Fracture behaviors of lightly reinforced concrete beams under different loading rates
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摘要: 动态加载下，混凝土中钢筋的阻裂性能一直是冲击动力学研究领域的难点之一。利用落锤试验机对含缺口的混凝土少筋梁进行三点弯曲试验，分析了不同加载速率下梁的冲击力、跨中挠度、混凝土起裂应变率和钢筋应变。实验结果表明：在一定加载速率范围内（0.885～1.252 m/s），混凝土预制裂缝尖端的裂纹起裂应变率、冲击力最大值、跨中挠度峰值与加载速率呈线性增长关系，当加载速率增至1.771 m/s时，增长趋势减弱；冲击力卸载时，钢筋部分弹性变形恢复导致裂纹产生闭合，裂纹嘴张开位移逐渐减小至恒定值，对裂纹嘴张开位移峰值前的部分曲线进行拟合后得到裂纹嘴张开位移率，结果表明裂纹嘴张开位移率随加载速率的提高而线性增大。Abstract: Under impact loads, the crack resistance of reinforcement in concrete is the focus of impact dynamics research. In this study, the three-point bending tests of lightly reinforce concrete beams with notch were conducted with a drop hammer testing machine. The high speed camera and digital image correlation (DIC) technology were applied to catch the fracture process and analyse the displacement field. The shape and geometry of the specimen followed the RILEM recommendation, i.e., 150 mm × 150 mm in cross section, 800 mm in length, notch-depth ratio was around 1/3 and span was kept constant 600 mm. And the impact force, mid-span deflection, steel strain beneath the loading point and concrete strain at the notch tip were measured over four loading rates. The experiment results showed that the crack initiation strain rate increased linearly with the loading rate, and the growth trend weakened when loading rate was 1.771 m/s. Based on the test results, the empirical formula of crack initiation strain rate with respect to loading rate was given, which was meaningful to simulate the crack initiation in concrete under dynamic loading. In addition, with the increase of loading rate, the steel yielded gradually, which resulted in the response time difference between impact force and steel strain decreased. With the recovery of elastic deformation of steel bars, the crack started to close and the macro crack became visible clearly. Meanwhile, the crack mouth opening displacement (CMOD) declined to a constant value after reaching the maximum value. Then, the crack mouth opening displacement rate was obtained from the fitting curve, and a linear growth relationship between CMOD rate and loading rate was founded. The failure process of the lightly reinforced beams was analyzed based on the CMOD rate, which provided an idea for comparing the fracture process of the beam under dynamic and static loads.
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图 5 加载速率和裂纹起裂应变率的关系
Figure 5. Relationship between crack initiation strain rate and loading rates
图 6 不同加载速率下的裂纹扩展
Figure 6. Crack propagation of concrete beams under different loading rates
图 9 冲击力峰值、跨中挠度最大值与加载速率的关系
Figure 9. Peak load and maximum of mid-span deflection versus loading rates
图 10 不同加载速率下冲击力、跨中挠度和钢筋应变时程曲线
Figure 10. Impact force, mid-span deflection and steel strain versus time under different loading rates
图 12 不同加载速率下裂纹嘴张开位移及其变化率
Figure 12. Crack mouth opening displacement and its changing rate at different loading rates
表 1 裂纹起裂应变率试验结果
Table 1. Crack initiation strain rate versus loading rates
v/（m·s−1） ${\dot \varepsilon _{{\rm{ini}}}}$/s−1 ${\dot \varepsilon _{{\rm{ini}}}}$平均值/s−1 $\Delta {\dot \varepsilon _{{\rm{ini}}}}/\Delta v$ 0.885 0.233 0.255 − 0.276 1.084 0.632 0.655 2.010 0.678 1.252 0.957 0.962 1.827 1.023 0.906 1.771 1.353 1.354 0.755 1.403 1.307 
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表 2 实验结果
Table 2. Experimental results
加载速率/(m·s−1) 冲击力 跨中挠度 钢筋应变 峰值/kN 峰值时刻/ms 峰值/mm 峰值时刻/ms 峰值/10−6 峰值时刻/ms 0.885 59.39 0.33 1.252 3.0 92.49 0.33 1.084 85.48 0.29 1.85 3.4 1489.27 0.54 1.252 130.16 0.14 2.53 4.0 3773.43 0.69 1.771 154.68 0.12 3.29 4.4 4473.32 0.71
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