Ductile-brittle Transition Temperature Region Research Articles

Evaluation of Plane Strain Fracture Toughness is the Ductile-Brittle Transition Temperature Region. 3rd Report, Evaluation by Using Small Fatigue Test Specimen.

Fracture toughness tests have been made in a transition temperature region at stress ratio R=-1.0 using 1TCT specimens which were cut out from SM41C carbon steel. Considerations of a plane strain fracture toughness and a plane strain condition at a brittle fracture were performed. The main results obtained were as follows. (1) The plane strain fracture toughness in the transition temperature region was simply obtained from a test which was carried out at R=-1.0 using a few small specimens. (2) A stress condition occurring in front of a crack at a brittle fracture satisfied the plane strain condition. (3) da/dN from the fatigue test data at stress ratio R=-1.0 can be extrapolated by ΔJ even in the transition temperature region. (4) The plane strain fracture toughness can be obtained from the minimum value of the fracture toughness test data using 20 three-point bend specimens in the transition temperature region.

Ductile-Brittle Transition Behavior in Sheet Specimens of Carbon Steels under Mode I-II Mixed Mode Loading.

This paper is concerned with the ductile-brittle transition behavior in sheet specimens of carbon steels under mode I -II mixed mode loading conditions. Mixed mode tests were carried out on 1.6 mm thick center-cracked sheet specimens of S50C and HT50 under various mixed mode loading conditions in the ductile-brittle transition temperature region using the testing method proposed by the present authors. In this method, the loading condition can be varied by changing the angle between the loading direction and the crack plane. The specimen width and the crack length were also changed. From the experiments, it was shown that the fracture appearance varied with decreasing test temperature, independent of the loading direction, as follows : ( a ) a ductile crack initiates and then extends, ( b ) a ductile crack initiates and then extends as a brittle cleavage crack, and ( c ) a cleavage crack initiates and extends. The upper limit temperature below which cleavage fracture can occur showed a tendency to decrease with increasing mode II component in the case of S50C, but it was almost constant and independent of the loading direction in the case of HT50.

Evaluation of Plane Strain Fracture Toughness in the Ductile-Brittle Transition Temperature Region. 1st Report. Plane Strain Condition considered with the Crack Initiation Point.

Fracture toughness tests were performed in the ductile-brittle transition temperature region using 65 three-point bend specimens which were cut out from SM41C carbon steel. Consideration of the probabilistic characteristics of fracture toughness, the crack initiation point of the brittle fracture and the plane strain condition gives the following results. (1) The probability distribution of fracture toughness shows the Weibull distribution. (2) The area of the plane strain condition is estimated from the crack initiation point and the shape of the crack front of the fibrous ductile fracture. The following equation giving the condition for plane strain in the center of the thickness is proposed : B≥0.0037{Kc(J)/σys}2+0.0092. (3) The thickness effect of the brittle fracture toughness is caused by the probability distribution of the weakest point in the crack front, namely, the Weibulltic volume effect.

Evaluation of Plane-Strain Fracture Toughness in the Ductile-Brittle Transition Temperature Region. 2nd Report. Effects of Plate Thickness on Both the Cleavage Crack Initiation Point and Fracture Toughness.

The fracture toughness test for specimens of various thicknesses is performed in the ductile-brittle transition temperature region. The statistical property of cleavage fracture toughness, and effect of thickness on the cleavage crack initiation point are investigated systematically. The following are the main results obtained in this study. (1) Cleavage cracks initiated at a certain distance (S') from the surface of specimens. (2) It is possible to determine the plane-strain region from the analysis of the cleavage crack initiation point. (3) The plane-strain condition at the center of the specimen is given by the following equation: B≥0.004(Kc(J)/σys)2+0.01. (4) The thickness effect of cleavage fracture toughness occurs with a statistical distribution of the weakest point ahead of the crack front.

The effects of hydrostatic pressure on fracture and the ductile-brittle transition in polycrystalline molybdenum

The tensile properties of polycrystalline molybdenum were investigated in the ductile-brittle transition temperature region at atmospheric and superposed hydrostatic pressures, H, extending to 275 MNm −2. Down to 180 K, at all pressures, the yield stress, σ Y , and (below the ductility transition temperature, T T the brittle fracture stress, σ F , increased with decreasing temperature. T T was lowered from ∼209 K by ∼0.17 K per MNm −2 of superposed pressure, which had no effect on the values of the stresses for yielding or brittle fracture, whichever occurred. There was a continuity, as a function of temperature (down to 180K), between the values of the tensile yield and brittle fracture stresses and the compressive yield stress. In the temperature region immediately above T T , at any pressure, failure took place by cleavage at approximately the same stress as at T T . The application of H raised the value of the required applied stress for failure by the magnitude of H, i.e. ductile cleavage occurred at a constant value of the maximum principal stress, postulated to be the crack propagation stress. Below T T the critical stage in brittle fracture, however, apperars to be crack nucleation, as the process took place at a constant shear stress. The data are consistent with T T , at all pressures, being determined by the condition that the stresses for yielding (approximately the same as for crack nucleation) and crack propagation are equal.