Fast Crack Growth Research Articles

Fast fracture in toughened glass when impacted randomly by Ice

Modelling the triggering of fracture at a pre-existing flaw is an evolving method of predicting ultimate failure in glass. This fracture mechanics approach of modelling has been shown to give more reliable predictions than a calibrated probabilistic distribution model (as is commonly adopted) when dealing with hail impact which is highly transient in nature. The dynamic stress intensity factor controlling fast crack growth is sensitive to the complex stress state surrounding the critical flaw. Finite element simulations of localised stresses in 3D could incur high computation cost which is compounded by the need to repeat computations until convergence and to simulate multiple strikes in emulating a storm scenario. In this study, closed-form expressions were developed to waive away the need of any simulations. With hail impact, boundary conditions of the glass panel need not be factored into the modelling, as the highly transient stresses are wave controlled. Input parameters are the thickness and level of prestress in glass; offset of position of strike from the known crack and its depth; and the size, velocity, and temperature of the ice impactor. Results from 40 test scenarios involving 5 offset distances, 3 crack depths, 2 glass thicknesses, and 2 sizes of ice were used to validate the prediction, and to reveal the sensitivity of the outcome of the impact to changes in the impact position relative to the known crack.

Rate-dependent augmented finite element method for fast arbitrary crack growth

Experimental observations indicate that fast crack growth often exhibits a rate dependency, which is caused by the viscosity of the material and the dissipation of energy resulting from the branching of microcracks. To address this phenomenon in simulation modeling, this study introduces a rate-dependent augmented finite element method designed for analyzing fast arbitrary crack growth. The method utilizes the maximum principal stress criterion to simulate arbitrary crack growth and predict the path of crack growth. The elements where crack growth is expected are divided into subelements, and they are re-assembled by incorporating rate-dependent cohesive elements between them to construct augmented finite elements. The cohesive elements are based on a rate-dependent trapezoid traction–separation relationship, where the critical traction and fracture toughness are modeled as functions of the separation rate and crack growth speed. The augmented finite elements only contain external nodal degrees of freedom (DoFs), while internal nodal DoFs are condensed. They can be utilized as conventional elements. A numerical algorithm based on the proposed method was developed in this article, and it was implemented in ABAQUS via user element subroutine. The effectiveness of this method and algorithm is validated through an example of dynamic crack growth in a double cantilever beam. The results demonstrate agreement with experimental and computational findings reported in the literature. Additionally, the method was applied to other structures, yielding reasonable outcomes. Analysis of these examples confirms the method’s utility in predicting fast arbitrary crack growth under dynamic load conditions.

Equivalent initial damage sizes (EIDS) for type 1C anodised aluminium alloy 7085-T7452 under variable amplitude loading

Aluminium alloy (AA) 7085-T7452 is a high strength, low weight alloy used in large forgings in primary airframe applications. For corrosion protection, airframe components are anodised and this study looks at the influence of type 1C anodising on the fatigue crack growth behaviour in AA 7085-T7452 specimens that were fatigue tested with fighter wing spectrum loading. Surface etch pits formed during essential anodising preparatory processes were the primary cause of fatigue crack nucleation. The severity of the early crack growth caused by those etch pits that nucleated the worst crack in each specimen was evaluated using quantitative fractography-based fatigue crack growth measurements and the equivalent initial damage size (EIDS) of those etch pits was determined. Characterising the EIDS distribution of aircraft alloys with representative surface treatments is a fundamental input into Linear Elastic Fracture Mechanics (LEFM) based fleet failure analysis of aircraft structures. In this study, a notable increase in the average EIDS value for the worst crack present was observed whenever material or loading condition changes led to a significantly increased number of crack nucleation sites. In particular, an increase in the cyclic stress spectrum’s severity increased the number of nucleation sites in a specimen and significantly increased the average EIDS. It appears that due to variable nucleating discontinuity properties (e.g. size, shape and orientation) and material property variations, an increased number of nucleation sites increases the probability that the worst crack present will nucleate at a location that is conducive for relatively fast early crack growth. This will potentially cause shorter overall airframe component fatigue lives and so both the mechanisms and conditions of etch pit fatigue nucleation in this material and material finish shall be discussed for the benefit of failure analysts evaluating unexpected fleet fatigue cracking.

A rate-dependent cohesive zone model for simulating fast crack evolution and growth

This paper proposes a rate-dependent cohesive zone model for simulating the evolution and growth of cracks under dynamic loading. This model takes into consideration the effects of cohesive separation rate and crack growth speed on the cohesive traction-separation relationship to reflect the rate-dependent characteristics of both material viscosity and energy dissipation of micro-crack branches. The evolution algorithm of the cohesive law is presented, wherein separation rate and crack growth speed are evaluated using the backward difference method explicitly in incremental computation, leading to the determination of rate-dependent critical traction and fracture toughness. The proposed model was implemented in ABAQUS software via UEL subroutine and utilized in an example of interfacial crack growth simulation, with satisfactory agreement found among the present results, experimental data, and numerical results from the literature. The effectiveness of the rate-dependent model and numerical algorithm in simulating high speed crack growth is validated through further numerical tests.

Abnormally fast crack propagation induced by short-range ordering in iron-cobalt alloys: A combined experiments and molecular dynamics simulations

We experimentally found that FeCo21.5Cr0.2Ni0.8 alloy quenched from 500 °C, slightly above the order-disorder transition temperature, surprisingly exhibited an extreme impact brittleness. Impact fracture morphology observed by scanning electron microscope (SEM) showed that the impact brittleness was caused by the fast crack growth and poor local plasticity. Short-range ordered (SRO) structures were found in the sample quenched from 500 °C by high-resolution transmission electron microscope (HRTEM) characterizations, and the average SRO size is ~1.58 nm. We contribute the impact brittleness to the formation of SROs. Molecular dynamics (MD) simulations were carried out to reveal the SRO-induced embrittlement mechanism caused by fast crack propagation under high-speed loading. MD simulations showed SROs significantly inhibited the deformation plasticity near crack tips by suppressing the dislocation proliferation, dislocation slip, and twinning. Generalized stacking fault energy (GSFE) curves were calculated to evaluate the effect of SROs on antiphase boundary (APB) energies, unstable stacking energy γusf, and intrinsic stacking energy γisf. SROs were found to prominently increase the APB energies and γisfγusf, fully supporting the SRO-induced poor dislocation and twinning plasticity near crack tips in MD simulations. These findings revealed the potential SRO-induced impact brittleness and its embrittlement mechanism, and provide a design consideration for the engineering application of materials under impact loading.

Impact of Strain-Induced Crystallization on Fast Crack Growth in Stretched cis-1,4-Polyisoprene Rubber.

cis-1,4-Polyisoprene (IR) elastomers harden via strain-induced crystallization (SIC) when the imposed stretch (λ) exceeds the onset value of SIC (λ*). We investigate the Mode-I fast crack growth in the IR sheets as a function of λ in a pure shear geometry. The steady-state crack velocity (V) increases with increasing λ, and V exceeds the shear wave speed of sound at λ > λs. Further stretch beyond λ* (>λs) causes SIC-driven hardening, resulting in a pronounced increase in V. The characteristics of the crack-tip strain field are also significantly influenced by the SIC-driven hardening: The crack-tip opening displacement increases with increasing λ at λ < λ* but exhibits an abrupt reduction beyond λ*. The crack-tip singularity and the area of strain increment caused by the crack growth change discontinuously around λ*. The abrupt variations in these crack-tip characteristics result from the considerable differences in the mechanical properties prior to the crack growth between the entirely amorphous state at λ < λ* and the partially crystallized state at λ > λ*.

Effects of heat treatment on corrosion fatigue and stress corrosion crack growth of additive-manufactured Alloy 800H in high-temperature water

Crack growth behaviour of additively manufactured (AM) Alloy 800H were studied under cyclic and static load in high-temperature water, focusing on heat treatment effects. Under cyclic loading, heat treatment gently affected corrosion fatigue. The growth rate is comparable to wrought 800H. As the load cycle frequency reduced to below 0.001 Hz, AM 800H exhibited inconsistent stress corrosion cracking response as compared to wrought 800H. Depending on heat treatment, inhomogeneous crack propagation and localized pinning were observed in stress-relieved and recrystallized AM 800H, attributed to carbides, microstructural inhomogeneity, and twin boundaries. Grain boundary sensitization at ˜10 wt% Cr caused fast crack growth.

Drying-Induced Pressure Rise and Fracture Mechanics Modeling of the Sphagnum Capsule

The Sphagnum capsule can disperse spores at an extraordinarily high velocity and acceleration during drying. Briefly, the pressure rise induced by the decrease in the environmental humidity inside the spore chamber causes crack growth between the lid and the capsule wall. At a critical condition, the lid of the capsule suddenly fractures, and the top spores are propelled by the high pressure. Motivated by this phenomenon, we develop a similar mechanics model to study the drying-induced pressure rise and the fracture mechanism of the Sphagnum capsule in this paper. We investigate the drying-induced pressure rise and obtain the deformation configuration for various stiffness ratios of different parts. We also establish a fracture mechanics model and calculate the energy release rate to study the lid separation during the ejection of spores. We find that the energy release rate increases with crack growth when the crack is short, maximizes at an intermediate central crack angle of around [Formula: see text], and gradually decreases with further increase in the central crack depending on the loading type. Such a nonmonotonic relationship between the energy release rate and the crack length can be readily used to explain the spontaneously fast unsteady crack growth and the following potential crack arrest reported in the literature. The results and the modeling method obtained in this paper can be used to explain similar fracture-related spore launching of plants and design bioinspired structures to realize the drying-induced fast movement.

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)].

Predicting ductile fracture of cracked pipes using small punch test data

This paper proposes a numerical method to simulate ductile fracture of a cracked pipe using small punch test data. The method is using FE damage analysis method based on the multi-axial fracture strain energy density model. The parameters in the damage model can be extracted solely from small punch test data, using which compact tension and circumferential through-wall cracked pipe tests are simulated. To validate the proposed method, simulation results are compared with TP316L experimental data. For the compact tension test, faster crack growth is predicted but for the pipe test, good agreement is obtained. The effect of the stress triaxiality on the prediction accuracy is discussed.

FRACTURE AND CRACK PROPAGATION OF METALLIC BILAYERS USING QUASI-CONTINUUM SIMULATIONS

The effects of the constituting material, initial crack length, and crystal orientation on the fracture and mechanics of bilayers with an interface crack under tension are studied using quasi-continuum simulations in terms of atomic trajectories, strain distribution, the stress-strain curve, and the crack growth-strain curve. The simulation results show that at the initial tension stage, the bilayers exhibit linear elasticity regardless of initial crack length and constituting material. With an increase in initial crack length at a layer interface, the yield stress, yield strain, and ultimate stress of the bilayers decrease. Bilayers with along er initial crack under tension have faster crack growth. Bilayers with a structural orientation of [020] versus [101̅] have lower mechanical strength compared to that of those with structural orientations of [100] versus [010] and [1̅10] versus [010]. The Ni/Ni bilayer has the highest yield stress and ultimate stress under tension.

Poroelastic effects on steady state crack growth in polymer gels under plane stress

Fracture experiments on polymer gels are often conducted with thin specimens, which are close to plane stress in two-dimensional models. However, many of the previous theoretical and numerical studies on fracture of polymer gels have assumed plane strain conditions. The subtle differences between the plane stress and plane strain conditions are elucidated in this paper based on a linear poroelastic formulation for polymer gels, including the asymptotic crack-tip fields and finite element simulations of steady-state crack growth in long strip specimens. Moreover, a poroelastic cohesive zone model is adopted to study the rate-dependent fracture process of polymer gels. It is found that, without the cohesive zone model, the normalized crack-tip energy release rate at the fast crack limit is greater than the slow crack limit, suggesting reduced poroelastic toughening for fast crack growth under plane stress conditions, while the two limits are identical under plane strain conditions. With a solvent-permeable cohesive zone for the case of immersed specimens, solvent diffusion within the cohesive zone enhances the poroelastic toughening significantly as the crack speed increases, leading to a rate-dependent traction-separation relation. On the other hand, with no solvent diffusion in the cohesive zone for the not-immersed case, the poroelastic toughening effect diminishes as the crack speed increases. Based on the present study, the intrinsic steady-state fracture toughness of a poroelastic gel can be determined using long-strip pure-shear specimens, which in general is smaller than the applied energy release rate.

Crack Propagation Behaviour in Thin Aluminium Alloy Sheet Repaired with Adhesively Bonded CFRP Patch

For reducing the crack tip stress intensity factor, adhesively bonded composite patches are employed for crack repair in thin structure. Experimental work was carried out to study environmental assisted cracking behaviour of crack repaired with single-sided bonded patch in aluminium alloy of 7xxx series in peak-aged condition. The observations were also made on crack in this alloy without any patch repair. The specimen developed for conducting the experiments simulated plane stress condition of thin structure. The patched and un-patched tensile test specimens were loaded simultaneously at constant load in the presence of 3.5% weight NaCl solution. The experimental set-up simulated the saline or corrosive environment. Aggressive effect of corrosion attack on crack due to anodic dissolution along with hydrogen embrittlement leading to faster crack growth in patched specimen was revealed compared to un-patched specimen. The present study is the improvement over similar researches carried out earlier in terms of design of test specimen under plane stress condition so as to obtain all the three regions of crack growth.

Crack growth behavior, fracture mechanism, and microstructural evolution of G115 steel under creep–fatigue loading conditions

Creep–fatigue crack growth (CFCG) behavior, fracture mechanism, and microstructural evolution of G115 steel under different hold times and initial stress intensity factor ranges (ΔKin) were investigated. The crack propagation durations increased significantly with decreasing ΔKin and increasing dwell time. In da/dN vs. ΔK plots, a “hook” was formed, and it entered the fast crack growth zone at approximately ΔK=36MPam, which corresponds to σref = σ0.2 under plane stress. In (da/dt)avg vs. (Ct)avg plots, data for a specific hold time in a wide load range exhibited a unique relationship. It appears that the parameter D0 in this relationship is affected by hold time and constraint, while φ is independent of these factors. Scanning electron microscopy analyses revealed that with increasing hold time (60–600 s), the fracture mode changed from transgranular-dominated to intergranular-dominated. Meanwhile, the fracture mode changed from intergranular-dominated to predominantly transgranular with high ductility when ΔKin increased (17–29 MPam). According to electron backscatter diffraction observations, many substructured and recrystallized grains formed after CFCG and the deformed grains further decreased as ΔKin increased. The increased crack resistance with decreased deformation grains may combine with the ductility fracture effect to decrease the CFCG rates under high load levels. Additionally, the local misorientation angle reduced significantly after CFCG, and the differences among various load levels were limited.

A numerical model based on ALE formulation to predict fast crack growth in composite structures

A novel numerical strategy to predict dynamic crack propagation phenomena in 2D continuum media is proposed. The numerical method is able to simulate the behavior of materials and structures affected by dynamic crack growth mechanisms. In particular, an efficient computational procedure based on the combination of Fracture Mechanics concepts and Arbitrary Lagrangian and Eulerian approach (ALE) has been developed. This represents a generalization of previous authors’ works in a dynamic framework with the purpose to propose a unified approach to predict crack propagation using dynamic or static fracture mechanics and a moving mesh methodology. The crack speed is explicitly evaluated at each time step by using a proper crack tip speed criterion, which can be expressed as function of energy release rate or stress intensity factor. In order to validate the formulation, experimental and numerical results available from the literature are considered. In addition, a parametric study to verify the prediction of proposed modeling in terms of mesh dependence phenomena, computational efficiency and numerical complexity is developed.

Effects of microstructure on fatigue crack propagation behavior in a bi-modal TC11 titanium alloy fabricated via laser additive manufacturing

In this study, the crack propagation behaviors in the equiaxed and equiaxed-columnar grain regions of a heat-treated laser additive manufacturing (LAM) TC11 alloy with a special bi-modal microstructure are investigated. The results indicate that the alloy presents a special bi-modal microstructure that comprises a fork-like primary α (αp) phase surrounded by a secondary α colony (αs) in the β phase matrix after the heat treatment is completed. The samples demonstrate a fast crack growth rate with larger da/dN values through the equiaxed grain sample versus across the equiaxed-columnar grain sample at low ΔK values (<13.8). The differences that are observed between the crack propagation behaviors (in the crack initiation stage) of the samples can be mostly attributed to the different size and morphology of the αp lamellae and αs colony within the grains in the equiaxed and columnar grain regions rather than the grain boundaries. The cracks prefer to grow along the α/β boundary with a smooth propagation route and a fast propagation rate in the equiaxed grain region, where the αp and α clusters have a large size. However, in the columnar grain region, small and randomly distributed αp lamellae generate a zigzag-shaped propagation path with a reduction in the da/dN value. Additionally, the change in the size of the αp lamellae in the equiaxed grains (heat affected bands, HAB) is also observed to influence the propagation behavior of the crack during the crack initiation stage.

Preparation and properties of hybrid epoxy/hydro-terminated polybutadiene/modified MMT nanocomposites

The effects of hydroxyl-terminated polybutadiene (HTPB) number average molecular weight ( $$ \overline{{\boldsymbol{M\fancyscript{n}}}} $$ ) and preparation temperature on montmorillonite (MMT) intercalation/exfoliation behavior were investigated. X-ray diffraction and Fourier transform infrared spectroscopy results indicated that 40 to 80°C were the preferred temperatures to obtain nanostructured HTPB/MMT. The higher $$ \overline{{\boldsymbol{M\fancyscript{n}}}} $$ (> 2800) of HTPB led to a more exfoliated nanostructure than lower $$ \overline{{\boldsymbol{M\fancyscript{n}}}} $$ as proven by transmission electron microscopy. The tensile strength, tensile modulus, and elongation at break of nanostructured HTPB/MMT/EP ternary composites were higher than those of the HTPB/EP binary blend, which was due to the HTPB/MMT hybrid structure. The effect of increasing molecular weight on impact strength in the ternary system was less than that in the binary system. The HTPB/MMT/epoxy hybrid composites with low concentration could improve the impact strength, which was due to the superposition effect of organic/inorganic nanostructures. In hybrid HTPB/MMT/epoxy composite system, the toughening was attributed to the slow crack growth and fast crack growth, and the dissipated energy of pulling or debonding nanoclay out from the matrix also contributed.

Steady-state crack growth in polymer gels: A linear poroelastic analysis

Based on a linear poroelastic formulation, we present an asymptotic analysis of the crack tip fields for steady-state crack growth in polymer gels. A finite element method is then developed for numerical analysis. A semi-infinite crack in a long strip specimen subject to plane-strain loading is studied in detail. The crack-tip fields from the numerical analysis agree with the asymptotic analysis with a poroelastic stress intensity factor, which is generally smaller than the stress intensity factor predicted by linear elasticity due to poroelastic shielding. Similarly, the crack-tip energy release rate calculated by a modified J-integral method is smaller than the applied energy release rate. The size of the poroelastic crack-tip field (or K-field) is characterized by a diffusion length scale that is inversely proportional to the crack speed. For relatively fast crack growth, the diffusion length is small compared to the strip thickness (small-scale diffusion), and the poroelastic K-field transitions to an elastic K-field at a distance proportional to the diffusion length. In this case, the crack-tip energy release rate decreases with increasing crack speed under the same loading condition. For relatively slow crack growth, the diffusion length is greater than the strip thickness (large-scale diffusion), and the poroelastic crack-tip field is confined and transitions to a one-dimensional diffusion zone ahead of the crack tip. In this case, the energy release rate increases with increasing crack speed. Both immersed and not-immersed crack face conditions are considered. Under the same loading conditions, the poroelastic stress intensity factor and the modified J-integral are higher for the immersed case than not-immersed, but they approach the same values at the fast and slow crack limits. In general, if the crack-tip energy release rate is taken to be the intrinsic fracture toughness of the gel, the applied energy release rate as the apparent fracture toughness is greater due to energy dissipation associated with solvent diffusion, which is referred to as poroelastic toughening. It is proposed that the modified J-integral can be used to determine the intrinsic fracture toughness of the gel in experiments, which may or may not depend on crack speed. Moreover, the present results are found to be qualitatively consistent with previous experiments on the effects of solvent viscosity and crack-tip soaking.

Experimental Study of High Temperature Fracture Behavior of A286 Superalloy at 650°C

In this paper an experimental work is done for investigation of high temperature fracture properties of A286 superalloy at 650°C. Stress intensity factor K and parameter C* for this superalloy are determined experimentally. For estimation of these parameters, an instrument is developed for investigation of high temperature fracture properties. For estimation of stress intensity factor, compliance method is used. For this purpose four different compact tension specimens are tested and the parameter K is estimated. Creep tests are done for the selected specimens and parameter C* is determined by semiempirical relationships at 650°C. In these tests it is concluded that the specimens are placed near the plane stress condition. Crack growth behavior of this alloy is also studied. High incubation time (600 h) leads to overaging and therefore this alloy after this time showed very ductile creep properties, and fast creep crack growth was the major result of this overaging phenomenon. Finally the obtained results are compared with well-known nonexperimental methods for determination of these parameters. The obtained results showed that the results are in good agreement with each other.

Environmental Degradation Effect of High-Temperature Water and Hydrogen on the Fracture Behavior of Low-Alloy Reactor Pressure Vessel Steels

Structural integrity of reactor pressure vessel (RPV) in light water reactors (LWR) is of highest importance regarding operation safety and lifetime. The fracture behaviour of low-alloy RPV steels with different dynamic strain aging (DSA) & environmental assisted cracking (EAC) susceptibilities in simulated LWR environments was evaluated by elastic plastic fracture mechanics tests (EPFM) and by metallo- and fractographic post-test analysis. Exposure to high temperature water (HTW) environments at LWR temperatures revealed only moderated reductions in the fracture initiation and tearing resistance of low alloy RPV steels with high DSA or EAC susceptibility, accompanied with a moderate, but clear change in fracture morphology, which indicates the potential synergies of hydrogen/HTW embrittlement with DSA and EAC under suitable conditions. The most pronounced degradation effects occurred in a) RPV steels with high DSA susceptibility, where the fracture initiation and tearing resistance reduction increased with decreasing loading rate and were most pronounced in hydrogenated HTW and b) high sulphur steels with high EAC susceptibility in aggressive occluded crevice environment and with preceding fast EAC crack growth in oxygenated HTW. The moderate effects are due to the low hydrogen availability in HTW together with high density of fine-dispersed hydrogen traps in RPV steels. Stable ductile transgranular tearing by microvoid coalescence was the dominant failure mechanism in all environments with additional varying few % of secondary cracks, macrovoids and quasi-cleavage in HTW. The observed behavior suggests a combination of plastic strain localisation by the Hydrogen-enhanced Local Plasticity (HELP) mechanism, in synergy with DSA, and Hydrogen-enhanced Strain-induced Vacancies (HESIV) mechanism with additional minor contributions of Hydrogen-enhanced Decohesion Embrittlement (HEDE) mechanism.