Shunmin Yang

To address the challenge of detecting internal defects in medium-thick titanium alloy laser welds, a combined simulation and experimental study on ultrasonic testing was conducted. A finite element model employing a 5 MHz shear wave angle transducer for inspecting titanium alloy welds was established. An ultrasonic testing system was developed, incorporating a DPR300 pulser-receiver (JSR Ultrasonics, Pittsford, NY, USA) and an MSO5204 oscilloscope (RIGOL, Suzhou, China), and was calibrated using standard reference blocks. The inspection results for four prefabricated internal defects at various depths demonstrated that all defects were effectively detected, with the minimum detectable equivalent defect size reaching 1 mm. The measured signal-to-noise ratio (SNR) averaged 17.6 dB, validating the high sensitivity of the proposed system. The mean absolute error for defect localization was 0.438 mm, achieving a positioning accuracy better than 0.5 mm. This study indicates that the pro-posed method enables effective detection and accurate localization of internal defects in titanium alloy laser welds, providing critical technical support for laser welding quality assessment.

Most existing carbon fiber composite materials are formed by high-temperature molding of multiple layers of fiber cloth. During the manufacturing and usage processes, materials are prone to defects such as voids, delamination, and inclusions, which seriously threaten their service life and safety performance. Ultrasonic testing is currently a widely adopted method for detecting defects in carbon fiber composite materials. However, existing narrow-pulse ultrasonic transducers often have to sacrifice emission energy to achieve narrow-pulse emission, which results in their limited ability to penetrate thicker carbon fiber composite materials. To address this issue, this paper proposes the design of a focused laminated transducer. By stacking and bonding lead titanate piezoelectric wafers and using a concave lens made of organic glass to focus ultrasonic waves, the emission sound intensity of the ultrasonic transducer is enhanced. The simulation results show that the designed focused double-stack transducer has a directivity gain that is 4.49 dB higher than that of the traditional single-piezoelectric-wafer transducer. The transducer fabricated based on this design has successfully achieved effective detection of internal defects in carbon fiber composite materials.

The magnetostrictive linear displacement sensor is a sensor for measuring absolute displacements. The sensor makes use of the magnetostrictive effect and inverse effect to complete the excitation and reception of ultrasonic guided waves, and from the computed time-of-flight of the guided wave the position of the cursor is determined. To improve the measurement accuracy of the sensor, this paper optimizes the system structure and the signal processing method. Improvement made to the bias magnetic field gave the best transduction efficiency; copper rings were used for accurate control of the dynamic magnetic field range, which enhanced the signal amplitude and effectively reduced the hysteresis impact; two-segment receiving coils were used to improve the SNR; and zero-phase filters were used to remove echo signal noise and phase shift. The delay time difference of the echo signal was determined by the cross-correlation algorithm, which determined accurately the value of displacement. The experimental results put the maximum error of the sensor at $\pm 25 ~\mu {m} $ .

Critically refracted longitudinal (L_CR) waves are commonly used to evaluate the residual stress of a material. The utilization of L_CR waves is advantageous in that these waves are not sensitive to the texture of the material. Thus, L_CR can be considered as a bulk longitudinal mode and can penetrate into the material well below its surface. However, while measuring the residual stress, the precision of the L_CR wave travel-time is influenced by several uncertain factors. In order to further improve the accuracy of test results, we developed a measurement approach based on three aspects. First, the distances between the transmitter and the receivers were determined by transducer signal analysis. Second, for the residual stress detection to be consistent, transducers with different frequencies presented similar beam divergence angles. Finally, three different frequencies were used to evaluate the residual stress throughout the plate thickness. Based on the results of the above analysis, we used L_CR waves to test 304 stainless steel plates. The detection error of residual stress measurement was ca. ±23 MPa. When compared with the X-ray diffraction approach, our method showed similar trends for the same regions of the specimens.