CN106769818A - A kind of aluminium alloy structure corrosion damage monitoring method based on algebraically iterative reconstruction algorithm - Google Patents

Abstract

A corrosion damage monitoring method for aluminum alloy structures based on an algebraic iterative reconstruction algorithm, the steps are as follows: 1: select the material aluminum alloy and piezoelectric sensors, and adopt a square array layout; 2: use a sine wave excitation signal, and the received Lamb wave It is one of them, that is, anti-symmetrical wave; Three: Based on the tomographic algorithm of algebraic reconstruction iteration, ART, the area to be detected of the plate is evenly divided into a network; Four: Carry out damage location imaging by analyzing the signal phase change of Lamb wave; Five: The test imaging damage location results are consistent with the actual damage location, which proves that the damage location method can accurately locate two damages; through the above steps, the characteristic parameters can be extracted through signal processing, and the corrosion damage degree of aluminum alloy components can be quantitatively characterized to solve the problem. It has the function of real-time monitoring of corrosion damage of aluminum alloy structure.

Description

A Corrosion Damage Monitoring Method for Aluminum Alloy Structures Based on Algebraic Iterative Reconstruction Algorithm

technical field

The invention belongs to the technical field of structural health monitoring, and in particular relates to a method for monitoring corrosion damage of an aluminum alloy structure based on an algebraic iterative reconstruction algorithm (ART).

Background technique

Structural health monitoring technology comprehensively uses sensor technology, signal processing and analysis technology to monitor the actual situation of the structure in real time, providing an important reference for the safety, reliability and durability of large-scale critical structures. Nondestructive testing based on Lamb waves has a wide range of applications due to its economy, and is increasingly used in structural health monitoring. Algebraic Iterative Reconstruction Algorithm (ART) has been widely used in medical computed tomography (CT), and this patent refers to it in the corrosion damage imaging of aluminum alloy plates with holes. When there is corrosion damage to the aluminum alloy plate, obtain the required waveform, and use appropriate data processing and analysis methods to process and analyze it, so as to extract the damage features, and obtain the damage position through the algebraic reconstruction technique (Algebraic Reconstruction Techniques, ART) Based on the information, the research on corrosion damage monitoring of aluminum alloy structure based on CT principle was realized.

Contents of the invention

(1. Purpose

Aiming at the problem that the corrosion damage of aviation aluminum alloys is difficult to monitor effectively in real time, this patent proposes a method for monitoring corrosion damage of aluminum alloy structures based on an algebraic iterative reconstruction algorithm, that is, a method for monitoring corrosion damage of aluminum alloy structures based on tomography. This patent uses piezoelectric sensor arrays for signal excitation and collection, and uses advanced signal processing methods and signal analysis methods to extract damage feature quantities.

(2) Technical solution

The invention aims at the monitoring of the corrosion damage of the aluminum alloy structure, and realizes the real-time monitoring of the corrosion damage of the aluminum alloy structure mainly through an algebraic iterative reconstruction algorithm.

A method for monitoring corrosion damage of an aluminum alloy structure based on an algebraic iterative reconstruction algorithm of the present invention specifically includes the following steps to realize:

step one:

This patent selects the material aluminum alloy, the grade is Al-2024-T3, the piezoelectric sensor of American Steminc Company, and adopts a square array layout; among them, four piezoelectric sensors are arranged on each side, and a total of 16 piezoelectric sensors are formed. 96 paths, as shown in Figure 2. This patent adopts the send-and-receive piezoelectric sensor sending and receiving method; because the excitation and receiving piezoelectric sensors are distributed on both sides of the test area, there will be less interference from boundary reflection signals, so the send-and-receive layout will affect the More sensitive to long-distance damage;

Step two:

The present invention has adopted the sine wave excitation signal of the central frequency of 50kHz, and the Lamb wave that receives is that Lamb wave is a kind of dispersion wave that multiple modes exist simultaneously, and what analyze in the present invention is antisymmetric wave;

Step three:

For the layout of the piezoelectric sensor network and the selected signal features, the tomographic algorithm based on the iterative algebraic reconstruction (ART) is used; the principle of the tomographic algorithm based on the iterative algebraic reconstruction (ART) is to divide the area to be detected of the plate evenly into the network; The piezoelectric sensor adopts the method of sending and receiving, and each piezoelectric sensor is used as a transmitting and receiving point respectively; the Lamb wave emitted by the piezoelectric sensor passes through each grid, and is imaged according to the attenuation coefficient value in each grid; when When the plate has no defects, the attenuation coefficients in each grid are basically the same. When there are defects, the attenuation coefficients in the grids at the defect are different from those at the non-defect. Based on the above principles, the damage is imaged;

For the algebraic reconstruction iterative algorithm, namely the ART algorithm, it is necessary to optimize the iterative algorithm according to the characteristics of corrosion damage monitoring, including the selection of the number of iterations and the termination condition based on semi-convergence judgment;

Step four:

The present invention uses hydrofluoric acid solution to corrode the aluminum alloy plate to produce corrosion damage, and when the corrosion damage is on the path of Lamb wave propagation, the phase of the signal direct wave changes;

The invention performs damage location imaging by analyzing the signal phase change of the Lamb wave; the amplitude of the Lamb wave is 5V, the magnification is 10, and the number of iterations of the ART algorithm is selected as 10;

Step five:

In order to make the imaging result smoother, the experiment adopts the mean value filtering process. For each current grid to be processed, a template is selected, which is composed of several adjacent grids, and the mean value of the template is used to replace the original grid. The value of the pixel, given by the formula

In the formula, M is the total number of the grid and its surrounding grids, f(x, y) is the pixel value of row x, column y, s is all grids, and g(x, y) is the calculated Final pixel value.

The test imaging damage location results are consistent with the actual damage locations, and the test results prove that the damage location method can accurately locate two damages.

Wherein, the "piezoelectric sensor of American Steminc Company" mentioned in step 1 refers to the SMD07T05R412WL piezoelectric ceramic disc, and its specific parameters are shown in Table 1.

Table 1

Among them, the "excitation signal" described in step 2 refers to the selection of the excitation signal. The excitation signal of the piezoelectric sensor is generated by an arbitrary signal generator, and the signal can be described by the following expression:

In the formula, f _c is the center frequency of the signal, N is the peak number of the signal, A is the amplitude of the signal, and H(t) is the Heaviside step function.

Wherein, the "ART algorithm" mentioned in step 3 refers to the algebraic reconstruction iterative algorithm,

The whole imaging method is to solve a statically indeterminate equation shown in the following formula,

Ax=b

In the formula, A represents the real shape of the area to be detected, x is the detection imaging result, and b is the projection of A;

Let b _i be the attenuation value of the i-th ray passing through the measured area, then there is a formula:

In the formula, X(s) is the attenuation coefficient; let X(s) be a constant on the j-th grid, and L _ij is the actual length of the i-th ray in the j-th grid;

The propagation path of the Lamb wave is actually the actual length L _ij of each ray in each grid; the specific idea of solving this L _ij is:

(1) Take the upper left corner of the grid as the origin, establish coordinate axes, the coordinates of each grid point and the piezoelectric sensor coordinates are known, and the number of piezoelectric sensors is m;

(2) Set up the straight line equation of the ray according to the coordinates of transmitting and receiving piezoelectric sensors, and obtain the intersection points of the straight line and each grid;

(3) Solve the distance between the next point and the previous point to get L _ij ;

Then there is the formula:

b _i =Σ _j L _ij x _j

In the formula, the matrix composed of L _ij becomes the A matrix; the final solution equation:

In the formula, m is the number of rays, n is the number of grids; the algebraic reconstruction iterative algorithm adopts the ART algebraic reconstruction iterative algorithm to solve the above equations, the basic idea is to give an initial estimated value (i=1,2,…,m), then according to Find an approximate projection value Then according to Find the reprojection value Continue in this way until the preset condition is met and stop, the formula is as follows:

x ^k,0 = x ^k ,

x ^k+1 = x ^k,m

In the formula, λ _k is the relaxation coefficient, and its role is to control the change of variables in each iteration, which mainly affects the convergence speed of iterations; λ _k generally takes a value between 0 and 1, and the smaller the value, the faster the change between two iterations. Small, the calculation is stable like this, but the calculation speed is slow; Fig. 4 shows an iteration of x when the relaxation coefficient λ _k =1;

Through the above steps, for the aluminum alloy plate structure, through the layout optimization and signal monitoring of multiple piezoelectric sensors, collecting different piezoelectric sensors and extracting characteristic parameters through signal processing, the corrosion damage degree of aluminum alloy components can be quantitatively characterized, which solves the problem It can monitor the corrosion damage of aluminum alloy structure in real time.

(3) Advantages of the present invention

The invention provides a method for monitoring corrosion damage of an aluminum alloy structure based on an algebraic iterative reconstruction algorithm. This method selects the experimental material, selects the layout of the piezoelectric sensor network, selects the excitation signal, and finally conducts the test and performs imaging through the algebraic iterative reconstruction algorithm to monitor the corrosion damage, locate and quantify the corrosion area, and quantify the results. It is relatively accurate, and the relative error of the corrosion damage area does not exceed 20%. And the effectiveness of this method is verified by comparing with the original image of the experiment. This patent has the characteristics of high accuracy and fast analysis speed. This patent applies the algorithm of tomography to the corrosion damage monitoring of aviation aluminum alloy structure as the innovation point of this patent.

Description of drawings

Figure 1(a): Schematic diagram of the left side view of the piezoelectric sensor.

Figure 1(b): Schematic diagram of the front view of the piezoelectric sensor.

Figure 2: Signal path diagram for a piezoelectric sensor.

Figure 3: Diagram of the monitoring method.

Figure 4: Schematic diagram of the ART algorithm.

Figure 5: Corrosion health and damage signals based on hydrofluoric acid solutions.

Figure 6: Piezoelectric sensor layout diagram.

Figure 7: Flowchart of the method of the present invention.

detailed description

A corrosion damage monitoring method for an aluminum alloy structure based on an algebraic iterative reconstruction algorithm of the present invention is shown in Figure 7, and its specific implementation steps are as follows:

step one:

This patent selects material Al-2024-T3, and its density, Young's modulus and Poisson's ratio are respectively 2.78g/cm3, 73.1GPa, 0.33. Its size is designed to be 500mm*500mm*2mm.

This patent selects the piezoelectric sensor of American Steminc Company for use. The size and appearance of the piezoelectric sensor are shown in Figures 1 and 2.

In this patent, the rectangular array is selected from linear array, rectangular array, circular array and parallel array according to the aluminum alloy material to be tested and the geometric characteristics of the piezoelectric sensor. Figure 4 shows the square array layout of piezoelectric sensors. Among them, four piezoelectric sensors are arranged on each side, and 16 piezoelectric sensors form 96 paths in total.

Step two:

Currently, the sending and receiving methods of piezoelectric sensors can be divided into two categories: pulse-echo (pulse-echo) layout and pitch-catch layout. For pulse-echo layouts, the exciting piezoelectric transducer and the receiving piezoelectric transducer must be on the same side of the test area. This arrangement is not suitable for long-distance damage detection because of the interference of superimposed boundary reflection signals. As for the send-and-receive layout, because the excitation and receiving piezoelectric sensors are distributed on both sides of the test area, there will be less interference from boundary reflection signals, so the send-and-receive layout will be more sensitive to long-distance damage. This patent adopts the mode of sending and receiving.

The received Lamb wave is a dispersive wave in which multiple modes exist simultaneously. What this patent analyzes is an antisymmetric wave (A0).

Step three:

A tomography algorithm based on Algebraic Reconstruction Iterative (ART) is used for the layout of the piezoelectric sensor network and the selected signal features.

The principle of the tomography algorithm based on the Algebraic Reconstruction Iterative Algorithm (ART) is to evenly divide the area to be inspected into a network, such as the grid in Figure 3. The piezoelectric sensor adopts one sending and one receiving, and each piezoelectric sensor is used as a transmitting and receiving point respectively. The Lamb wave emitted by the piezoelectric sensor passes through each grid, and is imaged according to the attenuation coefficient value in each grid. When the sheet has no defects, the attenuation coefficients in each grid are basically the same. When there are defects, the attenuation coefficients in the grids at the defect are different from those at the non-defective site. Based on the above principles, the damage is imaged.

For the ART algorithm, it is necessary to optimize the iterative algorithm according to the characteristics of corrosion damage monitoring, including the selection of the number of iterations and the termination condition based on semi-convergence judgment.

Step four:

This patent uses hydrofluoric acid solution to corrode the aluminum alloy plate to produce corrosion damage. Figure 5 shows the test results of two typical hydrofluoric acid corrosion damages on the Lamb wave propagation signal. From the comparison of the healthy and damaged signals in Figure 5, it can be seen that when the corrosion damage is on the path of Lamb wave propagation, the phase of the direct wave of the signal changes.

The layout of the piezoelectric sensors for the test is shown in Figure 6, which is a square layout with a total of 16 piezoelectric sensors. The specimens and piezoelectric transducers are the same as those proposed in this study. This patent performs damage localization imaging by analyzing the signal phase change of the Lamb wave. Lamb is a sinusoidal modulation wave of five cycles, the center frequency is 100kHz, the amplitude is 5V, the amplification factor is 10, and the number of iterations of the ART algorithm is selected as 10.

Step five:

In order to make the imaging result smoother, the experiment adopts the mean value filtering process. For each current grid to be processed, a template is selected, which is composed of several adjacent grids, and the mean value of the template is used to replace the original grid. The value of the pixel. Its formula is

In the formula, M is the total number of the grid and its surrounding grids, f(x, y) is the pixel value of row x, column y, s is all grids, and g(x, y) is the calculated Final pixel value.

Place the lesion on the lower right side of the area to be tested during the test. The test imaging damage location results were consistent with the actual damage location.

For two damages, the test results prove that the damage location method can accurately locate the two damages.

The effectiveness of the proposed tomography-based monitoring method for aluminum alloys is verified by laboratory analysis results.

The application verification of the corrosion damage monitoring of aluminum alloy structure based on tomography is aimed at the aluminum alloy plate structure. Through the layout optimization and signal monitoring of multiple piezoelectric sensors, different piezoelectric sensors are collected and the characteristic parameters are extracted through signal processing to quantitatively characterize the aluminum alloy structure. The degree of corrosion damage of alloy components, combined with the analysis results of corrosion damage in this patent, verifies the effectiveness and applicability of the proposed method.

Wherein, the "piezoelectric sensor of American Steminc Company" mentioned in step 1 refers to the SMD07T05R412WL piezoelectric ceramic disc, and its specific parameters are shown in Table 1.

Table 2

Among them, the "excitation signal" described in step 2 refers to the selection of the excitation signal. The excitation signal of the piezoelectric sensor is generated by an arbitrary signal generator, and the signal can be described by the following expression:

In the formula, f _c is the center frequency of the signal, N is the peak number of the signal, A is the amplitude of the signal, and H(t) is the Heaviside step function.

Wherein, the "ART algorithm" mentioned in step 3 refers to the algebraic reconstruction iterative algorithm,

The whole imaging method is to solve a statically indeterminate equation shown in the following formula,

Ax=b

In the formula, A represents the real shape of the area to be detected, x is the detection imaging result, and b is the projection of A.

Let b _i be the attenuation value of the i-th ray passing through the measured area, then there is a formula:

In the formula, X(s) is the attenuation coefficient. Let X(s) be a constant on the j-th grid, and L _ij be the actual length of the i-th ray in the j-th grid.

The propagation path of the Lamb wave is actually the actual length L _ij of each ray in each grid. Specific ideas for solving:

(1) With the upper left corner of the grid as the origin, the coordinate axes are established, the coordinates of each grid point and the piezoelectric sensor coordinates are known, and the number of piezoelectric sensors is m.

(2) Establish the line equation of the ray according to the coordinates of the transmitting and receiving piezoelectric sensors, and find the intersection points of the line and each grid.

(3) Calculate the distance between the next point and the previous point to get L _ij .

Then there is the formula:

b _i =Σ _j L _ij x _j

In the formula, the matrix composed of L _ij becomes the A matrix. The final solved equation:

In the formula, m is the number of rays, and n is the number of grids.

Using the ART algebraic reconstruction method to solve the above equations, the basic idea is to give an initial estimate first (i=1,2,…,m), then according to Find an approximate projection value Then according to Find the reprojection value Continue in this way until the preset condition is met and stop. The formula is as follows:

x ^k,0 = x ^k ,

x ^k+1 = x ^k,m

In the formula, λ _k is the relaxation coefficient, which is used to control the change of variables in each iteration, and mainly affects the convergence speed of iterations. The value of λ _k is generally between 0 and 1, and the smaller the value, the smaller the change between two iterations, so the calculation is stable, but the calculation speed is slow. FIG. 4 shows an iteration of x at a relaxation factor λ _k =1.

Through the above steps, the application verification of the corrosion damage monitoring of aluminum alloy structure based on tomography is aimed at the aluminum alloy plate structure, through the layout optimization and signal monitoring of multiple piezoelectric sensors, collecting different piezoelectric sensors and extracting characteristic parameters through signal processing, Quantitatively characterize the corrosion damage degree of aluminum alloy components, combined with the analysis results of corrosion damage in this patent, verify the effectiveness and applicability of the proposed method.

Claims (3)

1. a kind of aluminium alloy structure corrosion damage monitoring method based on algebraically iterative reconstruction algorithm, it is characterised in that：Its step It is as follows：

Step one：

The selection of material aluminium alloy and piezoelectric transducer, and using square array Column Layout；Wherein, four piezoelectricity are arranged on every side Sensor, 16 piezoelectric transducers constitute 96 paths altogether, and the piezoelectric transducer received using a hair one receives and dispatches mode；Because its Excitation and reception piezoelectric transducer are distributed in the both sides of test zone, the interference of less border reflected signal are had, so one Hair one receives layout and can damage more sensitive to remote；

Step 2：

Using the sine wave exciting signal of the centre frequency of 50kHz, the Lamb wave for receiving i.e. Lamb wave is one kind of multiple patterns Simultaneous dispersion wave, i.e. antisymmetry ripple；

Step 3：

For piezoelectric transducer network topology and the signal characteristic quantity chosen, with the tomography for based on algebraic reconstruction iteration being ART Scanning algorithm；It is that the region to be detected of plate is uniformly divided into network based on the tomoscan algorithm that algebraic reconstruction iteration is ART； The method that piezoelectric transducer is received using a hair one, each piezoelectric transducer is respectively as transmitting and receiving point；Piezoelectric transducer The Lamb wave of transmitting is passed through from each grid, the attenuation factor value imaging in each grid；When sheet material zero defect, each Attenuation coefficient in grid is essentially identical, and when existing defects, the attenuation coefficient in fault location grid is different from zero defect Attenuation coefficient, based on above reason, is imaged to damage；

It is ART algorithms, it is necessary to the characteristics of being monitored according to corrosion damage, is carried out excellent to iterative algorithm for algebraic reconstruction iterative algorithm Change, including iterations selection, based on semiconvergent judge end condition；

Step 4：

Corrode aluminium alloy plate using hydrofluoric acid solution to manufacture corrosion damage, when corrosion damage is on the path that Lamb wave is propagated When, the phase of signal direct wave there occurs change；

Being changed by the signal phase for analyzing Lamb wave carries out damage reason location imaging；The amplitude of Lamb wave is 5V, and multiplication factor is 10, ART algorithm iteration number of times elect 10 as；

Step 5：

In order that obtaining, imaging results are more smooth, experiment employs mean filter treatment, for each pending current grid, A template, the template will be selected to be constituted by its neighbouring a plurality of grid, original pixel is substituted with the average of template It is worth, its formula is

$g(x,y)=\frac{1}{M}\underset{f\∈s}{\Σ}f(x,y)$

In formula, M is the grid and the sum of the grid around it, and f (x, y) is the pixel value in xth row y row, and s is all of net Lattice, g (x, y) is the final pixel value tried to achieve；

Experiment imaging damage reason location result is consistent with actual damage position, and experiment results proved this damage positioning method energy is accurate Orient two damages；

By above step, for aluminium alloy plate structure, by the layout optimization and signal monitoring of a plurality of piezoelectric transducers, adopt Collect different piezoelectric transducers and characteristic parameter is extracted by signal transacting, energy quantitatively characterizing goes out the corrosion damage journey of aluminium alloy element Degree, solving can be to the function of the real-time monitoring of the corrosion damage of aluminium alloy structure.

2. a kind of aluminium alloy structure corrosion damage monitoring side based on algebraically iterative reconstruction algorithm according to claim 1 Method, it is characterised in that：" pumping signal " described in step 2, it refers to the selection for pumping signal, piezoelectric transducer Pumping signal produced by arbitrarily signal generating device, signal is described by following expression formula：

$u\left(t\right)=20\[H\left(t\right)-H(t-\frac{N}{f_c})\]\×(1-\cos \frac{2{\mathrm{\πf}}_{c}t}{N})\sin 2{\mathrm{\πf}}_{c}t$

In formula, f_cIt is the centre frequency of signal, N is the crest number of signal, and A is the amplitude of signal, and H (t) is Heaviside ladders Function.

3. a kind of aluminium alloy structure corrosion damage monitoring side based on algebraically iterative reconstruction algorithm according to claim 1 Method, it is characterised in that：" ART algorithms " described in step 3 refers to algebraically reconstruction iteration algorithm,

Whole imaging method is the static indeterminacy equation shown in one equation below of solution,

Ax=b

In formula, A represents the true form for needing detection zone, and x is detection imaging results, and b is the projection of A；

If b_iBe i-th ray by the pad value of tested region, then have formula：

$b_i={\∫}_{{\mathrm{ray}}_i}X\left(s\right)dl$

In formula, X (s) is attenuation coefficient；If X (s) is constant, L on j-th grid_ijIt is i-th ray in j-th grid Physical length；

The propagation path of Lamb wave is actual be exactly in each grid each bar ray it is actual walk length L_ij；The L_ijThe specific work for solving Method is：

(1) with the grid upper left corner as origin, reference axis, each grid point coordinates and piezoelectric transducer coordinate are set up, it is known that piezoelectricity is passed Sensor number is m；

(2) linear equation of the ray is set up according to the coordinate for launching and receiving piezoelectric transducer, straight line and each grid is obtained Intersection point；

(3) latter point and the distance of previous point are solved, L is obtained final product_ij；

Then there is formula：

b_i=∑_jL_ijx_j

In formula, L_ijThe matrix of composition turns into A matrixes；The final equation for solving：

$\left(\begin{array}{c}{b}_1\\ .\\ .\\ .\\ b_i\\ .\\ .\\ .\\ b_m\end{array}\right)=\left(\begin{array}{ccccc}{L}_{ij}& \mathrm{...}& L_{ij}& \mathrm{...}& L_{ij}\\ .& & .& & .\\ .& & .& & .\\ .& & .& & .\\ L_{ij}& \mathrm{...}& L_{ij}& \mathrm{...}& L_{ij}\\ .& & .& & .\\ .& & .& & .\\ .& & .& & .\\ L_{ij}& \mathrm{...}& L_{ij}& \mathrm{...}& L_{ij}\end{array}\right)*\left(\begin{array}{c}{x}_1\\ .\\ .\\ .\\ x_i\\ .\\ .\\ .\\ x_n\end{array}\right)$

M is the bar number of ray in formula, and n is the quantity of grid；Algebraic reconstruction iterative algorithm is asked using ART algebraic reconstruction iterative algorithms Solution above equation, first provides an initial estimate(i=1,2 ..., m), then basisObtain first approximation projection valueFurther according toTry to achieve reprojection's valueSo continue, until meeting pre-conditioned rear stopping, formula is as follows：

x^{K, 0}=x^k,

$x^{k,i}=x^{k,i-1}+{\mathrm{\λ}}_k\frac{b_i-(a^i,x^{k,i-1})}{\left|\right|a^i|{|}_2^2},$

x^k+1=x^{K, m}

In formula, λ_kIt is coefficient of relaxation, its change for acting as each iteration of control variables, the convergence rate of main influence iteration； λ_kBetween zero and one, the smaller representative iteration change twice of its value is smaller, so calculating stabilization for general value, but calculating speed Slowly.