Design method of multi-working condition constrained additive manufacturing lattice light weight structure mirror

Abstract

The application discloses a design method of a multi-working-condition-constrained additive manufacturing point array light-weight structure mirror, and simultaneously faces multiple scenes of machining stress, detection gravity action and assembly stress, so that the surface shape errors under different stresses are balanced and meet design indexes; the point array period, the point array rod diameter and the mirror surface thickness are determined according to the machining stress deformation; the back plate thickness and the side plate thickness are determined according to the detection gravity deformation; and the point array flexible structure is determined according to the assembly stress deformation. The design method of the multi-working-condition-constrained additive manufacturing point array light-weight structure mirror adopts the above steps, fills the structure and the flexible structure based on the point array structure design, compared with the existing mirror light-weight structure design method, the light-weight mirror obtained by the design balances the structure stiffness under different stresses, balances the surface shape errors of the mirror under different stresses, and the design parameters have a clear corresponding relationship with the stress scenes, and have the characteristics of easy parameter adjustment.

Description

Technical Field

[0001] This invention relates to the field of metal reflector manufacturing technology, and in particular to a design method for additive manufacturing lattice lightweight structure reflectors under multiple working conditions. Background Technology

[0002] Additive manufacturing offers greater structural design freedom for enclosed metal mirrors, meeting the needs for lightweight mirror fabrication and making them suitable for the aerospace field. The lightweight structural design of enclosed additively manufactured mirrors must balance weight reduction with structural stiffness requirements under specific stress conditions. The structural stiffness of this lightweight mirror is used to meet the mirror surface shape error specifications, ensuring the imaging quality of the optical system.

[0003] Current lightweight structural design methods for additive manufacturing mirrors are designed for single or limited stress scenarios, and the resulting mirror structures only meet the structural stiffness requirements for those single or limited stress scenarios. During actual processing, testing, and assembly, there are instances where the mirror surface shape error is poor at certain stages, failing to meet the surface shape specifications. For example, the surface shape error of a commonly used triangular lightweight closed additive manufacturing mirror is uneven under different stress conditions: during testing, the surface shape error meets the design specifications under gravity, but under processing and assembly stress scenarios, the surface shape error fails to meet the design specifications (exceeding them by 197% and 125.5%, respectively). This increases the difficulty of actual manufacturing of additive manufacturing mirrors. Summary of the Invention

[0004] The purpose of this invention is to provide a design method for additive manufacturing lattice lightweight structure reflectors under multiple working conditions, which can simultaneously address various scenarios such as processing stress, detection gravity, and assembly stress, thereby ensuring that the surface shape error is balanced under different stresses and meets the design specifications. The design parameters have a clear correspondence with the stress scenarios they are designed for, and the parameters are easy to adjust.

[0005] To achieve the above objectives, this invention provides a design method for additively manufactured lightweight lattice structure reflectors with multi-condition constraints, the steps of which are as follows:

[0006] S1. Determine the lens profile parameters, dot matrix unit type, lightweight index, and lens shape error index; the lens profile parameters include lens diameter, lens radius of curvature, total lens thickness, and mounting ear profile; the lens shape error index is defined as the peak and valley values ​​of surface shape error under processing, inspection, and assembly stress.

[0007] S2. Design variables are determined for processing deformation, gravity-induced deformation, and assembly deformation, respectively. These design variables include lattice period, mirror thickness, lattice rod diameter, side plate thickness, back plate thickness, and lattice flexible structure. The designs for processing deformation, gravity-induced deformation, and assembly deformation are parallel and specifically include:

[0008] S201. Determine the lattice period, lattice rod diameter, and mirror thickness based on processing deformation;

[0009] S202. Determine the thickness of the side plate and the back plate based on the detected gravity deformation.

[0010] S203. To design a flexible lattice structure for assembly deformation, the lattice elements are scaled unidirectionally along the radial, axial and orthogonal directions respectively to obtain three types of anisotropic lattice elements, and the three types of anisotropic lattice elements are combined to form a flexible structure.

[0011] S3. Establish the final finite element model of the reflector and determine the rod diameter of the flexible structure;

[0012] S4. Calculate the degree of lightweighting using the mass of the reflector and the mass of the solid mirror, and verify whether it meets the lightweighting index. If the degree of lightweighting does not meet the lightweighting index, adjust the parameters in S2. The main parameter affected is the diameter of the filling lattice rod. If the lightweighting index is met, proceed to step S5.

[0013] S5. Verify the surface shape error of the finite element model of the reflector structure under different forces. If the surface shape error does not meet the design specifications under processing force, detection gravity, or assembly force scenarios, return to step S2 to adjust the parameters, iterate and verify; if the surface shape error meets the design specifications under processing force, detection gravity, or assembly force scenarios, proceed to step S6.

[0014] S6. Regenerate the 3D model based on the finite element model of the reflector structure, set the powder leakage holes, and export it as the corresponding file format for additive manufacturing.

[0015] Preferably, the specific method for determining the lattice period, lattice rod diameter, and mirror thickness in S201 for processing deformation is as follows:

[0016] 1) Establish a simplified finite element model of the mirror support and calculate the periodic deformation of the mirror caused by the processing force. The simplified finite element model of the mirror support includes a planar mirror and a layer of lattice elements. The force scenario of the finite element model is: fix the bottom of the lattice elements and apply pressure to the surface.

[0017] 2) The numerical changes of the periodic deformation of the mirror surface with respect to the lattice period and the mirror thickness were obtained;

[0018] 3) Use Figure 1 The stringer effect formula shown is fitted with the numerical variation of the above-mentioned periodic deformation of the mirror surface to obtain the stringer effect parameter ψ of the lattice element.

[0019] 4) Use the string gap effect formula to determine the applicable lattice period and mirror thickness;

[0020] 5) Determine the appropriate lattice rod diameter using lightweight indexes;

[0021] 6) Establish a simplified finite element model of the curved mirror support to verify the mirror surface shape error caused by machining deformation.

[0022] Preferably, the specific method for determining the side plate thickness and back plate thickness in S202 for detecting gravity deformation is as follows:

[0023] 1) Establish a simplified finite element model of the reflector. The parameters of the simplified finite element model of the reflector include the mirror diameter, the mirror radius of curvature, the total thickness of the mirror, the back plate thickness, the side plate thickness, the mounting lug profile, the lattice element type, and the equivalent structure of the filled lattice. The equivalent structure of the filled lattice is a three-dimensional hexahedral finite element. The material parameters of the three-dimensional hexahedral finite element are the engineering Young's modulus, engineering Poisson's ratio, and engineering density of the filled lattice element. The specific values ​​are calculated based on the degree of lightweighting and the element type. The stress scenario of the simplified finite element model is that the structure is subjected to gravity in three orthogonal directions.

[0024] 2) By changing the thickness of the side plate and the back plate in the simplified finite element model of the reflector, the mirror deformation of the corresponding reflector structure under the influence of gravity is obtained.

[0025] 3) MATLAB was used to fit the mirror deformation to obtain the curves of surface shape error as a function of lattice rod diameter, side plate thickness and back plate thickness, respectively;

[0026] 4) Determine the side plate thickness and back plate thickness based on the above curves;

[0027] 5) Use a simplified finite element model of the reflecting mirror to verify whether the surface shape error of the mirror caused by gravity meets the design specifications.

[0028] Preferably, the specific method for establishing the final finite element model of the reflector and determining the diameter of the flexible structure rod in S3 is as follows:

[0029] 1) Based on the initial parameters determined in S1 and the design variables determined in S2, establish a finite element model of the reflector structure; the initial parameters include the mirror diameter, mirror radius of curvature, total mirror thickness, mounting lug profile, and lattice element type;

[0030] 2) Calculate the mirror deformation of the finite element model of the reflector under assembly stress. If the mirror deformation is greater than the design index, reduce the rod diameter of the lattice flexible structure; if the mirror deformation is much smaller than the design index, increase the rod diameter of the lattice flexible structure; if the mirror deformation is slightly smaller than the design index, that is, the assembly stress deformation meets the design index, proceed to the next step.

[0031] Therefore, the additive manufacturing method for lightweight lattice structure reflectors with multi-condition constraints using the above steps has the following advantages compared with the prior art:

[0032] 1) The mirror structure design method of the present invention simultaneously addresses multiple scenarios including processing stress, detection gravity, and assembly stress, thereby balancing the surface shape error under different stresses. Under the same weight, the lattice lightweight structure mirror manufactured by this method exhibits balanced surface shape error under different stresses and meets the design specifications.

[0033] 2) Flexible and rapid iteration: The reflector structure design adapts to different stress requirements, with corresponding design parameters matched to each stress scenario. The correspondence is clear, and the design can be completed by adjusting the corresponding parameters and structure. For reflectors with different stress requirements, the necessary structural adjustments can be achieved by adjusting the design parameters corresponding to that stress scenario.

[0034] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0035] Figure 1 A flowchart illustrating an embodiment of the design method for an additively manufactured lightweight lattice structure reflector under multiple working conditions constraints according to the present invention;

[0036] Figure 2 This is a simplified finite element model diagram of the reflector in an embodiment of the present invention;

[0037] Figure 3 This is a schematic diagram of a body-centered cubic unit according to an embodiment of the present invention;

[0038] Figure 4 This is a simplified finite element model diagram of the mirror support according to an embodiment of the present invention;

[0039] Figure 5 This is a simplified finite element model diagram of the support for the curved mirror according to an embodiment of the present invention;

[0040] Figure 6 This is a diagram showing the surface shape error of a curved mirror under processing stress conditions according to an embodiment of the present invention.

[0041] Figure 7 This is a simplified finite element model cross-sectional view of the reflector body according to an embodiment of the present invention;

[0042] Figure 8 This is a diagram showing the surface shape error of the mirror caused by axial gravity in an embodiment of the present invention.

[0043] Figure 9 This is a diagram showing the mirror surface shape error caused by radial gravity (Gv1) in an embodiment of the present invention.

[0044] Figure 10 This is a diagram showing the mirror surface shape error caused by radial gravity (Gv2) in an embodiment of the present invention.

[0045] Figure 11The diagrams show three anisotropic lattice elements according to embodiments of the present invention, where (a) is radially scaled, (b) is axially scaled, and (c) is in orthogonal direction.

[0046] Figure 12 This is a finite element model diagram of the flexible structure according to an embodiment of the present invention;

[0047] Figure 13 This is a finite element model diagram of a lattice lightweight structure mirror according to an embodiment of the present invention;

[0048] Figure 14 The mirror surface deformation of the dot matrix lightweight structure mirror under assembly stress according to an embodiment of the present invention;

[0049] Figure 15 The mirror surface shape error diagrams of the lattice lightweight structure reflector according to the embodiment of the present invention under assembly stress scenario (a), processing stress scenario (b), Gv1 radial detection gravity stress scenario (c), Gv2 radial detection gravity stress scenario (d), and Gp axial detection gravity stress scenario (e) are shown.

[0050] Figure 16 This is a structural diagram of a triangular weight-reducing closed additive manufacturing reflector according to an embodiment of the present invention.

[0051] Figure Labels

[0052] 101. Mirror body; 102. Mirror surface; 103. Mounting ear; 104. Dot matrix; 105. Side plate; 106. Back plate; 107. Flexible dot matrix structure; 201. Dot matrix unit type; 301. Planar mirror surface; 302. Single-layer dot matrix unit; 401. Filled dot matrix equivalent structure. Detailed Implementation

[0053] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0054] Example

[0055] like Figure 1 As shown, the design method for additive manufacturing lattice lightweight structure reflectors with multiple working conditions and constraints is as follows:

[0056] S1. Determine the lens contour parameters, dot matrix unit type 201, lightweighting index, and mirror surface 102 shape error index. In this embodiment, the mirror surface 102 has a diameter of 100mm, a radius of curvature of 500mm, and a total thickness of 10mm for the lens body 101. The dot matrix unit type 201 is body-centered cubic, such as... Figure 3 As shown. The shape error index for mirror 102 is: under the stress of processing, inspection, and assembly, the shape error index of mirror 102 is no greater than 20nm. The lightweight index is that the lightweight rate of mirror body 101 is no less than 70%.

[0057] S2. Design variables are determined for processing deformation, gravity-induced deformation, and assembly deformation, respectively. These design variables include the period of lattice 104, the thickness of mirror 102, the diameter of lattice 104 rods, the thickness of side plate 105, the thickness of back plate 106, and the lattice flexible structure 107. The designs for processing deformation, gravity-induced deformation, and assembly deformation are parallel and specifically include:

[0058] S201. To determine the period of lattice 104, the diameter of lattice 104 rods, and the thickness of mirror 102 for processing deformation, the specific method is as follows:

[0059] 1) Establish a simplified finite element model of the mirror 102 support and calculate the periodic deformation of the mirror 102 caused by processing forces. The simplified finite element model of the mirror 102 support includes a planar mirror 301 and a layer of lattice elements 302, such as... Figure 4 As shown. The stress scenario of the finite element model is as follows: the bottom of the 104-element lattice is fixed, and pressure is applied to the surface. The processing pressure is set to 1.5 kPa.

[0060] 2) By orthogonally changing the thickness of mirror 102 and the period of lattice 104, the simulation results show the numerical changes of the periodic deformation of the mirror as a function of the lattice period and the mirror thickness.

[0061] 3) Use Figure 1 The string gap effect formula shown is used to fit the numerical change of the periodic deformation of the mirror surface, and the string gap effect parameter ψ of the lattice 104 element is 0.00622. Using the string gap effect formula, it is calculated that when the thickness of the mirror 102 is 0.75 mm and the period of the lattice 104 is 7.5 mm, the design index of shape error PV not greater than 20 nm is met.

[0062] 4) Determine the diameter of the 104 lattice rod to be used: When the diameter of the 104 lattice rod is 0.3mm, the lightweight index of not less than 70% is met.

[0063] 5) Establish such Figure 5 The simplified finite element model of the curved mirror support shown verifies the surface shape error of mirror 102 due to machining deformation. The obtained surface shape error PV value of mirror 102 is 8 nm. Figure 6 As shown, the design specifications are met.

[0064] S202. To determine the thickness of the side plate 105 and the back plate 106 for detecting gravity deformation, the specific method is as follows:

[0065] 1) A simplified finite element model of the mirror body 101 is established using Inventor and Hypermesh, such as... Figure 2 and Figure 7As shown. The parameters of the simplified finite element model of the reflector 101 include the diameter of the mirror 102, the radius of curvature of the mirror 102, the total thickness of the mirror 101, the thickness of the back plate 106, the thickness of the side plate 105, the profile of the mounting ear 103, the lattice element type 201, and the equivalent structure 401 of the filled lattice.

[0066] The equivalent structure 401 with a filled lattice is a three-dimensional hexahedral finite element. The material parameters of the three-dimensional hexahedral finite element are taken as the engineering Young's modulus, engineering Poisson's ratio, and engineering density of the filled lattice element 104. The specific values ​​are calculated based on the degree of lightweighting and the element type. The stress scenario of the simplified finite element model is as follows: the structure is subjected to gravity in three orthogonal directions, which are applied along the radial, axial, and orthogonal directions, respectively.

[0067] 2) By changing the thickness of the side plate 105 and the back plate 106 in the simplified finite element model of the reflector body 101, the deformation of the mirror surface 102 under the influence of gravity of the corresponding reflector structure is obtained.

[0068] 3) MATLAB was used to fit the deformation of mirror 102, obtaining curves showing the variation of surface shape error with the diameter of lattice 104, the thickness of side plate 105, and the thickness of back plate 106, respectively. With the increase of the back plate 106 thickness, the shape error (Gp) of mirror 102 caused by gravity parallel to the optical axis increases; the shape errors (Gv1 and Gv2) of mirror 102 caused by gravity perpendicular to the optical axis decrease. With the increase of the side plate 105 thickness, Gp first decreases and then slightly decreases, with the point where the curve stabilizes is at a thickness of 3 mm for side plate 105; Gv1 and Gv2 are almost unaffected by the side plate 105 thickness.

[0069] 4) Based on the above curve, the thickness of the side plate 105 is determined to be 3mm, and the thickness of the back plate 106 is determined to be 0.5mm.

[0070] 5) Under this structure, gravity causes the surface shape error PV of mirror 102 to be 19.6nm, 4.2nm, and 4.3nm, respectively. Figure 8 , Figure 9 , Figure 10 As shown, the design specifications are met.

[0071] S203. For the assembly deformation design of the lattice flexible structure 107, the lattice 104 elements are unidirectionally scaled along the radial, axial, and orthogonal directions respectively, resulting in three types of anisotropic lattice 104 elements, such as... Figure 11 As shown, a flexible structure is composed of 104 elements from three different anisotropic lattice types, such as... Figure 12 As shown, the flexible structure is disposed between the side plate 105 and the mounting ear 103.

[0072] S3. Establish the final finite element model of the reflector and determine the rod diameter of the flexible structure. The specific method is as follows:

[0073] 1) Based on the parameters determined in S1 and the design variables determined in S2, establish a finite element model of the reflector structure, such as... Figure 13 As shown; among them, the earth-centered cubic lattice 104 of the internal filling region was created by nTopology and imported into Hypermesh in beam element format.

[0074] 2) Based on the finite element model of the reflector, the diameter of the flexible structure rod was determined, and the deformation of the mirror surface 102 under assembly stress was calculated. The simulated stress scenario was that two mounting ears 103 were fixed, and the other mounting ear 103 was forcibly displaced by 10 micrometers along the optical axis. The assembly stress surface shape was obtained by fitting with MATLAB, as shown below. Figure 14 As shown, the surface PV value is 18.5nm, which meets the design specifications.

[0075] S4. Verification of lightweighting: The reflector weighs 0.06419 kg, and the solid mirror body 101 weighs 0.2423 kg, resulting in a lightweighting rate of 74%. The lightweighting rate meets the lightweighting index.

[0076] S5. Verify the surface shape error of the finite element model of the reflector structure under different forces. Under processing stress, gravity detection, and assembly stress scenarios, the surface shape of the reflector is as follows: Figure 15 As shown in the figure, the values ​​are shown in the table below. The surface shape error meets the design specifications and ensures that the surface shape error is balanced under different forces.

[0077] Force conditions Gravity Gv1 Gravity Gv2 Gravity Gp Processing stress Assembly stress PV value of surface shape (nm) 8.0 9.0 10.6 14.1 18.5

[0078] For comparison, the surface shape data of a commonly used triangular weight-reduced closed additive manufacturing mirror of the same weight under processing stress, detection gravity, and assembly stress scenarios are shown in the table below. The mirror structure is as follows. Figure 16 As shown, the surface shape error under gravity during testing was better than the design specifications, but the surface shape error under processing and assembly stress conditions did not meet the design specifications (exceeding them by 197% and 125.5%, respectively); the surface shape error was uneven under different stress conditions.

[0079] Force conditions Gravity Gv1 Gravity Gv2 Gravity Gp Processing stress Assembly stress PV value of surface shape (nm) 3.5 3.6 9.5 59.4 45.1

[0080] S6. Regenerate the 3D model based on the finite element model of the reflector structure, set the powder leakage holes, and export it as the corresponding file format for additive manufacturing.

[0081] In summary, this embodiment realizes a design method for additive manufacturing lattice lightweight structure reflectors under multiple working conditions. Based on the lattice structure, the design incorporates filling and flexible structures, resulting in a lightweight reflector that balances structural stiffness under different forces, solving the problem of large differences in the shape accuracy of lightweight reflectors under different forces. Furthermore, it features a clear correspondence between design parameters and the target stress scenario.

[0082] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A design method for additively manufactured lightweight lattice structure reflectors under multiple working conditions, characterized in that: The steps are as follows: S1. Determine the lens profile parameters, dot matrix unit type, lightweight index, and lens shape error index; the lens profile parameters include lens diameter, lens radius of curvature, total lens thickness, and mounting ear profile; the lens shape error index is defined as the peak and valley values ​​of surface shape error under processing, inspection, and assembly stress. S2. Design variables are determined for processing deformation, gravity-induced deformation, and assembly deformation, respectively. These design variables include lattice period, mirror thickness, lattice rod diameter, side plate thickness, back plate thickness, and lattice flexible structure. The designs for processing deformation, gravity-induced deformation, and assembly deformation are parallel and specifically include: S201. Determine the lattice period, lattice rod diameter, and mirror thickness based on processing deformation; S202. Determine the thickness of the side plate and the back plate based on the detected gravity deformation. S203. To design a flexible lattice structure for assembly deformation, the lattice elements are scaled unidirectionally along the radial, axial and orthogonal directions respectively to obtain three types of anisotropic lattice elements, and the three types of anisotropic lattice elements are combined to form a flexible structure. S3. Establish the final finite element model of the reflector and determine the rod diameter of the flexible structure; S4. Calculate the degree of lightweighting using the mass of the reflector and the mass of the solid mirror, and verify whether it meets the lightweighting index. If the degree of lightweighting does not meet the lightweighting index, adjust the parameters in S2. The main parameter affected is the diameter of the filling lattice rod. If the lightweighting index is met, proceed to step S5. S5. Verify the surface shape error of the finite element model of the reflector structure under different forces. If the surface shape error does not meet the design specifications under processing force, detection gravity, or assembly force scenarios, return to step S2 to adjust the parameters, iterate and verify; if the surface shape error meets the design specifications under processing force, detection gravity, or assembly force scenarios, proceed to step S6. S6. Regenerate the 3D model based on the finite element model of the reflector structure, set the powder leakage holes, and export it as the corresponding file format for additive manufacturing. The specific method for determining the lattice period, lattice rod diameter, and mirror thickness in S201 for handling deformation is as follows: 1) Establish a simplified finite element model of the mirror support and calculate the periodic deformation of the mirror caused by the processing force. The simplified finite element model of the mirror support includes a planar mirror and a layer of lattice elements. The force scenario of the finite element model is: fix the bottom of the lattice elements and apply pressure to the surface. 2) The numerical changes in the periodic deformation of the mirror surface with respect to the lattice period and the mirror thickness were obtained; 3) The numerical variation of the periodic deformation of the mirror is fitted using the strut effect formula to obtain the strut effect parameter φ of the lattice element; 4) Determine the applicable lattice period and mirror thickness using the string gap effect formula; 5) Determine the appropriate lattice rod diameter using lightweight indexes; 6) Establish a simplified finite element model of the curved mirror support to verify the mirror surface shape error caused by machining deformation; The specific method for determining the thickness of the side plate and the back plate by detecting gravity deformation, as specified in S202, is as follows: 1) Establish a simplified finite element model of the reflector. The parameters of the simplified finite element model of the reflector include the mirror diameter, mirror radius of curvature, total mirror thickness, back plate thickness, side plate thickness, mounting lug profile, lattice element type, and infilled lattice equivalent structure. The infilled lattice equivalent structure is a three-dimensional hexahedral finite element. The material parameters of the three-dimensional hexahedral finite element are the engineering Young's modulus, engineering Poisson's ratio, and engineering density of the infilled lattice element. The specific values ​​are calculated based on the degree of lightweighting and the element type. The stress scenario of the simplified finite element model is that the structure is subjected to gravity in three orthogonal directions. 2) By changing the thickness of the side plate and the back plate in the simplified finite element model of the reflector, the mirror deformation of the corresponding reflector structure under the influence of gravity is obtained; 3) MATLAB was used to fit the mirror deformation to obtain the curves of surface shape error as a function of lattice rod diameter, side plate thickness and back plate thickness, respectively; 4) Determine the side plate thickness and back plate thickness based on the above curves; 5) Use a simplified finite element model of the reflector to verify whether the surface shape error of the mirror caused by gravity meets the design specifications.

2. The design method for a lightweight additive manufacturing lattice structure reflector with multi-condition constraints according to claim 1, characterized in that: The specific method for establishing the final finite element model of the reflector in S3 and determining the rod diameter of the flexible structure is as follows: 1) Based on the initial parameters determined in S1 and the design variables determined in S2, establish a finite element model of the reflector structure; the initial parameters include the mirror diameter, mirror radius of curvature, total mirror thickness, mounting lug profile, and lattice element type; 2) Calculate the mirror surface deformation of the finite element model of the reflector under assembly stress. If the mirror surface deformation is greater than the design index, reduce the rod diameter of the lattice flexible structure. If the mirror deformation is much smaller than the design specifications, then increase the rod diameter of the lattice flexible structure; If the mirror deformation is slightly less than the design specification, that is, the assembly stress deformation meets the design specification, proceed to the next step.

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