论文标题:3D-C/SiC的高温拉—拉疲劳性能研究
Tension-Tension Fatigue Behavior of 3D-C/SiC Composite at Elevated Temperature
论文作者 刘兴法
论文导师 乔生儒；吕宝桐,论文学位 硕士,论文专业 材料学
论文单位 西北工业大学,点击次数 608,论文页数 70页File Size5457k
2003-01-01论文网 http://www.lw23.com/lunwen_69690587/ 3D-C/SiC;高温疲劳;疲劳曲线;疲劳损伤机理;电阻;模量
3D-C/SiC composite,Elevated temperature fatigue,Fatigue life curve,Fatigue mechanics,Resistance,Modulus
陶瓷基复合材料在高温结构中有许多潜在的用途，3D-C／SiC又是目前这类材料的研究热点。但该材料的疲劳性能，特别是高温疲劳性能鲜有报道，限制了其设计、应用和进一步发展。因此本文研究了3D-C／SiC复合材料的高温拉-拉疲劳行为。所研究的3D-C／SiC由PAN碳纤维三维编织预制体化学气相渗透(CVI)SiC基体制成，其纤维体积分数约为40％。疲劳试验的应力主要范围为200MPa-300MPa，温度为室温、1100℃、1300℃和1500℃，应力比为0.1和0.5，频率为20HZ和60HZ，波形为正弦波。通过中间停机测量材料的弹性模量和电阻，并通过扫描照片观察裂纹扩展。研究发现： 试验表明，室温和高温下3D-C／SiC复合材料具有较高的抗疲劳性能，其疲劳极限约在拉伸强度的88％以上。该材料在室温、1100℃、1300℃和1500℃下的疲劳曲线和大多数材料的疲劳曲线一样，可划分为三个区域：短寿命区、长寿命区和无限寿命区。不同的是该材料的疲劳寿命曲线的应力范围比较窄。1100℃时的疲劳寿命最长，1300℃时的疲劳寿命居中，而室温和1500℃时的疲劳寿命最短。 3D-C／SiC因热膨胀不匹配所引起的热残余应力随温度不同而不同，界面上室温下沿径向表现为拉应力，在材料的制造温度以上表现为压应力，因此，不同温度下的疲劳断口上纤维束的拔出长度有着明显的不同，室温下拔出最长，1500℃下拔出最短。 通过试验发现，1500℃下的预疲劳(σ_(mc)＜σ_(max)＜200MPa)可以提高3D-C／SiC复合材料的抗拉强度、断裂应变和疲劳寿命，但弹性模量则呈现下降趋势。同时，该材料的疲劳寿命随着应力比的增大和频率的降低而减小，即该温度下对蠕变较为敏感。 3D-C／SiC在高温疲劳过程中，当基体裂纹密度达到饱和后，材料电阻变化率进入恒定阶段，材料弹性模量进入微小变化阶段。电阻和弹性模量变化走势基本一致，说明疲劳损伤、材料电阻、弹性模量之间有着必然的联系。用电阻和弹性模量变化规律均可表征疲劳中的损伤。 3D-C／SiC由于应力集中的原因，疲劳裂纹主要起源于纤维束编织交叉处，而裂纹的扩展方式主要取决于界面层的状态和基体本身所具有的缺陷。 由以上结果可知，3D一C/SIC复合材料具有较强的高温疲劳抗力。
CMC have a lot of latent useness at elevated temperature structure, as the same time, 3D-C/SIC composite is the focus among them. 3D-C/SiC composite has not been used and progressed widely because few people studied it especially its elevated fatigue properties in the world. So the tension-tension fatigue behavior of a 3D-C/SiC composite at elevated temperature has been investigated in present work. 3D-C/SiC composite, containing 40 Vol% T300 carbon fiber, was fabricated by Chemical Vapor Infiltration. The tension-tension fatigue tests have been performed under vacuum at room-temperature, 1100℃, 1300℃, 1500℃ , for stress from 200MPa to 300MPa, ratio of 0. 1 and 0. 5, sinusoidal loading frequency of 20HZ and 60HZ . Stopping fatigue machine during fatigue is to measure elastic modulus, resistance and photo crack by TEM. We found that:At room temperature and elevated temperature, 3D-C/SiC composite has revealed a good fatigue resistance compared with other ceramics matrix composites. The fatigue limit (based on 106 cycles) for 3D- C/SiC is about above 88% of its ultimate tensile strength (UTS).The fatigue-life curve of 3D-C/SiC has three regions that include short life region, long life region and infinite life region at room temperature, 1100℃, 1300 ℃ and 1500℃. But the stress changing range in fatigue life curve is narrow. 3D-C/S1C has the longest fatigue life at 1100℃, has the shortest fatigue life at room temperature and 1500℃.The stress in the interface of 3D-C/SiC is different at different temperature. Interface has tension-stress at room temperature, however has pressure-stress above 950℃-1000℃. So, fatigue fracture at different temperature has different fibers pull-out. Fracture has shortest fibers pull-out at 1500 ℃ and longest fibers pull-out at room temperature.Prefatigue ( σ mc< σmax<200MPa) at 1500℃ can improve tension strength, fracture strain and fatigue life of 3D-C/SJC composite, but in general,elastic modulus decreases.Fatigue life of 3D-C/SiC composite at 1500℃ decrease when stress ratio increases or fatigue frequency lessens. That is to say, 3D-C/SiC is sensitive to creep at this temperature.During elevated fatigue of 3D-C/SiC, when crack density of matrix reach to be saturated. Resistance and elastic modulus come into steady phase. So matrix cracks, resistance and elastic modulus are interrelated to each other.Because of stress concentration, fatigue cracks of 3D-C/SiC composite mainly initiate from crossover of fiber bundles.In a word, 3D-C/SiC composite is immune to fatigue.
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