【作者】 杨文；

【导师】 晁月盛； 李小武； 张广平；

【作者基本信息】 东北大学 ， 材料物理与化学， 2010， 博士

【摘要】 天然生物材料在经历大自然的选择后,其结构和功能不断优化,使之适应自然的能力趋于完美,而实际工程结构材料往往难以达到生物材料所具备的优异性能。因此,揭示天然生物材料的结构和性能显然对实际工程材料结构和性能的改进具有重要的指导意义。本论文工作选取大连海域的双壳纲贝壳紫石房蛤为研究材料,对其结构和力学性能,包括硬度、压缩、弯曲以及疲劳性能等进行系统的表征和分析,重点考察了各种力学性能在贝壳上的分布以及产生该分布的原因,分析讨论了贝壳的疲劳强度与断裂强度之间的关系。紫石房蛤贝壳的结构为分层结构,在厚度上分内、中、外三层,X射线数据显示这三层均为文石晶型的碳酸钙。根据贝壳各层结构形貌,构建了其整体结构示意图。贝壳外层是不含有机质的多孔块状结构,中层和内层是含有少量有机质的交错层状结构,相同取向的薄片(厚度约为200—-500 nm)构成了交错层状结构单元—“域”,其宽度约为20-80μm,相邻“域”中的薄片之间互成一定的角度。与紫石房蛤贝壳各层的结构特点密切相关,其外层的硬度较低,而中层和内层的硬度相差不大,均明显高于外层硬度。利用新设计的方法对压痕的横截面进行表征发现,硬度测试造成贝壳内部结构的损伤程度不仅与载荷的大小有关,还与载荷和结构中薄片之间的取向有关,当载荷垂直压入交错薄片表面时,贝壳内部结构损伤较严重。另外,通过硬度测试估算贝壳的断裂韧性约为2.21 MPa.m1／2。对干、湿两种状态的样品以不同加载方式进行压缩实验。结果发现,与干样品相比,湿样品的模量较高,抗压强度较低；当外加载荷以垂直于贝壳外表面方向加载时,湿样品的抗压强度略高于平行于贝壳外表面加载所得的湿样品强度。对干样品的压缩应力—应变曲线及断裂路径分析发现,曲线上台阶的出现对应了样品中裂纹的产生。台阶的连续出现意味着裂纹扩展阻力的增加、断裂强度提高。压缩实验结果的Weibull统计分析表明,湿样品抗压强度(72 MPa)与干样品(73 MPa)相比相差不大；垂直于贝壳样品外表面加载时的抗压强度(87 MPa)大于平行于贝壳外表面加载时的抗压强度(61 MPa)。分别对干、湿两种状态样品的三点弯曲力学行为进行研究,其结果表明,贝壳的生长方式特点导致贝壳边缘的弯曲强度高于贝壳内部的弯曲强度,外套膜位置附近干样品的弯曲曲强度较低,湿样品的弯曲强度较高,其它位置于样品的弯曲强度高于湿样品。相同状态样品的断裂能密度与弯曲强度在贝壳上分布的趋势一致,而弹性模量在贝壳上分布的规律性不明显,但干样品的弹性模量比湿样品的略大。贝壳样品的弯曲断裂路径主要受贝壳结构中的薄弱片层以及片层之间的薄弱界面的影响,另外,贝壳中闭壳肌等肉体组织对该位置样品的断裂路径也有一定的影响。Weibull统计分析结果表明：当断裂几率为50%[P(V)=0.5]时,同种贝壳干样品的弯曲强度(98 MPa)比湿样品的值(91MPa)略高。提出了疲劳强度的预估计方法,使贝壳样品高周弯曲疲劳测试的成功率大大提高。般来说,绝大部分样品的疲劳强度低于平均弯曲强度,疲劳寿命低于106周次。疲劳样品断裂路径明显的"zig-zag"模式和疲劳样品断口表面连续出现的小台阶形貌均表明,样品是逐渐发生疲劳断裂的。疲劳强度的Weibull统计分析结果表明：当断裂几率为50%[P(V)=0.5]时,干样品的疲劳强度为85 MPa,比静态弯曲强度(98 MPa)略低。更多还原

【Abstract】 Through a long process of natural selection, the structures and properties of natural biological materials could be greatly optimized, and their abilities to adapt the nature have tended to be perfect. In contrast, the artificial engineering materials normally cannot possess those superior properties owned by biological materials. Therefore, the investigations on the structure and mechanical properties of biological materials are significantly important and introductive for the improvement of the structures and properties of engineering materials. In the present work, the structure and mechanical properties (e.g., hardness, compression, bending and fatigue properties) of a bivalve clam, Saxidomus purpuratus shells, which were obtained from Dalian sea area, have been investigated systematically, focusing on the distribution of various mechanical properties at different positions in the shell. The relationship between the fatigue strength and the fracture strength of the shell is also discussed.The Saxidomus purpuratus shell has a hierarchical structure comprising three layers in thickness, i.e., inner, middle and outer layers, and X-ray diffraction data show that all the layers consist of aragonite. According to the structure of each layer, an overall structure profile of the shell was established. The shell shows a porous blocky structure without organic matters in the outer layer and a cross-lamellar structure in the inner and middle layers with little organic matters. The cross-lamellar structure of this shell is composed of numerous domains, each of which comprises parallel tiles with approximate thickness of 200-500 nm. The width of these domains ranges from 20μm to 80μm, and the tiles in any two neighboring domains make a certain angle mutually.Depending strongly upon the structural characteristics of each layer of the shell, the hardness of outer layer is obviously lower than those of middle and inner layers, for which the hardness is almost comparable. The characterization of the indentation using a newly-designed method shows that the damages induced by indentation relate not only to the magnitude of the indentation load, but also to the orientation between the load direction and the lamellae in the structure of the shell. When the indentation load is perpendicular to the surface of the crossed lamellae, the induced damage is more serious. Besides, the fracture toughness was roughly evaluated by hardness tests to be around 2.21 MPa·m1/2.Compression tests were performed with dry/wet shell specimens in different orientations. The results show that, as compared to the dry specimens, the wet have higher moduli and lower strengths. When the load is applied perpendicular to the surface of the shell, the compression strength is higher than that obtained with the load parallel to the surface of the wet shell. Analyses of compression stress-strain curves and the cracking paths demonstrate that the occurrence of steps in the curve should correspond to the formation of cracks. The continuous steps in the stress-strain curve imply that the crack propagation resistance becomes increased during the process of compressive deformation, causing an increase in the compression strength. Weibull statistics show that the compression strength (72 MPa) of wet specimens is quite close to that (73 MPa) of dry specimens, and that the compression strength (87 MPa) with the load perpendicular to the surface of the shell is higher than that (61 MPa) with the load parallel to the surface of the shell.The three-point bending mechanical behaviors of wet and dry specimens were investigated. It is found that the bending strength on the edge of the shell is higher; this is closely associated with the growth mechanism of the shell. The dry specimens near the mantle position have lower bending strengths than the wet, and the dry specimens in the other positions of the shell have higher strengths than the wet. Concerning the shell specimens of the same condition, the distributions of the fracture energy per area and the strength in the shell are identical. As compared to the case of fracture energy and strength, the distribution of the moduli is not of regularity, but the moduli of the wet specimens are a little higher than those of the dry specimens. The cracking paths are mainly affected by the weak lamellae and weak interfaces between lamellae and organic matters. Additionally, the tissues like adductor muscle etc. can also affect the cracking paths of the specimens. Weibull analysis indicates that the bending strength is 98 MPa for dry and 91 MPa for wet at a fracture probability of 50% [P(V)=0.5]. A prediction method was suggested to evaluate the fatigue strength in advance, so that the successful cases of high-cycle bending fatigue tests are obviously improved. Generally, the fatigue strengths of the shell specimens are lower than the relevant bending strengths, and the fatigue life is normally less than 106 cycles. The apparent "zig-zag" cracking path mode and the micro-step morphology on the fracture surface demonstrate that the shell has indeed undergone a process of fatigue failure. Weibull statistics show that at a fracture probability of 50%[P(V)=0.5], the fatigue strength of dry specimens is 85 MPa, a little lower than the bending strength (98 MPa).更多还原