CN113722831B - Beam bending energy absorption analysis method for multi-cell thin wall of two-end fixedly supported Z-direction rib plate - Google Patents

Abstract

The invention discloses a bending energy absorption analysis method for a multi-cell thin-walled beam with Z-direction ribs supported at both ends. When loading is performed and the cross section of the multicellular thin-walled beam enters a completely plastic state under the joint action of bending moment and axial force, the two-stage yield criterion is obtained; Step 2: Simplify the deformation and stress of the multicellular thin-walled beam, and obtain The relationship between the axial force at the plastic hinge and the displacement of the section centroid axis; Step 3: Obtain the bearable load of the thin-walled beam based on the yield criterion, the equilibrium equation and the relationship between the axial force at the plastic hinge and the displacement of the section centroid axis The relationship between external forces and. The invention has the characteristics of improving the accuracy of calculating the bending energy absorption characteristics of multi-cell thin-walled beams, shortening the development cycle and reducing design costs.

Description

A bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends

Technical field

The present invention relates to the technical field of automobile passive safety research. More specifically, the present invention relates to a bending energy absorption analysis method of a multi-cell thin-walled beam with Z-direction ribs fixed at both ends.

Background technique

Thin-walled beam structures are widely used as common load-bearing and energy-absorbing components in automobiles, ships, aviation and aerospace. The thin-walled beam structure on the car body is often subjected to lateral loads during collisions and undergoes bending deformation, such as the B-pillar in side collision accidents, the bumper beam in frontal pillar collision conditions, etc. The energy of the thin-walled beam structure when bending deformation occurs The absorption effect has received widespread attention from researchers.

At present, the main research method for the anti-collision design of thin-walled beams is to use a combination of experiments and finite element analysis. By using large deformation nonlinear finite element software to simulate and optimize the thin-walled beam structure, experiments are then used to analyze the design optimization results. Verification, but during the conceptual design stage, each structural design plan changes frequently, and using the finite element analysis method requires frequent changes to the finite element model, which consumes a lot of manpower and time.

With the continuous deepening of people's safety awareness and the continuous strengthening of safety regulations, how to improve the anti-collision performance of automobiles has become an important research topic. Polycellular thin-walled beams have better energy absorption effects than rectangular thin-walled beams, and are used in typical thin-walled beam safety components such as bumper beams and B-pillars in the body structure to improve the crash resistance of the body.

Current theoretical research on the energy absorption mechanism of multicellular thin-walled beams during bending deformation mainly focuses on the energy absorption effect of multicellular thin-walled beams under pure bending conditions or three-point bending conditions with simple supports at both ends. Since both ends of components such as B-pillars or bumper beams on the vehicle body are connected to other components on the vehicle body, they will be affected by axial force when bending and deforming. When the axial force and bending moment jointly act on the two ends of the multicellular thin-walled beam, the bending energy absorption behavior of the multicellular thin-walled beam is more complicated. At the same time, finite element calculations and experimental studies show that under the conditions of fixed support at both ends, multi-cell thin-walled beams with ribs along the Z direction can absorb more energy than multi-cell thin-walled beams with ribs along the Y direction. Therefore, the energy absorption when the multi-cell thin-walled beam with ribs along the Z direction undergoes transverse bending deformation under fixed support at both ends can be studied.

Contents of the invention

The purpose of this invention is to design and develop a bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends, and to establish the multi-cell thin-walled beam with ribs along the Z-direction under the combined action of bending moment and axial force. The energy absorption model when transverse bending deformation occurs, combined with various parameters of thin-walled beams, can accurately predict the bending energy absorption characteristics of multi-cell thin-walled beams with Z-direction ribs fixed at both ends.

The technical solution provided by the invention is:

A bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends, including the following steps:

Step 1: Use a hammer head to load the middle of the multi-cellular thin-walled beam with Z-direction ribs fixed at both ends, and when the cross-section of the multi-cellular thin-walled beam enters a completely plastic state under the combined action of bending moment and axial force, a multi-cellular thin-walled beam is obtained. The yield criterion of the section:

when When , the yield criterion of the cross-section of the multicellular thin-walled beam satisfies:

when When , the yield criterion of the cross-section of the multicellular thin-walled beam satisfies:

In the formula, a represents the first coefficient, H represents the height of the section, represents the distance between the neutral axis of the section and the centroidal axis of the section, t represents the wall thickness of the multicellular thin-walled beam, α ₁ represents the second coefficient, M represents the section bending moment, M ₀ represents the plastic limit bending moment of the section, and N represents Section axial force, N ₀ represents the plastic limit axial force of the section, α ₂ represents the third coefficient;

Step 2: Simplify the deformation and stress of the polycellular thin-walled beam and obtain the relationship between the axial force at the plastic hinge and the displacement of the section centroid axis:

In the formula, w represents the displacement of the section centroid axis, B represents the width of the section, and n represents the number of ribs along the Z direction in the thin-walled beam;

Step 3: Obtain the relationship between the external force that the thin-walled beam can withstand and the displacement of the hammer head based on the yield criterion, the equilibrium equation, and the relationship between the axial force at the plastic hinge and the displacement of the section centroid axis:

In the formula, P represents the force of the hammer head, L represents the length of the thin-walled beam, w _p represents the displacement of the hammer head, and k represents the fourth coefficient.

Preferably, the plastic limit bending moment of the section satisfies:

In the formula, σ ₀ represents the flow stress of the thin-walled beam.

Preferably, the plastic limit axial force of the section satisfies:

N ₀ =σ ₀ t[2B+(2+n)(H-2t)];

In the formula, N ₀ represents the plastic limit axial force of the section.

Preferably, the flow stress of the thin-walled beam satisfies:

In the formula, σ _y represents the yield stress of the thin-walled beam, and σ _u represents the ultimate stress of the thin-walled beam.

Preferably, the cross-sectional bending moment satisfies:

Preferably, the cross-sectional axial force satisfies:

N=(2+n)aHtσ ₀ .

Preferably, the generalized strain rate is obtained according to the yield criterion and the orthogonal law:

when When , the generalized strain rate satisfies:

when When , the generalized strain rate satisfies:

In the formula, represents the membrane strain rate,/> is the curvature change rate;

According to the velocity field at the plastic hinge during the deformation process of the thin-walled beam, the relationship between the generalized strain rate and displacement is obtained:

Preferably, the equilibrium equation is:

Preferably, the relationship between the displacement of the section centroid axis and the displacement of the hammer head satisfies:

w=kw _p .

Preferably, the relationship between the fourth coefficient and the number of ribs along the Z direction in the thin-walled beam satisfies:

Beneficial effects of the present invention:

(1) The bending energy absorption analysis method of a multi-cell thin-walled beam with Z-direction ribs fixed at both ends designed and developed by this invention establishes the joint action of bending moment and axial force on a multi-cell thin-walled beam with ribs along the Z-direction. The energy absorption model when transverse bending deformation occurs is used to calculate the yield criterion of the section when it enters a completely plastic state under the joint action of bending moment and axial force.

(2) The bending energy absorption analysis method of a multi-cell thin-walled beam with Z-direction ribs fixed at both ends designed and developed by this invention deduces the external force and hammer head displacement that the multi-cell thin-walled beam with ribs in the Z-direction can withstand The relationship between the multi-cell thin-walled beams along the Z direction of the ribs is obtained, and the mechanical relationship between the structural parameters (section size, number of ribs along the Z direction) and the bending performance of the multi-cell thin-walled beams along the Z direction can be accurately predicted. Bending energy absorption properties of thin-walled beams.

(3) The bending energy absorption analysis method of a multi-cell thin-walled beam with fixed Z-direction ribs at both ends designed and developed by this invention. In the concept of vehicle body crashworthiness, only the cross-sectional dimensions of the thin-walled beam and the ribs along the Z-direction are given. Based on the number of plates and the material properties of thin-walled beams, the bending performance of multi-cell thin-walled beams with ribs along the Z direction under fixed support at both ends can be quickly calculated. Compared with finite element simulation calculations and experiments, forward design of thin-walled beam structures can be realized. Greatly reduce simulation trial and error and the number of experiments, shorten the development cycle, and reduce design and development costs.

Description of the drawings

Figure 1 is a schematic flow chart of the bending energy absorption analysis method of a multi-cell thin-walled beam with Z-direction ribs fixed at both ends according to the present invention.

Figure 2 is a schematic structural diagram of a multi-cell thin-walled beam with ribs along the Z direction according to the present invention.

Figure 3 is a schematic diagram of the cross-sectional stress distribution in the first stage of the bending process of the multi-cell thin-walled beam with Z-direction ribs fixed at both ends of the present invention.

Figure 4 is a schematic diagram of the cross-sectional stress distribution in the second stage of the bending process of a multi-cell thin-walled beam with Z-direction rib plates fixed at both ends according to the present invention.

Figure 5 is a schematic diagram of the cross-sectional yield curve of a multi-cell thin-walled beam with Z-direction ribs fixed at both ends according to the present invention.

Figure 6 is a simplified schematic diagram of the deformation and stress of a multi-cell thin-walled beam with Z-direction ribs fixed at both ends according to the present invention.

Figure 7 is a schematic diagram of the finite element model of a multi-cell thin-walled beam with Z-direction ribs fixed at both ends according to the present invention.

Figure 8 is the load-displacement curve result of the theoretical calculation and finite element simulation of the bending energy absorption analysis of a multi-cell thin-walled beam with fixed Z-direction ribs at both ends when the number of ribs along the Z direction in the thin-walled beam of the present invention is 1. Comparison diagram.

Figure 9 is the load-displacement curve result of the theoretical calculation and finite element simulation of the bending energy absorption analysis of a multi-cell thin-walled beam with fixed Z-direction ribs at both ends when the number of ribs along the Z direction in the thin-walled beam of the present invention is 2. Comparison diagram.

Figure 10 is the load-displacement curve result of the theoretical calculation and finite element simulation of the bending energy absorption analysis of a multi-cell thin-walled beam with fixed Z-direction ribs at both ends when the number of ribs along the Z direction in the thin-walled beam of the present invention is 3. Comparison diagram.

Figure 11 is the load-displacement curve result of the theoretical calculation and finite element simulation of the bending energy absorption analysis of a multi-cell thin-walled beam with fixed Z-direction ribs at both ends when the number of ribs along the Z direction in the thin-walled beam of the present invention is 4. Comparison diagram.

Detailed ways

The present invention will be further described in detail below, so that those skilled in the art can implement it according to the text of the description.

As shown in Figure 1, the present invention provides a bending energy absorption analysis method for a multi-cell thin-walled beam with Z-direction ribs fixed at both ends. First, an ideal rigid-plastic model is used to simplify the stress-strain curve of the material, and the calculation is The flow stress of the thin-walled beam is then calculated based on the structural parameters of the section to calculate the plastic limit axial force and plastic limit bending moment of the multi-cell section. Then based on the distance between the neutral axis of the multi-cell section and the section centroid, the yield criterion of the multi-cell section is calculated. The solution is divided into two stages. On this basis, the deformation and force of the thin-walled beam with central loading and fixed support at both ends are simplified, and the relationship between generalized strain rate and displacement, as well as the plastic hinge force and hammer head displacement are calculated respectively. Finally, combined with the yield criterion, geometric equations and equilibrium conditions of thin-walled beams, the expression relationship between the external force that the thin-walled beam can withstand and the displacement of the hammer head is calculated. The present invention is implemented based on the Natural Science Foundation project (Project Approval Number: 51775228, Name: Research on the Theoretical Model of Collision Resistance of Multi-material Complex Section Thin-walled Beam Structures of Automobile Body). Specifically include:

As shown in Figure 2, the material of the polycellular thin-walled beam with Z-direction ribs fixed at both ends of the present invention is mainly a metal material. The ideal rigid-plastic model is used to simplify the stress-strain curve of the material, and the stress-strain curve of the thin-walled beam is calculated. Flow stress, the specific expression formula is as follows:

In the formula: σ ₀ represents the flow stress of the thin-walled beam, σ _y represents the yield stress of the thin-walled beam, and σ _u represents the ultimate stress of the thin-walled beam.

As shown in Figures 3 and 4, in the section of a multicellular thin-walled beam with Z-direction ribs fixed at both ends, the structural parameters of the multicellular section are used, combined with the flow stress of the multicellular thin-walled beam, to calculate the plastic limit axial force of the multicellular section. , the expression formula is as follows:

N ₀ =σ ₀ t[2B+(2+n)(H-2t)] (2)

In the formula: N ₀ represents the plastic limit axial force of the section, t represents the wall thickness of the multicellular thin-walled beam, B represents the width of the section, H represents the height of the section, and n represents the number of ribs along the Z direction in the thin-walled beam;

In the same way, the plastic limit bending moment of the section can be calculated, and the expression formula is as follows:

In the formula, M ₀ represents the plastic limit bending moment of the section.

As shown in Figure 5, when the section enters a completely plastic state under the joint action of bending moment and axial force, it is assumed that the distance between the neutral axis of the section and the centroid axis of the section is According to whether the neutral axis of the section enters the bottom edge of the section, the yield criterion of the section is divided into two stages, and the expression formulas of the yield criterion are calculated respectively;

The first stage: when the distance between the neutral axis of the section and the centroidal axis of the section is The cross-section yield criterion satisfies:

As shown in Figure 3, it is the cross-section stress distribution in the first stage, where the composite stress is the sum of the bending stress and the axial stress, and satisfies the cross-section axial force when the cross-section bending moment is the plastic limit bending moment of the cross-section. is 0, the yield criterion expression formula at this stage can be calculated as follows:

In the formula, a represents the first coefficient, α ₁ represents the second coefficient;

When the distance between the neutral axis position of the section and the centroidal axis of the section is When , the expression formulas for the section bending moment and section axial force that the section can bear are as follows:

N＝(2+n)aHtσ ₀ (6)

According to formula (2)(3)(4)(5)(6), the following dimensionless parameters can be derived, the ratio of the section bending moment to the plastic limit bending moment of the section and the ratio of the section axial force to the plastic limit axial force of the section The ratio expression formula is as follows:

In the formula: a represents the first coefficient, and

According to formula (4) (7) (8), the second parameter α ₁ can be calculated by eliminating the first parameter a, and the expression formula is as follows:

Substituting Equation (9) into Equation (4), the specific yield criterion expression at this stage can be obtained.

In the yield criterion, the generalized force and the generalized strain rate are orthogonal. According to the orthogonality relationship, the expression formulas of the generalized strain rate corresponding to the section bending moment and the section axial force, that is, the curvature change rate and the membrane strain rate, are as follows:

Second stage: When the distance between the neutral axis of the section and the centroid axis of the section is The cross-section yield criterion satisfies:

As shown in Figure 4, it is the cross-section stress distribution in the second stage. When the cross-section enters the plastic axial force state, the magnitude of the cross-section axial force is the plastic limit axial force of the cross-section, the magnitude of the cross-section bending moment is 0, and the yield at this stage The criterion expression formula is as follows:

In the formula, α ₂ represents the third coefficient;

When aH=H-2t, the expression formulas of section bending moment and section axial force are as follows:

According to formula (11) (12) (13), the third parameter α ₂ can be calculated, and the expression formula is as follows:

Similarly, according to the orthogonal law, the expression formulas of the curvature change rate and membrane strain rate can be calculated as follows:

In the bending condition discussed in this invention, the hammer head loading point is at the middle position of the thin-walled beam. During the bending process of the thin-walled beam, plastic deformation mainly occurs at the supports at both ends of the thin-walled beam and at the plastic hinge position in the middle. Therefore, as shown in Figure 6 , the deformation and stress conditions of thin-walled beams can be simplified.

According to the velocity field at the plastic hinge during the deformation process of the thin-walled beam, the relationship between the generalized strain rate and displacement can be obtained, and the expression formula is as follows:

In the formula, w represents the displacement of the section centroid axis;

According to the above formulas, the relationship between the axial force at the plastic hinge and the displacement of the section centroid axis can be calculated. The expression formula is as follows:

According to the stress at one end of the thin-walled beam, the equilibrium equation can be listed, and the expression formula is as follows:

F≈N;

From this, the equilibrium equation is obtained:

In the formula, P represents the force of the hammer head, and L represents the length of the thin-walled beam.

Considering that local depression occurs in the thin-walled beam where the hammer head is loaded, the displacement (downward) of the section centroid axis will be smaller than the displacement (downward) of the hammer head. The relationship between the two displacements is as follows:

w＝kw _p (19)

In the formula, k represents the fourth coefficient.

Since the fourth coefficient is related to the number of ribs along the Z direction in the thin-walled beam, the expression formula is as follows:

According to the yield criterion, geometric equations and equilibrium conditions of thin-walled beams, the relationship between the external force that the thin-walled beam can withstand and the displacement of the hammer head can be calculated. The expression formula is as follows:

Example

In this embodiment, the material used for the multi-cell thin-walled beams with ribs along the Z direction is aluminum alloy, and the grade is Al6063-T5. Its material mechanical properties are as shown in Table 1:

Table 1 Al6063-T5 material mechanical properties table

The schematic diagram of the thin-walled beam cross-section is shown in Figure 3 and Figure 4. The structural parameters of the multi-cell cross-section are as follows: the wall thickness of the multi-cell thin-walled beam t=2mm, the width of the cross-section B=60mm, the height of the cross-section H=60mm, and the length of the thin-walled beam L= 800mm, the diameter of the loaded hammer head is 30mm, and the number of ribs along the Z direction in the thin-walled beam is n=1.

1. According to equation (1), substitute σ _y =145MPa, σ _u =201MPa, and calculate the flow stress of the thin-walled beam σ ₀ =171MPa;

2. According to equations (2) and (3), substitute the flow stress σ ₀ of the thin-walled beam = 171MPa, the wall thickness of the multi-cell thin-walled beam t = 2mm, the width of the section B = 60mm, the height of the section H = 60mm, and the thickness of the thin-walled beam along the Z direction The number of ribs n = 1, the plastic limit axial force of the section N ₀ = 98496N, and the plastic limit bending moment M ₀ of the section = 2035584N·mm;

3. According to equations (4) (9) and (11) (14), substitute the plastic limit axial force N ₀ of the section = 98496N and the plastic limit bending moment M ₀ = 2035584N·mm to calculate the multicellular section respectively. The two-stage yield criterion;

4. According to equation (17), the relationship between the axial force at the plastic hinge and the displacement of the cross-section centroid is obtained;

5. According to equation (21), substitute the length L of the thin-walled beam = 800mm, and calculate the relationship between the external force that the thin-walled beam can withstand and the displacement of the hammer head;

The same steps were used to theoretically calculate the load-displacement curves of thin-walled beams for three groups of multi-cell thin-walled beams with the number n of ribs along the Z direction being 2, 3, and 4 respectively. At the same time, as shown in Figure 7, the commercial finite element software Ls-dyna was used. , use the Belytschko-Tsay shell element to divide the thin-walled beam into a 2×2mm ² -size grid, set 5 integration points in the thickness direction, and use stiffness-based hourglass control, and select the MAT_123 material model *MAT_MODIFIED_PIECEWISE_LINEAR_PLASTICITY model to simulate the aluminum alloy Al6063- The constitutive relationship of T5 establishes the finite element model of the multi-cell thin-walled beam with ribs along the Z direction under the condition of fixed support at both ends. As shown in Figure 8-11, it is the load-displacement curve obtained by finite element analysis and theoretical calculation. During the plastic deformation stage of the thin-walled beam, the maximum error between the theoretical prediction and the simulation model is 4.7%. The theoretically predicted load-displacement curve is consistent with the simulation. Highly consistent. The effectiveness of the method for analyzing the bending energy absorption characteristics of multi-cell thin-walled beams with ribs along the Z direction under fixed support at both ends according to the present invention has been verified through finite element simulation analysis.

This invention designs and develops a bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends. It establishes the transverse bending of multi-cell thin-walled beams with ribs along the Z-direction under the combined action of bending moment and axial force. The energy absorption model during deformation was used to calculate the yield criterion of the section when it enters a completely plastic state under the joint action of bending moment and axial force, and the relationship between the external force that the multi-cell thin-walled beam with ribs along the Z direction can withstand and the displacement of the hammer head was derived. Through the relational expression between By using the cross-sectional size, the number of ribs along the Z direction, and the material properties of thin-walled beams, the bending performance of multi-cell thin-walled beams with ribs along the Z-direction under fixed support at both ends can be quickly calculated, and the bending properties of multi-cell thin-walled beams with ribs along the Z-direction can be accurately predicted. Compared with finite element simulation calculations and experiments, the bending energy absorption characteristics can realize forward design of thin-walled beam structures, greatly reduce simulation trial and error and the number of experiments, shorten the development cycle, and reduce design and development costs.

Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the description and embodiments. They can be applied to various fields suitable for the present invention. For those familiar with the art, they can easily Additional modifications may be made, and the invention is therefore not limited to the specific details and embodiments shown and described herein without departing from the general concept defined by the claims and equivalent scope.

Claims (10)

1. A bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends, which is characterized by including the following steps: Step 1: Use a hammer head to load the middle of the multi-cellular thin-walled beam with Z-direction ribs fixed at both ends, and when the cross-section of the multi-cellular thin-walled beam enters a completely plastic state under the combined action of bending moment and axial force, a multi-cellular thin-walled beam is obtained. The yield criterion of the section: when When , the yield criterion of the cross-section of the multicellular thin-walled beam satisfies: when When , the yield criterion of the cross-section of the multicellular thin-walled beam satisfies: In the formula, a represents the first coefficient, H represents the height of the section, represents the distance between the neutral axis of the section and the centroidal axis of the section, t represents the wall thickness of the multicellular thin-walled beam, α ₁ represents the second coefficient, M represents the section bending moment, M ₀ represents the plastic limit bending moment of the section, and N represents Section axial force, N ₀ represents the plastic limit axial force of the section, α ₂ represents the third coefficient; Step 2: Simplify the deformation and stress of the polycellular thin-walled beam and obtain the relationship between the axial force at the plastic hinge and the displacement of the section centroid axis: In the formula, w represents the displacement of the section centroid axis, B represents the width of the section, and n represents the number of ribs along the Z direction in the thin-walled beam; Step 3: Obtain the relationship between the external force that the thin-walled beam can withstand and the displacement of the hammer head based on the yield criterion, the equilibrium equation, and the relationship between the axial force at the plastic hinge and the displacement of the section centroid axis: In the formula, P represents the force of the hammer head, L represents the length of the thin-walled beam, w _p represents the displacement of the hammer head, and k represents the fourth coefficient. 2. The bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends as claimed in claim 1, characterized in that the plastic limit bending moment of the section satisfies: In the formula, σ ₀ represents the flow stress of the thin-walled beam. 3. The bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends as claimed in claim 2, characterized in that the plastic limit axial force of the section satisfies: N ₀ =σ ₀ t[2B+(2+n)(H-2t)]; In the formula, N ₀ represents the plastic limit axial force of the section. 4. The bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends as claimed in claim 3, characterized in that the flow stress of the thin-walled beam satisfies: In the formula, σ _y represents the yield stress of the thin-walled beam, and σ _u represents the ultimate stress of the thin-walled beam. 5. The bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends as claimed in claim 4, characterized in that the cross-sectional bending moment satisfies: 6. The bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends as claimed in claim 5, characterized in that the cross-sectional axial force satisfies: N=(2+n)aHtσ ₀ . 7. The bending energy absorption analysis method for multicellular thin-walled beams with Z-direction ribs fixed at both ends as claimed in claim 6, characterized in that the generalized strain rate is obtained according to the yield criterion and the orthogonal law: when When , the generalized strain rate satisfies: when When , the generalized strain rate satisfies: In the formula, represents the membrane strain rate,/> is the curvature change rate; According to the velocity field at the plastic hinge during the deformation process of the thin-walled beam, the relationship between the generalized strain rate and displacement is obtained: 8. The bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends as claimed in claim 7, characterized in that the balance equation is: 9. The bending energy absorption analysis method for multi-cell thin-walled beams with Z-direction ribs fixed at both ends as claimed in claim 8, characterized in that the relationship between the displacement of the section centroid axis and the displacement of the hammer head satisfies : w=kw _p . 10. The bending energy absorption analysis method for multi-cell thin-walled beams with fixed Z-direction ribs at both ends as claimed in claim 9, characterized in that the relationship between the fourth coefficient and the number of ribs along the Z-direction in the thin-walled beam is The relationship satisfies: