高温后超高韧性水泥基复合材料冲击破碎#br# 分形特征分析

采用了Ф80mm的分离式Hopkinson压杆对经历不同温度（常温、200℃、400℃、500℃、600℃、800℃）后的混杂纤维UHTCC材料进行了三组冲击气压下(0.35MPa、0.45MPa、0.55MPa)的动态压缩试验，收集冲击破碎后的试块进行分形特征分析。利用X-CT和SEM扫描电子显微镜对高温后试样的内部结构进行观测，从微观上解释了碎片分形特征随温度变化的原因。结果表明：UHTCC的分形维数随着冲击气压的增大而增大，耗能能力随着分形维数的增加呈指数式上升。与传统混凝土材料不同，UHTCC的分形维数随着温度的升高而降低，800℃时其分形维数接近于混凝土。UHTCC中PVA和钢纤维的协同阻裂作用改变了裂纹的发展路径，造成断裂面粗糙度增加，这可能是分形维数高于混凝土的原因；随着温度的增大，纤维桥联作用的弱化、温度裂纹的出现以及水化产物的分解导致材料断裂面粗糙度降低，分形维数逐渐接近于混凝土。

Fractal characteristics of fire-damaged ultra high toughness cementitious composite after impact loading

The Ф80 mm splitting Hopkinson pressure bar was utilized to impact hybrid fiber ultra high toughness cementitious composite (UHTCC) under three sets of impact pressure (0.35MPa, 0.45MPa, 0.55MPa). The specimen was heated up to different temperatures (including ambient temperature, 200℃，400℃，500℃，600℃，800℃)before the impact loading, and the fragmentations after impact were collected to analyze the fractal characteristics. X-ray CT scanning and SEM (scanning electronic microscope) were utilized to observe the internal structures of the specimens after the high temperature, which could disclose the causes of the fractal characteristics of fragmentation changing with temperature. The results show that, the fractal dimension of UHTCC increases with the impact pressure, and the energy dissipation capacity exponentially increases with the fractal dimension. The fractal dimension of UHTCC decreases with the exposure temperature, which is different from that of concrete. At 800℃, the fractal dimension of UHTCC is close to that of concrete. The PVA and steel fibers in UHTCC have a synergistic crack blocking effect which may change the propagating path of cracks and result in increased roughness of fracture surface and higher fractal dimension. With the increase of temperature, the weakened bridging effect of fibers, the formation of cracks due to temperature and the decomposition of hydration products in UHTCC lead to the decrease of fractal dimension, which gradually close to that of concrete.