Effects of tensile prestrain on the notch toughness of low-alloy steel

Abstract

Tensile prestrains of various levels were applied to blank steel specimens. Four-point bend tests of notched specimens at various temperatures revealed an appreciable drop in the notch toughness of the specimens, which experienced 3 pct tensile prestrain. Further prestrains of up to 20 pct had a negligible effect on the notch toughness despite additional increases in the yield strength. Microscopic analyses combined with finite element method (FEM) calculations revealed that the decrease in toughness resulted from a change of the critical event controlling the cleavage fracture. The increase in yield strength provided by prestraining allowed the normal tensile stress at the notch tip to exceed the local fracture stress s f for propagating a just-nucleated microcrack. As a result, for the coarsegrained steel with low s f tested presently, the critical event was changed from tensile stress-controlled propagation of a nucleated microcrack to plastic strain-controlled nucleation of the microcrack at the notch tip. A reduction of toughness was induced as a result of this. The increase in yield strength provided by decreasing the test temperature acted in the same way.

Influence of TiN Inclusions on the Cleavage Fracture Behavior of Low-Carbon Microalloyed Steels

“…The results showed that the cleavage fracture initiation sites were located at both sides of the maximum principal tensile stress, but primarily on the left side (near to the notch) for low alloy steels and weld metals. [20][21][22][23][24]33,44] In the present work, it was found that the initiation sites were located in a zone where the triaxial-tensile stress elevated the yield stress to a level higher than the local fracture stress. The initiators can be the preferentially dangerous TiN inclusions in this zone.…”

“…Detailed investigations have been conducted on the stress distribution when a Charpy V-notch (CVN) specimen is in the completely elastic state, and many useful results have been achieved, which can help explain TiN cleavage initiation at a characteristic distance away from the notch tip at low temperature. [19][20][21][22][23][24][25] However, in the case of room temperature, where considerable plastic fracture is present on the fracture surface, less attention has been paid to the behavior of TiN inclusions. Therefore, a study was made of this aspect and is reported in the present article, in which it is desired to increase the understanding of the behavior of TiN inclusions in cleavage fracture by analyzing the stress state variation during the fracture of the specimens.…”

Effects of loading rate on fracture behavior of low-alloy steel with different grain sizes

“…[10,11,12] On the other hand, some researchers [24,25,26] suggested that the critical event can be changed from the propagation of a grain-sized crack through the propagation of a second-phase particle-sized crack to the nucleation of a crack by increasing the acuity of a defect [24,25] or decreasing the temperature. [26] In recent works, [27,28] it has also been found that with increasing grain sizes and decreasing second-phase particles, the critical event can be changed from the propagation of a grain-sized crack to a crack-nucleation-dominated model, [27] and increasing the prestrain and decreasing the temperature facilitates the critical event changing from the propagation of a nucleated microcrack to the nucleation of the microcrack and then deteriorates the toughness. [28] However, the question of whether the critical events for cleavage could be changed by various loading rates has not yet been clarified.…”

“…[26] In recent works, [27,28] it has also been found that with increasing grain sizes and decreasing second-phase particles, the critical event can be changed from the propagation of a grain-sized crack to a crack-nucleation-dominated model, [27] and increasing the prestrain and decreasing the temperature facilitates the critical event changing from the propagation of a nucleated microcrack to the nucleation of the microcrack and then deteriorates the toughness. [28] However, the question of whether the critical events for cleavage could be changed by various loading rates has not yet been clarified.…”

“…Because crack nucleation is plastic straincontrolled, the fact that cleavage fracture initiates at the notch tip implies that the local plastic strain for nucleating cracks plays an important role in the cleavage-fracture process, and the cleavage is nucleation controlled. [10,11,27,28] 1. Critical event for cleavage of CG specimens In this work, it is found in Table II that among the three specimens with loading rates of 30 mm/min, short fibrous cracks occurred before cleavage in two specimens, and, in one doubly notched specimen, a pearlite colony-sized crack (as shown in Figure 14(a)) being very close to the surviving notch tip remains, indicating that the critical event is propagation of a pearlite colony-sized crack controlled by tensile stress.…”