Method for measuring crack length of undetectable structure of airplane

Abstract

The invention belongs to the technical field of fatigue strength tests of aircraft parts, and discloses a crack length measuring method for an undetectable aircraft structure, which comprises the following steps: establishing a corresponding relation between a crack length and a strain value of a reference point through finite element detail stress / strain calculation analysis, and then actually measuring a strain change trend of the stress reference point through a test to measure the crack length of the undetectable aircraft structure. And the two are associated and compared, so that the position and length of the crack are determined. The method solves the problem that crack detection of an undetected part in an existing airplane fatigue strength test is not easy to implement, can efficiently and quickly measure cracks of the undetected part in the strength test, and is very practical.

Description

Technical Field

[0001] The invention belongs to the technical field of fatigue strength testing of aircraft parts, and in particular relates to a method for measuring the crack length of an untestable structure of an aircraft. Background Art

[0002] Fatigue strength testing is an indispensable task in the aircraft development process. Its main purpose is to expose crack damage in weak parts of the aircraft structure and provide reference for structural improvement design. An important issue that needs to be considered when formulating a test plan is how to detect fatigue cracks in various structures in a timely manner during the test, especially those parts that are blocked from visual inspection.

[0003] In existing fatigue strength tests, the detection of cracks in structures that cannot be inspected visually from the outside is mainly done by using special equipment to take X-ray films of the uninspectable parts and then find the specific location of the cracks through interpretation. The inspection process is cumbersome, and the test needs to be suspended during the inspection so that the inspectors can take pictures of the uninspectable parts one by one and find the cracks after a detailed interpretation of the films. This is time-consuming, labor-intensive, inefficient, and requires high inspection capabilities of the inspectors. Summary of the invention

[0004] In order to solve the problem of low test efficiency caused by the cumbersome detection process of cracks in parts that cannot be visually inspected from the outside in the existing fatigue strength test, the present invention proposes a method for measuring the crack length of an aircraft uninspectable structure. Through finite element detailed stress / strain calculation and analysis, a corresponding relationship between the crack length and the strain value of the reference point is established, and then the strain change trend of the stress reference point is measured through the test, and the two are correlated and compared to determine the crack position and length. The process is relatively simple and direct, and since there is no need to pause the test midway, the test efficiency is improved.

[0005] The technical solution of the present invention is achieved in this way:

[0006] A method for measuring the crack length of an untestable aircraft structure comprises the following steps:

[0007] 1) Based on the structural 3D model of the test piece, a detailed finite element model including untestable parts is established, and test constraints and test loads are applied to the finite element model;

[0008] 2) performing finite element detailed stress / strain calculation on the detailed finite element model in step 1) to obtain detailed stress / strain distribution of the undetectable part;

[0009] 3) In the detailed stress distribution cloud map obtained in step 2), select the area where cracks are likely to occur in the uninspectable part in the high stress or stress concentration area;

[0010] 4) setting a number of reference points at the external inspectable parts of the crack-prone area obtained in step 3);

[0011] 5) Arranging strain gauges on the fatigue strength test piece according to the position of the reference point obtained in step 4), and measuring the strain data of the strain gauges in real time during the test;

[0012] 6) When the strain gauge measurement data obtained in step 5) shows an obvious trend change, it can be determined that a crack occurs in the undetectable part near the reference point corresponding to the strain gauge;

[0013] 7) Corresponding to the reference points near the crack obtained in step 6), using the strain measurement data of these reference points, obtain the strain change curve at the reference point position;

[0014] 8) modifying the detailed finite element model in step 1): corresponding to the reference point position described in step 6), setting cracks of different shapes in the undetectable area near the position, thereby obtaining detailed finite element models of several different cracks;

[0015] 9) performing stress calculation on the detailed finite element model obtained in step 8) to obtain strain values ​​of reference points with different crack lengths;

[0016] 10) Correlating the strain value of the reference point obtained in step 9) with the crack morphology data to obtain strain variation curves of different cracks and strain values ​​of each reference point;

[0017] 11) comparing the strain change curve obtained in step 10) with the curve obtained by actual measurement in step 7);

[0018] 12) According to the comparison result of step 11), the strain bar variation curve that best matches the measured curve is selected to obtain crack information.

[0019] As a further solution of the present invention: the strain gauge in step 5) is a sensor for measuring small deformations of a structure. When it is pasted on the surface of the structure and powered on, the microstrain of the location can be obtained, and then the structural stress at the location of the strain gauge can be obtained.

[0020] As a further solution of the present invention: a plurality of reference points near the cracks in the undetectable part in step 7) are selected, and the number of reference points should not be less than 4. The more reference points there are, the easier it is to compare the strain change curves in step 11).

[0021] As a further solution of the present invention: the crack morphology in step 8) includes the length of the crack and the position of the crack tip.

[0022] As a further solution of the present invention: the process of establishing the strain change curve in step 10) is:

[0023] The details of several different crack shapes are calculated one by one in the finite element model to obtain the strain values ​​of several reference points in the vicinity of each crack shape;

[0024] The strain values ​​of each reference point under each crack morphology are placed in a line graph, the horizontal axis coordinates are the numbers of the reference points in a certain order, and the vertical axis is the strain value of the reference point with the corresponding number calculated in the detailed finite element model. The strain change curve of each reference point under this crack morphology is obtained by connecting the strain values ​​of each number on the vertical axis;

[0025] By analogy, a series of strain distribution curves of each reference point under different crack shapes can be obtained.

[0026] As a further solution of the present invention: the comparison process in step 11) is:

[0027] In step 9), the strain distribution curves of each reference point under a series of different crack morphologies obtained based on the detailed finite element calculation are compared with the strain change curve obtained in step 7), and the strain change curve with the closest value and change trend between the two is screened out. The crack morphology in the corresponding detailed finite element model can be obtained, thereby obtaining the actual crack morphology information of the uninspectable part on the test piece.

[0028] An electronic device for executing the above-mentioned measurement method comprises: a processor and a memory storing a computer program, and when the processor runs the computer program, the steps of the above-mentioned measurement method are implemented.

[0029] A computer storage medium stores a computer program, which implements the steps of the above-mentioned measurement method when executed by a processor.

[0030] The beneficial effects of this application are:

[0031] 1. Through the stress / strain calculation of the detailed finite element model, a series of strain change curves of each reference point under different crack shapes are obtained; then the strain distribution data of each reference point is obtained through actual measurement, and the two are compared to obtain the actual crack shape information of the undetectable parts on the test piece.

[0032] 2. There is no need for special tools such as X-ray equipment, which significantly saves crack detection time and reduces financial investment. It can efficiently and quickly measure cracks in undetectable areas during strength tests, which is very practical.

[0033] The present application is further described in detail below in conjunction with the accompanying drawings of the embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A schematic diagram of a test piece for a typical problem solved by the present invention;

[0035] Figure 2 Detailed finite element model diagram including test constraints and test loads;

[0036] Figure 3 Schematic diagram of reference points RP set near the uninspectable area of ​​the lower skin of the test piece;

[0037] Figure 4 Schematic diagram of the trend change of strain data measured by the strain gauge corresponding to the reference point RP in real time;

[0038] Figure 5 Schematic diagram of detailed finite element model corresponding to different crack morphologies;

[0039] Figure 6 Schematic diagram of strain distribution curves of each reference point RP under different crack shapes calculated by the detailed finite element model;

[0040] Figure 7 Schematic diagram of the strain distribution curve of each reference point RP obtained under normal loading of the specimen.

[0041] The accompanying drawings are marked: . DETAILED DESCRIPTION

[0042] The technical solution of the present application will be described clearly and completely below in conjunction with the accompanying drawings of the embodiments.

[0043] Figures 1 to 7 It is a schematic diagram for explaining an embodiment of a method for measuring crack length of an uninspectable structure of an aircraft according to the present invention.

[0044] The test piece in this example is Figure 1 As shown, at the connection area of ​​the upper skin, lower skin and inner lining plate of the test piece structure, the inner and outer surfaces of the lower skin are connected to the upper skin and the inner lining plate respectively. This area of ​​the lower skin is an external untestable part. The problem in this example is that in the fatigue test, it is confirmed Figure 1 Whether fatigue cracks occur in the external uninspectable parts of the test piece under the test load and the length of the cracks.

[0045] The specific steps are as follows:

[0046] Step 1: Establish a detailed finite element model of the test piece and apply test constraints and test loads to the finite element model. Figure 2 .

[0047] Step 2: Calculate the detailed finite element model to obtain the stress and strain numerical distribution of the undetectable parts of the lower skin.

[0048] Step 3: Select several rivet hole edges or other stress concentration areas in the uninspectable parts of the lower skin, and set 4 reference points RP at the corresponding external inspectable parts, see Figure 3 .

[0049] Step 4: On the actual fatigue strength test piece, Figure 3 Paste four strain gauges at the four reference points RP in the test, and then start the fatigue test to measure the strain data of the strain gauges in real time.

[0050] Step 5: When the strain data of the real-time measured strain gauge (e.g. at RP2) shows a trend change (see Figure 4 ), it can be determined that a crack of a certain length appears in the uninspectable part of the lower skin near the reference point RP2.

[0051] Step 6: For several reference points (RP2, RP1, RP3 and RP4) near the cracks in the undetectable part identified in step 5, record the strain data measured during the last loading, and make a strain change curve according to the position sequence of the reference points, see Figure 5 .

[0052] Step 7: Modify the detailed finite element model described in step 1: corresponding to the undetectable part of the lower skin near the reference point RP2 described in step 5 above, set cracks of different lengths and positions respectively, so as to obtain several detailed finite element models corresponding to different crack shapes (the more finite element model data containing different crack shapes, the more accurate the comparison result), see Figure 6 .

[0053] Step 8: Perform strain calculation on several detailed finite element models described in step 7 to obtain strain values ​​of reference points with different crack shapes, and associate the strain values ​​of these reference points with the crack shape data to obtain strain distribution curves of each reference point under different crack shapes, see Figure 7 .

[0054] Step nine: Compare the measured strain distribution curve obtained in step six with the strain distribution curves of each reference point under different crack shapes obtained in step eight.

[0055] Step 10: According to the comparison result of step 9, it is judged that curve 3 is most consistent with the measured strain distribution curve. Therefore, the crack of shape 3 corresponding to curve 3 is the crack shape of the undetectable part of the lower skin of the test piece.

[0056] Functions and effects of this embodiment:

[0057] It can be seen from the above embodiments that by using the method of the present invention, the strain curve of the reference point in the finite element model is compared with the strain curve obtained by loading and measuring the test piece, and the finite element strain distribution curve that best matches the measured curve is screened out, so that the crack morphology information of the undetectable part of the test piece can be obtained. There is no need to disassemble the test piece or other special detection means to find cracks. The application of this method can save test cycles and inspection costs, and is highly practical.

[0058] In a second aspect, the present invention provides an electronic device, comprising: a processor and a memory storing a computer program; in practical applications, the number of processors may be one or more; the number of memories may be one or more. When the processor runs the computer program, the measurement method applied to the electronic device is implemented.

[0059] The memory may be a volatile memory or a non-volatile memory, or may include both volatile and non-volatile memories. The non-volatile memory may be a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), a magnetic random access memory (FRAM), a flash memory, a magnetic surface memory, an optical disk, or a compact disc read-only memory (CD-ROM); the magnetic surface memory may be a disk memory or a tape memory. The volatile memory may be a random access memory (RAM), which is used as an external cache. By way of example and not limitation, many forms of RAM are available, such as static random access memory (SRAM), synchronous static random access memory (SSRAM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), double data rate synchronous dynamic random access memory (DDRSDRAM), enhanced synchronous dynamic random access memory (ESDRAM), synchronous link dynamic random access memory (SLDRAM, SyncLink Dynamic Random Access Memory), and direct RAM bus random access memory (DRRAM, Direct Rambus Random Access Memory).

[0060] The memory of the present invention is used to store various types of data to support the operation of the electronic device. Examples of these data include: any computer program used to operate on the electronic device, such as an operating system and an application; contact data; phone book data; messages; pictures; videos, etc. Among them, the operating system includes various system programs, such as a framework layer, a core library layer, a driver layer, etc., which are used to implement various basic services and process hardware-based tasks. The application program may include various applications, such as a media player (Media Player), a browser (Browser), etc., which are used to implement various application services. Here, the program that implements the method of the embodiment of the present invention may be included in the application program.

[0061] In a third aspect, the present invention further provides a computer storage medium, wherein a computer program is stored in the computer storage medium, and the computer storage medium may be a magnetic random access memory (FRAM), a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), a flash memory, a magnetic surface memory, an optical disc, or a compact disc read-only memory (CD-ROM); or various devices including one or any combination of the above memories, such as a mobile phone, a computer, a tablet device, a personal digital assistant, etc. When the computer program stored in the computer storage medium is executed by a processor, the measurement method applied to the above electronic device is implemented.

[0062] So far, the purpose of the present invention has been achieved.

[0063] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for measuring the crack length of an untestable aircraft structure, characterized in that: The steps include: 1) Based on the structural 3D model of the test piece, a detailed finite element model including untestable parts is established, and test constraints and test loads are applied to the finite element model; 2) performing finite element detailed stress / strain calculation on the detailed finite element model in step 1) to obtain detailed stress / strain distribution of the undetectable part; 3) In the detailed stress distribution cloud map obtained in step 2), select the area where cracks are likely to occur in the uninspectable part in the high stress or stress concentration area; 4) setting a number of reference points at the external inspectable parts of the crack-prone area obtained in step 3); 5) Arranging strain gauges on the fatigue strength test piece corresponding to the position of the reference point obtained in step 4), and measuring the strain data of the strain gauges in real time during the test; 6) When the strain gauge measurement data obtained in step 5) shows an obvious trend change, it can be determined that a crack occurs in the undetectable part near the reference point corresponding to the strain gauge; 7) Corresponding to the reference points near the crack obtained in step 6), using the strain measurement data of these reference points, obtain the strain change curve at the reference point position; 8) modifying the detailed finite element model in step 1): corresponding to the reference point position described in step 6), setting cracks of different shapes in the undetectable area near the position, thereby obtaining detailed finite element models of several different cracks; 9) performing stress calculation on the detailed finite element model obtained in step 8) to obtain strain values ​​of reference points with different crack lengths; 10) Correlating the strain value of the reference point obtained in step 9) with the crack morphology data to obtain strain change curves of different cracks and strain values ​​of each reference point; 11) comparing the strain change curve obtained in step 10) with the curve obtained by actual measurement in step 7); 12) According to the comparison result of step 11), the strain bar variation curve that best matches the measured curve is selected to obtain crack information.

2. The method for measuring crack length of an untestable aircraft structure according to claim 1, characterized in that: The strain gauge in step 5) is a sensor for measuring tiny deformation of a structure. When it is pasted on the surface of a structure and powered on, the microstrain of the location can be obtained, and then the structural stress at the location of the strain gauge can be obtained.

3. The method for measuring crack length of an untestable aircraft structure according to claim 1, characterized in that: In the step 7), a number of reference points are selected near the cracks in the undetectable part. The number of reference points should not be less than 4. The more reference points there are, the easier it is to compare the strain change curves in the step 11).

4. The method for measuring crack length of an untestable aircraft structure according to claim 1, characterized in that: The crack morphology in step 8) includes the crack length and crack tip position.

5. The method for measuring crack length of an untestable aircraft structure according to claim 1, characterized in that: The process of establishing the strain change curve in step 10) is as follows: The details of several different crack shapes are calculated one by one in the finite element model to obtain the strain values ​​of several reference points in the vicinity of each crack shape; The strain values ​​of each reference point under each crack morphology are placed in a line graph, the horizontal axis coordinates are the numbers of the reference points in a certain order, and the vertical axis is the strain value of the reference point with the corresponding number calculated in the detailed finite element model. The strain change curve of each reference point under this crack morphology is obtained by connecting the strain values ​​of each number on the vertical axis; By analogy, a series of strain distribution curves of each reference point under different crack shapes can be obtained.

6. The method for measuring crack length of an untestable aircraft structure according to claim 1, characterized in that: The comparison process in step 11) is as follows: In step 9), the strain distribution curves of each reference point under a series of different crack morphologies obtained based on the detailed finite element calculation are compared with the strain change curve obtained in step 7), and the strain change curve with the closest value and change trend between the two is screened out. The crack morphology in the corresponding detailed finite element model can be obtained, thereby obtaining the actual crack morphology information of the uninspectable part on the test piece.

7. An electronic device, characterized in that: include: A processor and a memory storing a computer program, wherein when the processor runs the computer program, the measurement method according to any one of claims 1 to 6 is implemented.

8. A computer storage medium, characterized in that: A computer program is stored, and when the computer program is executed by a processor, the measurement method according to any one of claims 1 to 6 is implemented.

Images