No description available.

No description available.

Determination of the stress distributions in post-core restored endodontically treated teeth is challenging due to the fact that the post and core systems, the root and its canal, and the bony structures supporting the root have small dimensions and are structurally complex. In this research, a 3D finite element model was developed to evaluate the stress distributions in a post-core restored endodontically treated maxillary incisor under various static loads. The physical model includes dentin, PDL, bone, post, core, gutta percha and crown. All materials are assumed to be homogenous, isotropic, and linear elastic. The effects of various factors on the stress distributions are investigated through simulations. These factors include post materials, post and core combinations, ferrule heights, post and dentin gaps at the coronal entrance of the canal, and canal diameters. It has been found that the horizontal loading is the most dangerous, which causes the highest stresses in dentin and posts, followed by the oblique loading and the vertical loading. The above listed factors, such as post materials, post and core combinations, ferrule heights, post and dentin gaps at the coronal entrance of the canal, and canal diameters, do not change the stress distributions and magnitudes significantly under horizontal and oblique loading. However, the stresses are sensitive to the above factors under the vertical loading, and it has been found that the stress distributions in both dentin and the post are the most uniform without stress concentrations when the elastic modules of the post and the core are similar to that of dentin. Regarding the effects of the gaps at the cervical region on the stress distributions in dentin, the high stresses at the apical portion of the root and the bottom of the gaps decrease as the increase of the depth of the gap under vertical loading. Overall, the sharp angle and notch of the gap at the coronal entrance of the canal should be avoided in tooth restoration since they can cause stress concentrations. On the effects of the ferrule heights, the changes of the stress distributions in dentin and the post are insignificant except that higher ferrule shows lower stresses at the top of the ferrule. Regarding the effects of the diameters of the posts, the results show that although the posts with large diameters support more loads, they cause high stress concentrations at the apical portion of the root, which is not desirable. / October 2005

Determination of the fatigue life of a component requires knowledge of the local maximum fluctuation stress and the through-thickness stress distribution acting at the critical cross-section. This has traditionally been achieved through the use of stress concentration factors. More recently finite element methods have been used to determine the maximum stress acting on a weldment. Unfortunately, meshing large and complicated geometries properly requires the use of fine meshes and can be computationally intensive and time consuming. An alternative method for obtaining maximum stress values using coarse three-dimensional finite element meshes and the hot spot stress concept will be examined in this paper. Coarse mesh stress distributions were found to coincide with fine mesh stress distributions over the inboard 50% of a cross-section. It was also found that the moment generated by stress distribution over the inboard half of the cross-section accounted for roughly 10% of the total moment acting in all of the cases studied. As a result of this, the total moment acting on a cross-section may be predicted using knowledge of the stress distribution over the inboard 50% of a structure. Given the moment acting on a cross-section, the hot spot stress may be found. Using bending and membrane stress concentration factors, the maximum stress value may be found. Finally, given the maximum stress data, the fatigue life of a component may be determined using either the strain-life approach or fatigue crack growth methods.

Determination of the fatigue life of a component requires knowledge of the local maximum fluctuation stress and the through-thickness stress distribution acting at the critical cross-section. This has traditionally been achieved through the use of stress concentration factors. More recently finite element methods have been used to determine the maximum stress acting on a weldment. Unfortunately, meshing large and complicated geometries properly requires the use of fine meshes and can be computationally intensive and time consuming. An alternative method for obtaining maximum stress values using coarse three-dimensional finite element meshes and the hot spot stress concept will be examined in this paper. Coarse mesh stress distributions were found to coincide with fine mesh stress distributions over the inboard 50% of a cross-section. It was also found that the moment generated by stress distribution over the inboard half of the cross-section accounted for roughly 10% of the total moment acting in all of the cases studied. As a result of this, the total moment acting on a cross-section may be predicted using knowledge of the stress distribution over the inboard 50% of a structure. Given the moment acting on a cross-section, the hot spot stress may be found. Using bending and membrane stress concentration factors, the maximum stress value may be found. Finally, given the maximum stress data, the fatigue life of a component may be determined using either the strain-life approach or fatigue crack growth methods.

Determination of the stress distributions in post-core restored endodontically treated teeth is challenging due to the fact that the post and core systems, the root and its canal, and the bony structures supporting the root have small dimensions and are structurally complex. In this research, a 3D finite element model was developed to evaluate the stress distributions in a post-core restored endodontically treated maxillary incisor under various static loads. The physical model includes dentin, PDL, bone, post, core, gutta percha and crown. All materials are assumed to be homogenous, isotropic, and linear elastic. The effects of various factors on the stress distributions are investigated through simulations. These factors include post materials, post and core combinations, ferrule heights, post and dentin gaps at the coronal entrance of the canal, and canal diameters. It has been found that the horizontal loading is the most dangerous, which causes the highest stresses in dentin and posts, followed by the oblique loading and the vertical loading. The above listed factors, such as post materials, post and core combinations, ferrule heights, post and dentin gaps at the coronal entrance of the canal, and canal diameters, do not change the stress distributions and magnitudes significantly under horizontal and oblique loading. However, the stresses are sensitive to the above factors under the vertical loading, and it has been found that the stress distributions in both dentin and the post are the most uniform without stress concentrations when the elastic modules of the post and the core are similar to that of dentin. Regarding the effects of the gaps at the cervical region on the stress distributions in dentin, the high stresses at the apical portion of the root and the bottom of the gaps decrease as the increase of the depth of the gap under vertical loading. Overall, the sharp angle and notch of the gap at the coronal entrance of the canal should be avoided in tooth restoration since they can cause stress concentrations. On the effects of the ferrule heights, the changes of the stress distributions in dentin and the post are insignificant except that higher ferrule shows lower stresses at the top of the ferrule. Regarding the effects of the diameters of the posts, the results show that although the posts with large diameters support more loads, they cause high stress concentrations at the apical portion of the root, which is not desirable.

No description available.

This thesis aims to develop a comprehensive usable engineering design approach to the fatigue analysis of threaded connections. Although primarily concerned with the fatigue-fracture mechanics behaviour of screw threads, a broad review of stress analysis investigations in such connections is reported. Connection types, their functions and standardisation authorities are presented with the purpose of familiarising the reader with the subject and the options available to the design of threaded fasteners. Fatigue crack initiation is discussed with reference to the specific setting of a critical thread root. A crack initiation model is adapted for employment in thread root design. A novel weight function approach is developed for use in the determination of stress intensity factors for threaded connections. A generic solution is proposed valid for the fatigue crack growth from any thread root under any symmetrical stress system. Its development and discussion is examined in detail, remaining close to its proposed application. Two engineering situations where the chief structural components are comprised of threaded members are taken as case studies. The background to each situation is elaborated in detail and full-scale fatigue tests were conducted on the critical components. In all, fourteen full-scale tests under constant and variable amplitude loading are reported. The results of these are analysed and used to validate the fatigue crack initiation and propagation models. Useful observations which are helpful to understanding the fracture mechanisms operating during the fatigue of threaded connections are reported. Material and environmental considerations are examined and a survey of relevant materials and their behaviour in environments associated with threaded fasteners is presented. The merits or otherwise of some common engineering practices are discussed with regard to fatigue. A method has been developed for predicting fatigue life in large threaded connections under random loading. Experimental results have been gathered on two types of components used on certain oil rigs, tether joints and drill strings. The agreement found between prediction and experiment is appreciably better than by previous methods of analysis and also points to aspects open to further improvement.

Determination of the stress distributions in post-core restored endodontically treated teeth is challenging due to the fact that the post and core systems, the root and its canal, and the bony structures supporting the root have small dimensions and are structurally complex. In this research, a 3D finite element model was developed to evaluate the stress distributions in a post-core restored endodontically treated maxillary incisor under various static loads. The physical model includes dentin, PDL, bone, post, core, gutta percha and crown. All materials are assumed to be homogenous, isotropic, and linear elastic. The effects of various factors on the stress distributions are investigated through simulations. These factors include post materials, post and core combinations, ferrule heights, post and dentin gaps at the coronal entrance of the canal, and canal diameters. It has been found that the horizontal loading is the most dangerous, which causes the highest stresses in dentin and posts, followed by the oblique loading and the vertical loading. The above listed factors, such as post materials, post and core combinations, ferrule heights, post and dentin gaps at the coronal entrance of the canal, and canal diameters, do not change the stress distributions and magnitudes significantly under horizontal and oblique loading. However, the stresses are sensitive to the above factors under the vertical loading, and it has been found that the stress distributions in both dentin and the post are the most uniform without stress concentrations when the elastic modules of the post and the core are similar to that of dentin. Regarding the effects of the gaps at the cervical region on the stress distributions in dentin, the high stresses at the apical portion of the root and the bottom of the gaps decrease as the increase of the depth of the gap under vertical loading. Overall, the sharp angle and notch of the gap at the coronal entrance of the canal should be avoided in tooth restoration since they can cause stress concentrations. On the effects of the ferrule heights, the changes of the stress distributions in dentin and the post are insignificant except that higher ferrule shows lower stresses at the top of the ferrule. Regarding the effects of the diameters of the posts, the results show that although the posts with large diameters support more loads, they cause high stress concentrations at the apical portion of the root, which is not desirable.

Thermal barrier coatings (TBCs) have been used widely in aerospace and land-based gas turbines. The TBC system consists of a top coat layer, a thermally grown oxide (TGO), a bond coat layer and a substrate. The growth kinetics of the TGO significantly affects the durability of TBCs. At a critical TGO thickness, the growth stresses exceed the ceramic-bond coat interface strength, resulting in TBC system failure. Regardless of the deposition method used, it is vitally important to accurately predict the TBC lifetime by investigating the determinants of the failure. The main objective of this study was to investigate the effect of oxidation stress induced by TGO layer in high temperature cycling environment through a series of reliable numerical simulations. Indeed, this oxidation stress is a known factor of interface degradation, and may result in failure of the ceramic-metal interface. A 2-D finite element model of the TBC was built via ANSYS APDL software, to conduct parametric studies of increasing complexity. The model accounted for elasticity first, before creep was integrated. Then, the model included swelling induced by phase transformation associated with oxidation, incorporating the effect of volumetric expansion of the newly grown TGO. This coupled oxidation constitutive approach was implemented for a typical air plasma spray deposited TBC coating. The interfacial radial stresses induced by the gradual oxidation were investigated. Different morphologies of the TBC interface were also considered to analyze the roughness effect on interface stresses. A complete model including swelling, creep, aging effects on the TBC layers at a given roughness was finally investigated.