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中文核心期刊

金属材料中低加载速率下的动态韧脆转变及断裂韧性测量

DYNAMIC DUCTILE-BRITTLE TRANSITION AND FRACTURE TOUGHNESS MEASUREMENT OF METAL UNDER INTERMEDIATE-LOW LOADING VELOCITIES

  • 摘要: 金属材料在冲击下的韧脆转变现象和动态断裂韧性的测量是金属材料冲击力学性能研究的重要组成部分.针对金属材料在冲击下的韧脆转变现象认识不足和韧性材料在较低加载率下动态J-R阻力曲线难以测量的现状,提出了采用高速材料试验机, 设计专用试验夹具,测量15MnTi钢和11MnNiMo钢在不同加载速率下的韧脆转变过程,以及裂尖约束对其动态韧脆转变速率变化的影响.在高速材料试验机上采用上夹具辊刹车,通过调节压缩杆长度改变试验中裂纹扩展量的试验方法,测量了15MnTi钢三点弯曲试样件在较低加载率下的动态断裂韧性.试验发现15MnTi钢CT试验件加载速率低于0.025 m/s时呈现出韧性断裂的特点,加载速率在0.1,\sim,0.5 m/s时为韧脆结合型断裂,加载速率高于0.5 m/s后进入脆性断裂区; 11MnNiMo钢CT试验件加载速率大于1.5 m/s后,分层断裂过程中出现先脆断后韧段的现象;发现15MnTi钢和11MnNiMo的动态韧脆转变速率受裂尖约束的影响非常明显,面内约束和面外约束的升高都会导致材料动态脆断速率出现明显降低;还发现三点弯曲试验中, 15MnTi钢在8788 MPa\cdotmm/s加载率内断裂韧性随加载率的提升呈现出缓慢下降的趋势.

     

    Abstract: The phenomenon of ductile-brittle transition and the measurement of dynamic fracture toughness of metallic materials under impact loading are important parts of the research on dynamic mechanical properties of metal materials. In view of the lack of understanding of ductile-brittle transition of metallic materials under impact loading and the difficulty in measuring the dynamic J-resistance curve of ductile materials at relative low loading rate, a method is proposed to measure the ductile-brittle transition process of 15MnTi and 11MnNiMo steels at different loading rates, and the effect of crack tip constraint on the rate change of dynamic ductile-brittle transition of the two materials, by using high-speed material testing machine and its corresponding special fixtures. The dynamic fracture toughness of 15MnTi steel under low loading rate was measured by adjusting the length of compression bar and changing the crack propagation by means of the brake of upper roller. The experimental results indicate that the CT specimen of 15MnTi steel is characterized as ductile fracture when loading velocity is lower than 0.025~m/s, and the fracture character of CT specimen of 15MnTi steel is ductile-brittle combination when loading velocity is between 0.1~m/s and 0.5~m/s, and brittle fracture of the CT specimen of 15MnTi steel starts from 0.5~m/s. The phenomenon of brittle fracture followed by ductile fracture for the CT specimen of 11MnNiMo steel occurs when the loading rate is greater than 1.5~m/s. The dynamic brittle fracture rate of 15MnTi and 11MnNiMo steels is significantly affected by crack tip constraint, and the dynamic brittle fracture rate of the material decreases obviously with the increase of in-plane constraint and out of plane constraint. It is also found that in the three-point bending tests, the fracture toughness of 15MnTi steel decreases slowly with the increase of loading rate, when the loading rate is lower than 8788~MPa\cdotmm/s.

     

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