A method for measuring the crack length of undetectable structures in aircraft

Abstract

This invention belongs to the field of fatigue strength testing technology for aircraft components. It discloses a method for measuring the crack length of undetectable structures in aircraft. Through finite element detailed stress / strain calculation and analysis, it establishes a correspondence between crack length and strain values ​​at a reference point. Then, by comparing the strain variation trend at the experimentally measured stress reference point, the two are correlated to determine the crack location and length. This method solves the problem of the difficulty in detecting cracks in undetectable parts of aircraft fatigue strength tests. It can efficiently and quickly measure cracks appearing in undetectable parts during strength tests, making it highly practical.

Description

Technical Field

[0001] This invention belongs to the field of fatigue strength testing technology for aircraft components, specifically relating to a method for measuring the crack length of undetectable structures in aircraft. Background Technology

[0002] Fatigue strength testing is an indispensable part of aircraft development. Its main purpose is to expose cracks and damage in weak points of the aircraft structure, providing a reference for structural improvement design. An important issue to consider when developing a test plan is how to detect fatigue cracks in various structures in a timely manner during the test, especially those areas that are not visible to the naked eye.

[0003] In existing fatigue strength tests, the detection of cracks in structures not visible to the naked eye mainly involves using specialized equipment to take X-ray films of the undetectable areas and then interpreting them to pinpoint the exact location of the cracks. This process is cumbersome, requiring the test to be paused so that personnel can take individual X-rays of the undetectable areas and carefully interpret the films to locate the cracks. It is time-consuming, labor-intensive, inefficient, and demands a high level of expertise from the personnel involved. Summary of the Invention

[0004] This invention addresses the problem of low testing efficiency in existing fatigue strength tests due to the cumbersome process of detecting cracks in areas undetectable by external visual inspection. It proposes a method for measuring the crack length of undetectable structures in aircraft. Through finite element detailed stress / strain calculations and analysis, a correspondence between crack length and strain values ​​at a reference point is established. Then, by comparing the strain variation trend at the measured stress reference point, the two are correlated to determine the crack location and length. The process is relatively simple and direct, and since it eliminates the need to pause the test midway, it improves testing efficiency.

[0005] The technical solution of this invention is implemented as follows:

[0006] A method for measuring the crack length of an undetectable structure in an aircraft, comprising the following steps:

[0007] 1) Based on the three-dimensional structural model of the test piece, establish a detailed finite element model including undetectable parts, and apply test constraints and test loads to the finite element model;

[0008] 2) Perform finite element stress / strain calculations on the detailed finite element model in step 1) to obtain the detailed stress / strain distribution of the undetectable parts;

[0009] 3) In the detailed stress distribution cloud map obtained in step 2), select areas prone to cracking in undetectable locations within high stress or stress concentration regions;

[0010] 4) Set several reference points on the external inspectable parts of the crack-prone area obtained in step 3);

[0011] 5) Based on the position of the reference point obtained in step 4), strain gauges are placed on the fatigue strength test specimen, and the strain data of the strain gauges are measured in real time during the test.

[0012] 6) When the strain gauge measurement data obtained in step 5) shows a significant trend change, it can be determined that a crack has appeared in the undetectable part near the reference point corresponding to this strain gauge.

[0013] 7) Using the strain measurement data of the reference points near the crack obtained in step 6), the strain change curve at the reference point location is obtained.

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

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

[0016] 10) Correlate the strain values ​​of the reference points 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) Compare the strain change curve obtained in step 10) with the curve obtained by actual measurement in step 7);

[0018] 12) Based on the comparison results of step 11), select the strain curve with the best match with the measured curve to obtain crack information.

[0019] As a further aspect of the present invention: the strain gauge in step 5) is a sensor for measuring minute deformations of a structure. After it is attached to the surface of the structure and energized, the micro-strain at the location can be obtained, and then the structural stress at the location of the strain gauge can be obtained.

[0020] As a further aspect of the present invention: in step 7), several reference points are located near the cracks in the undetectable parts. 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 aspect 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 aspect of the present invention: the process of establishing the strain change curve in step 10) is as follows:

[0023] The details of several different crack morphologies 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 morphology.

[0024] The strain values ​​of each reference point under each crack morphology are placed in a line graph. The horizontal axis is the number of the reference point according to a certain sorting, and the vertical axis is the strain value calculated by the reference point with the corresponding number in the detailed finite element model. Connecting the strain values ​​of each number on the vertical axis gives the strain change curve of each reference point under this crack morphology.

[0025] By analogy, strain distribution curves for various reference points under different crack morphologies can be obtained.

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

[0027] By comparing the strain distribution curves of each reference point under different crack morphologies obtained in step 9) with the strain change curves obtained in step 7), the strain change curves with the closest values ​​and trends are selected, and the crack morphology in the corresponding detailed finite element model can be obtained, thereby obtaining the actual crack morphology information of the undetectable parts on the test piece.

[0028] An electronic device for performing the above measurement method includes: a processor and a memory storing a computer program, wherein when the processor runs the computer program, the steps of the above measurement method are implemented.

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

[0030] The beneficial effects of this application are as follows:

[0031] 1. By calculating the stress / strain of the detailed finite element model, a series of strain variation curves of various reference points under different crack morphologies are obtained; then, the strain distribution data of each reference point is obtained by actual measurement, and the two are compared to obtain the actual crack morphology information of the undetectable parts on the test piece.

[0032] 2. It does not require special tools such as X-ray imaging equipment, which significantly saves crack detection time and costs. It can efficiently and quickly measure cracks in areas that cannot be detected during strength tests, making it highly practical.

[0033] The present application will be further described in detail below with reference to the accompanying drawings of the embodiments. Attached Figure Description

[0034] Figure 1 A schematic diagram of a test specimen for a typical problem solved by this invention;

[0035] Figure 2 A detailed finite element model diagram including experimental constraints and experimental loads;

[0036] Figure 3 A schematic diagram of reference points RP set up for the area near the undetectable parts of the lower skin of the test piece;

[0037] Figure 4 A schematic diagram showing the trend of strain data measured in real time at the reference point RP;

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

[0039] Figure 6 This is a schematic diagram of the strain distribution curves of each reference point RP under different crack morphologies, obtained by detailed finite element model calculation.

[0040] Figure 7 This is a schematic diagram of the strain distribution curves at various reference points RP obtained under normal loading of the test specimen. Detailed Implementation

[0041] The technical solution of this application will be clearly and completely described below with reference to the accompanying drawings of the embodiments.

[0042] Figures 1 to 7 This is a schematic diagram illustrating one embodiment of the method for measuring the crack length of an undetectable structure of an aircraft according to the present invention.

[0043] The test specimen in this example is as follows: Figure 1 As shown, at the connection area between the upper skin, lower skin, and inner liner plate of the test specimen structure, the inner and outer surfaces of the lower skin are connected to the upper skin and inner liner plate, respectively. This area of ​​the lower skin is an externally uninspectable part. The problem in this example is, in fatigue testing, confirming... Figure 1 The test specimen is shown to show whether fatigue cracks occur on the external, undetectable parts under the test load, and the length of the cracks.

[0044] The specific steps are as follows:

[0045] Step 1: Establish a detailed finite element model of the test specimen, and apply test constraints and loads to the finite element model. See [link to relevant documentation]. Figure 2 .

[0046] Step 2: Calculate the detailed finite element model to obtain the stress and strain numerical distribution of the undetectable parts of the lower skin.

[0047] Step 3: In the uninspectable areas of the lower skin, select several rivet hole edges or other stress concentration areas, and set 4 reference points RP at the corresponding externally inspectable areas. See Figure 3 .

[0048] Step 4: On the actual fatigue strength test specimen, corresponding to... Figure 3 Four strain gauges were attached to the four reference points RP, and then fatigue testing was carried out to measure the strain data of the strain gauges in real time.

[0049] Step 5: When the strain data of the real-time measured strain gauge (e.g., at RP2) shows a trend change (see...) Figure 4 This allows us to determine that a crack of a certain length has appeared in an undetectable part of the lower skin near reference point RP2.

[0050] Step Six: For several reference points (RP2, RP1, RP3, and RP4) near the undetectable crack location identified in Step Five, record the strain data measured during the last loading. Plot a strain change curve according to the positional order of the reference points. See... Figure 5 .

[0051] Step 7: Modify the detailed finite element model described in Step 1: For the undetectable area of ​​the lower skin near reference point RP2 described in Step 5, set cracks of different lengths and positions to obtain several detailed finite element models corresponding to different crack morphologies (the more finite element model data containing different crack morphologies, the more accurate the comparison results). See [link to relevant documentation]. Figure 6 .

[0052] Step 8: Perform strain calculations on the several detailed finite element models described in Step 7 to obtain strain values ​​at reference points with different crack morphologies. Correlate these strain values ​​at reference points with crack morphology data to obtain strain distribution curves for each reference point under different crack morphologies. See... Figure 7 .

[0053] Step 9: Compare the measured strain distribution curve obtained in Step 6 with the strain distribution curves of various reference points under different crack morphologies obtained in Step 8.

[0054] Step 10: Based on the comparison results in Step 9, it is determined that curve 3 is the closest to the measured strain distribution curve. Therefore, the crack in morphology 3 corresponding to curve 3 is the crack morphology of the undetectable part of the lower skin in the test piece.

[0055] The function and effect of this embodiment:

[0056] As can be seen from the above embodiments, by comparing the strain curve of the reference point in the finite element model with the strain curve obtained by loading the test piece, the finite element strain distribution curve that best matches the measured curve can be selected, and the crack morphology information of the undetectable parts in the test piece can be obtained. There is no need to disassemble the test piece or use other special detection methods to find cracks. Applying this method can save the test cycle and inspection cost, and it is highly practical.

[0057] Secondly, 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 above-described electronic device is implemented.

[0058] The memory can be volatile or non-volatile, or both. Non-volatile memory can be read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), ferromagnetic random access memory (FRAM), flash memory, magnetic surface memory, optical disc, or compact disc read-only memory (CD-ROM); magnetic surface memory can be disk storage or magnetic tape storage. Volatile memory can be random access memory (RAM), used as an external cache. By way of example, but 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), SyncLink Dynamic Random Access Memory (SLDRAM), and Direct Rambus Random Access Memory (DRRAM).

[0059] The memory of this invention is used to store various types of data to support the operation of the electronic device. Examples of such data include: any computer programs used to operate on the electronic device, such as operating systems and applications; contact data; phonebook data; messages; pictures; videos, etc. The operating system includes various system programs, such as a framework layer, core library layer, driver layer, etc., used to implement various basic services and handle hardware-based tasks. Applications can include various applications, such as media players, browsers, etc., used to implement various application services. Here, the program implementing the method of the embodiments of this invention can be included in the application.

[0060] Thirdly, the present invention also provides a computer storage medium storing a computer program. The computer storage medium can 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), etc.; or it can be various devices including one or any combination of the above-mentioned memories, such as mobile phones, computers, tablet devices, personal digital assistants, etc. When the computer program stored in the computer storage medium is executed by a processor, it implements the measurement method applied to the aforementioned electronic device.

[0061] Thus, the objective of this invention has been achieved.

[0062] The above are merely 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 within the protection scope of the present invention.

Claims

1. A method for measuring the crack length of an undetectable structure in an aircraft, characterized in that, Includes the following steps: 1) Based on the three-dimensional structural model of the test piece, establish a detailed finite element model including undetectable parts, and apply test constraints and test loads to the finite element model; 2) Perform finite element stress / strain calculations on the detailed finite element model from step 1) to obtain the detailed stress / strain distribution of the undetectable parts; 3) In the detailed stress distribution cloud map obtained in step 2), select the areas where cracks are likely to occur in the undetectable parts of the high stress or stress concentration areas; 4) Set several reference points on the external inspectable parts of the crack-prone area obtained in step 3); 5) Based on the position of the reference point obtained in step 4), strain gauges are placed on the fatigue strength test specimen, and the strain data of the strain gauges are measured in real time during the test. 6) When the strain gauge measurement data obtained in step 5) shows a significant trend change, it can be determined that a crack has appeared in the undetectable part near the reference point corresponding to this strain gauge. 7) Using the strain measurement data of the reference points near the crack obtained in step 6), the strain change curve at the reference point location is obtained. 8) Modify the detailed finite element model in step 1): corresponding to the reference point position described in step 6), set cracks of different shapes in the undetectable area near the position, thereby obtaining several detailed finite element models of different cracks. 9) Perform stress calculations on the detailed finite element model obtained in step 8) to obtain strain values ​​at reference points with different crack lengths; 10) Correlate the strain values ​​of the reference points obtained in step 9) with the crack morphology data to obtain strain variation curves of different cracks and strain values ​​of each reference point. 11) Compare the strain change curve obtained in step 10) with the curve obtained by actual measurement in step 7); 12) Based on the comparison results of step 11), select the strain change curve that best matches the measured curve to obtain crack information.

2. The method for measuring the crack length of an undetectable structure in an aircraft according to claim 1, characterized in that, The strain gauge in step 5) is a sensor for measuring minute deformations of a structure. When it is attached to the surface of the structure and energized, the micro-strain at 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 the crack length of an undetectable structure of an aircraft according to claim 1, characterized in that, In step 7), several reference points are located near the crack in the undetectable area. 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).

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

5. The method for measuring the crack length of an undetectable structure of an aircraft 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 morphologies 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 morphology. The strain values ​​of each reference point under each crack morphology are plotted in a line graph. The horizontal axis represents the reference point number according to a certain sorting, and the vertical axis represents the strain value calculated by the corresponding reference point in the detailed finite element model. Connecting the strain values ​​of each number on the vertical axis yields the strain change curve of each reference point under this crack morphology. By analogy, strain distribution curves for various reference points under different crack morphologies can be obtained.

6. The method for measuring the crack length of an undetectable structure of an aircraft according to claim 1, characterized in that, The comparison process in step 11) is as follows: By comparing the strain distribution curves of each reference point under different crack morphologies obtained in step 9) with the strain change curves obtained in step 7), the strain change curves with the closest values ​​and trends can be selected to obtain the crack morphology in the corresponding detailed finite element model, thereby obtaining the actual crack morphology information of the undetectable parts 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 executes 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, The device contains a computer program that, when executed by a processor, implements the measurement method according to any one of claims 1 to 6.

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