Ultrasonic flaw detection of metal materials has been widely used

1 A composite structure composed of multiple materials, to withstand high temperature, high speed, high pressure thermal ablation and erosion, the working environment is harsh, so each component is required to have a high degree of reliability, the nozzle metal shell and non-metal bonding The interface, due to the poor cleaning of the workpiece during bonding, the lack of tight coordination between the lining and the shell, and the lack of gas inside the adhesive layer, can easily cause debonding, especially the gap type large-area debonding, which is extremely harmful to product reliability. Therefore, the non-destructive testing of the bonding quality of its bonding interface is also a key process of product quality control in the production of solid rocket motors. Countries have invested a lot of manpower and material resources to carry out the research of non-destructive testing technology for the components of solid rocket motors. Ultrasonic flaw detection of metal materials has been widely used, and composite materials have the characteristics of high anisotropy, large sound attenuation, and complex structure. Ultrasonic flaw detection of composite materials and their components is a weak link, according to the Japan Institute of Aerospace Technology ” “Solid Rocket Motor Ultrasonic Flaw Detection” reported that they had successfully used the ultrasonic multiple reflection method to successfully detect the bonding surface of the non-metallic coating and the steel shell. After years of research, the ultrasonic longitudinal wave multiple reflection method was used to The bonding quality of the pipe metal shell and the non-metal bonding interface is tested, and it has been successfully used for batch flaw detection of products.

2 Detection principle Ultrasound is an elastic vibration wave with a frequency higher than 20000Hz. The so-called longitudinal wave means that the propagation direction of the wave is consistent with the movement direction of the particle. The debonding injury of the bonding interface refers to the air gap type formed by the interface of different materials. Defect, when the ultrasonic longitudinal wave passes through the metal and non-metal bonding surface vertically, due to the different acoustic impedances of the two media in the product, the sound wave will be reflected and transmitted at the interface. The propagation path of the sound wave is shown in Figure 1, in the figure L Is the incident wave; L is the reflected wave; t is the transmitted wave. Acoustic waves pass through each layer and interface of the product and respectively pass through three media, namely metal, air and non-metal. In these three media, the longitudinal wave sound velocity is 5900, 344 and 2200m/s respectively; when there is debonding at the acoustic impedance sub-adhesive interface, it is equivalent to the ultrasonic longitudinal wave incident perpendicularly to the metal-air interface, and the sound pressure reflectivity of the interface at this time Where P is the sound pressure of the incident wave; P is the acoustic impedance of the reflected wave at the debonding interface of the metal medium; Z is the acoustic impedance of the air medium.

substituted into the acoustic impedance data calculation shows that R≈1, that is, when there is a debonded solid rocket technology, the sound pressure reflectivity tends to 1, the transmittance tends to 0, and the sound wave is nearly 100% reflected here. But when the bonding is good, the sound pressure reflectance of the interface is not 0, and the calculation formula is as follows: where r is the sound pressure reflectance of a good bonding interface; P is the sound pressure of the reflected wave of a good bonding interface; Z is a non-metallic medium The acoustic impedance.

It is calculated that the reflected wave sound pressure accounts for 86% of the incident wave sound pressure, and the transmitted wave sound pressure accounts for 14%. In addition to reflection, the sound wave also has a part of transmission.

For an ultrasonic flaw detector with good amplification linearity, the wave height on the instrument screen is proportional to the sound pressure, that is, the ratio of any two adjacent wave heights is equal to the ratio of the corresponding sound pressure. The difference between the wave height and the good bonding zone is 1dB, and the multiple reflection method is adopted. When the ultrasonic wave is perpendicular to the product, the first reflected sound pressure is the second reflected sound pressure is the nth reflected sound pressure Where P is the sound pressure reaching the surface of the metal layer; P is the sound pressure received by the probe after n reflections; R is the sound pressure reflection of the A interface is the sound pressure reflectivity of the surface (bonding interface); T is the metal The attenuation coefficient of the piece; t is the thickness of the metal piece.

For a given metal material, its thickness is constant, so T and t are constant. After a fixed coupling agent is selected, R is also constant. The sound pressure P after multiple reflections is only related to R, that is It is only related to the bonding between the metal layer and the non-metal layer. When n is good and n=10, ΔdB=13dB, that is, after the ultrasonic longitudinal wave is reflected 10 times, the reflection wave amplitude of the debonding zone and the good bonding zone is 13dB different The above is reflected on the screen of the instrument, the debonding area has higher amplitude and more waves than the good adhesion area. Based on this, it is easy to distinguish the debonding area from the good adhesion area, and then evaluate the bonding quality.

3 Comparison of test blocks 3.1 Design and manufacture of ultrasonic flaw detection are to judge the quality of the detected part by observing the position and amplitude of the reflected echo on the fluorescent screen of the flaw detector. Taking into account the different shapes of defects formed in practice, the acoustics The relationship is complicated and it is difficult to quantitatively calculate and analyze. Therefore, in actual flaw detection, only artificial flaws with a known specific shape can be used to adjust flaw detection sensitivity, and use this as a scale to evaluate flaws to ensure the reproduction of inspection results. Use test blocks as a reference for comparison It is a feature of ultrasonic flaw detection.

In order to avoid the difference in acoustic performance between the test block and the tested product, the No. Ⅰ test block (Figure 2) has the same material type, thickness, curvature and surface finish as the tested product, and uses the same adhesive as the tested product. Debonding wounds of different shapes and sizes are made on the metal bonding surface. In Figure 2, A is a circular debonding (14mm is an ellipse). The comparison test block No. Ⅱ is a test block with natural defects cut from the product after the test (Figure 3).

Zhao Huirong: Ultrasonic inspection of solid rocket motor nozzle bonding interface 3.2 Inspection of test blocks 3.2.1 Adjustment of scanning speed The horizontal scale value of the time base scan line on the oscilloscope screen has a certain proportional relationship with the actual sound path. The thickness of the metal part of the probe nozzle is 4mm. The standard depth test block produced by Shantou Ultrasonic Instrument Research Institute is used to adjust the scanning speed. Adjust the level knob and the depth fine adjustment knob on the instrument panel to expand the scanning range. The time-base scanning line ratio is 1:2.5 , Receiving multiple reflection echoes.

3.2.2 The test result of the test block and the photo of the analysis test result are shown in Figure 4. The waveform analysis of the test result of the test block is shown in Table 1.

(A) Good bonding area of ​​the test block (b) A debonding area (circular) (c) B debonding area (ellipse) (d) C debonding area (irregular shape) can be seen from Figure 4 and Table 1. It can be seen that: a) The height of the reflected wave front is almost the same for several times. This is because the ultrasonic beam does not start to diffuse from the wave source, but there is a non-diffusion area near the wave source. In the non-diffusion area, the average sound pressure is basically unchanged. , The sound field radiated by the wafer is shown in Figure 5; b) In the good bonding area, in addition to the reflection of the sound wave, there is also a non-metallic transmission wave. Compared with the metal steel part, the non-metal heat insulation layer of the nozzle has a loose structure and a structure The attenuation of sound energy is more serious than that of steel. Under 5MHz frequency, through calculation, the attenuation coefficient of steel is less than 0.002dB, and the non-metallic insulation layer reaches 6dB/mm, which makes good bonding time. , The transmitted wave entering the non-metal parts is repeatedly absorbed, and the energy attenuation is large. Within 6 grids of the horizontal scale of the oscilloscope screen of the instrument, the reflected wave envelope appears as a smooth arc and drops rapidly; c) In the debonding zone, the incident wave is 100% For reflection, the wave frequency of the reflected wave increases, and the wave is emitted from the horizontal scale of the instrument’s oscilloscope screen to the full screen, which is better than the good bonding area. At the same horizontal scale of the oscilloscope screen, the wave amplitude increases, and the wave amplitude increases with the debonding area. Increasing, the more the waves, the higher the amplitude, and the reflected wave envelope will slowly decrease in a saw-tooth shape.

The solid rocket technology area flaw detector shows a good condition. The multiple reflected wave in the bonding zone is within “6” grid on the horizontal scale of the oscilloscope screen. A The multiple reflected wave in the debonding zone has a 30% amplitude at the horizontal scale “6” on the oscilloscope screen. Above, the multi-reflection wave in the debonding zone of the “8” grid has an amplitude of 30% at the horizontal scale “8” of the oscilloscope screen, and the multi-reflection wave in the debonding zone of the “10” grid has an amplitude of C in the debonding zone. The wave amplitude at the “10” grid on the horizontal scale of the wave screen is 10% to 30%. 3.3 Test block method to determine the flaw detection sensitivity The composite component uses test blocks for comparative testing to determine the flaw detection sensitivity. Debonding circle) as a reference, aim the probe at A debonding, adjust the instrument attenuation and gain knobs, so that the multiple reflection wave from A debonding can reach 10% at the horizontal scale “8” of the oscilloscope screen. At this time, the decibel value of the attenuator on the instrument panel is used as the detection sensitivity. After the sensitivity is adjusted, the decibel value is fixed.

4 Ultrasonic testing of nozzle bonding interface 4.1 Testing process The material of the tested product metal part is 30CrMnSiA, the material of the non-metal part is high silica/phenolic, the curvature of the tested part is 284mm, the thickness of the metal part is 4mm, the metal part and non-metallic insulation layer 944 glue is used for bonding. Product acceptance requirements: the debonding area shall not be greater than the total detection area. After the detection sensitivity is determined, the nozzle metal and non-metal bonding interface can be tested, taking the Υ14mm circular debonding as the benchmark. It is found that the multiple reflection wave at a certain position has a wave amplitude of more than 10% at the horizontal scale of the instrument’s oscilloscope screen. The sticky more than ten times 100% reflection is close to a spherical wave. The simplified formula can be used where P is the initial sound pressure; d is the diameter of the wafer; λ is the wavelength; s is the distance; A is the area of ​​the wafer.

If their reflections are regarded as a new sound source under the same conditions, the debonding area is equal to 100% of the area A emission, and the half-wave is equal to the area A emission, that is, the sound pressure at the center of the debonding and the debonding edge to the center of the wafer The difference is 6dB. The specific implementation method is: After finding the debonding, move the probe to make the reflected pulse amplitude on the phosphor screen reach the highest, and then move the probe up, down, left, and right. When the reflected wave amplitude is reduced to half of the original, the center line of the probe The position between is the debonded area, trace the debonded area with 1:1 ratio transparent paper, and calculate the debonded area with a planimeter. Table 2 shows the inspection results of the bonding surface of some nozzles in a certain batch.

4.2 Inspection results and analysis. Using the ultrasonic longitudinal wave multiple reflection method introduced in this article for the inspection of the metal and non-metal bonding interface of two actual nozzles, the internal debonding defects can be found relatively accurately. After anatomical control of the nozzle, the debonding position and debonding area are also consistent with the test results.

In the future application, after nearly a hundred products tested, it is proved that this method can more accurately find the internal debonding defects of the following workpieces: a. Gap type large area debonding. This type of debonding is mostly caused by improper cleaning of the workpiece, poor fit between the inner lining and the shell, and deformation of the workpiece. At this time, the bonding strength is lower than a good bonding strength, which is reflected on the oscilloscope screen of the instrument, and multiple reflections. The volatility is about 30% at the “10” grid on the horizontal scale.

B. Debonding of porous small pieces. This type of debonding is mostly caused by the ultrasonic testing of the bonding interface of the solid rocket motor nozzle with the glue, the gas inside the glue layer and the failure of the adhesive. At this time, the bonding strength is also lower than the good bonding strength. , Reflected on the oscilloscope screen of the instrument, the amplitude of multiple reflections is more than 10% at the horizontal scale “8” grid.

6 Concluding remarks Using the ultrasonic multiple reflection method, using conventional instruments and equipment, various debonding defects of the bonding interface can be found more accurately. The actual inspection of the bonding interface of the solid rocket motor nozzle proves that this method is suitable for on-site inspection on the production line and flaw detection on the position.

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