KR101964880B1 - System and method for diagnosing crack propagation of CFRP structures - Google Patents

Abstract

In the present invention, a strain rate can be calculated when an electrical resistance value of a carbon fiber reinforced plastic structure is measured by arranging a plurality of electrodes inside the carbon fiber reinforced plastic structure, obtaining a resistance change rate in a plurality of channels formed between the plurality of electrodes, and constructing a relational expression of the resistance change rate and the strain rate as a database; and stress can be diagnosed using the strain rate. In addition, by measuring an electrical resistance value of the carbon fiber reinforced plastic structure, the stress distribution can be derived, thereby replacing FEM.

Description

System and method for diagnosing crack propagation of CFRP structures

The present invention relates to a system and method for diagnosing stress of a carbon fiber reinforced plastic structure, and more particularly, to a carbon fiber reinforced plastic structure capable of diagnosing stress by using a resistance change behavior according to a deformation mode of a carbon fiber reinforced plastic structure. To a stress diagnosis system and method.

In general, carbon fiber reinforced plastic (CFRP) is a composite material using carbon fiber as a reinforcing material. Carbon fiber reinforced plastic is a high strength, high rigid and lightweight structural material that is actively used in various fields such as airplanes, automobiles, and robots. Since various structures such as airplanes, automobiles, and robots are structures that require high stability, there is a growing interest in technologies that can detect damage.

Conventionally, by additionally installing a sensor that can measure deformation or damage separately from the structure to perform inspection of a specific portion to be inspected, scan through inspection equipment such as acoustic sensors, or determine the damage with the naked eye.

However, there is a problem in that the additional installation of the sensor is not only costly, but also the installation of the sensor is limited because the area to be inspected is very extensive. In addition, when using inspection equipment or visual inspection, there is a problem in securing safety because it can be confirmed only after the damage or damage has already occurred.

In addition, there is a problem that the crack propagation cannot be predicted in advance.

Korea Patent Registration 10-1610710

It is an object of the present invention to provide a stress diagnosis system and method for a carbon fiber reinforced plastic structure which can predict failure by diagnosing stress using resistance change behavior.

In the method for diagnosing stress of a carbon fiber reinforced plastic structure according to the present invention, a plurality of electrodes are formed so that a plurality of carbon fiber reinforced plastic sheets are stacked and a plurality of channels are formed at positions spaced apart from each other by a predetermined distance from the inside and the bottom thereof. In the method for diagnosing stress of the carbon fiber reinforced plastic structure provided, at least one deformation mode of bending, tensioning, shearing and peeling is applied to the plurality of carbon fiber reinforced plastic structures through experiments, Measuring an electrical resistance value according to stress and strain in the channels, deriving a resistance-strain relationship representing a relationship between the strain and the resistance change rate for each deformation mode, and constructing a database; In the stress diagnosis, the user inputs shape information, elastic modulus, and deformation mode of the carbon fiber reinforced plastic structure to be diagnosed as a variable; The control unit finding the resistance-strain relationship for the strain mode from the database; Measuring an electrical resistance value for each of the plurality of channels formed in the carbon fiber reinforced plastic structure; Calculating strain in the channels by substituting the measured electrical resistance value into the resistance-strain relationship; Calculating strain in the channels, substituting the strain and elastic modulus into a relationship between stress and strain of the carbon fiber reinforced plastic structure to calculate stress at the positions of the channels; And representing the stress calculated at the location of the channels as a stress distribution within the carbon fiber reinforced plastic structure.

According to another aspect of the present invention, there is provided a method for diagnosing stress of a carbon fiber reinforced plastic structure, wherein a plurality of carbon fiber reinforced plastic sheets are stacked, and a plurality of channels are formed at positions spaced apart from each other by a predetermined distance from the inside and the bottom thereof. In the method for diagnosing stress of a carbon fiber reinforced plastic structure provided with electrodes of the electrode, through the experiment applying a deformation mode of at least one of bending, tensioning, shearing and peeling the plurality of carbon fiber reinforced plastic structures, Measuring electric resistance values according to stress and strain in the channels for each mode, deriving a resistance-strain relationship representing a relationship between the strain and the resistance change rate in each deformation mode, and constructing a database; In the stress diagnosis, the user inputs shape information, elastic modulus, and the plurality of deformation modes of the carbon fiber reinforced plastic structure to be diagnosed as a variable; The controller calculates weights of the plurality of deformation modes on the deformation of the carbon fiber reinforced plastic structure, respectively, by using the shape information input in the input step, and controls the plurality of deformation modes for the input deformation modes from the database. Finding resistance-strain relations and applying the weights to combine resistance-strain relations to derive one resistance-strain relation for the plurality of strain modes; Measuring a resistance change for each of the plurality of channels formed in the carbon fiber reinforced plastic structure; Calculating strain in the channels by substituting the measured electrical resistance change into the resistance-strain relationship; Calculating strain in the channels, substituting the strain and elastic modulus into a relationship between stress and strain of the carbon fiber reinforced plastic structure to calculate stress at the positions of the channels; Representing the stress calculated at the location of the channels as a stress distribution within the carbon fiber reinforced plastic structure; The control unit detects a channel in which compressive stress appears among the plurality of channels as a channel just before cracking, and converts a position of a channel in which the tensile stress increases above a predetermined ratio among the plurality of channels as a crack tip channel having a crack tip. The crack position is derived from the detected position information of the channel.

According to another aspect of the present invention, a stress diagnosis system for a carbon fiber reinforced plastic structure includes a plurality of carbon fiber reinforced plastic sheets stacked on top of each other at positions spaced apart from each other by a predetermined distance from an inside and a bottom of a carbon fiber reinforced plastic structure. A plurality of electrodes provided to form channels of the plurality of electrodes; Wires connected to the plurality of electrodes, respectively; A resistance measuring module measuring a change in resistance of the plurality of channels; A database storing a resistance-strain relationship representing a relationship between strain and resistance change measured in the channels according to deformation modes including bending, tension, shear and peeling applied to the carbon fiber reinforced plastic structures; When the resistance change is measured in the resistance measurement module, the electrical resistance change measured above is substituted into the resistance-strain relationship formula to calculate strain in the channels, and the strain and elastic modulus are the carbon fiber reinforced plastic structure. And a control unit for calculating the stress at the positions of the channels and determining the progress of the crack according to the calculated stresses, by substituting the relationship between the stress and the strain rate of the channel.

The present invention, by arranging a plurality of electrodes inside the carbon fiber reinforced plastic structure, by obtaining a resistance change rate in the plurality of channels formed between the plurality of electrodes, by building a relationship between the resistance change rate and strain rate as a database, By measuring the electrical resistance of the reinforced plastic structure, the strain can be calculated, and the strain can be used to diagnose stress.

In addition, by measuring the electrical resistance of the carbon fiber reinforced plastic structure, it is possible to derive the stress distribution, there is an advantage that can replace the FEM.

In addition, there is an advantage that can predict the crack growth.

1 is a view showing a stress diagnosis system of a carbon fiber reinforced plastic structure according to an embodiment of the present invention.
Figure 2 is a flow chart illustrating a stress diagnostic method of the carbon fiber reinforced plastic structure according to an embodiment of the present invention.
3 is a view schematically showing the crack propagation of the carbon fiber reinforced plastic structure according to an embodiment of the present invention as an electric resistance equivalent circuit.
Figure 4 is a graph showing the stress according to the crack growth in the stress diagnosis system of the carbon fiber reinforced plastic structure according to an embodiment of the present invention.
Figure 5 shows the stress distribution in the carbon fiber reinforced plastic structure according to an embodiment of the present invention.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

1 is a view showing a stress diagnosis system of a carbon fiber reinforced plastic structure according to an embodiment of the present invention.

Referring to FIG. 1, the carbon fiber reinforced plastic structure 10 according to the present invention is formed by stacking a plurality of carbon fiber reinforced plastic sheets.

The carbon fiber reinforced plastic structure 10 is described as an example formed in a beam shape.

A plurality of upper electrodes 11, 12, 13, and 14 are provided at an upper side of the carbon fiber reinforced plastic structure 10 to be spaced apart from each other by a predetermined distance, and a plurality of lower portions are also spaced apart from each other at a predetermined interval. Electrodes 15, 16, 17, 18 are provided. The separation distance between the upper electrodes 11, 12, 13, and 14 is set equal to the separation distance between the lower electrodes 15, 16, 17, and 18. In addition, when the separation distance is secured to be greater than or equal to a predetermined set distance, the resistance change pattern measured in the channels to be described later may obtain a linear increase pattern according to the crack propagation after the crack passes.

The upper electrodes 11, 12, 13, 14 and the lower electrodes 15, 16, 17, 18 are disposed to correspond to each other in the vertical direction. The upper electrodes 11, 12, 13, 14 and the lower electrodes 15, 16, 17, and 18 are paired in pairs to form a channel.

In the present embodiment, the upper electrodes 11, 12, 13, 14 are four first, second, three, and four electrodes 11, 12, 13, 14, and the lower The electrodes 15, 16, 17, and 18 are also described as four fifth, six, seven, and eight electrodes 15, 16, 17, and 18.

Wires are connected to the first, second, third, fourth, fifth, sixth, seventh, and eighth electrodes 11, 12, 13, 14, 15, 16, 17, and 18, respectively. The wire is connected to a resistance measurement module (not shown) which will be described later.

The channel may electrically connect the first channel C1 electrically connecting the first electrode 11 and the fifth electrode 15, and the second electrode 12 and the sixth electrode 16 to each other. A second channel C2 for connecting; a third channel C3 for electrically connecting the third electrode 13 and the seventh electrode 17; and the fourth electrode 14 and the eighth electrode. And a fourth channel C4 that electrically connects 18.

In addition, the channel may include a fifth channel C5 electrically connecting the first electrode 11 and the second electrode 12, and the second electrode 12 and the third electrode 13. The sixth channel C6 electrically connects to each other, and the seventh channel C7 electrically connects the third electrode 13 to the fourth electrode 14.

The first, second, third, and fourth channels C1, C2, C3, and C4 are perpendicular to the longitudinal direction of the carbon fiber reinforced plastic structure 10 in the carbon fiber reinforced plastic structure 10. It is formed in one vertical direction.

The fifth, sixth, seventh channels (C5), (C6), and (C7) are formed in the longitudinal direction of the carbon fiber reinforced plastic structure (10) in the upper side of the carbon fiber reinforced plastic structure (10).

The fifth, sixth, seventh channels (C5), (C6), and (C7) are formed in the longitudinal direction of the carbon fiber reinforced plastic structure (10) in the upper side of the carbon fiber reinforced plastic structure (10).

However, the present invention is not limited thereto, and the channel may include a channel connecting the fifth, six, seven, and eight electrodes 15, 16, 17 to each other.

The stress diagnosis method of the carbon fiber reinforced plastic structure configured as described above is as follows.

First, at least one deformation mode of bending, tensioning, shearing, and peeling is applied to the carbon fiber reinforced plastic structure, and a plurality of experiments are performed to construct a database.

Input parameters during the experiment include shape information of the carbon fiber reinforced plastic structure, deformation mode, stress, electrical conductivity of the carbon fiber reinforced plastic sheet, energy release rate, and the like.

Here, the energy release rate is a measure for comparing the crack propagation with respect to a specific tensile length and a specific tensile force during the delamination test. In other words, if the energy release rate is large at the same tensile length and tensile force, more crack propagation is obtained. Knowing the energy release rate of each material, it is possible to compare crack propagation to the same tensile length. Thus, the input variable includes electrical conductivity and energy release rate.

The electrical conductivity of the carbon fiber reinforced plastic sheet may be adjusted according to the type, size, and number of laminated carbon fibers.

Accordingly, different data are constructed according to the electrical conductivity of the carbon fiber reinforced plastic sheet and the energy release rate.

The deformation mode includes modes for bending deformation, tensile deformation, shear deformation and peel deformation. In the tensile test of the carbon fiber reinforced plastic structure, at least one or more bending deformation, tensile deformation, shear deformation, and peeling deformation are generated therein.

Therefore, the bending deformation, the tensile deformation, the shear deformation, and the peeling deformation are respectively tested, and data for each deformation mode are respectively constructed.

In the experiment, the measured value includes strain in each channel and an electrical resistance value according to the strain. That is, the electrical resistance value according to the stress and strain in the channel for each deformation mode is measured.

After measuring the electrical resistance value, the resistance change rate is calculated to derive a resistance-strain relationship representing the relationship between the strain rate and the resistance change rate.

The resistance-strain relationship according to the deformation mode is stored in a database.

The resistance-strain relationship is represented by Equation 1.

<Equation 1>

(α, β, γ) = f (ε)

Here, α, β, and γ are resistance change rates in each channel for each deformation mode, and ε is the strain rate.

The resistance-strain relationship is constructed in different ways according to the deformation mode.

As described above, the resistance-strain relationship for the deformation mode is stored in the database.

Thereafter, the user inputs variables including shape information, modulus of elasticity, and deformation mode of the carbon fiber reinforced plastic structure, which is a structure to be diagnosed, for stress diagnosis (S2).

The controller finds a resistance-strain relationship for the deformation mode input from the database (S3).

In this case, since the resistance-strain relation formula is constructed differently according to each deformation mode, a plurality of equations are derived when inputting a plurality of deformation modes.

The controller finds a plurality of resistance-strain relations respectively corresponding to the input deformation modes, and combines them to derive one resistance-strain relation.

For example, when the bending deformation mode and the tensile deformation mode are input in the input step, the first equation (f1), which is a resistance-strain relationship for the bending deformation mode, and a second resistance-strain relationship for the tensile deformation mode, may be used. Equation (f2) is derived respectively.

The controller calculates a weight of the bending deformation mode and the tensile deformation mode on the deformation of the carbon fiber reinforced plastic structure using the shape information input in the input step.

The controller applies the weight (a) of the bending deformation mode to the first equation, applies the weight (b) of the tensile deformation mode to the second equation, and then combines the weighted equations to combine one resistance. -Recalculate as strain relationship.

The recalculated resistance-strain relationship is given by Equation 2.

<Equation 2>

(α, β, γ) = f (ε) = a * f1 (ε) + b * f2 (ε)

Where a and b are weights and constants.

Thereafter, the resistance measurement module (not shown) measures an electrical resistance value in all channels of the carbon fiber reinforced plastic structure.

When the electrical resistance value is measured, the resistance change rate (α, β, γ) is calculated from the measured value.

Substituting the resistance change rate (α, β, γ) into Equation 2, which is the resistance-strain relationship, strain (ε) can be calculated in each channel (S5).

When the strain ε is calculated in the channels, the strain is substituted into the relationship between stress and strain of the carbon fiber reinforced plastic structure.

The relationship between stress and strain of the carbon fiber reinforced plastic structure is shown in Equation 3.

<Equation 3>

σ = Eε

Where E is Young's modulus and ε is the strain obtained from Equation 2.

Therefore, if the strain ε is known in the channels, the stress σ may be calculated in each channel (S6).

As shown in FIG. 5, the stresses calculated in all channels of the carbon fiber reinforced plastic structure may be combined to represent a stress distribution diagram of the entire carbon fiber reinforced plastic structure.

Looking at the stress distribution diagram, it is possible to confirm the presence or absence of cracking and the state of crack development.

Meanwhile, referring to FIG. 4, the controller detects a channel in which compressive stress appears among the plurality of channels as a channel just before cracking.

The location of the crack may be derived from the position information of the channel immediately before the crack when the channel immediately before the crack is detected.

In addition, the control unit detects a channel in which a crack tip is located in a crack tip channel in which a tensile stress of the plurality of channels increases by more than a set ratio.

The location of the crack may be derived from the location information of the crack tip channel.

On the other hand, Figure 3 is a diagram schematically showing the crack propagation of the carbon fiber reinforced plastic structure according to an embodiment of the present invention as an electric resistance equivalent circuit.

Referring to FIG. 3A, when the tensile force is not applied to the carbon fiber reinforced plastic structure, each initial resistance value R of the first, second, third and fourth channels C1 to C4 is the same.

Referring to FIG. 3B, when a tensile force is applied to the carbon fiber reinforced plastic structure, the resistance value of the first and second channels C1 and C2 located close to the portion where the tensile force is applied increases slightly. The resistance change rate of the first and second channels C1 and C2 is β.

Referring to FIG. 3C, when a tensile force is applied to the carbon fiber reinforced plastic structure to cause cracking, and when the crack approaches the first channel C1, the resistance value of the first channel C1 decreases. Indicates. That is, the resistance value decreases in the channel approaching the crack. The resistance change rate of the first channel C1 is ??, and the resistance change rate of the second and third channels C2 and C3 is β.

Referring to FIG. 3D, when the crack passes through the first channel C1, the resistance of the first channel C1 increases again, and the resistance of the second channel C2 to which the crack approaches is increased. The value indicates decreasing. The resistance change rate of the second channel C2 is ??, the resistance change rate of the third and fourth channels C3 and C4 is β, and the resistance change rate of the first channel C1 is α.

Referring to FIG. 3E, as the crack continues to progress through the first channel C1, the resistance value of the first channel C1 continues to increase linearly.

As described above, when a plurality of channels are formed to be spaced apart from each other by a predetermined interval in the direction of crack growth, and the resistance values are respectively measured in the plurality of channels, the resistance change rate measured in the plurality of channels according to the crack propagation is increased. You can see that it is different.

Therefore, the resistance change rate varies depending on the strain rate, and when the relation between the strain rate and the resistance change rate is constructed in a database, the strain rate can be obtained by knowing the resistance change rate later.

Knowing the strain, the stress can be calculated from Equation (3).

Since the stress distribution can be represented by the above method, it can be used in place of FEM.

When the stress distribution diagram is displayed, it is possible to confirm the presence or absence of cracks and the state of crack development.

Although the present invention has been described with reference to the embodiments shown in the drawings, this is merely exemplary, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

10: carbon fiber reinforced plastic structure
11, 12, 13, 14: upper electrode
15,16,17,18: lower electrode

Claims (8)

In the method for diagnosing stress of a carbon fiber reinforced plastic structure having a plurality of electrodes are laminated to a plurality of carbon fiber reinforced plastic sheets, and a plurality of electrodes to form a plurality of channels in a position spaced apart from each other at a predetermined interval from the inside ,
By applying a deformation mode of at least one of bending, tensioning, shearing and peeling to the plurality of carbon fiber reinforced plastic structures through an experiment, and measuring the electrical resistance value according to the stress and strain in the channels for each deformation mode, Deriving a resistance-strain relationship representing a relationship between the strain rate and the resistance change rate for each deformation mode and constructing a database;
In the stress diagnosis, the user inputs shape information, elastic modulus, and deformation mode of the carbon fiber reinforced plastic structure to be diagnosed as a variable;
The control unit finding the resistance-strain relationship for the strain mode from the database;
Measuring an electrical resistance value for each of the plurality of channels formed in the carbon fiber reinforced plastic structure;
Calculating strain in the channels by substituting the measured electrical resistance value into the resistance-strain relationship;
Calculating strain in the channels, substituting the strain and elastic modulus into a relationship between stress and strain of the carbon fiber reinforced plastic structure to calculate stress at the positions of the channels;
And representing the stress calculated at the positions of the channels as a stress distribution inside the carbon fiber reinforced plastic structure. The method according to claim 1,
When a plurality of the deformation modes are input in the input step,
The controller may be further configured to find and combine the plurality of resistance-strain relations for the input deformation modes from the database and recalculate the resistance-strain relations for the plurality of deformation modes. Stress diagnosis method of carbon fiber reinforced plastic structure comprising. The method according to claim 2,
In the reclaiming step,
Using the shape information input in the input step, the weight of the plurality of deformation modes to the deformation of the carbon fiber reinforced plastic structure, respectively,
And applying the weights to the plurality of resistance-strain relations for the input strain modes, respectively, and combining the weights into one resistance-strain relation. The method according to claim 1,
The controller detects a channel in which compressive stress appears among the plurality of channels as a channel just before cracking,
And a method for diagnosing stress of the carbon fiber reinforced plastic structure which derives the position of the crack from the position information of the channel immediately before the crack when the channel immediately before the crack is detected. The method according to claim 1,
The control unit detects a channel in which a crack tip is located as a crack tip channel in which a tensile stress of the plurality of channels increases by more than a set ratio,
Stress diagnosis method of the carbon fiber reinforced plastic structure for deriving the position of the crack from the position information of the crack tip channel. In the method for diagnosing stress of a carbon fiber reinforced plastic structure having a plurality of electrodes are laminated to a plurality of carbon fiber reinforced plastic sheets, and a plurality of electrodes to form a plurality of channels in a position spaced apart from each other at a predetermined interval from the inside, ,
By applying a deformation mode of at least one of bending, tensioning, shearing and peeling to the plurality of carbon fiber reinforced plastic structures through an experiment, and measuring the electrical resistance value according to the stress and strain in the channels for each deformation mode, Deriving a resistance-strain relationship representing a relationship between the strain rate and the resistance change rate for each deformation mode and constructing a database;
In the stress diagnosis, the user inputs shape information, elastic modulus, and the plurality of deformation modes of the carbon fiber reinforced plastic structure to be diagnosed as a variable;
The controller calculates weights of the plurality of deformation modes on the deformation of the carbon fiber reinforced plastic structure, respectively, by using the shape information input in the input step, and controls the plurality of deformation modes for the input deformation modes from the database. Finding resistance-strain relations and applying the weights to combine resistance-strain relations to derive one resistance-strain relation for the plurality of strain modes;
Measuring a resistance change for each of the plurality of channels formed in the carbon fiber reinforced plastic structure;
Calculating strain in the channels by substituting the measured electrical resistance change into the resistance-strain relationship;
Calculating strain in the channels, substituting the strain and elastic modulus into a relationship between stress and strain of the carbon fiber reinforced plastic structure to calculate stress at the positions of the channels;
Representing the stress calculated at the location of the channels as a stress distribution within the carbon fiber reinforced plastic structure;
The control unit detects a channel in which compressive stress appears among the plurality of channels as a channel just before cracking, and converts a position of a channel in which the tensile stress increases above a predetermined ratio among the plurality of channels as a crack tip channel having a crack tip. And detecting and deriving the position of the crack from the position information of the detected channel. A plurality of electrodes provided to form a plurality of channels therebetween at positions spaced apart from each other at upper and lower sides of the carbon fiber reinforced plastic structure formed by stacking a plurality of carbon fiber reinforced plastic sheets;
Wires connected to the plurality of electrodes, respectively;
A resistance measuring module measuring a change in resistance of the plurality of channels;
A database storing a resistance-strain relationship representing a relationship between strain and resistance change measured in the channels according to deformation modes including bending, tension, shear and peeling applied to the carbon fiber reinforced plastic structures;
When the resistance change is measured in the resistance measurement module, the electrical resistance change measured above is substituted into the resistance-strain relationship equation to calculate strain in the channels, and the strain and modulus of elasticity of the carbon fiber reinforced plastic structure And a control unit that calculates a stress at the positions of the channels and substitutes a relationship between stress and strain, and determines crack propagation according to the calculated stresses. The method according to claim 7,
And a display unit representing the stress calculated at the positions of the channels as a stress distribution inside the carbon fiber reinforced plastic structure.

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