剪切测试模型的优化及其在临床材料筛选中的应用
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摘要
粘结技术是现代口腔医学的重要支柱,剪切测试则是评价其核心指标的测试方法。然而在该测试过程中表现出测试结果稳定性差、数据变异大的不足,其标准差可高达20-50%(ISO数据)。因此,降低实验误差水平,提高实验的鉴别能力成为一项非常迫切的任务。许多学者都在为致力于剪切测试的标准化工作而努力,以增进研究结果之间的稳定性和可比性。目前的研究主要集中在牙齿的选择、保存,牙本质的取材,试件的表面处理,粘结过程的设计,测试前的临床环境模拟以及剪切加载因素等方面。现行国际标准对上述因素逐步进行了规范,然而对模型的设计及力学相关影响因素的关注尚不充分。而模型的设计对于测试中粘结界面的应力分布有着非常重要的影响,其意义甚至可能超出标本选择和试件处理等因素。因而有必要通过力学分析对剪切测试的可能影响因素进行深入研究,为剪切测试方法的优化提供理论基础;并建立能够被人们接受,且可以带来稳定测试结果的剪切强度测试模型。另外,我们尝试利用剪切测试的标准模型探讨临床材料力学属性对粘结界面应力分布的影响规律,为材料的筛选和研究工作提供参考。本研究共分三个部分:
第一部分剪切测试中的力学影响因素分析:本部分拟探讨几种模型因素对剪切测试应力分布的影响,其将有助于理解剪切测试结果产生差异的原因,并为剪切测试标准的改进提供参考。
实验一:针对粘结试件制作过程中可能出现的三种粘结剂边界形态(边缘缺陷型,规则界面和带有菲边的边缘形貌),应用二维有限元法进行剪切加载条件下的应力分布和峰值对比研究。选择具有代表意义的最大主应力和剪应力指标进行结果的评价。结果发现带有边缘缺陷的剪切模型会在牙本质层内产生最高水平的应力集中,而规则界面最大主应力和剪应力峰值均出现在粘结剂上边缘区域,应力的峰值也较高;带有边缘菲边的模型最大主应力和剪应力峰值分布在粘结剂的菲边上,应力的峰值水平最低。较大尺寸的菲边牙本质层内的应力水平也较低;对于一般尺寸大于5μm的菲边最大主应力的峰值位于菲边基部,这可能会影响到测试界面断裂的形式。边缘缺陷模型的最大应力较边缘菲边模型可高出40%以上,在试件的制作中避免或降低边缘形态的差异性是提高测试稳定性的一个重要因素。
实验二:应用二维有限元法分析了试件中牙本质自由端厚度及其直径大小对粘结界面最大主应力及剪应力水平的影响。结果发现模型中不同牙本质(自由端)厚度会对粘结界面的最大主应力峰值产生一定水平的影响(变化范围<14%),其随厚度增大敏感度逐渐下降。试件牙本质的直径大小对于应力峰值的影响则非常明显(变化范围<274%),在牙本质直径与粘结界面相等,实现平齐界面形式的情况下应力集中现象基本消失,应力分布的模式变得均匀平滑,这主要归功于这一极限模拟形式消除了界面形态的突变。这一界面分析的结果可以与临床粘结界面形式相互借鉴。分析表明牙本质的厚度和大小在操作中应该予以规范。
实验三:人们在拉伸测试实验中发现粘结面积会对粘结强度产生影响,因而设计了现行尺寸的微拉伸实验。然而对于剪切实验粘结面积的报道说法并不一致。为此我们在保持模型形状与加载比例的基础上,设计了不同粘结面积剪切、(微)拉伸测试模型的应力分析,以期揭示出现上述现象的力学机制。结果表明:减小模型粘结面积不会对模型的应力分布模式有明显的优化作用。减小微拉伸试件面积,反而使得粘结边界上最大主应力水平升高,应力分布趋向不合理;减小模型面积对于剪切测试模型的影响很小(3%);这从理论分析上来看是由于模型中相对较大的粘结剂厚度所致。微拉伸相对于普通拉伸测试所体现出的模型优势应归因于试件形状的改变。
实验四:有研究发现,加载方式的微小变化可以引起剪切测试强度高达1.7倍的差异性。为揭示这一现象发生的力学机制并探究适合剪切测试的最佳加载策略,我们利用Pro/E软件建立了剪切测试的三维有限元模型,并在Ansys Workbench中分别对该模型采用了点加载和线加载的方式进行对比研究。结果表明:线加载方式所获得的应力分布模式要显著优于点加载的实验模式,界面上的最大主应力和剪应力峰值也显著降低。因而认为线加载方式更加有利于剪切测试结果的稳定。
实验五:由于圆形粘结面在剪切加载条件下,不可避免的使得最大应力集中在圆形截面顶部的小范围内,我们设想通过改变模型截面的设计,获得有利于剪切测试结果稳定的应力分布模式。在Pro/E软件中分别建立圆形和方形两种粘结截面形状的模型,并导入Ansys Workbench中进行加载分析。结果表明:方形截面模型的应力峰值水平较圆形截面有所降低,在应力分布模式上形成了沿方形截面边缘平滑分布的连续应力区域,这有助于稳定的呈现粘结界面的缺陷水平。
第二部分剪切测试模型的改进与优化:本部分实验中尝试设计一种改进的剪切测试模型,对其进行三维自适应建模和多变量优化分析。进而设计相关模具、制作试件,对其检测效能进行验证。
实验一:新设计的模型以方形树脂端的参数为基础,分别以相邻接触面为参考平面依次建立相同边长的粘结剂层和牙本质层结构特征。将模型导入Ansys Workbench中实现模型的自适应组装。在相同表观强度的剪切加载中发现:新建的方形均匀截面模型在应力分布上明显优于传统的剪切测试模型,模型的上边缘应力分布平滑连续且应力最大值位于边缘中央区域。局部最大主应力和剪应力峰值水平显著下降约70%。
实验二:利用大型软件之间的无缝接口,定义最大主应力及其综合加权指标为目标变量,对包括加载距离,牙本质厚度和粘结面边长在内的多变量进行综合分析。结果发现:牙本质自由端厚度范围在0.5-1.5mm时对目标变量的影响很小。在这一结果基础上将牙本质自由端厚度设为1.0mm,基于尽量应用较大粘结面边长获得稳定粘结界面缺陷信息的原理,采用4.0mm的粘结面边长;最后经过单变量精确优化分析,推荐加载距离为1.87mm。各变量对结果的影响敏感性排序为:粘结面边长>加载距离>牙本质自由端厚度。
实验三:按照实验二的模型参数,设计相关金属模具,加工制作方形试件。对试件四周进行打磨修整,使其达到金属模具严格限定的尺寸(4.0mm)。按照推荐的加载距离进行加载并与传统剪切测试模型的测试结果进行比较。结果表明:优化后新模型的测试结果与传统模型的测试结果之间有显著性差异。新模型的结果稳定性明显优于传统模型,标准差由传统模型的38.5%下降至5.9%。
第三部分剪切测试有限元模型在临床材料筛选中的应用:实验中我们尝试利用标准剪切测试这一基本有限元模型对临床上承受剪切载荷的条件下,不同临床材料对粘结(或结合)界面的应力影响规律进行分析;以期对临床医生选择修复材料的种类以及相关材料的研究工作提供帮助。
实验一:在剪切力加载作用下,分析粘结剂弹性模量对粘结界面内应力大小和分布模式的影响。旨在反映剪切加载模式下粘结剂弹性模量影响粘结界面应力水平的一般规律。研究发现:随着粘结剂弹性模量的升高,牙本质和粘结剂层的总变形量逐渐减小,而两者的平均应力以及最大主应力峰值却迅速升高,尤其在低弹性模量范围内(1-5.5GPa),应力变化更为迅速。因此,对于主要承受剪切载荷的临床情况宜选择弹性模量较低的粘结剂材料。
实验二:不同的修复材料,其物理力学属性各不相同。那么材料所具有的不同力学属性(弹性模量,泊松比)究竟对牙本质粘结界面的稳定性影响如何呢?在本实验研究中,我们对一定范围内的弹性模量和泊松比进行了模拟,结果表明:修复材料的弹性模量对粘结界面的应力水平影响显著,而泊松比的作用微弱。并对临床常用的多种材料在剪切载荷条件下粘结界面上的应力分布进行了对比,结果表明:在相同剪切载荷条件下,弹性模量较高的金属和陶瓷材料可以在粘结界面的最薄弱部位,即牙本质-粘结剂界面上产生更低水平的破坏应力。
实验三:针对目前钛瓷结合效果欠佳的问题,本研究尝试对不同弹性模量钛合金表面烤瓷的受力情况进行分析,以期从机械力学角度揭示低弹性模量钛合金应用于改善钛瓷界面应力水平的可能性。结果表明:低弹性模量钛合金在剪切载荷模式下具有应力优化作用;然而在弯曲载荷下低弹性模量钛合金需要大于2mm的金属层厚度才能在变形最大的中央部分获得足够的刚度效果。低弹性模量钛合金在牙科烤瓷应用的综合力学性能有待进一步深入研究。
小结本研究得出以下结论:
1多种力学因素对剪切测试模型的应力分布有着不同的影响规律,这在测试的标准化工作中应该予以考虑。
2新设计推荐的方形均匀截面模型较传统剪切测试模型有显著的优势。
3利用剪切测试这一基本评价模型的力学分析为相应材料的选用和研究开发提供了参考。
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.
第一部分剪切测试中的力学影响因素分析:本部分拟探讨几种模型因素对剪切测试应力分布的影响,其将有助于理解剪切测试结果产生差异的原因,并为剪切测试标准的改进提供参考。
实验一:针对粘结试件制作过程中可能出现的三种粘结剂边界形态(边缘缺陷型,规则界面和带有菲边的边缘形貌),应用二维有限元法进行剪切加载条件下的应力分布和峰值对比研究。选择具有代表意义的最大主应力和剪应力指标进行结果的评价。结果发现带有边缘缺陷的剪切模型会在牙本质层内产生最高水平的应力集中,而规则界面最大主应力和剪应力峰值均出现在粘结剂上边缘区域,应力的峰值也较高;带有边缘菲边的模型最大主应力和剪应力峰值分布在粘结剂的菲边上,应力的峰值水平最低。较大尺寸的菲边牙本质层内的应力水平也较低;对于一般尺寸大于5μm的菲边最大主应力的峰值位于菲边基部,这可能会影响到测试界面断裂的形式。边缘缺陷模型的最大应力较边缘菲边模型可高出40%以上,在试件的制作中避免或降低边缘形态的差异性是提高测试稳定性的一个重要因素。
实验二:应用二维有限元法分析了试件中牙本质自由端厚度及其直径大小对粘结界面最大主应力及剪应力水平的影响。结果发现模型中不同牙本质(自由端)厚度会对粘结界面的最大主应力峰值产生一定水平的影响(变化范围<14%),其随厚度增大敏感度逐渐下降。试件牙本质的直径大小对于应力峰值的影响则非常明显(变化范围<274%),在牙本质直径与粘结界面相等,实现平齐界面形式的情况下应力集中现象基本消失,应力分布的模式变得均匀平滑,这主要归功于这一极限模拟形式消除了界面形态的突变。这一界面分析的结果可以与临床粘结界面形式相互借鉴。分析表明牙本质的厚度和大小在操作中应该予以规范。
实验三:人们在拉伸测试实验中发现粘结面积会对粘结强度产生影响,因而设计了现行尺寸的微拉伸实验。然而对于剪切实验粘结面积的报道说法并不一致。为此我们在保持模型形状与加载比例的基础上,设计了不同粘结面积剪切、(微)拉伸测试模型的应力分析,以期揭示出现上述现象的力学机制。结果表明:减小模型粘结面积不会对模型的应力分布模式有明显的优化作用。减小微拉伸试件面积,反而使得粘结边界上最大主应力水平升高,应力分布趋向不合理;减小模型面积对于剪切测试模型的影响很小(3%);这从理论分析上来看是由于模型中相对较大的粘结剂厚度所致。微拉伸相对于普通拉伸测试所体现出的模型优势应归因于试件形状的改变。
实验四:有研究发现,加载方式的微小变化可以引起剪切测试强度高达1.7倍的差异性。为揭示这一现象发生的力学机制并探究适合剪切测试的最佳加载策略,我们利用Pro/E软件建立了剪切测试的三维有限元模型,并在Ansys Workbench中分别对该模型采用了点加载和线加载的方式进行对比研究。结果表明:线加载方式所获得的应力分布模式要显著优于点加载的实验模式,界面上的最大主应力和剪应力峰值也显著降低。因而认为线加载方式更加有利于剪切测试结果的稳定。
实验五:由于圆形粘结面在剪切加载条件下,不可避免的使得最大应力集中在圆形截面顶部的小范围内,我们设想通过改变模型截面的设计,获得有利于剪切测试结果稳定的应力分布模式。在Pro/E软件中分别建立圆形和方形两种粘结截面形状的模型,并导入Ansys Workbench中进行加载分析。结果表明:方形截面模型的应力峰值水平较圆形截面有所降低,在应力分布模式上形成了沿方形截面边缘平滑分布的连续应力区域,这有助于稳定的呈现粘结界面的缺陷水平。
第二部分剪切测试模型的改进与优化:本部分实验中尝试设计一种改进的剪切测试模型,对其进行三维自适应建模和多变量优化分析。进而设计相关模具、制作试件,对其检测效能进行验证。
实验一:新设计的模型以方形树脂端的参数为基础,分别以相邻接触面为参考平面依次建立相同边长的粘结剂层和牙本质层结构特征。将模型导入Ansys Workbench中实现模型的自适应组装。在相同表观强度的剪切加载中发现:新建的方形均匀截面模型在应力分布上明显优于传统的剪切测试模型,模型的上边缘应力分布平滑连续且应力最大值位于边缘中央区域。局部最大主应力和剪应力峰值水平显著下降约70%。
实验二:利用大型软件之间的无缝接口,定义最大主应力及其综合加权指标为目标变量,对包括加载距离,牙本质厚度和粘结面边长在内的多变量进行综合分析。结果发现:牙本质自由端厚度范围在0.5-1.5mm时对目标变量的影响很小。在这一结果基础上将牙本质自由端厚度设为1.0mm,基于尽量应用较大粘结面边长获得稳定粘结界面缺陷信息的原理,采用4.0mm的粘结面边长;最后经过单变量精确优化分析,推荐加载距离为1.87mm。各变量对结果的影响敏感性排序为:粘结面边长>加载距离>牙本质自由端厚度。
实验三:按照实验二的模型参数,设计相关金属模具,加工制作方形试件。对试件四周进行打磨修整,使其达到金属模具严格限定的尺寸(4.0mm)。按照推荐的加载距离进行加载并与传统剪切测试模型的测试结果进行比较。结果表明:优化后新模型的测试结果与传统模型的测试结果之间有显著性差异。新模型的结果稳定性明显优于传统模型,标准差由传统模型的38.5%下降至5.9%。
第三部分剪切测试有限元模型在临床材料筛选中的应用:实验中我们尝试利用标准剪切测试这一基本有限元模型对临床上承受剪切载荷的条件下,不同临床材料对粘结(或结合)界面的应力影响规律进行分析;以期对临床医生选择修复材料的种类以及相关材料的研究工作提供帮助。
实验一:在剪切力加载作用下,分析粘结剂弹性模量对粘结界面内应力大小和分布模式的影响。旨在反映剪切加载模式下粘结剂弹性模量影响粘结界面应力水平的一般规律。研究发现:随着粘结剂弹性模量的升高,牙本质和粘结剂层的总变形量逐渐减小,而两者的平均应力以及最大主应力峰值却迅速升高,尤其在低弹性模量范围内(1-5.5GPa),应力变化更为迅速。因此,对于主要承受剪切载荷的临床情况宜选择弹性模量较低的粘结剂材料。
实验二:不同的修复材料,其物理力学属性各不相同。那么材料所具有的不同力学属性(弹性模量,泊松比)究竟对牙本质粘结界面的稳定性影响如何呢?在本实验研究中,我们对一定范围内的弹性模量和泊松比进行了模拟,结果表明:修复材料的弹性模量对粘结界面的应力水平影响显著,而泊松比的作用微弱。并对临床常用的多种材料在剪切载荷条件下粘结界面上的应力分布进行了对比,结果表明:在相同剪切载荷条件下,弹性模量较高的金属和陶瓷材料可以在粘结界面的最薄弱部位,即牙本质-粘结剂界面上产生更低水平的破坏应力。
实验三:针对目前钛瓷结合效果欠佳的问题,本研究尝试对不同弹性模量钛合金表面烤瓷的受力情况进行分析,以期从机械力学角度揭示低弹性模量钛合金应用于改善钛瓷界面应力水平的可能性。结果表明:低弹性模量钛合金在剪切载荷模式下具有应力优化作用;然而在弯曲载荷下低弹性模量钛合金需要大于2mm的金属层厚度才能在变形最大的中央部分获得足够的刚度效果。低弹性模量钛合金在牙科烤瓷应用的综合力学性能有待进一步深入研究。
小结本研究得出以下结论:
1多种力学因素对剪切测试模型的应力分布有着不同的影响规律,这在测试的标准化工作中应该予以考虑。
2新设计推荐的方形均匀截面模型较传统剪切测试模型有显著的优势。
3利用剪切测试这一基本评价模型的力学分析为相应材料的选用和研究开发提供了参考。
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.
引文
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