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§1 引言在球铁断裂韧性研究中,有二方面问题特别引起人们的注意。一是测量方法的问题;另一是球化状态对断裂的影响,或者说石墨在断裂过程作用的问题。国内现在对球铁断裂韧性测量普遍采用ASTM E399-72(或74)标准中三点弯曲试样。北京钢研院在他们与北京第一机床厂等单位合作测球铁断裂韧性时,认为用疲劳裂纹与用顶端宽度小于0.15mm的线切割的裂纹,所得K_(lc)值是一样的。在后来工作中进一步从理论上得G_c(ρ_0)=G_0+π/4ρ_0σ_Fε_F的公式。这里ρ_0为裂纹顶端曲率半径,Gc为临界裂纹扩展力,G_0为常数,σ_F为材料断裂真应力(经缩颈修正),ε_F为断裂的真应变。因此认为对脆性材料G_c(ρ_0)基本上为一与ρ_0无关的常数。因此,他们认为测球铁K_(lc)不必开疲劳裂纹。但国内其它单位在测球铁K_(lc)时还是普遍地开疲劳裂纹。测量方法的另一问题是负荷—位移(P—V)曲线中P_(max)/P_σ往往大于1.1。按E399—72,这样的结果无效,必须用更大尺寸试样重新测量。但是多数单位现在仍把它看成有
§ 1 Introduction In the study of ductile iron fracture toughness, there are two aspects of particular concern. First, the problem of measurement methods; the other is the impact of the state of the ball on the fracture, or the role of graphite in the fracture process. China now commonly used in the determination of ductile iron ductile toughness ASTM E399-72 (or 74) standard three-point bending test specimens. When they cooperated with Beijing No. 1 Machine Tool Plant and other units to determine the fracture toughness of the ball, Beijing Institute of Steel Technology believed that the value of Kc (lc) was the same with the crack with the crack of the line with the tip width less than 0.15 mm. In the later work, the formula of G_c (ρ_0) = G_0 + π / 4ρ_0σ_Fε_F is further theoretically obtained. Where ρ_0 is the radius of curvature at the tip of the crack, Gc is the critical crack propagation force, G_0 is constant, σ_F is the true stress of the material fracture (necking), and ε_F is the true strain of the fracture. Therefore, it is considered that the brittle material G_c (ρ_0) is basically a constant unrelated to ρ_0. Therefore, they think that the measuring iron K_ (lc) does not need to open the fatigue crack. However, other domestic units in the test of iron K_ (lc) is still generally open fatigue cracks. Another problem with the measurement method is that the P_ (max) / P_σ in the load-displacement (P-V) curve tends to be greater than 1.1. Press E399-72, this result is invalid, you must re-measure the larger size of the sample. But most organizations still see it now