Ductile Crack Research Articles

Torsional Behaviour of Steel Fibre Reinforced Alkali Activated Concrete.

Nine alkali-activated concrete beams were produced and tested under pure torsional load to failure. The alkali-activated concrete beams were produced with following variables: (i) fibres only, (ii) conventionally reinforced or (iii) a hybrid of both fibres and conventional steel reinforcement. The fibres only beams were found to have approximately 20% higher cracking torque than conventionally reinforced beams. However, fibres only beams were observed to have lower post crack ductility and inconsistent post crack behaviour, in comparison to conventionally reinforced alkali-activated concrete (AAC) beams. On the other hand, the hybrid reinforcements in AAC beams were found to demonstrate more ductile post crack behaviour consistently of the beams tested. Hybrid reinforcement was also shown to have 20% and 25% improvement in cracking and ultimate torque compared to conventionally reinforced, which suggests that it is suitable for industrial applications to improve structure capacity. The ultimate torque results of the beams were compared to an analytical model that considered the contribution of fibres. It was found that the ultimate torque of the hybrid reinforced beam has good correlation with the model but overestimated conventionally reinforced beams.

A modified Coffin-Manson model for ultra-low cycle fatigue fracture of structural steels considering the effect of stress triaxiality

The ductile crack initiation of structural steels due to ultra-low cycle fatigue (ULCF) loading often governs the limit state in steel structures. This paper proposes a modified Coffin-Manson model for ULCF fracture of structural steels considering the effect of stress triaxiality. To this end, tests on coupon specimens and circular notched specimens made of Chinese Q345qC steel were conducted to form different stress triaxiality states. Based on test results and corresponding finite element analysis, the parameters of modified Coffin-Manson model were determined, and empirical formulas were established between model parameters and stress triaxiality. Finally, detailed finite element analysis was conducted on circular notched specimen tests to validate the proposed model. The results show that the proposed model can be used to predict ULCF fracture of structural steels at different stress triaxialities with high accuracy.

Fracture Modeling Of Cracked Pipe Under Monotonic And Cyclic Loading

ABSTRACT The present work presents experiment and numerical modelling to quantify the effect of the loading type on ductile crack growth in circumferential through-wall cracked SA508 Gr.1a pipes. In tests, three types of loading conditions are considered; monotonic, displacement-controlled and load-controlled large-amplitude cyclic loading. Experimental data are compared with numerical crack growth simulation results based on the multiaxial fracture strain energy model. Simulated results show overall good agreements with experimental data for all loading types. Based on FE results, the effects of the loading type and cyclic hardening model on ductile crack growth are discussed.

Static Crack Resistance of Heat-Resistant KhN43MBTYu Nickel-Chromium Alloy in Gaseous Hydrogen

The effect of hydrogen at a pressure of up to 35 MPa and a content of up to 29 ppm on the strength, ductility, short-term and long-term static crack resistance of four KhN43MBTYu (EP-915VD) alloy modifications with different heat treatment modes and chemical composition has been studied. It has been found that the critical stress intensity factor KIc in the presence of hydrogen, just as the ductility characteristics of smooth specimens, depends on the deformation rate, reaching minimum values at rates of less than 0.1 mm/min. The fracture toughness decreases under the action of hydrogen by a factor of 2.5, and the plane strain state occurs at a much smaller specimen thickness. An optimal combination of high strength, ductility, short- and long-term static crack resistance in air and hydrogen has been achieved in a fine-grained alloy with low carbon and sulfur content. Based on the results of long-term static crack resistance tests at the predetermined maximum fatigue test duration of 300 h, the invariant characteristics of crack resistance, the threshold values of stress intensity factor in hydrogen, have been determined, which vary from 23 to 48 MPa ∙ m1 / 2 depending on the alloy heat treatment mode.

Modeling ductile fracture using critical strain locus and softening law for a typical pressure vessel steel

This work describes the framework to model ductile damage based on phenomenological stress-modified critical strain criterion (SMCS) coupled with a softening law to predict the onset of ductile initiation and ductile propagation in typical fracture specimens, extracted from a flat plate made of ASTM A285 Gr. C steel. A very detailed and well-illustrated methodology has been developed for the identification of material parameters. Laboratory testing of cylindrical tensile bars and SE(B) specimens, at room temperature, provides necessary and sufficient information to calibrate the numerical parameters in the proposed model. For either geometry, the applied loading is measured by a continuous record of the load (P) and displacement (Δ). After the model parameters have been set, verification studies are carried out for SE(B) specimens having shallow cracks with and without side-grooves. Consequently, parameter transferability, outside small-scale yielding condition between specimens having different crack tip conditions, can be addressed and the constraint influence on the driving force better understood. An additional check is performed by comparing the final crack front profile measured on the fracture surface of the SE(B) specimens with the numerical calculated in the finite element analyses. The phenomenological model adopted herein can reproduce and predict reasonably well the experimental data obtained for specimens with different levels of stress triaxiality (constraint). Overall, it is shown that the SMCS criterion combined with a softening law can be used to study and to predict the influence of stress state on ductile failure initiation and ductile crack growth by identifying nine model parameters through testing notched round bar geometries and SE(B) specimens. The proposed methodology shows great potential as an engineering tool for assessing the integrity of complex structures such as welded pipelines and pressure vessels.

Effect of Hydrogen Corrosion on the Brittle Fracture Resistance of an Oil Line Pipe 530 mm in Diameter Made of 12GSB Ferritic–Pearlitic Steel

The structure, the static tensile properties, the impact toughness, and the fracture mechanisms of the 12GSB pipe steel specimens cut from the defect regions in a pipeline 530 mm in diameter are studied. Hydrogen corrosion is found to affect the anomalous character of the temperature dependences of the impact toughness and the fraction of ductile component in a fracture surface. Sulfur is shown to substantially influence the ductile crack propagation energy in the transverse direction and to weakly influence this energy along the rolling direction. The development of hydrogen corrosion in the pipeline is found to be accompanied by a significant increase in the reduced fraction of intergranular fracture (15.8–35%) in brittle fracture surfaces, and this fraction in the separation plane reaches 50.5%. The relation between the impact toughness of the transverse specimens and the fraction of ductile component in a fracture surface has an anomalous character: this relation has two linear segments, the fracture energies in which differ by an order of magnitude. The segment with a low fracture energy (4.4–16.1 J/cm2) characterizes the action of the sear component of separation in the rolling plane, and the segment with a high fracture energy (38–246 J/cm2), the action of a ductile mode I fracture mechanism. The impact toughness anisotropy coefficient is found to be high, Ka = 2.31.

Effects of stress triaxiality ratio on the heat build-up of polyamide 11 under loading

Adiabatic heat build-up in polymers attributed to the conversion of plastic work into dissipative heat is a well known phenomenon. The temperature in the vicinity of a crack tip due to heat build-up may be exceptionally high so that it can locally reach the glass transition temperature Tg, even though the ambient testing temperature is lower than Tg. A significant alteration of the local material response and damage mechanisms is then induced. By simultaneous measurement of temperature during experimental tests under quasi-static loading and for a wide range of stress triaxiality ratios, a Gurson-Tvergaard-Needleman based thermo-mechanical constitutive model, integrating temperature-dependent coefficients, has been developed. Predictive capabilities of the proposed thermo-mechanical model to simulate the isothermal behaviour of PolyAmide 11 (PA11) have led to adiabatic simulations, to account for the heat build-up highlighted experimentally, of ductile crack extension. The model parameters were identified using experimental data obtained from PA11 samples with a given stress triaxiality ratio. Predicted evolutions given by the proposed constitutive model for other stress triaxiality ratios and geometries are found to be in good agreement with experimental data.

The low temperature mechanical properties of a Nitronic 40 forging

Nitronic 40 forged shells are typically used for structural reinforcement in high field pulse magnet design and applications. To better understand the mechanical performance of this versatile high strength austenitic steel a series of mechanical tests were conducted. Tensile were performed at 295 K, 77 K and 4 K, and cryogenic fracture mechanics tests were performed at 77 K and 4 K. The effect of temperature on strength, ductility, toughness and fatigue crack growth rate are evaluated. Microstructure and composition effects are also presented and discussed.

Assessment of the defect stability for large structures submitted to thermal shock loading through the Gfr criterion

This paper proposes a formulation and a numerical protocol to allow the application of the Gfr criterion for thick stainless steel components submitted to thermal shock loading. In a first part of the paper, a short description of the criterion initially defined in the S. Marie's PhD is proposed. The proposition to extend the criterion to thermal loading is then made based on the elastic-plastic stresses determined on the crack free model and the classical influence function formulation.Following this description and proposition of extension, two numerical applications are proposed in order to illustrate the applicability of the modified protocol and its coherence with already existing schemes.In a third part, a validation through the comparison to a large scale test performed on CF8M cast stainless steel material is made. This test was chosen since the Gfr criterion is a candidate for defect stability evaluation in cast components. It is then shown that the parameter can be determined on standard specimen, then used to predict the ductile crack growth in large component made of complex materials such the duplex cast stainless steel.

Numerical Investigation of Strength Mismatch Effect on Ductile Crack Growth Resistance in Welding Pipe

The effect of strength mismatch (ratio between the yield stress of weld metal and base metal, My) on the ductile crack growth resistance of welding pipe was numerically analyzed. The ductile fracture behavior of welding pipe was determined while using the single edge notched bending (SENB) and single edge notched tension (SENT) specimens, as well as axisymmetric models of circumferentially cracked pipes for comparison. Crack growth resistance curves (as denoted by crack tip opening displacement-resistance (CTOD-R curve) have been computed using the complete Gurson model. A so-called CTOD-Q-M formulation was proposed to calculate the weld mismatch constraint M. It has been shown that the fracture resistance curves significantly increase with the increase of the mismatch ratio. As for SENT and pipe, the larger My causes the lower mismatch constraint M, which leads to the higher fracture toughness and crack growth resistance curves. When compared with the standard SENB, the SENT specimen and the cracked pipe have a more similar fracture resistance behavior. The results present grounds for justification of usage of SENT specimens in fracture assessment of welding cracked pipes as an alternative to the traditional conservative SENB specimens.

Ductile crack initiation evaluation in stiffened steel bridge piers under cyclic loading

Although detailed shell analysis is suitable to predict the ductile crack initiation life of steel members, such detailed method adds time expense and complexity. In order to simply predict the ductile crack initiation life of stiffened steel bridge piers, a total of 33 cases are simulated to carry out the parametric analyses. In the analysis, the effects of the width-to-thickness ratio, slenderness ratio, plate thickness and so on are considered. Both shell analyses and beam analyses about these 33 cases are conducted. The plastic strain and damage index obtained from shell and beam analyses are compared. The modified factor Bs is determined based on the predicted results obtained from both shell and beam analyses in order to simulate the strain concentration at the base corner of the steel bridge piers. Finally, three experimental results are employed to verify the validity of the proposed method in this study.

Prediction of ductile crack growth resistance using mechanical properties of material

Prediction of ductile crack growth resistance using mechanical properties of material

Novel ductile wellbore cementitious composite for geologic CO2 storage

CO2 leakage through damaged wellbore cement sheaths is a major risk of geologic CO2 storage (GCS), as conventional wellbore cement is brittle and can be damaged due to acid attack and downhole stresses during CO2 injection. Here we examine a novel fiber-reinforced engineered cementitious composite (ECC) proposed as a substitute to conventional wellbore cement due to its superior ductility and intrinsic crack width control. ECC and conventional wellbore cement coupons were exposed to water in equilibrium with CO2 at 50 °C and 10 MPa. The samples were retrieved after several days and their mechanical performance was evaluated using a four-point bending test, microhardness, and compressive strength analyses. Optical microscopy and mercury intrusion porosimetry were used to characterize the progression of the carbonation front and pore structures of the specimens. Control experiments were conducted under the same temperature and pressure conditions but with a N2 headspace to isolate the impact of CO2. It was found that carbonation increased the ultimate flexural strength of ECC but decreased its ductility. However, the ductility of carbonated ECC remained higher than that of conventional wellbore cements that exhibited brittle failure under all test conditions. Additionally, ECC exhibited minimal material loss and continued resistance to deformation in comparison to conventional wellbore cements. This suggests that while the exposure of ECC to CO2 will alter its mechanical properties, altered ECC will continue to exhibit mechanical performance superior to conventional wellbore cement, and therefore shows promise as a highly durable wellbore cementing material for GCS applications.

Wavelet-enriched adaptive hierarchical FE model for coupled crystal plasticity-phase field modeling of crack propagation in polycrystalline microstructures

This paper develops a novel, wavelet-enriched adaptive finite element model for solving coupled crystal plasticity-phase field models to simulate crack propagation in polycrystalline microstructures. No a-priori assumption of the crack path is needed. Crack propagation under conditions of finite deformation is driven by stored elastic energy that accounts for material anisotropy and tension–compression asymmetry, and defect energy resulting from slip system dislocation glide and hardening. The resulting finite element model is capable of simulating both brittle and ductile crack propagation in material microstructures. A major contribution of this work is the creation of the adaptive, multi-resolution wavelet-based hierarchical enrichment of the FE model. The adapted enrichment follows the path of crack growth and is able to successfully overcome the challenges of high resolution required for the regularized crack in the coupled model. The multi-resolution wavelet basis functions adaptively construct optimal enrichment basis for the high gradients in the phase field order parameter near the crack path. The wavelet-enriched adaptive finite element model is found to be robust with excellent convergence characteristics in multiple validation tests conducted with the polycrystalline Ti–6V–4Al alloy.

Influence of Grain Size Distribution on Ductile Intergranular Crack Growth Resistance

AbstractThe influence of grain size distribution on ductile intergranular crack growth resistance is investigated using full-field microstructure-based finite element calculations and a simpler model based on discrete unit events and graph search. The finite element calculations are carried out for a plane strain slice with planar grains subjected to mode I small-scale yielding conditions. The finite element formulation accounts for finite deformations, and the constitutive relation models the loss of stress carrying capacity due to progressive void nucleation, growth, and coalescence. The discrete unit events are characterized by a set of finite element calculations for crack growth at a single-grain boundary junction. A directed graph of the connectivity of grain boundary junctions and the distances between them is used to create a directed graph in J-resistance space. For a specified grain boundary distribution, this enables crack growth resistance curves to be calculated for all possible crack paths. Crack growth resistance curves are calculated based on various path choice criteria and compared with the results of full-field finite element calculations of the initial boundary value problem. The effect of unimodal and bimodal grain size distributions on intergranular crack growth is considered. It is found that a significant increase in crack growth resistance is obtained if the difference in grain sizes in the bimodal grain size distribution is sufficiently large.

An h-adaptive meshfree-enriched finite element method based on convex approximations for the three-dimensional ductile crack propagation simulation

A new numerical approach for modeling the three-dimensional ductile crack propagation problem is presented. The approach is obtained by the introduction of an h-adaptive finite element refinement scheme using the element-wise meshfree enrichments and meshfree visibility criteria to achieve the desired accuracy due to the material nonlinearity and topological changes in the presence of displacement discontinuity. To ensure the element compatibility condition, meshfree convex approximations are employed to the finite element refinement scheme. A crack propagation algorithm combining the meshfree discretization and the crack surface tracking scheme is proposed. To further simplify the freeform surface generation in this moving discontinuity problem, a meshfree particle integration method is considered. Finally, two numerical benchmarks are studied to examine the effectiveness of present method in the three-dimensional ductile crack propagation analysis.

CPOT: A suitable tool for crack propagation path optimization based on image recognition

A Crack Path Optimization Tool (CPOT) is developed in this study, and it is designed as a plugin for ABAQUS/CAE. The CPOT provides a friendly graphical user interface, which can control crack propagation along the specified path effectively. With the arrangement strategy of holes, the tool can deflect the crack propagation path and avoid the critical domains or structures. In this application, optimization algorithm assisted extended finite element method (XFEM) is applied to crack propagation analysis. Meanwhile, the image recognition technique is employed to extract the related crack coordinates from crack path image. The grayscale processing and binarization methods are used to obtain the pixels of crack path in the image processing module. Then, a conversion coefficient is proposed to convert the pixel coordinate into physical coordinate. Compared with the single crack optimization strategy, a recognition method for multiple cracks is added to distinguish different cracks path. Additionally, three-dimensional optimization problem is also considered in the application. Finally, several examples which utilize the deformation plasticity theory for the ductile crack problem are employed to illustrate the superiority of the application. The results indicate the high accuracy and efficiency of the application for controlling arbitrary crack propagation along the specified path.

Effects of crystalline orientation, twin boundary and stacking fault on the crack-tip behavior of a mode I crack in nanocrystalline titanium

In this work, molecular dynamics simulation and linear elastic fracture mechanics were employed to analyze the crack-tip behavior of a Mode I crack in nanocrystalline titanium. The effects of crystalline orientation, twin boundary and stacking fault on crack propagation were taken into account. Simulation results demonstrate that the crack-tip behavior and thus the crack propagation mode strongly depend on crystalline orientations and plane defects. Cracks lying on the hexagonal close-packed basal plane, the prismatic plane, or along a stacking fault plane defect propagate in a brittle manner, without involving proper dislocation emissions and twin nucleations in the crack-tip vicinity. In contrast, cracks show a ductile propagation behavior when aligned along the pyramidal plane, the {101¯2} plane, or the {101¯2}〈1¯011〉 twin boundary. For these cases, local crack-tip plasticity and crack-tip reconstructions are found to play significant roles. The impact of strain rate on the crack-tip behavior of a basal crack was also investigated in detail. Five strain rates varied between 109 s−1 and 1010 s−1 were considered. With increased strain rates, a brittle-to-ductile transition was clearly observed for crack propagation. The desired transition can be attributed to the high stress and energy concentrations near the crack-tip under elevated strain rates, leading to the successive emission and propagation of partial dislocations. To verify and validate the simulation results, a theoretical analysis on the competition between brittle and ductile crack propagations was also implemented. The theoretical predictions based on the linear elastic fracture mechanics were found to reasonably agree with the simulation results. The observations and conclusions deduced from the combined modeling and theoretical study are helpful to the better understanding of fracture mechanics in nanocrystalline titanium.

Effect of crystal orientation on initiation and propagation of crack: Phase field crystal model study

The crystal phase field (PFC) method is used to simulate the propagation of the nano-crack of samples with different crystal orientations under the strain of the uniaxial tensile. The results show that the different crystal orientations have a significant effect on the initiation and propagation of the cracks. For the samples with the orientation angles of 5° and 20°, the notch is directly cracked due to the strain concentration at the dislocation of the crack tip. The cracks mainly show a mode of the brittle expansion, and its edges show smooth planar features. For the samples with the orientation angles of 10° and 15°, the dislocation is firstly emitted at the notch to generate vacancies by dislocation slipping. The vacancies grow and connect to form cracks. This process of the crack propagation belongs to the mode of the ductile crack with the rough edges. The results are consistent with that of the molecular dynamic and experimental results.

Prediction of ultimate resistance by application of damage theory with special regard to HSS

ABSTRACTIt is a general question as to whether the established rules for predicting ultimate resistance under global tension can also be applied to structures made from HSS, i.e. up to = 960 MPa. Apart from the welding issue, for the base material, this question refers to several items, e.g. the plastic flow curve, the yield stress level, the yield ratio, the upper‐shelf ductility and strainability and the lower‐shelf toughness. It reveals that predicting the ductile fracture of steel components can only be an approximation, and neither allows for a deeper understanding of the real ductile crack development up to ultimate load nor leads to a realistic extrapolation if high‐strength steels are used. The research work presented therefore suggests an innovative extension of the FEM analysis by implementing appropriate damage mechanics based failure criteria. After an introduction to the principles of this methodology, possibilities for its use in structural steel research and design as well as further insights into the structural behaviour of steel components, especially HSS up to S960, are presented – insights that are not possible by traditional testing or simulation.