Dynamic Crack Growth Research Articles

Dependence of crack growth kinetics on dendrite orientation and water chemistry for Alloy 182 weld metal in high-temperature water

Stress corrosion cracking growth rates of Alloy 182 weld metals in T–S and T–L orientations in 288°C pure water with various dissolved oxygen and hydrogen concentrations were measured. Extensive inter-dendritic stress corrosion cracking paths on the side surfaces and fracture surfaces were observed. The crack growth path in the T–S orientation specimen was perpendicular to the applied loading direction, and parallel to the loading direction in the T–L specimen. Crack growth rates of the T–S specimen were significantly higher than those of the T–L specimen under the same test conditions. The crack growth rate decreased significantly with decreasing dissolved oxygen concentration. Adding dissolved hydrogen in water caused an apparent decrease of the alternating current potential drop signal during crack growth monitoring.

Investigating the effects of micro-defects on the dynamic properties of rock using Numerical Manifold method

In this study, the Numerical Manifold method (NMM) is extended to investigate the effects of micro-defects on the dynamic mechanical properties of rock under different strain rates. The displacement decomposition technique is incorporated in the NMM to estimate the mixed mode stress intensity factors at the crack tip. A dynamic crack growth criterion is also incorporated in the NMM for crack growth analysis. The developed NMM is first validated by a simple sliding crack model. The developed model is then applied to investigate the effects of the micro-cracking properties such as initial micro-crack length, initial micro-crack inclination angle and initial micro-crack separation distance on the dynamic mechanical properties of a granite under different strain rates ranging from 10−4/s to 100/s. The effect of confining stress on the granite dynamic strength is also investigated. Simulation results illustrated that the initial micro-crack length and the confining stress have a significant effect on the dynamic strength. The effect of micro-crack separation distance, on the other hand, is heavily dependent on the ratio of separation distance to the initial micro-crack length.

A recurrent neural network for modeling crack growth of aluminium alloy

A new recurrent neural model for crack growth process of aluminium alloy is developed in this work. It is shown that a recurrent neural network with the feedback loops at the output layer is constructed to model the dynamic relationship between the crack growth and cyclic stress excitations of aluminium alloy. The output feedback loops in the neural model play the role of capturing the fine changes of crack growth dynamics. The Extreme Learning Machine is then used to uniformly randomly assign the input weights in a proper range and globally optimize both the output weights and feedback parameters, to ensure that the dynamics of crack growth under variable-amplitude loading can be accurately modeled. The simulation results with the averaged experimental data of the 2024-T351 aluminium alloy show that the excellent modeling and prediction performance of the recurrent neural model can be achieved for fatigue crack growth of aluminium alloys.

Fatigue Behavior of Nickel Based Super Alloy 718 in a Hot Corrosive Environment

Abstract Nickel based super-alloys when exposed to a combination of high temperature and low melting point fused corrosion products, result in early fatigue failure compared to their response in high temperature benign environment. The high cycle fatigue (HCF) (at 550 and 625°C) as well as fatigue crack growth (at 550°C) behavior of a nickel based super alloy 718 in hot corrosive environment (Na2SO4+NaCl salt coating), typical of marine engine environment, is presented in this paper. At least an order of magnitude decrease in fatigue life is noticed when the temperature is changed from 550 to 625°C at stress levels below 450 MPa. The statistical analysis of scatter in fatigue lives at different stress levels is performed and a Weibull–Inverse power law model is fitted to the stress–fatigue life data of alloy 718 in hot corrosive environment. Fracture surface examination of hot corrosion fatigue failures showed higher crack growth rates compared to uncoated high temperature HCF fracture surfaces. Fatigue crack growth rate at 550°C in a hot corrosive environment increases by an order of magnitude at 0.5 Hz in the Paris region compared to crack growth kinetics at 2 Hz in lab air environment at the same temperature. The fracture surface shows a mix of transgranular and intergranular mode of crack in the propagation region at 0.5 Hz. A fatigue failure diagram is proposed combining the thresholds of maximum stress level from endurance tests and maximum stress intensity factor from fatigue crack growth tests to demarcate the regions of propagating and non-propagating cracks during hot corrosion fatigue. The complexities in incorporating the effect of loading frequency in fatigue failure diagram are highlighted.

Yielding and crack growth testing of polymers under severe liquid media conditions

Several techniques for testing under superimposed mechanical and environmental loading are described in terms of test devices and test arrangements implemented on conventional mechanical test equipment (universal screw-driven test machines or electro-dynamic test machines). As to the application of mechanical loads, monotonic, static and cyclic tests are covered, including standard tensile tests and fracture mechanics based experiments. The former case emphasizes the determination of liquid environments on modulus, yield and post-yield behavior using conventional dumbbell specimens of the 5A Type (ISO 527) or notched pipe ring tensile (NPR-T) specimens. The latter methods are designed to obtain fatigue life and crack growth kinetics data using cracked round bar (CRB) or compact type (CT) specimens. Selected examples of material characterization for various loading conditions in air, water and liquid hydrocarbon environments are described and discussed with respect to predict material behavior under simultaneous mechanical and environmental loads.

Wallner lines, crack velocity and mechanisms of crack nucleation and growth in a brittle bulk metallic glass

Mode I fracture experiments were conducted on brittle bulk metallic glass (BMG) samples and the fracture surface features were analyzed in detail to understand the underlying physical processes. Wallner lines, which result from the interaction between the propagating crack front and shear waves emanating from a secondary source, were observed on the fracture surface and geometric analysis of them indicates that the maximum crack velocity is ∼800ms−1, which corresponds to ∼0.32 times the shear wave speed. Fractography reveals that the sharp crack nucleation at the notch tip occurs at the mid-section of the specimens with the observation of flat and half-penny-shaped cracks. On this basis, we conclude that the crack initiation in brittle BMGs is stress-controlled and occurs through hydrostatic stress-assisted cavity nucleation ahead of the notch tip. High magnification scanning electron and atomic force microscopies of the dynamic crack growth regions reveal highly organized, nanoscale periodic patterns with a spacing of ∼79nm. Juxtaposition of the crack velocity with this spacing suggests that the crack takes ∼10−10s for peak-to-peak propagation. This, and the estimated adiabatic temperature rise ahead of the propagating crack tip that suggests local softening, is utilized to critically discuss possible causes for the nanocorrugation formation. Taylor’s fluid meniscus instability is unequivocally ruled out. Then, two other possible mechanisms, viz. (a) crack tip blunting and resharpening through nanovoid nucleation and growth ahead of the crack tip and eventual coalescence, and (b) dynamic oscillation of the crack in a thin slab of softened zone ahead of the crack-tip, are critically discussed.

Fatigue life prediction for aging RC beams considering corrosive environments

A new crack growth-based corrosion fatigue life prediction method for aging reinforced concrete beam is proposed in this paper. The proposed method couples the corrosion growth kinetics and fatigue crack growth kinetics together. The relationship between corrosion damage morphology and corrosion loss is investigated by the experimental results. A phenomenological model is proposed to obtain the stress concentration factor model under different corrosion loss conditions. Following this, the developed model is integrated with an asymptotic method to calculate the stress intensity factor for the crack at corrosion pit roots. The fatigue life is predicted by the integration of the fatigue crack growth rate curve from the equivalent initial flaw size to the critical length. Probabilistic analysis methodology is proposed to consider various sources of uncertainties for the fatigue life prediction. Fatigue life prediction results are validated with experimental observations for various corroded steel bars and beams available in the literatures.

Dynamic crack growth modeling technique based upon the SGBEM in the Laplace domain

This paper presents a new dynamic crack growth prediction tool based upon the symmetric Galerkin boundary element method (SGBEM) in the Laplace domain, combined with the maximum hoop stress (MHS) criterion. The fast Laplace inverse transform developed by Durbin is employed to obtain time-domain solutions of the stress intensity factors required by the MHS criterion. In this work, the proposed Laplace SGBEM tool is used to predict crack trajectories in homogeneous and non-homogeneous isotropic brittle materials under dynamic loading conditions. The effect of the elastic constant mismatches is investigated for the influence of crack–inclusion interaction on crack growth.

Three Dimensional Dynamic Analysis of Crack Growth in Unreinforced Baked Brick Shear Wall

This paper presents the 3D dynamic crack growth simulation of unreinforced baked brick shear wall by using particle discretization scheme finite element method (PDS-FEM), which is efficient and capable of computing bifurcation/branching in cracking. The technology of fast modelling of bricks and cements by applying VB script in AUTOCAD is illustrated briefly. The shear wall including mortar joints is modelled in detail. The model parameters are calibrated by using standard static tests. Since the computation cost is high in structural level fracture analysis, parallel computation technology is employed. Finally, with two-phase failure criterion of mortar under multi-dimension stress state, the performance of low and high loading speed is compared. The numerical results verify the availability of dynamic fracture analysis of masonry structure by using PDS-FEM.

Nonlinear fracture dynamics of laminates with finite thickness adhesives

Finite thickness interfaces, such as structural adhesives, are often simplified from the modeling point of view by introducing ideal cohesive zone models that do not take into account the finite thickness properties in the evaluation of the interface stiffness and inertia. In the present work, the nonlinear dynamic response of those layered systems is numerically investigated according to the finite element method. The weak form of the dynamic equilibrium is written by including not only the contribution of cohesive interfaces related to the virtual work exerted by the cohesive tractions for the corresponding relative displacements, but also considering the work done by the dynamic forces of the finite thickness interfaces resulting from their inertia properties. A fully implicit solution scheme both in space and in time is exploited and the numerical results for the double cantilever beam test show that the role of finite thickness properties is not negligible as far as the crack growth kinetics and the dynamic strength increase factor are concerned.

Quasi-static and dynamic fracture behaviour of rock materials: phenomena and mechanisms

An experimental investigation is conducted to study the quasi-static and dynamic fracture behaviour of sedimentary, igneous and metamorphic rocks. The notched semi-circular bending method has been employed to determine fracture parameters over a wide range of loading rates using both a servo-hydraulic machine and a split Hopkinson pressure bar. The time to fracture, crack speed and velocity of the flying fragment are measured by strain gauges, crack propagation gauge and high-speed photography on the macroscopic level. Dynamic crack initiation toughness is determined from the dynamic stress intensity factor at the time to fracture, and dynamic crack growth toughness is derived by the dynamic fracture energy at a specific crack speed. Systematic fractographic studies on fracture surface are carried out to examine the micromechanisms of fracture. This study reveals clearly that: (1) the crack initiation and growth toughness increase with increasing loading rate and crack speed; (2) the kinetic energy of the flying fragments increases with increasing striking speed; (3) the dynamic fracture energy increases rapidly with the increase of crack speed, and a semi-empirical rate-dependent model is proposed; and (4) the characteristics of fracture surface imply that the failure mechanisms depend on loading rate and rock microstructure.

Modal analysis of the dynamic crack growth and arrest in a DCB specimen

This paper discusses dynamic crack growth and arrest in an elastic double cantilever beam (DCB) specimen, simulated using the Bernoulli–Euler beam theory. The specimen is made from two different materials. The section of interest, where the dynamic crack growth takes place, is made from a material, the fracture energy of which will be denoted \(2\Gamma _1 \). The initial crack grows slowly in a starter material with a fracture energy \(2\Gamma _{0} \;(\Gamma _{0} >\Gamma _1 )\), while opposed displacements on both arms of the specimen are continuously increased. As the crack reaches the material interface at \(\ell =\ell _c \), the loading displacement is instantly suspended, and the crack suddenly propagates through the test zone, until it stops at \(\ell =\ell _A \). During this process, the energy \(2\Gamma _1 (\ell _A -\ell _c )\) is dissipated. The beam motion and the fracture process during the fast crack growth stage are investigated, based on the balance energy associated to the Griffith criterion. The motion equations are approximated using a modal decomposition up to order \(N\) of the beam deflection (the analysis has been performed up to \(N=10\) but in most cases \(N=5\) is sufficient to obtain an accurate solution). This process leads to a set of N second order differential equations whose unknowns are the mode amplitudes and their derivatives, and another equation the unknowns of which are the current crack length \(\ell (t)\), velocity \(\dot{\ell }(t)\) and acceleration \(\ddot{\ell }(t)\). To demonstrate the accuracy of this method, it is first tested on a one dimensional peeling stretched film problem, with an insignificant bending energy. An exact solution exists, accurately approximated by the modal solution. The method is then applied to the DCB specimen described above. Despite the rather crude nature of the Bernoulli–Euler model, the results crack kinematics, and specially the arrest length, correspond well to those obtained by the combined use of finite elements and cohesive zone models, even for a few modes. Moreover, for the basic mode \(N=0\) (also referred to as Mott solution), even if the crack kinematics is not accurately reproduced, the prediction of the crack arrest length remains correct for moderate ratios. Some parametric studies about the beam geometry and the initial crack velocity are performed. The relative crack arrest \(\ell _A /\ell _c \) appears to be almost insensitive to these parameters, and is mainly governed by the ratio \(R=\Gamma _{0} /\Gamma _1 \) which is the key parameter to predict the crack arrest.

Numerical Simulations of Dynamic Fracture Growth Based on a Cohesive Zone Model with Microcracks

AbstractA cohesive finite element model is employed to study the dynamic crack growth mechanisms in different materials. Dynamic crack propagation is analyzed numerically for a 2D square specimen with prescribed initial microcracks subjected to tensile loading conditions. In the cohesive zone model, the initial microcracks or defects are set up as traction-free interfacial surfaces in the specimen plane. The phenomena of microcrack initiation, nucleation, growth, coalescence, and propagation are captured from the simulation. The numerical simulation results have shown that the initially prescribed mircocrack or defect direction will result in a different macrocrack propagation path and crack branching path.

Cavitation resistance of epoxy-based multilayer coatings: Surface damage and crack growth kinetics during the incubation stage

Four epoxy-based multilayer coating systems, with thicknesses of 380±20μm, 650±10μm, 720±30μm and 920±20μm, were applied manually onto stainless steel samples and subjected to vibratory cavitation tests according to ASTM G32-09 standard. In order to correlate the cavitation resistance of the coating systems with some of their mechanical properties, instrumented micro indentation tests were performed to determine hardness, resilience, total plastic work, among others, as a function of the thickness of the coatings. Examination of the surfaces by Scanning Electron Microscopy (SEM) revealed that the surface damage in all the coatings was caused by incubation and growth of cracks. Statistical analysis of crack growth data allowed determining a behavior law characteristic for each coating system, which was adjusted with proper parameters related to the mechanical properties measured by micro indentation. In particular, a good correlation was obtained among cavitation resistance, coating thickness and hardness-to-Young modulus ratio H/E.

Experimental and computational investigation of the dynamic behavior of Al–Cu–Li alloys

A dislocation-density based crystalline plasticity formulation, finite-element techniques, rational crystallographic orientation relations and a new fracture methodology were used to predict the failure modes associated with the high strain rate behavior of high strength Al–Cu–Li alloys. Widely used aluminum alloy 2195 (AA2195) was taken as the representative of Al–Cu–Li alloys. Experimental characterization using Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) were performed to gain insights on microstructural behavior. The alloy aggregate was modeled with representative microstructures that included precipitates, dispersed particles, and different grain boundary (GB) distributions. The new fracture methodology, based on overlapping elements and phantom nodes, was used with a fracture criteria specialized for fracture on different cleavage planes to investigate dynamic crack nucleation and growth. The compressive behavior of AA2195 under high strain rate loading was compared with that of Al–Cu alloy 2139 to further understand the behavior of the AA2195 with the more ductile AA2139. The predictions quantify how local microstructural effects, due to precipitates and dispersed particles, have a dominant effect on crack initiation and growth.

Fatigue life prediction of a rubber material based on dynamic crack growth considering shear effect

This paper suggests a fatigue life calculation method (A fatigue life calculation method is suggested) for rubber components based on the dynamic crack growth considering shear effect. Dynamic tearing tests were carried out, and the crack length was measured using an optical microscope to calculate the dynamic crack growth rate which characterizes and determines the fatigue life. The algorithm was numerically implemented in finite element code, ABAQUS standard, by using the user subroutine and applied to several rubber components. In the finite element analysis, deformation mode of an element was classified into tension and shear, and a weighting factor was multiplied to a strain energy density according to the degree of shear strain. Tension and compression of an elliptic dumbbell specimen was simulated in order to verify the material parameters of the suggested fatigue life prediction equation and to enhance the reliability of the algorithm. Finally, the fatigue life of a vehicle suspension bushing was calculated and compared with test. There were good agreements in the failure location and the magnitude of the fatigue life.

Simulation of Dynamic Crack Propagation under Quasistatic Loading

Simulation of dynamic crack growth under quasistatic loading was performed using finite element method with embedded incubation time fracture criterion [. Experimental data, used for comparison was taken from [. ANSYS finite element software package was used in order to receive FEM solutions. The fracture criterion was implemented as an external procedure written in C++. The developed model is not using and trimming parameters. Only initial experimental conditions and material properties measured in separate experiments are used. Received dependencies for crack velocities as a function of time closely follow those observed in experiments by J.Finberg. Simulation results provide a possibility to conclude that the incubation time approach is an effective method to predict fracture initiation as well as crack propagation at various loading rates. Dependencies of an instant crack velocity on the current level of stress intensity factor received in this work for quasistatic loads and in [ for high-rate loads is discussed and compared to those experimentally observed by K. Ravi-Chandar and W.G. Knauss [ and J. Finberg [.

Dynamic behavior of short aramid fiber‐filled elastomer composites

Short fiber reinforcement is a suitable way to improve the tribological properties of elastomers. However, rubbers products are often exposed to highly dynamic mechanical loadings. Hereby it is crucial to study the change in dynamic behavior due to the addition of short fibers. Therefore, these properties were investigated in terms of dynamic mechanical thermal analysis, heat build‐up (HBU), and fatigue crack growth resistance under cyclic loadings for two different rubber compounds. A peroxide cured ethylene–propylene–diene rubber (EPDM) and a sulfur cured natural rubber (NR) compound were chosen and reinforced with two types of short aramid fibers. It was found that the short fibers could contribute to the improvement in the crack growth resistance, HBU, and the dynamic mechanical behavior of the composites depending on the testing conditions. POLYM. ENG. SCI., 54:2958–2964, 2014. © 2014 Society of Plastics Engineers

182 合金と希釈模擬材および低合金鋼の界面近傍でのき裂進展挙動

182 合金と希釈模擬材および低合金鋼の界面近傍でのき裂進展挙動

Dynamic Nonlinear Crack Growth at Interfaces in Multi-layered Materials

Finite thickness interfaces, such as structural adhesives, are often simplified from the modelling point of view by introducing ideal cohesive zone models that do not take into account the finite thickness properties in the evaluation of the interface stiffness and inertia. In the present work, the nonlinear dynamic response of those layered systems is numerically investigated according to the finite element method. The weak form of the dynamic equilibrium is written by including not only the contribution of cohesive interfaces related to the virtual work exerted by the cohesive tractions for the corresponding relative displacements, but also considering the work done by the dynamic forces of the finite thickness interfaces resulting from their inertia properties. A fully implicit solution scheme both in space and in time is exploited and the numerical results for the double cantilever beam test show that the role of finite thickness properties is remarkable as far as the crack growth kinetics and the dynamic strength increase factor are concerned.