The Effects Of Initial Damage On The Cleavage Fracture Behavior Of Notched Specimens With Different Microstructures Of Steels

In this paper, a series of four point normal and reverse bending experiments are carried out on the notched specimens of material A(16MnR fine grain), B(CF coarse grain) and C(16MnR coarse grain), and different levels of micro-cavity damage are introduced and the effect of the notch root radius is eliminated. To separate the effect of damage from residual stress and work hardening behavior, the specimens are tempered after pre-loading. Then the specimens are fractured by four point bending at a low temperature of -196°C. By measurement of mechanical parameters, microscopic observation, and finite element methods (FEM) calculation, the effects of initial damage on the cleavage fracture toughness of notched specimens with different microstructures of steels are experimentally investigated. The main results are as follows:(1) In the case of pre-load ratio P₀ IP_gy =0.861, the damage area fraction f_a of material A and C increases little with the increment of P₀/P_gy,. In the case of P₀/P_gy =0.861, the damage area fraction f_a of material A and C increases quickly with the increment of P₀/ P_gy. At the same P₀ /P_gy , the damage level of material A is obviously higher than that of material C. For material B, the damage area fraction f_a increases slowly with the increment of P₀/P_gy ,but at the same P₀/P_gy, the damage area fraction of material B is lower than that of material A and C.(2) When P₀/P_gy is less than 0.861, the fracture toughness of material A decreases slowly with the increment of P₀/P_gy. In the case of P₀/P_gy =0.861, the fracture toughness decreases quickly with the increment of P₀/P_gy .For material B, the fracture toughness keeps near constant with the increment of P₀/P_gy. For material C, when P₀/P_gy is less than 0.861,the fracture toughness keeps near constant. When P₀/P_gy is bigger than 0.861, the fracture toughness decreases quickly with the increment of P₀/P_gy.When the damage area fraction f_a is low, the fracture toughness of material C is lower than that of material A, the difference of the fracture toughness between material B and C is little. But when the damage area fraction f_a is big, the difference between the fracture toughness of material B and C is little, the fracture toughness of material C is lower than that of material B.(3) At -196°C, the reason that the fracture toughness of material A and C decreases with the increment of initial damage area fraction is that there is high local stress and strain concentration around the void which is introduced by the pre-loading, especially the big string void. The local stress and strain concentration makes up the insufficiency of normal stressa^, then the cleavage fracture occurs when Pf is low(SIC), then the external field value a^ = a} is low. The reason that the fracture toughness of material B keeps near constant with the increment of initial damage level is that the main initiation source is small size spherical voids(IC) or particles(IP), and the kinds of the initiation source don't change with the increment of P^IP^ . Although the damage area fraction fa increases with the increment of Po/i^'but the absolute value of fa is low, and the main damage kind is spherical void with sjnall size, and the local stress and strain concentration around the spherical void is much lower than that of big string void. So, the fracture toughness of material B keeps near constant.(4)Material B is not sensitive to the pre-damage, that is to say, at the same P0IPgy, the damage of material B is little, and the size and the number of initial defect is little, the fracture toughness of material B is constant with the increment of Po I Pgy. But at the same P0/Pgy, the damage of material C is big, and the size and the number of initial defect is big, the fracture toughness of material C decreases with the increment of P0IPgy -So in the case of near the same grain sizes, the fracture toughness of the steel with much inclusions is decided by the big initial damage defect, which decreases the fracture toughness.(5)For the materials with the same chemical composition and inclusions, in the case of Po IP^ <0.861, the fracture toughness is decided by the grain size, and the fracture toughness of material A with fine grain is bigger than that of the material C with coarse grain. When P0/Pgy is bigger than 0.861, the fracture toughness is decided by the initial damage, and the effect of grain size decreases. The damage level of material A is high, the size of the defect is big, then there is more higher stress and strain concentration around the defects, then the probability of nucleation and propagation of the crack of material A is bigger than that of material C. Then the fracture toughness of material A decreases, that is to say, because of the big initial damage, the fracture toughness is not sensitive to the grain sizes.(6) The reason that material A is sensitive to initial damage is that when the specimens are under the same P^IP^, because the yield strength oy of material A with fine grains is higher than that of material B and C. So, the local normal stress ax^. a2> a3 of material A is higher, the normal stress of both the inner of inclusions and the interface between the inclusions and matrix is higher. So, the inclusion of material A is easier to be fractured or to be separated from the matrix, then, the level of initial damage of material A is higher.