Initiation Crack Growth Research Articles

Corrosion fatigue life prediction of crude oil storage tank via improved equivalent initial flaw size

Corrosion fatigue is identified as the main failure mechanism for structures working in severe corrosive medium subjected to cyclic rotating bending fatigue, for example crude oil storage tank (COST). Emphasis is placed on the study of corrosion pit formation and the development of cracks from pits. An improved equivalent initial flaw size (I-EIFS) is proposed for corrosion fatigue life prediction. Based on the concept of equivalent initial flaw size (EIFS), pitting corrosion and small crack growth are equivalent to a part of long crack growth process in corrosion fatigue process. Pitting and crack propagation are quantified throughout the fatigue loading thereby allowing a model to be developed that included the stages of pit development, pit-to-crack transition and crack growth in order to predict the fatigue life. A corrosion fatigue life prediction case is adopted to demonstrate the effectiveness of the proposed model. Based on the proposed model, failure analysis and stress calculation are performed to predict the corrosion fatigue life of COST, which provides a method for the life prediction of COST. The validity of proposed method is verified by comparing the service life of COST.

Effects of surface treatments on environmentally-assisted cracking susceptibility of Alloy 182 in BWR environment

The effects of four machining surface preparations on the environmentally-assisted cracking (EAC) initiation and early crack growth were evaluated for Alloy 182 flat tapered specimens exposed to a simulated boiling water reactor (BWR) environment under constant extension rate tensile test conditions. Results showed that the apparent EAC critical threshold stress for initiation was highly dependent on the surface treatment. Detailed microstructural characterization revealed the nature of the machining-induced deformation associated with the four surface treatments. Surface treatments affected the surface oxidation during exposure in the simulated BWR environment. The machining-induced deformation structure affected the EAC initiation susceptibility.

Scaling relations for brittle fracture of entangled polystyrene melts and solutions in elongational flow

The criterion for brittle fracture of entangled polymer liquids [Wagner et al., J. Rheol. 62, 221–223 (2018)] is extended by including the effects of finite chain extensibility and polymer concentration. Crack initiation follows from rupture of primary C–C bonds, when the strain energy of entanglement segments reaches the energy of the covalent bond. Thermal fluctuations will concentrate the strain energy on one C–C bond of entanglement segments, leading to bond scission and rupture of polymer chains followed by crack initiation and fast crack growth. In start-up flows, entanglement segments characterized by long relaxation times, i.e., predominantly those in the middle of the polymer chain, will be the first to reach the critical strain energy and will fracture. Recent experimental data of Huang [Phys. Fluids 31, 083105 (2019)] of fracture of a monodisperse polystyrene melt and of several solutions of monodisperse polystyrenes dissolved in oligomeric styrene are in agreement with the scaling relations for critical Weissenberg number as well as Hencky strain and stress at fracture derived from this fracture criterion and the extended interchain pressure model [Narimissa, Huang, and Wagner, J. Rheol. 64, 95–110 (2020)].

Regarding fracture plane with minimum toughness and fracture anisotropy associated with elongated grain boundaries in ultra-fine-grained Cu

This study is mainly focused on the relationship between the microstructure and fracture toughness of ultrafine-grained Cu produced by equal channel angular pressing. Crack initiation toughness (KIC) and crack growth toughness (TR) are quantitatively evaluated along different crack planes to identify the crack plane with minimum toughness. The results show that grain elongation plane (GEP), making an angle of 29° with the extrusion direction, exhibits the lowest fracture toughness (KIC = 54 MPa m1/2 and TR = 6), while the crack extending normal to GEP confronts the greatest fracture resistance. Both KIC and TR increase monotonically with the inclination angle between the crack plane and the GEP. The preferential micro-void nucleation and growth along the deformation-induced elongated grain boundaries lead to the formation of finely dimpled fracture facets on the fracture surface and favorable crack extension. The presence of distributed regions with elongated grains efficiently contributes to the enhancement of fracture toughness by promoting microscopic crack deflections and crack path tortuosity.

Approximations for Stress-Intensity Factors and Crack Propagation of Box Beams

The stress-intensity factors of box beams under torsion and crack propagation under torsion or/and bending moment are discussed here. This study is motivated by a previous work [1] that derived a closed-form expression of the mode II stress-intensity factor for thin-walled beams with a centered longitudinal crack and subjected to torsion. Naturally, the way in which mode I stress-intensity factors are influenced by torsion is of interest. The influence of parameters such as crack length, crack angle, width and depth of beam, wall thickness, and stiffener size on mode I and mode II stress-intensity factors has been studied using the finite element method and represented the data in a surrogate model using two approaches: 1) a Fourier-series-based correction factor, and 2) an artificial neural network. The crack propagation is also of interest. Specifically, the crack propagation study focuses on both the limit loadings and the angle in which the initial crack growth occurs.

Experimental and XIGA-CZM based Mode-II and mixed-mode interlaminar fracture model for unidirectional aerospace-grade composites

This article presents a fracture model for evaluation of Mode-II and Mixed-Mode (Mode-I + II) interlaminar fracture properties of unidirectional laminated composites. Mode-II and Mixed-Mode (Mode-I + II) interlaminar fracture tests have been carried out on End-Notched Flexure and Mixed-Mode bending specimens, respectively. The non pre-craked specimens are tested under pure Mode-II and Mixed-Mode loading conditions for the estimation of energy required to initiate crack growth from the natural crack tip. Pre-cracked specimens have been tested under pure Mode-II loading conditions to investigate the nature of delamination propagation from the sharp crack tip. A new set of interfacial fracture properties has been evaluated for AS4/914 laminates. An expression has been developed to correlate the experimentally obtained energy release rate (ERR) and equivalent crack growth (Δa) using a compliance based beam method. The curve-fitted expression between ERR and Δa is further introduced to the cohesive zone model for numerical modelling. Fractured surfaces at the final fracture load point in non pre-cracked specimens during Mode-II fracture tests are examined using scanning electron microscopy technique. Numerical simulations of crack initiation and propagation for all fracture tests have been performed using a combined formulation of fracture mechanics and damage mechanics. The introduction of experimentally obtained interlaminar interfacial properties into a combined formulation of extended isogeometric analysis and cohesive zone model shows a good agreement with experimentally obtained results.

Effect of thermomechanical processing defects on fatigue and fracture behaviour of forged magnesium

The microstructural origins of premature fatigue failures were investigated on a variety of forged components manufactured from AZ80 and ZK60 magnesium, both at the test specimen level and the full-scale component level. Both stress and strain-controlled approaches were used to characterize the macroscopically defect-free forged material behaviour as well as with varying levels of defect intensities. The effect of thermomechanical processing defects due to forging of a industrially relevant full-scale component were characterized and quantified using a variety of techniques. The fracture initiation and early crack growth behaviour was deterministically traced back to a combination of various effects having both geometric and microstructural origins, including poor fusion during forging, entrainment of contaminants sub-surface, as well as other inhomogeneities in the thermomechanical processing history. At the test specimen level, the fracture behaviour under both stress and strain controlled uniaxial loading was characterized for forged AZ80 Mg and a structure-property relationship was developed. The fracture surface morphology was quantitatively assessed revealing key features which characterize the presence and severity of intrinsic forging defects. A significant degradation in fatigue performance was observed as a result of forging defects accelerating fracture initiation and early crack growth, up to 6 times reduction in life (relative to the defect free material) under constant amplitude fully reversed fatigue loading. At the full-scale component level, the fatigue and fracture behaviour under combined structural loading was also characterized for a number of ZK60 forged components with varying levels of intrinsic thermomechanical processing defects. A novel in-situ non-contact approach (utilizing Digital-Image Correlation) was used as a screening test to establish the presence of these intrinsic defects and reliably predict their effect on the final fracture behaviour in an accelerated manner compared to conventional methods.

Analysis of Concrete Tensile Failure using Dynamic Particle Difference Method under High Loading Rates

This study presents the use of the dynamic particle difference method (PDM) to analyze tensile failure in concrete subjected to high loading rates. In general, strong form-based meshfree methods suffer from limitations pertaining to the material modeling of concrete because concrete exhibits both softening and damage behaviors that initiate crack growth under an impact load. These methods are generally based on the direct discretization of the governing equations, such as Navier's equation, which involves second-order differentiation. However, conventional material models are based on the first-order derivatives of displacement. The newly developed dynamic PDM can effectively address the limitations of material modeling using a combination of first-order derivative approximations. This circumvents the requirement for high-order derivative approximations, which are essential in strong formulations, such as the finite difference method and point collocation method. The strain rate effect caused by an extremely high loading speed was successfully modeled by accurately reflecting the energy dissipations that result from the cohesive property of the concrete and brittle cracking. Although the developed method incorporates the elastic constitutive relation, it enables the effective modeling of these nonlinear effects. In addition, it can reduce the computational effort. The proportional damping algorithm simulates the effect of velocity in the equation of motion and the cohesion effect in the concrete material. The damage model and the visibility criterion adequately handle crack initiation and propagation in the concrete member. Furthermore, it is noteworthy that the final discrete forms of the dynamic PDM are similar to the integrands of the weak form in the conventional finite element formulation. We ascertained that the stiffness and mass proportional damping effects are related to the inertia and strain of the material, respectively. It was confirmed that the location and direction of crack propagation in concrete varied with the strain rate. Hence, the accuracy and robustness of the proposed method were successfully verified by simulation, and the strain-rate dependency of concrete fracture was efficiently simulated using the proposed method.

A fatigue damage-cumulative model in peridynamics

While the present structural integrity evaluation method is based on the philosophy of assumed similitude, Fatigue and Damage Tolerance (F&DT) evaluations for next generation of air-vehicles require high-fidelity physical models within cyberspace. To serve the needs of F&DT evaluation in digital twin paradigm, a fatigue damage-cumulative model within peridynamic framework is proposed in this paper. Based on the concept of fatigue element block and damage accumulation law in form of Coffin-Manson relationship, the proposed model applies to both fatigue crack initiation and fatigue crack growth; fatigue crack growth rates under constant-amplitude and simple variable-amplitude block loading cases can be well predicted for three common structural materials without inputs of Paris law parameters. Additionally, the proposed model can also be easily extended to a probabilistic version; for verification, multiple-site-damage problems are simulated and the statistic nature of fatigue process in experiments can be well captured. In the end, main features of the proposed model are summarized, and distinctions from the other models are discussed. There may be a potential for the peridynamic damage-cumulative model proposed in this work to numerically predict fatigue problems in digital twin paradigm for future generations of aerospace vehicles.

Effects of solution-aging treatments on microstructure features, mechanical properties and damage behaviors of additive manufactured Ti–6Al–4V alloy

Hot isostatic pressing (HIPing) treatment was generally carried out to eliminate possible defects and to homogenize the microstructure for Ti–6Al–4V alloy fabricated by the Electron beam rapid manufacturing (EBRM) process. But the HIPed alloy with relative coarse lamellar microstructure exhibited a low strength compared to the as-built one. Post solution-aging treatments were necessary to optimize the HIPed alloy's tensile properties. So far, the relationship between the heat-treated microstructure features and corresponding mechanical properties of EBRM-produced Ti–6Al–4V has not been completely understood. Therefore, in the present work, solution-aging treatments under various solution temperatures (930 °C–990 °C) were applied to the HIPed Ti–6Al–4V produced by the EBRM, and the corresponding microstructures and mechanical properties were investigated. The results showed that solution-aging treatments could increase the HIPed alloy's tensile strength but decrease its ductility. With increasing solution temperature, the tensile strength presented an upward tendency until to 980 °C, whereas a decrease after 990 °C solution treatment. The variation in tensile properties was caused by the microstructures evolved with solution temperatures. In-situ tensile results showed that the deformation and damage behaviors of the alloy were also highly affected by the solution temperatures. Owning to a decrease of content of the αp with increase of solution temperature, the alloy at high solution temperature presented superior resistance of crack initiation but inferior resistance of crack growth than that at low solution temperature.

Effect of a large population of seeded alumina inclusions on crack initiation and small crack fatigue crack growth in Udimet 720 nickel-base disk superalloy

Crack initiation, crack coalescence and small crack growth behavior were monitored for over 400 seeded inclusions during interrupted low cycle fatigue testing conducted on the P/M Udimet 720 nickel disk alloy at 650 °C. Two types of seeded alumina inclusions with average sizes of 54 µm and 122 µm were used in the study performed at varying loading conditions resulting in LCF lives ranging from 2,000 cycles to over 1,000,000 cycles. The fatigue behavior was sub-categorized into four groups. Visual maps detailing inclusion size/cycle history were developed. The effect of surface residual stresses on the fatigue life was also investigated.

The effect of loading direction on the fracture behaviors of cortical bone at a dynamic loading rate

In this study, the dynamic cracking processes in porcine cortical bone were visualized in real-time using the high-speed synchrotron X-ray phase-contrast imaging (PCI) technique in three osteon orientations: in-plane transverse, out-of-plane transverse and in-plane longitudinal. The dynamic flexural loading applied on the pre-notched bone specimens was introduced by a modified Kolsky compression bar. High-speed X-ray images of the entire loading events were documented with a high-speed camera. Three-dimensional X-ray micro-computed tomography was conducted to examine the intact microstructures and obtain the basic material properties of the bone material used for mechanical characterizations. The onset location, where crack initiated, and the subsequent direction, along which the incipient crack propagated, were measured quantitatively using the high-speed X-ray images and the latter was found dependent on the osteon direction significantly. The crack propagation velocities were dependent on crack extension over the entire crack path significantly for all the three directions while the initial velocity for in-plane longitudinal direction was lower than the other two directions. Straight-through crack paths were observed for in-plane longitudinal specimens while the cracks were deflected and twisted in the in-plane transverse direction. For out-of-plane transverse direction, the cracks follow paths with tortuosity fall in between the other two directions, showing a mixed mode of fractures of the former two extreme cases. The toughening mechanisms, visualized by the high-speed X-ray images, and the corresponding fracture toughness, evaluated in terms of fracture initiation toughness and crack growth resistance curve (R-curve), were also found significantly different among the three osteon directions, suggesting an overall transition from brittle to ductile-like fracture behaviors at the dynamic displacement rate (5.4 m/s) as the osteon orientation varies from in-plane longitudinal to out-of-plane transverse, and to in-plane transverse eventually.

Size effect of deformation nanotwin bundles on their strengthening and toughening in heterogeneous nanostructured Cu

Herein, we fabricated a heterogeneous nanostructured Cu with deformation nanotwin bundles (NTBs) embedded in a matrix of nanograins by means of dynamic plastic deformation at liquid nitrogen temperature. We conducted fracture mechanical measurements to investigate the effect of longitudinal length of the NTBs on their strengthening and toughening. Results suggest that an increase in the NTB length had a marginal influence on the tensile strength; however, it remarkably promoted both fracture initiation toughness and crack growth toughness. Longer NTBs are more effective not only in intrinsically enhancing crack tip plasticity by suppressing the strain localization and void nucleation but also in serving as crack bridges behind the crack front to extrinsically resist the fracture by shielding the crack tip.

An assessment of the J-integral test for a metallic foam

An assessment is made of the J-integral test procedure for initial crack growth in an open-cell aluminium alloy foam by combining finite element (FE) simulations with experiment. It is found experimentally that a zone of randomly failed struts develops ahead of the primary crack tip, and is comparable in size to that of the plastic zone. Hence, a crack tip J-field is absent at the initiation of crack growth from the primary crack tip. This implies that the measured JIC value and the J versus crack extension Δa curve cannot be treated as material properties despite the fact that the specimen size meets the usual criteria for J validity. The toughness tests were performed on a single-edge notched bend specimen, and crack extension was measured by the direct current potential drop method, by digital image correlation and by X-ray computed tomography. The crack growth resistance of the foam is associated with two distinct zones of plastic dissipation: (i) a bulk plastic zone emanating from the crack tip (containing a cluster of randomly failed struts), and (ii) a crack bridging zone behind the advancing crack tip. The applicability of a cohesive zone model to predict the fracture response is explored for the observed case of large scale bridging. To do so, FE simulations are performed by replacing the discrete lattice of the open-cell metallic foam by a compressible, elastic-plastic hardening solid while the fracture process zone in the foam is represented by a cohesive zone, as characterised by a tensile traction versus separation law. A detailed comparison of the cohesive zone model with experimental observations reveals that it is possible to capture the load versus displacement response but not the details of the fracture process zone using a single set of process zone parameters.

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 phase in surface layers on rotating-bending fatigue strength of SCM415 steel after austenitic nitriding

The rotating bending fatigue strength of JIS-SCM415 steels after austenitic nitriding was evaluated to find the surface phase that gave the highest strength. Nitriding was performed at 903 K under conditions of KN = 0.15, 0.25 and 0.35 atm-1/2. In addition, specimens which were tempered at 523 K for 7.2 ks after nitridings with KN = 0.15 and 0.25 atm-1/2 were prepared. A specimen prepared by nitrocarburizing at 853 K was also evaluated as a comparison. Specimens nitrided at 903 K at KN = 0.15 atm-1/2 showed the highest fatigue limit because the 10 μm thick surface layer was a dual phase of austenite and martensite that might have a fine microstructure which inhibited initial crack generation and growth. Specimens nitrided at 903 K at KN = 0.35 atm-1/2 showed the second highest fatigue limit because the ε compound layer that was 20 μm thick had a high compressive residual stress. Specimens nitrides at 903 K at KN = 0.25 atm-1/2 showed the third highest fatigue limit because of the formation of a 6 μm thick compound layer which was a dual phase of ε and γ’ that had few voids and a high toughness (low hardness) in addition to its compressive residual stress. Tempered specimens showed a lower fatigue limit than that of non-tempered specimens. This was because the surface layer became brittle in tempered specimens. Specimens other than the tempered specimen in KN = 0.25 atm-1/2showed a fatigue strength higher than that of a sample nitrocarburized at 853 K.

Slow crack growth resistance of modern PA-U12 grades measured by cyclic cracked round bar tests and strain hardening tests

A critical failure mechanism in long-term applications of plastic structures, such as piping systems, is considered to be crack initiation and subsequent Slow Crack Growth (SCG). Thus, safe installation and operation during service lifetime of such structures are not conceivable without the knowledge of the resistance against SCG for any new material, like Unplasticized PolyAmide 12 (PA-U12). Unfortunately, long-term static tests at elevated temperatures lead to unreasonably long test times and measurements may also be affected by thermal aging and hydrolysis. Against this backdrop, the current study examines two accelerated test methods, namely the cyclic Cracked Round Bar (CRB) test as well as the Strain Hardening (SH) test, in order to characterize the SCG behavior. Both were originally developed for PolyEthylene (PE) and successfully implemented in recent years. While the cyclic CRB test measures SCG directly, accelerated through cyclic loading, the SH test quantifies the molecular disentanglement resistance, which determines craze formation and breakdown. The focus of this work was initially put on the extension of the CRB test towards PA-U12 grades, checking the occurrence of actual crack initiation and propagation. Afterwards a correlation to SH test results was done in terms of SCG resistance. Thereby, adequate test conditions were developed and the influence of the Molecular Weight (MW), expressed by the Viscosity Number (VN) of the PA-U12 grades, was considered. Results confirm the suitability of each method to rank SCG resistance and show a good correlation of the abovementioned SMall scale Accelerated Reliable Test (SMART) methods, highlighting their sensitivity to long-term relevant molecular parameters.

Local and non-local modeling aspects of three-dimensional cracks growth initiation

Three-dimensional crack growth initiation is examined under the assumption that the crack front remains smooth. Two important modeling issues, specific to three-dimensional rather than two-dimensional cracks, are addressed. First, it is established that, at each point along the crack front, the velocity and configurational force are two-dimensional vectors, lying in the local normal plane. This allows one to generalize any two-dimensional crack growth criterion to three dimensions. Second, a simple mesoscopic model to account for along-the-front non-locality is proposed. This model eliminates pathological growth patterns ubiquitous to basic models applied to three-dimensional cracks. Further, the model is straightforward to use as it relies on standard fracture properties only.

Initiation and Early-Stage Growth of Internal Fatigue Cracking Under Very-High-Cycle Fatigue Regime at High Temperature

The initiation and early-stage crack growth under very-high-cycle fatigue (VHCF) at room temperature, 750 °C, and 850 °C on directionally solidified Ni-base superalloy have been investigated. There was little frequency effect of 20 kHz on fatigue lives when compared with 100 Hz, nor did the deformation and fracture mechanisms. Dislocation tangles re-arranged themselves to form well-defined networks at interface of γ/γ′, accounting for the enhanced fatigue strength at 850 °C in VHCF regime when compared to that at 750 °C. In most cases, internal casting pore was the crack initiation site. Crack initiation and early-stage growth occurred on one of the {111} planes or their intersecting planes, a characteristic of Stage I cracking. With the use of optimized intermittent loading conditions, both the initiation and early-stage crack growth processes were successfully tracked on the basis of fine but visible beach marks within the Stage I cracking region. The first registered fatigue beach mark can be as close as only 86 μm to the crack initiation site and the crack length increased steadily over the whole early-stage crack growth stage. The enhanced fatigue strength at 850 °C can be rationalized with the higher threshold for propagating the early-stage crack. The fraction of fatigue life consumed for early-stage crack growth reduces with the decreasing stress, eventually leading to the initiation-controlling VHCF fatigue failure. The implications of these results are discussed with respect to the model prediction of fatigue life and fatigue strength.

Modelling the micro-damage process zone during cortical bone fracture

Cortical bone employs intrinsic toughening mechanisms to delay crack growth initiation and propagation hence increasing its fracture toughness. Computational models of the bone fracture process though do not explicitly capture these intrinsic local toughening mechanisms. Such models could provide insights into possible sub-microscale mechanisms involved in the bone fracture process. Therefore, in this study, the intrinsic toughening mechanism referred to as the micro-damage process zone (MDPZ) was modelled using a bi-linear continuum damage law. This model was then experimentally validated using single edge notch bending specimens and digital image correlation for strain field measurements. The size and shape of the micro-damage process zone as well as the load-deflection curves generated by the model reasonably replicated those measured experimentally. The results indicate that the continuum damage mechanics approach is a robust means of modelling the MDPZ at the continuum level and with further development of the model can provide a useful tool for studies of the fracture process in cortical bone.