Shear behavior of TC4 alloy under dynamic loading
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摘要: 使用分离式霍布金森压杆(SHPB)对2种TC4(Ti-6Al-4V)试样(单边剪切试样与双边剪切试样)在应变率104 s-1下进行动态剪切加载，利用SIM D8高速照相系统捕捉了绝热剪切带扩展的整个历程，得到了TC4在拍照时刻的应力应变曲线；使用金相显微镜和SEM扫描电镜对TC4绝热剪切带的微观形貌进行观察，发现绝热剪切带宽度为5~12 μm，断口从韧窝断裂演变为解理断裂，可观测到韧窝状与河流花样断口形貌，但是并未看到相变的发生；对2种试样就产生绝热剪切带的形式与敏感性进行了分析，实验表明双边试样更易产生绝热剪切带；通过高速照相系统的标定换算，得到TC4绝热剪切带产生的临界剪切应变在78%~88%之间。在SHPB动态加载条件下，TC4绝热剪切带的扩展速度在460~1 250 m/s之间，且应变率越高，剪切带扩展越快，扩展平均速度与名义应变率近似呈线性关系；另外，在同一加载速率下，剪切带并不是匀速扩展，其扩展速度随载荷的增加而不断增加。Abstract: Two types (single-edged and double-edged) of TC4 (Ti-6Al-4V) alloy samples are tested by using the split-Hopkinson bar system under the strain rate of 104 s-1. By using ultra-high speed photography, the initialization and propagation of adiabatic shear band (ASB) are obtained with sufficient resolution. The relationship of the mechanical properties is built. The width of the shear band is observed to be 5-12 μm by using metallurgical microscope and scanning electron microscopy (SEM). Shear dimples and smooth regions are observed in the fracture surfaces as well. However, no obvious phase-transfer are detected. The double-edged samples have shown a better performance because of the relatively negligible bending during experimenting. The critical shear strain at which ASB appears is between 78%-88% and the propagation speed is estimated to be in the range of 460-1 250 m/s. It is also found that when strain rate rises, the shear bands propagate faster. Moreover, the propagation speed increases as the loading process proceeds and the speed is linearly proportional with the nominal strain rate.
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图 3 剪切带扩展高速照片(TC4-B6, 时间间隔4 μs)
Figure 3. Ultra-high speed photographs of extending shear zone(TC4-B6, time interval: 4 μs)
图 10 剪切带扩展高速照相机照片(TC4-A6，时间间隔8 μs)
Figure 10. Ultra-high speed photographs of the shear band (TC4-A6, time interval: 8 μs)
图 12 名义应变率与剪切带扩展平均速度的关系
Figure 12. Average propagation speed-nominal strain rate curve
表 1 实验结果
Table 1. Experimental results
编号 v/(m·s-1) εc ${{\dot \varepsilon }_{\rm{s}}}$/(104) W/μm TC4-A-1 21.4 2.5 3.0 - TC4-A-2, 3, 4 23.6 3.0 3.3 - TC4-A-5, 6, 7 28.2 3.2 3.5 5~10 TC4-A-8, 9, 10 29.6 3.2 3.6 5~10 TC4-B-1, 2, 3 22.3 3.0 4.0 - TC4-B-4, 5, 6 25.5 3.1 5.0 8~12 TC4-B-7, 8, 9 29.1 3.2 6.0 8~12 TC4-B-10, 11, 12 33.9 3.4 6.3 8~12 
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表 2 局部剪切应变最大值
Table 2. The maximum of local shear stain
编号 ${\dot \varepsilon }$/(104) εp d e f TC4-B6 5.0 0.78 2.21 3.07 TC4-B9 6.0 0.79 2.27 3.11 TC4-B12 6.3 0.81 2.32 3.20
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