CN108896664A - The velocity of sound and attenuation coefficient integrated detection method in a kind of polymer - Google Patents

Abstract

The invention discloses a method for integrated detection of sound velocity and attenuation coefficient in polymers, which is characterized in that it comprises: (1) sending an ultrasonic probe facing a sample to be detected; Get the amplitude spectrum of the reflected echo, the side of the sample facing the ultrasonic probe is defined as the upper surface, and the side facing away from it is the lower surface; (3) according to the amplitude spectrum of the obtained reflected echo, obtain the Detect the ultrasonic propagation velocity of the sample, the thickness of the sample and the attenuation coefficient of the sample. The detection method of the invention is simple in operation, convenient in calculation, low in detection cost, does not damage the sample to be tested, is easy to realize on-line monitoring, and can simultaneously detect the sound velocity, attenuation coefficient and wall thickness of polymer parts.

Description

An integrated detection method of sound velocity and attenuation coefficient in polymer

technical field

The invention belongs to the technical field of detection methods, and in particular relates to an integrated detection method of sound velocity and attenuation coefficient in polymers.

Background technique

Polymer parts are mostly formed by injection molding, and the propagation speed and attenuation coefficient of ultrasonic waves in them are important indicators of their microstructure. The speed of sound can characterize the degree of crystallinity and orientation of polymer parts, and the attenuation coefficient is closely related to the grain size of crystalline parts. For the sound velocity of polymer parts, currently the transit time method is mainly used to measure the sound velocity, that is, to measure the wall thickness of the part and the ultrasonic propagation time to calculate the sound velocity; for the attenuation coefficient, the current calculation is mainly based on the amplitude of the echo signal combined with the definition of the attenuation coefficient. These detection methods have the following disadvantages:

(1) The transit time method needs to know the wall thickness of the polymer to be tested, and this method cannot be used for online monitoring of the polymer injection molding process.

(2) Calculating the attenuation coefficient based on the amplitude of the echo signal requires less information and the amplitude of the sound pressure is disturbed by many factors, so the calculation accuracy is low.

Contents of the invention

Aiming at the problems of the existing detection methods, the present invention proposes an integrated detection method of ultrasonic sound velocity and attenuation coefficient based on the ultrasonic propagation theory, which can be used for online monitoring and has high precision.

An integrated detection method for sound velocity and attenuation coefficient in a polymer, comprising the following steps:

(1) The ultrasonic probe is facing the sample to be tested to emit ultrasonic waves;

(2) collect the reflected echoes of the upper and lower surfaces of the sample, and obtain the amplitude spectrum of the reflected echoes, the side of the sample facing the ultrasonic probe is defined as the upper surface, and the side facing away from it is the lower surface;

(3) According to the obtained amplitude spectrum of the reflected echo, the ultrasonic propagation velocity of the sample to be tested, the thickness of the sample and the attenuation coefficient of the sample are obtained.

As preferably, in step (3), specifically include:

(3-1) find the logarithm to the amplitude spectrum H (f) that obtains; Wherein U ₁ (f) is the spectrum of the echo reflected from the upper surface, and U ₂ (f) is the spectrum of the echo reflected from the lower surface;

(3-2) obtain the relation diagram of the logarithm of the amplitude spectrum of the reflection echo and the frequency f, and carry out linear fitting to it to obtain the linear function relation diagram; Wherein, f is the discrete frequency;

(3-3) Find the vertical axis intercept D of the linear function diagram obtained, and obtain the propagation velocity c of the ultrasonic wave at the sample to be tested according to the following formula:

Z = ρc

D=lnK

In the above formula: _Z0 is the acoustic impedance of the medium between the upper surface of the sample to be tested and the ultrasonic probe, Z is the acoustic impedance of the sample to be tested, ρ is the density of the sample to be tested, and c is the propagation velocity of ultrasonic waves in the sample to be tested;

(3-4), the thickness x of the sample is calculated by the time difference Δt of the reflected echoes on the upper and lower surfaces, and its expression is as follows:

x=c×Δt/2

(3-5) Find the slope k of the obtained linear function diagram, and obtain the attenuation coefficient α according to k=-2mxα÷f _c ; wherein m=0.115 is the unit of the attenuation coefficient converted from dB/cm to Np/cm Coefficient, f _c is the center frequency of the ultrasonic probe.

In the present invention, said f is a discrete frequency, specifically when receiving ultrasonic echoes, according to a certain sampling frequency f _s and sampling points: 2 ^N for sampling, f is exactly the discrete value between 0-f _s , step size It is f _s /2 ^N , that is to say, f represents a series of discrete frequencies: 0, f _s /2 ^N , (f _s /2 ^N )*2, (f _s /2 ^N )*2*2,.. ...., f _s .

The present invention can detect off-line samples (parts), preferably, the sample is an off-line sample, and the sample is placed in a medium liquid before detection, and during detection, the ultrasonic probe is immersed in the medium liquid, and the Z ₀ is the acoustic impedance of the medium liquid.

The medium liquid acts as a coupling agent to ensure that the ultrasonic wave can effectively enter the sample to be tested. The coupling agent is water or glycerin, machine oil and the like.

The present invention can also be used for the detection of online samples, as preferably, the sample is an online sample, the ultrasonic probe is close to the mold cavity side wall outside the online sample, and the Z ₀ is the acoustic impedance of the mold cavity side wall . Optionally, the coupling agent can be applied between the ultrasonic probe and the side wall of the mold cavity to improve detection accuracy.

Preferably, the sample to be tested is a polymer object with a flat surface, such as an offline plastic part or an online injection-molded plastic sample.

Taking the detection of offline plastic parts and using water as the medium liquid as an example, the above-mentioned integrated detection method of sound velocity and attenuation coefficient based on the ultrasonic propagation theory works as follows: when ultrasonic waves propagate in plastic parts, echoes will be generated on the upper and lower surfaces of the plastic. Wave signal, the relationship between the upper and lower surface echo signals is determined by the transfer function:

Where H(f) is the transfer function (i.e. the echo amplitude spectrum), U ₁ (f) is the spectrum of the upper surface echo (U ₁ (f)=FT(u ₁ (t))), U ₂ (f ) is the spectrum of the subsurface echo. H(f) is related to the density, sound velocity and attenuation coefficient of the medium:

In the formula: Z ₀ is the acoustic impedance of water, Z is the acoustic impedance of plastic parts, Z=ρc, ρ is the density of plastic, and c is the propagation speed of ultrasonic wave in plastic. m=0.115 is the coefficient for converting the attenuation coefficient unit from dB/cm to Np/cm. x is the thickness of the plastic part, α is the attenuation coefficient, f _c is the center frequency of the ultrasonic probe, f is the discrete frequency (for example, when collecting data, the sampling frequency is 50MHz, and the number of sampling points is 128 points, then this f means to use 128 Points divide 0-50MHz into equal equal parts for discrete frequency values). Φ ₁ (f), Φ ₂ (f) are the phase spectra of the reflected echoes from the upper and lower surfaces, respectively. Take the logarithm on both sides of the transfer function equation at the same time:

Where |H(f)| means to take the modulus of H(f);

When making the f-ln|H(f)| curve, lnK is the intercept of the vertical axis, and -2mxa÷f _c is the slope. Therefore, after obtaining the upper and lower surface echoes of the sample, the logarithm of the echo amplitude spectrum and the frequency relationship diagram can be made, and the linear function relationship can be obtained by linear fitting. The K value is obtained through the fitting straight line, and then the ultrasonic propagation velocity c can be calculated, and the wall thickness can be calculated by the time difference of reflected echoes from the upper and lower surfaces, and the expression is as follows:

x=c×Δt/2

In the formula, x is the wall thickness of the plastic part, and Δt is the time difference of reflected echoes from the upper and lower surfaces.

The above-mentioned integrated detection method of sound velocity and attenuation coefficient based on ultrasonic propagation theory should know the density of the plastic to be tested before the detection.

Compared with prior art, the beneficial effect of the present invention is:

The detection method of the invention is simple in operation, convenient in calculation, low in detection cost, does not damage the sample to be tested, is easy to realize on-line monitoring, and can simultaneously detect the sound velocity, attenuation coefficient and wall thickness of polymer parts.

Description of drawings

Fig. 1 is a schematic diagram of the ultrasonic detection principle of the present invention.

Fig. 2 is an actual measurement diagram of reflected echoes used in PMMA detection according to the present invention.

Fig. 3 is a graph showing the relationship between the logarithmic value and the frequency of the echo amplitude spectrum used for PMMA detection according to the present invention.

Detailed ways

In order to make the present invention more clearly understood, the present invention will be further described below according to specific examples and accompanying drawings of the present invention.

As shown in FIG. 1 , it is a principle diagram of ultrasonic detection of the present invention, including an ultrasonic probe 1 , a container 2 , water 3 , and a sample 4 .

Ultrasonic testing methods for plastic parts, including:

Place the sample in a container filled with water.

Use a water immersion ultrasonic probe to emit pulse waves with a certain center frequency. When testing, ensure that the ultrasonic probe is facing the upper surface of the sample.

Use an oscilloscope to collect the reflected echoes from the upper and lower surfaces of the receiving sample.

Calculate the frequency spectrum of the reflected echoes from the upper and lower surfaces of the sample according to the Fourier transform.

Make a graph of the relationship between the logarithmic value of the reflected echo amplitude spectrum and frequency, and perform linear fitting.

Find the vertical intercept of the fitted line and calculate the K value.

Calculate the ultrasonic propagation speed c according to the K value.

The wall thickness is calculated from the sound velocity c and the time difference between the reflection echoes from the upper and lower surfaces.

The attenuation coefficient is calculated from the wall thickness and the slope value of the fitted curve.

Figure 2 is a schematic diagram of the actual measurement of the reflected echo of PMMA. The first waveform is the echo reflected from the upper surface, and the second waveform is the echo reflected from the lower surface. The time difference between the two waveforms is 3.28 μs. The center frequency of ultrasound is 2.5MHz.

Fig. 3 is a graph showing the relationship between the logarithmic value and the frequency of the echo amplitude spectrum used for PMMA detection. Through linear fitting, lnK=-0.1245 is obtained, and the density of PMMA is known to be 1.18g/cm ³ , and then the propagation speed of ultrasonic wave in PMMA is calculated to be 2547m/s, and the wall thickness of PMMA is calculated according to the time difference to be 4.18mm. The thickness is 4.46mm, the relative error is 6.3%, and the attenuation coefficient is 7.25dB/cm. The comparison shows that the method has high precision.

The above description is only an application example of the present invention, and does not limit the range of applicable samples to be tested. The parts that can be measured by the present invention need not be exhausted here, and any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (5)

1. the velocity of sound and attenuation coefficient integrated detection method in a kind of polymer, which is characterized in that include the following steps：

(1) ultrasonic probe face sample to be tested is emitted into ultrasonic wave；

(2) reflection echo for acquiring sample upper and lower surface, seeks the amplitude spectrum of reflection echo, the sample face ultrasonic probe Side is defined as upper surface, back to side be lower surface；

(3) according to the obtained amplitude spectrum of reflection echo, acquire the ultrasonic propagation velocity of sample to be tested, the thickness of sample and The attenuation coefficient of sample.

2. the velocity of sound and attenuation coefficient integrated detection method in polymer according to claim 1, which is characterized in that step (3) it in, specifically includes：

(3-1) seeks logarithm to obtained amplitude spectrum H (f)；WhereinU₁It (f) is the frequency of upper surface reflection echo Spectrum, U₂It (f) is the frequency spectrum of lower surface reflection echo；

(3-2) seeks the logarithm of the amplitude spectrum of reflection echo and the relational graph of frequency f, and carries out linear fit to it and obtain linearly Function relation figure；Wherein, f is discrete frequency；

The vertical axis intercept D for the linear functional relation figure that (3-3) is sought seeks ultrasonic wave in sample to be tested according to the following formula Spread speed c：

Z=ρ c

D=lnK

In above-mentioned formula：Z₀The acoustic impedance of medium between sample to be tested upper surface and ultrasonic probe, Z are the sound of sample to be tested Impedance, ρ are the density of sample to be tested, and c is spread speed of the ultrasonic wave in sample to be tested；

(3-4), the thickness x of sample is calculated by time difference Δ t of upper and lower surface reflection echo, and expression formula is as follows：

X=c × Δ t/2

The slope k for the linear functional relation figure that (3-5) is sought, according to k=-2mx α ÷ f_c, acquire attenuation coefficient α；Wherein m =0.115 is converted into the coefficient of Np/cm, f for attenuation coefficient unit by dB/cm_cFor the centre frequency of ultrasonic probe.

3. the velocity of sound and attenuation coefficient integrated detection method in polymer according to claim 2, which is characterized in that described Sample is offline sample, and the sample is placed in dielectric fluid before detection, and when detection, the ultrasonic probe is impregnated in the medium In liquid, the Z₀For the acoustic impedance of dielectric fluid.

4. the velocity of sound and attenuation coefficient integrated detection method in polymer according to claim 2, which is characterized in that described Sample is on-line sample, and the ultrasonic probe is close to the die cavity side wall on the outside of on-line sample, the Z₀For the die cavity side wall Acoustic impedance.

5. the velocity of sound and attenuation coefficient integrated detection method in polymer according to claim 1 or 2, which is characterized in that The sample to be tested is the polymer articles of surfacing.