Ultrasonic resonance-based inspection of ultra-thin nickel sheets bonded to silicone

In the field of non-destructive testing (NDT), The detection of bonding defects between ultra-thin metal and silica gel is a difficult problem. In this study, In this study, ultrasonic resonance method was used to evaluate the bonding strength of ultra-thin metal to silica gel bonding structure. The composite parts of ultra-thin nickel sheet and silicon sheet with three different bonding states were studied. The bonding state of nickel sheet and silica gel is different, and the absorption of ultrasound is different. Using the resonance generated by high-frequency ultrasound in ultra-thin nickel sheet, the acoustic attenuation of the combination of ultra-thin nickel sheet and silicon rubber sheet was analyzed by resonance signal, and the bonding state between ultra-thin nickel sheet and silicon rubber sheet was characterized by bonding coefficient. Through experimental comparison, the results showed that the attenuation of ultrasonic signal in the nickel sheet and silicon film with different adhesive states characterize the adhesive state of ultra-thin nickel sheet and silicon film by the bonding coefficient, the bonding coefficient of good parts, weak adhesive parts and debonded parts is reduced successively. By setting an appropriate determination threshold value, the bonding state between the ultra-thin nickel sheet and the silicon film can be accurately determined according to the bonding coefficient obtained by detection.


Introduction
Nickel sheet and silicone composite parts made of the composite conductive sheet in the automotive and aerospace industries have a wide range of applications. During the bonding process, due to the pollution ( such as dust particles) on the nickel sheet or silica gel bonding surface or the uneven coating of the binder, defects such as debonding and weak bonding will occur, affecting the stability of the product [1][2][3].
The bond strength between different materials can be evaluated by x-ray [4], Lamb wave [5] and ultrasonic [6][7][8][9]. Jasiūnienė [4] used x -ray to scan the composite fault, To evaluate the effect of the adhesive layer pores on the properties of the adhesive layer, but this method requires the destructive testing of the sample; Ren et al [5] used Lamb wave to study the metal bonding layer. However, in practical application, the change of various structural dispersion curves will limit the lamb wave; Titov S A et al [10] using the ultrasonic pulse echo method, based on the plane wave model, the bond strength of two thin metals was detected. It was found that the ultrasonic echo signal of the bond was the sum of two responses, which were related to the ultrasonic reflection signals of the first metal-binder interface and the second metal-binder interface of the bond. By comparing the pulse echo signal of the test area with the echo signal of the first metal layer of the adhesive, the bonding state of the adhesive can be effectively judged. This method can detect the bonding strength of the adhesive with a single layer thickness of 0.1 mm-1 mm. Challis R E et al and Robinson A M et al [11][12][13] found that the attenuation rate of ultrasonic signal is greater when the composite is well bonded, and developed a signal processing method based on adaptive inversion. Although the algorithm can detect the bonding strength of the bonded part, it is only suitable for thicker composite parts. When the composite part is thinner, the echo signal will overlap, which Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
will make the detection very difficult. Jeenjitkaew C et al [14] When the bond strength of the composite piece changes, The lateral lateral strain of the adhesive layer will change accordingly, but this method is only suitable for large artifacts; Guyott c.c. C H et al [15] used the ultrasonic pulse echo method to determine the bonding strength of the composite by comparing the amplitude of the echo signal, but the intensity of the echo signal will be affected by the pressure strength between the fusion agent and the ultrasonic probe and the test piece. Goglio L et al [16] used the ultrasonic pulse echo method to analyze the ultrasonic echo signal of the composite part, calculated the reflection coefficient of the composite part with good bonding and bonding defects, and used statistical analysis to analyze the reflection coefficient, set the appropriate confidence to judge the quality of the composite part. The study found that this method is applicable to the composite part with a single layer thickness of 2 mm-20 mm. However, when calculating the reflection coefficient, the acoustic impedance of the composite part is difficult to calculate, and the method depends on selecting the appropriate confidence, so it is difficult to use in practical engineering.Wang et al [17] used the air-coupled ultrasonic analysis model to calculate the interface stiffness and lateral stiffness to evaluate the bond strength of the composite. However, the results are not always consistent with experimental studies. Due to the inhomogeneity and diversity of composites, it is unlikely to be applied to composites. Due to the high attenuation, high wavelength and low bandwidth characteristics in the air, the application of air-coupled ultrasound in evaluating bonding quality is limited, especially for composite bonding structures. Wu et al [18] recently used the air-coupled ultrasonic nondestructive testing system to calculate the interface stiffness of the adhesive layer during the curing process of the composite adhesive structure. However, the sensitivity of this study is limited to curing-related defects, and the accuracy needs to be improved.
The detection objects involved in the above studies are all adhesive bodies with single layer thickness over 0.1 mm, and there is no effective destructive assessment method for the bond between single layer thickness less than 0.1 mm, especially the bond between ultra-thin metal and non-metal.
The purpose of this study is to use high-frequency ultrasonic resonance method to detect the bonding strength between nickel sheet with thickness of 0.07 mm and silicon sheet with thickness of 0.47 mm. Under laboratory conditions, three kinds of composite parts with different bonding strength are studied. The results show that high-frequency ultrasonic resonance method can achieve effective non-destructive evaluation of the bonding state between ultra-thin metal and non-metal.

Materials and methods
The composite formed by the nickel sheet and the silicone is shown in figure 1.
The ultrasound was detected by water immersion pulse-echo. In the experiment, NI Company PCI-51354 dual-channel high-speed A/D acquisition card (the highest sampling frequency is 2GHz) was used to collect echo signal and Olympus 5072PR ultrasonic signal transmitting receiver. The center frequency of the ultrasonic probe was 50 MHz, the effective detection area diameter was 10 mm, and the probe distance was 1 mm from the surface of the nickel sheet. The ultrasonic incident from the nickel sheet end. The sound speed in nickel is 5600 m s −1 , and the ultrasonic wavelength in nickel is about 0.112 mm. Because the thickness of the nickel sheet is less than the acoustic wavelength, the ultrasound reflects back and forth between the two surfaces of the nickel sheet to form a standing wave and produce resonance. The bond state between the nickel sheet and the silica gel is different, and the attenuation of the resonance ultrasound in the nickel sheet can effectively characterize the bond strength between the nickel sheet and the silica gel.
Nickel sheet and silicone sheet 3 pieces each, length and width are 50 mm, respectively, to produce three bonding state composite parts. The first is bonded strictly in accordance with the normal process (nickel sheet and silicone film bonding surface clean and then coated with binder); the second in the nickel sheet bonding surface paste transparent tape, and then coated with the binder, nickel sheet and silicone film bonding remove the transparent tape, and then nickel sheet and silicone film bonding, at this time paste transparent tape area due to no binder and lead to nickel sheet and silicone surface to form a weak bond; the third in the nickel sheet

Theory
When resonating, the quality factor of the resonance is defined by the following equation [19,20]: In equation (1): Q is the quality factor, E is the total energy per cycle of resonance, E´is the energy lost per cycle, w is the angular frequency and ΔE is the energy lost per second. In equation (2): f is the resonance point frequency and Δf´is the bandwidth between the half-power resonance points.
Based on the relationship between angular frequency and frequency, equation (2) can be rewritten as follows.
w is the angular frequency of the resonance point and Δw is the angular frequency of the bandwidth between the half-power resonance points. As the speed of ultrasound propagating in the workpiece is related to the time as d=ct, t is the propagation time of ultrasound from the surface to the bottom of the workpiece. According to Parseval's theorem, the energy loss per second of ultrasound is Eβc/d, with β being the attenuation constant. Equation (1) can be rewritten as follows.
Combining equations (3) and (4) yields: The ultrasonic attenuation coefficient is the following equation: α is the ultrasonic attenuation coefficient. The final equation for the ultrasonic attenuation coefficient is obtained as: where Δf is the interval between two adjacent resonant frequencies, and Δf is numerically equal to the resonant frequency f. Because the thickness of the nickel sheet is 0.07 mm, the bond between the nickel sheet and the silicone is characterized by the bond coefficient I according to equation (7), ignoring 1/d:

Experimental testing
Three samples are divided into 25 square areas and numbered, and each area is inspected individually by ultrasound. This is shown in figure 3(a). The ultrasound signals for the three bonded states are shown in figure 3(b). FFT was performed on each of the three ultrasound signals. Figure 3(c) shows that the high-frequency ultrasound resonance frequency in wellbonded, debonded and weakly bonded composite nickel is 40.1MHz, of which the debonded state due to the gap between the nickel and silicon film, ultrasound penetration through the nickel into the air with very little energy, its attenuation is the lowest; weakly bonded state from the bonded layer through the ultrasound and debonded state relatively more, its attenuation is higher than the debonded state; the well-bonded state in the three state, from the bonded surface through the most ultrasound, its attenuation will be the highest.
The results of the ultrasonic frequency domain signal processing calculations for the three states of good bonding, weak bonding and debonding according to equation (8) are shown in table 1.
Based on the above method, each of the three samples was tested and processed in turn according to the divided areas, and the bonding coefficients were calculated, as shown in figure 4. Figure 4 shows that the bonding coefficients for well-bonding composite parts range from 1 to 1.6, for weakly bonding composite parts from 0.9 to 1 and for debonding composite parts from 0.7 to 1. The bonding coefficient of well-bonding composite parts is greater than 1, so 1 can be used as the basis for determining whether the nickel sheet is well bonding to the silicone sheet. It should be noted that the ultrasonic resonance signal is not reflected between the two surfaces and the surface of the nickel sheet, the thickness has to affect the adhesive coefficient I.

Conclusion and future scope
The ultra-thin composite conductive sheet made of nickel sheet and silicone is widely used in industry. In this study, based on high-frequency ultrasonic resonance, the well-bonded, weakly bonded and debonded composite parts were examined separately. The effect of the bonding state on the bonding coefficient is analyzed, as shown in figure 4, and it is found that the bonding coefficient decreases with the weakening of the bonding strength. The bond strength of the composite parts can be accurately distinguished by setting the appropriate bond coefficient. Due to the limitation of experimental conditions, the specific relationship between adhesion coefficient and defects has not been established, which needs further research. According to the results of this experimental study, the method of this study has useful exploration and reference significance for the non-destructive testing of the bonding state between ultra-thin metals and non-metals.

Data availability statement
Any data that support the findings of this study are included within the article. The data cannot be made publicly available upon publication because no suitable repository exists for hosting data in this field of study. The data that support the findings of this study are available upon reasonable request from the authors.