CN114492128A - Self-adaptive generation method of finite element model for FFF thermal stress analysis - Google Patents

Self-adaptive generation method of finite element model for FFF thermal stress analysis Download PDF

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CN114492128A
CN114492128A CN202210088991.8A CN202210088991A CN114492128A CN 114492128 A CN114492128 A CN 114492128A CN 202210088991 A CN202210088991 A CN 202210088991A CN 114492128 A CN114492128 A CN 114492128A
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潘万彬
王姝钫
占钰琪
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Hangzhou Dianzi University
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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F30/00—Computer-aided design [CAD]
    • G06F30/20—Design optimisation, verification or simulation
    • G06F30/23—Design optimisation, verification or simulation using finite element methods [FEM] or finite difference methods [FDM]
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C64/00—Additive manufacturing, i.e. manufacturing of three-dimensional [3D] objects by additive deposition, additive agglomeration or additive layering, e.g. by 3D printing, stereolithography or selective laser sintering
    • B29C64/10—Processes of additive manufacturing
    • B29C64/106—Processes of additive manufacturing using only liquids or viscous materials, e.g. depositing a continuous bead of viscous material
    • B29C64/118—Processes of additive manufacturing using only liquids or viscous materials, e.g. depositing a continuous bead of viscous material using filamentary material being melted, e.g. fused deposition modelling [FDM]
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C64/00—Additive manufacturing, i.e. manufacturing of three-dimensional [3D] objects by additive deposition, additive agglomeration or additive layering, e.g. by 3D printing, stereolithography or selective laser sintering
    • B29C64/30—Auxiliary operations or equipment
    • B29C64/386—Data acquisition or data processing for additive manufacturing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B33—ADDITIVE MANUFACTURING TECHNOLOGY
    • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
    • B33Y50/00—Data acquisition or data processing for additive manufacturing
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F30/00—Computer-aided design [CAD]
    • G06F30/10—Geometric CAD
    • G06F30/17—Mechanical parametric or variational design
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2111/00—Details relating to CAD techniques
    • G06F2111/04—Constraint-based CAD
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2113/00—Details relating to the application field
    • G06F2113/10—Additive manufacturing, e.g. 3D printing
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2119/00—Details relating to the type or aim of the analysis or the optimisation
    • G06F2119/08—Thermal analysis or thermal optimisation
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2119/00—Details relating to the type or aim of the analysis or the optimisation
    • G06F2119/14—Force analysis or force optimisation, e.g. static or dynamic forces

Abstract

The invention discloses a finite element model self-adaptive generation method for FFF thermal stress analysis. The method comprises the following steps: step 1: generating a multi-resolution voxelization model of a geometric model to be simulated; step 2: attribute configuration is carried out on the voxelized model; and step 3: based on the attribute configuration and the multi-resolution voxel model, generating a multi-resolution finite element model in a self-adaptive mode; and 4, step 4: and solving the generated finite element model. The invention adopts a multi-resolution voxelization method to realize the self-adaptive generation of an integrated finite element model, thereby realizing the simulation reproduction of the FFF process. The method comprises the steps of preprocessing a boundary representation model to obtain physical attributes of the boundary representation model, generating multi-resolution voxel models, establishing a hierarchical relationship and an attribute transfer relationship between the multi-resolution voxel models, generating a finite element model for thermal stress analysis with corresponding resolution in a self-adaptive mode, improving model construction efficiency and enhancing reusability and universality of the finite element model.

Description

Finite element model self-adaptive generation method for FFF thermal stress analysis
Technical Field
The invention belongs to the field of finite element simulation analysis, and particularly relates to an integrated finite element model self-adaptive generation method for FFF thermal stress analysis.
Background
FFF (Fused deposition printing) is a well-developed additive manufacturing technology that is most widely used in the market. The technological principle of the technology is that materials are heated to a molten state through a spray head, the spray head moves along the outline and the internal structure of the model under the instruction of a control system, the molten materials are extruded while moving, and the materials are rapidly solidified to finish the deposition at the current position. The time consumed by this process is generally strongly related to the size of the model, the area of the overhanging region, the material filling rate, etc., and the larger the model, the more overhanging regions and the longer the printing time. In order to determine whether a model to be printed can be effectively printed before real printing, the finite element analysis technology is widely applied mainly to the problem of part forming accuracy reduction caused by uneven shrinkage or curling of materials.
Before finite element simulation analysis is performed on a CAD model to be printed, a finite element (analysis) model corresponding to the model is established, and usually, a plurality of preliminary preparation works are included, such as grid division; setting parameters such as material properties, ambient temperature, time step number, step length and the like; adding thermal convection and constraint conditions; loading a moving heat source to the corresponding grid surface simulation spray head; the "unit alive" attribute is set to simulate a material deposition process, etc. The preparation work is mainly completed by manual interaction at present, and time and efficiency are wasted. At the same time, the above manual effort increases not only with the increase of the complexity of the model, but also according to the increase of the requirement of the analysis accuracy (i.e. using the finite element mesh with higher resolution). This seriously affects the widespread use of finite element simulation analysis in FFF.
Additionally, existing CAD models that perform finite element analysis for FFF typically cannot include hole features. This is mainly because such structures often correspond to the printer head running without load, and the simulation time is difficult to obtain accurately, which may cause that the heat source movement and the activation of the life and death unit in the simulation process cannot be effectively performed, so that the validity of the simulation result is not good.
Aiming at the problems, the invention provides an integrated finite element model self-adaptive generation method for FFF thermal stress analysis. By establishing the multi-resolution voxel model and establishing a transmission rule for the physical attributes of the multi-resolution voxel model, the efficient and convenient self-adaptive generation and reconstruction of the multi-resolution finite element model are realized. Voxels with different resolutions are divided into a bounding box of the model, and the 'idle driving' and 'deposition' states of a spray head are simulated by distinguishing the types of the voxels, so that the model can simulate a part printing process with any geometric shape.
The identification method of the volatile failure area proposed by Jaiswal comprises the following steps: jaiswal P, Rai R.A geographic recycling approach for additional manufacturing quality assessment and automatic model correction [ J ]. Computer-air designed, 2018,109.
Disclosure of Invention
The invention mainly aims to provide an FFF thermal stress analysis-oriented integrated finite element model self-adaptive generation method, so that the problems that manual simulation model establishment is time-consuming and labor-consuming, and the establishment of a geometric model and the establishment of a corresponding FFF thermal stress analysis finite element model are independent are solved, and the reusability and universality of a simulation model are improved. And generating a multi-resolution voxel model to obtain the geometric content of the model, and then acquiring the physical properties of each unit, thereby automatically constructing the finite element model. And a multi-resolution finite element model is generated in a self-adaptive manner through the transmission of physical attributes among voxel models with different resolutions.
The self-adaptive generation method of the finite element model for FFF thermal stress analysis comprises the following steps:
step 1: generating a multi-resolution voxelization model of a geometric model to be simulated;
considering that the filament output shape of each unit time can be similar to a cube in the FFF-based printer nozzle filament output deposition process, and the shapes of all finite element grid units are conveniently unified and modeled, the invention further constructs a finite element model by adopting a voxelization model. In order to realize the generation of the integrated multi-resolution finite element model, a plurality of resolutions are set to carry out voxelization on the geometric model to be simulated. And in addition, a voxel is established according to the AABB bounding box of the model to be used as a voxel model. After obtaining the multi-resolution voxel model, establishing a hierarchical relationship: the first layer, namely the root node, is used for representing a model bounding box body voxel model and belongs to the bottommost layer voxel model. The second layer represents the other resolution voxel model. The third level, leaf node, represents the highest resolution voxel model, and is in the highest level voxel model.
Step 2: attribute configuration is carried out on the voxelized model;
the voxel units in each multi-resolution voxel model are numbered in the order of their generation, in the order of the print path. In order to set the parameters of the finite element model, a resolution voxel model is selected, and the semantics of each voxel, namely the physical properties, are obtained. The physical properties of the finite element model for FFF include: voxel type, time step attribute, "unit life and death" attribute, material attribute, constraint condition, moving heat source and loading surface thereof. The generated physical properties can be mutually transmitted among multi-level voxel models.
And step 3: based on the attribute configuration and the multi-resolution voxel model, generating a multi-resolution finite element model in a self-adaptive mode;
and establishing a transmission relation and a numbering relation of physical attributes among voxel models with different resolutions. If the physical properties of the voxel model of the required resolution have been obtained in step 2, they are directly imported into the finite element model. And if the physical attribute of the voxel model with the required resolution ratio is the physical attribute, acquiring the physical attribute corresponding to the voxel model with the required resolution ratio according to the known physical attribute and the transfer relation.
And establishing an empty temperature-structure coupling model by using simulation software, and importing the geometric model and the physical properties thereof by using a script language to establish a finite element model with the current resolution.
And 4, step 4: and solving the generated finite element model.
And adding a required result object into a solver of the finite element model according to the requirement, and solving to obtain a result.
The invention has the following beneficial effects:
aiming at the problems that the manual establishment of a finite element model is time-consuming and labor-consuming, and the establishment of a boundary representation model and the establishment of a corresponding FFF thermal stress analysis finite element model have independence at present, the invention adopts a multi-resolution voxelization method to realize the self-adaptive generation of an integrated finite element model. The method comprises the steps of preprocessing a boundary representation model to obtain physical attributes of the boundary representation model, generating multi-resolution voxel models, establishing a hierarchical relationship and an attribute transfer relationship between the multi-resolution voxel models, generating a finite element model for thermal stress analysis with corresponding resolution in a self-adaptive mode, improving model construction efficiency and enhancing reusability and universality of the finite element model.
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FIG. 1 is a flow chart of the present invention;
FIG. 2 is a boundary representation model of an embodiment;
FIG. 3 is a top resolution voxel model of an embodiment;
FIG. 4 is a schematic diagram of a hierarchical relationship of a multi-resolution voxel model;
FIG. 5 is a schematic view of voxel cell types;
FIG. 6 is a diagram illustrating voxel mapping between different resolutions;
FIG. 7 is a simulation result of a finite element model with a voxel size of 0.4 mm;
FIG. 8 is a simulation result of a finite element model with a voxel size of 0.2 mm.
Detailed Description
The invention is further illustrated by the following figures and examples.
As shown in fig. 1, the method for automatically and rapidly generating a finite element model for FFF thermal stress analysis mainly includes the following 3 steps:
step 1: and generating a multi-resolution voxelization model of the geometric model to be simulated.
1-1, determining the highest resolution.
The highest resolution, namely the voxelization size is the smallest, and the model is voxelized by taking the printable diameter of the volatile and useless area of the model as the voxelization size under the highest resolution. If the model does not have a volatile failure region, 0.2mm is taken as the voxel size of the highest resolution.
According to the volatile failure region identification method proposed by Jaiswal, firstly, the easy failure region is defined as a thin-wall structure, a sharp-corner structure, a small hole and a narrow gap.
The thin-wall structure is a part with the wall thickness smaller than the diameter (0.4mm) of the spray head, and the requirement on the printing resolution ratio of the thin-wall structure is too high, so that errors can occur in the printing process. And judging whether the wall thickness of each part of the model is smaller than the diameter of the spray head or not by obtaining the wall thickness of each part of the model so as to determine the thin-wall structure.
The sharp corner structure is a sharp corner angle in the model, and the sharp corner structure is difficult to print due to the limited resolution of the printer. And judging whether the region is a sharp-angled structure or not by acquiring the included angle of the adjacent edges in each layer of the slices.
The reason why small holes and narrow gaps are difficult to print is also the limited resolution of the printer. When too much or too little material is deposited, the contours of the holes and narrow gaps can distort or even disappear. Pores and narrow gaps were identified by morphological erosion and dilation methods.
After the volatile failure region is determined, the voxel size of the voxel model with the highest resolution is determined as the size that enables the region (mainly thin-wall structure, small hole and narrow gap) to be printed successfully. A geometric model to be simulated (a boundary representation model, fig. 2 is a boundary representation model of an embodiment) is introduced into the spacecollaim, and an AABB bounding box of the model is voxelized according to a set size, as shown in fig. 3. The effect of successful printing of the model can be analyzed based on the finite element model established by the resolution voxel model, and if the finite element model is established based on the voxel model with lower resolution, the validity of the simulation result cannot be ensured.
And 1-2, determining other resolutions and generating a corresponding voxel model.
In order to realize FFF simulation under different printing accuracies, common nozzle diameters of 0.4mm, 0.6mm and 0.8mm are used as voxel sizes of voxel models with other resolutions, and the models are subjected to voxelization respectively to generate voxel models corresponding to the printing accuracies. It is noted that the voxel size of the other resolution voxel model is an integer multiple of the voxel size of the highest resolution voxel model. In addition, a voxel is established according to the AABB bounding box of the model and is used as a model bounding box voxel model.
1-3, hierarchical relationship of constructors and prime models.
The multi-resolution voxel model corresponds to a hierarchical relationship and is represented by a tree structure, as shown in fig. 4 (the voxel size of the second layer voxel model is 0.4mm as an example), and each node in the tree represents a voxel of the corresponding voxel model. The first layer, namely the root node, is used for representing a model bounding box body voxel model and belongs to the bottommost layer voxel model. The second layer represents the other resolution voxel model. The third level, leaf node, represents the highest resolution voxel model, and is in the highest level voxel model. According to the hierarchical relationship, a transmission rule of physical properties between layers can be established, so that a finite element model is established. The first layer is a model bounding box body element model and only consists of one voxel, so that the physical properties and the propagation relation with other layers are not considered.
Step 2: and carrying out attribute configuration on the voxelized model.
2-1, numbering the voxels according to the print path.
The voxels in each multi-resolution voxel model are numbered in the order of their generation, in the order of the print path. This operation facilitates defining the geometry and state of its effect later on the "element life and death" attribute. In the example, a unidirectional printing direction in the positive X-axis direction is taken as an example.
And 2-2, acquiring the physical attribute of each voxel in a resolution voxel model.
One voxel in the multi-resolution voxel model corresponds to one solid element in the finite element model, and one solid element is activated in one unit time. In order to set the parameters of the finite element model, the semantics, i.e. physical properties, of each voxel in a voxel model need to be obtained in advance. The voxel model is a highest resolution voxel model or other resolution voxel models determined according to the actual printer nozzle diameter.
The physical attributes of the voxels include: voxel type, time step attribute, "unit life and death" attribute, material attribute, constraint condition, moving heat source and loading surface thereof.
(1) Voxel type:
the voxel types are grouped into three types, as shown in fig. 5:
model internal voxels: voxels in which all points of the voxel interior or surface are located inside the model (excluding the model surface) are called model interior voxels.
Model boundary voxels: the intersection of a voxel with the outer surface of the model (i.e., the interior or surface of the voxel contains at least one point on the outer surface of the model) is called a model boundary voxel.
Empty voxels: the eight vertices of a voxel, called empty voxel, are all located outside the model and do not intersect the model.
(2) Time step attribute:
the time step attributes of the voxel comprise the total time step number, the step length and the activation time step, and depend on the last number (namely the total voxel number) of the voxel model, the moving speed of the nozzle, the voxel size and the current voxel number. The total number of time steps is the total number of voxel models +1, where 1 is the time step of the cooling process. Each voxel in the same resolution voxel model has the same total number of time steps. The step size is the generation time of a voxel and is determined by dividing the side length of the voxel by the moving speed of the nozzle. The activation time step is the current voxel number, meaning that all voxels before the number are completely activated. The activation of the voxels inside the model and the voxels at the boundary of the model represents the generation process of the voxels, and the activation of the empty voxels represents the idle driving process of the sprayer.
(3) The "unit life and death" attribute:
the voxel 'cell life' attribute corresponds to the time step attribute and is used to simulate the deposition process of FFF. The "cell alive" attribute includes two states: the "inhibit" state corresponds to the "active" state, and corresponds to a certain state at each time step. The "inhibit" state indicates that the cell has not yet been deposited, and the "activate" state indicates that the cell has been generated. Since during FFF, the cell indicates that printing is complete at that location once deposition is complete. Therefore, the initial state of the 'cell life and death' attribute of each voxel unit is 'inhibition', and the states corresponding to the activation time step and all the time steps after the activation time step are converted into 'activation' according to the activation time step to represent the generation of the unit.
(4) Material properties:
the material properties of the voxel unit are the physical properties of raw materials used for printing, are customized by a user according to the requirements of the user, and generally comprise specific heat capacity, density, elastic modulus, poisson ratio, thermal expansion coefficient and the like.
(5) Constraint conditions are as follows:
the constraint condition refers to a support effect of the printing bottom plate and the support structure on the model in the printing process, so that the constraint condition of 'fixed support' needs to be applied to the lower surface of a voxel attached to the printing bottom plate and the support structure. Generally, only model boundary voxels have "fixed support" constraints.
(6) Moving the heat source and the loading surface thereof:
the activation process of the voxel unit is initiated by the action of a moving heat source on the upper surface of the voxel unit, and the loading surface is the upper surface of each layer of voxels. For the simulation of the FFF process, the embodiment of the present invention uses (but is not limited to) a gaussian heat source to establish the nozzle heat source model, and since the printing direction of the embodiment is X-axis positive printing, the heat source moves along the X-axis positive direction parallel to the XY plane, and the formula is:
Figure BDA0003488400160000071
wherein Q is the heat value of the coordinate point, QmAnd v is the heat source moving speed, t is time, and R is the heat source radius.
And step 3: and adaptively generating a multi-resolution finite element model based on the attribute configuration and the multi-resolution voxel model.
3-1, determining the numbered relation among multi-resolution voxel models:
the interrelationship of numbering between two different resolution voxel models for voxel sizes P (other resolutions), T (highest resolution), respectively, is described below, where the voxel size between the different resolutions is a N (P/T, by default an integer) times relationship. The bounding box size of the part is X Y Z, and the specification of the P resolution voxel model is
Figure BDA0003488400160000072
The specification of the T resolution voxel model is
Figure BDA0003488400160000073
First, for any resolution voxel model (S is the voxel size), the voxel number R is related to the corresponding position in the model (the R-th layer t is arranged in v columns) as follows:
Figure BDA0003488400160000074
Figure BDA0003488400160000075
if the voxel number in the P voxel model is known as I, the corresponding position in the voxel model is the ith layer, j, row and k according to the formula (1), and the voxel corresponds to N in the T voxel model3Individual voxels (these voxels are a cube). From voxel I, its location in the T voxel model corresponding to the voxel can be determined. As shown in FIG. 6, the 1 st layer, 1 st row, 1 st column voxels of the cube correspond to the ith in the T voxel model1Layer j1Row k1Column, Nth layer, Nth row and Nth column of the cube corresponding to ith voxel in the T voxel model2Layer j2Row k2Columns, wherein:
Figure BDA0003488400160000081
i2＝i1+N-1,j2＝j1+N-1,k2＝k1+ N-1, formula (4)
The number is determined from the position by equation (2):
conversely, if a voxel is known in the T voxel model, which is numbered M, and its position is in the q-th layer, M, and n, calculated according to equation (1), the position of the voxel in the corresponding P voxel model is q ' layer, M ', and n ':
Figure BDA0003488400160000082
the number can be obtained from equation (2).
3-2, establishing a transmission rule of physical attributes among multi-level voxel models:
and establishing a corresponding relation through the voxel numbers among the multi-resolution voxel models, thereby transmitting the physical attributes among the related voxels. The physical attributes that can be transferred are as follows:
(1) voxel type:
for the voxel type transmission of different levels, if the high-level voxel model comprises a model boundary voxel, the corresponding low-level voxel type is the model boundary voxel; if the high-level voxel model only contains the voxels inside the model, the corresponding type of the low-level voxel model is the voxels inside the model; if the high-level voxel model only contains empty voxels, the corresponding low-level voxel model is an empty voxel.
(2) Time step attribute:
for voxel models of different levels, the relation between the step sizes can be expressed as L ═ N3L, wherein L is the step size of the low-level voxel model, L is the step size of the high-level voxel model, and N is the ratio of the voxel sizes of the low-level voxel model and the high-level voxel model.
(3) Material properties:
for each level of the voxel model, the material properties can be transferred to each other. I.e. the material properties of a voxel in the low-level voxel model are the material properties of its corresponding voxel in the high-level voxel model and vice versa.
(4) Constraint conditions are as follows:
for the transfer of the constraint, if a "fixed support" constraint for the lower surface exists in the higher-level model boundary voxel, the constraint also exists for the lower surface of the voxel corresponding to the lower-level voxel model. On the contrary, if the lower surface constraint exists in the boundary voxel of the model at the lower level, the same "fixed support" constraint exists only in the lower surface of the voxel at the lowest level corresponding to the voxel in the voxel model at the higher level.
(5) Moving the heat source and the loading surface thereof:
the mobile heat sources conform to the same heat source formula, the loading surfaces of the mobile heat sources are similar to the constraint conditions, and when the upper surface of the high-level model voxel is the loading surface, the upper surface of the corresponding low-level voxel is also the loading surface; conversely, when the upper surface of the low-level voxel is a loading surface, the upper surface of the uppermost layer of voxels of the voxels corresponding to the high-level voxel model is the loading surface.
And 3-3, generating a finite element model with the required resolution.
If the required resolution is the resolution of the voxel model with the physical attributes acquired in the step 2, the physical attributes are known; if not, acquiring the physical attribute of the voxel model with the required resolution according to the acquired physical attribute and the transfer relation thereof.
Finite element models were created using simulation software, in this example using the Ansys Workbench. An empty temperature-structure coupling model is established through an Additive Wizard plug-in, and a script file is used for automatically importing a geometric model and physical attributes thereof into the empty model. Finite element modeling of the required resolution is complete.
And 4, step 4: and solving the generated finite element model.
And adding a required result object into a solver of the generated finite element model, and solving.
Fig. 7 shows the temperature field and the total deformation result of the finite element model (at a certain time) having a voxel size of 0.4mm, and fig. 8 shows the temperature field and the total deformation result of the finite element model (at a certain time) having a voxel size of 0.2 mm.

Claims (5)

1. The method for adaptively generating the finite element model for FFF thermal stress analysis is characterized by comprising the following steps of:
step 1: generating a multi-resolution voxelization model of a geometric model to be simulated;
a voxel model is adopted to further construct a finite element model, and a plurality of resolutions are set to carry out voxel formation on the geometric model to be simulated; in addition, a voxel is established according to the AABB bounding box of the model and is used as a voxel model; after obtaining the multi-resolution voxel model, establishing a hierarchical relationship: the first layer is a root node and is used for representing a model bounding box body voxel model and belongs to a bottommost layer voxel model; the second layer represents other resolution voxel models; the third layer, namely a leaf node, represents the highest resolution voxel model and is positioned in the highest layer voxel model;
step 2: attribute configuration is carried out on the voxelized model;
numbering voxel units in each multi-resolution voxel model according to the generation sequence of the voxel units according to the sequence of the printing paths; in order to set parameters of the finite element model, a resolution voxel model is selected, and the semantics of each voxel, namely the physical attributes, are obtained; the physical properties of the finite element model for FFF include: voxel type, time step attribute, "unit life and death" attribute, material attribute, constraint condition, mobile heat source and loading surface thereof; the generated physical attributes can be mutually transmitted among the multi-level voxel models;
and step 3: based on the attribute configuration and the multi-resolution voxel model, generating a multi-resolution finite element model in a self-adaptive mode;
establishing a transmission relation and a numbering relation of physical attributes among voxel models with different resolutions; if the physical attributes of the voxel model with the required resolution are obtained in the step 2, directly introducing the physical attributes into the finite element model; if the physical attribute of the voxel model with the required resolution ratio is found, acquiring the physical attribute corresponding to the voxel model with the required resolution ratio according to the known physical attribute and the transfer relation;
establishing an empty temperature-structure coupling model by using simulation software, and importing a geometric model and physical properties thereof to establish a finite element model of the current resolution;
and 4, step 4: solving the generated finite element model;
and adding a required result object into a solver of the finite element model according to the requirement, and solving to obtain a result.
2. The method of claim 1, wherein the step 1 comprises the following steps:
generating a multi-resolution voxelization model of the geometric model to be simulated in a self-adaptive manner;
1-1, determining the highest resolution;
the highest resolution, namely the voxelization size is minimum, the printable diameter of a volatile failure area of the model is used as the voxelization size of the model under the highest resolution, and the voxelization is carried out on the model; if the model does not have a volatile failure area, taking 0.2mm as the voxel size of the highest resolution;
according to the identification method of the volatile failure region provided by Jaiswal, firstly, defining the easy failure region as a thin-wall structure, a sharp-corner structure, a small hole and a narrow gap;
the thin-wall structure is a part with the wall thickness smaller than the diameter of the spray head, and the requirement on the printing resolution ratio of the thin-wall structure is too high, so that errors can occur in the printing process; judging whether the wall thickness of each part of the model is smaller than the diameter of the spray head or not by obtaining the wall thickness of each part of the model so as to determine a thin-wall structure;
the sharp-angle structure is a sharp vertex angle in the model, and the sharp-angle structure is difficult to print due to the limited resolution of the printer; judging whether the region is a sharp corner structure or not by acquiring an included angle of adjacent edges in each layer of slices;
the reason that the small holes and narrow gaps are difficult to print is that the resolution of the printer is limited; when the deposited material is too much or too little, the outlines of the small holes and the narrow gaps are distorted or even disappear; identifying small holes and narrow gaps by a morphological corrosion and expansion method;
after a volatile failure area is determined, taking the size which can enable the area to be printed successfully as the voxel size of the voxel model with the highest resolution; introducing a geometric model to be simulated into the SpaceClaim, and carrying out voxelization on the AABB bounding box of the model according to a set size;
1-2, determining other resolutions, and generating a corresponding voxel model;
in order to realize FFF simulation under different printing accuracies, the diameters of the spray heads are 0.4mm, 0.6mm and 0.8mm, which are used as the voxel sizes of voxel models with other resolutions, and the models are respectively voxelized to generate voxel models corresponding to the printing accuracies; it should be noted that the voxel size of the other resolution voxel model is an integer multiple of the voxel size of the highest resolution voxel model; in addition, a voxel is established according to the AABB bounding box of the model and is used as a model bounding box body voxel model;
1-3, hierarchical relationship of constructor prime model;
the multi-resolution voxel model corresponds to a hierarchical relation and is represented by a tree structure, and each node in the tree represents a voxel of the corresponding voxel model; the first layer is a root node and is used for representing a model bounding box body voxel model and belongs to a bottommost layer voxel model; the second layer represents other resolution voxel models; the third layer, namely a leaf node, represents the highest resolution voxel model and is positioned in the highest layer voxel model; according to the hierarchical relationship, a transmission rule of physical attributes between layers can be established, so that a finite element model is established; the first layer is a model bounding box body element model and only consists of one voxel, so that the physical properties and the propagation relation with other layers are not considered.
3. The method of claim 2, wherein the step 2 is as follows:
2-1, numbering the voxels according to the printing path;
numbering the voxels in each multi-resolution voxel model according to the generation order thereof according to the order of the printing paths;
2-2, acquiring the physical attribute of each voxel in a resolution voxel model;
one voxel in the multi-resolution voxel model corresponds to one solid unit in the finite element model, and one solid unit is activated in unit time; in order to set parameters of a finite element model, the semantics, namely physical attributes, of each voxel in a voxel model need to be acquired in advance; the voxel model is a highest-resolution voxel model or other resolution voxel models determined according to the actual diameter of a printer nozzle;
the physical attributes of the voxels include: voxel type, time step attribute, "unit life and death" attribute, material attribute, constraint condition, mobile heat source and loading surface thereof;
(1) voxel type:
the voxel types are grouped into three types:
model internal voxels: voxels in which all points of the voxel interior or surface are located inside the model are called model interior voxels;
model boundary voxels: the voxel and the outer surface of the model have an intersection relation, namely the interior or the surface of the voxel at least comprises one point which is positioned on the outer surface of the model and is called as a boundary voxel of the model;
empty voxels: eight vertexes of the voxel are positioned outside the model and do not intersect with the model, and the voxel is called a null voxel;
(2) time step attribute:
the time step attributes of the voxels comprise the total time step number, the step length and the activation time step, and depend on the last number of the voxel model, namely the total voxel number, the moving speed of the sprayer, the voxel size and the current voxel number; the total time step number is the total number +1 of the voxel model, wherein 1 is the time step of the cooling process; each voxel in the same resolution voxel model has the same total time step number; the step length is the generation time of a voxel and is determined by dividing the side length of the voxel by the moving speed of the sprayer; the activation time step is the current voxel number, which means that all the voxels before the number are completely activated; the activation of the voxels inside the model and the voxels at the boundary of the model represents the generation process of the voxels, and the activation of the empty voxels represents the idle driving process of the sprayer;
(3) the "unit life and death" attribute:
the voxel 'unit life and death' attribute corresponds to the time step attribute and is used for simulating the deposition process of the FFF; the "cell alive" attribute includes two states: the 'inhibit' state and the 'activate' state correspond to a determined state at each time step; the "inhibit" state indicates that the cell has not yet been deposited, and the "activate" state indicates that the cell has been generated; since during FFF, the cell indicates that printing is complete at that location once deposition is complete; therefore, the initial state of the 'unit life and death' attribute of each voxel unit is 'inhibition', and the states corresponding to the activation time step and all the subsequent time steps are converted into 'activation' according to the activation time step so as to represent the generation of the unit;
(4) material properties:
the material attribute of the voxel unit is the physical attribute of the raw material used for printing, and is self-defined by a user according to the self requirement;
(5) constraint conditions are as follows:
the constraint condition refers to the support effect of the printing bottom plate and the support structure on the model in the printing process, so that the constraint condition of 'fixed support' needs to be applied to the lower surface of a voxel attached to the printing bottom plate and the support structure;
(6) moving the heat source and the loading surface thereof:
the activation process of the voxel unit is initiated by the action of a moving heat source on the upper surface of the voxel unit, and the loading surface is the upper surface of each layer of voxels.
4. The method of claim 3, wherein for FFF process simulation in a moving heat source and a loading surface thereof, a Gaussian heat source is used to build a nozzle heat source model, and when printing is performed in an X-axis positive direction, the heat source moves in the X-axis positive direction parallel to an XY plane, and the formula is as follows:
Figure FDA0003488400150000041
wherein Q is the heat value of the coordinate point, QmIs the maximum of the heat source centerAnd the heat flow density, v is the moving speed of the heat source, t is time, and R is the radius of the heat source.
5. The method for adaptively generating a finite element model for FFF thermal stress analysis according to claim 3 or 4, wherein the specific method in step 3 is as follows:
3-1, determining the numbered relation among multi-resolution voxel models:
the interrelation of the numbers between two voxel models with different resolutions, wherein the voxel sizes are respectively P and T, is described as follows, wherein the voxel sizes between different resolutions are N times of the relationship; the bounding box size of the part is X Y Z, and the specification of the P resolution voxel model is
Figure FDA0003488400150000051
The specification of the T resolution voxel model is
Figure FDA0003488400150000052
Firstly, for any resolution voxel model, S is the voxel size, and the voxel number R is arranged in v-line with the corresponding position R-th layer t in the model, and the relation is as follows:
Figure FDA0003488400150000053
Figure FDA0003488400150000054
if the voxel number in the P voxel model is known as I, the corresponding position in the voxel model is the ith layer, j, row and k according to the formula (1), and the voxel corresponds to N in the T voxel model3A voxel size; by the voxel I, the position of the corresponding voxel in the T voxel model can be determined; as shown in FIG. 6, the 1 st layer, 1 st row, 1 st column voxels of the cube correspond to the ith in the T voxel model1Layer j1Row k1The Nth row and the Nth column of the Nth layer of the cube correspond to the T voxel modelIth of (2)2Layer j2Row k2Columns, wherein:
Figure FDA0003488400150000055
i2＝i1+N-1,j2＝j1+N-1,k2＝k1+ N-1, formula (4)
The number is determined from the position by equation (2):
conversely, if a voxel is known in the T voxel model, which is numbered M, and its position is calculated from equation (1) in the q-th layer, M, n, then the voxel position in the corresponding P voxel model is q 'layer, M', n:
Figure FDA0003488400150000056
then the serial number can be obtained by the formula (2);
3-2, establishing a transmission rule of physical attributes among multi-level voxel models:
establishing a corresponding relation through voxel numbers among the multi-resolution voxel models so as to transmit physical attributes among related voxels; the physical attributes that can be transferred are as follows:
(1) voxel type:
for the voxel type transmission of different levels, if the high-level voxel model comprises a model boundary voxel, the corresponding low-level voxel type is the model boundary voxel; if the high-level voxel model only contains the voxel inside the model, the corresponding low-level voxel model type is the voxel inside the model; if the high-level voxel model only contains empty voxels, the corresponding low-level voxel model is an empty voxel;
(2) time step attribute:
for voxel models of different levels, the relation between the step sizes can be expressed as L ═ N3L, where L is the step size of the low-level voxel model, L is the step size of the high-level voxel model, and N is a low-level voxel model and a high-level voxel modelA ratio of model voxel sizes;
(3) material properties:
for each level of voxel model, the material properties can be mutually transmitted; that is, the material property of a voxel in the low-level voxel model is the material property of its corresponding voxel in the high-level voxel model, and vice versa;
(4) constraint conditions are as follows:
for the transmission of the constraint condition, if the 'fixed support' constraint for the lower surface exists in the boundary voxel of the high-level model, the constraint also exists on the lower surface of the voxel corresponding to the low-level voxel model; on the contrary, if the lower surface constraint exists in the boundary voxel of the low-level model, the same 'fixed support' constraint exists only in the lower surface of the lowest-level voxel of the corresponding voxel in the high-level voxel model;
(5) moving the heat source and the loading surface thereof:
the movable heat sources all follow the same heat source formula, the loading surface of the movable heat sources is similar to the constraint condition, and when the upper surface of the high-level model voxel is the loading surface, the upper surface of the low-level voxel corresponding to the high-level model voxel is also the loading surface; on the contrary, when the upper surface of the low-level voxel is a loading surface, the upper surface of only the uppermost layer of voxels corresponding to the high-level voxel model is the loading surface;
3-3, generating a finite element model with the required resolution;
if the required resolution is the resolution of the voxel model with the physical attributes acquired in the step 2, the physical attributes are known; if not, acquiring the physical attribute of the voxel model with the required resolution according to the acquired physical attribute and the transfer relation thereof;
establishing a finite element model by using simulation software Ansys Workbench; establishing an empty temperature-structure coupling model through an Additive Wizard plug-in, and automatically introducing a geometric model and physical attributes thereof into the empty model by using a script file; finite element modeling of the required resolution is complete.
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