A Collaborative Framework for CAD–FEA Interoperability

Computer Mediated Communication [CE6014] Final Project Report - 2012

Student: William Buckley UCC ID: 112223669 Institution: University College Cork Course: Masters in Information Technology in Architecture, Engineering and Construction Lecturer: Dr. Matevž Dolenc, University of Ljubljana Date: 05/12/2012 Declaration Sheet

I declare that this project, in whole or in part, has not been submitted to any University as an exercise for a qualification. I further declare that, except where reference is made in the text, the contents are entirely my own work. The author agrees to the lending or copying of this document upon request for study purposes, subject to the normal conditions of acknowledgement.

Author: William Buckley WILLIAM BUCKLEY November 2012

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Abstract 3D CAD software has become established in providing models for downstream CAM/CAE applications. When those involved can work directly with the original, clean CAD model the results are significant boosts to product quality, production costs, and time to market.

The growing power of 3D CAD modelling is leading to greater complexity in geometry and its expression in file formats. This complexity leads to a greater number of file exchange issues, requiring the models to be reworked by the downstream users. Studies show that FEA users, for example, are spending as much as 70% of their time fixing CAD models. (McKenney, 1998)

This report discusses these issues in the context of a hypothetical Irish Small-Medium Enterprise (SME). The key issues regarding this SME’s interoperability between its CAD and FEA departments are isolated and identified. It will show how file format testing can aid in choosing the correct file transfer format. The SME design process methodology is also reviewed, and a Use Case Model for interoperability is developed.

The results of this report indicate the ACIS SAT file format is the best option for file transfer between the CAD and FEA software due to the required model topology and the ACIS derived graphics kernels the software operates with. The Spatial Data Management Environment (SDME) is developed for the SME design process methodology and a UML Use Case Model is formed to highlight the issue of interoperability in the system.

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List of Figures

Figure 1: File formats AutoCAD 2012 can open normally ...... 4 Figure 2: Importable file types in AutoCAD 2012 ...... 5 Figure 3: Exportable file types in AutoCAD 2012 ...... 5 Figure 4: List of file types Inventor 2012 can import ...... 6 Figure 5: List of file types Inventor 2012 can export ...... 6 Figure 6: Model to undergo file interoperability testing; isometric view rendering ...... 9 Figure 7: Model to undergo file interoperability testing, front elevation rendering ...... 10 Figure 8: Inventor 2012 model attributes...... 10 Figure 9: /CAE 6.11 GUI with imported Test Model undergone distortion ...... 12 Figure 10: Use Case Diagram in UML of proposed SDME process ...... 13

List of Tables

Table 1: Common CAD software and corresponding Geometric modelling kernels ...... 1 Table 2 : File formats Abaqus/CAE 6.11 can import per module ...... 3 Table 3 : File formats Abaqus/CAE 6.11 can export per module ...... 4 Table 4: Original Test Model data ...... 9 Table 5: Results of importing model in Abaqus/CAE 6.11 ...... 11

1 List of Abbreviations

Symbol Description 2D Two Dimensional 3D Three Dimensional ACIS Geometric Kernel ASCII American Standard Code for Information Interchange CAD Computer Aided Drawing CAE Computer Aided Engineering CAM Computer Aided Manufacturing CSCW Computer Supported Collaborative Work FEA Finite Element Analysis GUI Graphical User Interface IAI International Alliance for Interoperability IGES Initial Graphics Exchange Specification KBE Knowledge Based Engineering

OLE Object Linking and Embedding PDM Product Data Management SAT Standard ACIS Text SDME Spatial Data Management Environment SME Small and Medium Enterprises STEP Standard for the Exchange of Product Model Data

UML Unified Modelling Language

XML Extensible Markup Language

2 Table of Contents Declaration Sheet ...... i Abstract ...... ii List of Figures ...... 1 List of Tables ...... 1 List of Abbreviations ...... 2 Introduction ...... 1 The Interoperability Issue...... 1 Graphics Kernels ...... 1 Case Study ...... 2 Case Study Constraints ...... 2 Methodology ...... 3 File exchange Assessment ...... 3 FEA Software ...... 3 Abaqus/CAE 6.11-PR3 ...... 3 CAD Software ...... 4 AutoCAD 2012 ...... 4 Inventor 2012 ...... 5 Interoperability Methods ...... 6 Standardised CAD/FEA software ...... 6 Standardised CAD/FEA Graphics Kernel ...... 7 Neutral CAD File Format ...... 7 Object Linking and Embedding ...... 7 Model Translation...... 8 Modular Design ...... 8 Development of a Use Case Model for design process ...... 8 Results & Discussion ...... 9 File Format Interoperability ...... 9 Test Model ...... 9 Test results and discussion ...... 11 Use Case Model for SDME ...... 12 Conclusions & Recommendations ...... 13 Bibliography ...... 15

3 Introduction The Interoperability Issue Interoperability is the ability for software to interface, couple, and integrate with other software. The issue of Interoperability has long been found in Computer aided design and engineering. However, due to a multitude of reasons, the issue has never been adequately resolved on a global scale. The downstream use of CAD models is of critical importance to FEA personnel. FEA personnel spend as much as 70% of their time repairing CAD models from upstream in order to proceed with meshing and simulation. (McKenney, 1998)

According to the 2010 "Collaboration & Interoperability Report" by Longview Advisors, 41 per cent of professionals design, development, and engineering play some sort of role in CAD interoperability or data exchange activities (Stackpole, 2011). An understanding of the required interoperability is needed to eliminate these chokepoints in design. The first step in eliminating this issue is realising that the main cause of interoperability problems arise from the geometric modelling kernels.

Graphics Kernels Geometric modelling kernels are 3D solid modelling software components used in computer- aided design packages. As shown in Table 1, most common CAD software for mid to low end applications do not share the same Graphical modelling kernel.

Company/ Application ACIS derived Proprietary Autodesk/ AutoCAD Dassault Systèmes / Catia V5 SolidWorks Corp./ SolidWorks CADKEY Corp./ CADKEY Unigraphics/ Parametric Technology Corp./ Pro/ENGINEER IMS/ TurboCAD Table 1: Common CAD software and corresponding Geometric modelling kernels Table 1 records Catia V5 as operating with a proprietary kernel. This kernel is the Convergence Geometric Modeller (CGM) which is considered to be the most advanced kernel in the industry and will be seen in more next gen-products. (Stackpole, 2011)

ACIS is an object oriented C++ architecture that enables robust, 3D modelling capabilities. In late 2000, Dassault Systèmes acquired Spatial Technology, the company that created and develops ACIS. This was perceived as a move against their main competitor, Autodesk, who ran the ACIS kernel in the AutoCAD software package. (Suhanova, 2005) In response, AutoCAD went on to develop its own in-house 3D geometric modelling kernel now known as Autodesk ShapeManager. While this kernel diverged from ACIS 7.0 in 2001, it still shares a closely related topology. (Cross, 2001)

The Parasolid kernel has until recently been one of the most widely used graphics kernels. It is developed by Siemens who also publish the SolidEdge 3D modelling software. However, in recent years, the Parasolid kernel has declined in use, with ACIS and ACIS derived proprietary kernels becoming more widespread.

1 Evidently, with the proliferation of various kernels come interoperability issues. This project aims to highlight this with its case study, highlighted below.

Case Study This project focuses on the interoperability issues between CAD and CAE for a typical SME in the Irish Engineering sector. As a typical SME will have limited resources, certain constraints will be applied to this project’s scope. The importation of CAD models to the FEA software requires a high degree of interoperability. The best solution to this issue is the focus of this report.

Case Study Constraints The first constraint is continued usage of current CAD and CAE commercial software suite packages currently under license to the SME. This will ensure resources are not used in re- skilling the workforce at large to operate with new software.

For the purposes of this project AutoCAD 2012 and Inventor 2012, both produced by Autodesk, will be the CAD packages of choice for the theoretical SME. Autodesk is globally known to be the leading low to mid-range CAD software provider. A review of up to date CAD software and applications was carried out in assessing the options available to the SME. It was decided on the basis of Puodžiūnienė’s (2012) review of current trends that AutoCAD 2012 was still relevant and capable of performing into well into the near future.

The chosen CAE package is ABAQUS 6.11, published by Dassault Systèmes, a world-leading French multinational in the area of CAE. (Suhanova, 2005)

Autodesk AutoCAD 2012 is used for 2D drafting, 2012 is a 3D solid modeller, and Simulia Abaqus 6.11 is Finite Element Analysis software. The 2D models from AutoCAD are imported to Abaqus as ‘sketches’ while the 3D solid, shell or wire models from Inventor are imported as ‘parts’. (Simulia 2012)

Simulia Abaqus 6.11 contains a CAD interface for drafting directly in the software. However, this CAD module is limited in its drafting capability and it is preferable to import geometric models previously created in dedicated CAD software, as described above.

As both Autodesk and Dassault Systèmes also produce CAE and CAD software respectively, they are competing for the same clientele. The issue of interoperability will be most evident between such large competitors.

2 Methodology File exchange Assessment In order to verify the effectiveness of file transfer between Autodesk Inventor 2012 and Simulia Abaqus/CAE, and between Autodesk AutoCAD 2012 and Simulia Abaqus/CAE, a detailed test of a complex artefact was undertaken. Some file formats are more successful than others at translating data between CAD systems. This test will establish which file format is best suited to transfer a topologically solid 3D artefact with attributed data.

The effectiveness of each file type for the transfer of artefact data was assessed. The transfer focused on two movements:  AutoCAD to Abaqus  Inventor to Abaqus

The movements in reverse, i.e. Abaqus to Inventor, were not assessed as alterations to model geometry are typically carried out in the CAD packages. As such, it is mostly a one way interoperability issue.

FEA Software

Abaqus/CAE 6.11-PR3 Abaqus/CAE 6.11-PR3 allows the import of artefacts in several different ways. This is due to its modular format whereby an artefact can be imported at varying stages in the simulation timeline. The modules which allow import of geometric data are Sketch, Part, Assembly, and Model. Table 2 indicates which file formats may be imported under each module of Abaqus/CAE 6.11, while Table 3 highlights the exportable file formats.

Simulia Abaqus 6.11 has an ACIS CAD geometry kernel (.sat).

File Type File Extension Sketch Part Assembly Model ACIS SAT .sat IGES .igs, . STEP .stp, .step AutoCAD DXF .dxf VDA .vda Catia V4 .model, .catdata, .exp Catia V5 .CATPart, .CATProduct Parasolid .x_t, .x_b, .xmt Elysium Neutral .enf Output Database .odb Substructure .sim Assembly Neutral .eaf Abaqus/CAE Database .cae Abaqus Input File .inp, .pes Abaqus Output Database .odb Input File .dat, .nat, etc. Input File .cdb Table 2 : File formats Abaqus/CAE 6.11 can import per module

3 File Type File Extension Sketch Part Assembly Not Module Specific ACIS SAT .sat IGES .igs STEP .stp VDA .vda Output Database .odb Substructure .sim OBJ .obj VRML .wrl Compressed VRML .wrz 3DXML .3dxml Table 3 : File formats Abaqus/CAE 6.11 can export per module Dassault Systems utilises a neutral file type for the transfer of CAD files safely into the Abaqus environment. This file type is the Elysium Neutral File. Its file extension is .enf. ENF holds not only geometry information but also 3D annotation and attribute information. ENF is developed by Elysium Co. Ltd. Elysium is quickly becoming the leading software company specializing in the handling of 3D geometry in all its guises.

CAD Software

AutoCAD 2012 AutoCAD 2012 greatly extended its interoperability with its market competition through support of their file type import options. This can be seen as a concession from the community in its repeated calls for cross platform usage. Figure 1 and Figure 2 show the file formats AutoCAD 2012 can open and import respectively. While AutoCAD converts these imported files to the Autodesk kernel taxonomy, it does so with minimal loss of geometric data and error.

Figure 1: File formats AutoCAD 2012 can open normally

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Figure 2: Importable file types in AutoCAD 2012 AutoCAD 2012 can export models in the file formats shown in Figure 3.

Figure 3: Exportable file types in AutoCAD 2012

Autodesk Inventor 2012 Autodesk Inventor is 3D solid modelling design software for creating 3D digital prototypes used in the design, visualization and simulation of products. Figure 4 shows the file formats Inventor 2012 can import.

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Figure 4: List of file types Inventor 2012 can import As the primary 3D modelling software used by this Project’s SME, the export capability of Inventor 2012 is of primary importance. Figure 5 shows all file types Inventor 2012 can export 3D artefacts.

Figure 5: List of file types Inventor 2012 can export

Interoperability Methods The challenges of interoperability between CAD and CAE have been well documented (Rowell, 1997). Several methods to facilitate the interoperability of files between CAD and FEA have been developed. Hickey (2001), Mocko & Fenves (2003), and Rowell (1997) elaborated on the challenge of CAD interoperability. They pay particular reference to the various options available to successfully interoperate CAD with CAE. Though dated, these methods have remained the most widely used options available to a typical Irish SME. The methodologies that may be relevant to the case study SME are outlined in this section. Indeed, a combination of all these options should be used to varying degrees in order to ensure perfect interoperability.

In order to ensure interoperability, regardless of the chosen method, all relevant parties should have a basic understanding of FEA. Williams (2000) wrote an informative article on these knowledge requirements for CAD technicians and non-FEA practicing design team members.

Standardised CAD/FEA software This method is most suited to an SME as it is easier to control intra-company file types by dedicating all concerned software to one supplier’s software suite; e.g. Autodesk, SolidWorks, etc. However, problems arise when the SME is required to work with larger companies as a sub-contractor or similar inter-company scenario as frequently arises in the Engineering sector. This inflexibility in file interoperability would limit business opportunities and require more resources.

6 From an economic perspective, this method is also found wanting. The widespread adoption of a company’s software in a sector such as Engineering in Ireland would lead to a ‘cornering of the market’, inevitably leading to increased prices and loss of innovation. A competitive CAD/FEA software market is more beneficial to a typical SME than everyone adopting one company’s software for ease of interoperability.

Standardised CAD/FEA Graphics Kernel Similar to the above methodology, the standardisation of the underlying graphical modelling kernel in CAD and FEA software would solve most interoperability problems concerning file types. This would be due to all software using a common kernel, eliminating the requirement of multiple file extensions. This method is more economically sound than the above methodology as companies would continue marketing their brands and software suites while at their core they were all fully interoperable. While this method is seeing some adoption, it is limited to parent companies adopting their own kernel for all CAD, CAE and CAM software they own, allowing high end CAD software to interoperate with mid and low end CAD within the same company etc.

Neutral CAD File Format As the most used format for CAD to FEA interoperability, the use of neutral file formats such as the Initial Graphics Exchange Specification (IGES) and the Standard for the Exchange of Product Model Data (STEP) are very common. STEP is generally regarded as the best neutral file format as it is an international standard (ISO 10303). It is reliable in transferring most 3D geometry correctly from and to any software package that supports it.

Interoperability with and neutral data formats was assessed by González et al (2007). They used the STEP and XML formats for interoperability in in multi-body system simulation and results were positive when used together.

Industry Foundation Classes (IFC) was developed by the International Alliance for Interoperability (IAI). Lee et al (2003) developed a framework for designing with IFC at its centre. However, Pazlar and Turk (2008) demonstrated the inadequacies of IFC for the exchange of CAD data for Building Information Models (BIMs). This is analogous to the presentation of IFCs to the CAD to FEA exchange. Due to the lack implementation of IFC in the CAD/CAE community, it is not suitable for such an SME.

Park & Kim (2012) proposed a new format for improving the exchangeability of FEA data in a collaborative design environment. The format, named PAM, is an example of interoperability between FEA software; an often overlooked area. Though PAM was shown to work effectively with many established FEA programmes, it will not be used in this project due to the scenario of a single FEA package used by the SME.

Object Linking and Embedding The use of object linking embedding can be used to facilitate the use of multiple files to relate to an artefact through the use of embedded links. The Windows Object Linking and Embedding (OLE) technology is the most common example of such an approach. The open approach of OLE has recently declined, however, with most new OLEs originating from companies wishing to complement their various software packages.

7 Model Translation Translation of CAD and FEA models through the use of an intermediary is possible. However, as most companies are unwilling to release source code for their product, the creation of such translator technology is resource heavy.

Modern CAD and FEA software have developed sophisticated geometric healing methods for imprecise imported models. Products such as Elysium's CADdoctor which is built into Abaqus/CAE go a long way in patching up loose geometry. (Stackpole, 2011) Third party intermediary products such as Elysium will always exist to solve the more difficult interoperability issues when they arise. (Stackpole, 2011)

There exists a common misconception that translational software can fundamentally alter a model, losing a designers intent. Machine Design (2008) highlights the fact that FEA is at best only 5-10% accurate in any practical engineering model. As such, the cleaning of CAD models by Third party products such as Elysium has a negligible effect on results. Anything beyond clearly visible inconsistencies with the original model can also be safely ignored as the tolerance for FEA in this field is orders of magnitude greater than the 0.01mm required of a mid-range CAD package. (Machine Design, 2008)

Modular Design The notion of combining CAD and FEA as a modular software package has seen some success (Dearth, 2006). Aifaoui et al (2006) introduced a new method improving the interoperability of the parallel processes of design and analysis. The method they described was to streamline design and analysis by actively combining them through interoperability based on the concept standard analysis features and a semantically rich product model. The idea of modular analyses is used to cut down on design/analysis iteration by applying these modules to frequently required analysis tasks; thus eliminating the requirements for a formal analysis and design revision. This would require a sophisticated database to be developed and ordered modularly.

Development of a Use Case Model for design process Katranuschkov & Schere (1997) developed a modular framework for interoperability between operational, modelling and functional aspects of the design environment from the research of the previous 20 years. While it is now dated, the main theory behind the issue of interoperability is essentially the same and forms the basis for many use case models of interoperability in use today.

Gujarathi1 & Ma (2011) presented a parametric data model repository to act as the supply source of input for CAD and CAE models and thus maintain the associative dependences. This database would act as a catchall for files concerning a model and would allow design iterations without the need for email and memos as updates would be visible, similar to Google Docs and DropBox.

For the purposes of this project, a use case diagram developed in UML to illustrate the main functionality of the proposed design process was created. The basis of this Use Case Model came from the work of Charles et al. (2006).

8 Results & Discussion

File Format Interoperability

Test Model The chosen model was of typical front door with panelling acquired from the GrabCAD.com library (GrabCAD, 2012). This model had a reasonably high level of detail including irregular curved and angled geometry while remaining a manageable file size at 982kB in its native Inventor Part File (.ipt) file format. Shown in Table 4 is the data concerning the model in its original format, as shown in Figure 8. File Type Size (kB) Face Density Volume Mass (g) Count (g/mm^3) (mm^3) Test Model Inventor Part 982 417 0.001 94541226 94541.226 File (.ipt) Table 4: Original Test Model data

Figure 6: Model to undergo file interoperability testing; isometric view rendering

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Figure 7: Model to undergo file interoperability testing, front elevation rendering

Figure 8: Inventor 2012 model attributes

10 Test results and discussion As the model was originally designed in Inventor 2012, this was the original starting point for this interoperability test. From here, the file was exported as different compatible file types to Abaqus/CAE 6.11. The part module of Abaqus was the point of entry for the import. The results of importing in Abaqus recorded in Table 5 for all file formats assessed:

Test File Size Topology Face Edge Vertex Abaqus Comment Model Type (kB) Count Count Count File Size (kB) Original (.ipt) 982 Solid 417 N/A N/A N/A N/A Inventor Part File Parasolid (.x_t) 636 Shell 422 1016 600 876 Large Text distortion Parasolid (.x_b) 393 Shell 422 1016 600 872 Large Binary distortion IGES (.iges) 1503 Solid 422 1152 740 912 Some distortion on door handle curvature STEP (.stp) 1044 Solid 422 1051 639 1192 Some distortion on door handle curvature ACIS (sat) 1589 Solid 417 1130 728 908 None SAT Table 5: Results of importing model in Abaqus/CAE 6.11 The results of the test indicate there was no way of transferring additional information to the FEA software with the geometric data.

The imported topology is of vital importance to the overall simulation. As this model was to replicate a typical solid object, e.g., a timber door, the solid topology was the preferred option. The requirement of a solid topology was a primary factor on assessing a preferred transfer file for the SME as most of its work comprised of solid three dimensional models.

The ACIS SAT file was found to be the most stable format in transferring topological and geometric data. This was expected as both software share a common ancestry in the ACIS kernel originally developed by Spatial Technology. However, the loss of all attribute data still occurred as with all other file formats tested. The modular interface of Abaqus/CAE prevents properties of imported artefacts from being conserved from Autodesk Inventor 2012. Further study is required to assess the level of data file format stores on attributes. Shown in Figure 9 is the imported test model in the Abaqus/CAE environment undergone geometric distortion. (Note the difference in the glass panelling with respect to Figure 6 and Figure 7)

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Figure 9: Abaqus/CAE 6.11 GUI with imported Test Model undergone distortion In order to assess the export capabilities of AutoCAD 2012, the same model described above was utilised. As it is a 3D model in the Autodesk Inventor Part File format, a conversion to .dwg was required to allow opening in AutoCAD 2012. Though the .new .dwg file was larger at 1569kB than the original Inventor file, the test was not concerned with the interoperability between AutoCAD and Inventor. Consequently, this AutoCAD Drawing file was deemed a standalone from the Inventor Part File for the sake of assessing the interoperability between AutoCAD 2012 and Abaqus/CAE 6.11.

On assessing the interoperability between AutoCAD 2012 and Abaqus/CAE 6.11, it was found that only 2D geometric data can be transferred. This is due to AutoCAD 2012 lacking the ability to model in 3D, as discussed previously.

It is the Author’s opinion that AutoCAD 2012 should only be used for importing 2D ‘sketches’ into the Abaqus environment where further three dimensional operations can be carried out. The import of 2D geometry is far less complex than 3D geometry and as such a full test was not required.

Autodesk Inventor and AutoCAD run Autodesk Shape Manager, which is derived from ACIS and is fully compatible with ACIS 7.0 and prior versions. As Abaqus FEA runs on standard ACIS, it stands to reason that the file type native to ACIS should be used. This file type is ACIS SAT and has a .sat file extension. SAT files are simply ASCII text files which are readable with any text editor.

Use Case Model for SDME

Figure 10 shows the proposed Spatial Data Management Environment (SDME) process developed for this project and originally outlined in Charles et al (2006). While this project

12 focused on the relationship (highlighted in red in Figure 10) between the CAD Actor and the ‘Manage and Structure CAD and FEA data’ use case, it was deemed prudent to frame this issue in context. The SME would also benefit greatly by noting precisely where the interoperability issue arose with respect to the rest of the design process.

Figure 10: Use Case Diagram in UML of proposed SDME process

Conclusions & Recommendations

The working relationship between a designer and simulator will always require a direct method of communication due to requirements of design loops and revision. The iterative nature of this design process has been shown to be undermined by failures in the transfer of artefacts between CAD and CAE software.

ACIS SAT has been shown to be the best transfer file type between Autodesk Inventor and Simulia Abaqus due to the common ACIS kernel ancestry. Its use as a transfer file between CAD and FEA leads to minimal loss or distortion of geometric data attributed to a regular SME engineering artefact with a solid topology.

A Use Case Diagram was developed around a Spatial Data Management Environment (SDME). This is proposed to the SME as a method of highlighting the areas of interoperability within the company’s structure. It is recommended the SME adopts the SDME process in its company structure.

The work of Li et al (2005) reviewed the major methodologies and technologies of collaborative CAD systems to support design teams geographically dispersed. Their comment on the availability of 3D visualisation capabilities across the internet can be seen as advantageous for an SME where cost and distance is concerned. Such new, cheap technology

13 should be utilised by the SME in order to remain competitive. Further study is required in this area.

14 Bibliography

1. Aifaoui, N, Deneux, D, and Soenen, R 2006, ‘Feature-based interoperability between design and analysis processes’, Journal of Intelligent Manufacturing, 17, pp. 13-27.

2. Charles, S, Ducellier, G & Eynard, B 2006, ‘CAD and FEA integration in a simulation data management environment based on a knowledge based system’, Proceedings of TMCE 2006, Ljubljana, Slovenia.

3. Cross R 2001, ‘Autodesk Announces Plan to Develop A New 3D Solid-Modeling Kernel Named "Shape Manager" Based on ACIS 7.0 Kernel Licenced From ’, retrieved from Geocities October, 2009[online], available: http://www.oocities.org/wpsmoke/inventormdt/inventorkernel.html, accessed 12 November 2012.

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15 10. Lee, K, Chin, S, & Kim, J 2003, ‘A Core System for Design Information Management Using Industry Foundation Classes’, Computer-Aided Civil & Infrastructure Engineering, 18, 4, pp. 286-298, Academic Search Complete, EBSCOhost, viewed 30 November 2012.

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16 20. Suhanova, A 2005, ‘The Stand of Autodesk: Good to Great’, CAD/CAM/CAE Observer, 6, 24, pp. 7-12.

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