Crack Tip Opening Displacement Values Research Articles

Study on crack propagation behaviors of three-point bending concrete beams with small span-depth ratios

Three-point bending (TPB) beams with small span-depth ratios, i.e. TPB beams with the same span of 400 mm but various depths (100 mm, 150 mm and 200 mm), were tested to study the crack propagation behaviors of concrete. The evaluations of crack extension resistance curve (KR-curve), fracture process zone (FPZ) length, lFPZ, and crack tip opening displacement (CTOD) were studied. The digital image correlation (DIC) technique was utilized to monitor the entire loading process. Moreover, the effects of strain rates (10−5/s, 10−4/s, 10−3/s and 10−2/s) on the fracture process of TPB beams with small span-depth ratios (2, 2.5 and 3) were discussed based on findings in literature. The results shown that (1) with the increase of specimen depth from 100 mm to 200 mm, the obtained KR-curves and initial fracture toughness were not influenced by the small span-depth ratios, while different increase trends were observed for the unstable fracture toughness, the average maximum value of lFPZ and the value of CTOD. (2) With the loading rate increasing from 10−5/s to 10−2/s, the obtained KR-curve and the maximum length of FPZ increased slightly, while the evolution of CTOD was not affected by the loading rate.

A prediction model of brittle crack arrest toughness in TMCP heavy plates

Micro-alloyed low carbon MnCrNiCuMo steels were proposed to produce heavy plates with a maximum thickness of 100 mm. An integrated industrial route including a thermo-mechanical control process (TMCP) based on a study on continuous cooling transformation was applied in industrial production. Comprehensive mechanical properties tests including the measurement on resistance against crack initiation using crack tip opening displacement (CTOD) approach were conducted. Large scale wide-plate temperature gradient double tension (DT) technique was employed to evaluate the crack arrest toughness Kca in the heavy plates under different conditions. Nil-ductility transition temperature (NDTT) was also tested across the entire thickness section of the plates. The results showed that heavy steel plates were qualified being brittle crack arrest (BCA) products at EH47 grade with yield strength high than 460 MPa. High crack initiation toughness was exhibited by both Charpy-V notch impact property and CTOD values. Crack arrest toughness at −10 °C greater than 253 MPa √m was achieved in a plate even with thickness of 100 mm. The mean value of crack arrest toughness at different temperature measured by the DT approach can be indexed by the highest NDTT in the interior section of the heavy plates leading to an empirical prediction model. Further, fracture mechanics based master curve approach was used to statistically predict crack arrest toughness distribution over a wide range of temperatures showing improved predictability over the empirical model and an existing model in the literature.

Crack-Tip Opening Displacement of Girth Welds in a Lean X70 Pipeline Steel.

Crack-tip opening displacement (CTOD) tests were conducted on girth welds of two API 5L X70 pipeline steels (pipe A and pipe B) to investigate the influence of base metal composition on the fracture toughness of the joint. CTOD measurements across the weld showed that the weld fusion zone had the lowest CTOD values for both pipes, with pipe B having a higher CTOD value than pipe A. Detailed microstructure characterization of the multi-pass weld showed that the fusion zone in both pipes consisted of three distinct zones: the columnar zone, the coarse equiaxed zone, and the fine equiaxed zone. Both the columnar zone and coarse-grained equiaxed zone had acicular ferrite and grain boundary ferrite microstructures, whereas the fine-grained equiaxed zone had a finer ferrite microstructure compared to the other two zones. The main difference between the two pipes was the variation in ferrite grain sizes and the volume fractions of grain boundary ferrite and acicular ferrite. In comparison to pipe B, pipe A, with a higher concentration of Mo, Ni, and Cu in both the base metal and the weld fusion zones, consisted of a higher volume fraction of grain boundary ferrite and a lower volume fraction of acicular ferrite in the columnar and coarse-grained equiaxed zones. The lower concentration of Mo, Ni, and Cu in pipe B likely resulted in the formation of a predominantly acicular ferrite microstructure in the fusion zone, thereby improving the toughness of the weld joint in comparison to pipe A.

Three-dimensional numerical study of thickness effect on plastic CTOD and monotonic plastic zone in an aluminium CT specimen

This work is a numerical study of CT specimens with straight cracks and different thickness values submitted to different load levels, which aims to analyse the shape and dimension variations of the monotonic yielded area near the crack tip, examining the impact of variables such as load magnitude, thickness of the specimen and position along the crack front. Additionally, it intends to explore the correlation between the size of the monotonic plastic zone and the plastic CTOD (Crack Tip Opening Displacement) to determine if a well-defined relationship exists. Furthermore, the impact of specimen thickness and load level on plastic CTOD will be explored. A complex plastic zone was found along the crack front, with the maximum area positioned at some distance from the crack front. A plane stress scale was determined considering the size of the equivalent bi-dimensional yielded area. When examining the behaviour across the thickness, no conclusive correlation was identified between the plastic CTOD and the monotonic yielded area. This suggests that utilising this parameter may not be appropriate for studies related to fatigue crack growth. However, when considering average values, a good correlation was found. The increase in thickness decreased the plastic CTOD values at the surface, while the interior values were unaffected.

Effect of microstructure heterogeneity on the cryogenic fracture toughness of the dissimilar joint between SA645 and 304L made by laser welding

The cryogenic fracture toughness of the SA645/304L dissimilar weld, produced using continuous wave IPG fiber laser welding with a 0.3 mm beam offset towards the 304L side, and the microstructural effects on the fracture behavior were systematically investigated. The weld metal consists of ∼89% columnar dendritic martensite and ∼11% retained austenite (RA), where the ununiform distribution of constituent phases as well as the distinctions between martensite and RA in terms of morphology and mechanical properties lead to the microstructure heterogeneity of weld metal. Pop-in phenomenon appears on the Force-Displacement (F-V) curve during the crack tip opening displacement (CTOD) test for the weld. The CTOD value for the weld is ∼0.057 mm, about 8 times less than that when pop-in effect is ignored (∼0.563 mm). Rapid propagation of the pre-fatigue crack tip along the center of the weld leads to the pop-in phenomenon. Fracture surface in pre-fatigue crack tip (PCT) region shows quasi-cleavage features, while fracture surface in stable crack propagation (SCP) region presents a ductile-fractured surface with dimples. The crack propagation path in SCP region is frequently deflected. Inhibition effect of grain boundaries on crack propagation and TRIP effect of retained austenite (RA) result in the improvement in fracture toughness of the SCP region.

The effect of mechanical inhomogeneity in microzones of welded joints on CTOD fracture toughness of nuclear thick-walled steel

This study employs the microshear test method to examine the local mechanical properties of narrow-gap welded joints, revealing the mechanical inhomogeneity by evaluating the microshear strength, stress‒strain curves, and failure strain. On this basis, the influence of weld joints micromechanical inhomogeneity on the crack tip opening displacement (CTOD) fracture toughness is investigated. From the root weld layer to the cover weld layer, the fracture toughness at the center of the weld seam demonstrates an increasing trend, with the experimental and calculated CTOD values showing a good correspondence. The microproperties of the welded joints significantly impact the load-bearing capacity and fracture toughness. During the deformation process of the “low-matching” microregions, the plastic zone expansion is hindered by the surrounding microregion strength constraints, thus reducing the fracture toughness. In contrast, during the deformation of the “high-matching” microregions, the surrounding microregions absorb some of the loading energy, partially releasing the concentrated stress at the crack tip, which in turn increases the fracture toughness.

Novel Characterizations of Effective SIFs and Fatigue Crack Propagation Rate of Welded Rail Steel Using DIC

The fatigue crack growth-rate test of rail head, waist, and bottom material for U71Mn welded rail was carried out. Digital image correlation (DIC) was used to capture the full-field displacement. The crack-tip position was accurately obtained based on the full-field displacement data, and an accurate crack-tip opening displacement (CTOD) measurement point was found. The CTOD values of the welded rail head under overloaded and unloaded condition were extracted, and the area size of elastic CTOD and plastic CTOD was obtained. According to COD data under different experimental conditions, the corresponding crack opening force was extracted, the crack opening function introduced based on the Elber model, and a calculation method of effective stress-intensity factors (SIFs) considering the plasticity-induced crack closure proposed. The results in this paper provide some references for accurately assessing the fatigue life of welded rail.

Comparison of Fracture Toughness in the Coarse-Grain Heat-Affected Zone of X80 Pipelines Girth-Welded under Conventional and Ultra-Low Heat Input.

The coarse-grain heat-affected zones (CGHAZs) of X80 girth-welded steel pipelines are prone to embrittlement, which has an extremely adverse effect on their structural integrity. In the present work, the fracture behavior of the CGHAZs of X80 girth welds under the conditions of conventional and ultra-low heat input was studied. The fracture toughness of CGHAZs was evaluated using the crack tip opening displacement (CTOD) test at -10 °C, and the fracture behavior mechanism of CGHAZs were clarified by analyzing microstructural characteristics at prefabricated fatigue cracks containing fracture cloud image, scanning electron microscopy (SEM), and electron back-scatter diffraction (EBSD) figures. The results illustrate that the average fracture toughness (CTOD) value of the ultra-low heat input CGHAZ is 0.6 mm, and the dispersion of CTOD values is small, while the CTOD value of conventional heat input is only 0.04 mm. The ultra-low heat input makes the high-temperature residence time of the coarse-grained region short, reduces the proportion of prior austenite grain boundaries, and inhibits the formation of strip-like bainite and island-like M-A components. The reduction of these deleterious ductile microstructures increases the plastic reserve and deformation capacity of the CGHAZ.

Cleavage fracture micromechanisms in thick-section quenched and tempered S690 high-strength steels

For structural assessment and optimal design of thick-section high-strength steels in applications under harsh service conditions, it is essential to understand the cleavage fracture micromechanisms. In this study, we assess the effects of through-thickness microstructure of an 80-mm-thick quenched and tempered S690 high-strength steel, notch orientation, and crack tip constraint in cleavage nucleation and propagation via sub-sized crack tip opening displacement (CTOD) testing at −100 °C. The notch was placed parallel and perpendicular to the rolling direction, and the crack tip constraint was analysed by varying the a/W ratio: 0.5, 0.25, and 0.1. The notch orientation does not play a role, and the material is considered isotropic in-plane. Nb-rich inclusions were observed to act as the weak microstructural link in the steel, triggering fracture in specimens with the lowest CTOD values. While shallow-cracked specimens from the top section present larger critical CTOD values than deep-cracked ones due to stress relief ahead of the crack tip, the constraint does not have a significant influence in the middle due to the very detrimental microstructure in the presence of Nb-rich inclusions. Some specimens show areas of intergranular fracture due to the combined effect of C, Cr, Mn, Ni, and P segregation along with precipitation of Nb-rich inclusions clusters on the grain boundaries. Several crack deflections at high-angle grain boundaries were observed where the neighbouring sub-structure has different Bain axes.

Effect of Post-Weld Heat Treatment on Microstructure and Fracture Toughness of X80 Pipeline Steel Welded Joint.

In the current study, post-weld heat treatment (PWHT 580 °C) was used for an X80 pipeline steel-welded joint, and the fracture toughness of the welded joint was investigated using a crack tip opening displacement (CTOD) test. The relationship between microstructure evolution and fracture toughness is also discussed in this study. The results showed that the weld center mainly consisted of acicular ferrite (AF). The subcritical heat-affected zone (SCHAZ) consisted of a large amount of fine polygonal ferrite and some AF, and it maintained the rolling state of the base metal. The microstructure of the coarse-grained heat-affected zone (CGHAZ) was composed of granular bainite (GB) and M/A constituents, the latter of which decreased after the PWHT. The CTOD values of the weld center were in the range of 0.18–0.27 mm, while those of the CGHAZ were in the range of 0.02–0.65 mm. A brittle fracture occurred in the CGHAZ for both the as-welded and PWHT samples; the CTOD values were 0.042 mm and 0.026 mm, respectively. The CTOD values of the SCHAZ’s location were in the range of 0.8–0.9 mm. The PWHT did not deteriorate the microstructure of the CGHAZ and had little influence on the fracture toughness of the X80 pipeline steel-welded joint; it ensured the fracture toughness of the welded joints and reduced the welding residual stress.

Upgrading the mechanical properties of marine structural steel by enhancement of acicular ferrite formation

AbstractThis work reports the results of an investigation of the effect of magnesium addition on microstructure evolution, acicular ferrite formation together with the change in the mechanical properties of micro‐alloyed steel plates. The investigated steel is subjected to high heat input (4.5 kJ mm−1) simulating the thermal cycle of heat affected zones, conducted by induction heating in a computer controlled weld thermal cycle simulator. Both magnesium and magnesium free steel samples have carbon and aluminum contents of 0.08 wt.% and 0.009 wt.%, respectively, are investigated. The change in microstructure, hardness and crack tip opening displacement fracture toughness properties due to magnesium addition is evaluated, and compared with the other steel having the same chemical compositions but without Magnesium. The fracture toughness tests reveal the increase of magnesium steel‘s crack tip opening displacement values. In contrast, the hardness measuring presents the decrease of its hardness due to the decrease of martensite volume fraction and reduction in ferrite grain size. Optical and transmission electron microscopy observations detect some microstructural morphology that can affect the mechanical properties of such steels. Hence, the improved toughness of magnesium containing steel are attributed to both the generation of acicular ferrite nucleated within the matrix, refined microstructure and the bainite dominated microstructure in steel.

Damage Assessment for Steel Structures Subjected to cyclic pre-strain

Plastic deformation provoked by unexpected events, such as earthquakes, can deteriorate the steel fracture toughness, and it can also induce premature failure of a structure and accidents. This study tries to formulate new equations to quantify the fracture toughness degradation of steel that was cyclic pre-strained. Four point bending tests were used to carry out the cyclic plastic pre-strain and Crack Tip Opening Displacement (CTOD) tests were conducted to compare pre-strained specimens with the steel without plastic deformation (as-received). The cumulative damage law was employed to account for the difference between each specimen, associated with Weibull stresses and critical CTOD values. Furthermore, additional Finite Element Method (FEM) simulations were conducted, using the new approach for the establishment an accurate Weibull Stress criterion, which can represent different cleavage fracture resistance according to the last pre-strain cycle. In the end, a new CTOD equation was also formulated using macro parameters, and it can account for the fracture toughness degradation of material that was subjected to cyclic plastic deformation.

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.

FRACTURE PROPERTIE ANALYSIS OF RECYCLED AGGREGATE CONCRETE BASED ON DIGITAL IMAGE CORRELATION TECHNIQUE

Three point bending fracture tests of recycled aggregate concrete beams with different recycled coarse aggregate replacement ratio (0%, 30%, 50%, 70%, 100%) have been carried out, with results analyzed by digital image correlation (DIC) method and the traditional electrometric method. The double-K fracture parameters, crack-mouth-opening displacement (CMOD), crack-tip-opening displacement (CTOD), crack extension length ( \begin{document}$\Delta a$\end{document} ) and fracture growth path were obtained, and the validity of DIC method was verified. The results show that when the replacement rate is 100%, the crack initiation fracture toughness and unstable fracture toughness of recycled aggregate concrete decrease by 15.81% and 15.39%, respectively. With the increase of the replacement rate of recycled aggregate, the slopes of the descending sections of P-CMOD, P-CTOD and P- \begin{document}$\Delta a$\end{document} curves increase. DIC method can effectively capture the fracture propagation process. The CMOD values measured by DIC method and clip type extenometer are in good agreement. The variation trends of DIC measured value and formula-calculated value of CTOD are relatively consistent. Based on the test results, the estimated values, \begin{document}$\Delta a$\end{document} , of specimens with different replacement rates of recycled aggregate under different loads can be obtained by fitting the curve between \begin{document}$P$\end{document} / \begin{document}${P_{\max }}$\end{document} and \begin{document}$\Delta a$\end{document} / \begin{document}$\Delta {a_{\text{c}}}$\end{document} .

Investigation of Correlation Between Fracture Toughness and Charpy Impact Energy of Cryogenic Steel Welds.

The high manganese steel was developed to improve the fracture toughness and safety at cryogenic temperatures, the austenite structure was formed by increasing the manganese (Mn) content. The developed weld high manganese steel was alloyed with austenite stabilizing elements (e.g., C, Mn, and Ni) for cryogenic toughness and fluxes contained less than 10% of acidic slag formers such as rutile (TiO₂) and silica (SiO₂). This paper describes the work carried out to enhance the fracture toughness of Mn contents in an economical way by means of increase of manganese up to 23% instead of using nickel (Ni) which has unique element to improve fracture toughness especially at cryogenic steel. The new cryogenic steels should be carefully evaluated in terms of safety for application in real structures including LNG ships. In this study, the fracture toughness performance was evaluated for recently developed cryogenic steels (high-Mn steels), especially the crack tip opening displacement (CTOD) parameter was evaluated using the prediction formula proposed by conventional equation. The CTOD value was investigated the effect of microstructure and mechanical properties of Fe-C-Mn and Fe-C-Mn-Ni high manganese steel, it was revealed that the e-martesnsite phase formed in high manganese steel of 0.2C-20Mn and 0.4C-20Mn as a result of a low stability of austenite upon strain-induced phase transformation.

Influence of the Welding Process on the Mechanical Characteristics and Fracture of the S700MC High Strength Steel under Various Types of Loading.

This paper focuses on the mechanical properties analysis of the high strength S700MC steel applied in welding joints. The research comprised mechanical tests for checking what the changes of tensile characteristics, mechanical parameters, resistance to impact, and fracture toughness look like in selected regions of the welding joint. Stress-strain curves have shown significant differences in the tensile characteristic shape and the values of Young’s modulus, yield stress, ultimate tensile strength, and ductility due to the welding process applied. In the case of Charpy tests, the courses of the accumulated energy, force, deflection, and project velocity are presented, indicating the maximum value of absorbed energy, the same level of force during the first contact of the projectile with the specimens, and the significant variation of the velocity for the impact energy ranging from 50 J up to 300 J. On the basis of the fracture toughness tests, the distributions of the CTOD (Crack Tip Opening Displacement) values are presented for the parent material, HAZ (Heat Affected Zone) and weld. Moreover, the characteristic features of the fatigue pre-crack, transient and crack propagation zones are identified and discussed.

Critical Ultimate Load and Ultimate Bearing Failure of CT110 with Initial Crack Defect

CT110 is ultra-high-strength coiled tubing (CT). It has been applied in the drilling, completion, fracturing and stimulation process of deep wells, ultra-deep wells and large displacement horizontal wells. Under complex load, the CT will crack and eventually failure. Fracture toughness of CT materials can be measured using Crack Tip Opening Displacement (CTOD). In this paper, a three-point bending finite element model (FEM) of CT110 material was established, and its critical CTOD value is obtained by calculation as 0.732 mm. The FEM of three-dimensional CT with initial crack was established and its feasibility was verified. The ultimate bearing failure analysis of CT with initial crack defects shows that under the same working condition, internal pressure accelerates the crack initiation and propagation speed, so that the crack of tubing body reaches the critical value in advance, and the maximum value at 130 MPa is close to the critical value. Under the combined loads, the maximum CTOD value of the CT with a crack increases with the increase in the crack depth and length, and the deeper the crack depth, the greater the influence of the crack length on its maximum value. This study on crack initiation and propagation of CT with defects provides a basis for predicting the fatigue life and revealing the failure mechanism of CT.

Determination of fracture toughness of polymer coating using micro-scale digital image correlation technique

This paper presents a comprehensive study on determining the interface adhesive strength of the polymer coating/substrate system to improve its durability. Interfacial stress intensity factors based on the crack tip opening displacement (CTOD) method were theoretically analysed and verified by the finite element simulation. Thereafter, an experimental setup was established with the micro-scale digital image correlation (DIC) method to measure the CTODs and the designed loading fixture for multi-angle loading. The apparent specimen rotation and out-of-plane displacement were significantly restrained in the test, and the measured displacement accuracy of the micro-scale DIC method was approximately 1/10 of the minimum CTOD value. Mixed-mode stress intensity factors and an interfacial failure criterion curve in terms of stress intensity factors were obtained. Furthermore, the test method was applied to evaluate the interfacial fracture toughness of nanoparticle-reinforced epoxy coatings. The experimental results demonstrate that determining the stress intensity factors on the interface of the polymer coating/substrate system based on the CTODs is an effective and reliable approach.

Fracture mechanical analyses of high strength steels applying experiments and simulation

The paper contains information about the experimental and simulation test results of thermomechanically rolled high strength steel. The S960TM material grade was produced by a modern technological process, which provide longer estimated life time, better performance, and due to higher strength enable smaller applied wall thickness, which require less welding activity. The only disadvantage of this material grade is that they can contain material discontinuities in their microstructure. The characterisation of the response of these materials against load can be analysed by fracture mechanical investigation. Fracture mechanical experiments were carried out with three-point bending method, calculating fracture toughness value of crack-tip opening displacement (CTOD). To reduce the complicated real experiment in fracture mechanics, the simulation possibility of fracture mechanical behaviour of the investigated material was applied, using DEFORM software, which gave an acceptable narrowing to the measured phenomenon.

Effect of Structural State of Metal in Weld Fusion Zone of Large-Diameter pipes on Fracture Mechanism and Results of Crack-Resistance (CTOD) Tests

We describe a study of fractures and metal structure in the fusion zone (FZ) and weld center (WC) for longitudinal welds and base metal (BM) in large-diameter pipe with strength class K60 (Grade X70) tested for crack resistance including a determination of the crack tip opening displacement (CTOD) on through-cut SENB samples (N-P per BS EN ISO 15653). Tests indicated that CTOD–20 (at temperature–20 °C) is generally more stable for BM and WC metal than for FZ metal. The effect of crack front displacement from the specified location on measured CTOD–20 values was studied for FZ samples. The CTOD–20 values for the FZ metal were found to depend on the failure mechanism caused by initiation of the fatigue crack, including maximum fatigue-crack length and maximum ductile-failure segment length. The structure of the FZ metal along the fatigue crack front was studied at various crack resistance levels using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The failure mechanism and resultant CTOD values for FZs in double-sided welds depend not only on the structural characteristics of the metal in the heat-affected zone (HAZ) (ratio of lath-like bainite to granular bainite, the martensitic-austenitic (MA) component and cementite morphology), but also on whether or not a fatigue crack is present in the section of pipe base metal where the deformation is largest. This differentiates the CTOD samples from the KCV samples. The fact that the BM area is located at the crack front has a substantial effect on the fracture mechanism in a FZ CTOD.