A test apparatus for simulating large-amplitude free vibration of bending-torsional coupled nonlinear stiffness

Abstract

This invention belongs to the technical field of bridge wind tunnel testing devices, and provides a large-amplitude free vibration testing device for simulating bending-torsional coupled nonlinear stiffness. The device includes a rigid model, a rigid rod, a rigid boom, a bearing pulley, a linear tension spring, and a lightweight, high-strength rope. By adjusting the original length, stiffness, and initial strain of the spring, as well as the spatial positions of the boom's suspension point and the bearing pulley, the target nonlinear stiffness in both vertical and torsional directions can be achieved, facilitating large-amplitude bending-torsional coupled flutter vibration tests under the nonlinear stiffness of bridge structures. During the large-amplitude vibration of the model, the spring remains vertical, avoiding instability and damping caused by local vibrations. This invention's large-amplitude free vibration testing device only adds a bearing pulley and a lightweight, high-strength rope compared to traditional devices, making it simple in design, low in cost, easy to manufacture, and highly efficient.

Description

Technical Field

[0001] This invention belongs to the technical field of bridge wind tunnel testing devices, specifically relating to a wind tunnel testing device that can simulate large-amplitude free vibration of a rigid bridge model under vertical bending and torsional coupled nonlinear stiffness. Background Technology

[0002] Long-span bridges are characterized by their lightweight, low-damping structures and high wind sensitivity, making them susceptible to large-amplitude bending-torsional coupled nonlinear flutter. The geometric stiffness of bridge structures experiencing large-amplitude flutter varies with displacement, and the greater the displacement, the greater the change in geometric stiffness—in other words, geometric nonlinearity objectively exists. Ignoring this geometric nonlinearity could significantly affect the accuracy of flutter performance tests on long-span bridges.

[0003] Traditional bridge main girder segment coupled free vibration tests employ a vertically suspended horizontal boom device with upper and lower springs. When the model and boom experience torsional displacement, the springs tilt, causing the device to develop geometrically nonlinear stiffness. This torsional stiffness decreases nonlinearly with increasing torsional displacement and is relatively weakly affected by vertical displacement; therefore, the variation of vertical stiffness with both torsional and vertical displacements is negligible.

[0004] However, traditional test devices are not suitable for conducting large-amplitude tests where stiffness changes with displacement. The main reasons include: (1) The nonlinear stiffness characteristics of traditional devices and actual bridges do not match. For example, for the first-order positive symmetric vertical bending mode of cable-stayed bridges, the vertical stiffness of the actual bridge generally increases with the increase of vertical downward displacement and decreases with the increase of vertical upward displacement, while the vertical stiffness of traditional test devices remains basically unchanged. (2) The torsion of the boom will cause the spring to tilt, and the tilted spring will vibrate locally during the vibration of the model, causing uncontrollable nonlinear stiffness, unstable damping, and non-constant mass and mass moment to the device.

[0005] Sébastien Maheux et al. developed a test device that considered the nonlinear stiffness of the structure by setting up an initially inclined spring (Nonlinear Wind Tunnel Tests of Cable-Supported Bridges. Journal of Structural Engineering, 2023, 149(10): 04023137). However, the inclined spring is inevitably affected by sag, and local vibration will occur during the model vibration process, which will lead to uncontrollable stiffness nonlinearity, unstable damping, and non-constant mass and mass moment, affecting the test accuracy. Therefore, it is urgent to develop a wind tunnel test device that can realize large amplitude and stable low damping free vibration with vertical and torsional stiffness varying with displacement. Summary of the Invention

[0006] The technical problem this invention aims to solve is that traditional free-vibration wind tunnel testing devices cannot accurately simulate the geometric stiffness that changes with displacement in rigid segmental models of bridge main beams and other structural components. This invention improves upon traditional devices to accurately simulate the vertical and torsional stiffness variations of actual bridges with displacement, while ensuring that the springs remain vertical and the device maintains stable low damping under large-amplitude coupled vibration. The device includes a rigid model, a rigid rod, a rigid boom, a bearing pulley, a linear tension spring, and a lightweight, high-strength rope.

[0007] The technical solution of the present invention:

[0008] A large-amplitude free vibration test device for simulating bending-torsional coupled nonlinear stiffness includes a rigid model 1, a rigid rod 2, a rigid boom 3, a first bearing pulley 4, a first linear tension spring 5, a first lightweight high-strength rope 6, a second bearing pulley 7, a second linear tension spring 8, and a second lightweight high-strength rope 9.

[0009] Rigid model 1 is fixed at both ends with rigid rods 2, ensuring that the torsional center line of rigid model 1 is collinear with the axis of rigid rod 2. The free end of rigid rod 2 passes vertically through the center of rigid boom 3 and is fixed thereto. Two first bearing pulleys 4, one first linear tension spring 5, and one first lightweight high-strength rope 6 are arranged symmetrically on one side of rigid model 1, ensuring that the first bearing pulley 4 is directly below the first linear tension spring 5. The upper end of the first linear tension spring 5 is fixed, and its lower end is fixedly connected to the upper end of the first lightweight high-strength rope 6. The first lightweight high-strength rope 6 passes vertically downwards through the first bearing pulley 4, and its lower end, whether inclined or vertical, is connected to the upper suspension point of rigid boom 3. Two second bearing pulleys 7, two second linear tension springs 8, and two second lightweight high-strength ropes 9 are arranged symmetrically on one side of rigid model 1, ensuring that the second bearing pulley 7 is directly above the second linear tension spring 8. The lower end of the second linear tension spring 8 is fixed, and its... The upper end is fixedly connected to the lower end of the second lightweight high-strength thin rope 9; the second lightweight high-strength thin rope 9 passes vertically upward through the second bearing fixed pulley 7, and the upper end of the inclined or vertical second lightweight high-strength thin rope 9 is connected to the lower suspension point of the rigid boom 3; this ensures that during the vertical and torsional coupled free vibration of the rigid model 1 and the rigid boom 3, the first lightweight high-strength thin rope 6 above the first bearing fixed pulley 4 and the second lightweight high-strength thin rope 9 below the second bearing fixed pulley 7 remain vertical, so that the first linear tension spring 5 and the second linear tension spring 8 only undergo vertical expansion and contraction deformation; by utilizing the nonlinear change of the inclination angle of the first lightweight high-strength thin rope 6 below the first bearing fixed pulley 4 and / or the second lightweight high-strength thin rope 9 above the second bearing fixed pulley 7 with the vertical and torsional displacement of the rigid model 1, the deformation of the first linear tension spring 5 and the second linear tension spring 8 is nonlinearly related to the vertical and torsional displacement of the rigid model 1, thereby realizing the geometric nonlinear stiffness of vertical and torsional coupling.

[0010] The geometric nonlinear stiffness of the large-amplitude free vibration test device is related to the following parameters: the vertical and torsional displacements of the rigid model 1; the original length, initial strain, and stiffness coefficient of the first linear tension spring 5 and the second linear tension spring 8; the distance between the two upper suspension points, the distance between the two lower suspension points, and the height difference between the upper and lower suspension points of the rigid boom 3; the distance between the two first bearing fixed pulleys 4 and their height difference with the upper suspension point of the rigid boom 3; the distance between the two second bearing fixed pulleys 7 and their height difference with the lower suspension point of the rigid boom 3; the initial tilt angle of the first lightweight high-strength thin rope 6 is determined by the spatial position of the first bearing fixed pulley 4 and the upper suspension point of the rigid boom 3; the initial tilt angle of the second lightweight high-strength thin rope 9 is determined by the spatial position of the second bearing fixed pulley 7 and the lower suspension point of the rigid boom 3; by increasing the initial tilt angle of the first lightweight high-strength thin rope 6 and the second lightweight high-strength thin rope 9 and decreasing the initial strain of the first linear tension spring 5 and the second linear tension spring 8, the degree of stiffness nonlinearity is improved.

[0011] The first bearing fixed pulley 4 and the second bearing fixed pulley 7 are arranged on a rigid frame outside the wind tunnel for suspending the rigid model 1. Their spatial positions are adjusted according to the target geometric nonlinear stiffness, while ensuring that they do not collide with the rigid boom 3 within the design amplitude. For the condition where the vertical stiffness increases with increasing downward vertical displacement and decreases with increasing upward vertical displacement, the spatial positions of the two upper suspension points of the rigid boom 3 and the two first bearing fixed pulleys 4 can be adjusted to create an appropriate tilt angle for the first lightweight high-strength thin rope 6 below the first bearing fixed pulley 4. The distance between the two lower suspension points of the rigid boom 3 and the distance between the two second bearing fixed pulleys 7 are adjusted to be the same, ensuring that the second lightweight high-strength thin rope 9 is entirely vertical. For the condition where the vertical stiffness decreases with increasing downward vertical displacement and increases with increasing upward vertical displacement, the distance between the two upper suspension points of the rigid boom 3 and the distance between the two first bearing fixed pulleys 4 can be adjusted to be the same, ensuring that the first lightweight high-strength thin rope 6 is entirely vertical. The spatial positions of the two lower suspension points of the arm 3 and the two second bearing fixed pulleys 7 are adjusted to create an appropriate tilt angle for the second lightweight high-strength thin rope 9 above the second bearing fixed pulley 7. For working conditions where the vertical stiffness increases or decreases with the increase of the absolute value of the vertical displacement, the spatial positions of the upper and lower suspension points of the rigid arm 3, and the spatial positions of the first bearing fixed pulley 4 and the second bearing fixed pulley 7 are adjusted to create appropriate tilt angles for both the first lightweight high-strength thin rope 6 and the second lightweight high-strength thin rope 9. Based on basically satisfying the target vertical geometric nonlinear stiffness, the original length, stiffness coefficient, and initial strain of the first linear tension spring 5 and the second linear tension spring 8, the spatial positions of the upper and lower suspension points of the rigid arm 3, and the spatial positions of the first bearing fixed pulley 4 and the second bearing fixed pulley 7 are further adjusted to ultimately achieve the target vertical torsional coupling nonlinear stiffness characteristics. The first lightweight high-strength thin rope 6 and the second lightweight high-strength thin rope 9 can be tilted towards the inside or outside of the model according to the actual situation.

[0012] The first bearing pulley 4 and the second bearing pulley 7 are lightweight and high-strength, with a bearing rotation friction coefficient as low as possible. During system vibration, the first lightweight high-strength rope 6 and the second lightweight high-strength rope 9 will respectively press against the first bearing pulley 4 and the second bearing pulley 7, causing them to rotate. Using a small-diameter bearing and a large-diameter pulley scheme can reduce the bearing rotation distance, reduce system energy consumption, and lower the vibration system damping under the condition that the first lightweight high-strength rope 6 and / or the second lightweight high-strength rope 9 undergo the same displacement.

[0013] The first lightweight high-strength thin rope 6 and the second lightweight high-strength thin rope 9 are lightweight, high-strength, high-modulus elasticity, and have no bending or torsional resistance. The material is not limited. The first lightweight high-strength thin rope 6 and the second lightweight high-strength thin rope 9 should have sufficient length to ensure that within the designed amplitude, the first linear tension spring 5 does not collide with the first bearing pulley 4, and the second linear tension spring 8 does not collide with the second bearing pulley 7.

[0014] The beneficial effects of the present invention are as follows: (1) By adjusting the original length, stiffness and initial strain of the linear tension spring, the spatial position of the rigid boom suspension point and the bearing pulley, the target nonlinear stiffness in the vertical and torsional directions can be achieved, so as to facilitate the large amplitude bending-torsional coupling flutter vibration test under the nonlinear stiffness of the bridge structure; (2) During the large amplitude vibration of the model, the spring can always maintain a vertical state, avoiding the instability damping and stiffness caused by its local vibration; (3) The device only adds a bearing pulley and a lightweight high-strength thin rope compared to the traditional device. It is simple in design, low in cost, easy to process, and efficient and feasible. Attached Figure Description

[0015] Figure 1 This is a structural diagram of the test device when a lightweight, high-strength thin rope tilts outward.

[0016] Figure 2 This is a structural diagram of the test device for the first lightweight, high-strength thin rope when tilted.

[0017] Figure 3 This is a structural diagram of the test device for the second lightweight, high-strength thin rope when tilted.

[0018] Figure 4 This is a structural diagram of the test device when a lightweight, high-strength thin rope is tilted inward.

[0019] In the diagram: 1. Rigid model; 2. Rigid rod; 3. Rigid boom; 4. First bearing pulley; 5. First linear tension spring; 6. First lightweight high-strength rope; 7. Second bearing pulley; 8. Second linear tension spring; 9. Second lightweight high-strength rope. Detailed Implementation

[0020] The specific embodiments of the present invention will be described in detail below with reference to the technical solutions and accompanying drawings.

[0021] like Figure 1As shown, a large-amplitude free vibration test device for simulating bending-torsional coupled nonlinear stiffness includes a rigid model 1, a rigid rod 2, a rigid boom 3, a first bearing pulley 4, a first linear tension spring 5, a first lightweight high-strength rope 6, a second bearing pulley 7, a second linear tension spring 8, and a second lightweight high-strength rope 9. Rigid model 1 is fixed at both ends with rigid rods 2, ensuring that the torsional center line of rigid model 1 is collinear with the axis of rigid rod 2. The free end of rigid rod 2 passes vertically through the center of rigid boom 3 and is fixed thereto. Two first bearing pulleys 4, one first linear tension spring 5, and one first lightweight high-strength rope 6 are arranged symmetrically on one side of rigid model 1, ensuring that the first bearing pulley 4 is directly below the first linear tension spring 5. The upper end of the first linear tension spring 5 is fixed, and its lower end is fixedly connected to the upper end of the first lightweight high-strength rope 6. The first lightweight high-strength rope 6 passes vertically downwards through the first bearing pulley 4, and its lower end, whether inclined or vertical, is connected to the upper suspension point of rigid boom 3. Two second bearing pulleys 7, two second linear tension springs 8, and two second lightweight high-strength ropes 9 are arranged symmetrically on one side of rigid model 1, ensuring that the second bearing pulley 7 is directly above the second linear tension spring 8. The lower end of the second linear tension spring 8 is fixed, and its... The upper end is fixedly connected to the lower end of the second lightweight high-strength thin rope 9; the second lightweight high-strength thin rope 9 passes vertically upward through the second bearing fixed pulley 7, and the upper end of the inclined or vertical second lightweight high-strength thin rope 9 is connected to the lower suspension point of the rigid boom 3; this ensures that during the vertical and torsional coupled free vibration of the rigid model 1 and the rigid boom 3, the first lightweight high-strength thin rope 6 above the first bearing fixed pulley 4 and the second lightweight high-strength thin rope 9 below the second bearing fixed pulley 7 remain vertical, so that the first linear tension spring 5 and the second linear tension spring 8 only undergo vertical expansion and contraction deformation; by utilizing the nonlinear change of the inclination angle of the first lightweight high-strength thin rope 6 below the first bearing fixed pulley 4 and / or the second lightweight high-strength thin rope 9 above the second bearing fixed pulley 7 with the vertical and torsional displacement of the rigid model 1, the deformation of the first linear tension spring 5 and the second linear tension spring 8 is nonlinearly related to the vertical and torsional displacement of the rigid model 1, thereby realizing the geometric nonlinear stiffness of vertical and torsional coupling.

[0022] The geometric nonlinear stiffness of the large-amplitude free vibration test device is related to the following parameters: the vertical and torsional displacements of the rigid model 1; the original length, initial strain, and stiffness coefficient of the first linear tension spring 5 and the second linear tension spring 8; the distance between the two upper suspension points, the distance between the two lower suspension points, and the height difference between the upper and lower suspension points of the rigid boom 3; the distance between the two first bearing fixed pulleys 4 and their height difference with the upper suspension point of the rigid boom 3; the distance between the two second bearing fixed pulleys 7 and their height difference with the lower suspension point of the rigid boom 3; the initial tilt angle of the first lightweight high-strength thin rope 6 is determined by the spatial position of the first bearing fixed pulley 4 and the upper suspension point of the rigid boom 3; the initial tilt angle of the second lightweight high-strength thin rope 9 is determined by the spatial position of the second bearing fixed pulley 7 and the lower suspension point of the rigid boom 3; by increasing the initial tilt angle of the first lightweight high-strength thin rope 6 and the second lightweight high-strength thin rope 9 and decreasing the initial strain of the first linear tension spring 5 and the second linear tension spring 8, the degree of stiffness nonlinearity is improved.

[0023] The first bearing fixed pulley 4 and the second bearing fixed pulley 7 can be installed on the rigid frame outside the wind tunnel to suspend the rigid model 1. The vertical and horizontal positions are adjusted according to the target nonlinear stiffness characteristics, while ensuring that they do not collide with the rigid boom 3 within the design amplitude.

[0024] like Figure 2 As shown, for the working condition where the vertical stiffness increases with the increase of vertical downward displacement and decreases with the increase of vertical upward displacement, the spatial positions of the two upper suspension points of the rigid boom 3 and the two first bearing fixed pulleys 4 can be adjusted so that the first lightweight high-strength thin rope 6 under the first bearing fixed pulley 4 produces an appropriate tilt angle, and the distance between the two lower suspension points of the rigid boom 3 and the distance between the two second bearing fixed pulleys 7 are adjusted to be the same so that the second lightweight high-strength thin rope 9 is in a vertical state.

[0025] like Figure 3 As shown, for the working condition where the vertical stiffness decreases with the increase of vertical downward displacement and increases with the increase of vertical upward displacement, the distance between the two upper suspension points of the rigid boom 3 and the distance between the two first bearing fixed pulleys 4 can be adjusted to be the same, so that the first lightweight high-strength thin rope 6 is in a vertical state. The spatial position of the two lower suspension points of the rigid boom 3 and the two second bearing fixed pulleys 7 can be adjusted so that the second lightweight high-strength thin rope 9 above the second bearing fixed pulley 7 has an appropriate tilt angle.

[0026] like Figure 1 and Figure 4As shown, for working conditions where the vertical stiffness increases or decreases with the increase of the absolute value of the vertical displacement, the spatial positions of the upper and lower suspension points of the rigid boom 3, and the spatial positions of the first bearing fixed pulley 4 and the second bearing fixed pulley 7 are adjusted respectively to make the first lightweight high-strength thin rope 6 and the second lightweight high-strength thin rope 9 produce appropriate tilt angles. On the basis of basically satisfying the target vertical geometric nonlinear stiffness, the original length, stiffness coefficient and initial strain of the first linear tension spring 5 and the second linear tension spring 8, the spatial positions of the upper and lower suspension points of the rigid boom 3, and the spatial positions of the first bearing fixed pulley 4 and the second bearing fixed pulley 7 are further adjusted to finally achieve the target vertical torsional coupling nonlinear stiffness characteristics. The first lightweight high-strength thin rope 6 and the second lightweight high-strength thin rope 9 can be tilted towards the inside or outside of the model according to the actual situation.

[0027] The first bearing fixed pulley 4 and the second bearing fixed pulley 7 are lightweight and high-strength, and the bearing rotation friction coefficient is minimized. Using a small-diameter bearing and a large-diameter pulley scheme can reduce the bearing rotation distance, reduce system energy consumption, and lower the damping of the vibration system under the condition that the first lightweight high-strength rope 6 and / or the second lightweight high-strength rope 9 undergo the same displacement.

[0028] The first lightweight high-strength thin rope 6 and the second lightweight high-strength thin rope 9 possess characteristics such as lightweight, high strength, high elastic modulus, and no resistance to bending or torsion; the material is not limited. The first lightweight high-strength thin rope 6 and the second lightweight high-strength thin rope 9 should have sufficient length to ensure that within the designed amplitude, the first linear tension spring 5 does not collide with the first bearing fixed pulley 4, and the second linear tension spring 8 does not collide with the second bearing fixed pulley 7.

[0029] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any equivalent changes, modifications, or variations made by those skilled in the art to the above examples using the technical solutions of the present invention shall still fall within the scope of the technical solutions of the present invention.

Claims

1. A test apparatus for simulating large-amplitude free vibration with coupled bending and torsional stiffness, characterized in that, The large amplitude free vibration test device includes a rigid model (1), a rigid rod (2), a rigid boom (3), a first bearing fixed pulley (4), a first linear tension spring (5), a first lightweight high-strength thin rope (6), a second bearing fixed pulley (7), a second linear tension spring (8), and a second lightweight high-strength thin rope (9). Rigid rods (2) are fixed at both ends of the rigid model (1), ensuring that the torsion center line of the rigid model (1) is collinear with the axis of the rigid rod (2). The free end of the rigid rod (2) passes vertically through the center of the rigid boom (3) and is fixed thereto. There are two first bearing pulleys (4), first linear tension springs (5), and first lightweight high-strength thin ropes (6) on one side of the rigid model (1), arranged symmetrically to ensure that the first bearing pulleys (4) are directly below the first linear tension springs (5). The upper end of the first linear tension springs (5) is fixed, and its lower end is connected to the first linear tension springs (5). The upper end of the first lightweight high-strength thin rope (6) is fixedly connected; the first lightweight high-strength thin rope (6) passes vertically downward through the first bearing fixed pulley (4), and the lower end of the inclined or vertical first lightweight high-strength thin rope (6) is connected to the upper suspension point of the rigid boom (3); the second bearing fixed pulley (7), the second linear tension spring (8), and the second lightweight high-strength thin rope (9) on one side of the rigid model (1) are all two in number and arranged symmetrically to ensure that the second bearing fixed pulley (7) is directly above the second linear tension spring (8); the lower end of the second linear tension spring (8) The upper end of the first lightweight high-strength rope (6) is fixedly connected to the lower end of the second lightweight high-strength rope (9); the second lightweight high-strength rope (9) passes vertically upward through the second bearing pulley (7), and the upper end of the inclined or vertical second lightweight high-strength rope (9) is connected to the lower suspension point of the rigid boom (3); this ensures that during the vertical and torsional coupled free vibration of the rigid model (1) and the rigid boom (3), the first lightweight high-strength rope (6) above the first bearing pulley (4) and the second lightweight high-strength rope (9) below the second bearing pulley (7) remain vertical. The first linear tension spring (5) and the second linear tension spring (8) undergo only vertical expansion and contraction deformation. By utilizing the nonlinear change of the tilt angle of the first lightweight high-strength thin rope (6) below the first bearing fixed pulley (4) and / or the second lightweight high-strength thin rope (9) above the second bearing fixed pulley (7) with the vertical and torsional displacement of the rigid model (1), the deformation of the first linear tension spring (5) and the second linear tension spring (8) is nonlinearly related to the vertical and torsional displacement of the rigid model (1), thereby realizing the geometric nonlinear stiffness of vertical and torsional coupling. The first bearing fixed pulley (4) and the second bearing fixed pulley (7) are arranged on the rigid frame outside the wind tunnel for suspending the rigid model (1). Their spatial positions are adjusted according to the target geometric nonlinear stiffness, while ensuring that they do not collide with the rigid boom (3) within the design amplitude. For the working condition where the vertical stiffness increases with the increase of vertical downward displacement and decreases with the increase of vertical upward displacement, the spatial positions of the two upper suspension points of the rigid boom (3) and the two first bearing fixed pulleys (4) can be adjusted so that the first lightweight high-strength thin rope (6) under the first bearing fixed pulley (4) has an inclination angle. The distance between the two lower suspension points of the rigid boom (3) and the distance between the two second bearing fixed pulleys (7) are adjusted to be the same so that the second lightweight high-strength thin rope (9) is in a vertical state. For the vertical stiffness increasing with the increase of vertical downward displacement, For the condition where the vertical stiffness increases with the increase of vertical displacement, the distance between the two upper suspension points of the rigid boom (3) and the distance between the two first bearing fixed pulleys (4) can be adjusted to be the same, so that the first lightweight high-strength thin rope (6) is in a vertical state. The spatial positions of the two lower suspension points of the rigid boom (3) and the two second bearing fixed pulleys (7) can be adjusted so that the second lightweight high-strength thin rope (9) above the second bearing fixed pulley (7) produces an inclination angle. For the condition where the vertical stiffness increases with the increase of the absolute value of vertical displacement or decreases with the increase of the absolute value of vertical displacement, the spatial positions of the upper and lower suspension points of the rigid boom (3) and the spatial positions of the first bearing fixed pulley (4) and the second bearing fixed pulley (7) can be adjusted so that the first lightweight high-strength thin rope (6) and the second lightweight high-strength thin rope (9) both produce an inclination angle.

2. The large-amplitude free vibration test device according to claim 1, characterized in that, The geometric nonlinear stiffness of the large amplitude free vibration test device is related to the following parameters: the vertical and torsional displacements of the rigid model (1), the original length, initial strain, and stiffness coefficient of the first linear tension spring (5) and the second linear tension spring (8), the distance between the two upper suspension points, the distance between the two lower suspension points, and the height difference between the upper and lower suspension points of the rigid boom (3), the distance between the two first bearing fixed pulleys (4) and their height difference with the upper suspension point of the rigid boom (3), and the distance between the two second bearing fixed pulleys (7) and their height difference with the lower suspension point of the rigid boom (3). The height difference of the suspension points; the initial tilt angle of the first lightweight high-strength thin rope (6) is determined by the spatial position of the upper suspension point of the first bearing fixed pulley (4) and the rigid boom (3), and the initial tilt angle of the second lightweight high-strength thin rope (9) is determined by the spatial position of the lower suspension point of the second bearing fixed pulley (7) and the rigid boom (3); by increasing the initial tilt angle of the first lightweight high-strength thin rope (6) and the second lightweight high-strength thin rope (9) and reducing the initial strain of the first linear tension spring (5) and the second linear tension spring (8), the degree of stiffness nonlinearity is improved.

3. The large-amplitude free vibration test apparatus according to claim 1, characterized in that, Based on satisfying the target vertical geometric nonlinear stiffness, the original length, stiffness coefficient and initial strain of the first linear tension spring (5) and the second linear tension spring (8), the spatial positions of the upper and lower suspension points of the rigid boom (3), and the spatial positions of the first bearing fixed pulley (4) and the second bearing fixed pulley (7) are further adjusted to finally achieve the target vertical torsional coupling nonlinear stiffness characteristics.

4. The large-amplitude free vibration test apparatus according to claim 3, characterized in that, Depending on the actual situation, the first lightweight high-strength rope (6) and the second lightweight high-strength rope (9) may be tilted towards the inside or outside of the model.

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