Non-destructive testing technology and its application (4)

Ultrasonic diffraction and scattering characteristics:

When the ultrasonic wave propagates through the medium, it encounters a heterogeneous interface (such as a defect). According to the huygens principle, a diffraction phenomenon occurs at the edge thereof, and a newly excited diffraction wave is generated. from the apparent point of view, the original ultrasonic wave can continue to advance around the defect, but an acoustic shadow (space without ultrasonic waves) is formed behind the defect. The new diffracted wave can be used to evaluate the surface crack depth or the height of the internal crack. In China, this method is called the edge regenerative wave method, and the foreign country is called the tip diffraction wave method. The phenomenon of sound shadow formation is used for ultrasonic penetration detection, that is, when ultrasonic waves encounter defects on their sound paths, due to reflection, diffraction, scattering, etc., and because of the abnormal microstructure of the material of the workpiece to be inspected, It will cause the attenuation of the ultrasonic propagation energy, so that the acoustic energy received at the other end of the acoustic path is lower than the acousticenergy received under normal conditions, and the difference can be reflected by using the ultrasonic flaw detector display or directly using the electric meter indication.Used as a basis for inspection and evaluation, ultrasonic thickness measurement gauge can be used for defect detection of sheet, composite or bonded structure, such as delamination, debonding etc., and can also be used for cracking tips of small electrical switches. Ultrasonic diffraction (regenerative wave) determines the crack depth.

Silver plated contacting quality inspection and more. The advantage is that it is easy to implement automatic detection, but the disadvantage is that the size of the defect and the location of the defect cannot be known, and the relative positions of the two probes are strictly required. When the ultrasonic wave propagates in the medium, its own wavefront diffusion will cause the sound energy passing through the unit area perpendicular to the direction of the sound beam to decrease as the propagation distance increases, which is called diffusion attenuation, which is the ultrasonic itself. The characteristic is related to the beam spread angle 2θ (θ is the semi-diffusion angle of the ultrasound beam). In addition, the ultrasonic wave is in the grain boundary of the material, the phase point, or the acoustic impedance of the suspended particles, impurities, bubbles, etc. in the medium (the value is equal to the product of the speed of sound and the density) (even if it is a slight difference). The scattering state is related to the wavelength of the ultrasonic wave and the magnitude of the scattering particle ( the average crystal grain diameter). In the metal material, the ratio of the wavelength λ to the average diameter of the crystal grains can be divided into three scattering states: Rayleigh scattering: "when λ, the degree of scattering is proportional to the fourth power of the frequency, which is the majority of the metal. random scattering: ≈λ, the degree of scattering is proportional to the square of the frequency, such as is usually the case in coarse-grain castings; diffuse scattering: ≥ λ, the degree of scattering is inversely proportional to, which is often expressed in In the case where the surface of the detected surface of the workpiece is rough, the diffuse scattering loss of the incident acousticenergy at the interface is caused. A similar metaphor for this situation can be as if the car lights were scattered in foggy weather and could not shine through the fog. Due to the existence of the scattering phenomenon, the acoustic energe throughthe unit area is perpendicular to the sound path is reduced, that is, the scatteringattenuation is caused. Although the existence of this scattering phenomenon in the ultrasonic pulse reflection detection method not only reduces the penetration ability of the ultrasonic wave but also interferes with the echo discrimination, it can also be returned to the ultrasonic wave by the superimposed reverberation of the scattered ultrasonic wave in the metal material. After the probe is received, it is displayed on the ultrasonic flaw detector display in the form of weed echo. By evaluating the level of clutter, the microstructure of the metal material can be judged and evaluated. Especially in the aerospace industry, the evaluation of clutter levels has become an important indicator in the acceptance criteria for ultrasonic testing of titanium alloy forgings.

Ultrasonic attenuation characteristics In addition to the scattering attenuation described in the previous section, another important cause of energy attenuation when ultrasonic waves are transmitted through the material is the attenuation due to internalabsorption, which is related to the viscosity of the material, heat conduction, boundary friction, The relaxation phenomenon is related to the loss of ultrasonic energy in the form of heat and solute atom migration, in addition to dislocation motion (such as dislocation density, length change, presence of holes and impurities) and magnetic domain wall motion, Residual stress causes sound field disturbances...etc. They can cause the attenuation of ultrasonic energy, which corresponds to the scattering attenuation in the upper section. We refer to the ultrasonic energy attenuation caused by these reasons as absorption absorption. It can be seen that the attenuation mechanism of ultrasonic waves in the material is very complicated. We consider comprehensive attenuation. Assume that the sound pressure amplitude at the distance source X=0 is P0, and the sound pressure amplitude after the distance X is PX, then: PX =P0·e-αx, where α is called the attenuation coefficient, which can be divided into two parts, namely: α=αs+αa, where αs is the scattering attenuation coefficient and αa is the absorption attenuation coefficient. Therefore, the attenuation coefficient expressed in α is a comprehensive parameter of a material, whichgenerally increases as the ultrasonic frequency increases. In the ultrasonic testing, it is possible to determine the degree of reduction of acoustic energy after the ultrasonic wave passes through the material (for example, the evaluation of the degree of reduction of the echo amplitude of the bottom surface of the workpiece in the ultrasonic pulse reflection method) is called the bottom wave loss evaluation or the bottom reflection loss, or ultrasonic wave. Penetration method can be used to assess the nature, morphology and distribution of the material microstructure, such as the detection of coarse crystals of metal materials, overheating and over-burning, (anoverheated structure in metal forgings), carbides. Uniformity, carbide spheroidization rate of ductile iron, room temperature tensile strength of carbon steel, stress measurement, and the like. 

The available data introduces the use of the clutter display caused by scattering and the attenuation evaluation of the echo amplitude to judge the spacing of the cementite layer in the pearlite structure of the locomotive wheel (the pearlite steel with a carbon content of 0.53~0.61%). Determine the yield limit and wear resistance of the wheel. There are also reports on the use of ultrasonic attenuation characteristics in fatigue testing of materials (in the fatigue test, the internal friction and lattice distortion inside the specimen can cause ultrasonic scattering, and the local plastic deformation of the fractured surface can cause the ultrasonic energy to be absorbed). Used for the evaluation of fracture toughness of steel. Combining the ultrasonic attenuation characteristics with the sound velocity characteristics can be used to determine, for example, the hydrogen content in titanium alloys (reducing the risk of hydrogen in titanium alloys) and to assess the aging quality of aluminum alloys, Velocity characteristics of ultrasonic waves of the same wave type have different propagation speeds in the different materials, and in the same material, ultrasonic waves of different wave types also have different propagation speeds. When the composition, microstructure, density, inclusion ratio, concentration, polymer conversion rate, strength, temperature, humidity, pressure (stress), flow rate of the material vary or change, the speed of sound will also vary.Using a special sound velocity tester or a conventional ultrasonic pulse reflection type flaw detector or thickness gauge to compare the material of unknown sound velocity with a standard sample of known sound velocity, so that the sound velocity or the speed of sound of the material can be measured and can be applied: (1) Determination of physical constants of materials, such as: according to the relationship in physics, generally: sound velocity C = (E / ρ) 1/2, where ρ is the material density, E is the elastic modulus of the material . Since the speed of sound is affected by the anisotropy, shape and interface of the material, and the respective elastic moduli are used depending on the vibration form of the ultrasonic wave, the longitudinal wave velocity in gas and liquid (only in gas and liquid) The longitudinal wave has: CL = (K / ρ0) 1/2, where K is the capacitive elastic modulus (volumetric elastic modulus) of the material, and ρ0 is the original static density of the medium in the presence of the acoustic wave. In solids: the ultrasonic longitudinal wave velocity propagating axially in a thin rod having a diameter smaller than the ultrasonic wavelength is: Cl = (E / ρ) 1/2, where E is the Young's modulus of the material, and ρ is the material density diameter.Ultrasonic longitudinal wave propagation in the axial direction of a thick rod larger than the ultrasonic wavelength. CL={[K+(4/3)G]/ρ}1/2={[E(1-σ)]/ρ(1+σ) (1-2σ)} K in the 1/2 formula is the capacitive elastic modulus (volumetric elastic modulus) of the material, G is the shear elastic modulus of the material, and σ is the poisson's ratio of the material (the material is in the force,When longitudinal strain occurs in the direction, lateral strain is also generated in the vertical direction, and the ratio between them is called poisson's ratio, which is one of the physical properties of the material). The shear wave sound velocity is: Cs=(G/ρ)1/2={E/[ρ·2(1+σ)]}1/2 The Rayleigh wave sound velocity is: CR=[(0.87+1.12σ)/(1 +σ)]·(G/ρ)1/2. when the sound velocity is measured and another parameter is known, other parameters can be calculated.

(2) Measuring temperature: The speed of sound in the medium is related to the temperature of the medium. This characteristic can be used to measure the temperature of the non-contact medium. It can further be used to indicate the melting point,boiling point and phase change of the medium, and to measure the specific heat of the medium. The heat of fusion is the heat of reaction and the heat of combustion are measured, and the purity and molecular weight of the medium are measured.

(3) Measuring flow rate: When ultrasonic waves propagate in a flowing medium (such as gas, liquid or fluid transfer pipes containing a certain proportion of solid particles, or water channels.), the propagation speed is different from that under static conditions with respect to a fixed coordinate system. It is related to the flow rate of the medium, so that the flow rate can be determined based on the change in the speed of sound and the flow rate (fluid cross-sectional area x flow rate) can be further determined. (4) Measuring the viscosity of the liquid η: According to the shear acoustic impedance Z and (η·ρ) 1/2 (η is the viscosity of the liquid, ρ is the density of the liquid), and the acoustic impedance Z=ρ·C, therefore By measuring the speed of sound and determining the density of the liquid, the density of theliquid can be determined. (5) Stress measurement: The propagation velocity of ultrasonic waves in the material has an approximately linear change with the applied stress (called ultrasonic stress effect), so it can be used to measure the strength of concrete prestressed, the strength and residual stress of the metal, and the fastening. Tensile stress on a piece (such as a fastening bolt). (6) Hardness measurement: The hardness of the metal surface hardened layer can be determined by using the speed change characteristic of the wave in the hardened layer of the metal surface.

(7) Determining the depth of the crack on the surface of the metal: the difference between the time when the wave is transmitted directly along the metal surface and the time when the surface crack is present and the wave is bypassed by the crack. According to the propagation speed of the Rayleigh wave, it can be calculated by the depth of the crack. This method is called time delay method or transit time method, Δt method.

(8) Measurement thickness: According to the relationship between the ultrasonic propagation distance X and the sound velocity C and the transmission time t: X=C·t, for example, when is measuring thickness by ultrasonic pulse reflection method, workpiece thickness d=C·t/2. The reason for using the denominator 2 here is that the ultrasonic probe emits an ultrasonic pulse to the bottom surface of the workpiece and the reflective return probe is received, so that the sound path passes are twice of the thickness of the workpiece.

Using the velocity characteristics of ultrasonic waves, it can also be applied to, themeasurement of the strength of spheroidal graphite cast iron and the degree of spheroidization of graphite, determining the humidity of ceramic adobe to determine the timing of firing in the kiln, and the analysis of the characteristics of the gaseous medium (for example, the purity of industrial oxygen and nitrogen). the metabolic rate of animal respiration has the change in the content of a component in the gas, etc.as well as the density of the petroleum fraction, the neoprene latex.

The ultrasonic time delay method is used to determine the density of the surface crack depth liquid and the like. In summary, the application of ultrasonic velocity characteristics, especially in industrial measurement technology is numerous. Ultrasonicis a kind of mechanical vibration wave. We can use the ultrasonic resonator to inject the ultrasonic wave with adjustable frequency (mainly using longitudinal wave) into the workpiece to be inspected. When the ultrasonic wave resonates with the natural frequency of the workpiece, the incident wave of the opposite direction propagates. The reflected waves are superimposed on each other to form a standing wave, which is the thickness resonance of the longitudinal wave perpendicularly incident. With this resonance characteristic, it can be applied to the following aspects:

(1) Thickness measurement:
The thickness of piezo ceramic disc transducer is d, and the wavelength of the ultrasonic wave propagating therein is λ, which is obtained when resonance occurs: d=λ1/2=2λ2/2=3λ3/2=...=n·λn/2, where n is Any positive integer, that is, the thickness of the workpiece to be inspected at this time is equal to an integral multiple of the half wavelength of the resonant ultrasonic wave. When the ultrasonic velocity C of the test piece material is known, according to the relationship between the speed of sound, the wavelength and the frequency: C = λ · f, the ultrasonic frequency at the time of thickness resonance can be obtained: fn = C / λn = n · C / 2d When n=1, f1=C/2d, which is the fundamental frequency of the thickness resonance. Since the difference between the frequencies of any two adjacent harmonics is equal to the fundamental frequency, there are: fn-fn-1=nf1-( N-1) f1=f1, so the frequency of two adjacent harmonics in the thickness resonance can be determined by the resonator, and the thickness of the workpiece is: d=C/[2(fn-fn-1)], When the frequencies of non-adjacent harmonics are fm and fn, respectively, since: fm-fn=(mn)f1.

(2) Detection of defects:
When there is a defect in the workpiece to be inspected, the national frequency will change as compared with the same workpiece without defects, and the resonance state will also change (resonance frequency changes), so that the existence of the defect can be detected accordingly. For example, it is used to measure the hardness of metals, to inspect the quality of sheet spot welding, especially for the bonding defects of composite materials and bonded structures (such as unbonded, debonded, poor gel, etc.) and the detection of bonding strength. acoustic vibration detection method is designed to check the quality of glue joints.

A typical application of ultrasonic resonance characteristics is an ultrasonic hardness tester, which measures the hardness by means of a change of the resonant frequency of the ultrasonic sensor bar. It is mainly used to determine the hardness of a metal, and can also be used for other measurements by a comparison method. Ultrasonic hardness measurement has the advantages of minimal damage to the surface of the test piece, fast measurement speed and simple operation procedure. It is especially suitable for 100% inspection of finished workpieces, and can directly detect the workpiece by holding the probe, especially suitable for large scales that are difficult to move. Workpieces parts that are not easily disassembled,which are measured. The following is an example of the ultrasonic hardness tester,which produced.Under the uniform contact pressure, the tip of the sensor is in contact with the surface of the test piece, and the resonant frequency of the sensor will follow the test piece. The hardness of the test piece is determined by measuring the change in the resonant frequency of the sensor.