AutoFEM Analysis ShipConstructor Software: …autofem.com/downloads/files/SC+AUTOFEM_working_together.pdfAutoFEM Analysis & ShipConstructor Software: Working Together 3 Shipbuilding,

AutoFEM Analysis & ShipConstructor Software: Working Together Performing the finite element analysis of ShipConstructor structures

2014

AutoFEM Software LLP 20.01.2014

AutoFEM Analysis & ShipConstructor Software: Working Together 2

Performing the finite element analysis of ShipConstructor structures

Contents

Shipbuilding, engineering and calculations ......................................................................................................................... 3

ShipConstructor (SSI-Corporation, Canada) ....................................................................................................................... 3

AutoFEM Analysis (AutoFEM Software, United Kingdom) ................................................................................................... 3

Joint use of AutoFEM Analysis and ShipConstructor software ............................................................................................. 3

ShipConstructor & AutoFEM: How does it work?................................................................................................................. 3

Input data ...................................................................................................................................................................... 3

Preparation of the calculation model ............................................................................................................................... 3

Settings of integration with ShipConstructor .................................................................................................................... 6

1. Direct reading of data from the library materials ShipConstructor. ............................................................................ 7

2. Mapping the ShipConstructor material on the material in the database of AutoFEM Analysis. ................................... 7

Stages of finite-element analysis .................................................................................................................................... 9

Creating a set of objects for finite element study ............................................................................................................. 9

Creating study ............................................................................................................................................................. 11

Generation of finite-element mesh ................................................................................................................................ 12

Assigning materials ...................................................................................................................................................... 13

Defining fixation/restraints ............................................................................................................................................ 14

Applying loads ............................................................................................................................................................. 15

Solving the study ......................................................................................................................................................... 16

Studying the results ..................................................................................................................................................... 17

Investigation of the colour diagrams .......................................................................................................................... 18

Animation of the result on the screen ........................................................................................................................ 18

Creating animation video .......................................................................................................................................... 19

Creating Report ....................................................................................................................................................... 19

Conclusion .................................................................................................................................................................. 21

Supplement. Helpful tips and advices ............................................................................................................................... 22

Creating materials in AutoFEM Analysis material library ............................................................................................... 22

A typical order of actions to create the new material: ................................................................................................. 22

Validation of the model and correcting intersections ...................................................................................................... 23

Diagnostics of 3D model (study) ................................................................................................................................... 24

Multi-volume bodies ................................................................................................................................................. 24

Non-contacting groups of bodies .............................................................................................................................. 25

Mode Intersections of the bodies .............................................................................................................................. 26

Contacts between the bodies.................................................................................................................................... 26

Check of PLC representation .................................................................................................................................... 27

Creating finite-element mesh ........................................................................................................................................ 27

Decreasing the finite element size ............................................................................................................................ 28

Increasing the curvature for cylindrical 3D models ..................................................................................................... 28

Flag Thin-walled structure ........................................................................................................................................ 28

Errors of mesh generation ........................................................................................................................................ 28

Node tolerance for surface 3D models ...................................................................................................................... 29

Use of the option "Stabilize the unfixed model" ............................................................................................................. 30

What method of solving equation is better to use? ........................................................................................................ 30

Working with ShipConstructor Pipes ............................................................................................................................. 32


Shipbuilding, engineering and calculations Shipbuilding relates to the field of industrial activity, in which, in addition to developing the project, taking into account all the details, design calculations for strength, stability, dynamic effects play an important role. The main tools of modern engineers, using computer-aided design (CAD), became techniques of finite element analysis of products (FEM). These methods are being actively developed the last 50 years and have gained a new impetus to the development in connection with the emergence and spread of available computer-aided design (CAD-systems).

ShipConstructor (SSI-Corporation, Canada) ShipConstructor software has been deliberately designed to match standard industry processes, terminology and concepts of modern shipbuilding. With technology designed to scale with the largest shipbuilding projects, associative and parametric ship specific 3D modeling capabilities, and industry specific production output that promotes modular construction, ShipConstructor truly thinks shipbuilding. AutoCAD provides the underlying CAD engine and drafting tools for ShipConstructor products. More information about the ShipConstructor software can be found on the official website of the company (www.ssi-corporation.com).

AutoFEM Analysis (AutoFEM Software, United Kingdom) AutoFEM Analysis is a comprehensive system of finite-element analysis, which, as well as ShipConstructor, uses AutoCAD as a geometric modeling kernel. The system is focused on the wider community of users of AutoCAD and allows carrying out the most requested calculations without leaving AutoCAD and enjoying all the benefits of an integrated solution. More information about the AutoFEM Analysis software can be found on the official website of the company (www.autofemsoft.com).

Joint use of AutoFEM Analysis and ShipConstructor software Since both systems are based on the same system (AutoCAD), SC users have a natural ability to use AutoFEM Analysis for solutions of strength and other computational tasks in shipbuilding. To better meet the needs of shipbuilders working with ShipConstructor, in AutoFEM Analysis (since version 2.2) added additional capabilities to process data structures developed in the SC, such as for example the import of names of parts and materials in the finite element study.

ShipConstructor & AutoFEM: How does it work?

Input data Initial data for calculation are a three-dimensional model of the structure and information about its materials (name of grade), of which the parts are made. AutoFEM Analysis 2.2 has the ability to read the names of the materials assigned to the parts in the basis of SC and match these names with the materials in the database AutoFEM Analysis, using data the latter for calculation. In addition, user can specify structural materials directly in the finite element study in AutoFEM Analysis. Thus, in order to proceed with the finite element study, user must:

have a solid or surface 3D structural model, created in ShipConstructor (obligatory) assign the materials to the parts of the structure (optional).

Preparation of the calculation model Generally, any drawing of ShipConstructor, containing three-dimensional data about the product, - unit, object of ProductHierarchy or assembly - can be used as a base of the finite-element calculation model. ShipConstructor user himself determines the 3D part models, which will be modeled by finite element method. Possible to recommend a few, generally equivalent, ways to prepare 3D models for computational analysis.


Using the Product Hierarchy Drawing The user can create Product Hierarchy Drawings specifically for calculation, in which is convenient to enable or disable different levels of detail involved in the calculation.

Custom Product Hierarchy Drawing can be created to determine the set of objects for the calculation,

including construction details to be calculated

Using the Unit Initial 3D model of a unit may be used for finite-element modelling.

Finite-element study can be created for a specific Unit


Use of Assembly Production drawing (Assembly) may be used for finite-element modelling.

Finite element analysis can be performed directly in the production assembly.

Using the Model Link command The user can create the calculation 3D model from several units using Model Link command of ShipConstructor.

Creating the study on the basis of Model Link assembly


Exporting the 3D model into external dwg file User can export the 3D model of structure into dwg file and perform the analysis outside the ShipConstructor environment.

Finite-element modelling of exported ShipConstructor 3D model

Thus, the ShipConstructor user can choose the most convenient for him way in specific situation to prepare the 3D model of structure for calculations. The main thing is that the user continues to use his usual design environment AutoCAD. Settings of integration with ShipConstructor AutoFEM Analysis has the special settings for integration with the ShipConstructor database. These options are active in the command dialog "Set of objects for FEA" when the document is opened in ShipConstructor.

Settings of integration AutoFEM - ShipConstructor in command dialog "Set of objects for FEA"


Integration of AutoFEM Analysis with ShipConstructor software includes following features: • the system reads the unit of length, in which the ShipConstructor model was made, and uses them for the finite element model; • the system reads the names of structure parts as they are identified in the ShipConstructor database and uses them to identify objects in the finite element studies; • the system reads the mechanical properties of the material from the ShipConstructor database and uses them in the finite element model (first version of integration); • the system reads the name of the ShipConstructor material from the database and matches it with the name of the material from AutoFEM Analysis material library (second variant of integration).

Let us more closely consider the integration options for the transfer of materials data.

1. Direct reading of data from the library materials ShipConstructor. The mode "take properties from SC base" is used. Firstly, it is necessary to specify the mechanical properties of structural materials that are used for structural parts in the ShipConstructor materials library. For structural strength analysis and stability (buckling) analysis, the minimum required is the value of the modulus of elasticity (Young's modulus, the dimension N/mm2) and Poisson's ratio (dimensionless). Frequency analysis (calculation of the resonance frequencies), in addition to the above parameters, also requires the density of the material.

Parameters of structural material in the ShipConstructor database

To facilitate the analysis of safety factor results in the static structural analyzes, it is also recommended to additionally define: • yield strength of the material, • tensile strength, • compressive strength. When the option "take properties from SC base" is active, the system will read the material data and will use them to calculate. This approach is useful for everyday work, as it is sufficient only once set the parameters of the standard material in the database and then you can reuse it for calculations in all your projects.

2. Mapping the ShipConstructor material on the material in the database of AutoFEM Analysis. The mode "use AutoFEM materials by name" is used. In AutoFEM Analysis material library, ShipConstructor user can create a material which name (grade) coincides with the name of material in the ShipConstructor database of materials, set correct physical properties of it, and use this material for construction details. Or, otherwise, ShipConstructor user can create a material in ShipConstructor database of materials which name (grade) coincides with the name of the material in the AutoFEM Analysis library, and use this material for construction details. When the second mode of integration is used, the system will use for calculation the material properties from the AutoFEM Analysis material library, finding in AutoFEM the material this the same name as is used in the ShipConstructor drawing.


Creating a material whose name coincides with the name of the material in AutoFEM Analysis base

Assigning the part material

This approach is convenient because it allows to use materials with complex properties (eg, anisotropic) for the analysis of ShipConstructor models, and also to solve the problems of heat conduction and fatigue, in which the nonlinear material properties can depend on various parameters.

AutoFEM Analysis library of materials allows specifying materials with complex properties and using them to

calculate the ShipConstructor models.


Stages of finite-element analysis There are several typical stages of finite element modelling of arbitrary structure using AutoFEM Analysis: • Creating a set of objects for finite element study • Creating the study of desired type • Generating a finite element mesh • Assigning the boundary conditions (fixations, loads) • Assigning materials • Performing calculation • Analysis of the results So how exactly does the strength calculations are most often required in shipbuilding, let us consider a typical sequence of actions to carry out finite-element analysis of ship structure, using AutoFEM Analysis, on the example of static strength analysis.

Creating a set of objects for finite element study The first stage of the finite element analysis is to create a set of objects for finite element study. The set may include all or selected design details. In the process of creating the set of objects, AutoFEM Preprocessor imports 3D geometry of the structure from AutoCAD. In the sequel, all finite element studies are based on this geometry. Only one set of objects to FEA can be defined in a document. All studies are based on this single set. However, the composition and type of each study are independent from each other. The set of objects may include all of the three-dimensional bodies in the drawing or the selected part or objects. Furthermore, there are two types of set of objects:

• A set of 3D solid models (AutoCAD 3D solids) • A set of surface objects (AutoCAD 3D surfaces)

For users ShipConstructor greatest interest is the use of three-dimensional solid model, available to them, so the example below illustrates mainly work with a set of 3D solid objects. In the ShipConstructor drawing, go to the tab AutoFEM and run the command "Create a study". If the document does not have an existing set of objects for FEA, a dialog appears to create it.

Running the command "Create a study"


Command dialog to create a set of objects for finite element analysis

Request to check intersections between bodies

In the process of creating a set of objects, initial verification of 3D geometry models, finding and correcting the intersections between the bodies are made. If the intersections between the bodies are found, the following message appears:


The system offers to automatically fix all found intersections

If to refuse removal of intersections, a set of objects will contain the intersecting bodies. Set of objects with intersecting bodies can be used for modeling in the plate / shell formulation based on the faces of solid bodies. However, volumetric (tetrahedral) mesh for such studies is not built as a three-dimensional space at each point must comply only one point of a single body. In most practical cases, intersections between construction details are not allowed when the simulation in the volumetric formulation (tetrahedrons) is carried out. After elimination of intersections, the system automatically continues the command of creating the first study.

Creating study After the successful creation of a set of objects, the command dialog "Creating Study" automatically appears. System allows for finite element modeling in two basic formulations:

1. Volumetric problem formulation. The geometry of the design is approximated using tetrahedral finite elements.

2. Plate (shell) problem formulation. It is used to calculate the thin-walled structures. In this case, the geometry is approximated by triangular finite elements of the plates / shells. Problems of the second type can be defined in two ways: • on the basis of three-dimensional surface AutoCAD models (AutoCAD surfaces); • on the basis of solid AutoCAD 3D models, when the geometry of the computational model is defined by facets of a solid 3D model.

The study can include all bodies from the set of objects for FEA, or only some of them, chosen by cursor in the Preprocessor. window. The user can multiply edit the composition of the study, i.e. remove some objects from the study, or, otherwise, add new items. Studies can be copied and their type may be modified. For example, the user can create and fulfill a study of the Static analysis type. Then, user can copy this study, change its type on Buckling analysis, and analyze the stability of the structure for the same boundary conditions. It is very convenient and saves a good time to prepare the calculation model.


Dialog for creating the study

Generation of finite-element mesh By default, after the establishment of research, dialogue of the finite element mesh generation is automatically opened. For models based on 3D solids, tetrahedral finite element mesh is constructed. For models based on 3D surfaces, the finite-element mesh consisting of triangular finite elements is created.

Dialogue of finite element mesh generation


For typical ShipConstructor models (usually, relatively thin and spatially extended), it is recommended to build the finite element mesh with the active option "Thin-walled structure (regular)". This allows to get a grid with acceptable for practical calculations the quantity and quality of finite elements. By choosing suitable parameters of the grid in the dialog, it is possible to evaluate mesh quality and then finish its creating:

Tetrahedral grid model of the ShipConstructor design

Assigning materials AutoFEM Analysis has the library of materials, which is composed of common construction materials that can be used for the calculation. The system supports the work with anisotropic materials, and allows to set graphs of nonlinear dependence of ultimate strength of the material on the number of loading cycles (SN-curves). As noted above, the special integration tool with ShipConstructor allows the use of materials from the library AutoFEM Analysis and this way is convenient for "complex" cases, when the material or study require additional material properties or data. As for the routinely used structural steels, it is advisable to adjust the characteristics of the material directly in the ShipConstructor database of materials.

Access to AutoFEM material library


Materials of parts are displayed next to the names of items in the study

Defining fixation/restraints In most mechanical calculations, the structure, to which loads are applied, should be fixed in space, so as to prevent its free movement for an indefinite length under the action of applied forces. Special commands (Fixture, Plane of Symmetry, Elastic base) allow defining the fixed elements of the construction and setting the parameters of these restrictions. In the command dialog box, the user specifies the type of restrictions (full or partial) and selects the fixed structural elements in the preprocessor. Special filters (selectors) facilitate the selection of model elements - faces, edges, vertices, and user coordinate systems allow specifying the direction of the moving degrees of freedom. Fixture is displayed in the Preprocessor conventional signs, the relative size of which is adjusted in the command dialog.

Command dialog of Restraint


Defining the restraints for ShipConstructor model

Applying loads For mechanical calculations as loads are available: force, pressure, acceleration, torque hydrostatic load, centrifugal force, and the additional mass. The user determines the magnitude and direction of loads and their application zones to the system. For each load, own specific symbol is used to reflect it in the Preprocessor window. Size and color of the conventional symbol are adjusted in the dialogue of the corresponding boundary condition.

Applying loads to a structure

There is a special window "AutoFEM Palette", which displays all the studies existing in the document, as well as data on each element of the study: the finite element mesh, materials, boundary conditions, the results of calculations. Contextua menu with a list of suitable commands is available on each item in the tree of studies. AutoFEM Palette and tree of studies are the main tools for working with AutoFEM Analysis.


Tree of study, prepared for calculation

Solving the study After you creating the finite element mesh, assigning materials, restraints and loads, it is possible to carry out calculations. Before the calculation a special dialog appears, in which the user can select the method for solving equations, set a list of necessary results, and specify additional analysis options, depending on the type of analysis (stabilization of the system, nonlinearity, thermoelasticity and others).

Running the solving process from contextual menu (left) and AutoFEM Ribbon (right)

After starting the calculation, a window appears that displays system messages about the process of solving equations.


Properties of static analysis study

Message box of the fifnite-element solver

Studying the results After completion of the calculation, list with the results appears in the tree of study. From the contextual menu on the result, user can open the Postprocessor window with the selected result. The result is displayed as colored three-dimensional model of structure where each colour corresponds to a specific value which can be estimated with the help of colour scale. User has the following additional tools to investigate results:

tooltips with the result value at the cursor; labels with the result value (double click); sensors - special tool to assess results at a preselected point; graphics, allowing to reflect the change of the result; section, allowing to look into the interior of the model and estimate the situation there; report - independent html-based document, containing all important data of study with the colour

diagrams and 3D VRML models of the results; animation of the result on the screen; creating video file with the animation of the result in several video-formats;


Investigation of the colour diagrams In static strength analysis, estimation of the results begins with the investigation of the result "Displacement". User must make sure that calculated displacements of the structure correspond to the expected and have a physical sense. You can then proceed to the analysis of the stress state of the structure.

Calculated deflections of design

Diagram of safety factor on equivalent stress

Animation of the result on the screen


User has special tools to animate the result on the computer screen and investigate how it changes with the change of applied load

. Tools to display screen animation of the result

Creating animation video User can create video file of animated result.

Creating the video file of animated result

Creating Report There is a special command "Report" creating the independent html-based document with the main data about current study. This document can be sent to a customer or saved for latter comparison.


Command "Report" in contextual menu and its dialog box

Sections of the report


VRML 3D model of the result in the html-report

Thus, we have found the maximum displacements and safety factors for structure that allows us to make conclusions about the reliability of the designed system and its suitability for use in the real world. Also, we have got the presentational materials such as films and html-report that can be shown to our customer and defend our design decision.

Conclusion Thus, ShipConstructor users have easy to use, convenient and integrated tool for different types of finite element simulation of their structures. More about AutoFEM Analysis can be found on the website www.autofemsoft.com.


Supplement. Helpful tips and advices

Creating materials in AutoFEM Analysis material library As it was mentioned in the first part of the article, to adjust mapping the materials from ShipConstructor onto finite-element in AutoFEM Analysis, it is necessary, in AutoFEM material library, to create the material which name coincides with the material name in the ShipConstructor drawing. The typical seqience of actions is shown below.

A typical order of actions to create the new material: 1. Create the new user’s library of materials or open the existing one.

Creating the user’s library of materials

2. In the user’s library of materials, select the existing group of materials or create a new one.

Creating new group

3. When the cursor is on the group of materials, activate command "Create a new material" in the context menu.

Creating a new material


4. Choose the material type: “Isotropic”, “Orthotropic” or “Transversally isotropic” and set up the name of the material. After OK is pressed, the new material will appear in the selected group of materials.

Typing the name of the material

5. Select the newly-created material from the list of materials and fill in the material’s parameters, choosing them by a single click in the window of the material’s properties contrariwise each property.

6. Press OK in the dialog to complete the editing.

Validation of the model and correcting intersections In the process of creating a set of objects for FEA, system checks imported for calculation geometry for correctness in terms of finite element modelling. There are several widespread problems that are often encountered in models of AutoCAD, which should be resolved in order to achieve a positive result. Here are the problems:

Intersections of 3D solids. The presence of intersections between solid bodies makes it impossible to adequately simulate the real physical system. Indeed, in a single point of space may be present only one physical body. Fortunately, AutoFEM Analysis can automatically eliminate intersections, found in the 3D model. Of course, we should take into account that the elimination of intersections changes the original geometry of the 3D model, but if the errors are relatively small, the automatic removal of intersections can cope with this problem.

Automatic detecting and removing intersections


Warning! Removal of intersections provides by using the sequence of AutoCAD commands. It can take a significant time for such large 3D models as ShipConstructor models often are. Sometimes, in the case of emerging incorrect geometry, crash of AutoCAD may occur.

"Flying" parts - objects which have no spatial fixation as a whole body. They are erroneous for mechanical studies such as static strength and buckling analysis but admissible for frequency and thermal studies.

Multi-volume bodies. From the point of view of mesh generator, each part of structure must have only one closed volume. Otherwise, in AutoCAD, multi-volume solids can be created easily when the Boolean union of non contacting bodies and arrays of solids are created. In current AutoFEM version, we must eliminate emerging such bodies if the tetrahedral mesh is intended to be created for them.

Non-contacting solids. Sometimes, when the 3D model is transferred from AutoCAD into AutoFEM, the system cannot properly detect the contact between the faces which, as assumed, must have the contacting faces. It may occur the problems with the mesh creation or influence on the result of calculation.

Errors of piece linear complex (PLC) representation of AutoCAD 3D model. These errors may occur on complicated 3D geometry as an unpredictable result of 3D modelling operations and prevent the tetrahedral mesh creation. Usually, initial 3D model should be changed in such case.

Diagnostics of 3D model (study) The specific command, Study Diagnostics, is designed to help the user find erroneous objects in the study and test the correctness of a 3D model of a structure from the point of the finite-element modelling on the whole. The command is performed automatically if the Preprocessor finds mistakes in the process of study creation. For the active study, this command is also available in the context menu in the study tree.

Contextual menu of Diagnostics command on Set of objects for FEA (left) and Study (right)

In all modes of work of the diagnostics command, the drop-down list "Group of the bodies" allows you to select the group of erroneous objects, while the list Geometry reflects names of the objects in the selected group. The following diagnostic modes are available:

Multi-volume bodies Mode Multi-volume bodies reflects multi-volume bodies if any in the study. The multiple-volume body is the

term meaning the AutoCAD object which consists of several non-contacting volumes. Multi-volume bodies are perceived by AutoCAD as single object. One requirement set by the AutoFEM Preprocessor is the one-volume nature of solid bodies involved in the study, i.e. one closed area in the 3D space must correspond to a single body.


Image of the multi-volume body

Multi-volume bodies are formed, e.g., when using copying by massive in AutoCAD (use the AutoCAD command EXPLODE to separate the massive into separate bodies), and also can be obtained when a body is created with the help of Boolean operation (in this case, for the division of the multi-volume body, the AutoCAD command SOLIDEDIT/BODY/SEPARATE is used). In addition, the multi-volume bodies may emerge in the process of automatic correction of the model’s intercections by the command of study creation. Appearance of the multi-volume bodies post automatic correction indicates significant inaccuracy of the 3D modeling of the real structure and requires correction using AutoCAD tools.

Appearance of the multi-connection body in the process of intersection

corrections evidences errors in the 3D modeling process

Non-contacting groups of bodies Mode Non contacting bodies reflects groups of bodies not having physical contact with other bodies. If the

body which does not contact the others is characterized by insufficient boundary conditions (e.g., it is not fixed in the space in the case of static analysis), an algebraic system of equations may prove to be degenerative, and the process of calculations may stop. Therefore, in some cases, the presence of independent groups of bodies in the study can indicate incorrectness of the problem setting.


Non conacting parts in the Preprocessor window

Mode Intersections of the bodies Mode Intersections of the bodies illuminates pairs of intersecting bodies. Creating the finite-element mesh

for the intersecting bodies is not permitted. Actually, the read solid-body physical object does not permit the presence of more than one body and material at one spatial point (AutoCAD and other CAD systems allow for it). As we are concerned with the reliability of calculated results, we prohibit the creation of the mesh for solid-body objects when we find the intersections of the bodies. One can check the intersections using also the AutoCAD command INTERFERE.

Reflecting pairs of intersecting objects in the diagnostics window.

Contacts between the bodies Mode Show the contacts between the bodies reflects all facets by which the bodies contact with each other

in the Preprocessor window. Using this command, you may check whether the determination of the body contacts by the Preprocessor is correct or not. Sometimes, when modeling in AutoCAD, invisible gaps can emerge between bodies. These gaps will be perceived by the system as no dense contact between the bodies. This can lead to unreliable results of the modeling. With the help of this mode, the user may check the presence of gaps between the bodies which must contact firmly. Besides, in this mode you may check whether automatic improvement of intersections was correct.


Parts in the model are joined by surface (shown green)

Surfaces of the contact are not revealed. The model’s correctness requires checking.

Check of PLC representation Mode Check PLC representation activates the regime of testing the geometry of the 3D model using its

current PLC presentation. When possible inaccuracies in the presentation of the 3D model are found, they are shown in the Preprocessor window; also the "Geometry" window shows the number of suspicious objects being found. If these errors do not vanish at the change of mesh construction parameters, the user must analyze the initial AutoCAD 3D model in the fields, marked during the diagnosis process, and correct the 3D model to eliminate the errors.

Possible problem areas are shown in the model

Note that the retrieval of the command FEMAMESH to create the finite-element mesh is impossible prior to removal of all multi-volume bodies from the study.

Creating finite-element mesh The most important stage in the preparation of the finite-element study is to construct a finite element mesh that approximates the geometry of the structure. Taking into account a sufficiently large complexity of three-dimensional models of ShipConstructor, not in all cases it is possible to build affordable finite element mesh


without additional 3D model modifications. The system offers a set of tools (such as command "Diagnostics of 3D model"), to facilitate search and diagnose of the causes of potential problems, affecting on the process of creating the finite-element mesh. Also, parameters of mesh construction, specified in the command dialog can affect the possibility of getting the grid.

Parameter "Edge length"

Parameter "Curvatura"

Decreasing the finite element size Normally, the system offers too large size of the finite element of mesh. User should define acceptable size of the finite element using a special slider (Edge length) or typing the value in the text box.

Increasing the curvature for cylindrical 3D models For 3D models, which have in their composition round or cylindrical geometrical elements, often is useful to increase the accuracy of curve elements representation.

Flag Thin-walled structure Flag "Thin-walled structure (regular)" switches on the mode of mesh generation with the simplified control of quality of finite elements. If the "Mesh quality" parameter is set as "Dis", the mesh will be constructed on the basis of the initial PLC model with minimal modifications (usually it looks like a "regular" mesh with square cells). Such mesh contains a lesser number of the elements and may be used to make calculations for thin-walled structures, as well as in many cases for preliminary calculations with the decreased number of elements.

Errors of mesh generation If, in the process of constructing the tetrahedron mesh, the algorithm encounters the insuperable obstacle, the diagnostic message is given, and the system points at the area of geometry or shows the number of the “bad” body at the model. The user can try and eliminate the problem in the 3D model geometry or exclude the "bad" body from the finite-element study.

Indication of the place where there is an error of 3D model geometry


Node tolerance for surface 3D models the possibility of managing the accuracy of nodes’ coupling was added. It can be useful from the points of obtaining mesh models by surfaces, butt-jointed by segments, and of eliminating possible ruptures at the borders of segments.

Rupture of the mesh model

Removing the gaps by increasing the node tolerance


Use of the option "Stabilize the unfixed model" Somitimes, during the analysis of large ShipConstructor assemblies, a situation may occur when fixations, applied to the system of do not restrict any structural element of free movement. Usually in this case, the solver produces the following diagnostic message and stops the calculation.

Use of option "Stabilize the unfixed model"

In many cases, the option "stabilize the system" helps to find not fixed element of design or carry out a successful calculation.

What method of solving equation is better to use? Generally, there are two main methods of equation systems solving. They are direct and iterative methods. The default mode of automatically choosing one of these methods is turned on in AutoFEM Analysis. If the number of equations is more than the number, specified in Processor settings (which equals 100,000 on default), the iterative method is used for solving. If the system is smaller, the direct method is used. You can set your own criterion for choosing a method of solving that is determined by parameters of your computer system. Also you can manually set whether the direct or iterative method should be used for solving the equation system, choosing the desirable mode directly before solving.

Selecting solving equetion method and threshold


General recommendations of choosing and using these methods are as follows. The first criterion is power of your computer system. The iterative method has fewer requirements for the amount of memory (RAM) than the direct method. The next criteria are the quality of finite-element mesh and "thickness" of a problem. The direct method is based on the Gauss method and its modifications. Full inversion of matrix occurs. This means that the large amount of memory is needed for storing the inverse matrix. In addition the time of calculation increases as N3 with number of equations N. Usually the direct method works well and it is preferred when you have:

relatively small equation system or a powerful computer system, based on x64-platform, and sufficient memory (RAM) amount (at least 8GB), which allows to place the inverse matrix fully in RAM. If you have such a computer system, you can efficiently solve problems up tol 1.5 -2 million degrees by the direct method for the "thin" 3D model.

mesh has many "stretched" elements study has contact restrictions

The iterative method. The main advantage of iterative method is its posing ow demands on the RAM amount in comparison with the direct method. For the iterative method, the amount of necessary additional RAM is comparable with the amount of RAM for initial stiffness matrix location. Therefore, even relatively weak computer system (such as an office computer with Windows 32-bit operational system and no more than 3GB RAM) is able to solve up tol 1 million equations. But there is a certain drawback. Its performance depends on such property of tetrahedral finite elements as aspect ratio. When aspect ratio grows, the number of iterations increases and time of solving may significantly increase too. Therefore, if the mesh has many stretched elements (this often occurs if your model is slim and thin) or even "incorrect" ones, you may face the slowdown of the solving rate. Usually the iterative method works well and is preferred when you have:

good mesh with total aspect ratio less than 8-4 and small number of stretched elements. "thick" model, meaning that you have several layers of tetrahedral elements per section. study has no contact restrictions.

A "thick" finite element model. There are several layers of the finite elements. The Iterative method works better in comparison with the direct method in the medium and large models.

A "thin" finite element model. Mesh consists of one or two layers of finite elements. The direct method works well, and the iterative method may be down if quality of the mesh is poor.


Working with ShipConstructor Pipes Some types of ShipConstructor 3D models (such as pipes, for example) have no the true 3D solid geometry in ShipConstructor drawing. The user can export such 3D models into DWG file and then simulate them as ordinary AutoCAD models.

Shipcontstructor Pipes are not true 3D solid objects and cannot be directy used in finite-element studies

Finite-element analysis of exported ShipConstructor Pipe

Documents

AutoFEM Analysis ShipConstructor Software: …autofem.com/downloads/files/SC+AUTOFEM_working_together.pdfAutoFEM Analysis & ShipConstructor Software: Working Together 3 Shipbuilding,