Ding Xia shaw * teclab.cn Ruitaike (China) Co., Ltd

Summary:Ultrasonic nonlinear analysis, as a non-destructive testing technique, is mainly used for risk assessment of microcracks, weak bonding, residual stress, and fatigue strength in materials. Unlike conventional ultrasound based on time-domain signal amplitude detection technology, nonlinear ultrasound detection technology is based on non-destructive evaluation of fundamental frequency and harmonic frequency intensity. This article uses an HPA high-power amplifier to excite the fundamental frequency probe with a Hanning window sine signal, and analyzes the nonlinear coefficients of rock samples by receiving the transmitted signal through a broadband harmonic probe. Verified the feasibility of the ultrasonic nonlinear research platform based on HPA high-power amplifier.

key word:Nonlinear ultrasonic phased array microcrack non-destructive testing

1. Research background

The conventional nonlinear ultrasonic harmonic analysis method is based on second harmonic detection. By exciting the tested material with high-energy ultrasound, the nonlinear coefficients β of the fundamental and second harmonic analysis materials are measured

（1）

A1 and A2 are the absolute amplitude values at the fundamental and second harmonic frequencies of the received signal, respectively. K=2 π/λ wavenumber.

Through controlled variable experiments, it can be found that there is a linear relationship between the nonlinear number β and the fatigue strength, adhesive strength, microcracks, residual stress, etc. of the material within a certain range. Thus achieving non-destructive evaluation of weak adhesion and fatigue strength of materials, and even predicting the lifespan of materials. This article designed an ultrasonic nonlinear testing platform for rock samples to verify the feasibility of the platform. A 250 kHz fundamental frequency probe is used to excite ultrasonic signals in rock samples, and a 500 kHz broadband ultrasonic transducer is used to receive fundamental and harmonic frequency signals in transmission detection mode. By applying a uniaxial external load perpendicular to the direction of sound wave propagation in the middle of the rock sample, the variation of the acoustic nonlinear coefficient of the rock sample under different loads was measured to obtain the variation law of the acoustic nonlinear coefficient of the rock sample under different loads.

2. Experimental design

The HPA high-power amplifier used in this experimental platform has a maximum output of 1500 Vpp, a maximum power of over 5 KW, and a bandwidth ranging from 50 kHz to over 5 MHz. The excitation probe is a narrowband 250 kHz center frequency piezoelectric composite transducer, and the receiving probe is a wideband piezoelectric composite transducer with a center frequency of 500 kHz (bandwidth>70%). The length of the rock sample is 10 cm. To minimize nonlinear effects in the experimental setup that do not originate from rock samples, consider:

1. Select a 10 period sine signal with a Hanning window as the excitation signal to make the frequency of the excitation signal purer (as shown in Figures 3 and 4);

2. Due to the significant nonlinear effects caused by water or coupling agents containing water, phenyl salicylate (C) was used₁₃H₁₀O_3，A white powder crystal with a melting point of 42 ℃ was used as a coupling agent to bond two probes and the rock sample.

Figure 1. Acoustic Nonlinear Experimental Platform

3. Experimental process

By testing the frequency spectrum of the excitation signal, it was found that adding a Hanning window to a sine signal can make the frequency components of the signal cleaner (as shown in Figures 3 and 4). Therefore, the experiment used a signal generator to generate this signal as the excitation signal, which was input into an HPA high-power amplifier to amplify it to greater than 1000 Vpp and excite a 250 kHz narrowband ultrasonic transducer.

Figure 2. 10 period 250kHz sine time-domain signal and its spectrum

Figure 3. Time domain signal and spectrum of 10 period 250kHz sine wave (with Hanning window)

By collecting the projected ultrasound signal of the rock sample through an oscilloscope and performing FFT analysis, the fundamental frequency and harmonic peaks can be observed. By applying uniaxial external loads perpendicular to the propagation direction in the middle of the rock sample: 33.5 N, 67 N, 95.5 N, 174.5 N, 256 N. After each external load is applied, the current projected time-domain signal is measured and saved without changing any other parameters or experimental conditions.

Figure 4. Time domain signal and spectrum of rock sample transmission

4. Experimental results

By applying 5 different external loads, a total of 6 sets of experimental data were obtained. According to formula 1, the relative acoustic nonlinearity coefficient can be calculated to obtain the following curve (Figure 5). It can be observed that when the applied external load increases by an equal amount, the nonlinear coefficient forms a relatively intact linear relationship with the external load. The average increase in external load applied in the first three times is about 31.8 N, and the average increase in external load applied in the last two times is about 80.25 N. The relative nonlinearity coefficient has an average increase of about 1.41X10 in the first three times^-3,The average increase in the last two times is about 0.55X10^-3.

Figure 5. Nonlinear coefficient variation curve under uniaxial external load

5. Conclusion

The acoustic nonlinear measurement system based on HPA high-power amplifier has achieved the measurement of acoustic nonlinear coefficients of rock samples and obtained the relationship curve between external loads and nonlinear coefficient changes. The entire system is simple and practical, with high cost-effectiveness. This experiment is only to verify the feasibility of the system. Although nonlinear effects outside the detection object were excluded as much as possible in this experiment, due to the fact that nonlinear effects are related to many factors (such as environmental temperature, load application method, etc.), further improvement of experimental conditions is needed for more accurate measurements.

reference:

1: Zhou Zhenggan, Liu Siming Nonlinear Ultrasonic Non destructive Evaluation Method for Initial Plastic Deformation and Fatigue Damage of Aluminum Alloy, Journal of Mechanical Engineering, 2011, 47 (6), DOI：10.3901/JME.2011.06.001