

Comparison of longitudinal critically refracted ultrasonic waves and magnetic Barkhausen noise for tensile stress measurement

Longitudinal critically refracted (LCR) ultrasonic waves and magnetic Barkhausen noise (MBN) are two commonly used methods for measuring the residual stress of steel. Both of these methods have distinct advantages and disadvantages that need to be taken into consideration when selecting the most appropriate technique for measuring residual stress under specific circumstances. This issue is experimentally investigated in this paper by comparing the linearity, sensitivity and repeatability of these two measurement techniques for evaluating the stress within Q345 steel samples subjected to different levels of applied tensile stress. The time-of-flight (TOF) of LCR waves is demonstrated to conform almost perfectly with the applied stress in a linear relationship, while a similarly perfect confirmation is obtained for the root mean square (RMS) voltage of MBN signals to a quadratic curve. The results demonstrate that the RMS voltage of MBN signals is much more sensitive than the TOF of LCR waves to variations in surface stress, where the application of a 240 MPa tensile stress increases the RMS voltage of MBN signals by about 140% relative to that of the unstressed sample, while the TOF of LCR waves increases by less than 0.15% under corresponding conditions. Multiple testing of the same specimen demonstrates that MBN measurements more closely coincide than those of LCR waves. Therefore, MBN exhibits superior repeatability to LCR waves.

Longitudinal critically refracted (LCR) ultrasonic wave for residual stress measurement

The article deals with the state-of-the-art in the field of Longitudinal critically refracted (LCR) ultrasonic wave, for non-destructive material evaluation. It checks its capability for residual stress identification, and reviews positives and negatives related to its use. Obtained information within the article, are used for the understanding of essence of method and for the evaluation of its use in the engineering practice. The article can be the source of information about the LCR wave measurement technology, which is the part of the complex ultrasonic testing method. For the frequency of using this technology for surface residual stress measurement, it is appropriate to have this information in one whole, which are gathered of the outputs of researches by various authors. The paper is divided in few sections and sub-sections. In the first section, information about LCR wave technique and factors correlated with this method, are provided. The next section writes about residual stresses and the importance of their identification. Next, the principal of residual stresses measurement and basic structure of measurement device, is described. A significant part of study, describes the state so far of theoretical and practical researches within the use of this method, in the technological practice of residual stress identification in surface layers of engineering components. In the conclusion, obtained knowledges are summarised and evaluated. Related positive and negative aspects are included, with a verifying the need of future researches.

Experimental Measurement of Residual Stress Distribution in Rail Specimens Using Ultrasonic LCR Waves

Longitudinal critically refracted (LCR) waves are considered bulk longitudinal waves and penetrate into an effective depth beneath the surface parallel to the material surface. Such LCR waves can be employed to measure residual stresses because the acoustoelastic effect is the basis for ultrasonic residual stress measurements. This effect is described by the relationship between change of wave travel time and stress applied when such waves propagate in a stressed medium. In this paper, stresses applied in a rail were evaluated by using a developed LCR probe. With this transducer, it was verified how the difference in the arrival times of the LCR waves showed a trend as the tensile stresses increased. The acoustoelastic coefficients were calculated using the relationship between the stresses and the travel times, and the residual stresses of the used rails were measured using these coefficients. In addition, the difference in residual stress distribution according to the characteristics of the wheel-rail contact surface was analyzed from the obtained residual stress value. It was concluded that this non-destructive evaluation technique using LCR waves could be employed for accurate stress measurement of rails because differences in stress applied to the rail can be detected.

Denoising of LCR Wave Signal of Residual Stress for Rail Surface Based on Lifting Scheme Wavelet Packet Transform

According to the acousto elastic effect, the residual stress on the surface of the rail can be evaluated by measuring the change in the propagation velocity of ultrasonic waves, such as longitudinal critically refracted (LCR) waves on the surface of the rail. The LCR wave signal is often polluted by a variety of noise sources, coupled with the influence of the poor surface condition of the inspected component, which greatly reduces the detectability and online measurement ability of the LCR wave signal. This paper proposes the application of the lifting scheme wavelet packet transform (LSWPT) denoising method to solve the noise suppression problem of LCR wave signal. The traditional wavelet transform (WT), wavelet packet transform (WPT), as well as the lifting scheme wavelet transform (LSWT) and lifting scheme wavelet packet transform are compared and analyzed in the soft thresholding and hard thresholding processing of denoising ability and efficiency of the noisy LCR wave signal. The experimental results show that the LSWPT method has the characteristics of fast calculation speed and a good denoising effect, and it is an efficient method of denoising signals for on-line ultrasonic measurement of residual stress on the rail surface.

Analysis of the directivity of Longitudinal Critically Refracted (LCR) waves

The use of ultrasonic longitudinal critically refracted (LCR) waves is one approach used for near surface material characterization. It has been shown to be sensitive to stress and, in general, less sensitive to the effects of the texture of the material. Although the LCR wave is increasingly widely applied, in experiments the factors that influence the formation of the LCR beam are seldom discussed. This paper reports a new numerical model used to investigate the transducers’ parameters that can contribute to the directionality of the LCR wave and hence enable performance optimization when used for industrial applications. An orthogonal experimental method is used to study the sensitivity to the transducer parameters which influence the LCR wave beam characteristics. This method provides a design tool used to study and optimize multiple parameter experiments and it can identify which parameter or parameters are of most significance. The effects of incident angle, the aperture and the center frequency of the transducer were all studied. It is shown that the aperture of the transducer, the center frequency and the incident angle are the most important factors in controlling the directivity of the resulting LCR wave fields. The model was validated by comparision of data to those obtained with a finite element model. Experiments were also performed to confirm the numerical results. The model and experimental data provided improve understanding of the transducer selection and positioning in the optimization of LCR wave fields in experiments, which can be used to give signals which exhibit higher sensitivity for near-surface stress characterization.

Study on Propagation Depth of Ultrasonic Longitudinal Critically Refracted (LCR) Wave.

The accurate measurement of stress at different depths in the end face of a high-pressure compressor rotor is particularly important, as it is directly related to the assembly quality and overall performance of aero-engines. The ultrasonic longitudinal critically refracted (LCR) wave is sensitive to stress and can measure stress at different depths, which has a prominent advantage in stress non-destructive measurements. In order to accurately characterize the propagation depth of LCR waves and improve the spatial resolution of stress measurement, a finite element model suitable for the study of LCR wave propagation depths was established based on a wave equation and Snell law, and the generation and propagation process of LCR waves are analyzed. By analyzing the blocking effect of grooves with different depths on the wave, the propagation depth of the LCR wave at seven specific frequencies was determined in turn. On this basis, the LCR wave propagation depth model is established, and the effects of wedge materials, piezoelectric element diameters, and excitation voltages on the propagation depth of LCR waves are discussed. This study is of great significance to improve the spatial resolution of stress measurements at different depths in the end face of the aero-engine rotor.

Subsurface Stress Measurement in GIS Epoxy Composite by Using LCR Waves

Internal stress in basin insulators of gas-insulated metal-enclosed switchgear (GIS) can lead to cracks, which affects the safe operation of these apparatuses. In this research, we proposed a subsurface internal stress measurement method for GIS epoxy composites. This method is based on an ultrasonic longitudinal critically refracted (LCR) wave technique. In this study, some epoxy composite specimens were synthesized with similar materials and manufacturing processes to those of 252 kV GIS basin insulators. An ultrasonic stress measurement system that utilized the LCR wave technique was set-up to investigate the relationship between stress and LCR wave propagation time, as well as to measure the compressive stress of the epoxy specimen within 0–50 MPa. The results show that LCR wave propagation time linearly decreased when stress increased in the subsurface zone and the acoustoelastic coefficient was −4.95. We found the relative errors of stress measurements to be less than 13%.