Nondestructive Testing and Characterization of Residual Stress Field Using an Ultrasonic Method

To address the difficulty in testing and calibrating the stress gradient in the depth direction of mechanical components, a new technology of nondestructive testing and characterization of the residual stress gradient field by ultrasonic method is proposed based on acoustoelasticity theory. By carrying out theoretical analysis, the sensitivity coefficients of different types of ultrasonic are obtained by taking the low carbon steel（12%C） as a research object. By fixing the interval distance between sending and receiving transducers, the mathematical expressions of the change of stress and the variation of time are established. To design one sending-one receiving and oblique incidence ultrasonic detection probes, according to Snell law, the critically refracted longitudinal wave（LCR wave） is excited at a certain depth of the fixed distance of the tested components. Then, the relationship between the depth of LCR wave detection and the center frequency of the probe in Q235 steel is obtained through experimental study. To detect the stress gradient in the depth direction, a stress gradient LCR wave detection model is established, through which the stress gradient formula is derived by the relationship between center frequency and detecting depth. A C-shaped stress specimen of Q235 steel is designed to conduct stress loading tests, and the stress is measured with the five group probes at different center frequencies. The accuracy of ultrasonic testing is verified by X-ray stress analyzer. The stress value of each specific depth is calculated using the stress gradient formula. Accordingly, the ultrasonic characterization of residual stress field is realized. Characterization results show that the stress gradient distribution is consistent with the simulation in ANSYS. The new technology can be widely applied in the detection of the residual stress gradient field caused by mechanical processing, such as welding and shot peening.

TECHNOLOGY OF QUANTITATIVE ANALYSIS ON VIBRATORY STRESS RELIEF BASED ON ULTRASONIC TIME-OF-ARRIVAL METHOD

The effect of vibratory stress relief （VSR） is usually evaluated with the indirect method of observing the change of amplitude frequency response characteristics of structures. A new kind of evaluating method of VSR based on the ultrasonic time-of-arrival method （UTM）, which can obtain the residual stress directly through measuring the propagation time of ultrasonic wave in the material, is presented. At first, the principle of the measuring method of residual stress based on UTM is analyzed. Then the measuring system of the method is described, which is in virtue of ultrasonic flaw detector and high-sampling-rate digital oscillograph. And a set of calibration system that contains a piece of standard specimen is also introduced. Experimental results prove the relation between the residual stress and the propagation time of ultrasonic in workpieces. Finally, the measuring and calibration systems are applied in evaluating the effect of VSR. The final test results show that the method is effective.

Nonlinearity analysis of piezoelectric micromachined ultrasonic transducers based on couple stress theory

This paper studies the static deformation behavior of a piezoelectric micromachined ultrasonic transducer （PMUT） actuated by a strong external electric field. The transducer membrane consists of a piezoelectric layer, a passive layer and two electrode layers. The nonlinearities of the piezoelectric layer caused by electrostriction under a strong electric field are analyzed. Because the thickness of the transducer membrane is on the microscale, the size dependence of the deformation behavior is evaluated using the couple stress theory. The results show that the optimal ratio of the top electrode diameter and the membrane diameter is around 0.674. It is also found that this optimal value does not depend on any other parameters if the thicknesses of the two electrodes are negligible compared with those of the piezo- electric and passive layers. In addition, the nonlinearities of the piezoelectric layer will become stronger along with the increase of the electric field, which means that softening of the membrane stiffness occurs when a strong external electric field is applied. Meanwhile, the optimal thickness ratio for the passive layer and the piezoelectric layer is not equal to 1.0 which is usually adopted by previous researchers. Because there exists size dependence of membrane deforma-tion, the optimal value of this thickness ratio needs to be greater than 1.0 on the microscale.

Reduction of internal stress in SU-8 photoresist layer by ultrasonic treatment

At present, high internal stress in SU-8 photoresist layer has become one of the most primary problems and restricted structure stability and higher aspect ratio of SU-8 photoresist. However, the existing studies all focused on optimizing the process pa-rameters or mask design, and could not be popularized. The purpose of this paper is to reduce the internal stress in cured SU-8 photoresist layer by ultrasonic stress relief technology. The ultrasonic stress relief mechanism in SU-8 photoresist layer was presented. Based on improved Stoney’s formula, the profile mothod calculation model for SU-8 internal stress was proposed. The effect of ultrasonic stress relief on SU-8 layers was studied by experiments. Some important factors, such as amplitude of vibration, power input and relief time, were discussed. The values of internal stress before and after the ultrasonic stress relief process were compared. The research results show that the internal stress in cured SU-8 layer can be reduced effectively if the proper experimental parameters are chosen, and ultrasonic stress relief technology in nonmetal field has some practical value.

I– Ions as Obstacles to Dislocation Motion in NaCl:I– Single Crystals

Strain-rate cycling tests associated with the ultrasonic oscillation were conducted for the purpose of investigation on the interaction between dislocation and I– ions during plastic deformation of NaCl:I– (0.5 mol% in the melt) at 77 K to room temperature. The relative curves of stress decrement (Δt) due to the oscillation and strain-rate sensitivity ( ) have stair-like shape for NaCl single crystals doped with I– at low temperatures. There are two bending points and two plateau regions. λ decreases with Δt between the two bending points. τp at Δt of first bending point and λp between λ at first plateau place and at second one depend on the dopant ions as weak obstacles to dislocation motion. Not only temperature dependence of τp and λp but also τp versus V (activation volume) reflects the interaction between dislocation and I– ions. On the basis of the data (i.e. τp and λp) analyzed in terms of the relative curves of Δt and λ, the activation energy, G0, for the overcoming of dislocation from the dopant ion is found to be 0.47 and 0.53 eV for NaCl:Br– and NaCl:I–, respectively. This result that G0 for NaCl:I– is somewhat larger than for NaCl:Br– leads to the phenomenon that I– ions are slightly stronger than Br– ones as weak obstacles to dislocation motion because of the difference between isotropic strains around I– ion and around Br– in NaCl single crystal. Furthermore, the values of τp0 and Tc are also obtained for the two kinds of specimens. τp0 and Tc are the value of τp at absolute zero and critical temperature at which τp becomes zero.