Summary:Ultrasonic nonlinear analysis, as a non-destructive testing technique, is mainly used for risk assessment of microcracks, weak bonding, and fatigue strength in materials. Unlike conventional ultrasonic nonlinear analysis and harmonic measurement of nonlinear coefficients, this paper proposes a new method for intuitive imaging of internal microcracks in materials using ultrasonic phased array nonlinear imaging. By detecting AISI304 and AL7075 specimens and comparing them with traditional ultrasonic phased array linear imaging, it is found that this method can effectively improve the contrast noise ratio and help detect internal microcracks in materials.

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

1. Research background

1.1 Nonlinear Ultrasonic Harmonic Analysis Method¹

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

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 or adhesive strength 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. However, this method cannot achieve fast and intuitive imaging, and requires corresponding calibration research on the measured material in advance to obtain the linear relationship between the nonlinear coefficient and the required evaluation variable before subsequent detection can be carried out. The bigger problem is that the nonlinear coefficient is very sensitive to temperature, residual stress, grain size, and the nonlinearity of the experimental equipment itself. It is difficult to achieve effective detection through variable control in actual engineering testing.

1.2 Principle of Nonlinear Ultrasonic Phased Array Imaging²

Nonlinear phased array ultrasound imaging is a method based on amplitude modulation imaging. The ultrasonic phased array nonlinear imaging method in this article is implemented through four steps:

Stimulate all chips to obtain linear images of ultrasonic phased array under full aperture

Excitation of odd numbered chips to obtain half aperture ultrasonic phased array images

Excitation of even numbered chips to obtain half aperture ultrasonic phased array images

By calculating the [full aperture] - ([odd order aperture]+[even order aperture]), a nonlinear image is obtained

Figure 1: Linear ultrasound imaging (left) and nonlinear phased array ultrasound imaging (right)

If the material exhibits acoustic linearity, the following calculation method should be used to obtain:

[Full aperture] - ([Odd order aperture]+[Even order aperture])=0 (2)

When the material exhibits acoustic nonlinearity

[Full aperture] - ([Odd order aperture]+[Even order aperture]) ≠ 0 (3)

2. Preparation for the experiment

2.1 Sample

2.2 Device

The sample was scanned and imaged using a programmable ultrasonic phased array system (AOS company, OEM PA128/128145V excitation) and an ultrasonic phased array probe (Imasonic company, 64 chip, component 1mm). Develop nonlinear imaging software by utilizing the openness of programmable ultrasound phased array systems.

Figure 4. OEMPA 128/128 Programmable Ultrasonic Phased Array System Test Device

3. Experimental methods and results

For sample 1, use a 64 chip 1mm phased array probe to scan in two ways

a. The probe scans parallel cracks along the X-axis without any angular deviation or dynamic focusing. (Δ z=57mm, Δ e=250um Step=1mm)

1. Probe Y-axis vertical crack scanning, without angular deviation and dynamic focusing. (Δ z=57mm, Δ e=250um step size 1mm)

Figure 5. Two scanning methods for sample 1

Figure 6. Comparison of Linear Ultrasound and Nonlinear Ultrasound (AMI) Phased Array Imaging (Left X-axis, Right Y-axis)

Figure 7. Comparison of Linear Ultrasound and Nonlinear Ultrasound (AMI) Phased Array Imaging

From the perspective of imaging effect, the nonlinear ultrasound phased array imaging is significantly better than linear imaging, with a contrast noise ratio 18-19 dB higher than linear imaging, and cracks are more easily detected from background noise.

4. Establishment of Nonlinear Ultrasonic Phased Array Model

For ultrasonic phased arrays, we change the number of excitation apertures to alter the excitation energy, resulting in varying degrees of nonlinear response. We assume that the nonlinear response Anl increases exponentially with the maximum aperture A. As the effective aperture decreases, the number of excitations increases and the nonlinear response strengthens. Assuming the following relationship exists:

                         A_nl=Aⁿ-N(A/N)ⁿ                                   (4) 

A represents the number of all array elements, and N represents the ratio of probe excitation aperture (Ratio)=maximum aperture/effective aperture.

chart9. Aperture ratioNdefinition

chart10. Schematic diagram of nonlinear response under different aperture ratios

Figure 11. Nonlinear response curve as a function of aperture ratio N

Through data fitting, it was found that when the index n=1.7, the nonlinear response of the crack conforms to the hypothesis published. From the graph, it can be seen that as the ratio increases, the nonlinear response gradually becomes stronger, and when N=16, the nonlinear response tends to saturate. This theoretical model provides a reference direction for future quantitative analysis.

Conclusion:

The nonlinear ultrasonic phased array amplitude modulation imaging method, as an effective supplementary means, can obtain more obvious responses from background noise when detecting microcracks. The imaging accuracy is consistent with conventional linear ultrasound imaging. And it is possible to use nonlinear ultrasound phased array imaging for quantitative analysis. It should be noted that exciting the nonlinear response of ultrasound requires the probe to emit sufficient energy. In the future, we will focus on studying the relationship between aperture ratio and nonlinear imaging, and further improve the hypothesis formula.

Thank you: This project was jointly completed by Sorbonne University in France, Northeastern University in Japan, and AOS Corporation in the United States.

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

2：Sylvain Hauperta, Guillaume Renauda,Andreas Schummb,  Ultrasonic imaging of nonlinear scatterers buried in a medium,NDT&E International 87 (2017) 1–6