Local Fracture Stress Σf Research Articles

Effects of sizes of ferrite grains and carbide particles on toughness of notched and precracked specimens of low-alloy steels

Four point bending (4PB) tests of notched specimens and COD tests of precracked specimens were carried out on two steels; one steel was treated into two groups with the same ferrite grain size but different carbide sizes, the other steel with different ferrite grain sizes but similar carbide sizes. The results of the tests show that the toughness measured in notched specimens is mainly determined by the grain sizes, which define the local fracture stress σf; the size of carbide particle plays a minor role. However, on the contrary, in precracked specimens the toughness is sensitive to the carbide sizes, which affect the critical plastic strain epc for initiating a crack nucleus; the effect of grain size is indistinct. By these inferences the behavioral discrepancy of large grain steel in improvement of crack fracture toughness while reducing the notch toughness is explained.

Study on cleavage fracture criteria of the quasi-brittle and micro-inhomogeneous materials

On the bases of recent achievements on the micro-mechanism of cleavage this paper analyses the inherent deficiencies of the stress intensity factor KI which is used to evaluate the fracture toughness of quasi-brittle and micro-inhomogeneous materials. The KI parameter can uniquely determine the field intensity ahead of a crack tip in the condition of elastic and small scale yielding (SSY). However, the KI cannot uniquely determine the critical condition triggering the cleavage fracture in a quasi-brittle and inhomogeneous steel where the cleavage fracture process is not a direct extension of the precrack but is initiated at a variable distance from the precrack tip. The variable distances of cleavage initiations invoke varied critical values of KI. On the bases of authors' experiments, this paper analyses the physical meaning of the local fracture stress σf, its stability and the feasibility to be used as an engineering parameter for assessing the fracture toughness.

Cleavag Fracture Criterion of Low Alloy Steal and Weld Metal in Notched Specimens

The cleavage fracture criterion of low alloy steel and weld metal in notched specimens is investigated in detail based on a great number of experimental data. It has been found that the most cleavage fractures initiate at a distance shorter (left side) than that of the peak stress location below a notch root, and the cleavage fracture in notched specimens must satisfy a dual criterion, i.e., a critical plastic strain (ep ≥ epc) for initiating a crack nucleus, and a critical tensile stress (σyy ≥ σf) for its propagation. According to the dual criterion model, the great number of experimental data of 4PB (four-point bending) tests for the low alloy steel and weld metal and their statistical distribution are explained. The effects of temperature , the local fracture stress σf and the critical plastic strain epc on the locations of cleavage initiation sites and the controlling steps of cleavage fracture process are discussed.