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用ASTM A508C1.3和A470C1.6钢的约300个0.5T、1T、2T和4T的紧凑拉伸试样,在转变区进行了断裂韧性试验。由小试样的J_c换算的K_J表现出从低的K_(1c)值到比较高韧性值的很宽的分散区。用扫描电子显微镜详细研究了断裂行为和小试样的韧性分散的原因。转变区快速脆性断裂行为可以分为两个区:①在钝化线上产生的区域(区域Ⅰ)和②在R-曲线上产生断裂的区域(区域Ⅱ)。K_J值的分散是由于试样中裂纹扩展的差异而引起的。提出了由K_J值得到等价于用ASTM方法求得的K_(1c)的断裂韧性的方法。在区域Ⅱ采用修正的R-曲线,在区域Ⅰ推荐用满足NB≥3000Jc/σ_y要求的试样数目求得K_J的下限值,这一方法也适用于区域Ⅱ。
Fracture toughness tests were conducted in the transition zone using approximately 300 compact tensile specimens of 0.5T, 1T, 2T and 4T from ASTM A508C1.3 and A470C1.6 steels. K_J in terms of J_c for the small sample showed a wide dispersion from a low K 1c value to a relatively high toughness value. The fracture behavior and the reason of the ductile dispersion of the small sample were investigated in detail using a scanning electron microscope. The rapid brittle fracture behavior of the transition zone can be divided into two zones: ① the zones created in the passivation line (zone I) and ② the zones in the R-curve where the zone breaks (zone II). The dispersion of K_J values is due to the difference in crack propagation in the specimen. A method of obtaining the fracture toughness equivalent to K_ (1c) obtained by the ASTM method from the value of K_J is proposed. The modified R-curve is used in Zone II, and the lower limit of K_J is recommended for Zone I using the number of samples satisfying NB ≥ 3000Jc / σ_y. This method also applies to Zone II.