Formation of three dimensional crack patterns

该项目的目的是详细了解由于不均匀收缩过程而形成三维（3D）裂纹模式的机制。在第一阶段，成功地开发了基于裂纹轮廓本身序列展开的断裂力学三维分岔分析方法。这种方法现在也适用于陶瓷瞬态热冲击载荷的情况。从而可以对超载情况下裂纹的侵彻深度和残余强度进行计算预测。此外，应检查保温层热溅射过程中产生的内应力对裂纹扩展的影响。陶瓷前驱体材料在小管中的干燥过程导致星形裂纹。对这一现象的研究也应采用上述的分岔分析。在工程的最后阶段，首次计算了具有弯曲裂纹表面的三维裂纹模式。通过计算，成功地验证了玄武岩柱状结构中的六角形截面在发育过程中具有能量优先性。在此模拟中，假定裂纹前缘沿释放能量最大的方向扩展。现在，玄武岩柱的振动裂纹面这一无法解释的现象将采用同样的方法进行研究。在项目的实验部分，需要开发分辨率显著提高的成像技术，以改善陶瓷热冲击裂纹的成像。这样，小开口的裂缝就可以被检测到。这种有条理的改进旨在裂纹间距和裂纹深度的测量质量，足以验证模拟的预期结果。为了更清晰地描绘裂缝，对计算机层析成像技术、染色渗透连续切片技术和层析显微镜技术进行了比较。此外，代替以前使用的具有微小裂纹开口的陶瓷，现在更合适的陶瓷，例如瓷器或陶器，将受到热冲击。此外，处理由x射线分析技术获得的3D图像的软件将得到增强，以更可靠地检测裂缝。为此，每个层析层的评估将包含相邻层析层中的信息。一个软件解决方案，跟踪在裂缝模式发展过程中发生的柱过渡，通过几个柱之间的过渡安排被识别和它们的频率被量化。因此，对陶瓷系统和干淀粉-水悬浊液中裂纹模式的实验获得的三维图像的分析将有助于对三维裂纹模式发展的一般理解。

The aim of the project is a detailed understanding for the mechanisms of the formation of three dimensional (3D) crack patterns as a result of inhomogeneous shrinkage processes. During the 1st phase the method of fracture mechanical 3D bifurcation analysis based on a series expansion of the crack contour itself was developed successfully. This method now shall also be applied to cases with transient thermal shock loads at ceramics. Thereby both, the penetration depth of the cracks at overload cases and the residual strength can be predicted computationally. In addition, the impact on the crack propagation by internal stresses that develop during the thermal sputtering of insulation layers shall be examined. The drying procedure of ceramic precursor material in small tubes leads to star-shaped crack patterns. The mentioned bifurcation analysis shall be applied to investigate this phenomenon as well.For the first time, 3D crack patterns with curved crack surfaces were computed in the last period of the project. This calculation verified successfully that hexagonal cross sections in the columnar structures of basalt are energetically preferred during the process of development. In this simulation it is assumed that the crack front propagates in a direction at which the released energy is at a maximum. Now, the unexplained phenomenon of the oscillating crack surfaces of basalt columns shall be investigated using the same approach.In the experimental part of the project imaging techniques with significantly higher resolution shall be developed in order to improve the imaging of thermal shock cracks in ceramics. That way, cracks with small openings will become detectable. This methodical improvement aims at a quality of the measurement of crack spacings and crack depths that suffices for a verification of the expected results of the simulation. In order to map the cracks more clearly the techniques of computer laminography, serial sectioning technique with dye penetrant testing and tomographic microscopy are compared. Furthermore, instead of the formerly used ceramics with their tiny crack openings, now more suitable ceramics like for example porcelain or pottery shall be subjected to thermal shocks.Moreover, the software that processes the 3D images obtained by X-ray analyses techniques will be enhanced to detect the cracks more robust. For this purpose, the evaluation of each tomographic slice will incorporate the information in the adjacent tomographic slices. A software solution that tracks the column transitions occurring during the development of crack patterns is planned by which the transitions between several column arrangements are identified and their frequencies are quantified . In so doing, the analyses of experimentally obtained 3D images of crack patterns in ceramic systems and in dried starch-water-suspensions will contribute to the general understanding of the development of 3D crack patterns.