CN110044682B - Method for monitoring fatigue crack propagation of single-side notch aluminum alloy test piece based on FBG sensor - Google Patents
Method for monitoring fatigue crack propagation of single-side notch aluminum alloy test piece based on FBG sensor Download PDFInfo
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Abstract
The invention discloses a fatigue crack propagation monitoring method for a single-side notch aluminum alloy test piece based on an FBG (fiber Bragg Grating) sensor, which is characterized in that a distributed fiber Bragg Grating sensor network is adopted to sense the fatigue crack propagation in a structure, the central wavelength offset of the FBG sensor is extracted as a characteristic parameter, and a fatigue crack length prediction relation model is established by utilizing the change rule of the central wavelength offset of the FBG sensor at different positions along with the crack propagation, so that the fatigue crack propagation length prediction is realized. The fatigue crack propagation monitoring method realizes the prediction of the crack propagation position by calculating the central wavelength offset of the FBG sensor, and has the characteristics of quickness, intuition, no need of a large amount of priori knowledge and the like.
Description
Technical Field
The invention relates to the technical field of fatigue damage monitoring of structural health monitoring, in particular to a method for monitoring fatigue crack propagation of a single-side notch aluminum alloy test piece based on an FBG (fiber Bragg Grating) sensor.
Background
At present, the consumption of structural materials of the aerospace vehicle accounts for a great proportion of the consumption of the structural materials of the aerospace vehicle, and in complex mechanical structures of the aerospace vehicle, the aircraft and the like made of the aluminum alloy materials, the key structures and parts of the machinery are easy to cause structural wear and failure due to the effect of various alternating fatigue loads such as external heat or force and the like for a long time, so that fatigue damage and fracture are caused. Fatigue cracks are the initial state form of fatigue damage failure. Although the aerospace vehicle can still be used when tiny cracks exist, the performance of the aerospace vehicle is reduced, and if the aerospace vehicle is not monitored and maintained for a long time, the disassembly of the body structure and more serious catastrophic results can be finally caused.
With the rapid development of the aerospace field, the monitoring of the structural health of aerospace vehicles is becoming more and more urgent and important. The fiber grating sensor has the unique advantages of light weight, good flexibility, small volume, electromagnetic interference resistance, integration of signal transmission and sensing, easiness in distributed networking and the like, overcomes many defects of the traditional electrical sensor, meets the requirements of sensing technology in the field of modern aerospace, and particularly shows the unique advantages under extremely complex application environments. Therefore, in recent years, the fiber grating sensing technology is rapidly developed, and the fiber grating sensor has become one of the most mainstream sensors for structural health monitoring in the aerospace field.
Currently, in fatigue crack damage monitoring and prediction, a neural network, a support vector machine and other modes are mostly adopted, a network is trained by a large amount of sample data, although the precision is higher, the generalization performance is poor, and the real-time performance and the practicability are poor due to the fact that the network is based on a large sample.
Disclosure of Invention
The invention aims to solve the technical problem of providing a method for monitoring the fatigue crack expansion of a single-side notch aluminum alloy test piece based on an FBG (Fiber Bragg Grating) sensor aiming at the defects involved in the background technology.
The invention adopts the following technical scheme for solving the technical problems:
the method for monitoring the fatigue crack propagation of the single-side notch aluminum alloy test piece based on the FBG sensor comprises the following steps:
step 1), carrying out finite element modeling on the aluminum alloy test piece with the single-side notch:
constructing a finite element model of the single-side notch aluminum alloy sample piece according to a preset elastic modulus, a Poisson ratio, a size, a diameter of a round hole, a length of a single-side notch and a length of a prefabricated crack, wherein the direction of the single-side notch is vertical to the edge of the sample piece, and the direction of the crack is the same as that of the single-side notch;
step 2), defining an intersection point of a central connecting line of two circular holes of the test piece and the crack propagation direction as a crack starting point, sequentially arranging three strain sensors FBG1, FBG2 and FBG3 at equal intervals along the crack direction, setting the vertical distance between the sensors and the crack propagation path as a preset distance threshold, and setting the axial direction of the three sensors FBG1, FBG2 and FBG3 to be vertical to the crack direction;
step 3), collecting reflection center wavelengths of three strain sensors of FBG1, FBG2 and FBG3 before fatigue test as original reference signals, and recording the original reference signals as lambdaRiWherein i is 1,2,3, i represents a sensor number; and connecting the adhered FBG sensors into a fiber bragg grating demodulator in series through jumper wires, clamping two round holes of the test piece, stretching the test piece by using a constant load, obtaining central wavelength data of three strain sensors, namely FBG1, FBG2 and FBG3, corresponding to different crack lengths, and recording the central wavelength data as { W }i,jWhere i ═ 1,2,3, denotes the sensor number, j denotes the crack length;
step 4), according to { Wi,jH and three strain sensor original center wavelength reference signals λ of FBG1, FBG2 and FBG3RiThen, the central wavelength offset of three strain sensors FBG1, FBG2 and FBG3 are calculated, and a central wavelength offset response characteristic set { S } of each sensor corresponding to different crack lengths is obtainedi,jWhere i ═ 1,2,3, denotes the sensor number, j denotes the crack length;
step 5), according to { Si,jDescribing points one by one in a two-dimensional rectangular coordinate system by taking the offset of the central wavelength as a dependent variable y and the length of the crack as an independent variable x, and utilizing an exponential function expression
Fitting the relationship between the central wavelength offset and the crack length of the FBGs 1 and 2, and calculating a parameter a of a fitting expression1、a2、b1、b2、c1、c2、c3(ii) a Reusing polynomial expression y ═ a3x6+b3x5+c4x4+d1x3+d2x2+d3x+d4Fitting the relation between the central wavelength offset of the FBG3 and the crack length, and calculating a fitting expression parameter a3、b3、c4、d1、d2、d3、d4Thus, a fatigue crack propagation prediction function relation model corresponding to the three FBG sensors is established;
step 6) according to the central wavelengths of the three strain sensors of the FBG1, the FBG2 and the FBG3 responding to the unknown cracks and the original central wavelength reference signals lambda of the three strain sensors of the FBG1, the FBG2 and the FBG3 in the step 3)RiCalculating the central wavelength offset of three strain sensors FBG1, FBG2 and FBG3 corresponding to the unknown crack length, and respectively recording the central wavelength offset as delta lambdaFBG1、△λFBG2、△λFBG3；
Step 7), dividing a crack expansion region according to the crack length prediction relation model in the step 5), comparing the central wavelength offset of three strain sensors FBG1, FBG2 and FBG3, and determining the region where the unknown crack expands, wherein the method specifically comprises the following steps:
step 7.1), along the increasing direction of x in the two-dimensional rectangular coordinate system, sequentially marking the intersection point of the FBG1 fitting curve and the FBG2 fitting curve as (x)a,ya) The intersection of the curve fitted to FBG1 and the curve fitted to FBG3 is denoted as (x)b,yb) The intersection of the curve fitted to FBG2 and the curve fitted to FBG3 is denoted as (x)c,yc) The intersection of the curve fitted to FBG2 and the curve fitted to FBG1 is denoted as (x)d,yd) Dividing a crack expansion area, wherein the principle of area division is as follows:
x≤xais marked as an areaI；xa<x<xbThe region of (1), denoted as region II; x is the number ofb≤x≤xcThe region of (1), denoted as region III; x is the number ofc<x<xdThe region of (1), denoted as region IV; x is greater than or equal to xdThe region of (a), denoted as region V;
step 7.2), according to the central wavelength offset Delta lambda of the 3 FBG sensors in the step 6)FBG1、△λFBG2、△λFBG3Determining the region where the crack propagates by fitting a functional relation model with the sensor in the step five according to a specific determination principle:
if Δ λFBG1>△λFBG2>△λFBG3Determining that the crack propagates in the region I;
if Δ λFBG2>△λFBG1>△λFBG3Determining that the crack propagates in the region II;
if Δ λFBG2>△λFBG3>△λFBG1Determining that the crack propagates in the region III;
if Δ λFBG3>△λFBG2>△λFBG1Determining that the crack propagates in the region IV;
if Δ λFBG3>△λFBG1>△λFBG2Determining that the crack propagates in the region V;
step 8), calculating the central wavelength offset Delta lambda of the 3 FBG sensors obtained in the step 6) according to theFBG1、△λFBG2、△λFBG3Substituting the three FBG central wavelength offsets obtained in the step 5) into a fitting function relation curve of the crack length to determine three initial crack length values of the unknown crack, and calculating to obtain the final unknown crack extension length by taking the absolute value ratio of the FBG sensor central wavelength offsets as a weight:
step 8.1), in the area where the unknown crack is spread and determined in the step 7), the central wavelength shift quantity delta lambda of the three FBG sensors calculated in the step 6 is usedFBG1、△λFBG2、△λFBG3Substituting the three function fitting curve relational expressions obtained in the step 5) to calculate three initial crack length values;
step 8.1.1), when a certain fitting function curve is non-monotonous, namely the same central wavelength offset corresponds to two different crack lengths, in the determined section, the coordinates with the same central wavelength offset on the non-monotonous curve are respectively recorded as (x)q,yq)、(xq’,yq) Wherein q is 1,2,3, q represents the sensor number, and the peak coordinate of the curve is (x)pq,ypq) Has xq<xpq<xq'; defining the relationship between the crack length and the central wavelength offset on the fitting function curve corresponding to the other two FBG sensors in the interval as (x)i,yi)、(xj,yj) And i and j are 1,2,3, i and j denote sensor numbers, then:
if xi、xj≤xpqWith xq、xi、xjAs a preliminary crack length calculation;
if xi、xj>xpqWith xq’、xi、xjAs a preliminary crack length calculation;
if xi≤xpq≤xjCompare | xi-xqI and | xj-xq' | size, when | xi-xqIf | is smaller, then xqAnd xi、xjAs a preliminary crack length calculation value, | xj-xq' if smaller, then xq' and xi、xjAs a preliminary crack length calculation;
if xj≤xpq≤xiCompare | xj-xqI and | xi-xq' | size, when | xj-xqIf | is smaller, then xqAnd xi、xjAs a preliminary crack length calculation value, | xi-xq' if smaller, then xq' and xi、xjAs a preliminary crack length calculation;
step 8.1.2), when all the determined sections are monotonous curves, recording the transmission of FBG1, FBG2 and FBG3The relationship between the crack length and the central wavelength offset on the fitting function curve corresponding to the sensor is respectively (x)1,y1)、(x2,y2)、(x3,y3) Directly with x1、x2、x3As a preliminary crack length calculation;
step 8.2), taking the absolute value ratio of the central wavelength offset of the three strain sensors of FBG1, FBG2 and FBG3 to the unknown crack response as a weighting factor, namely
Wherein Δ λFBG1、△λFBG2、△λFBG3Respectively representing the FBG1, FBG2 and FBG3 sensor center wavelength offsets associated with unknown crack lengths calculated by the step 6; weighting the calculated value of the initial crack length calculated by each FBG fitting curve in the step 8.1) to determine a final unknown crack length value X:
Compared with the prior art, the invention adopting the technical scheme has the following technical effects:
1. the invention uses the fiber Bragg grating sensor, only 3 fiber Bragg grating sensors form a series network, the fatigue crack propagation monitoring of the aluminum alloy unilateral notch test piece can be realized, compared with a piezoelectric sensing array, the system load is reduced, and the invention has the obvious advantages of simple structure, electromagnetic interference resistance and the like;
2. the fiber Bragg grating sensor reflection center wavelength is used as a characteristic parameter to carry out fatigue crack propagation monitoring and prediction, the linear relation between the fiber Bragg grating sensor reflection center wavelength and strain is good, and compared with other signal characteristic processing methods, the fiber Bragg grating sensor reflection center wavelength is a simpler, more convenient and more visual physical monitoring method by using the optical characteristics of the sensor;
3. in many current methods for monitoring and researching fatigue crack damage, the method is generally required to be established on the basis of a great deal of prior knowledge. The method utilizes the relation between the central wavelength offset of the sensor at different positions and the crack length, can establish a crack propagation prediction fitting function relation model by only needing fewer sample points, and performs crack length prediction by combining weight factors, thereby enhancing the engineering practicability.
Drawings
FIG. 1 is a schematic diagram of the structure and specification of a single-side notched aluminum alloy test piece according to the present invention;
FIG. 2 is a schematic layout of three strain sensors of FBG1, FBG2 and FBG3 in the present invention;
FIG. 3 is a flow chart of a method for monitoring fatigue crack propagation of a single-edge notched aluminum alloy test piece based on an FBG sensor;
FIG. 4 is a schematic illustration of the fatigue crack growth zone division in the present invention;
FIG. 5 is a schematic diagram of the identification of unknown crack lengths in the present invention.
Detailed Description
The technical scheme of the invention is further explained in detail by combining the attached drawings:
the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, components are exaggerated for clarity.
The flow chart of the method for monitoring the fatigue crack propagation of the single-side notch aluminum alloy test piece based on the FBG sensor is shown in FIG. 3, and the specific implementation steps are as follows:
step 1), finite element modeling is carried out on the test piece:
constructing a finite element model of the aluminum alloy sample with the single-side notch, wherein the elastic modulus of the finite element model is 70Gpa, the Poisson ratio is 0.33, the size of the finite element model is 240mm multiplied by 200mm multiplied by 15mm, the diameter of a round hole is 25mm, the length of the single-side notch of the sample is 37mm, the length of a prefabricated crack is 2.5mm, the direction of the prefabricated crack is the same as that of the single-side notch, and the finite element model starts from the tip of the single;
step 2), as shown in fig. 2, defining an intersection point of a central connecting line of two circular holes of the test piece and the crack propagation direction as a crack starting point, sequentially arranging three strain sensors of FBG1, FBG2 and FBG3 at positions 20mm, 40mm and 60mm away from the notch, namely at positions 32.5mm, 52.5mm and 72.5mm away from the crack starting point, setting the vertical distance between the sensors and the crack propagation path to be 10mm, and setting the axial direction and the crack direction of the three sensors of FBG1, FBG2 and FBG3 to be vertical;
step 3), collecting reflection center wavelengths of three strain sensors of FBG1, FBG2 and FBG3 before fatigue test as original reference signals, and recording the original reference signals as lambdaRiWherein i is 1,2,3, i represents a sensor number; and connecting the adhered FBG sensors into a fiber bragg grating demodulator in series through jumper wires, clamping two round holes of the test piece, stretching the test piece by using a 20KN constant load, acquiring central wavelength data of three strain sensors, namely FBG1, FBG2 and FBG3, corresponding to different crack lengths, and recording the central wavelength data as { W }i,jWhere i ═ 1,2,3, denotes the sensor number, j denotes the crack length;
step 4), according to { Wi,jAnd three strain sensor original center wavelength reference signals R of FBG1, FBG2 and FBG3fThen, the central wavelength offset of three strain sensors FBG1, FBG2 and FBG3 are calculated, and a central wavelength offset response characteristic set { S } of each sensor corresponding to different crack lengths is obtainedi,jWhere i ═ 1,2,3, i denotes the sensor number, j denotes the crack length;
step 5), according to { Si,jDescribing points one by one in a two-dimensional rectangular coordinate system by taking the offset of the central wavelength as a dependent variable y and the length of the crack as an independent variable x, and utilizing an exponential function expression
Fitting the relationship between the central wavelength offset and the crack length of the FBGs 1 and 2, and calculating a parameter a of a fitting expression1、a2、b1、b2、c1、c2、c3(ii) a Reusing polynomial expression y ═ a3x6+b3x5+c4x4+d1x3+d2x2+d3x+d4Fitting the relation between the central wavelength offset of the FBG3 and the crack length, and calculating a fitting expression parameter a3、b3、c4、d1、d2、d3、d4Thus, a fatigue crack propagation prediction function relation model corresponding to the three FBG sensors is established;
step 6) according to the central wavelengths of the three strain sensors of the FBG1, the FBG2 and the FBG3 responding to the unknown cracks and the original central wavelength reference signals lambda of the three strain sensors of the FBG1, the FBG2 and the FBG3 in the step 3)RiCalculating the central wavelength offset of three strain sensors FBG1, FBG2 and FBG3 corresponding to the unknown crack length, and respectively recording the central wavelength offset as delta lambdaFBG1、△λFBG2、△λFBG3；
Step 7), dividing a crack propagation region according to the crack length prediction relation model in the step 5), comparing the central wavelength offset of three strain sensors FBG1, FBG2 and FBG3, and determining the region where the unknown crack propagates, as shown in FIG. 4, specifically comprising:
step 7.1), along the increasing direction of x in the two-dimensional rectangular coordinate system, sequentially marking the intersection point of the FBG1 fitting curve and the FBG2 fitting curve as (x)a,ya) The intersection of the curve fitted to FBG1 and the curve fitted to FBG3 is denoted as (x)b,yb) The intersection of the curve fitted to FBG2 and the curve fitted to FBG3 is denoted as (x)c,yc) The intersection of the curve fitted to FBG2 and the curve fitted to FBG1 is denoted as (x)d,yd) Dividing a crack expansion area, wherein the principle of area division is as follows:
x≤xathe region of (a), denoted as region I; x is the number ofa<x<xbThe region of (1), denoted as region II; x is the number ofb≤x≤xcThe region of (1), denoted as region III; x is the number ofc<x<xdThe region of (1), denoted as region IV; x is greater than or equal to xdThe region (c) is denoted as a region V.
Step 7.2), according to the central wavelength offset Delta lambda of the 3 FBG sensors in the step 6)FBG1、△λFBG2、△λFBG3The numerical value relationship of (3) and the sensor in the step 5) are fitted with a functional relationship model, and the region where the crack propagates is determined, wherein the specific determination principle is as follows:
if Δ λFBG1>△λFBG2>△λFBG3Determining that the crack propagates in the region I;
if Δ λFBG2>△λFBG1>△λFBG3Determining that the crack propagates in the region II;
if Δ λFBG2>△λFBG3>△λFBG1Determining that the crack propagates in the region III;
if Δ λFBG3>△λFBG2>△λFBG1Determining that the crack propagates in the region IV;
if Δ λFBG3>△λFBG1>△λFBG2Determining that the crack propagates in the region V;
step 8), calculating the central wavelength offset Delta lambda of the 3 FBG sensors obtained in the step 6) according to theFBG1、△λFBG2、△λFBG3Substituting the three FBG central wavelength offsets obtained in the step 5) into a fitting function relation curve of the crack length to determine three primary crack length calculated values of the unknown crack, and determining the final unknown crack propagation length by combining the absolute value ratio of the FBG central wavelength offsets as a weight;
step 8.1), let step 6) of Δ λFBG1＝y1，△λFBG2＝y2，△λFBG3＝y3The coordinate points corresponding to the three fitting curves of FBG1, FBG2 and FBG3 are respectively (x)1,y1)、(x2,y2) And (x)3,y3) The coordinates having the same y value are respectively (x)1’,y1)、(x2’,y2) And (x)3’,y3) This is illustrated in FIG. 5:
(a) when the crack propagates to the region I, the curve fit to the FBG1 has a non-monotonic interval, and the intersection point of the boundary between the FBG1 curve and the FBG2 curve in the region I, II is marked as (x)i,yi) The peak coordinate of the FBG1 curve is noted as (x)p1,yp1)；
If y1<yiThen x is1、x2、x3As a preliminary crack length calculation;
if yi≤y1<yp1Then for the same y1Value, there are two different crack lengths x1And x1', according to x2、x3And xp1The size relationship is determined to be x1Or x1' and further determining a preliminary crack length calculation by:
if x2、x3≤xp1Then x is1、x2、x3As a preliminary crack length calculation;
if x2、x3>xp1Then x is1’、x2、x3As a preliminary crack length calculation;
x if2≤xp1≤x3Compare | x1-x2I and | x3-x1' | size, when | x1-x2If | is smaller, then x1And x2、x3As a preliminary crack length calculation value, | x3-x1If' | is smaller, then x is used1' and x2、x3As a preliminary crack length calculation;
fourthly if x3≤xp1≤x2Compare | x3-x1I and | x2-x1' | size, when | x3-x1If | is smaller, then x1And x2、x3As a preliminary crack length calculation value, | x2-x1If' | is smaller, then x is used1' and x2、x3As a preliminary crack length calculation;
if y1＝yp1Then x isp1、x2、x3As a preliminary crack length calculation;
(b)when the crack propagates to region II, x is directly introduced1、x2、x3As a preliminary crack length calculation;
(c) when the crack propagation is predicted to region III, similar to the region I solving method, the peak coordinate of the FBG2 curve is recorded as (x)p2,yp2) The abscissa of the intersection point of the boundary of the II and III regions of the FBG1 fitted curve and the FBG3 fitted curve is marked as xjThen x is addedjSubstituting the curve into the FBG2 to calculate a corresponding coordinate point as (x)j,yj)；
If y2<yjThen x is1、x2、x3As a preliminary crack length calculation;
if yj≤y2<yp2Then for the same y2Value, there are two different crack lengths x2And x2', according to x1、x3And xp2The size relationship is determined to be x2Or x2' and further determining a preliminary crack length calculation by:
if x1、x3≤xp2Then x is1、x2、x3As a preliminary crack length calculation;
if x1、x3>xp2Then x is1、x2’、x3As a preliminary crack length calculation;
x if1≤xp2≤x3Compare | x2-x1I and | x3-x2' | size, when | x2-x1If | is smaller, then x2And x1、x3As a preliminary crack length calculation value, | x3-x2If' | is smaller, then x is used2' and x1、x3As a preliminary crack length calculation;
fourthly if x3≤xp2≤x1Compare | x3-x2I and | x1-x2' | size, when | x3-x2If | is smaller, then x2And x1、x3As a preliminary crack length calculation value, | x1-x2If' | is smaller, then x is used2' and x1、x3As a preliminary crack length calculation;
if y2＝yp2Then x isp2、x1、x3As a preliminary crack length calculation;
(d) when the crack propagates to region IV, similar to the region III solution method, the peak coordinate of the FBG3 curve is recorded as (x)p3,yp3) The abscissa of the intersection point of the boundary line of the IV and V regions of the FBG1 fitted curve and the FBG2 fitted curve is marked as xkThen x is addedkSubstituting the curve into the FBG3 to calculate a corresponding coordinate point as (x)k,yk)；
If y3<ykThen x is1、x2、x3As a preliminary crack length calculation;
if yk≤y3<yp3Then for the same y3Value, there are two different crack lengths x3And x3', according to x1、x2And xp3The size relationship is determined to be x3Or x3' and further determining a preliminary crack length calculation by:
if x1、x2≤xp3Then x is1、x2、x3As a preliminary crack length calculation;
if x1、x2>xp3Then x is1、x2、x3' as a preliminary crack length calculation;
x if1≤xp3≤x2Compare | x3-x1I and | x2-x3' | size, when | x3-x1If | is smaller, then x3And x1、x2As a preliminary crack length calculation value, | x2-x3If' | is smaller, then x is used3' and x1、x2As primary crack growthCalculating a value;
fourthly if x2≤xp3≤x1Compare | x3-x2I and | x1-x3' | size, when | x3-x2If | is smaller, then x3And x1、x2As a preliminary crack length calculation value, | x1-x3If' | is smaller, then x is used3' and x1、x2As a preliminary crack length calculation;
if y3＝yp3Then x isp3、x1、x2As a preliminary crack length calculation;
(e) when the crack propagates in the region V, x is directly introduced1、x2、x3Calculated as the primary crack length.
Step 8.2), taking the ratio of the absolute values of the central wavelength offsets of the three strain sensors of FBG1, FBG2 and FBG3 to the unknown crack response as a weight, namely taking the ratio as a weight
Wherein Δ λFBG1、△λFBG2、△λFBG3The central wavelength offset of three strain sensors of FBG1, FBG2 and FBG3 for the unknown crack calculated in the step six; the calculated values of the length of the primary crack obtained by fitting the curves to the FBG1, FBG2 and FBG3 in the step 8.1) are recorded as x respectivelyFBG1、xFBG2And xFBG3And weighting the three primary crack length calculated values to determine a final unknown crack length calculated value:
it will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The above-mentioned embodiments, objects, technical solutions and advantages of the present invention are further described in detail, it should be understood that the above-mentioned embodiments are only illustrative of the present invention and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (1)
1. The method for monitoring the fatigue crack propagation of the single-side notch aluminum alloy test piece based on the FBG sensor is characterized by comprising the following steps of:
step 1), carrying out finite element modeling on the aluminum alloy test piece with the single-side notch:
constructing a finite element model of the single-side notch aluminum alloy sample piece according to a preset elastic modulus, a Poisson ratio, a size, a diameter of a round hole, a length of a single-side notch and a length of a prefabricated crack, wherein the direction of the single-side notch is vertical to the edge of the sample piece, and the direction of the crack is the same as that of the single-side notch;
step 2), defining an intersection point of a central connecting line of two circular holes of the test piece and the crack propagation direction as a crack starting point, sequentially arranging three strain sensors FBG1, FBG2 and FBG3 at equal intervals along the crack direction, setting the vertical distance between the sensors and the crack propagation path as a preset distance threshold, and setting the axial direction of the three sensors FBG1, FBG2 and FBG3 to be vertical to the crack direction;
step 3), collecting reflection center wavelengths of three strain sensors of FBG1, FBG2 and FBG3 before fatigue test as original reference signals, and recording the original reference signals as lambdaRiWherein i is 1,2,3, i represents a sensor number; connecting the adhered FBG sensors into a fiber bragg grating demodulator through jumper wires in series, clamping two round holes of the test piece, stretching the test piece by using constant load, and obtaining the corresponding crack lengthsThe central wavelength data of three strain sensors, FBG1, FBG2 and FBG3, are recorded as Wi,jWhere i ═ 1,2,3, denotes the sensor number, j denotes the crack length;
step 4), according to { Wi,jH and three strain sensor original center wavelength reference signals λ of FBG1, FBG2 and FBG3RiThen, the central wavelength offset of three strain sensors FBG1, FBG2 and FBG3 are calculated, and a central wavelength offset response characteristic set { S } of each sensor corresponding to different crack lengths is obtainedi,jWhere i ═ 1,2,3, denotes the sensor number, j denotes the crack length;
step 5), according to { Si,jDescribing points one by one in a two-dimensional rectangular coordinate system by taking the offset of the central wavelength as a dependent variable y and the length of the crack as an independent variable x, and utilizing an exponential function expression
Fitting the relationship between the central wavelength offset and the crack length of the FBGs 1 and 2, and calculating a parameter a of a fitting expression1、a2、b1、b2、c1、c2、c3(ii) a Reusing polynomial expression y ═ a3x6+b3x5+c4x4+d1x3+d2x2+d3x+d4Fitting the relation between the central wavelength offset of the FBG3 and the crack length, and calculating a fitting expression parameter a3、b3、c4、d1、d2、d3、d4Thus, a fatigue crack propagation prediction function relation model corresponding to the three FBG sensors is established;
step 6) according to the central wavelengths of the three strain sensors of the FBG1, the FBG2 and the FBG3 responding to the unknown cracks and the original central wavelength reference signals lambda of the three strain sensors of the FBG1, the FBG2 and the FBG3 in the step 3)RiCalculating the central wavelength offset of three strain sensors FBG1, FBG2 and FBG3 corresponding to the unknown crack length, and respectively recording the central wavelength offset as delta lambdaFBG1、△λFBG2、△λFBG3；
Step 7), dividing a crack expansion region according to the crack length prediction relation model in the step 5), comparing the central wavelength offset of three strain sensors FBG1, FBG2 and FBG3, and determining the region where the unknown crack expands, wherein the method specifically comprises the following steps:
step 7.1), along the increasing direction of x in the two-dimensional rectangular coordinate system, sequentially marking the intersection point of the FBG1 fitting curve and the FBG2 fitting curve as (x)a,ya) The intersection of the curve fitted to FBG1 and the curve fitted to FBG3 is denoted as (x)b,yb) The intersection of the curve fitted to FBG2 and the curve fitted to FBG3 is denoted as (x)c,yc) The intersection of the curve fitted to FBG2 and the curve fitted to FBG1 is denoted as (x)d,yd) Dividing a crack expansion area, wherein the principle of area division is as follows:
x≤xathe region of (a), denoted as region I; x is the number ofa<x<xbThe region of (1), denoted as region II; x is the number ofb≤x≤xcThe region of (1), denoted as region III; x is the number ofc<x<xdThe region of (1), denoted as region IV; x is greater than or equal to xdThe region of (a), denoted as region V;
step 7.2), according to the central wavelength offset Delta lambda of the 3 FBG sensors in the step 6)FBG1、△λFBG2、△λFBG3Determining the region where the crack propagates by fitting a functional relation model with the sensor in the step five according to a specific determination principle:
if Δ λFBG1>△λFBG2>△λFBG3Determining that the crack propagates in the region I;
if Δ λFBG2>△λFBG1>△λFBG3Determining that the crack propagates in the region II;
if Δ λFBG2>△λFBG3>△λFBG1Determining that the crack propagates in the region III;
if Δ λFBG3>△λFBG2>△λFBG1Determining that the crack propagates in the region IV;
if Δ λFBG3>△λFBG1>△λFBG2Determining that the crack propagates in the region V;
step 8), according toStep 6) calculating the central wavelength offset Delta lambda of the 3 FBG sensorsFBG1、△λFBG2、△λFBG3Substituting the three FBG central wavelength offsets obtained in the step 5) into a fitting function relation curve of the crack length to determine three initial crack length values of the unknown crack, and calculating to obtain the final unknown crack extension length by taking the absolute value ratio of the FBG sensor central wavelength offsets as a weight:
step 8.1), in the area where the unknown crack is spread and determined in the step 7), the central wavelength shift quantity delta lambda of the three FBG sensors calculated in the step 6) is addedFBG1、△λFBG2、△λFBG3Substituting the three function fitting curve relational expressions obtained in the step 5) to calculate three initial crack length values;
step 8.1.1), when a certain fitting function curve is non-monotonous, namely the same central wavelength offset corresponds to two different crack lengths, in the determined section, the coordinates with the same central wavelength offset on the non-monotonous curve are respectively recorded as (x)q,yq)、(xq’,yq) Wherein q is 1,2,3, q represents the sensor number, and the peak coordinate of the curve is (x)pq,ypq) Has xq<xpq<xq'; defining the relationship between the crack length and the central wavelength offset on the fitting function curve corresponding to the other two FBG sensors in the interval as (x)i,yi)、(xj,yj) And i and j are 1,2,3, i and j denote sensor numbers, then:
if xi、xj≤xpqWith xq、xi、xjAs a preliminary crack length calculation;
if xi、xj>xpqWith xq’、xi、xjAs a preliminary crack length calculation;
if xi≤xpq≤xjCompare | xi-xqI and | xj-xq' | size, when | xi-xqIf | is smaller, then xqAnd xi、xjAs a preliminary crack length calculation value, | xj-xq' if smaller, then xq' and xi、xjAs a preliminary crack length calculation;
if xj≤xpq≤xiThen | x is comparedj-xqI and | xi-xq' | size, when | xj-xqIf | is smaller, then xqAnd xi、xjAs a preliminary crack length calculation value, | xi-xq' if smaller, then xq' and xi、xjAs a preliminary crack length calculation;
step 8.1.2), when all the determined sections are monotonous curves, recording the relation between the crack length and the central wavelength offset on the fitting function curves corresponding to the FBG1, FBG2 and FBG3 sensors as (x)1,y1)、(x2,y2)、(x3,y3) Then directly with x1、x2、x3As a preliminary crack length calculation;
step 8.2), taking the absolute value ratio of the central wavelength offset of the three strain sensors of FBG1, FBG2 and FBG3 to the unknown crack response as a weighting factor, namely
Wherein Δ λFBG1、△λFBG2、△λFBG3Respectively representing the FBG1, FBG2 and FBG3 sensor center wavelength offsets associated with unknown crack lengths calculated by the step 6); weighting the calculated value of the initial crack length calculated by each FBG fitting curve in the step 8.1) to determine a final unknown crack length value X:
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