Crack arrest in modern steels and their weldments—Comparison between small and large scale experiments

The results of a range of different small scale material characterisation tests were correlated with the crack arrest temperature at the yield strength of a number of modern ferritic steels and their weldments. The crack arrest temperature was determined experimentally using large scale structurally representative double tension tests. The comparison of small and large scale tests indicates that a safe estimate of the crack arrest temperature for the steels and welds investigated can be obtained using the nil-ductility transition temperature plus 40°C. The 50% fracture appearance transition temperature or the 20% fracture appearance transition temperature plus 20°C obtained from full thickness drop weight tear test are also reliable measures of the lower bound crack arrest temperatures.     

Static and impact crack properties of a high-strength steel welded joint

In order to gain the benefits of weldable high-strength steels in pressurized equipment applications, satisfactory toughness and crack properties of the welded joint, both in the weld metal and the heat-affected –zone (HAZ), are required. Experimental investigations of toughness and crack resistance parameters through static and impact tests of a high-strength, low-alloy steel (HSLA) with a nominal yield strength of 700 MPa and its welded joint, were performed on Charpy-sized specimens, V-notched and pre-cracked, of the parent metal, weld metal and HAZ. The selected electrode produced slight undermatching and enabled the welded joints to be manufactured without cold cracks. The impact energy and its parts responsible for crack initiation and propagation were determined by toughness evaluation. Crack sensitivity, defined as the ratio of the impact energy for V-notched and for pre-cracked specimens, enabled a comparison of the homogeneous microstructure of the parent metal and the weld metal, and of the heterogeneous microstructure of the heat-affected-zone (HAZ), which indicated a better crack toughness behaviour of the HAZ. The results obtained showed that the toughness and crack resistance of the weld metal were significantly lower than those of the parent metal and the HAZ. The fracture mechanics parameters, J_Ic integral, and plane strain fracture toughness, K_Ic, as well as J resistance curves expressed the degradation less.     

Measurement of crack propagation velocity in nuclear pressure vessel steel (SA516 gr. 70)

An attempt has been made to develop a simple, reliable and cost-effective device for measuring the dynamic crack propagation velocity in a nuclear pressure vessel steel (SA516 gr. 70). The experimental method is described and a simple digital approach is proposed. The experimentally determined dynamic crack velocity has been utilized to obtain elastic dynamic stress intensity factors by INSAMCR (a two-dimensional dynamic finite element code which is a modified version of SAMCR developed by Dr Schwartz at the University of Maryland). A relationship between instantaneous crack tip velocities and dynamic stress intensity factors for pressure vessel steels is estimated using dynamic crack propagation velocities determined by a proposed measuring device. The relationship between the dynamic stress intensity factor and time history and the dynamic arrest toughness for each test are obtained using the generation mode dynamic finite element analysis. A function ƒ(å) = 1·356 − 2·672å + 6·494å² − 4·539å³ + 1·461å⁴ is suggested which may be useful to predict the relationship between the dynamic fracture toughness (K(å)) and the dynamic crack arrest toughness (K_Ia) for SA516 gr. 70 steel (say K(å) = K_Ia ƒ(å) where å is the dynamic crack propagation velocity).     

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.     

Hydrogen embrittlement of structural steels: Effect of the displacement rate on the fracture toughness of high-pressure hydrogen pre-charged samples

The aim of this paper is to study the effect of the displacement rate on the fracture toughness under internal hydrogen of two different structural steels grades used in energy applications. To this end, steel specimens were pre-charged with gaseous hydrogen at 19.5 MPa and 450 °C for 21 h and then fracture toughness tests were carried out in air at room temperature. Permeation experiments were also conducted to obtain the hydrogen diffusion coefficients of the steels. It was observed that the lower the displacement rate and the higher the steel yield strength, the stronger the reduction in fracture toughness due to the presence of internal hydrogen. A change in the fracture micromechanism was also detected. All these findings were justified in terms of hydrogen diffusion and accumulation in the crack front region in the different steel specimens.     

Fatigue crack growth behavior of reactor pressure vessel steels in air and high-temperature water environments

Fatigue tests under constant amplitude load were conducted on CT specimens of A533B3 steels with four levels of sulfur content at different temperatures in air and high-temperature water environments. A modified capacitance-type COD gauge was shown to be suitable for fatigue crack length measurement at high temperatures in air. The observation that the Young's moduli measured at a strain rate of 4 × 10⁻³ s⁻¹ for the A533B3 steels at 150 °C and 300 °C did not decrease with an increase in temperature seemed to be related to the presence of dynamic strain aging. The fatigue crack growth rates at 150 °C and 300 °C in air were about two and half times slower than those tested at 400 °C, because dynamic strain aging prevailed at 150 °C and 300 °C. Fractographic examination results suggested that inclusions embedded in secondary cracks enhanced the fatigue crack initiation rather than the fatigue crack growth. The fatigue crack growth rates taken in the oxygen-saturated water environment were one order of magnitude faster than those obtained in air.     

Dynamic energy absorption and perforation of ductile structures

This article focuses on the energy absorption of structural systems and components subjected to extreme loadings which can arise from impact, explosion and various other dynamic events. The dynamic properties of high-strength steels are discussed briefly because they have been proposed for energy-absorbing systems and other components in place of the more common mild steel and aluminium alloys. An energy-absorbing effectiveness factor is proposed which compares the dynamic energy absorbed in a device with the energy consumed up to rupture in a uniaxial tensile test specimen, which is made from the same volume of the material. This dimensionless factor is examined for the axial impact loading of thin-walled tubes having several cross-sectional shapes and made from different materials. It transpires that the aluminium alloys are more effective than the mild and high-strength steels and that circular cylindrical shells are more effective than square tubes and thin-walled tubes having other cross-sectional shapes.     

CRACK ARREST ANALYSIS UNDER CYCLIC THERMAL SHOCK FOR AN INNER-SURFACE CIRCUMFERENTIAL CRACK IN A FINITE-LENGTH THICK-WALLED CYLINDER

This article presents a crack arrest depth analysis under cyclic thermal shock for an inner-surface circumferential crack in a finite-length thick-walled cylinder with rotation-restrained edges. The inside of the cylinder is cooled from a uniform temperature distribution. The effects of heat transfer conditions on the maximum transient stress intensity factor for the problem were investigated with systematical evaluation methods formerly developed. Then, under an assumption of a tentative threshold stress intensity range j K th together with the Paris law, the crack arrest depth under cyclic thermal stress was evaluated. The results suggested the existence of an upper limit for the normalized crack arrest depth, independent of the cylinder material in an engineering sense. Finally, the validity of applying j K max h j K th as a crack arrest criterion under cyclic thermal shock was confirmed by fatigue tests under mechanical loads equivalent to those induced by cyclic thermal shock.     

A quantitative description on fracture toughness of steels in hydrogen gas

Fracture toughness or critical stress intensity factor of many steels can be reduced by hydrogen gas. In this paper, a simple quantitative model to predict the fracture toughness of steels in gaseous hydrogen is proposed. This model is based on the assumption that fracture of a cracked body occurs when the maximum principal stress ahead of the crack tip reaches the critical cohesive stress for crack initiation. The critical stress is inversely proportional to the accumulated hydrogen concentration. The notion is that the crack will initiate at the elastic-plastic boundary ahead of the crack tip when hydrogen concentration reaches a maximum value after a long-term hydrogen diffusion assisted by the hydrostatic stress. The model describes the dependence of fracture toughness on hydrogen pressure, temperature and yield strength of steels. It can be used to quantitatively predict fracture toughness of steels in hydrogen gas, particularly in high pressure. Some experimental data reported in literature were used to validate the model, and a good agreement was obtained.     

Use of the reflectionless stress intensity factor procedure to predict crack arrest during a hypothetical pressurized thermal shock event: The effect of initial crack size

A dynamic methodology based on the reflectionless stress intensity factor concept is used to examine the combined effects of pressure and thermal loads on crack arrest in a nuclear reactor pressure vessel that is subjected to a hypothetical pressurized thermal shock event. Earlier work, for the case of an initially infinitesimally shallow crack, has shown that the currently accepted ASME Code procedure is overly conservative in its arrest predictions when applied to the arrest of a crack deep in the vessel thickness. The present paper shows that this conclusion is also applicable to the case where the initial crack is not necessarily infinitesimally shallow.     

Correction factors for safe performance of API X65 pipeline steel

Prediction of required Charpy energy for fracture arrest is vital for safe performance of gas transportation pipelines. This is commonly estimated through failure models calibrated in the past on fracture data from combined Charpy tests and full-thickness burst experiments. Unfortunately, such pipeline failure models are unable to correctly predict the minimum arrest toughness of thermo-mechanical controlled rolled (TMCR) steels. To refine the existing failure models, different empirical adjustments have been proposed in recent years. In this paper, similar correction factors were derived from fracture information of instrumented Charpy impact tests on API X65 steel. The contribution of different fracture mechanisms of impact test specimens was determined through energy partitioning analysis. Parts of the energy contribution were correlated then to the source of uncertainty observed in similar experiments. The applied technique was similar to that of previous studies on X70 and X100 steels, and proved to be encouraging in giving consistent results compared to available test data.     

Numerical analysis of IPIRG cracked pipe experiments subjected to dynamic and cyclic loading

This report contains results of a finite element study aiming to identify the influence of loading history and geometry for cracked pipes subjected to complex loading. The experiments have been performed within the International Piping Integrity Research Group (IPIRG) Program. The majority of the numerically analyzed experiments were conducted on straight pipes with an outside diameter of 168 mm and containing a large circumferential through-wall crack. The considered pipes were loaded in four-point bending under displacement control and at a temperature of 288°C. The types of loading were combinations of either quasi-static or dynamic and also monotonic or cyclic loading with different loading ratios R. Some analyses were also performed on surface-cracked pipes subjected to slow, monotonic loading.

In the finite element study, 20-node solid elements were used for the through-wall cracked pipes and a combination of shell and non-linear line spring elements for the surface-cracked pipes. Stable crack growth was simulated by gradual node relaxation and crack closure is accounted for by using simple contact elements. The J-integral for a remote contour is calculated and used as a characterizing fracture parameter although the cyclic loading violates the theoretical basis for this procedure. The near-tip J can not be used for growing cracks because of the weak energy singularity. The results of the numerical study confirm the trends from the experiments in that a high loading rate has a negative influence on the fracture properties of the studied carbon steel and that large cyclic loading, especially at R = −1, lowers the apparent J_R-curve for both carbon and stainless steels. To some extent geometry effects appear to be present when comparing the results from pipes containing surface cracks and through-wall cracks with results from CT specimens. These effects are more pronounced for large amounts of stable crack growth than at initiation.     

Progressive crack extension due to local cooling of a crack in LNG storage tank material

This paper details an experimental programme designed to simulate the effects of cold gas escape from a crack in the outer mild steel containment tank of an LNG storage tank. Tests involving stressed flat plates in which an artificial defect was locally cooled showed that crack extension occurred when a critical local temperature range was reached. Temperature and strain investigations showed that the crack driving force was generated by thermally induced contraction stresses. In practice, further crack extension could be prevented by application of local heating. The cracks so initiated arrested after travelling a short distance and the arrest temperature conditions are related to stress level. The crack arrest temperature is sub-zero for normal tank design stresses and extensive fracture would not occur in the material investigated.     

Results of a dynamic analysis of crack propagation and arrest data in the modified compact tension geometry

Crack arrest toughness is of relevance to the integrity assessment of nuclear pressure vessels. Hence a detailed analysis has been performed on the dynamic behaviour of two proposed crack arrest specimen geometries. A one-dimensional finite difference computer program has been used to determine the instantaneous dynamic stress intensity factor, $\text{K}{}_{\mathrm{<mtext>ID</mtext>}}$, during fast crack growth. Experimental data used as input were in the form of crack length/time pairs determined in specimens of the compact tension geometry. An analysis of the errors involved in this technique suggests that they are part systematic and part random and each typically at the 5% level.Values of $\text{K}{}_{\mathrm{<mtext>ID</mtext>}}$ showed similar trends to those observed by previous workers using different experimental techniques. Within the first few millimetres of growth $\text{K}{}_{\mathrm{<mtext>ID</mtext>}}$ dropped to about 0·7 times the nominal stress intensity at initiation from a blunt notch and decayed further to about half of the value. Average values of $\text{K}{}_{\mathrm{<mtext>ID</mtext>}}$ and crack speed were much less than values predicted from an analysis using the assumption of a velocity-independent fracture toughness.Load line displacements and specimen energy were also calculated in the present analysis. This indicated that there were repeated exchanges between potential and kinetic energy during crack growth. These data taken together with those obtained by previous workers allow a more complete understanding of the dynamic behaviour of the crack arrest specimen geometry.     

Effect of hydrogen in advanced high strength steel materials

The effect of hydrogen in AHSS material (automobile and structural component) was discussed. Dual Phase steels were highly susceptible to hydrogen-related failure when working on hydrogen environment. The influence of hydrogen on TRIP steel was seen during fractographic examination where the brittle transgranular fracture was presented. TWIP steels results were inconsistent. The mechanisms which were responsible for crack growth are discussed. LIST and SSRT testing were performed for mechanical properties evaluation and SEM and TEM were used for microstructural examination of fractured samples. Simultaneous preventing methods to reduce hydrogen embrittlement such as coating, alloying and providing diffusion layer were discussed.     

Analysis of crack arrest event in NESC-1 spinning cylinder experiment

In the NESC-1 spinning cylinder test, a large surface-breaking flaw in a thick steel cylinder component was subjected to high primary and secondary stresses produced by combined rotation and thermal shock loading. The crack was arrested after relatively small amounts of ductile tearing and cleavage crack extension. Finite element analyses have been carried out to obtain static elastic stress intensity factors for the initial and arrested crack under constant load and constant displacement boundary conditions. Applied static elastic stress intensity factors for the arrested crack have been compared with the plane strain crack arrest toughness values measured using small-scale compact crack arrest (CCA) specimens. The present analyses of the crack arrest event in the NESC-1 spinning cylinder test have concluded: (1) Applied static elastic stress intensity factors are reduced significantly for the lobe-shaped arrested crack which developed from the initial semi-elliptical surface crack as a result of the localised cleavage crack propagation. This reduction in crack driving force is likely to be the main reason for crack arrest. (2) The analysis carried out and comparison with the full-scale experiment confirm the prevailing approach to the assessment of crack arrest that brittle propagation will stop if the applied crack driving force falls below the crack arrest toughness. (3) The results justify the use of the static elastic stress intensity factor as the crack propagation driving force parameter and the static plane strain crack arrest toughness as the resistance parameter for crack arrest evaluation for small relative crack jump dimensions. (4) The small-scale CCA tests can be employed to evaluate crack arrest in a large cylinder of the same material.     

Modeling for ductile-to-brittle transition under ductile crack growth for reactor pressure vessel steels

The local stress–strain state (SSS) near the crack tip is investigated by the finite element method in the finite strain statement (with regard to a change of the crack tip blunting) for both stationary cracks and crack growing by a ductile mechanism. Using the revealed particularities of SSS near the stationary and growing crack tips and the local cleavage fracture criterion the phenomenon of the ductile-to-brittle transition is explained for reactor pressure vessel steels. The model is proposed to predict the amount of ductile crack extension preceding the ductile-to-brittle transition as a function of the test temperature. The procedure for calculation of the cleavage fracture toughness is also elaborated with regard to ductile crack extensions. Analysis of the obtained calculated results and available experimental data is made. Alternative approaches for the interpretation of the ductile-to-brittle transition are discussed.     

The reasonable design of Charpy-size specimen for fracture toughness test in nuclear surveillance

Use of a precracked and side-grooved Charpy-size specimen is an economical and convenient single-specimen method of evaluating the elastic-plastic fracture toughness of nuclear pressure vessel steels. This paper has studied the influence of side-groove depth on fracture toughness and stable increment of crack on several pressure vessel steels in detail. Test results are compared with those of large-size specimens which the National Standard of China (i.e. GB 2038) requires. The research results indicate that using a precracked Charpy-size specimen with side-groove depth 30% and adopting the energy before the maximum load of a three-point bending test curve, the elastic-plastic fracture toughness of materials can be evaluated conservatively when the crack begins to propagate, and the reasonable design of a Charpy-size specimen for fracture toughness tests in nuclear surveillance has been established. At the same time, the constraint effect and thickening action of the side-groove on a Charpy-size specimen have also been discussed, and theoretically explained.     

Fracture initiation in a thermoelastic solid with relaxation: Transient three-dimensional analysis

Transient, 3D growth of a plane, brittle crack in an isotropic, thermoelastic solid is considered. The solid is initially at rest at uniform (absolute) temperature and contains a semi-infinite, plane crack. Growth is caused by the application of in-plane and normal point forces to each face of the crack. The related problem of displacement discontinuities that exist on regions that exhibit dynamic similarity is first considered. An exact solution in integral transform space leads to asymptotic forms, valid for short times. These are used to generate equations of the Wiener–Hopf type for the fracture problem. Analytic solution expressions are obtained and, upon inversion, subjected to a dynamic energy release rate criterion that includes kinetic energy. Results show that a particular form of rapid growth in time of the forces can, in the crack initiation phase, cause crack growth rates that vary with position, but not with time.