Ductile Failure Initiation Research Articles

Coupled effects of crystallographic orientation and void shape on ductile failure initiation using a CPFE framework

Ductile failure reveals to be an anisotropic phenomenon, for which the proper mechanism has not been clearly addressed yet in the literature. In this paper, the effects of some key anisotropy factors on ductile failure initiation, detected by void coalescence and plastic strain localization, are investigated using unit-cell computations based on crystal plasticity finite element method. The studied anisotropic effects are induced by the combination of initial crystallographic orientations and void shapes. Therefore, single crystals with three different initial orientations and polycrystalline aggregates with three different initial crystallographic textures are respectively considered. A single void with either spherical, prolate or oblate shape is assumed to be preexisting at the center of each unit cell. By contrast to previous analyses in the literature, plastic strain localization is predicted in the present study on the basis of bifurcation theory. To cover a wide range of stress states, the simulations are performed under two macroscopic loading configurations: proportional triaxial stressing, characterized by constant stress triaxiality and Lode parameter, and proportional in-plane straining, specified by constant strain-path ratio. The obtained results show that the combined anisotropic effects play an important role in the occurrence of void coalescence and plastic strain localization, as well as in the competition between them.

A machine learning based sensitivity analysis of the GTN damage parameters for dynamic fracture propagation in X70 pipeline steel

The Gurson–Tvergaard–Needleman (GTN) damage model is widely used to predict ductile failure initiation and propagation. However, the material-dependent parameters can show a significant spread when determined for the same steel grade material. Different calibration procedures and optimisation processes cause a significant variation in the obtained parameter values. Furthermore, there is no clear consensus on which parameters require calibration. In this study, the influence of the material-dependent parameters used to model the dynamic ductile fracture behaviour of X70 grade pipeline steel is investigated. A sensitivity analysis is performed on a finite element model of a Charpy V-Notch (CVN) specimen. Seven GTN model parameters are considered in a total of 70 simulations. A feedforward back-propagating artificial neural network (ANN) is constructed and trained using data obtained through the numerical simulations. A connected weights (CW) algorithm allows to determine the relative influence of each parameter on the fracture energy. It was observed that the void growth acceleration factor plays an important role with respect to the parameter influences. Remarkably, the mean nucleation strain, \( \varepsilon_{N} \) has the highest relative importance whilst the critical void volume fraction, \( f_{c} \)—which is considered as a crucial damage parameter—showed the smallest influence when the acceleration factor is low. On the contrary, when considering a high acceleration factor, \( f_{c} \) becomes the most influential parameter. Based on the obtained importance for each parameter, it is suggested that parameters \( f_{0} \), \( f_{c} \), \( f_{F} \), and \( f_{N} \) should be selected for calibration in each individual application. Finally, the applied machine learning approach is used to predict the fracture energy for a given set of damage parameters for X70 grade steel. It is observed that the trained neural network is able to provide a satisfactory approximation of the CVN fracture energy.

A strain based criterion to assess ductile failure initiation in dented pipelines under tensile loading

This work addresses the problem of describing ductile fracture behavior and predicting ductile failure initiation in dented pipelines under tensile loading based upon a 3-D computational cell approach coupled with a stress-modified, critical strain (SMCS) criterion for void coalescence. A series of tension tests conducted on notched tensile specimens with different notch radius for a carbon steel pipe provides the stress–strain response of the tested structural steel from which the SMCS criterion is calibrated. Full scale cyclic bend tests also performed on a 165 mm O.D tubular specimen with 11 mm wall thickness enable verification of the proposed approach in assessing ductile cracking behavior in damaged pipelines. These exploratory analyses predict the tensile failure load for the pipe specimen associated with ductile crack initiation in the highly damaged area inside the denting and buckling zone which are in good agreement with experimental measurements.

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.

Finite Fracture Mechanics Assessment in Moderate and Large Scale Yielding Regimes

The coupled Finite Fracture Mechanics (FFM) criteria are applied to investigate the ductile failure initiation at blunt U-notched and V-notched plates under mode I loading conditions. The FFM approaches are based on the simultaneous fulfillment of the energy balance and a stress requirement, and they involve two material properties, namely the fracture toughness and the tensile strength. Whereas the former property is obtained directly from experiments, the latter is estimated through the Equivalent Material Concept (EMC). FFM results are presented in terms of the apparent generalized fracture toughness and compared with experimental data already published in the literature related to two different aluminium alloys, Al 7075-T6 and Al 6061-T6, respectively. It is shown that FFM predictions can be accurate even under moderate or large scale yielding regimes.

Numerical study of ductile failure under non-proportional loading

This paper investigates two numerical methods for predicting the initiation of ductile failure under moderately and strongly non-proportional loading paths. Two distinct phenomena are considered as indicators for the initiation of ductile failure: (i) the localization of deformation into a narrow band and (ii) the coalescence of microscopic voids. Recent experimental data in the literature from various axisymmetric tension tests on a high-strength steel are used to calibrate and validate the two methods. In the first method, which is based on the imperfection band approach, strain localization analyses are conducted using the deformation history extracted from finite element simulations of the tension tests. In the second method, axisymmetric unit cells are utilized to evaluate the onset of void coalescence using the stress history extracted from the same finite element simulations of the experiments. The various uniaxial tension tests yield different moderately and strongly non-proportional loading paths that are used to evaluate the predictive capabilities of the two methods. The numerical results are further used to discuss the similarities and differences between the two methods. Both the strain localization analyses and the coalescence analyses are found capable of predicting the initiation of failure in the experiments with good accuracy; however, the coalescence analyses are generally in closer agreement to the experiments.

Determination of ductile damage behaviors of high strain rate compression deformation for Ti-6Al-4V alloy using experimental-numerical combined approach

Ductile damage behavior of Ti-6Al-4V alloy was characterized by the evolution of flow curves and deformed specimens in compression tests at high strain rates and various temperatures. Cracks along the shear bands of deformed specimens were observed at lower temperatures (20 °C and 100 °C) and high strain rates (2500–10,000 s−1). The failure initial strains (FIS) during crack generation were calculated according to the measured flow stresses at the transition from stable plastic to damage evolution stage. Special emphasis is placed on the influence of strain rate and temperature on the ductile failure initiation. The failure initiation parameters of Johnson-Cook (JC) damage model were obtained by genetic algorithm (GA) optimization using a combined experimental–numerical approach. That is, the ductile damage parameters were optimized according to the measured FIS and the simulated stress triaxiality, equivalent strains, stresses and temperatures, using compression simulation without damage model. At lower temperatures (T < 300 °C), the failure parameters were used to simulate the flow stresses and deformation of specimen by the JC plastic and damage models. To further validate the failure parameters, at higher temperatures (T ≥ 300 °C), a coupling modified JC (JCM) plastic and energy density based damage model was proposed to characterize the flow curves at damage evolution stage by compression simulation using a subroutine program. The predicted flow stresses and the deformed specimens were consistent with the experimental results.

Ductile fracture of an ultra-high strength steel under low to moderate stress triaxiality

Aeronautical structures such as landing gears, wheels or turbine shaft are designed to withstand predefined loadings corresponding to critical scenarios. This work addresses the definition of a ductile failure initiation criterion under multiaxial loading for ML340©™ ultra high strength steel employed in such structures. The experimental testing campaign includes tension–torsion tests under proportional loading, standard tensile tests, tensile tests on notched round and flat specimens and plane strain tests. The experimental database covers a stress triaxiality range from 0 to 1.3 and various positive Lode parameter stress states. A von Mises quadratic yield criterion with a combined Voce/linear hardening function is employed to describe plastic behavior. The hardening function is identified using an inverse method to fit cross section reduction in order to take into account large strains. A simple non-linear damage cumulation rule based on Rice and Tracey/Johnson Cook formulation with a Lode-dependent term is proposed. Good agreement was found between predicted and observed failure initiation load steps and failure locations.

Modelling of the CO2-Induced Degradation of a Fractured Caprock During Leakage: Potential for a Mechanical Self-Limiting Process

In the present study, we assess the potential for initiating ductile failure in a fractured caprock due to the chemical alteration of its mechanical properties under pressure increase induced by CO2 leakage and fixed in situ boundary conditions. In this view, 2D numerically coupled reactive-transport simulations were set up by using the Opalinus Clay formation as an analogue for a caprock layer. The fractured system was viewed as a compartmentalised system that consists of a main highly permeable pathway, a moderately permeable damage zone and the intact rock. The outputs of the numerical simulations (mineral fraction, porosity changes, gas saturation, pore-fluid pressure) were converted into parameter changes of the yield surface by viewing the rock material of the three compartments (fault, damage zone and intact rock) as a composite system that consists of a clayey solid material, pores and mineral inclusions (such as carbonate and quartz). Three alteration processes were considered: (1) the effect of the mineral fraction and porosity evolution on the yield surface, (2) changes in the resulting poro-elastic properties and (3) the suction effect, i.e. the bounding effect induced by the presence of two phases, water and CO2. Our numerical investigations showed that the decrease in the friction coefficient remained negligible during leakage, while the pre-consolidation stress mainly decreased. Consequently, the damage zone of the fractured system became more collapsible over time, which was driven by low-to-moderate pressure build-up of the fluid penetrating the fault (1 MPa in our case). For the considered case, the initiation of ductile failure is likely under conditions of fixed vertical stress and zero lateral strain. This process could potentially limit the spatial spreading of CO2-induced alteration, although this remains very site specific. We recommend that characterisation efforts be intensified to obtain better insight into the properties of fracture systems in caprock-like formations (with special attention to their initial over consolidation ratio).

On the conversion of stable tearing into unstable cleavage in spheroidized steels

This paper describes a method for ascertaining how, in spheroidized steels, the probability of cleavage initiation and ductile failure initiation in the transition temperature region varies with the distance from a pre–crack tip. In this method, criteria for the propagation of microcracks initiated at carbides (Godse and Gurland. 1989), and for coalescing of voids (Garrison and Thompson, 1986) are applied to the conditions found in the non–homogenous stress–strain field ahead of a crack tip. Temperature dependences have been utilized for estimating the distance at which stable crack propagation is converted into a cleavage process and the lowest temperature at which ductile tearing can occur.

Modeling and simulation of 2012 Sandia fracture challenge problem: phantom paired shell for Abaqus and plane strain core approach

The prediction of crack initiation, propagation, and ductile fracture in a large scale metallic structure has posed a great challenge to the design and certification community. The complexity in component geometry, material heterogeneity, and the 3D stress distribution, will likely make the crack growth curvilinear. An accurate 3D stress prediction is essential to predict the ductile fracture that is controlled by void nucleation, growth and coalescence. To alleviate the computational burden associated with the use of a 3D solid finite element modeling coupled with a remeshing of the ductile failure prediction of a large scale structure, a novel approach based on the coupling of an extended finite element for shell elements and a plane strain core characterization of a cracked region is developed to efficiently simulate an arbitrary crack growth in a large scale thin-walled structure and its associated load deflection curve while retaining a sufficient level of computational efficiency. The methodology is implemented in Abaqus’ explicit solver via its VUEL where a kinematic description of a cracked shell is accomplished via the use of phantom-paired elements. A plane strain core that controls the mixture of plane stress and plane strain components is introduced for a rational representation of localized plasticity induced 3D stress state for the prediction of a ductile failure initiation. A cohesive injection is applied on the newly created crack surface to dissipate the energy during the crack propagation. The resulting XSHELL toolkit for Abaqus is applied for the ductile failure prediction of the 2012 Sandia fracture challenge problem. A blind prediction is performed first using geometry independent plane strain core parameters followed by a refined analysis based on a set of well-calibrated material and plane strain core parameters from Sandia’s tensile and compact testing data.

Ductile tensile failure in metals through initiation and growth of nanosized voids

We here reveal the initiation of ductile failure in metals at the nanometer scale by molecular dynamics simulations coupled with a novel analytical model. This proceeds by the emission of a special type of dislocation shear loop, which can expand as a partial or perfect dislocation, evolve into a prismatic loop through reaction, or develop into twins. Molecular dynamics (MD) simulations predict a strong dependence of the stress required for the initiation of plastic flow at the surface of the void for both Cu (a model fcc metal) and Ta (a model bcc metal). The decrease in stress with increasing void size is also analyzed in terms of a new analytical approach based on the energetics of dislocation loop emission. For both fcc (copper) and bcc (tantalum) metals initiation of plastic flow in MD simulations takes place at voids as small as a tri-vacancy (radius R≈0.1nm). Extensive calculations for tantalum combined with the analytical model, which tracks the simulations, enable extrapolation to R≈300nm, in the realm of second phase particles and inclusions. Thus we conclude that this is a general mechanism of tensile failure in pure monocrystalline metals where other initiation sites are absent.

Failure mode of laser welds in lap‐shear specimens of high strength low alloy (HSLA) steel sheets

ABSTRACTIn this paper, the failure mode of laser welds in lap‐shear specimens of non‐galvanized SAE J2340 300Y high strength low alloy steel sheets under quasi‐static loading conditions is examined based on experimental observations and finite element analyses. Laser welded lap‐shear specimens with reduced cross sections were made. Optical micrographs of the cross sections of the welds in the specimens before and after tests are examined to understand the microstructure and failure mode of the welds. Micro‐hardness tests were also conducted to provide an assessment of the mechanical properties in the base metal, heat‐affected and fusion zones. The micrographs indicate that the weld failure appears to be initiated from the base metal near the boundary of the base metal and the heat‐affected zone at a distance away from the pre‐existing crack tip, and the specimens fail due to the necking/shear of the lower left load carrying sheets. Finite element analyses based on non‐homogenous multi‐zone material models were conducted to model the ductile necking/shear failure and to obtain the J integral solutions for the pre‐existing cracks. The results of the finite element analyses are used to explain the ductile failure initiation sites and the necking/shear of the lower left load carrying sheets. The J integral solutions obtained from the finite element analyses based on the 3‐zone finite element model indicate that the J integral for the pre‐existing cracks at the failure loads are low compared to the fracture toughness and the specimens should fail in a plastic collapse or necking/shear mode. The effects of the sheet thickness on the failure mode were then investigated for laser welds with a fixed ratio of the weld width to the thickness. For the given non‐homogenous material model, the J integral solutions appear to be scaled by the sheet thickness. With consideration of the plastic collapse failure mode and fracture initiation failure mode, a critical thickness can be obtained for the transition of the plastic collapse or necking/shear failure mode to the fracture initiation failure mode. Finally, the failure load is expressed as a function of the sheet thickness according to the governing equations based on the two failure modes. The results demonstrate that the failure mode of welds of thin sheets depends on the sheet thickness, ductility of the base metal and fracture toughness of the heat‐affected zone. Therefore, failure criteria based on either the plastic collapse failure mode or the fracture initiation failure mode should be used cautiously for welds of thin sheets.

Modeling of failure mode of laser welds in lap-shear specimens of HSLA steel sheets

Failure mode of laser welds in lap-shear specimens of high strength low alloy (HSLA) steel sheets is investigated in this paper. The experiments for laser welds in lap-shear specimens under quasi-static loading conditions are briefly reviewed first. The experimental results showed that the laser welds failed in a ductile necking/shear failure mode and the ductile failure was initiated at a distance away from the crack tip near the boundary of the base metal and heat affected zone. In order to understand the failure mode of these welds, finite element analyses under plane strain conditions were conducted to identify the effects of the different plastic behaviors of the base metal, heat affected zone, and weld zone as well as the weld geometry on the ductile failure. The results of the reference finite element analysis based on the homogenous material model show that the failure mode is most likely to be a middle surface shear failure mode in the weld. The results of the finite element analysis based on the multi-zone non-homogeneous material models show that the higher effective stress–plastic strain curves of the weld and heat affected zones and the geometry of the weld protrusion result in the necking/shear failure mode in the load carrying sheet. The results of another finite element analysis based on the non-homogeneous material model and the Gurson yield function for porous materials indicate that the consideration of void nucleation and growth is necessary to identify the ductile failure initiation site that matches well with the experimental observations. Finally, the results of this investigation indicate that the failure mode of the welds should be examined carefully and the necking/shear failure mode needs to be considered for development of failure or separation criteria for welds under more complex loading conditions.

Effect of void growth and shape on the initiation of ductile failure of steels

Notched tensile bars of a high strength structural steel were loaded to first void coalescence. The results show that damage curves determined by using proportionally loaded specimens are not valid for specimens under non-proportional loading. The results of the experiments with different load histories of notched tensile bars show, that a single parameter criterion like reaching a critical void volume is not sufficient to describe the initiation process of void coalescence. The stress-triaxiality at initiation and the shape of the voids exhibit a strong influence on the initiation values. Two ways to describe initiation are shown: A simple one considers a critical combination of void volume, stress-triaxiality and plastic strain at initiation. The other one uses a shape factor to describe the shape of the voids and calculates the initiation values. Both models work with an accuracy in the order of the experimental scatter.

Damage work as ductile fracture criterion

This paper treats the ductile failure initiation in circumferentially-notched tension specimens and explores the local damage model that is able to represent the continuous degradation of the deforming material. With the aid of finite element calculations, the notched specimens have been simulated numerically and the whole strain-stress history for each geometry derived. This allows determination of the evolution of strain-stress fields until fracture occurs. Two damage models were evaluated: the Rice and Tracey cavity growth model and a model which combines the latter with the plastic strain work, to derive an intrinsic parameter called ‘damage work’. These models could predict the location where the crack will initiate as well as the crack initiation step which is reached for a relatively constant value of the critical damage.

Dynamic fracture characteristics of CuCo alloys

The fracture behavior of heat-treatable CuCo alloys has been examined for conditions of quasi-static and dynamic loading. The finer dimple structures and intergranular fracture surfaces observed after dynamic loading suggest that failure under these conditions is controlled primarily by microvoid nucleation along grain boundaries and at incoherent particle-matrix interfaces. Alloys in the peakaged condition exhibit a superior resistance to dynamic fracture when compared with solution-treated and with overaged alloys, suggesting vacancy trapping by coherent precipitates. Transmission electron microscopy studies revealed microvoid formation at incoherent precipitates during deformation in overaged alloys, leading to the conclusion that voids too small for detection by optical microscopy assume an important role in the initiation of ductile failure during dynamic loading.

Material characterization by J-R curves of a 20MnMoNi55 forging

J-R curves have been determined for one specific forging of the reactor pressure vessel steel 20MnMoNi55 to characterize its fracture resistance in the upper shelf toughness regime. The multiple specimen unloading (MSU), a direct current potential drop (DCPD), and the single specimen partial unloading compliance (SSPUC) methods have been applied to test different CT-specimen geometries at temperatures between 25 and 300°C. The J–R curves are temperature dependent showing a minimum of slope and absolute values at 200°C. The critical J-values for onset of stable crack growth in the scatter of the results are independent of temperature and geometry: J i ≈ 180 kJ/m 2. The values of the technical initiation toughness J IC according to ASTM E813 7 and according to Loss et al. (1979), 8 the tearing modulus T, and the instability parameter J 50 show corresponding temperature trends as the J-R curves with a minimum at about 200°C. The material data have been used to evaluate the ductile failure initiation and tearing instability of several structural tests. The quality of agreement between calculable predictions and experiments is shown to depend on the input material J-R curve and evaluation of the loading which requires detailed knowledge about the material stress-strain behaviour and the relevant crack tip constraint.

On the influence of state of stress on ductile failure initiation in high strength steels

The effect of stress state on the effective plastic strain to initiate ductile failure in three high strength steels is investigated. Circumferentially notched tension specimens were used, and failure initiation strains were correlated with a parameter which is a measure of the “triaxiality” of the stress state. Results are given for both in-plane and through-thickness directions in rolled plate. The failure initiation data, together with appropriate stress-strain fields, and estimates of characteristic lengths over which failure initiates, have been used to predict failure initiation at notches and at crack tips; conventional fracture mechanics parameters, such as K I at crack extension in small scale yielding are estimated for one of the high strength steels.

The initiation of ductile failure by fractured carbides in an austenitic stainless steel

Fracture of rod-shaped chromium carbide particles in a stainless steel, first observed between 5% and 10% tensile strain, occurs as a result of the concentrated local stresses from matrix slip lines, and a theory describing the condition for fracture of the carbides is in good agreement with the experimental results. Final fracture of the material occurs by the linking-up of the voids formed by the carbide fractures, and consequently the elongation to fracture of the steel is substantially lower than for the same steel without carbide precipitates. The results for this material suggest that the fracture of brittle fibres in a composite material may also occur as a consequence of the concentrated local stresses from matrix slip lines.