分形维数和弹性模量衰减表征2D-C/SiC的拉伸蠕变损伤

分形维数和弹性模量衰减表征2D-C/SiC的拉伸蠕变损伤

  • 摘要: 真空条件对2D-C/SiC复合材料在1300℃和1500℃进行了高温拉伸蠕变试验,蠕变进行到0、0.5h、2h、10h、25h、50h中断试验,用SEM观察表面形貌,用盒维数法计算试样表面裂纹的分形维数;同时测量试样的弹性模量。结果表明,由于2D-C/SiC特有的蠕变损伤形式,所形成的损伤尺度都较短,其分形维数介于0~1之间。用分形维数和弹性模量衰减都可表征2D-C/SiC的蠕变损伤,两种损伤参量所描述的蠕变损伤总的发展趋势基本一致,即蠕变开始阶段损伤发展较快,随后进入缓慢发展的第二阶段。在第二阶段中,分形维数表征的损伤持续单调增加;而用弹性模量衰减表征的损伤在该阶段出现先下降随后升高的现象。以基体裂纹为主要损伤形式的条件下,分形维数主要反映蠕变试样局部的损伤,而弹性模量衰减反映的是蠕变试样整体的损伤。

     

    Abstract: Tensile creep tests of 2D-C/SiC composites were carried out at 1300℃ or 1500℃ under vacuum. When the creep time reached 0h, 0.5h, 2h, 10h, 25h, 50h, the testing was stopped and the specimen was removed from the furnace to examine the surface morphology by SEM, and the elastic modulus by a vibration device. The fractal dimension of the surface micro-cracks was evaluated from SEM micrographs. It was found that during creep many micro-cracks are produced on the sample surface and the fractal dimension of the cracks increased from 0 to 1. Both the fractal dimension and elastic modulus changes can be used to evaluate the creep damage of the composites, and they give similar damage curves. Both the damage parameter D vs. creep time and the fractal dimension vs. creep time curves include two stages: a fast developing stage of creep damage followed by a slower changing stage. In the slow stage, the damage curves evaluated from elastic modulus show an initial decline and then rise again. However, those evaluated from the fractal dimension monotonically increase with creep time. Considering matrix cracks as the main form of creep damage, the variation of fractal dimension mainly reflects the local damage of crept specimens while the variation of elastic modulus reflects the damage in the whole specimen.

     

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