Fracture Toughness Of Structural Steels Research Articles

Effect of fatigue-crack closure on the near-threshold fracture toughness of structural steels

1. The method used here for fatigue tests-providing for loading of specimens with constant values of ΔK and an asymmetric cycle—is very effective when studying the conditions of occurrence of different mechanisms of crack closure. 2. The main reason for localization of crack closure due to oxides in the central part of the fracture is nonuniformity of the stress-strain state along the crack front. This non-uniformity is present even at levels of ΔK close to the threshold value. 3. The thickening of the oxide layer on the fracture surface with fatigue-crack growth at low rates may be autocatalytic in nature and may lead to disturbance of the invariance of the near-threshold section of the fatigue curves. 4. An increase in the degree of constraint of the material near the tip of a fatigue crack with an increase in specimen thickness facilitates and intensifies crack closure due to oxides, so that the threshold values of ΔKth are reached at a higher nominal SIF range. 5. The predominance of one of the crack-closure mechanisms (associated with plastic strain or oxide formation) in the case of near-threshold growth of a fatigue crack leads to inversion of the effect of specimen thickness on the parameter ΔKth.

Working Load on Specimen in Rapid-Fracture Toughness Test

In order to evaluate dynamic fracture toughness of structural steel, three point bending (3 PB-COD) or compact tension (CT) test is usually carried out by using drop weight or high tensile rate testing machine.Since, in static test, applied load to the specimen can easily measured exactly by load cell of test system, but in dynamic test it is difficult to get or decide working load on test specimen because the effect of acceleration affects on the output of load cell in test system.In this paper, the estimation of working load on test specimen was investigated applying a simple mass and spring model to the loading system of test equipment.

Evalution of micro-fracture mechanisms and fracture toughness of cryogenic structural steels and weldment.

In order to evaluate the fracture toughness of structural steels for superconducting magnets of fusion reactor, elastic-plastic fracture toughness testing was performed at 4.2K on AISI 310S base metal, weld metal and 20Mn steel. Fracture toughnesses were evaluated by a single specimen technique using the Key Curve reported previously. Elastic-plastic fracture toughness JIC and tearing modulus Tmat of weld metal were significantly lower than those of base metal. As the result of AE measurement, microfracturing was detected at the early stage of loading, and pop-in cracking was observed at the later stage. Fractographic and metallographic examination of fracture surfaces and fracture paths revealed a difference in dimple sizes between the weld metal and the base metal, and also micro void formation ahead of a growing crack tip, which resulted in smaller crack opening angle of the weld metal than the base metal. Finally, the significance of crack growth analysis of weld metal was emphasized to ensure the structural integrity of superconducting magnet.

Effect of the dimensions of specimens on the crack resistance of creep-resisting hull steels

1. Increased thickness of specimens leads to reduced rate of fatigue crack growth on the first section of the diagram of fatigue failure and to increased value of Kth of the investigated steels at 293°K. This is probably due to the increased residual compressive stresses in the crack mouth and the increased time necessary for the emergence of a crack, originating in the central regions of the specimen, onto its lateral surface when the dimensions of the specimen are increased. 2. Increased thickness of the specimen practically does not cause a change of the rate of fatigue crack growth on the Paris section, but it increases the characteristics Kfc and KQ of the investigated steels under cyclic and static loading, respectively. 3. The effect of the scale on the characteristics of fracture toughness of structural steels is apparently connected with the proneness of the material to strain-hardening.

The effect of microstructure on the fracture toughness of structural steels

Comprehensive research has demonstrated that fracture toughness, K IC , may be uniquely related to the mechanical properties describing a material's crack tip behaviour and microstructure. In the work described in the present paper the behaviour of cracks in seven weldable low alloy structural steels with tensile strengths, σ TS , in the range 455–765 MPa was investigated by measurement and observation of plane-strain fracture toughness, tensile properties and the microstructure. The information so obtained was used to establish a sophisticated K IC calculation model. It is shown that the process zone size is the dominant microstructural factor controlling the fracture toughness and that it varies from two to six grain sizes, depending on the particular microstructure. The model proposed matches the experimental data well and can be applied to find an optimum steel microstructure for given brittle fracture design requirements.

The influence of mechanical twinning on the fracture toughness of structural steels

The influence of mechanical twinning on the fracture toughness of structural steels

Resistance to fracture of steel with a ferritic-pearlitic structure

1. The increase of the fracture toughness with increasing amounts of pearlite up to 20% coincides with the rapid increase in strength due to the sharp reduction of the distance between pearlite inclusions. With larger amounts of pearlite, leading to an increase in the size of the inclusions, the inclusions tend to split under the influence of local stress concentrations at the tip of the moving crack, facilitating fracture. 2. The fracture toughness of structural steel with a polygonal ferritic—pearlitic structure is highest with about 20% pearlite.

鋼材の破壊靱性値評価法と溶接構造物の欠陥評価

鋼材の破壊靱性値評価法と溶接構造物の欠陥評価

Assessing the fracture toughness of structural steels by the critical crack opening method

1. By defining changes in the angle of crack opening before the crack starts to move, CCO essentially defines the work expended on deformation of a preliminary crack zone, i.e., it correlates to some extent with the magnitude ofap.c and contains no information on the work of crack propagationap. The CCO method is applicable to the qualitative assessment ofap.c. 2. Impact tests with evaluation ofap andap.c yield substantially more information on the toughness properties of the metal being tested than can be obtained in tests to evaluate CCO and with much less labor.

The relationship between the plasticity characteristics and fracture toughness of structural steel

The relationship between the plasticity characteristics and fracture toughness of structural steel

Fracture Toughness of Structural Steels as a Function of the Rate Parameter T ln A/ε˙

The influence of temperature and strain rate upon the fracture toughness of structural steel is the question considered in this paper. The hypothesis is proposed that fracture toughness, KIc, for initial crack extension is a single-valued function of the rate parameter T ln A/ε˙. Measurements made by Krafft and Sullivan of fracture toughness, KIc, for three steels covering a range of low temperatures and high strain rates are presented as a function of this rate parameter. Two of the materials support the contention that a single-valued relation exists between KIc and the parameter, while scatter in the data for the third steel does not allow a conclusion. The complexity of design against fracture in structural steels is reviewed. For conservative design it must be assumed that a crack is present in the structure. Design against fracture must insure that the minimum fracture toughness at the service temperature, strain rate and stress is sufficient to prevent this crack from extending rapidly through the base metal. A means of using low temperature laboratory fracture toughness tests for estimating a minimum fracture toughness corresponding to the service conditions is discussed.