A new stress measurement method using single source high-frequency cylindrical guided waves is put forward to promote the efficiency and precision of the contact-type ultrasonic testing technique. The propagation patterns and acousto-elasticity of cylindrical guided waves, traveling in a loaded steel rod, are analyzed under the theoretical framework of the Pochhammer-Chree equation and nonlinear acoustics. According to the theoretical derivations, the ratio of the maximum group velocities of different modes is selected as the measurement criterion of stress. Then, a stress calibration/measurement experiment is conducted to prove the validity of the method. A single-source ultrasonic transducer driven with an impulsive excitation is introduced in the experiment and the generated multimode guided waves in the received signals are analyzed. It is observed from experimental results that the group speed ratio of two different modes is linearly proportional to the axial stress and the evaluation errors of the proposed method is less than 6%.

In this work, we propose a novel coarse-to-fine method for object pose estimation coupled with admittance control to promote robotic shaft-in-hole assembly. Considering that traditional approaches to locate the hole by force sensing are time-consuming, we employ 3D vision to estimate the axis pose of the hole. Thus, robots can locate the target hole in both position and orientation and enable the shaft to move into the hole along the axis orientation. In our method, first, the raw point cloud of a hole is processed to acquire the keypoints. Then, a coarse axis is extracted according to the geometric constraints between the surface normals and axis. Lastly, axis refinement is performed on the coarse axis to achieve higher precision. Practical experiments verified the effectiveness of the axis pose estimation. The assembly strategy composed of axis pose estimation and admittance control was effectively applied to the robotic shaft-in-hole assembly.

The detection of wear condition is critical for the prediction of mechanical equipment failure and the avoidance of catastrophic accidents. A large-diameter inductive debris detection sensor was previously presented to satisfy the requirement of high throughput, and the characteristics of induced voltage generated by single debris were analyzed. However, in practice, plenty of wear particles carried by lubricant flowing through the sensors probably form the aliasing signal, thus affecting the evaluation of concentration and size of particles. In order to analyze the mechanism of aliasing, this paper studies the influence of the motion state of wear debris, such as velocity, relative spatial position and trajectory, on the waveform of the aliasing induction signal. The experimental results show that the aliasing induced voltage is proportional to the debris velocity and magnetic induction intensity. Additionally, the voltage peak-peak of aliasing signal is approximately proportional to the radius of the debris instead of the volume, from which the relational expression about the aliasing signal and component singles derives. Finally, the theoretical results and experimental results are performed to validate this relation.

Accurate assessment and monitoring of axial stress during the service life of the bolt connection is of great significance in ensuring the stability and reliability of the aerospace structure. The current acoustoelastic method requires both transverse and longitudinal mode waves and is less sensitive to small-size bolts, resulting in relatively larger errors in the evaluation of axial stress. In this paper, an axial stress measurement method with only a single longitudinal/transverse transducer for tightened bolts based on a novel attenuation-based model is introduced. The ultrasonic responses under axial preload in different frequency bands are comprehensively reflected through the energy attenuation coefficient matrix. In addition, an experimental method for selecting the optimal frequency band is given. Furthermore, the semi-permanent coupling method is proposed to provide a stable coupling condition to verify the feasibility of the attenuation-based model. Loading tests and ultrasonic tests were conducted on two kinds of strength-grade materials to obtain the calibration curves of both the longitudinal and transverse attenuation models. The experimental results show that the energy attenuation coefficient is positively correlated with the axial stress in a specific frequency range and the calibrated curve has a high fitting rate with a quadratic fitting function. The average relative error of the prediction results is less than 7.5%, which is better than the conventional acoustoelastic effect model.

Based on the scattering attenuation theory of loaded polycrystalline metals, a bolt axial stress measurement method using ultrasonic echo attenuation is proposed for the problem that the conventional time of flight (TOF) method is difficult to measure the stress of high-strength short bolts. Firstly, the attenuation coefficients of body-centred cubic crystalline materials in the Rayleigh scattering range are described, and the mathematical model between the ratio of the peak of the two bottom echoes and the axial stress of the bolt is further derived. Then, the effects of the transducer wafer diameter, mounting eccentricity and bolt thread on the ultrasonic signal were analysed by finite element simulation, to provide a strategy for transducer selection and mounting. In addition, a magnetic ring clamping device was designed to ensure a tight fit between the transducer and the bolt, and to improve the alignment of both. Finally, a bolt axial stress ultrasonic measurement system was built, and a comparison experiment between TOF method and attenuation method was conducted to predict the axial stress of bolts. The experimental results shown that the average percentage measurement error of the attenuation method proposed in this paper is 4.42%–5.76% lower than that of the conventional TOF method, which is more suitable for measuring the axial stress of bolts.

The reliability and stability of a mechanical system generally counts on the effective detection and control of the stress state of its bolted connections. In this paper, a novel bolt axial stress evaluation method based on the scattering attenuation of polycrystal using the energy ratio of multiple echoes is proposed. Above all, the longitudinal-incident attenuation coefficients for preload cubic crystalline materials in the Rayleigh regime are introduced. In view of the estimation of frequency-dependent attenuation coefficients are prone to suffer from the disturbance of the cylindrical guided waves, the energy ratios of the first two back-end echoes within the signals are applied instead. Subsequently, a stress measurement experiment is conducted to validate the proposed method, in which the accurate energies of the echoes are evaluated using the S transform. The results reveal that, as long as the probe is firmly bonded, the proposed method can achieve better sensitivity and reliability than that of the existing methods using TOF (Time of flight), especially for short or full-threaded bolts.