Plane strain fracture toughness of modern ferritic structural steels

Abstract

The temperature dependence of the plane strain fracture toughness of a low carbon, fine grain, ferritic steel for structural applications is investigated. The ductile–brittle transition is found to occur in the interval between 160 and 184 K. The experimental results are interpreted by an analytical model which permits calculation of the plane strain fracture toughness K _1c in the brittle domain as a function of the tensile properties and the cleavage fracture stress, making use of a piecewise approximation for the distribution of tensile stress on the crack axis and applying a deterministic fracture criterion at the stress peak. A similar criterion, which consists of equating the severest strain on the crack axis to a critical strain for cavity nucleation, provides the upper shelf fracture toughness. A relatively simple figure for predicting the transition temperature of steels in this family as a function of material properties can be obtained in this way.     

舰船结构钢的夏比冲击韧性与断口形貌

论述了从夏比冲击韧性分解出来的断裂扩展功与断口形貌的关系，指出冶金因素对夏比冲击韧性α＿ｋ值和扩展功的影响不完全是一致的，提出采用α＿ｋ，值和断口纤维率作为韧性指标的互补性，建议在我国的舰船结构钢韧性指标中增加断口纤维率的要求。     

Effect of pre-strain on fracture toughness of ductile structural steels under static and dynamic loading

The ductile fracture process consists of void nucleation, growth and coalescence. The whole ductile process can be divided into two successive steps: (I) the initial state to void nucleation, followed by (II) void growth up to void coalescence. Based on this suggestion, resistance to ductile fracture could be divided into the resistance to stage I and stage II, and accordingly the whole fracture toughness could be regarded to be due to contributions from stages I and II. The fracture toughness contributed from the two steps is, respectively, denoted as void nucleation-contributed fracture toughness and void growth-contributed fracture toughness. The effect of plastic pre-strain on the fracture toughness of ductile structural steels under static and dynamic loading (4.9 m/s) within the ductile fracture range was evaluated by summing contributions due to void nucleation-contributed and void growth-contributed fracture toughness. The effect of strain rate on fracture toughness was also investigated by the same means. The results show that both plastic pre-strain and high-speed loading decrease the void nucleation-contributed fracture toughness while their effects on the void growth-contributed fracture toughness depend on the variations in strength and ductility. Moreover, fracture toughness of structural steels generally decreases with increasing strain rate.     

Effect of desulphurization on the fracture toughness of structural steels at climatic temperatures

The results show that desulphurization of 09G2 and 10KhSND steels by blowing silicocalcium into the ladle and by treatment in the ladle with a synthetic slag with the sulphur content varying from 0.04–0.04 to 0.003–0.005% increases several times the impact toughness, 1.2–1.5 times the critical stress intensity factor, reduces the temperature of transition from ductile to quasibrittle failure, and increases the critical crack opening displacement two to six times in the temperature range 213–293 K. Desulphurization is an important method of increasing the brittle failure resistance of structural steels at reduced temperatures.Translated from Problemy Prochnosti, No. 11, pp. 14–21, November, 1990.     

Assessment of the fracture toughness of cast steels

Estimates of the fracture toughness in terms of the critical stress intensity factorsK _C andK _IC are made for a 1Cr steel, a 1/2Cr-1/2Mo-1/4V steel, a 1 1/2Mn-Ni-Cr-Mo steel and a 1 1/2 Ni-Cr-Mo steel all in cast form. The methods used are linear elastic fracture mechanics,J-integral and crack opening displacement methods. The last two methods are applied in combination with an electrical potential method to detect the initiation of fracture.     

Evaluation and modeling of hydrogen-induced fracture in structural steels

From the macroscopic point of view, the results of hydrogen embrittlement of pearlitic steel strongly depend on compressive residual stresses generated in the vicinity of the crack tip by fatigue precracking of the specimens (the relevant variable is the maximum stress intensity factor during the last stage of fatigue precracking. As far as the kinetic crack growth curve is concerned, the threshold stress intensity value for hydrogen-assisted cracking does not have an intrinsic character but depends on the distribution of compressive residual stresses in the vicinity of the crack tip and, therefore, on the maximum stress intensity factor during the last stage of fatigue precracking. The fractographic analysis of hydrogen embrittlement tests on precracked and notched specimens of high-strength pearlitic steel reveals the existence of a special microscopic mode of fracture associated with hydrogen-induced microdamage: the so-called tearing topography surface or TTS, which can be regarded as the process zone. Hydrostatic stress is the macroscopic variable governing the extension of the TTS zone. It is demonstrated that the distribution of hydrostatic stresses in notched specimens is practically independent of the loading process and the point of maximum hydrostatic stress is a characteristic of the geometry of the problem and never changes its position. The depth of the TTS zone is a function of the electrochemical potential, the maximum fatigue precracking load, and the duration of the test, which reveals its correlation with the process of hydrogen embrittlement. These phenomenological relations can be explained by using a model of stress-assisted diffusion of hydrogen. The role of diffusion as the main mechanism of hydrogen transport in pearlitic steel is demonstrated by comparing the hydrogen-affected region and the plastic zone, which reveals no relationship between them. Hydrostatic stresses play an important role in accelerating the diffusion of hydrogen. The hydrogen-induced fracture of notched specimens of pearlitic steel is a time-dependent phenomenon admitting the kinematic modeling of the process as a function of the strain rate. In this conceptual framework, the local strain rate in the vicinity of the notch tip is the relevant variable controlling the process. For cold-drawn prestressed steel wires, the chemical modeling of the diffusion of hydrogen emphasizes the role of residual stresses generated in smooth wires in the process of manufacturing as well as the role of stresses induced by fatigue loads in precracked wires. In this case, their magnitude depends on the load (maximum stress intensity factor). In notched specimens of austenitic stainless steel, hydrogen damage can be described as multicracking in the area surrounding the notch and failure is induced by plastic instability. In this case, the action of hydrogen can be mechanically simulated as a geometric enlargement of the notch in the form of extended microdamage.     

HAZ fracture toughness of titanium containing steels

Abstract

Steels containing various combinations of microalloying elements (Nb, V, and Ti) were welded at heat inputs from 3 to 6 kJ mm^?1. It was shown by detailed crack tip opening displacement fracture toughness testing of coarse grained heat affected zone (HAZ) regions in single pass weld deposits that the poorest toughness properties were exhibited by steel containing a combination of Nb, V, and Ti. Steel microalloyed with only titanium had the best HAZ fracture toughness at all heat input levels. Detailed microstructural analysis, grain size measurement, hardness, and precipitation in HAZ regions were evaluated to explain the fracture toughness properties observed.

MST/887     

On a relationship between stretched zone parameters and fracture toughness of ductile structural steels

Quantitative stereofractographic analysis of the stretched zone at the crack tip is performed on specimens of two ductile steels used for determining fracture toughness. The effect of temperature, loading rate, stress-state mode and specimens size on the stretched zone formation is studied. A correlation is obtained of the stretched-zone height and width with fracture toughness of materials and the crack tip opening displacement value which is determined by means of various mathematical models.