Thin-wall material bending viscoelasticity performance testing method based on CBT device

Abstract

The invention discloses a thin-wall material bending viscoelasticity performance testing method based on a CBT device, and belongs to the technical field of bending tests. Based on a CBT test device, the test is changed from constant-speed loading and unloading to variable-speed loading and unloading, and a test machine loading and unloading speed function expression for equal bending deformation rate of a bending test piece is established; a test scheme is designed by combining the viscoelasticity theory, the influence of the bending deformation rate on the bending performance of the characterization test piece can be tested, the bending relaxation rigidity is obtained, and the testing capability of the CBT test on the bending performance of the thin-wall viscoelastic material is expanded.

Description

Technical Field

[0001] The invention relates to the technical field of bending tests, and in particular to a method for testing the bending viscoelastic properties of thin-walled materials based on a CBT device. Background Art

[0002] The Counterweight-balanced column Bending Test (CBT) was proposed by Fernandez and Murphey of NASA Langley Research Center in 2018 to study the large deformation bending properties of composite materials. Figure 1 ). The CBT device is installed on a uniaxial testing machine. By loading the machine vertically and rotating the two rotating arms, it can approximately apply a pure moment load to the bending specimen and achieve a bending of about 170°, thereby achieving the experimental effect of large deformation bending. According to the post-processing method of CBT experimental data, the "force-displacement" response of the specimen can be converted into the "bending moment-curvature" response of the specimen, achieving the experimental purpose of measuring the large deformation bending performance of the specimen.

[0003] Compared with the traditional three-point bending and four-point bending test methods, the outstanding feature of the CBT device is that it can measure the bending response process of the specimen at about 170°. The CBT device adopts a central symmetrical design, which can effectively eliminate the influence of the deadweight of the tooling while realizing the bending test. Studies have shown that when the ratio of the length L of the rigid rotating arm to the length S of the bending specimen is L / S>2, the measurement error can be controlled within 5.7%, which is suitable for the testing and characterization of the large deformation bending mechanical properties of materials.

[0004] In the CBT test process of the prior art, the testing machine generally adopts equal rate loading, which causes the bending specimen to be in a non-uniform bending process, and the bending deformation rate of the specimen is in a non-constant state of "first fast and then slow". The uncontrollable bending loading rate in the CBT test cannot accurately characterize the rate correlation of the bending performance of the specimen, that is, the bending performance is affected by the bending deformation rate, which limits its ability to test the bending performance of viscoelastic materials. To this end, a method for testing the bending viscoelastic properties of thin-walled materials based on a CBT device is proposed. Summary of the invention

[0005] The technical problem to be solved by the present invention is: how to change the constant speed loading of the CBT test into variable speed recording, so as to achieve a controllable bending deformation rate, and establish a thin-walled material bending viscoelastic performance testing method based on a CBT device.

[0006] The present invention solves the above technical problems through the following technical solutions, and the present invention comprises the following steps:

[0007] Step S1: Determine the relevant test parameters of the CBT device and set a fixed bending deformation rate Get the bending angular velocity Substitute each parameter into the loading and unloading speed function expression, and then obtain the loading and unloading speed function of the testing machine at a fixed bending deformation rate;

[0008] Step S2: During the loading and unloading stage, the “force-displacement-time” test data of the testing machine is recorded and converted into the “bending moment-curvature-time” relationship through a relationship, and then the bending relaxation stiffness during the loading and unloading stages is calculated through a formula;

[0009] Step S3: In the relaxation and recovery stage, when the target curvature is reached, a fixed relaxation and recovery time is set, the "force-time" test data is recorded and converted into a "bending moment-time" relationship through a relationship, and then the bending relaxation stiffness in the relaxation and recovery stage is calculated;

[0010] Step S4: Repeat the process of steps S1-S3, and each bending specimen is subjected to multiple loading and unloading cycles.

[0011] Furthermore, in step S1, the relevant test parameters of the CBT device include the length L of the rotating arm, the initial angle γ of the rotating arm, and the length S of the bending specimen.

[0012] Furthermore, in step S1, the loading and unloading speed function expression at a fixed bending deformation rate is as follows:

[0013]

[0014] in, is the initial bending angle of the bending specimen, which is equal to zero only during the first loading.

[0015] Furthermore, in the steps S2 and S3, the relationship during conversion is as follows:

[0016]

[0017]

[0018]

[0019] Where, M is the bending moment of the bending specimen, κ is the curvature of the bending specimen, It is the bending angle of the bending specimen after bending, that is, the bending angle, R is the bending curvature radius of the bending specimen, δ is the vertical loading displacement, and P is the vertical force applied by the testing machine to the tooling, that is, the vertical load.

[0020] Furthermore, in step S2, the bending specimen is at a fixed bending deformation rate during the loading and unloading stage. The calculation formula for bending and bending relaxation stiffness is as follows:

[0021]

[0022] Furthermore, in the step S3, the bending specimen is bent at a fixed curvature κ during the relaxation and recovery stages, and the calculation formula for the bending relaxation stiffness is as follows:

[0023]

[0024] Compared with the prior art, the present invention has the following advantages: the method for testing the bending viscoelastic properties of thin-walled materials based on the CBT device, based on the CBT test device, changes the test from uniform speed loading and unloading to variable speed loading and unloading, and establishes a loading and unloading speed function expression of the testing machine for bending specimens with equal bending deformation rates; and combined with the viscoelasticity theory, a test scheme is designed, which can test and characterize the influence of the bending deformation rate on the bending performance of the specimen, obtain the bending relaxation stiffness, and expand the testing capability of the CBT test on the bending performance of thin-walled viscoelastic materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic diagram of the loading and deformation process of a CBT device made of aluminum alloy, where the bending specimen in a is in the initial state, b is a parameter schematic diagram, c and d are bending process diagrams, and the bending specimen in e is bent to about 170°;

[0026] Figure 2 is a curvature-time curve in an embodiment of the present invention;

[0027] Figure 3 It is a speed-time curve in the embodiment of the present invention. DETAILED DESCRIPTION

[0028] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given, but the protection scope of the present invention is not limited to the following embodiment.

[0029] like Figure 1 The figure shows the loading deformation process of the CBT device made of aluminum alloy, which shows the design parameters of the CBT device. In the figure, S is the length of the bending specimen, L is the length of the rotating arm, γ is the initial angle of the rotating arm, is the bending angle of the bending specimen after bending, i.e., the bending rotation angle, R is the bending curvature radius of the bending specimen, δ is the vertical loading displacement, and P is the vertical force applied to the fixture, i.e., the vertical load. During the CBT test, the bending moment of the specimen has a relationship with the vertical load (1), the vertical loading displacement has a relationship with the bending rotation angle of the specimen (2), and the curvature of the specimen has a relationship with the bending rotation angle (3).

[0030] In the CBT test, the “force-displacement” relationship of the testing machine can be transformed into the “bending moment-curvature” relationship of the bending specimen through equations (1)-(3), thereby enabling the study of the large deformation bending properties of the material.

[0031]

[0032]

[0033]

[0034] Where M is the bending moment of the bending specimen, and κ is the curvature of the bending specimen.

[0035] It should be noted that, in this embodiment, the CBT test is changed from uniform speed loading and unloading to variable speed loading and unloading. The bending deformation rate of the CBT test is controlled by reasonably designing the vertical loading and unloading speeds. In combination with the viscoelasticity theory, the corresponding loading rules are designed, and then a method for testing the bending viscoelastic properties of thin-walled materials based on the CBT device is proposed.

[0036] In order to achieve the equal deformation rate bending of the bending specimen based on the CBT device, it is necessary to establish a suitable loading and unloading speed function of the testing machine. They are all single-valued continuous functions of time t. By taking the derivative of equation (2) with respect to time t, we get equation (4):

[0037]

[0038] in, is the bending angular velocity of the bending specimen;

[0039] Initial loading and unloading process is 0, It has the following relationship with t:

[0040]

[0041] when When the positive and negative angular velocities are fixed, substitute equation (5) into equation (4) and simplify to obtain the vertical loading and unloading velocity v of the testing machine and the bending angular velocity of the bending specimen: The relationship between (6):

[0042]

[0043] Generally, when the initial loading and unloading process When it is not 0, It has the following relationship with t:

[0044]

[0045] when When the positive or negative angular velocity is fixed, is the initial bending angle of the specimen. Substitute equation (7) into equation (4) and simplify to obtain the vertical loading and unloading speed v of the testing machine and the bending angular velocity of the bending specimen. The relationship between (8):

[0046]

[0047] Expression (8) is the constant bending angular velocity of the bending specimen. When bending, the loading and unloading speed of the testing machine is a function of the bending deformation rate of the bending specimen. When the bending specimen is subjected to the angular velocity When bending, the bending deformation rate of the bending specimen is is also a fixed value. Then, the loading and unloading speed function is designed according to expression (8) to achieve the equal bending deformation rate. The test effect.

[0048] In viscoelastic theory, the bending viscoelastic properties of thin-walled materials are mainly characterized by bending relaxation stiffness. According to viscoelastic theory, two methods can be used to calculate the bending relaxation stiffness D(t): according to expression (9), the derivative of the moment response with respect to curvature is calculated during loading and unloading; according to expression (10), the ratio of the moment response to the curvature is calculated under fixed curvature deformation. The former focuses on the viscoelastic response during bending loading and unloading, while the latter focuses on the viscoelastic response under fixed curvature deformation.

[0049] Loading and unloading section:

[0050] Relaxation segment:

[0051] In order to realize the large deformation bending viscoelastic performance test of thin-walled materials based on the CBT device, a thin-walled material bending viscoelastic performance test method based on the CBT device is provided in this embodiment to obtain the viscoelastic response of the bending specimen during the bending process and under fixed bending deformation. According to expressions (4)-(8) and relaxation stiffness calculation expressions (9) and (10), the test loading and unloading diagram is designed, see Figure 2 , the testing process is divided into the following 6 steps:

[0052] 1) Determine the L, γ, and S design parameters of the CBT device. Set a fixed bending deformation rate Bending angular velocity Substituting the loading and unloading speed function expression (8) into the loading and unloading speed function, the loading and unloading speed function of the test machine at a fixed bending deformation rate can be obtained. Taking the CBT device parameters L = 70mm, γ = 5°, S = 22mm as an example to illustrate the specific test process, the test process is selected as shown in Table 1. Figure 2 Substituting the loading and unloading deformation rate and time parameters in Table 1 into formula (8), the loading and unloading “speed-time” relationship of the test process is obtained, see Figure 3 .

[0053] Table 1 Example test process data

[0054]

[0055] 2) Equal bending deformation rate During the loading and unloading stage, the "force-displacement-time" test data of the testing machine are recorded and converted into the "bending moment-curvature-time" relationship through equations (1), (2), and (3). The bending relaxation stiffness during the loading and unloading stage is calculated using expression (9), and the difference quotient method in numerical differentiation can be used to achieve the derivative operation.

[0056] 3) During the relaxation and recovery phase, under the condition of fixed curvature κ deformation, set fixed relaxation and recovery time, record the “force-time” test data and convert them into “bending moment-time” relationship through equations (1), (2) and (3), and calculate the bending relaxation stiffness during the relaxation phase using equation (10);

[0057] 4) Repeat the above process and perform three loading and unloading cycles for each specimen. Different bending deformation rates and relaxation and recovery curvatures can be used in the loading and unloading stages of each cycle.

[0058] In summary, the method for testing the bending viscoelastic properties of thin-walled materials based on the CBT device in the above embodiment is based on the CBT test device, changes the test from uniform speed loading and unloading to variable speed loading and unloading, and establishes a function expression of the loading and unloading speed of the testing machine for bending specimens with equal bending deformation rates; and combined with the viscoelasticity theory, a test scheme is designed to test and characterize the influence of the bending deformation rate on the bending properties of the specimens, obtain the bending relaxation stiffness, and expand the testing capability of the CBT test on the bending properties of thin-walled viscoelastic materials.

[0059] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A method for testing the bending viscoelastic properties of thin-walled materials based on a CBT device, characterized in that: The following steps are involved: Step S1: Determine the relevant test parameters of the CBT device and set a fixed bending deformation rate Get the bending angular velocity Substitute each parameter into the loading and unloading speed function expression, and then obtain the loading and unloading speed function of the testing machine at a fixed bending deformation rate; Step S2: During the loading and unloading stage, the "force-displacement-time" test data of the testing machine is recorded and converted into the "bending moment-curvature-time" relationship through a relationship, and then the bending relaxation stiffness during the loading and unloading stages is calculated through the formula; Step S3: In the relaxation and recovery stage, when the target curvature is reached, a fixed relaxation and recovery time is set, the "force-time" test data is recorded and converted into a "bending moment-time" relationship through a relationship, and then the bending relaxation stiffness in the relaxation and recovery stage is calculated; Step S4: Repeat the process of steps S1-S3, and each bending specimen is subjected to multiple loading and unloading cycles.

2. A method for testing the bending viscoelastic properties of thin-walled materials based on a CBT device according to claim 1, characterized in that: In step S1, the relevant test parameters of the CBT device include the length L of the rotating arm, the initial angle γ of the rotating arm, and the length S of the bending specimen.

3. A method for testing the bending viscoelastic properties of thin-walled materials based on a CBT device according to claim 2, characterized in that: In step S1, the loading and unloading speed function expression at a fixed bending deformation rate is as follows: in, is the initial bending angle of the bending specimen, which is equal to zero only during the first loading.

4. A method for testing the bending viscoelastic properties of thin-walled materials based on a CBT device according to claim 3, characterized in that: In the steps S2 and S3, the relationship during conversion is as follows: Where, M is the bending moment of the bending specimen, κ is the curvature of the bending specimen, It is the bending angle of the bending specimen after bending, that is, the bending angle, R is the bending curvature radius of the bending specimen, δ is the vertical loading displacement, and P is the vertical force applied by the testing machine to the tooling, that is, the vertical load.

5. A method for testing the bending viscoelastic properties of thin-walled materials based on a CBT device according to claim 4, characterized in that: In step S2, the bending specimen is at a fixed bending deformation rate during the loading and unloading stage. The calculation formula for bending and bending relaxation stiffness is as follows:

6. A method for testing the bending viscoelastic properties of thin-walled materials based on a CBT device according to claim 4, characterized in that: In step S3, the bending specimen is bent at a fixed curvature κ during the relaxation and recovery phases, and the calculation formula for the bending relaxation stiffness is as follows:

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