Special finite elements: Theoretical background and application

Specijalni konačni elementi (KE) koriste se za modeliranje posebnih fenomena u ponašanju konstrukcija. Izraz "specijalni" označava KE sa specifičnim svojstvima (parametri krutosti, ponašanje pod opterećenjem, itd.) i posebnom ulogom u modeliranju, po čemu se i bitno razlikuju od "standardnih" KE. Postoji više tipova specijalnih KE, ali se najčešće koriste: "nelinearna opruga" KE, "zazor" KE i "veza" KE. Na različitim će primjerima biti pokazane prednosti primjene specijalnih KE, posebno u modeliranju konturnih i prijelaznih uvjeta. Teorijska podloga data je samo u neophodnom obimu, zbog dobrog razumijevanja numeričkih aspekata primjene specijalnih KE.Special finite elements (FEs) are used for modeling of certain phenomena in structural behavior. The term "special" denotes FEs with specific characteristics (stiffness parameters, behavior under load, etc.) and a specific role in modeling, which is their main difference from "common" FEs. There are many types of special FEs, but the following are often used: so-called "nonlinear spring" FE, "gap" FE and "link" FE. Various examples will show many advances in the application of these FEs, especially in modeling of boundary and interface conditions. Theoretical background is given only in the amount necessary for proper understanding of numerical aspects of the application of special FEs

An Innovative Photogrammetric System for 3D Digitization of Dental Models

This paper presents an innovative system for 3D reconstruction of a physical dental model. The innovative system is based on close-range photogrammetry and enables the projection of digital light texture on the objects surface. It is based on the application of mirrors that direct the digital light texture to the vertical surfaces of the physical model. In this way, high coverage of the object is achieved, and 3D reconstruction from one set of photographs is possible. 3D digitization, verification and comparison of the proposed methodology was performed on dental models that are characterized by extremely complex surfaces. It was performed by comparing the proposed approach with active stereovision, and the efficiency was evaluated in relation to the reference 3D model obtained by the structured light 3D scanner. The comparison of the results was performed on the basis of the mean deviation and standard deviation for the 3D model with combined teeth and for the 3D model with metal caps. The absolute mean deviations for the 3D model with combined teeth are 0.004-0.021 mm, with a standard deviation of 0.055-0.058 mm, and for the 3D model with metal caps absolute mean deviations are 0.015-0.033 mm, with a standard deviation of 0.095-0.113 mm, respectively. Absolute minimum values of mean deviation of 0.004 mm and standard deviations of 0.055 mm were obtained by 3D model with combined teeth,which was reconstructed by the proposed innovative approach. The obtained results indicate a higher accuracy of the innovative approach in relation to the use of a commercial 3D scanner that uses active stereovision principle

Multi-Criteria Evaluation of Design Complexity for Patient-Specific Bone Graft

With the rise of modern computer-aided technologies, their use in various different fields is becoming more and more apparent, but more profoundly in the field of medicine. The use of such technology enables the design of complex anatomical structures, often found in different areas of medicine. Maxillofacial and oral fields are becoming more and more popular with the use of such technologies, all leaning toward designing and fabrication of patient-specific implants from a biocompatible material. The level of complexity in personal graft design depends on criteria that describe the bone graft\u27s various properties. This research applies multi-criteria decision aiding in selection of patient-specific bone graft optimal design.Twelve different patient-specific bone-grafts designs have been evaluated by four decision makers who expressed their preferences with direct weighting and revised Simos procedure. Well known VIKOR method was used for multi-criteria decision aiding and the final results verified that the fully curved shape graft design is the least complex while the complex shape is the most demanding from the graft design perspective

Prepoznavanje cilindara i ravni u trodimenzionim oblacima tačaka

U radu se predlaže metod za prepoznavanje cilindara i ravni u nestruktuiranim oblacima tačaka. Predloženi proces prepoznavanja se može podeliti u tri osnovne faze. Prvu fazu predstavlja automatska segmentacija širenjem regiona počev od jedne karakteristične tačke. Kriterijumi širenja regiona zasnovani su na osobinama singularnosti informacione matrice sistema kao i pripadnosti tačaka površi čiji su parametri estimirani metodom najmanjih kvadrata. Druga faza algoritma se odnosi na grupisanje presegmentiranih oblasti i estimaciju parametara prepoznatih cilindara i ravni. Dobre performanse ovoj fazi obezbeđuje upotreba unapređenog algoritma robusnog prepoznavanja cilindara iz oblaka tačaka kao i uvođenje procesa precizne estimacije parametara ravni. Na samom kraju, odnosno u trećoj fazi procesa upotrebom predloženog algoritma vrši se ponovna obrada celokupnog polaznog oblaka tačaka u cilju ekstrakcije prepoznatih primitiva i obezbeđivanja preciznih krajnjih rezultata. Predloženi metod je pre svega namenjen prepoznavanju cilindara i ravni u oblacima tačaka koji reprezentuju određene mašinske delove, pa je u skladu sa tim i eksperimentalno verifikovan na većem broju odgovarajućih sintetizovanih oblaka

Computer-aided methods for single stage fibrous dysplasia excision and reconstruction in the zygomatico-orbital complex

Computer Aided Design and Additive Manufacture (CAD/AM) technologies are sufficiently refined and meet the necessary regulatory requirements for routine incorporation into the medical field, with long-standing application in surgeries of the maxillofacial and craniofacial region. They have resulted in better medical care for patients, and faster, more accurate procedures. Despite ever-growing evidence about the advantages of computer aided planning, CAD and AM in surgery, detailed reporting on critical design decisions that enable methodological replication, and the development and establishment of guidelines to ensure safety, are limited. This paper presents a novel application of CAD and AM to a single stage resection and reconstruction of fibrous dysplasia in the zygoma and orbit. It is reported in sufficient fidelity to permit methods replication and design guideline developments in future cases, wherever they occur in the world. The collaborative approach included engineers, designers, surgeons and prosthetists to design patient-specific cutting guides and a custom implant. An iterative design process was used, until the desired shape and function were achieved, for both of the devices. The surgery followed the CAD plan precisely and without problems. Immediate post-operative subjective clinical judgements were of an excellent result. At 19 months post-op, a CT scan was undertaken to verify the clinical and technical outcomes. Dimensional analysis showed maximum deviation of 4.73 mm from the plan to the result, while CAD-Inspection showed that the deviations range between -0.1 and -0.8 mm, and that the majority of deviations are located around the –0.3 mm. Improvements are suggested and conclusions drawn regarding the design decisions considered critical to a successful outcome for this type of procedure in the future

Fuzzy Hybrid Method for the Reconstruction of 3D Models Based on CT/MRI Data

This research proposes a hybrid method for improving the segmentation accuracy of reconstructed 3D models from computed tomography/magnetic resonance imaging (CT/MRI) data. A semi-automatic hybrid method based on combination of Fuzzy C-Means clustering (FCM) and region growing (RG) is proposed. In this approach, FCM is used in the first stage as a preprocessing step in order to classify and improve images by assigning pixels to the clusters for which they have the maximum membership, and manual selection of the membership intensity map with the best contrast separation. Afterwards, automatic seed selection is performed for RG, for which a new parameter standard deviation (STD) of pixel intensities, is included. It is based on the selection of an initial seed inside a region with maximum value of STD. To evaluate the performance of the proposed method, it was compared to several other segmentation methods. Experimental results show that the proposed method overall provides better results compared to other methods in terms of accuracy. The average sensitivity and accuracy rates for cone-beam computed tomography CBCT 1 and CBCT 2 datasets are 99 %, 98.4 %, 47.2 % and 89.9 %, respectively. For MRI 1 and MRI 2 datasets, the average sensitivity and accuracy values are 99.1 %, 100 %, 75.6 % and 99.6 %, respectively. The average values for the Dice coefficient and Jaccard index for the CBCT 1 and CBCT 2 datasets are 95.88, 0.88, 0.6, and 0.51, respectively, while for MRI 1 and MRI 2 datasets, average values are 0.96, 0.93, 0.81 and 0.7, respectively, which confirms the high accuracy of the proposed method

Fuzzy Hybrid Method for the Reconstruction of 3D Models Based on CT/MRI Data

This research proposes a hybrid method for improving the segmentation accuracy of reconstructed 3D models from computed tomography/magnetic resonance imaging (CT/MRI) data. A semi-automatic hybrid method based on combination of Fuzzy C-Means clustering (FCM) and region growing (RG) is proposed. In this approach, FCM is used in the first stage as a preprocessing step in order to classify and improve images by assigning pixels to the clusters for which they have the maximum membership, and manual selection of the membership intensity map with the best contrast separation. Afterwards, automatic seed selection is performed for RG, for which a new parameter standard deviation (STD) of pixel intensities, is included. It is based on the selection of an initial seed inside a region with maximum value of STD. To evaluate the performance of the proposed method, it was compared to several other segmentation methods. Experimental results show that the proposed method overall provides better results compared to other methods in terms of accuracy. The average sensitivity and accuracy rates for cone-beam computed tomography CBCT 1 and CBCT 2 datasets are 99 %, 98.4 %, 47.2 % and 89.9 %, respectively. For MRI 1 and MRI 2 datasets, the average sensitivity and accuracy values are 99.1 %, 100 %, 75.6 % and 99.6 %, respectively. The average values for the Dice coefficient and Jaccard index for the CBCT 1 and CBCT 2 datasets are 95.88, 0.88, 0.6, and 0.51, respectively, while for MRI 1 and MRI 2 datasets, average values are 0.96, 0.93, 0.81 and 0.7, respectively, which confirms the high accuracy of the proposed method

Copyright Protection of 3D Digitized Sculptures by Use of Haptic Device for Adding Local-Imperceptible Bumps

This research aims to improve some approaches for protecting digitized 3D models of cultural heritage objects such as the approach shown in the authors\u27 previous research on this topic. This technique can be used to protect works of art such as 3D models of sculptures, pottery, and 3D digital characters for animated film and gaming. It can also be used to preserve architectural heritage. In the research presented here adding protection to the scanned 3D model of the original sculpture was achieved using the digital sculpting technique with a haptic device. The original 3D model and the model with added protection were after that printed at the 3D printer, and then such 3D printed models were scanned. In order to measure the thickness of added protection, the original 3D model and the model with added protection were compared. Also, two scanned models of the printed sculptures were compared to define the amount of added material. The thickness of the added protection is up to 2 mm, whereas the highest difference detected between a matching scan of the original sculpture (or protected 3D model) and a scan of its printed version (or scan of the protected printed version) is about 1 mm

A Novel Method for Determination of Kinetic Friction Coefficient using Inclined Plane

Presented in this paper is the theoretical background for a novel method for determination of kinetic friction coefficient. The method is based on the equation of movement of a rigid body along an inclined plane and has not been previously discussed in literature. According to the theoretical results presented in this paper, mean kinetic friction coefficient can be determined based on the time it takes a rigid body to travel a distance along an inclined plane. Experimental results show that the mean kinetic friction coefficient increases with increasing mean sliding velocity, i.e. with decreasing sliding time. Increasing sliding velocity increases deviation of friction coefficients. This method also allows the determination of energy consumed due to friction along the travelled distance. Increasing mean kinetic friction coefficient and mean sliding velocity increases energy consumed due to friction. Practical calculation requires application of the distance law, which, in turn, requires accurate measurement of the sliding time and distance

Copyright Protection of 3D Digitized Artistic Sculptures by Adding Unique Local Inconspicuous Errors by Sculptors

In recent years, digitization of cultural heritage objects, for the purpose of creating virtual museums, is becoming increasingly popular. Moreover, cultural institutions use modern digitization methods to create three-dimensional (3D) models of objects of historical significance to form digital libraries and archives. This research aims to suggest a method for protecting these 3D models from abuse while making them available on the Internet. The proposed method was applied to a sculpture, an object of cultural heritage. It is based on the digitization of the sculpture altered by adding local clay details proposed by the sculptor and on sharing on the Internet a 3D model obtained by digitizing the sculpture with a built-in error. The clay details embedded in the sculpture are asymmetrical and discreet to be unnoticeable to an average observer. The original sculpture was also digitized and its 3D model created. The obtained 3D models were compared and the geometry deviation was measured to determine that the embedded error was invisible to an average observer and that the watermark can be extracted. The proposed method simultaneously protects the digitized image of the artwork while preserving its visual experience. Other methods cannot guarantee this