High-precision extensible three-point bending test clamp

Abstract

The utility model belongs to the technical field of aviation structure strength tests, and particularly relates to a high-precision extensible three-point bending test clamp which comprises a lower cross beam, a lower support, an upper cross beam and a positioning block. A test piece is arranged between the lower supports and the positioning blocks, the lower cross beams are arranged below the lower supports, scales are arranged between the lower cross beams and the lower supports, the two groups of lower supports are symmetrically arranged on the two sides of the lower cross beams, and the upper cross beams are connected with the positioning blocks through bolts; and the upper cross beam and the lower cross beam are connected with a testing machine. The test fixture is integrated and matched through the bolt, the positioning block and the wedge-shaped block, the high-precision slide tolerance of the fixture is ensured directly depending on the centering property of a standard detection testing machine, and the assembly problems of miscentering, mismatching of a test piece and the section of the fixture and the like caused by man-made reasons in the fixture assembly process are solved; the assembling efficiency of the clamp is improved, the mounting accuracy of the clamp is improved, and more accurate test data can be conveniently obtained.

Description

Technical Field

[0001] This application belongs to the field of aerospace structural strength testing technology, and specifically relates to a high-precision, expandable three-point bending test fixture. Background Technology

[0002] Due to their lightweight advantages, honeycomb sandwich structures and polymer-based composite materials have begun to gradually replace many load-bearing structures in military and civilian aircraft. Examples include the ailerons, horizontal and vertical stabilizer leading edges, and rudder of the F-35; the over 300m² belly fairing and floor of the A380; and the passenger and cargo cabin floors of the C919. In actual use, both inevitably experience bending loads. Polymer-based composite materials, lacking any thickness reinforcement and relying solely on the matrix to transfer loads, are less capable of withstanding out-of-plane loads and are more prone to delamination and debonding damage. Sandwich structures also often bear bending loads in a beam-like manner, frequently experiencing shear failure within the honeycomb core, resulting in a significant decrease in structural load-bearing capacity. This damage is often undetectable to the naked eye or manifests as irreversible damage to the lower panel, posing unpredictable risks to structural safety. Therefore, characterization testing of the bending properties of both types of materials is essential.

[0003] (1) Polymer-based bending performance test

[0004] The standard currently used for testing the flexural properties of polymer-based composite materials is ASTM D 7264, such as... Figure 1 As shown.

[0005] The requirements for the loading head and support are as follows: the loading head and support should be... Figure 1 The cylindrical contact surface shown has a radius of 5mm, a hardness of ≥55HRC, and a smooth bottom surface, free of scratches, burrs, and sharp edges. The loading head and support can be fixed, rotatable, or rolling, while rotatable or fixed devices are often used for composite materials.

[0006] (2) Bending performance test of honeycomb sandwich structure

[0007] The most commonly used testing standard for honeycomb sandwich structures is ASTM C 393. Three-point bending loading is considered the standard loading configuration for determining the shear strength and shear modulus of the honeycomb core. However, this method requires specific failure modes to obtain valid data: shear failure of the honeycomb core or debonding of the panel from the core. If panel failure occurs before these modes, the test method is not met, and ASTM D 7249 must be used to calculate the panel's failure strength; the shear strength of the honeycomb core cannot be obtained. Although the test methods are similar, ASTM C 393 recommends three-point loading as a more accurate standard configuration for testing the shear strength of the core, while ASTM D 7249 considers this loading method a non-standard configuration.

[0008] The requirements for the test fixture in the standard are as follows:

[0009] 1. Support and loading rod - the loading rod should have sufficient stiffness so as not to deform significantly under load; the loading rod should allow the specimen to rotate freely between the loading point and the support point. The recommended configuration is a 25mm wide flat steel (contacting the specimen via a pressure pad) which can be loaded via a V-groove with a V-shaped loading rod (the radius of the V-groove should be greater than the radius of the V-shaped loading rod and greater than 3mm) or a cylindrical loading rod; the second configuration is a 25mm diameter steel cylinder directly contacting the specimen, but due to the line contact, this loading method has the risk of local collapse, and as the specimen deflects, the span is also constantly increasing.

[0010] 2. Pressure pad - a rubber pressure pad should be used between the loading block and the specimen to prevent damage due to local stress concentration; the pressure pad used should have a Shore A hardness of 60, a width of 25mm, a nominal thickness of 3mm, and extend the full width of the specimen.

[0011] The current test device has the following problems: 1. The positioning of the test device is insufficient, and accurate behavior tolerances cannot be guaranteed during assembly, such as the parallelism of the upper loading head and the lower support plane, the symmetry of the two lower side supports, etc.; 2. The displacement collection method is unreasonable, the clamp displacement is used as the midpoint deflection in the bending test, the collection accuracy level is low, the influencing factors are more and obvious, and the accurate midpoint displacement-load curve cannot be obtained, which is not conducive to the test of bending modulus; 3. The installation of the test piece is not convenient and is prone to angular, since the width of the fixture support is much larger than that of the test piece, it is impossible to ensure that the cross section of the test piece is parallel to the loading head during the installation of the test; 4. The function is single, the current test fixture can only complete the bending performance test of polymer matrix composites, and cannot meet the three-point bending test requirements of the honeycomb structure. SUMMARY

[0012] The purpose of the present application is to provide a high-precision expandable three-point bending test fixture to solve the problems of low precision, poor applicability and inaccurate deflection measurement of the current three-point bending test device.

[0013] The technical scheme of the present application is: a high-precision expandable three-point bending test fixture, comprising a lower crossbeam, a lower support, an upper crossbeam and a positioning block; the test piece is arranged between the lower support and the positioning block, the lower crossbeam is arranged below the lower support, a scale is arranged between the lower crossbeam and the lower support, the lower support is symmetrically arranged on both sides of the lower crossbeam, and the upper crossbeam is connected with the positioning block through bolts; the upper crossbeam and the lower crossbeam are connected with the testing machine.

[0014] Preferably, the testing machine is an INSTRON testing machine, and the upper cross beam and the lower cross beam are directly connected to the INSTRON testing machine through wedge-shaped clamps.

[0015] Preferably, the top of the lower support and the bottom of the positioning block are both provided with arc-shaped stop blocks.

[0016] Preferably, the top of the lower cross beam is provided with an inverted T-shaped groove, and the bottom of the lower support is provided with a T-shaped block that is in sliding fit with the inverted T-shaped groove.

[0017] Preferably, the top of the lower support and the bottom of the positioning block are both provided with pads that are in arc-shaped connection with the lower support and the positioning block and are in bolted fit.

[0018] Preferably, the lower cross beam is provided with a lifting eye screw, and the lifting eye screw is connected with a pull-wire type displacement sensor.

[0019] The high-precision expandable three-point bending test fixture of the present application is integrated through bolts, positioning blocks and wedge-shaped blocks, directly relies on the centering property of the testing machine to ensure the high-precision positioning tolerance of the fixture, avoids the problems of misalignment and mismatch between the test piece and the fixture section caused by human factors during the assembly of the fixture, improves the assembly efficiency of the fixture and increases the accuracy of the installation of the fixture, and facilitates obtaining more accurate test data. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions provided by the present application, the following will briefly introduce the drawings. Obviously, the drawings described below are only some embodiments of the present application.

[0021] Figure 1 It is a schematic diagram of the three-point bending loading configuration structure in the background art;

[0022] Figure 2 It is a front view of the three-point bending fixture for polymer matrix composites of the present application;

[0023] Figure 3 It is an axonometric view of the three-point bending fixture for polymer matrix composites of the present application;

[0024] Figure 4 It is a schematic diagram of the three-point bending fixture for sandwich structures of the present application with pads;

[0025] Figure 5 It is a schematic diagram of the displacement collection of the three-point bending fixture for sandwich structures of the present application.

[0026] 1, lower cross beam; 2, lower support; 3, upper cross beam; 4, positioning block; 5, stop block; 6, pad; 7, lifting eye screw; 8, pull-wire type displacement sensor. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0028] A high-precision scalable three-point bending test fixture for testing polymer matrix composites, honeycomb sandwich structures.

[0029] As Figures 2-3 , including a lower beam 1, a lower support 2, an upper beam 3 and a positioning block 4. The test piece is arranged between the lower support 2 and the positioning block 4, the lower beam 1 is arranged below the lower support 2, a scale is arranged between the lower beam 1 and the lower support 2, the lower support 2 has two groups and is symmetrically arranged on both sides of the lower beam 1, and the upper beam 3 is connected with the positioning block 4 through bolts; the upper beam 3 and the lower beam 1 are connected with a testing machine. The lower support 2 is used to provide a support mode meeting the requirements of the test standard, and the positioning block 4 is used to meet the front and rear centering of the standard width (13mm) bending test piece; before the test, the polymer matrix composite can directly place the test piece on the lower support 2 and abut against the positioning block 4, adjust the upper chuck of the testing machine so that it will touch the upper surface of the test piece, adjust the load and displacement zero of the testing machine, and then start the test.

[0030] Preferably, the testing machine is an INSTRON testing machine, and the upper beam 3 and the lower beam 1 are directly connected with the INSTRON testing machine through wedge-shaped clamping blocks.

[0031] Preferably, the top of the lower support 2 and the bottom of the positioning block 4 are both provided with arc-shaped stop blocks 5, which are used to cross-span centering when in use and are used for detection by vernier caliper and the like to reduce errors.

[0032] Preferably, the top of the lower beam 1 is provided with an inverted T-shaped groove, and the bottom of the lower support 2 is provided with a T-shaped block in sliding cooperation with the inverted T-shaped groove, so that the lower support 2 can move horizontally on the lower beam 1 by loosening the bolts, thereby adjusting the support position of the test piece.

[0033] The test fixture is integrated by bolts, positioning blocks 4 and wedge-shaped blocks, and directly depends on the centering of the testing machine to ensure the high-precision position tolerance of the fixture, avoids the problems such as misalignment and mismatching of the test piece and the fixture section caused by human factors during the assembly of the fixture, improves the assembly efficiency of the fixture and increases the accuracy of the installation of the fixture, and facilitates to obtain more accurate test data.

[0034] In combination with Figure 4In one specific embodiment, the top of the lower support 2 and the bottom of the positioning block 4 are both provided with the pad 6, which is arcuately connected with the lower support 2 and the positioning block 4 and is bolted, and the width of the pad 6 is adjusted to meet the test requirements of the honeycomb sandwich structure, and the test piece is often of different width due to the absence of standard test piece configuration, so the test tool can meet the test requirements by adjusting the width of the pad 6, thereby improving the efficiency and ensuring the applicability of the clamp.

[0035] In combination Figure 5 In one specific embodiment, the lower cross beam 1 is provided with the lifting ring screw 7, and the pull wire type displacement sensor 8 is connected to the lifting ring screw 7 and is arranged on the lower surface of the test piece through the lifting ring screw 7 and is connected through an adhesive block to collect the midpoint deflection, and compared with the original use of the cross beam displacement as the midpoint displacement for calculation, the measurement method has higher accuracy and is simple to operate and is more conducive to accurately characterizing the bending performance of the material.

[0036] Finally, it should be noted that: the drawings of the disclosed embodiments only involve the structures involved in the disclosed embodiments, and other structures can be referred to the usual design, and in the case of no conflict, the same embodiment and different embodiments of the present application can be combined with each other;

[0037] Finally: the above only describes the preferred embodiments of the present application and is not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A high precision scalable three-point bend test fixture, characterized by: Including lower crossbeam (1), lower support (2), upper crossbeam (3) and positioning block (4);Test piece is located between lower support (2) and positioning block (4), the lower crossbeam (1) is located below lower support (2), the lower crossbeam (1) is equipped with scale between lower support (2), the lower support (2) has two groups and is symmetrically arranged on the both sides of lower crossbeam (1), the upper crossbeam (3) is connected with positioning block (4) by bolt;The upper crossbeam (3) and lower crossbeam (1) are connected with testing machine.

2. The high precision extensible three-point bend test fixture of claim 1, wherein: The testing machine is INSTRON testing machine, the upper crossbeam (3) and lower crossbeam (1) are connected with INSTRON testing machine through wedge-shaped clamp block clamping.

3. The high precision extensible three-point bend test fixture of claim 1, wherein: The top of lower support (2) and the bottom of positioning block (4) are provided with arc-shaped stop block (5) in the middle position.

4. The high precision extensible three-point bend test fixture of claim 1, wherein: The top of lower crossbeam (1) is provided with inverted T-shaped groove, the bottom of lower support (2) is provided with T-shaped block matched with inverted T-shaped groove.

5. The high precision extensible three-point bend test fixture of claim 1, wherein: The top of lower support (2) and the bottom of positioning block (4) are provided with pad block (6), the pad block (6) is connected with lower support (2) and positioning block (4) in arc shape and is matched with bolt.

6. The high precision extensible three-point bend test fixture of claim 1, wherein: The lower crossbeam (1) is provided with lifting ring screw (7), the lifting ring screw (7) is connected with pull wire type displacement sensor (8).

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