Force For Fatigue Crack Growth Research Articles

Modeling of crack tip fields and fatigue crack growth in fcc crystals

Predicting the mechanical behavior of polycrystalline materials containing a crack under both monotonic and cyclic loading conditions is crucial for accurately assessing the integrity of engineering materials. This study focuses on the deformation characteristics of face-centered cubic (fcc) grains within the crack tip field and their significant role in governing the driving force for fatigue crack growth during cyclic loading. We employ a cyclic crystal plasticity finite element (CPFE) model to analyze the mechanical response of austenitic 316L stainless steel polycrystals by accounting for nonlinear kinematic hardening effects. Through CPFE simulations, we investigate the deformation fields in 316L grains at the crack tip, considering two different grain orientations under plane strain conditions. Our CPFE results under monotonic loading align consistently with previous theoretical and experimental findings, particularly in comparing CPFE-simulated and experimentally observed plastic sectors consisting of different slip traces on the specimen surface near the crack tip. Based on a critical plastic work criterion for crack advancement, cyclic CPFE simulations are used to determine the fatigue crack growth rate as a function of stress intensity factor range for the two crack tip grain orientations in stainless steel. The simulated Paris law exponent matches experimental values. Furthermore, we compare cyclic CPFE results with those from cyclic J2 plasticity finite element simulations. This study demonstrates a cyclic CPFE approach for determining crack tip fields, accounting for crystallographic effects on plastic deformation of crack tip grains. Our approach can be applied to effectively evaluate fatigue crack growth rates in fcc polycrystalline metals.

Modelling of time‐dependent fatigue crack growth of hydrogels

AbstractThe service life of hydrogel‐based devices is strongly influenced by the anti‐fatigue capability of hydrogels. While recent advancements in the synthesis of hydrogels have improved their fatigue resistance, the underlying mechanisms that dominate the time‐dependent fatigue crack growth of hydrogels are still poorly understood. In this study, a computational method is developed to predict the fatigue crack of hydrogels. A physics‐based poro‐visco‐hyperelastic model is developed within the framework of the Theory of Porous Media at finite strains to describe the mechanical behaviour of hydrogels. A Bell model‐based chain evolution law is adopted to describe the time‐dependent breaking/reforming kinetics of physical chains. An energy‐based fatigue crack growth model is proposed to predict the fatigue crack growth of hydrogels. The averaged elastic energy density surrounding the crack tip is calculated as the driving force of fatigue crack growth. The effects of viscoelasticity, physical chain kinetics and fluid transport on the fatigue crack growth are analysed. The results indicate that these time‐dependent mechanisms cannot be ignored in predicting the fatigue crack behaviour of hydrogels.

Challenging the “ΔKeff is the driving force for fatigue crack growth” hypothesis

Tests are used to verify the hypothesis “the fatigue crack growth (FCG) driving force is the effective stress intensity factor range ΔKeff″. The tests are performed measuring FCG rates in steel and aluminum C(T) and DC(T) specimens under fixed {ΔK, Kmax} loading conditions and during FCG delays induced by single overloads. Crack-opening loads Pop are redundantly measured along the crack path in all tests, using independent near and far field strain-gages, as well as Digital Image Correlation (DIC) techniques. Finally, elastoplastic strain loops within the reverse plastic zone ahead of crack tips are measured using a stereo microscope DIC system.

Особливості росту втомних тріщин у псевдопружному NiTi сплаві

There have been analyzed the peculiarities of fatigue crack growth in pseudoelastic NiTi alloy at stress ratios R = 0,2 and 0,5. The fatigue crack growth resistance was determined on cylindrical specimens with a diameter of 8 mm with a unilateral segmental notch to a depth of 0,6 mm in their diametrical cross section at a temperature of 20°C in air. Using the servohydraulic testing machine STM-100, the specimens were preloaded by three-point bending in order to grow a fatigue crack from the notch, and the fatigue crack growth kinetics was determined on a cylindrical specimen with one lateral crack under uniaxial tension at a frequency of 25 Hz. The length of crack on the specimen surface was determined using the binocular microscope with the precision not less than 0,02 mm. To calculate the crack depth b in the deepest point of the crack front, the crack shape b/a of the initial and final cracks was determined using the linear interpolation. The fatigue crack growth rate was determined using this value in the deepest point of the crack front after certain number of loading cycles. It was found out that, unlike the traditional concepts of fracture mechanics, the mechanical driving force of fatigue crack growth, that unambiguously describes the failure kinetics despite of R is not the stress range, but a maximal value of stress intensity factor. According to the microfractographic studies, the main mechanism of crack propagation is connected with the brittle failure of martensite alloy structure elements. Therefore, the strain austenite–martensite transformation at the crack tip is responsible for its kinetics. The intensity of such transformation is determined mostly by the stress levels, and not by its range, that determines the mechanical driving force of fatigue crack growth.

Verification of the ΔKeff hypothesis along the fatigue crack path in thin and thick Al specimens

Elber assumed that the actual driving force for fatigue crack growth (FCG) is the effective stress intensity factor ΔKeff. To verify this hypothesis, both DC(T) and C(T) specimens are cut from a 6351-T6 Al alloy circular bar with two different thicknesses, 2 and 30mm, tested under fixed ΔK and Kmax to simulated plane stress and plane strain FCG conditions. A strain-gage bonded on the back face of the specimens is used to measure the crack length and a custom-made Labview program is used to control the applied load, maintaining ΔK and Kmax constant along the crack path. Moreover, the crack opening load is redundantly measured during the FCG tests, using far field strains from the back face gage and near field strains from a series of gages bonded along the crack path, as well as an independent digital image correlation system to measure displacement/strain fields on the face of the specimens. These tests show that the Al specimens reproduce the behavior previously observed in similar tests in 1020 steel: a significant decrease of the opening load as the cracks grow along the specimens, while maintaining a FCG rate essentially constant under the fixed {ΔK, Kmax} loading, a behavior that cannot be explained by the ΔKeff hypothesis.

Crack closure effects on fatigue damage ahead of crack tips

Elber’s assumed long ago that the effective stress intensity factor (SIF) range ΔKeff = Kmax − Kop is the actual driving force for fatigue crack growth (FCG), where Kop is the SIF that fully opens the crack, and his idea still is widely used to predict residual lives of cracked components. However, although crack closure can affect the FCG process, the ΔKeff idea cannot explain many of its peculiarities. To try to understand why this happens, the actual Kop role in FCG is questioned comparing ΔKeff–based predictions with similar predictions obtained using an alternate model that estimates crack increments assuming they are caused instead by the accumulated damage ahead of the crack tip. To be fair, this damage is calculated by the very same strip-yield mechanics used to calculate Kop and ΔKeff, i.e. the deformations predicted by the strip-yield model are used to describe the cyclic strain field ahead of the crack tip as well. Hence, the main goal of this exercise is to compare two different hypotheses for the actual FCG driving force using the same formulation basis. Both models are tested for different materials, constraint factors, and stress to yield strength ratios combinations, considering and neglecting the effect of crack closure. This exercise indicates that the effects of crack closure predicted by the critical damage model can be significantly lower than those predicted by the ΔKeff model.

Effect of tempering time on fatigue crack growth behavior of CLAM steel

Effect of tempering time on fatigue crack behavior of China Low Activation Martensitic (CLAM) steel was studied at room temperature in air. The fatigue crack growth rates subjected to the tempering time range of 90–9000 min were investigated. The results showed that the fatigue crack growth decelerated in the near-threshold region as the tempering time increased. The fatigue crack growth rate after tempering for 9000 min was approximately one-third of that of the specimen tempered for 90 min with ΔK of 14 MPa m0.5. The improved fatigue crack growth resistance could be explained by the influence of variation in microstructure, i.e., martensitic laths and precipitates. This study suggested that the effective driving force for fatigue crack growth was reduced in the near-threshold region by the activated roughness-induced crack closure mechanism as a result of rougher fracture surface caused by coarsened martensitic laths and precipitates.

Fatigue crack growth predictions based on damage accumulation ahead of the crack tip calculated by strip-yield procedures

Elber assumed a long time ago that ΔKeff is the driving force for fatigue crack growth (FCG), and his hypothesis is the basis for strip-yield models widely used to predict residual lives of cracked components. However, this hypothesis cannot explain many load sequence effects observed in practice. Hence, it is at least worth to verify if FCG models based on ΔKeff are indeed intrinsically better than concurrent models based on other principles. To do so, the same mechanics is used to predict FCG rates based both on Elber's ideas and on the alternative view that FCG is instead due to damage accumulation caused by the cyclic strain history ahead of the crack tip, an idea does not need or use the ΔKeff hypothesis. To compare both approaches fairly, FCG rates are estimated by damage accumulation using the cyclic strain ranges induced by plastic displacements calculated by the very same procedures used by strip-yield models, assuming there are strain limits associated both with FCG thresholds and with material toughness. Despite based on apparently conflicting principles, both models can reproduce quite well FCG curves, a somewhat surprising result. Besides confirming that data fitting cannot be used to prove any model superiority, this result indicates that the ΔKeff hypothesis is not a necessary requirement to explain the FCG behavior.

A model to quantify fatigue crack growth by cyclic damage accumulation calculated by strip-yield procedures

Elber's hypothesis that DeltaKeff can be assumed as the driving force for fatigue crack growth (FCG) is the basis for strip-yield models widely used to predict fatigue lives under variable amplitude loads, although it does not explain all load sequence effects observed in practice. To verify if these models are indeed intrinsically better, the mechanics of a typical strip-yield model is used to predict FCG rates based both on Elber's ideas and on the alternative view that FCG is instead due to damage accumulation induced by the cyclic strain history ahead of the crack tip, which does not need or use DeltaKeff ideas. The main purpose here is to predict FCG using the cyclic strains induced by the plastic displacements calculated by strip-yield procedures, assuming there are strain limits associated both the with the FCG threshold and with the material toughness. Despite based on conflicting principles, both models can reproduce quite well FCG data, a somewhat surprising result that deserves to be carefully analyzed.

Fatigue crack growth in miniature specimens: The equivalence of ∆K-correlation and that based on the change in net-section strain energy density

In fatigue crack growth experiments of miniature specimens of Ti-6Al-4V alloy, it is shown that the stress intensity factor range (∆K) cannot uniquely correlate the crack growth rates based on the standard stress intensity factor solution, but can only on the basis of the solution for uniformly displaced ends. It also is shown that, remarkably, a similar and strong correlation exists between the change in the net section strain energy amplitude and the rate of fatigue crack growth. The proposed approach is direct and simpler and it throws a new light on the nature of driving force for fatigue crack growth in structural materials.

Dynamic Strain Evolution around a Crack Tip under Steady- and Overloaded-Fatigue Conditions

We investigated the evolution of the strain fields around a fatigued crack tip between the steady- and overloaded-fatigue conditions using a nondestructive neutron diffraction technique. The two fatigued compact-tension specimens, with a different fatigue history but an identical applied stress intensity factor range, were used for the direct comparison of the crack tip stress/strain distributions during in situ loading. While strains behind the crack tip in the steady-fatigued specimen are irrelevant to increasing applied load, the strains behind the crack tip in the overloaded-fatigued specimen evolve significantly under loading, leading to a lower driving force of fatigue crack growth. The results reveal the overload retardation mechanism and the correlation between crack tip stress distribution and fatigue crack growth rate.

Can ?Keff be assumed as the driving force for fatigue crack growth?

This work raises some new questions about the validity of blindly assuming that Elber’s effective stress intensity factor is the actual fatigue crack driving force, and that as so it can be used to explain all load sequence effects on fatigue crack growth (FCG). Although plasticity-induced crack closure can be a quite reasonable heuristic explanation for many non-elementary FCG behaviors, it has some limitations that cannot be ignored. In fact, this never settled discussion is particularly important for the simulation of FCG lives under real service loads, a most important practical issue. After arguing that ?Keff can spoil the use of the most important similitude principle in FCG problems, simple but convincing experimental data that cannot be explained by this classical idea is presented here. This data involves the shape of fatigue crack fronts and the FCG behavior under nominally plane stress and plane strain conditions.

Low-cycle fatigue crack growth prediction by strain intensity factor

The validity of the strain intensity factor for representing driving force of fatigue crack growth under the large scale yielding condition was shown in this study. First, a crack growth test technique using plate specimens was developed in order to apply a fully-reversed cyclic load and to measure the global strain for calculating the strain intensity factor. Fatigue crack growth tests using Type 316 stainless steel revealed that the growth rates correlated well with the strain and effective strain intensity factor ranges for various stress or strain ranges, specimen geometries and loading modes (stress- and strain-controlled). Based on the test results, validity and physical meaning of the strain intensity factor were discussed. It was concluded that the strain intensity factor was an effective parameter for predicting crack growth under the small and large scale yielding conditions.

Fatigue crack retardation by the application of repair coatings to gas pipelines under pressure

We describe additional capabilities of the repair insulation coatings applied to main pipelines (without taking them out of operation) in increasing the durability of these pipelines. As a specific feature of this method, we can mention the retardation of the subsequent propagation of surface fatigue cracks as a result of the artificial formation of the effect of their closure by filling the crack cavity with liquid epoxy resins. After solidification, it plays the role of a wedge decreasing the range of cyclic plastic strains near the crack tip, i.e., the mechanical driving force of fatigue crack growth. By using the obtained experimental results, we analyze the mechanical situation formed at the crack tip by using the outlined method of retardation of the crack growth.

Modeling of Fatigue Crack Closure by Finite Element Method

Crack closure concept is often used to explain the crack propagation behavior in cracked components. The effective stress intensity factor range is considered as a driving force of fatigue crack growth based on the traditional crack closure concept. The crack closure process and the plastic deformation near the crack tip were discussed in this paper. The standard compact tension specimen with the plane-stress condition was used to study the crack closure. A dynamic crack propagation method was proposed to simulate the effect of previous fatigue crack growth on the successive crack growth behavior. To obtain the accurately numerical results of stress and strain components, the Jiang and Sehitoglu cyclic plasticity model was implemented into ABAQUS as UMAT. With the detailed stress and strain response taken from the finite element (FE) simulation, the whole process of crack closure was described by the load curve. The load corresponding to maximum crack closure length is firstly proposed to describe the effect of fatigue damage. According to the results of FE simulation, the cyclic plasticity of the material near the crack tip persists during the crack closure period and should not be ignored.

Ratchetting strain as a driving force for fatigue crack growth

Ratchetting deformation has significant implications on material damage and fatigue life of components under service loading conditions. In this work, we explore the concept of ratchetting strain as a driving force in controlling crack growth, both time-independent and time-dependent, utilising elasto-plastic, visco-plastic and crystal-plasticity constitutive models. Characteristics of crack tip deformation were examined for both stationary and growing cracks using the finite element method. Whilst the strain range and the stress range remained largely unchanged throughout the loading cycles, distinctive strain ratchetting behaviour near the crack tip occurred in all cases, leading to progressive accumulation of tensile strain normal to the crack growth plane. It seems plausible that this tensile strain, or ratchetting strain, may be responsible for material separation leading to crack growth. The effects of material constitutive model, grain orientation and loading frequency on the development of ratchetting strain were also examined. Ratchetting strain was found to be related to accumulated plastic strain; and both were used in a crack growth criterion with which the crack growth rates of a nickel alloy were predicted. The preliminary results seem to be encouraging.

Assessment of effective stress intensity factors using thermoelastic stress analysis

It has been demonstrated that thermoelastic stress analysis (TSA) can provide accurate information about the real crack-driving force for fatigue crack growth. Experiments were conducted using aluminium 2024 compact tension specimens which were initially pre-cracked to different crack lengths at a constant R ( R = 0). Subsequently, thermoelastic images were captured at increasing R values from 0 to 0.5 and Δ K values from 3 to 6 MPa m0.5 were calculated. The images showed a dramatic change in the stress pattern ahead of the crack tip as R decreased which was always associated with an increase in phase difference (loss of adiabatic conditions) ahead of the crack tip. This indicates that the technique is able to account for the change in conditions that arise from contact between the crack faces. To support this observation, thermoelastic results have been compared with those obtained using compliance methods. The results showed a very good level of agreement, illustrating the ability of TSA to infer successfully the effective Δ K.

Fatigue Crack Propagation at Polymer Adhesive Interfaces

Fatigue (slow) crack growth in epoxy/glass, epoxy acrylate/glass and epoxy/PMMA interfaces was studied under constant and cyclic loading at both high and low humidities using the interfacial, four-point flexure test. Finite element analysis was used to determine the energy release rate and phase angle appropriate for the different crack geometries observed. The experimental results show that for the polymer/glass interfaces, the primary driving force for fatigue crack growth is the applied energy release rate at the crack tip and that increasing test humidity enhances crack growth under constant loading but has an insignificant effect under cyclic loading. At low humidity the crack growth rates under cyclic loading are significantly greater than under constant loading. For epoxy/PMMA interfaces the crack growth results were independent of the applied energy release rate, relative humidity, and cyclic vs. constant loading, within experimental scatter. In addition, for polymer/glass interfaces the effect of phase angle (13 to 54°) on crack growth rates is not significant. However, for epoxy/PMMA interfaces the applied energy release rate for the initiation of crack growth is considerably greater for a phase angle of 66° than for 5°, indicating that increasing shear at the crack tip makes the initiation of crack growth more difficult. These results are discussed in terms of possible mechanisms of fatigue crack growth at polymer adhesive interfaces.

The effective fatigue threshold: significance of the loading cycle below the crack opening load

Experiments were carried out to demonstrate the validity of a new concept for the evaluation of the effective fatigue threshold values for loading conditions with K min below the crack opening value. For this, load-CTOD curves were determined for different R-ratios keeping K th, max constant and reducing K min below K th, min, which resulted in a resumption of the fatigue crack growth. This observation is explained on the basis of a new closure evaluation, which takes into account the total range CTOD to affect the driving force for fatigue crack growth. Three methods to measure the effective threshold values are proposed and critically compared. Experimental data are given for Cu, high-strength Al alloys and a dual-phase steel.

欠陥を有するセラミックスの疲労寿命推定法

A new method of predicting the fatigue lifetime of ceramics with various sizes of defects has been developed. In this method, first, specimens each of which has an artificial crack of constant size are used to estimate fatigue crack propagation parameters. In the vicinity of the tip of an artificial crack induced by Knoop indentation, a tensile stress field is generated. Then, the stress intensity factor acting as the driving force of fatigue crack growth was considered the sum of two terms associated with the applied stress and residual stress field. By numerical computation inserting the stress intensity factor into the power law of crack propagation, crack propagation parameters are estimated. Subsequently, using the obtained parameters, the fatigue lifetimes of the smooth specimens with fracture originating from a small defect, are predicted.This method was applied to three types of silicon nitride ceramics. As a result, the cyclic fatigue lifetimes of precracked specimens, which were set free from residual stress field by surface removal, were precisely predicted using the obtained parameters. Thus, the values of these parameters were proved to be adequate. Furthermore, the predicted fatigue lifetimes of the smooth specimens were in good agreement with the experimental results, considering the scatter of static strength.