Effect of loading rate on the modeⅡ dynamic fracture characteristics of 40Cr steel
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摘要: 采用新型Ⅱ型动态断裂测试技术，对高强钢40Cr在高加载速率下的Ⅱ型动态断裂特性进行了测试研究。基于新设计的Ⅱ型动态断裂试样和分离式霍普金森压杆（split Hopkinson pressure bar， SHPB）技术，通过实验-数值方法确定了裂尖在加载过程中的应力强度因子曲线。采用应变片法确定了试样的起裂时间，最终得到40Cr的Ⅱ型动态断裂韧性值，并对其加载速率相关性和材料的失效机理进行了研究。结果表明，在1.08～5.53 TPa·m1/2/s的加载速率范围内，40Cr的Ⅱ型动态断裂韧性基本表现为与加载速率成正相关的变化趋势。通过对试样断口形貌的分析，确定了材料的失效模式及机理，发现随着加载速率的增加，存在拉伸型失效向绝热剪切型失效模式转变的现象。Abstract: 40Cr high strength steel is often used in aerospace and national defense fields due to its excellent mechanical properties. Therefore, the research on the mode Ⅱ dynamic fracture characteristics and failure mechanism of 40Cr high strength steel under high-speed impact has important scientific significance and engineering value. Experiments and numerical simulations were carried out to study the mode Ⅱ dynamic fracture properties of 40Cr material under high loading rates. Based on the newly designed specimen and the novel test technique for mode Ⅱ dynamic fracture, the experimental-numerical method was used to determine the dynamic stress intensity factor curve of the crack tip during the loading process. The crack initiation time of the specimen was obtained by the strain gauge pasted on the specimen, and the mode Ⅱ dynamic fracture toughness of 40Cr was finally determined. The effect of loading rate and the failure mechanism of the material were also studied. The results show that within the present loading rate range of this work (1.08−5.53 TPa·m1/2/s), the mode Ⅱ dynamic fracture toughness of 40Cr presents a positive correlation with the loading rate. Through the analysis of the fracture morphology of the recovered specimen, it is found that there exists a transition from tensile failure mode to adiabatic shear failure mode with the increase of loading rate, and the critical loading rate is about 2.92 TPa·m1/2/s. When KⅡd≤1.70 TPa·m1/2/s, the 40Cr steel mainly exhibits brittle fracture; when KⅡd is in the range of 2.12−2.92 TPa·m1/2/s, the fracture mainly presents the morphological characteristics of ductile fracture; when KⅡd ≥ 3.19 TPa·m1/2/s, the material failure is mainly caused by adiabatic shear bands. Brittle fracture is dominated by strain rate hardening mechanism, ductile fracture is dominated by strain/strain rate hardening and thermal softening mechanisms, and adiabatic shear fracture is dominated by thermal softening mechanism.
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图 1 Ⅱ型断裂试样几何尺寸(单位：mm)
Figure 1. Geometric dimensions of the mode Ⅱ fracture specimen (unit: mm)
图 3 试样拉伸起裂的典型信号（1.25 TPa·m1/2/s）
Figure 3. Typical signals of tensile fracture initiation of a specimen (1.25 TPa·m1/2/s)
图 4 试样绝热剪切起裂的典型信号（4.86 TPa·m1/2/s）
Figure 4. Typical signals of ASB initiation of a specimen (4.86 TPa·m1/2/s)
图 5 拉伸型起裂（1.25 TPa·m1/2/s）与ASB型起裂（4.86 TPa·m1/2/s）的高速摄影图
Figure 5. High-speed photographic images of tensile fracture initiation (1.25 TPa·m1/2/s) and ASB initiation (4.86 TPa·m1/2/s)
图 8 子弹速度与起裂时间的关系
Figure 8. Relationship between bullet velocity and crack initiation time
图 10 不同加载方式下的DFT与加载速率关系图
Figure 10. Relationship between DFT and loading rate under different loading methods
图 11 不同断口形貌特征的DFT与加载速率关系图
Figure 11. Relationship between DFT and loading rate of different fracture morphology characteristics
图 12 40Cr试样沿晶断裂及A区放大图（1.25 TPa·m1/2/s）
Figure 12. Intergranular fracture of 40Cr specimen and magnification of location A (1.25 TPa·m1/2/s)
图 13 40Cr试样韧性断裂特征（2.92 TPa·m1/2/s）
Figure 13. Ductile fracture characteristics of 40Cr specimen (2.92 TPa·m1/2/s)
图 14 40Cr试样绝热剪切型断裂特征（3.87 TPa·m1/2/s）
Figure 14. Adiabatic shear fracture characteristics of 40Cr specimen (3.87 TPa·m1/2/s)
表 1 高强钢40Cr的元素成分及质量分数
Table 1. Composition and mass fraction of high-strength steel 40Cr
w(C)/% w(Mn)/% w(Si)/% w(Cr)/% w(Ni)/% 0.37～0.45 0.50～0.80 0.20～0.40 0.80～1.10 − 
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表 2 高强钢40Cr的力学性能参数
Table 2. Mechanical properties of high strength steel 40Cr
材料 ρ/(kg·m−3) E/GPa μ σb/MPa 40Cr 7820 199 0.3 1987
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表 3 入射杆、透射杆的力学性能参数
Table 3. Mechanical properties of the incident and transmission bars
材料 ρ/（kg·m−3） E/GPa μ 18Ni 8000 190 0.3
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表 4 高强钢40Cr的动态断裂韧性值
Table 4. Dynamic fracture toughness values of high strength steel 40Cr
L/m p/MPa tf/µs KⅡd/（MPa·m1/2） ˙KⅡd/（TPa·m1/2/s） 0.380 0.18 27 29.2 1.08 0.380 0.19 26 32.6 1.25 0.380 0.22 22 37.5 1.70 0.203 0.18 19 40.2 2.12 0.203 0.20 17 49.4 2.92 0.203 0.22 16 51.0 3.19 0.100 0.14 13 50.3 3.87 0.100 0.16 11 53.5 4.86 0.100 0.18 10 55.3 5.53 注: L为子弹长度，p为气压，tf为起裂时间，KⅡd为Ⅱ型断裂韧性，˙KⅡd为加载速率
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表 5 不同断口形貌下DFT与加载速率的线性拟合参数
Table 5. Linear fitting parameters of DFT and the loading rates under different fracture morphology
材料 断口类型 A b 40Cr 脆性 12.88 15.80 韧性 9.98 19.94 绝热剪切型 2.09 43.39
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