Crack Tip Opening Displacement Tests Research Articles

A new understanding of hydrogen-assisted cracking of coarse-grained heat-affected zone in X65 pipeline steel through {110} lath bainite boundary texture

The susceptibility of subzones of the coarse-grained heat-affected zone (CGHAZ) in X65 pipeline steel to hydrogen embrittlement was studied through an in situ crack-tip opening displacement test in an H2S-containing environment. The intercritically reheated CGHAZ (IC-CGHAZ) exhibited the highest embrittlement factor of 89.4%, compared to 64.0% for the sub-critically reheated CGHAZ (SC-CGHAZ). Hydrogen transportation by dislocations to massive-slender martensite/austenite (M/A) constituents initiated cracks in IC-CGHAZ. Deterioration of lath bainite (LB) boundaries, with separation of M/A-ferrite interface, accelerated crack propagation across prior austenite grains and promoted hydrogen-assisted degradation of fracture toughness. Reheated CGHAZ at a peak temperature of 620 °C led to an increased intensity of the {110} LB boundary texture by up to 52% in SC-CGHAZ. This enhanced texture facilitated dislocation slipping along {110} LB boundary planes, thereby promoting deformation compatibility and preventing the deterioration of LB boundaries in the presence of hydrogen.

Liquid Hydrogen Properties Affecting Fracture Toughness

In this study, fracture toughness with the base metal of 9% Ni steel, a cryogenic steel, was evaluated under conditions without hydrogen charging (WO-H) and with hydrogen charging (W-H). Hydrogen charging was performed using an electrochemical cathodic charging method in a electrolyte of 3% NaCl + 0.3% NH4SCN at 19℃ with a current density of 50 A/m². Fracture toughness was assessed at -80℃, -100℃, -130℃, and -160℃ using CTOD (Crack Tip Opening Displacement) tests. The results for WO-H indicated a decrease in fracture toughness values with decreasing temperature, while the results for W-H showed an increase in fracture toughness values as the temperature decreased. In addition, the fracture surfaces and fracture toughness of WO-H and W-H became increasingly similar as the temperature decreased, as observed through scanning electron microscopy (SEM). At -80℃, there was a significant difference in fracture toughness between the WO-H and W-H conditions. WO-H was influenced only by the low temperature, whereas W-H was affected by both low temperature and hydrogen, showing combined effects that led to a decrease in fracture toughness due to hydrogen embrittlement. However, at -160℃, the fracture toughness values for both WO-H and W-H conditions were nearly identical. This suggests that the temperature effect on fracture toughness reduction is greater than hydrogen embrittlement at very low temperatures.

An in-depth investigation into the implications of alleviating banded microstructures on crack-tip opening displacement test of API 5LX60 linepipe steel plate

The present study seeks to fill the research gap regarding the correlation between crack-tip opening displacement (CTOD) and banded microstructures of high-strength low-alloy steel. For this purpose, a distinct casting procedure and thermo-mechanical controlled process route were implemented to produce an API 5LX60 MS linepipe steel plate free of banded microstructures and suitable for sour environments. According to the results of optical microscopy, the volume fraction of quasi-polygonal ferrite increased by 59% and the average grain size of the microstructure declined, whereas the volume fraction and size of pearlite constitute decreased compared to the API 5LX60 M linepipe steel plate, which is suitable for sweet environments. As opposed to the API 5LX60 M steel plate, no evidence of banded microstructures was observed, and in the CTOD test, the fracture was ductile with a surface exhibiting a higher percentage of side grooves. Moreover, no secondary cracks or river patterns were observed in the secondary electron imaging of the API 5LX60 MS steel plate. The CTOD and KIC fracture toughness increased from 1.32 mm to 1.73 mm and from 54.01 MPa m0.5–58.19 MPa m0.5, respectively. Moreover, the force vs. notch-opening displacement curves of both API 5LX60 M and API 5LX60 MS steel plates showed stable crack growth, with API 5LX60 MS having a higher plastic deformation prior to the final fracture. The rational correlation between the microstructure and CTOD test results of API 5LX60 MS steel plate approved improvements in this plate.

Effect of Vanadium and Rare Earth on the Structure, Phase Transformation Kinetics and Mechanical Properties of Carbide-Free Bainitic Steel Containing Silicon

Carbide-free bainitic (CFB) steels with a matrix of bainitic ferrite and thin layers of retained austenite, to reduce the manufacturing costs, usually do not contain alloying elements. However, a few reports were presented regarding the effect of alloying elements on the properties of these steels. Thus, this study evaluates the effects of vanadium and rare earth (Ce-La) microalloying elements on the structure, phase transformation kinetics, and mechanical properties of carbide-free bainite steel containing silicon fabricated by the casting and austempering procedure. Optical and scanning electron microscopy (OM and SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were used to study the microstructure and phase structure. The transformation kinetics were examined by a dilatometry test. Hardness, tensile, and impact tests evaluated the mechanical properties. Due to adding alloying elements, the fracture toughness and change in matrix phases relation was studied by the crack tip opening displacement (CTOD) test and SEM fractography. The microstructure of the silicon added sample was completely carbide-free bainite. The test results showed vanadium helped CFB formation, even in continuous cooling. The primary austenite grain (PAG) size grew by vanadium addition. The EBSD phase map illustrates an increment in the percentage of retained austenite by vanadium. In contrast, the addition of 0.03 wt% rare earth reduced the primary austenite grain size and reduced the retained austenite content. The results of the dilatometry test confirmed that vanadium and rare earth addition both reduced the critical cooling rate of the bainite transformation. Vanadium leads to an earlier cessation of bainite transformation, while rare earth elements postpone this transformation. Mechanical tests showed that the tensile strength of carbide-free bainite steels was strongly influenced by the morphology and volume fraction of austenite. Retained austenite, when transformed to martensite during the transformation-induced plasticity (TRIP) phenomenon, leads to increased tensile strength and fracture toughness, or retained austenite with a film-like shape prevents the growth of cracks by blinding the crack tip. The result of the CTOD test exhibited that retained austenite plays the leading role in increasing crack resistance when TRIP occurs.

Simplified Prediction Method of Stress Intensity Factor in Mid-thick Plane in 3D Cracked Body and Its Difference from 2D Handbook Formula

Abstract The authors have been working to improve the crack tip opening displacement (CTOD) evaluation standard as a fracture parameter for the cleavage-type brittle fracture critical condition of carbon steel. In 2016, WES1108, Standard Test Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurement, which is the CTOD test standard in Japan, was revised, and subsequently, the CTOD calculation formula of ISO 15653:2010, Metallic Materials – Method of Test for the Determination of Quasistatic Fracture Toughness of Welds, was revised in 2018, focusing on the study of the formula that constitutes the basis of the standard. Similar to the original British Standards equation, the proposed CTOD calculation formula consists of a small-scale yield (SSY) term and a plastic term, but in the SSY term, the two-dimensional (2D) stress intensity factor (SIF) is used. The authors focused on the incorrectness of this treatment derived from the difference between 2D and three-dimensional (3D). Here, two accurate relationships have been explored through systematic finite element method analyses; one is the correlation of the 3D SIF values in the mid-thickness plane and 2D SIF, and the second is the relationship between the actual 3D SIF in mid-thickness and the CTOD in mid-thickness. The KI ratio (K3D/K2D) sharply coincides with the value 1.11, as inferred in several previous pieces of research in the idealistic infinite plate with a crack. However, if the ligament size is finite as in the actual fracture toughness test, the KI ratio drops significantly from 1.11, and in some cases, the KI ratio is less than 1. By unifying these findings, a new precise CTOD formula has been established. Lastly, the study inferred that the error of the calculation formula is improved, especially in small thickness regions, as compared with the current standard proposed in 2016 and 2018 by the authors.

Effect of residual stress modification technique on CTOD test results in heavy thick welded joints

ABSTRACT In order to investigate the effects of residual stress modification on the results of crack-tip opening displacement (CTOD) tests in heavy thick welded joints, experiments using a submerged-arc welding (SAW) joint of an EH47 steel plate with the thickness of 70 mm and residual stress analyses by the finite element method (FEM) were performed. It was confirmed that appropriate reversed bend conditions improved fatigue precrack front shapes, and the low ratio of the fatigue precrack length to the size of the compressive plastic zone induced by reversed bending contributed to the fatigue crack shape improvement. Although the local compression with a large diameter platen was effective in improving the fatigue precrack front shape, small diameter platens were not effective even if they applied to wide ranges of specimens. In addition, specimens treated by the reversed bending showed higher critical CTOD values than those treated by local compression. According to the FEM analysis, the reversed bending introduced tensile residual stress at the mechanical notch root, although the initial residual stress distribution of the welded joint still remained a little away from the notch root. On the other hand, the initial residual stress was almost removed by the local compression with a large diameter platen. It was presumed that the initial residual stress distribution affected fatigue precrack front shapes and the critical CTOD in welded joints. The reversed bending is very useful if appropriate conditions can be used.

Experimental and Numerical Investigation on Specimen Geometry Effect on the CTOD Value For VL-E36 Shipbuilding Steel

Abstract There are special cases in the marine industry, where additional material tests, such as the fracture toughness test, must be performed. Additional fracture toughness tests, such as CTOD (Crack Tip Opening Displacement), are typically performed on three-point bend specimens. The dimension that defines all the specimen dimensions is the thickness of the material to be tested. It is recommended by classification societies (e.g. DNVGL) to test specimens that are twice as high as the material thickness. The width determines the length and, therefore, the weight of the specimen which, for a 100 mm plate is over 140 kg. Current ASTM E1820, BS7448-1 and ISO 12135 testing standards also allow for proportions other than those recommended. This results in a much smaller test piece. Reducing the specimen size allows the testing machine to achieve lower forces than a specimen with a width to thickness ratio of two. This paper presents the effect of changing the specimen geometry on CTOD test results. Research was performed for specimens with a height to thickness ratio of one and two. Abaqus software was used for numerical calculations. The numerical results were, at selected points, verified experimentally.

Microstructure and Mechanical Properties of Nb-API X70 Low Carbon Steel

The oil and gas production has grown steadily, and the care with its transportation grid needs to be maintained and expanded. The pipeline used in the transport of gas and oil by-product requires materials with high mechanical strength, toughness, and fatigue resistance. The chemical composition and distribution, amount, and morphology of the final microconstituents of the steel are of paramount importance once it will directly influence the mechanical properties. The fracture toughness is one of the most important mechanical properties for the suitability of the steel for pipeline usage, and is most of the literature with pipelines study does not present tests such as crack tip opening displacement (CTOD), with a steady and controlled crack growth behavior, to evaluate its fracture toughness. This study compares the fracture toughness of the API 5L X70 steel between two plates of steel with different Nb content, 0.06 and 0.09 Nb wt.% by CTOD tests. The mechanical response was correlated to the grain size and distribution. The CTOD parameter was used to assess the fracture toughness at 25 °C and − 60 °C. The microstructures and microconstituents of the two plates of steel were similar. The matrices were composed of ferrite, with bands of secondary products, including degenerated pearlite. Results showed a prone to the formation of a bimodal microstructure, coarse and fine grains, due to the addition of Nb. High-Nb steel presented better fracture toughness than normal-Nb steel at low-temperature.

Test Research on Toughness Evaluation of Long-span Bridge Welded Joint

Aiming at the inherent unavoidable defects of welded joints, a CTOD (Crack Tip Opening Displacement) test was carried out on the E43 steel weld specimen under the fatigue state. The research results provide experimental and calculation basis for the use of bridge steel. In this paper, a CTOD fatigue pre-crack test study was carried out on the welded joint of E43 steel.

Fracture toughness assessment at different regions in an inertial friction welded Ti-5Al-2Sn-2Zr-4Mo-4Cr alloy plate

Abstract The study aims to ascertain the influences of position on fracture toughness and fracture mechanism of inertial friction welded Ti-5Al-2Sn-2Zr-4Mo-4Cr joints. The room-temperature fracture toughness values of the parent material and three other regions in the weld were evaluated by standard crack tip opening displacement tests. The micro-structure and tensile properties of the welds were also investigated. Based on the observation of fracture surface and crack propagation path, a schematic illustration of the crack propagation was formed. The results suggest that the weld metal had the worst fracture toughness. The individual fracture toughness of different regions in the weld could be explained by the various modes of crack propagation, which were affected by different microstructures.

The investigation of fatigue growth and CTOD test of structure steel

Due to the inherent and unavoidable defects of the steel structure of the bridge, CTOD (crack tip opening displacement) tests were carried out on E43 steel specimen under a condition of fatigue. The results provide an experimental base and computing foundations for the bridge steel-using. In this paper, CTOD fatigue pre-cracking tests for E43 steel was studied.

Investigation of Hydrogen Embrittlement Susceptibility and Fracture Toughness Drop after in situ Hydrogen Cathodic Charging for an X65 Pipeline Steel.

The present research focuses on the investigation of an in situ hydrogen charging effect during Crack Tip Opening Displacement testing (CTOD) on the fracture toughness properties of X65 pipeline steel. This grade of steel belongs to the broader category of High Strength Low Alloy Steels (HSLA), and its microstructure consists of equiaxed ferritic and bainitic grains with a low volume fraction of degenerated pearlite islands. The studied X65 steel specimens were extracted from pipes with 19.15 mm wall thickness. The fracture toughness parameters were determined after imposing the fatigue pre-cracked specimens on air, on a specific electrolytic cell under a slow strain rate bending loading (according to ASTM G147-98, BS7448, and ISO12135 standards). Concerning the results of this study, in the first phase the hydrogen cations’ penetration depth, the diffusion coefficient of molecular and atomic hydrogen, and the surficial density of blisters were determined. Next, the characteristic parameters related to fracture toughness (such as J, KQ, CTODel, CTODpl) were calculated by the aid of the Force-Crack Mouth Open Displacement curves and the relevant analytical equations.

Additive Manufacturing with Superduplex Stainless Steel Wire by CMT Process

For many years, the oil and gas industry has utilized superduplex stainless steels due to their high strength and excellent corrosion resistance. Wire arc additive manufacturing (WAAM) was used with superduplex filler wire to create walls with different heat input. Due to the multiple heating and cooling cycles during layer deposition, brittle secondary phases may form such as intermetallic sigma (σ) phase. By inspecting deposited walls within wide range of heat inputs (0.40–0.87 kJ/mm), no intermetallic phases formed due to low inter-pass temperatures used, together with the high Ni content in the applied wire. Lower mechanical properties were observed with high heat inputs due to low ferrite volume fraction, precipitation of Cr nitrides and formation of secondary austenite. The walls showed good toughness values based on both Charpy V-notch and CTOD (crack tip opening displacement) testing.

Multivariate Analysis to Relate CTOD Values with Material Properties in Steel Welded Joints for the Offshore Wind Power Industry

The increasingly mechanical requirements of offshore structures have established the relevance of fracture mechanics-based quality control in welded joints. For this purpose, crack tip opening displacement (CTOD) at a given distance from the crack tip has been considered one of the most suited parameters for modeling and control of crack growth, and it is broadly used at the industrial level. We have modeled, through multivariate analysis techniques, the relationships among CTOD values and other material properties (such as hardness, chemical composition, toughness, and microstructural morphology) in high-thickness offshore steel welded joints. In order to create this model, hundreds of tests were done on 72 real samples, which were welded with a wide range of real industrial parameters. The obtained results were processed and evaluated with different multivariate techniques, and we established the significance of all the chosen explanatory variables and the good predictive capability of the CTOD tests within the limits of the experimental variation. By establishing the use of this model, significant savings can be achieved in the manufacturing of wind generators, as CTOD tests are more expensive and complex than the proposed alternatives. Additionally, this model allows for some technical conclusions.

Effects of heat input on microstructure and fracture toughness of simulated coarse-grained heat affected zone for HSLA steels

In order to clarify the relationships among welding heat input, microstructural evolution and fracture toughness of coarse-grained heat affected zone (CGHAZ) for HSLA steels, the microstructure of simulated CGHAZ with heat input of 10–100 kJ/cm was quantified and the crack tip opening displacement (CTOD) at −20 °C was evaluate. With the increase in heat input, the bainite transformation temperature and duration transformation time increase, which lead to the higher contents of granular bainite and M-A constituents. The fracture toughness of simulated CGHAZ is highly related with the morphologies of M-A constitute. During CTOD test, the island M-A constituents can arrest microcracks and increase the CTOD value, and the fine-stringer ones can deform without excessive damage to fracture toughness. However, the coarse-stringer and massive M-A constituent would crack and debond from the matrix, which seriously decreases the fracture toughness. When the heat input increases from 10 kJ/cm to 100 kJ/cm, the proportion of island M-A constituent decreases from 86% to 35%, while that of coarse-stringer and massive M-A constituent increases from 1% and 6% to 7% and 57%, respectively, which result in the significant deterioration of fracture toughness. The fracture toughness is optimal at heat input of 30 kJ/cm, because the packets of lath bainite are segmented smaller by granular bainite and effectively improved the cracking resistance.

Fracture Assessment of Weld Joints of High-Strength Steel in Pre-Strained Condition

Unstable fractures tend to occur after ductile crack initiation or propagation. In most collapsed steel structures, a maximum 15% pre-strain was recorded, at the steel structural connections, during the great earthquake of 1995, in Japan. Almost-unstable fractures were observed in the beam-to-column connections, where geometrical discontinuities existed. Structural collapse and unstable failure occurred after large-scale plastic deformations. Ship structures can also suffer from unstable fractures in the welded joints. The fracture resistance of butt-welded joints subjected to tension in the pre-strained condition was estimated by considering the toughness deterioration, due to pre-strain and toughness correction for constraint loss in a tension specimen. The target specimen for this fracture assessment was a double-edged, through-thickness crack panel, with a crack in the weld joint (heat-affected zone (HAZ)). The critical fracture toughness value (crack tip opening displacement (CTOD)) of a large structure with pre-strain, which was applied to the HAZ region, was estimated from a small-scale, pre-stained, three-point bend specimen. Fracture toughness values, evaluated by a CTOD test, were recently mandated for shipbuilding steel plates. The critical fracture toughness value is a very useful parameter to evaluate the safety of huge ship structures.

The Relationship of Fatigue and CTOD about E43 Steel

Because of the inevitable defects inherently exist in the bridge steel structure, the steel E43 specimen was under fatigue and CTOD (Crack tip opening displacement) test. The results provide an experimental base and computing foundations for the bridge steel-using. This paper studies the CTOD fatigue precracking experiment on steel E43.

The Analysis of Low Temperature Toughness on X80 Pipeline Steel Welded Joint

Aiming at the security problems of pipeline steel application, the different positions of the welded joints of circumferentially welding pipeline of X80 steel were investigated by microstructure observation, the hardness, Charpy impact toughness and crack tip opening displacement (CTOD) test at low temperature. The Vickers hardness test results show that there are local softened regions in heat-affected zone (HAZ). Charpy impact test indicate that the ductile–brittle transition temperature of weld is below − 60 °C, the ductile–brittle transition temperature of HAZ is around − 38 °C. CTOD test reveal that the fracture toughness of HAZ shows a large fluctuation since it is in the ductile–brittle transition temperature regime.

Redistribution of Welding Residual Stresses of Crack Tip Opening Displacement Specimen by Local Compression.

The demand of crack tip opening displacement (CTOD) test which evaluates fracture toughness of a cracked material is very important to ensure the stability of structure under severe service environment. The validity of the CTOD test result is judged using several criterions of the specification standards. One of them is the artificially generated fatigue pre-crack length inside the specimen. For acceptable CTOD test results, fatigue pre-crack must have a reasonable sharp crack front. The propagation of fatigue crack started from the tip of the machined notch, which might have propagated irregularly due to residual stress field. To overcome this problem, test codes suggest local compression method, reversed bending method and stepwise high-R ratio method to reduce the disparity of residual stress distribution inside the specimen. In this paper, the relation between the degree of local compression and distribution of welding residual stress has been analyzed by finite element analyses in order to determine the amount of effective local compression of the test piece. Analysis results show that initial welding residual stress is dramatically varied three-dimensionally while cutting, notch machining and local compressing due to the change of internal restraint force. From the simulation result, the authors find that there is an optimum amount of local compression to modify regularly for generating fatigue pre-crack propagation. In the case of 0.5% compressions of the model width is the most effective for uniforming residual stress distribution.

On the fracture toughness of bulk metallic glasses under Berkovich nanoindentation

Measurements of fracture toughness from the crack tip opening displacement (CTOD) testing methods are complex and paramount important for a range of bulk metallic glasses (BMGs). In this work, we attempted to propose an indentation protocol to evaluate indentation fracture toughness based on two key considerations: First, the plastic deformation induced damage to the characteristic fracture initiation point calculated from integrating the indentation load-displacement curves is obtained and used to estimate indentation fracture toughness. Second, the continuum damage mechanics theory has been adopted to identify the characteristic fracture initiation point. To verify the applicability of the suggested approach, fracture toughness values of BMGs were estimated and compared with those from CTOD tests in the literature. The estimated fracture toughness values KC obtained from the indentation tests agreed well with those KQ from the CTOD tests within an approximately relative difference of 10%. We also found that KQ linearly correlates with KC (KQ=1.1KC−3.7). The value of KQ in Al-based BMG is predicted to be ~35MPa√m.