CN109682680B - Complex composite material mechanical connection nail load measurement test method - Google Patents
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Abstract
The invention discloses a complicated composite material mechanical connection nail load measurement test method, and belongs to the technical field of mechanical devices and transportation. The method comprises the following steps: applying loads at two ends of the test piece, and measuring strain fields of the front surface and the side surface of the laminated board at the outer side of the test piece by adopting DIC equipment; respectively selecting sections at different load transmission positions of two outer side laminated plates of a test piece, wherein the sections are intersected with the surfaces of the outer side laminated plates to form lines along the width and thickness directions of the outer side laminated plates as strain extraction lines; extracting strain along a strain extraction line in a width direction of the outer laminate surface for each cross section; extracting, for each cross section, strains at both ends of a strain extraction line in the thickness direction of the outer laminate; calculating the average strain of each section of the outer laminate; calculating the load transmitted by the outer laminated plate at each section; and calculating the load distribution ratio of the bolt to the laminated plate outside the test piece at different sections. The method has high calculation precision and low cost, and is more beneficial to large-scale application.
Description
Technical Field
The invention relates to the technical field of mechanical devices and transportation, in particular to a nail load measurement test method for mechanical connection of complex composite materials.
Background
The use level of composite materials in the airplane structure is higher and higher, and the composite materials are applied to main load-bearing structures such as airframes, wings and the like at present. Mechanical bonding is an important form of bonding for composite structures. Nail load distribution is the basis for composite mechanical joint design and strength analysis. The pin load distribution of the mechanical connection of the composite material is significantly different relative to the metallic structure, since the composite material appears brittle until failure. The test method for measuring the mechanical connection nail load distribution of the composite material has important significance for verifying the design and analysis method of the composite material connection structure.
At present, the composite material mechanical connection nail load test method mainly comprises a strain gauge measurement method and a nail load sensor method. In the strain gauge measuring method, a strain gauge is adhered to the surface of a mechanically connected plate, and the nail load distribution ratio is deduced according to the difference of average strain values among different nail rows. In the nail load sensor method, a bolt-shaped sensor is adopted to replace a real bolt, and the extrusion load of each nail hole is obtained through a sensor load signal output by the sensor during loading.
For the strain gauge measurement method, because the strain field variation gradient of the connected plate between the fasteners is large, the strain gauge can only measure the strain value of a plurality of discrete points, and the deviation between the strain mean value and the true mean value of the cross section is large, so that a large error is generated. In addition, due to the eccentric action, the connecting structure generates a secondary bending moment, which causes the connected piece to bend. The strain on the outer upper and lower surfaces of the connected member is different, and the error is further increased by calculating the average strain of the entire section using only the strain on the outer surface. Practice has proved that this method has major drawbacks and the range of application is limited.
For the nail load sensor method, the sensor is both a gauge and a fastener. The load borne by the sensor is represented by a strain rosette output signal stuck inside the metal rod, so that four grooves are formed around the metal rod of the sensor. On one hand, after slotting, the matching states of the metal rod and the screw hole in the real situation are different, and the measurement result cannot reflect the distribution characteristics of the nail loads in the actual structure; on the other hand, four grooves seriously impair the load-bearing capacity of the sensor rod, and the method can only measure the load distribution in a low-load condition. In addition, the sensor is difficult to manufacture, and the technology is not mature and expensive.
For the direct nail load measurement test method, all fasteners of the test piece must be identical and the thickness of the joined plates must not vary in the joining area. Whereas in aircraft structures the configuration of the connecting structure is much more complex. For these structures, the non-contact direct measurement test method is no longer applicable.
In addition, the inventor proposes a direct measurement test method for the nail load of the mechanical connection of the composite material, and calculates the nail load distribution ratio of the connection structure by directly measuring the connection deformation of the composite material. The method is convenient to operate and reliable in result when used for mechanical connection with the same fasteners and simple structure; however, this method is no longer suitable for connection structures with complex configurations.
Disclosure of Invention
In order to solve the problems, the invention provides a nail load measurement test method for mechanical connection of complex composite materials, which is used for nail load distribution test of complex-structure composite material mechanical connection structures with variable thicknesses of connected plates, different bolt diameters and the like. The invention adopts non-contact measurement technology to measure the strain fields of different sections of the connected plate and calculates the nail load distribution proportion in the connecting structure. The test method has the advantages of high measurement precision, wide range, wide application range and universal applicability.
According to a first aspect of the present invention, there is provided a method of nail load measurement testing of a mechanical connection of a complex composite, the mechanical connection of the complex composite being a double shear connection comprising a first outer laminate, a second outer laminate and an intermediate laminate, and a plurality of bolts connecting the laminates, the two outer laminates may be of the same or different lengths, the bolts may be of the same or different diameters and types, characterised in that the method comprises measuring the outer surface and side strain fields of the mechanical connection of the two outer laminates by means of a Digital Image Correlation (DIC) apparatus, calculating the average strain of the two outer laminates at different sections accurately, calculating the load transmitted by the two outer laminates and deriving therefrom a nail load distribution ratio.
Further, the method comprises the following steps:
step 2: respectively selecting sections at different load transmission positions of two outer side laminated plates mechanically connected by a complex composite material, wherein the sections are intersected with the surfaces of the outer side laminated plates to form lines along the width and thickness directions of the outer side laminated plates as strain extraction lines;
and step 3: extracting strain along a strain extraction line of the outer laminate for each section, and calculating an average strain of each section of the outer laminate;
and 4, step 4: calculating the load transmitted by the first and second outer laminates at each cross section;
and 5: and calculating the load distribution ratio of the bolt to the laminated plate outside the test piece at different sections.
Further, the step 3 specifically includes:
step 301: for the i-th cross section, the strain is extracted along the strain extraction line in the width direction of the outer laminate surface, which is recorded as
Step 302: for the ith cross section, the strains at both ends of the strain extraction line in the thickness direction of the outer laminate were extracted, and the strain at the outer surface edge of the outer laminate was recorded as
Strain at the edge of the inner surface of the outer laminate is noted
Step 303: calculating the average strain of the i-th cross section of the outer laminate
Further, in step 303, the average strain of the i-th cross section of the outer laminate is calculated
The method specifically comprises the following steps:
wherein n is an integer greater than 2, wiIs the laminate width.
Further, in step 4, calculating the load transmitted by the first outer laminated plate and the second outer laminated plate at the ith cross section specifically as follows:
in the formula, F1iFor loads transferred in the i-th cross-section of the first outer laminate,
average strain for the ith cross section of the first outer laminate obtained by step 3; k is a radical of1iAn in-plane stiffness of the first outer laminate at the i-th cross-section; f2iFor the loads transmitted by the second outer laminate in the i-th cross-section,
is the average strain of the ith cross-section of the second outer laminate obtained by step 3; k is a radical of2iN is an integer greater than 2 for the in-plane stiffness of the second outer laminate at the i-th cross-section.
Further, in step 5, calculating the load transmitted by the ith bolt to the first and second outer laminates of the test piece specifically as follows:
in the formula (I), the compound is shown in the specification,
for the i-th bolt to transmit to the first outer laminate, F1iThe load transmitted in the ith section of the first outer laminated plate obtained in the step 4;
for the load transmitted by the ith bolt to the second outer laminate, F2iIs the load transmitted in the ith cross-section of the second outer laminate obtained by step 4.
Further, in step 2, the number of sections taken on the outer laminate is the same as the number of bolts on the outer laminate.
Further, in step 2, the cross-section taken on the outer laminate is located between adjacent bolts and outside the outermost bolt.
Further, the DIC device is fixed on an external support and connected with a computing device provided with image acquisition software and strain analysis software.
Further, the DIC equipment is provided with image acquisition software and strain calculation software.
The invention has the beneficial effects that:
the invention relates to a complicated composite material mechanical connection nail load measurement test method, which is used for complicated configuration connection nail load distribution test and provides test data for verification of a composite material mechanical connection structure design and analysis method. The technology uses Digital Image Correlation (DIC) as a tool, calculates the load transmitted by a connecting plate by measuring strain fields of the outer surface and the side surface of the connecting plate mechanically connected by the composite material, and further deduces the distribution condition of the nail load.
In addition, compared with the prior art, the technical scheme of the invention has the following advantages:
1) compared with a strain gauge measuring method, the nail load measurement test method provided by the invention has the advantages that the average strain is calculated according to the strain distribution result of the outer surface of the connected plate along the width, and the average strain calculation precision is obviously improved through strain measurement at a plurality of discrete points compared with the strain gauge method; in addition, the average strain in the width direction is corrected according to the strain result of the outer laminated plate along the thickness direction, so that the influence of bending deformation caused by eccentric bending moment is eliminated, the calculation accuracy is further improved, and the error cannot be eliminated by a strain gauge measuring method.
2) Compared with a nail load sensor method, the method provided by the invention has the advantages that the mechanical connection of the composite material is measured in a non-contact manner, the test piece is not interfered, the measured result is closer to the nail load state of a real structure, and the method can be used for the test verification of the structure; in addition, the invention can measure the nail load distribution of the composite material from mechanical connection to damage, and the nail load sensor method can only carry out low-load measurement due to the limitation of the sensor range. Finally, compared with a nail-carried sensor method, the method has low cost and is more beneficial to large-scale application.
3) Compared with the direct nail load measurement method which can only be used for the composite material mechanical connection structure with simple structure, the method can be used for the connection structure with the thickness and width change of the connected piece or different diameters of the fastener, and has wider application range.
Drawings
FIG. 1 shows a schematic flow diagram of a complex composite mechanical connection nail load measurement test method according to the present invention;
FIG. 2 shows a schematic view of a mechanical connection of composite materials;
FIG. 3 shows a schematic view of composite material mechanical connection load transfer and deformation;
FIG. 4a shows a schematic diagram of a test piece strain front measurement;
FIG. 4b shows a schematic of a test piece strain backside measurement;
fig. 4c shows a schematic diagram of a lateral measurement of the strain of the test piece.
Detailed Description
The complex composite mechanical connection nail load measurement test method according to the present invention will be described in detail with reference to the accompanying drawings.
The invention relates to a nail load measurement test method for mechanical connection of a complex composite material. The method is used for designing the composite material connecting structure of the airplane and analyzing the strength.
The technical principle of the invention is as follows:
(1) averaging after the full-field strain integral of the front face of the outer laminated plate to obtain the average strain of the outer laminated plate, combining the rigidity of the outer laminated plate to obtain the load transmitted by different positions of the connecting plate, and further obtaining the nail load according to the load difference of the outer laminated plates on two sides of the fastener;
(2) on the basis of the key point (1), correcting the average strain of the outer laminated plate by the full-field strain of the side surface of the outer laminated plate, and eliminating the influence of bending moment, namely calculating the strain data in the thickness direction of the laminated plate;
(3) the non-contact measurement method is adopted for measuring the full-field strain of the connected plate, the strain data is continuous, no interference is caused to a test piece, and the test load range is larger.
Referring to fig. 1, the complicated composite material mechanical connection nail load measurement test method according to the invention comprises the following steps:
and step 101, applying loads to two ends of the test piece, and measuring strain fields of the front surface and the side surface of the first outer laminated plate and the second outer laminated plate of the test piece by using DIC equipment. The DIC equipment is fixed on an external bracket and is provided with picture acquisition software and strain calculation software.
Step 102: the cross sections were taken on each of the two outer laminates of the test piece, and the cross sections intersected with the outer laminate surface to form lines in the width and thickness directions of the outer laminates as strain extraction lines. The cross-section is located between adjacent bolts and outside the outermost bolt. So that the number of cross-sections selected is the same as the number of bolts on the outer laminate.
Step 103: for each cross section, strain is extracted along a strain extraction line in the width direction of the outer laminate surface. For the ith section, the strain extraction line along the width of the outer laminate surface extracts the strain as
Step 104: for each cross section, the strains at both ends of the strain extraction line in the thickness direction of the outer laminate are extracted. For the ith cross section, the strains at both ends of the strain extraction line in the thickness direction of the outer laminate were extracted, and the strain at the outer surface edge of the outer laminate was recorded as
Strain at the edge of the inner surface of the outer laminate is noted
Step 105: the average strain per cross-section of the outer laminate was calculated. Of the i-th section of the outer laminate
Average strain
Wherein n is an integer greater than 2, wiIs the laminate width.
Step 106: the load transmitted by the first and second outer laminates at each section was calculated.
The first and second outer laminates transmit a load at the ith cross-section of:
in the formula, F1iFor the first outer laminate in the i-th sectionThe load transmitted in the plane is transmitted,
average strain for the ith cross-section of the first outer laminate resulting from step 5; k is a radical of1iAn in-plane stiffness of the first outer laminate at the i-th cross-section; f2iFor the loads transmitted by the second outer laminate in the i-th cross-section,
average strain for the ith cross-section of the second outer laminate resulting from step 5; k is a radical of2iN is an integer greater than 2 for the in-plane stiffness of the second outer laminate at the i-th cross-section.
Step 107: and calculating the load distribution ratio of the bolt to the laminated plate outside the test piece at different sections.
Example 1
The following description will explain the present invention in detail by taking a typical mixed-and-overlapped connection structure as an example, but not by way of any limitation to the present invention.
A typical composite lay-up mechanical connection is shown in fig. 2, where three composite panels are bolted together. The load transfer path between the composite panels is shown in figure 3, with the load being gradually transferred from the middle composite panel to the outer two composite panels by the respective bolts. The load transmitted by the bolt to the first outer laminate is marked F in turn1、F2And F3The load transferred by the bolts to the second outer laminate is denoted in turn as F4And F5. The loads transmitted in the first outer laminate are denoted in the order Fu1、Fu2And Fu3The load transmitted in the second outer laminate is denoted F in sequenceb1And Fb2。
The invention provides a composite material mechanical connection nail load indirect measurement test method, and a test schematic diagram is shown in figure 4. In the test process, DIC equipment is used for simultaneously measuring strain fields of the front surface, the back surface and the side surface of the composite material mechanical connection test piece, strain results of 1-3 sections of the outer surface and the side surface of the first outer laminated plate (upper plate) are extracted, and strain results of 4 and 5 sections of the outer surface and the side surface of the second outer laminated plate (lower plate) are extracted. The positions of the sections 1-5 are shown in FIG. 4, and there is no requirement for precise positions as long as the positions are near the positions shown in the figure.
In the test piece, strain of the outer laminated plate is not uniformly distributed along the width of the plate under the action of bolt extrusion load. The outer laminates are subjected not only to in-plane loads but also to eccentric bending moments. There is some degree of bending deformation of the outer laminate. In the case of a low bending moment level, the outer laminate bending deformation satisfies the straight normal assumption.
The distribution of the strain on the outer surface and the side surface of the outer laminated plate in each section can be directly measured by a DIC device. The average strain of each cross-section of the outer laminate can be calculated as
In the formula (I), the compound is shown in the specification,
for the strain extracted along the strain extraction line in the width direction of the outer laminate surface, wiFor the width of the laminate,
is the strain at the edge of the outer surface of the outer laminate,
is the strain of the inner surface edge of the outer laminate.
Further, the load transmitted by each section of the first and second outer laminates can be obtained:
in the formula, FuiFor loads transferred in the i-th cross-section of the first outer laminate,
is the average strain for the ith cross section of the first outer laminate from equation 1; k is a radical ofuiAn in-plane stiffness of the first outer laminate at the i-th cross-section; fbiFor the loads transmitted by the second outer laminate in the i-th cross-section,
is the average strain of the ith cross-section of the second outer laminate from equation 1; k is a radical ofbiN is an integer greater than 2 for the in-plane stiffness of the second outer laminate at the i-th cross-section.
The load transmitted by the bolts to the upper and lower plates and the load transmitted in the plates have the following relationship:
thus, the distribution ratio of the nail loads transmitted by the bolts to the upper and lower plates is given by
The invention provides a testing method for measuring the nail load of the mechanical connection of composite materials, which is used for measuring the strain distribution of the outer surface and the side surface of a connected plate between bolts by a non-contact deformation measuring technology and calculating the load transmitted in the connected plate so as to indirectly obtain the nail load distribution of a connection structure. According to the invention, the average strain of the surface is calculated according to the strain distribution of the connected plate along the width, and the influence of bending deformation caused by eccentric bending moment is eliminated according to the side strain result, so that the test precision is improved; meanwhile, the invention can be used for mechanical connection of composite materials with complex configuration and has wide application range.
Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.
Claims (9)
1. A nail load measurement test method for mechanical connection of complex composite materials, wherein the mechanical connection of the complex composite materials is double-shear connection and comprises a first outer laminated plate, a second outer laminated plate, a middle laminated plate and a plurality of bolts for connecting the laminated plates, the two outer laminated plates have the same or different lengths, the bolts have the same or different diameters and types, the method is characterized in that digital image correlation equipment is used as a tool, strain fields of the outer surfaces and the side surfaces of the two outer laminated plates which are mechanically connected by the complex composite materials are measured, the average strain of the different sections of the two outer laminated plates is accurately calculated, so that the load transmitted by the two outer laminated plates is calculated, and the nail load distribution proportion is deduced,
wherein the method comprises the steps of:
step 1, applying loads at two ends of a test piece, and measuring strain fields of the front and the side surfaces of a first outer side laminated plate and a second outer side laminated plate which are mechanically connected by a complex composite material by adopting digital image related equipment;
step 2: respectively selecting sections at different load transmission positions of two outer side laminated plates mechanically connected by a complex composite material, wherein the sections are intersected with the surfaces of the outer side laminated plates to form lines along the width and thickness directions of the outer side laminated plates as strain extraction lines;
and step 3: extracting strain along a strain extraction line of the outer laminate for each section, and calculating an average strain of each section of the outer laminate;
and 4, step 4: calculating the load transmitted by the first and second outer laminates at each cross section;
and 5: and calculating the load distribution ratio of the bolt to the laminated plate outside the test piece at different sections.
2. The method according to claim 1, wherein step 3 specifically comprises:
step 301: for the i-th cross section, the strain is extracted along the strain extraction line in the width direction of the outer laminate surface, which is recorded as
l is a position coordinate along the width direction of the surface of the laminated board;
step 302: for the ith cross section, the strains at both ends of the strain extraction line in the thickness direction of the outer laminate were extracted, and the strain at the outer surface edge of the outer laminate was recorded as εi t0And the strain at the edge of the inner surface of the outer laminate is recorded as εi t；
Step 303: calculating the average strain of the i-th cross section of the outer laminate
3. The method of claim 2 wherein in step 303, the average strain of the i-th cross-section of the outer laminate is calculated
The method specifically comprises the following steps:
wherein n is an integer greater than 2, wiIs the laminate width.
4. A method according to claim 3, wherein in step 4, the calculation of the load transmitted by the first and second outer laminates in the ith section is carried out by:
in the formula, F1iFor loads transferred in the i-th cross-section of the first outer laminate,
average strain for the ith cross section of the first outer laminate obtained by step 3; k is a radical of1iAn in-plane stiffness of the first outer laminate at the i-th cross-section; f2iFor the loads transmitted by the second outer laminate in the i-th cross-section,
is the average strain of the ith cross-section of the second outer laminate obtained by step 3; k is a radical of2iN is an integer greater than 2 for the in-plane stiffness of the second outer laminate at the i-th cross-section.
5. The method according to claim 4, wherein in step 5, the load transmitted by the ith bolt to the test piece first and second outer laminates is calculated as:
in the formula (I), the compound is shown in the specification,
for the i-th bolt to transmit to the first outer laminate, F1iThe load transmitted in the ith section of the first outer laminated plate obtained in the step 4;
for the load transmitted by the ith bolt to the second outer laminate, F2iIs the load transmitted in the ith cross-section of the second outer laminate obtained by step 4.
6. A method according to claim 1, wherein in step 2, the cross-section taken on the outer laminate is located between adjacent bolts and outside the outermost bolt.
7. The method of claim 1, wherein the digital image correlation device is secured to an external support.
8. The method of claim 7, wherein the digital image correlation device is installed with picture taking software and strain calculation software.
9. Method for measuring the nail load distribution of composite materials in an aircraft structure, said composite materials being mechanically connected, characterized in that it measures the nail load distribution of composite materials in an aircraft structure using a complex composite material mechanical connection nail load measurement test method according to any one of claims 1 to 8.
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