| Adhesive technology is the one of the foundation subjects of modern dentistry. Shear test is the measuring method for the major technical criteria. However, shear test results are indicated to be instable with great variance. The standard deviation listed in ISO files ranges in 20-50%. It is now an urgent task to lower the deviation level and improve the discrimination capability of shear test. Many researchers have worked in this field for standardization of the test, by which more stable and comparable results can be expected. The studies about shear test focused on the selection, conservation and utilization of tooth, surface treatment of samples, adhesive procedure, simulation of oral environment and loading factors. By literatures it has been found that the study on the mechanical factors of shear test was scarce and insufficient attention was given to the standardization of these factors. However, mechanical design of the model could have great influence to stress distribution of bond interface and the significance may surpass the influence of selection of solution of test samples. Therefore, it is necessary to carry out systematic study of shear test model in the view of mechanics, which is expected to suggest some helpful procedures for shear test. Besides, shear test model is attempted to analysis the influence of clinical materials on stress distribution of bond interface, targating to provide some guide for selection and study of these materials. This study is composed of three parts.Part one: Analysis of influencing mechanical factors in shear test. This part was aimed to investigate the role of several influencing factors in shear test, which could help us understand the deviation of shear test results and provide some guide for improvement of shear test standard.In experiment one, 2-D finite element analysis was used to simulate three possible adhesive bond interface geometries (edge with defection, ideal clear edge and edge with fillet). The stress distribution and peak stress values of Maximum principle stress and shear stress were used for comparison. The results indicated that edge with defection caused the highest stress intensification level in dentin substrate. The peak value ofσ1 andτfor ideal clear edge were also high and positioned in adhesive layer approaching to dentin substrate. The peak stress level ofσ1 andτfor edge with fillet positioned within the fillet and its value was the lowest. The larger fillet would cause the lower stress level. For fillet above 5um, peak stress positioned at base area of the fillet, which might be associated with the fracture style. Stress level of defected model can be 40% higher than the model with edge fillet. Eliminating or reducing difference in interface geometry is a critical factor to improve test stability.In experiment two, 2-D finite element analysis was also adopted to examine the influence of dentin thickness and diameter. The results showed that the thickness of dentin substrate can have some influence on peak value of Maximum principle stress (Variation <14%). The influence sensitivity decreased with the increase of dentin thickness. The influence of dentin diameter is much more significant (Variation <274%). When dentin diameter equals to bond interface, stress intensification phenomenon can be eliminated and the stress pattern is quite smooth on bond interface. The above results can be referenced to and validated with clinical condition. The thickness and diameter of dentin substrate should both be given enough attention in experimental design.In experiment three, the influence of bond area was investigated. During tensile test, it was argued that bond area can have some influence to tensile bond strength and so micro tensile test was devised. As for shear test, there came to different reports. So shear test and (micro) tensile test with different bond areas were devised based on similar model ratio in this experiment to investigate the mechanisms. The results indicated that smaller bond area will not make the stress distribution more rational. Bond area has little influence over shear test, and negative effect for micro tensile test (caused improved peak stress level). The above results were actually caused by the relatively increased thickness of adhesive layer. The advantage of micro tensile test over traditional tensile test is due to the change of model shape and therefore evenly distributed stress pattern.In experiment four, the influence of loading style was analyzed. It was reported that slight difference in loading style can cause results variation of 171%. In order to elaborate the mechanism of this phenomenon and explore the best loading style in shear test, Pro/E was used to build 3-D model and Ansys Workbench to compare the difference between point loading and line loading. The results indicated that line loading produced evener stress distribution pattern than point loading. It is suggested that line loading is beneficial for the stability of shear test.In experiment five, influence of interface shape was investigated. Since round shape interface will inevitably cause a very restricted stress intensifying area at the top of bond interface, an alternated interface shape was attempted to improve this condition. Separately round and square bond interface model was devised in Pro/E, and then imported into Ansys Workbench for further solution. It was indicated that square interface produce lower stress level. More importantly, the stress level along the upper edge of square interface was even and smooth which is beneficial for exhibiting defect level and density consistently.Part 2: Design and optimization of new shear test model. We attempted to devise a novel shear test model, build 3-D self-consistent finite element model and carry out a multi-variables optimization for this model. Metal mould is to be devised and the efficacy of test model to be evaluated primarily.In experiment one, the newly devised model was based on the parameter of resin substrate. Adhesive layer and dentin substrate were built in order according to the reference plane. The model was imported into Ansys Workbench and self-consistent was achieved successfully. Under the same nominal bond strength, newly devised model achieved much evener stress distribution with the peak stress level positioned in the middle area of (stress smoothly distributed) upper edge. The peak stress level ofσ1 andτdecreased about 70% compared to traditional model.In experiment two, multi-variable (loading distance, dentin thickness and interface edge size) analysis was carried out through the seamless interface withσ1 or the integratedσindex set as target variable. The results indicated that dentin thickness within 0.5-1.5mm had little influence on target variable. And then dentin thickness was set at 1.0mm. Interface edge size was set at 4.0mm based on the principle of exhibiting stable defect information with larger edge size. At last the optimization of loading distance suggested the value of 1.87mm. Sensitivity of target variable to each variable is listed as below: interface edge size > loading distance >dentin thickness. In experiment three, according to suggested parameter the metal mould was prepared. Polishing method was used to plane the four sides of test samples to obtain the assumed edge size (4.0 mm). The optimized loading distance was adopted and the results were compared with traditional model. It was shown that there were significant difference between the results of square model and traditional model. The novel shear model produced more stable results with SD decreasing from 38.5% (traditional model) to 5.9%.Part three: Application of standard shear test finite element model in the optimization of clinical materials. Standard shear test model was adopted to analyze the influence of various clinical materials to bond interfaces under shear load. It was aimed to provide some help for selection of various clinical materials in clinic and also for the development of novel clinical materials.In experiment one, shear test model was used to analyze the influence of adhesive elasticity on stress distribution at bond interface. It was targeted to find the general role of adhesive layer under shear load. It was found that with the increase of adhesive elasticity total deformation of dentin and adhesive layer decreased gradually, while equivalent stress and maximum principle stress increased quickly especially true for low elastic modulus range (1-5.5 GPa). As for the condition mainly bearing shear load, adhesive with low elasticity is preferred.In experiment two, the influence of different restorative materials with various mechanical properties on the stability of dentinal bond interface was investigated. Certain range of elastic modulus and Poisson's ratio was simulated. Some commonly used restorative materials were analyzed in detail for comparison. It was shown that elastic modulus had more influence over Poisson's ratio. As for commonly used materials, metal and ceramic with comparatively higher stiffness can produce lower stress level at the critical point of bond interface. The resin materials within low elastic modulus range showed more sensitive result to concerned equivalent stress level. It is suggested that restorative materials similar to dentin tissue in mechanic are not always the sound choice.In experiment three, different titanium alloy with various elastic modulus were analyzed to explore the possibility to improve stress distribution on titanium-porcelain interface. The results indicated that the titanium alloy with low elastic modulus can play positive role in lowering stress level on titanium- porcelain interface. But the titanium alloy with low elastic modulus should have a thickness over 2mm to obtain enough stiffness in the middle area, where the maximum deformation occurs. The application of titanium alloy with low elastic modulus in titanium-porcelain field awaits further research.Conclusions:1 Several mechanical factors have different influence on stress distribution of shear test model which should be paid enough attention in standardization procedure of shear test.2 The suggested square-sectioned model is superior over traditional shear test model.3 The mechanical analysis with standard shear test model provides helpful guide for the utilization and research work of the clinical materials.
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