Fracture Toughness Of Structural Steels Research Articles

Estimation of the Ultimate Fracture Toughness of Structural Steels under Stress Concentrations

Considering a steel type based on the behavior pattern of an alloy’s basic mechanical characteristics, a correlation has been found between the following: the characteristics of the ultimate fracture toughness of structural steels at the critical embrittlement onset temperature Tb in the compound stress state BrNFb and BrC0b and the fracture toughness in the uniaxial stress state Brb through parameters that reflect the metal state in the loading elastic–plastic range. A method for the rational selection of structural steels for products that operate under the action of nonuniform force fields of different strengths has been developed.

Analysis on Critical CTOD of Long-Term Used Penstock

Many studies have been conducted to characterize the fracture toughness of structural steels and their welded joints. However, most studies focus on newly developed steels, and the number of studies on the fracture toughness of long-term used steels in structural components is rather limited. Furthermore, a lack of data on the fracture toughness causes difficulties in evaluating the structural integrity of existing steel structures. In this study, CTOD tests were performed to characterize the fracture toughness of penstock that has been in service for 50 years. By measuring the critical crack tip opening displacement in conjunction with analysis for chemical compositions, the characteristics of fracture toughness were investigated.

On the statistical modeling of cleavage fracture toughness of structural steels

The prevailing statistical models of cleavage fracture of structural steels for probabilistic risk assessment all infer a two- or three- parameter Weibull distribution of fracture toughness with a modulus of 4 and a fixed-value threshold independent of temperature and plastic constraint. This work starts with a critical review of three major statistical models of cleavage fracture toughness, namely the Beremin model, the Master Curve approach and the Prometey Unified Curve model, with a focus on their theoretical foundations, followed by a brief introduction and further extension of a newly developed statistical cleavage fracture toughness model. Then the Euro fracture toughness dataset is employed to assess these four models. The basic formulations of all the three existing models are not normative and defy the assumption of plastic yielding as a prerequisite to cleavage fracture. The key points of the new model are validated by the Euro dataset that cleavage fracture toughness does not necessarily obey the Weibull statistics while the threshold fracture toughness varies with temperature and plastic constraint.

WES 2808 for brittle fracture assessment of steel components under seismic conditions Part III: Change in CTOD fracture toughness of structural steels by pre-strain and dynamic loading

It has been reported that dynamic and cyclic large deformation the main factor, and as for the brittle fracture of the weld steel structure at the Hanshin-Awaji earthquake in 1995. As for this issue, fracture toughness deteriorates by the change in tensile properties of the steel materials by the dynamic and/or cyclic strain, and it is thought that fracture was initiated from weld defects. Fracture toughness change due to dynamic and/or cyclic strain was quantified at FTE Committee in Japan welding engineer society (JWES), and the result is taken in an assessment procedure standard of WES2808(2003). Thereafter, the FTE committee examined the expansion of application steel materials continuously and was able to adapt HT780 as a target range in this standard revision. In this report, a temperature shift concept that is the basic of the fracture toughness evaluation of WES2808 is described the influence of the dynamic loading and pre-strain. And it is included the comparison of strain-history effect such as monotonic vs. cyclic strain and tensile vs. compressive strain. In addition, it is examined the influence of the dynamic and pre-strain history on fracture toughness change for HT780 and confirmed that a temperature shift equations in WES2808 (2003) can be applicable to HT780 steel.

Fracture toughness of steels with nickel content in respect of carbide morphology

This paper is focused on the influence of Ni addition on the microstructure and fracture toughness of structural steels after tempering. Nickel is known to increase the resistance to cleavage fracture of steel and decrease a ductile–brittle transition temperature. The medium carbon, low alloy martensitic steels attain the best combination of properties in low tempered condition, with tempered martensite, retained austenite and transition carbides in the microstructure. In the present research, four model alloys of different Ni contents (from 0·35 to 4·00%) were used. All samples were in as quenched and tempered condition. Quenching was performed in oil at room temperature. After quenching, samples were tempered at 200°C for 2 h. An increase in nickel content in the investigated model structural steels causes a decrease in ε carbide volume fraction in their microstructure. Cementite nucleates independently in the boundaries of martensite laths and in the twin boundaries in the areas where the ε carbide has been dissolved. It was stated that stress intensity factor KIc significantly decreases in the case of the presence of dispersive elongated cementite precipitations at the boundaries of the prior austenite grains.

Estimation of the Fracture Toughness of Structural Steels by Means of the CTOD Evaluation on Notched Small Punch Specimens

By means of the experimental analysis of two structural steels, one with a ductile behaviour and the other one with brittle behaviour, this paper compares the results obtained by means of small punch test (SPT) performed on notched samples with those obtained in standard fracture toughness tests, in order to obtain a relationship between them and to analyse the suitability of the SPT for estimating the fracture toughness. With the aim of analysing the evolution of the defect tip opening displacement (δSPT) during the test, loading was interrupted at different levels of punch displacement (d) and these specimens were analysed under SEM.In the case of the ductile steel, δSPT at the onset of crack growth was larger than the value of CTOD obtained in conventional fracture tests, but this could be expected due to the lower constraint condition of the SPT. By the other hand, the brittle steel exhibits ductile fracture micromechanisms in the SPT, so fracture mechanisms have changed from the standard to the small punch test, due to the different load conditions. Relationship between d and δSPT seems to be material independent.

Prediction of temperature and prestraining effects on fracture toughness of high-strength structural steel by the local approach

To investigate the tensile properties and facture toughness of high-strength structural steel of Q420 under different conditions, tensile tests and crack tip opening displacement (CTOD) tests were conducted. The test results revealed that temperature significantly influenced the fracture toughness of structural steel and that fracture toughness was significantly reduced with the decrease in temperature, resulting in brittle fractures. Prestraining increased the yield stress and tensile strength of the structural steel. However, it significantly reduced the plasticity and fracture toughness, and further increased the probability of brittle fractures. As determined by FE-analysis, temperature and prestraining facilitated brittle fractures due to the activation of the resulting near crack tip stress fields. Based on the Weibull stress fracture criterion, the temperature and prestraining effects on the fracture toughness were predicted from fracture toughness results of the virgin material at room temperature by the local approach. The prediction was in good agreement with the experimental results. It certified that the critical Weibull stress obeys the two-parameter Weibull distribution in the local approach and the fracture behavior of the prestrained material at different temperatures can be characterized by the local approach.

Experimental and numerical study of ball indentation for evaluation of mechanical properties and fracture toughness of structural steel

Determination of mechanical properties of materials using non-conventional techniques has been an active area of research for a long time. Among various small specimen techniques to determine mechanical properties of materials, the ball indentation technique (BIT) has proved to be advantageous. BIT is used to measure material’s mechanical properties including fracture toughness when a tensile test cannot be performed: on welded joints or critical locations of components under service. The present work highlights the applicability of BIT for evaluating flow behaviour of engineering structural steels En24. Standard mechanical properties like ultimate tensile strength, yield stress, strain hardening coefficient evaluated for steels with varying microstructures generated through heat treatment. To determine fracture toughness from the flow behaviour, non linear damage models have been utilized. Using this model, fracture toughness of the En24 steel at different heat treated conditions has been computed using the results generated by the BIT. These results are verified with the already established correlation in literature. Numerical validation of the results generated by the BIT has been carried out by Finite Element Modelling (FEM) using standard ABAQUS software package. The FE model of the BI simulation helps to compute sub indenter stress-strain fields which determine the extent of pile-up in highly ductile materials.

The effect of carbide precipitate morphology on fracture toughness in low‐tempered steels containing Ni

Nickel is known to increase the resistance to cleavage fracture of iron and decrease a ductile-to-brittle transition temperature. The medium-carbon, low-alloy martensitic steels attain the best combination of properties in low-tempered condition, with tempered martensite, retained austenite and transition carbides in the microstructure. This paper is focused on the influence of Ni addition (from 0.35 to 4.00%) on the microstructure and fracture toughness of structural steels after tempering. In this research, four model alloys of different concentration of Ni and constant concentration of carbon and other elements were used. All samples were in as-quenched and tempered conditions. Quenching was performed in oil at room temperature. After quenching, samples were tempered at 200 degrees C for 2 h. The microstructure of the investigated steels was analyzed using JEM200CX transmission electron microscope. An increase of nickel content in the investigated structural steels causes a decrease of epsilon carbide concentration in their microstructure after tempering. In these steels, cementite precipitates independently in the boundaries of martensite needles and in the twin boundaries in the areas where the Fe(2.4)C carbide has been dissolved. These results will be used to design new technologies of tempering of structural steels with nickel addition.

Effects of Cyclic Plastic Strains on Fracture Toughness of Structural Steels

The effects of cyclic plastic strains on the fracture toughness of structural steels are investigated by Charpy V-notch impact tests. It is shown that strain aging reduces the fracture toughness of structural steels subjected to cyclic plastic strains. Charpy absorbed energy is examined on the top and bottom flanges of a girder of a highway bridge hit by the January 17, 1995 Kobe Earthquake. It is shown that the girder was not subjected to cyclic plastic strains during the Earthquake.

Development of the Local Approach to Fracture over the Past 25 years: Theory and Applications

This review paper is devoted to the local approach to fracture (LAF) for the prediction of the fracture toughness of structural steels. The LAF has been considerably developed over the past two decades, not only to provide a better understanding of the fracture behaviour of materials, in particular the failure micromechanisms, but also to deal with loading conditions which cannot easily be handled with the conventional linear elastic fracture mechanics and elastic–plastic fracture mechanics global approaches. The bases of this relatively newly developed methodology are first presented. Both ductile rupture and brittle cleavage fracture micromechanisms are considered. The ductile-to-brittle transition observed in ferritic steels is also briefly reviewed. Two types of LAF methods are presented: (i) those assuming that the material behaviour is not affected by damage (e.g. cleavage fracture), (ii) those using a coupling effect between damage and constitutive equations (e.g. ductile fracture). The micromechanisms of brittle and ductile fracture investigated in elementary volume elements are briefly presented. The emphasis is laid on cleavage fracture in ferritic steels. The role of second phase particles (carbides or inclusions) and grain boundaries is more thoroughly discussed. The distinction between nucleation and growth controlled fracture is made. Recent developments in the theory of cleavage fracture incorporating both the effect of stress state and that of plastic strain are presented. These theoretical results are applied to the crack tip situation to predict the fracture toughness. It is shown that the ductile-to-brittle transition curve can reasonably be well predicted using the LAF approach. Additional applications of the LAF approach methods are also shown, including: (i) the effect of loading rate and prestressing; (ii) the influence of residual stresses in welds; (iii) the mismatch effects in welds; (iv) the warm-prestressing effect. An attempt is also made to delineate research areas where large improvements should be made for a better understanding of the failure behaviour of structural materials.

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.

Fatigue and Fracture of Steel Girders

This paper presents an overview of materials selection, design, and detailing of steel girders for fatigue and fracture limit states. The historical context of the fracture control plan for bridges is presented. A discussion of fracture toughness of structural steel and weld metal is presented along with typical Charpy and fracture-toughness test data, including the new high-performance steel A709 HPS 485W. Fatigue of cover plate details and distortion-induced cracking are discussed. Methods of dealing with variable-amplitude loading are then compared to test data.

Evaluation of Prestraining and Dynamic Loading Effects on the Fracture Toughness of Structural Steels by the Local Approach

On the occasion of recent great earthquakes, great concern is focused on the prevention of unstable fracture of steel structures against the seismic loading. This paper employs the local approach for the evaluation of prestraining and dynamic loading effects, experienced during an earthquake, on the fracture toughness of structural steels. The prestraining and dynamic loading lead to a similar result: increasing the yield stress and tensile strength and decreasing the fracture toughness. It is shown, however, that the combined effects of prestraining and dynamic loading is not equivalent to the sum of each individual effect. The analysis using the local approach demonstrates that the critical Weibull stress at brittle fracture initiation is independent of prestraining and dynamic loading. Based on the Weibull stress fracture criterion, the prestraining and dynamic loading effects on the fracture toughness can be predicted from static toughness results of the virgin material. As an engineering application, a simplified method is proposed for the estimation of fracture toughness under the seismic condition. This method uses a reference temperature concept: the dynamic fracture toughness at the service temperature T with prestrain is displaced by the static toughness of the virgin material at a lower temperature T−ΔTPD, where ΔTPD is a temperature shift of the fracture toughness caused by prestraining and dynamic loading. The temperature shift ΔTPD is provided as a function of the flow stress elevation in the seismic condition.

Effects of temperature and loading rate on fracture toughness of structural steels

The fracture characteristics of body-centered-cubic (BBC) metallic materials is highly dependent on temperature and strain rate. To study the effects of temperature and loading rate on the dynamic fracture behavior of a high-strength low-alloy steel, a series of fracture toughness experiments were carried out using three-point-bend specimens and impact specimens. The results show that with the increase of temperature and decrease of loading rate, dynamic fracture toughness rises. Furthermore, the study indicates that dynamic fracture toughness is controlled by dislocation thermal activation motion and may be quantitatively expressed by introducing the Arrhenius equation.

Fracture Mechanics Testing for Structural Steels

Abstract Fracture mechanics is widely used to evaluate the fracture behavior of structural steels. However, the fracture toughness of structural steels is affected significantly by temperature, loading rate, and constraint. Because of the effect of these factors, the fracture behavior of structural steels varies considerably from linear elastic to general yielding. Accordingly, there are numerous fracture mechanics tests of structural steels—each of which is applicable to the general regime of linear-elastic, elastic-plastic, or general yielding behavior. This paper describes the fracture testing and behavior of structural steels in each of these three general domains and briefly describes the ASTM test methods used in each domain.

Principle and method for determining fracture toughness of structural steel by side-cut se (B)

In this paper, the principle and method for determining fracture toughness indexes K IC , δ C and J IC of structural steel by side-cut SE (B) are introduced. This method has many advantages: clear critical point, simple operation, stable date, less cost and will be valuable to use more frequently.

Evaluation of the fracture toughness of structural steels under high-speed deformation

We investigated the dynamic crack resistance (fracture toughness) of low-alloy structural steels within a broad range of change of temperature (173–323°K) and loading rate (10−5−120 m/sec) on standard specimens tested under conditions of three-point bending by a drop weight. As velocity parameters determining the conditions of high-speed deformation of metal in the zone of prefailure we considered the speed of opening of the crack tip δ and the rate of local plastic deformation έ on the contour of its blunted tip. To calculate έ in dependence on the loading conditions, the corresponding expression is presented. We evaluated experimentally the mechanical properties of the investigated steels at different temperatures T and strain rates, and we also considered the possibility of determining theoretically the dynamic yield strength $$\sigma _{0.2} ^d $$ that is used in establishing the value of $$\dot \varepsilon $$ . We present the correlation between the transition temperature of the metal to ductile failure Tv and the velocity parameters $$\dot \delta , \dot \varepsilon $$ .

Arrest and propagation of a brittle crack in structural steels. Report 2

The authors describe a laboratory method of determining the fracture toughness of structural steel in the crack propagation stage KID. The KID — a dependences were obtained for three structural ferritic-pearlitic and bainitic steels at different temperatures. The KID — a dependences are analyzed from the viewpoint of microstructural features of failure of these materials.

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.