Crack Tip Blunting Research Articles

Experimental investigation of the time and temperature dependent growth of fatigue cracks in Inconel 718 and mechanism based lifetime prediction

The following contribution deals with the growth of cracks in low-cycle fatigue (LCF) and thermomechanical fatigue (TMF) tested specimens of Inconel 718 measured by using the replica method. The specimens are loaded with different strain rates. The material shows a significantly higher crack growth rate if the strain rate is decreased. Electron backscatter diffraction (EBSD) is adopted to identify the failure mechanism and the misorientation relationship of failed grain boundaries in secondary cracks. The analyzed cracks propagated mainly transgranular but also intergranular failure can be observed in some areas. It is found that grain boundaries with coincidence site lattice (CSL) boundary structure are generally less susceptible for intergranular failure than grain boundaries with random misorientation. For modeling the experimentally identified crack behavior an existing model for fatigue crack growth based on the mechanism of time dependent elastic–plastic crack tip blunting is enhanced to describe environmental effects based on the mechanism of oxygen diffusion at the crack tip. For the diffusion process the temperature dependent parabolic diffusion law is assumed. As a result, the time dependent cyclic crack tip opening displacement (ΔCTOD) is used as representative value to describe both mechanisms. Thus, most of the included model parameters characterize the deformation behavior of the material and can be determined by independent material tests. With the determined material properties, the proposed model describes the experimentally measured crack growth curves very well. The model is validated based on predictions of the number of cycles to failure of LCF as well as in-phase and out-of-phase TMF tests in the temperature range between room temperature and 650°C.

A numerical study of the effect of single overloads on plasticity induced crack closure

In this work, the effect of single overloads on plasticity induced crack closure is studied. An elastic-plastic finite element model was developed and the crack opening level was calculated from the contact forces along the crack flank. The effects of the loading parameters and stress state are analysed, and the mechanisms behind crack closure variations are identified. An overload is a traumatic event that eliminates material’s memory relative to the load history. Crack tip blunting is the mechanism behind this memory loss, since it eliminates crack closure. Material hardening has a major relevance on the evolution of plastic blunting, which was evident in the variation of the CTOD parameter. On the other hand, the overload produces strong plastic deformation ahead of the crack tip, giving rise to conditions for the rapid generation of crack closure higher than before the event. The peak of crack closure was found to increase linearly with the load increase above the maximum baseline value. The crack is totally closed for overload ratios of about 2.5. Empirical models were developed for the peak of crack closure, for the delay of this peak and for the stabilization distance after the overload. Finally, the stress state was found to have a major effect on crack closure level after an overload.

Corrosion Fatigue Behavior of Low-Carbon Structural Steel in NaCl Solution

Corrosion fatigue is one of the environmental degradations of metallic materials and it has become a great concern as the cause of failure of infrastructures such as bridges and offshore structures. Although it is well-known that corrosion can enhance the degradation of fatigue life, the detailed mechanism of corrosion fatigue is still unclear. Therefore, it is needed to clarify the effects of corrosion on corrosion fatigue life and on the behavior of crack initiation and propagation under cyclic deformation. In this study, corrosion fatigue behavior of low-carbon structural steel is investigated by 4-point bending tests in aqueous NaCl. The material used in this study was low-carbon structral steel (JIS SM490). The chemical composition of the steel is listed in Table 1. The material was heat-treated at 845 OC for 45 min and then quenched in an oil bath. Furthermore, the material was tempered at 550 OC for 60 min and then cooled in a water bath. The mechanical properties of the heat-treated material were evaluated to be 442 MPa of yield stress and 614 MPa of tensile stress. The heat-treated material was machined into a V-notched 4-point bending specimens in the size of 8 mm wide, 100 mm long, and 4 mm thick. The V-shaped notch of 1 mm in depth was introduced at the center of the specimens. The 4-point bending tests were performed in 1 M and 0.1 M NaCl solutions. The applied stress waveform was triangular at the stress ratio of 0.18 and the frequency of 0.167 Hz. The maximum stress applied to the specimens was changed in the range from 500 MPa to 600 MPa. In this study, fatigue life of the steel was investigated as a function of applied maximum stresses. It was found from the results that fatigue life of the steel was reduced in aqueous NaCl compared with that in air. In addition, the fatigue life in aqueous NaCl was improved by applying a cathodic potential to the steel. These results indicate that anodic dissolution of iron can enhance corrosion fatigue damage of the steel in NaCl environment. In addition, although corrosion fatigue life evaluated in 1 M NaCl was almost the same as that in 0.1 M NaCl, crack initiation was accelerated in 1 M NaCl compared with that in 0.1 M NaCl. This suggests that crack propagation is retarded due to the blunting of crack tips in the higher concentration of NaCl. Figure 1

Path-dependent J-integral evaluations around an elliptical hole for large deformation theory

AbstractAn exact expression is obtained for a path-dependent J-integral for finite strains of an elliptical hole subject to remote tensile tractions under the Tresca deformation theory for a thin plate composed of non-work hardening material. Possible applications include an analytical resistance curve for the initial stage of crack propagation due to crack tip blunting.

Synergy of crack closure, near-tip residual stress and crack-tip blunting in crack growth under periodic overloads – A fractographic study

Fatigue crack growth tests were performed on an Al-Cu alloy under specially designed load sequences with a periodic overload cycle. These overloads were either immediately followed by or preceded by underloads of controlled magnitude. Baseline loading magnitude was designed to induce near-threshold conditions well below the Paris Regime. The fatigue fracture surfaces obtained from the tests were subject to fractographic analysis. The study shows that irrespective of their magnitude, underloads preceding tensile overloads do not affect crack growth rate after the overload. However, if the underload follows an overload, it carries a deleterious effect on retardation, whose magnitude is determined by the extent to which the underload drops below baseline cycling level. In either case, observed effects do not seem to have anything to with crack closure, but rather, are attributable to the effect of underloads on near-tip residual stress during baseline cycling that in turn controls threshold stress intensity range. Post-overload crack closure is reduced by crack-tip blunting, but rapidly recovers.

Strain gradient plasticity-based modeling of hydrogen environment assisted cracking

Finite element analysis of stress about a blunt crack tip, emphasizing finite strain and phenomenological and mechanism-based strain gradient plasticity (SGP) formulations, is integrated with electrochemical assessment of occluded-crack tip hydrogen (H) solubility and two H-decohesion models to predict hydrogen environment assisted crack growth properties. SGP elevates crack tip geometrically necessary dislocation density and flow stress, with enhancement declining with increasing alloy strength. Elevated hydrostatic stress promotes high-trapped H concentration for crack tip damage; it is imperative to account for SGP in H cracking models. Predictions of the threshold stress intensity factor and H-diffusion limited Stage II crack growth rate agree with experimental data for a high strength austenitic Ni-Cu superalloy (Monel®K-500) and two modern ultra-high strength martensitic steels (AerMet™100 and Ferrium™M54) stressed in 0.6 M NaCl solution over a range of applied potential. For Monel®K-500, KTH is accurately predicted versus cathodic potential using either classical or gradient-modified formulations; however, Stage II growth rate is best predicted by a SGP description of crack tip stress that justifies a critical distance of 1 μm. For steel, threshold and growth rate are best predicted using high-hydrostatic stress that exceeds 6 to 8 times alloy yield strength and extends 1 μm ahead of the crack tip. This stress is nearly achieved with a three-length phenomenological SGP formulation, but additional stress enhancement is needed, perhaps due to tip geometry or slip-microstructure.

Influence of dynamic strain ageing on fracture behaviour and stretch zone formation of a reactor pressure vessel steel

Fracture behaviour of a reactor pressure vessel steel has been studied under dynamic strain ageing (DSA) conditions. Severity of DSA is varied by changing both the loading rate and test temperature. Significant reduction in fracture toughness is noted at \(235\,{^{\circ }}\hbox {C}\), which is recovered with further increase of temperature at slow loading rate. DSA influence on fracture behaviour diminished with temperature at fast loading rate. DSA led to repeated crack tip blunting and “ridge-valley” fractographic features. Defining the stretch zone boundary was difficult in such cases, and applying stretch zone method for determination of initiation fracture resistance seemed to be inappropriate.

Role of Chloride in the Corrosion and Fracture Behavior of Micro-Alloyed Steel in E80 Simulated Fuel Grade Ethanol Environment.

In this study, micro-alloyed steel (MAS) material normally used in the production of auto parts has been immersed in an E80 simulated fuel grade ethanol (SFGE) environment and its degradation mechanism in the presence of sodium chloride (NaCl) was evaluated. Corrosion behavior was determined through mass loss tests and electrochemical measurements with respect to a reference test in the absence of NaCl. Fracture behavior was determined via J-integral tests with three-point bend specimens at an ambient temperature of 27 °C. The mass loss of MAS increased in E80 with NaCl up to a concentration of 32 mg/L; beyond that threshold, the effect of increasing chloride was insignificant. MAS did not demonstrate distinct passivation behavior, as well as pitting potential with anodic polarization, in the range of the ethanol-chloride ratio. Chloride caused pitting in MAS. The fracture resistance of MAS reduced in E80 with increasing chloride. Crack tip blunting decreased with increasing chloride, thus accounting for the reduction in fracture toughness.

Proposal for a new CTOD calculation formula

A new CTOD calculation method is investigated in this work, considering the variation of crack tip blunting due to strain hardening. The calculation formula is constructed assuming that CTOD45FEM based on three-dimensional elasto-plastic FEM is the target CTOD to be calibrated. A new factor f is introduced to correct the plastic term. In this factor, the blunted crack tip shape is considered to depend on the strain hardening exponent, and f is given as a function of the yield-to-tensile ratio (YR) of the material and the specimen thickness. The appropriateness of the investigated CTOD calculation is verified numerically and experimentally.

Molecular dynamics simulations of crack growth behavior in Al in the presence of vacancies

Fracture properties of a material are strongly influenced by the presence of defects. In view of this, molecular dynamics simulations were performed within the framework of embedded atom method to investigate the crack tip behavior under mode I loading in FCC Al with and without defects. Defects consisting of a random distribution of vacancies were introduced in the precracked matrix. The results reveal a significant change in the fracture behavior of Al with defects. The interactions between the crack tip and vacancies were found to modify the stress concentrations at the crack tip. Increase in vacancy content lead to increased crack tip blunting and reduced crack propagation speeds as compared to the vacancy-free material, thereby toughening the material to fracture. The significance of such studies towards gaining deeper insights into the fracture behavior at higher scales is highlighted.

Mechanical strength and damage tolerance of highly porous alumina ceramics produced from sintered particle stabilized foams

Highly porous alumina particle stabilized foams were prepared by combining the concepts of particle stabilized foams and gelcasting, using sulfonate surfactants and poly vinyl alcohol (PVA) as the gelcasting polymer. The ceramic samples sintered at 1500°C for 2h had porosities from 65% to 93%, with pore sizes in two categories: “big pore” around 300 μm and “small pore”, around 100–150μm, depending on the type and amount of surfactant added. The mechanical behaviour of the foams (axial and diametral compression) depended on the overall porosity and pore size. On average, tensile and compressive strengths around 5 and 16MPa respectively were measured for samples with bigger pore sizes and larger porosities. Samples with smaller pore sizes and lower porosities produced average values of 12 and 57MPa for tensile and compressive strengths, respectively. The elastic modulus reached a maximum around 3GPa for “small pore” size samples. The effect of increasing amount of PVA in the samples had a strong effect on the green mechanical strength, but it did not significantly affect the mechanical response of the sintered alumina foams. Large and complex shape sintered components produced using this route showed a remarkable damage tolerance, due to crack tip blunting.

Flaw tolerance of metallic glasses

The flaw tolerance of bulk metallic glasses (BMGs) is evaluated using a thermoplastic synthesis approach. We found that flaw tolerance quantified by the notch toughness decreases apparently with decreasing radius until a critical value. Below this critical value, measured notch toughness is independent of its radius, revealing a flaw tolerance behavior of BMGs. We explain such flaw tolerance by a critical plastic zone originating from the BMGs' inherent crack tip blunting capability. This zone defines a characteristic distance over which stable shear banding plastic process develops prior to fracture instability. The specific characteristic distance and crack blunting capability vary widely among BMGs, which rationalizes the vast variety in their fracture behavior and suggest specific flaw tolerance. Our finding is encouraging for BMGs' structural applications since flaws smaller than the critical value are increasingly difficult to avoid but are “indistinguishable” in their influence to fracture toughness.

Challenges in fracture toughness modelling of nanocomposites

ness of hybrid polymer composites is whether or not the particles’ agglomerations are considered. Note that even in microparticle reinforced polymers the distribution of the particles is locally inhomogeneous because they get into contact as a consequence of production. In nanoparticle filled elastomers particles agglomerate and the agglomerates build even larger structures. Consequently, if we consider the close neighbourhood of a single particle there exists a much higher particle density than in larger distances. Such behaviour can be described for example by pair distribution functions (DOI: 10. 1016/j.compositesb.2014.01.002). Close particles cause high stress or strain concentrations in the matrix in between. This fact must be considered in strength and fracture toughness modelling. Filled polymer composites have been studied extensively because of their technological and scientific importance (DOI: 10.1007/BF01160743; DOI: 10.016/j. compositesb.2008.01.002; DOI: 10.1016/0266-3538 (88)90009-7). Generally, crack tip blunting, crack pinning, localized shear bands, debonding of particles as plastic voiding and localized plastic shear yielding were found to be responsible for the increase of toughness in thermoset composites. Models that consider such effects are able to describe some of the experimental results properly (DOI: 10.1007/s10853009-4064-9; DOI: 10.1016/j.compscitech.2009.12. 024). Also the effect of particle size on plastic void growth has been taken in account (DOI: 10.1016/ j.mechrescom.2015.2.002). Homogeneous particle distribution cannot be expected in ‘real’ materials. Increased number of nearest neighbours in microparticle filled composites and agglomerate formation in nanocomposites must be considered. Stiffness is not influenced by these events because at small deformations no debonding occurs. However, if strength or fracture toughness are computed, such local heterogeneities should be considered. As basic mathematical and experimental data are available, the modelling should involve this structural level. In the first step any mathematical pair distribution function can be used to calculate the number of particles close to the chosen one in the ensemble. High stress concentration in between such a pair may initiate debonding. The ligament in between the debonded particles becomes unloaded and their outer region overloaded accompanied with yielding. Debonding does not contribute much to energy dissipation but triggers matrix yielding that enhances the toughness. Debonding of particles leads to a reduction of tensile strength, too. Let us go in this direction to get a deeper understanding in the mechanical and fracture mechanical behaviours!

高張力鋼板における応力負荷時の水素濃度分布評価

In order to evaluate hydrogen concentration under mechanical loading, a stress-hydrogen diffusion or distribution coupling computing method was developed based on Oriani's theory. The method was implemented to commercial finite element (FE) analysis software, Abaqus, with its user subroutine UMAT and UMATHT. This method was applied to analyze concentration of hydrogen near a blunting crack tip. It is shown that the results of hydrogen distribution were nearly identical compared to the Krom's study, and it is also shown that hydrogen concentration was higher supposing high strength steels.

Investigation on the rotational deformation of SEB specimens with various crack length to width ratio

A new CTOD calculation formula was proposed by using a correction factor for the blunted crack tip in the authors’ previous report[Kawabata et al(2016)], and the calculated CTOD by the formula significantly corresponded to the actual CTOD obtained by FE analysis in the wide range of specimen thicknesses. However, the use of the CTOD formula is limited to the frequently used crack depth-to-width ratio, a0/W, between 0.45 and 0.55, and this does not satisfy a strong demand for wider a0/W range. Meanwhile, ISO15653 covers a wider range of a0/W between 0.10 and 0.70, where J-based CTOD calculation formula has been specified in the annex-E of ISO15653. In this context, the plastic hinge model was investigated for a0/W between 0.05 and 0.70 by using FE analysis, and the analytical deformation of SEB specimens revealed that the plastic hinge model was applicable to the specimens even though their a0/W were out of the frequently used a0/W range from 0.45 and 0.55. Their rotational center points were almost steady when changing the applied load and the work hardening coefficient.

A numerical analysis of the mechanisms behind plasticity induced crack closure: Application to variable amplitude loadings

The effect of loading parameters on fatigue crack growth has been explained using the concept of crack closure. Plasticity induced crack closure (PICC) is linked to the crack tip plastic deformation, which becomes residual with crack propagation. The objective here is to identify the main mechanisms behind PICC, and for that different loading cases were considered namely overloads and load blocks. An analytical model was used to isolate the effect of residual plastic deformation on PICC, however significant differences were obtained relatively to finite element results. A second mechanism, which is crack tip blunting, was used to explain the transient behaviour observed after overloads and load blocks. For overloads and low–high load sequences there is a sudden increase of crack tip blunting with load increase which explains the sudden decrease of crack opening level. For high–low load sequences there is a sudden decrease of crack tip blunting which enhances the effect of residual plastic wake. Finally, the partial closure concept was tested looking to non-linear crack tip parameters but no evidences of Donald’s effect were found for the material studied.

The typical manners of dynamic crack propagation along the metal/ceramics interfaces: A molecular dynamics study

This paper addresses the issue of crack propagation mechanisms of various interfaces using molecular dynamics (MD) simulations. Four different interfacial crack propagation behaviors are identified: (1) the strictly interfacial crack propagation (type I); (2) the crack tip blunts due to the nucleation of a twinning at the crack tip (type II); (3) crack propagates with daughter cracks ahead of the mother crack coalescing to the mother crack (type III); (4) the stacking fault nucleation appears at the crack tip and leads to the crack tip blunting (type IV). Furthermore, the adhesive strength coefficient λ is used to identify the type of the interface. A transition zone around λ=0.9 is found. Interface with λ beyond the transition zone means a “nearly clear interface”, where interfacial crack propagates in type I or type II. While the interfaces with λ below this zone are shown to be a “relative rough interface”, then type III and IV of the interfacial cracks are preferred in them. The effect of adhesive strength of interface on the mechanism of crack propagation along the interface of metal/ceramics nanocomposites is illustrated in detail. Our work can be used to actively control and design the high performance nanocomposites.

Investigation on crack propagation in single crystal Ag with temperature dependence

Abstract

Fracture Energy Evaluation Using J-Integral in Orthogonal Microcutting

Fracture in cutting of ductile as well as brittle materials can be characterized using parameters such as K, G, R, and J-integral; of these, R has been widely used. To accurately evaluate the contribution of fracture energy in total cutting energy, J-integral would provide a more comprehensive basis as it encompasses several fracture modes, material plasticity, and nonlinear behavior. Therefore, this work adopts J-integral to evaluate the contribution of fracture energy to the size effect during microcutting of AISI 1215 steel. The work uses explicit integration method within ansys/ls-dyna to simulate two-dimensional (2D) orthogonal microcutting. U- and V-shaped cutting edges were used to represent a sharp crack-tip and a blunt crack-tip, respectively. Considering several alternative contours around crack-tip, covering the plastic zone, J-integral was calculated. Upon benchmarking J-integral values with other simulations in the literature, the approach was adopted for microcutting simulations in this work. It is observed that J-integral increases with uncut chip thickness, whereas it decreases with cutting speed, rake angle, and tool edge radius. The term (J/t0) defines contribution of fracture to the size effect in terms of J-integral, which is in the range of 4–24% under various parametric conditions. The corresponding values of R were always found to lie above those of the J-integral indicating that J-integral is relatively more appropriate parameter to quantify the fracture energy during microcutting.

Nanocomposite hydrogels: Fracture toughness and energy dissipation mechanisms

In this study, fracture toughness of nanocomposite hydrogels is quantified, and active mechanisms for dissipation of energy of nanocomposite hydrogels are ascertained. Poly(N,N-dimethylacrylamide) nanocomposite hydrogels are prepared by in situ free radical polymerization with the incorporation of Laponite, a hectorite synthetic clay. Transmission electron microscopy proves exfoliation of clay platelets that serve as multifunctional crosslinkers in the created physical network. Extraordinary high fracture energies of up to 6800 J m−2 are determined by the pure shear test approach, which shows that these soft and stretchable hydrogels are insensitive to notches. In contrast to single- and double-network hydrogels, dynamic mechanic analysis and stress relaxation experiments clarify that significant viscoelastic dissipation occurs during deformation of nanocomposite hydrogels. Similar to double-network hydrogels, crack tip blunting and plastic deformation also contribute to the observed massive fracture energies. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1763–1773