Plastic strain-controlled short crack growth and fatigue life

Constant plastic strain-controlled and constant stress-controlled tests were performed on smooth and lightly notched specimens machined from a massive forging of 42CrMo4 steel. Comparison of the fatigue life curves plotted as function of the plastic strain amplitude and stress amplitude shows a decisive role of plastic strain amplitude. Crack initiation and the kinetics of short crack growth were studied in constant plastic strain amplitude loading and the relation between the crack growth coefficient and plastic strain amplitude was established. This is equivalent to the Coffin–Manson law and shows that the Coffin–Manson law can be interpreted in terms of short crack growth.     

Variable amplitude crack growth in notched specimens

Predictions of variable amplitude fatigue crack growth in notched tensile specimens at an elevated temperature and relative high loads were conducted. The predictions were based on constant amplitude fatigue data and mean crack growth rates were calculated. Crack closure levels were calculated by comparison of crack closure free experimental data obtained with the so called constant K_I,max-method and nominal data. Also, numerical simulations of the closure level throughout the specimens were performed. Although experimental data showed rather large a scatter it was concluded that variable amplitude crack growth rates can be predicted with reasonable agreement using constant amplitude fatigue data. Also, crack growth rates cannot be predicted by LEFM in the inelastic notch affected zone, even if absence of crack closure is assumed.     

Prediction of shape change of corner crack by fatigue crack growth circles

As a fatigue crack grows, its shape changes. Conventional method for predicting the shape change typically requires the computation of several hundred increments to get accurate results. In this paper, a new finite element simulation technique that uses fatigue crack growth circles is developed. Since the circles are perpendicular to the new crack front as well as to the current crack front, they can represent the real path of the fatigue crack well and yield more accurate results. The new technique is validated by applying it to the case of a quarter-elliptical corner crack in a plate with an open hole subjected to tension loading. The effect of the Paris–Erdogan exponent is also investigated.     

Abaqus implementation of extended finite element method using a level set representation for three-dimensional fatigue crack growth and life predictions

A three-dimensional extended finite element method (X-FEM) coupled with a narrow band fast marching method (FMM) is developed and implemented in the Abaqus finite element package for curvilinear fatigue crack growth and life prediction analysis of metallic structures. Given the level set representation of arbitrary crack geometry, the narrow band FMM provides an efficient way to update the level set values of its evolving crack front. In order to capture the plasticity induced crack closure effect, an element partition and state recovery algorithm for dynamically allocated Gauss points is adopted for efficient integration of historical state variables in the near-tip plastic zone. An element-based penalty approach is also developed to model crack closure and friction. The proposed technique allows arbitrary insertion of initial cracks, independent of a base 3D model, and allows non-self-similar crack growth pattern without conforming to the existing mesh or local remeshing. Several validation examples are presented to demonstrate the extraction of accurate stress intensity factors for both static and growing cracks. Fatigue life prediction of a flawed helicopter lift frame under the ASTERIX spectrum load is presented to demonstrate the analysis procedure and capabilities of the method.     

Effect of the stress distribution in simple welded specimens and complex components on the crack propagation life

While a long stable crack propagation phase was observed during experiments of complex welded components, very conservative estimations of the fatigue life were achieved in the past. The difference was explained by the stress gradient occurring over the plate thickness. This paper deals with numerical crack propagation simulations which were performed for geometrically different variants. The variants differ in global geometry, boundary conditions and weld shape. The analyses aim to investigate how the crack propagation is altered if the structural configuration gets more complex. In conclusion, the stress gradient over the plate thickness, the effective plate thickness due to vertical web plates and high notch effects slow down the crack propagation rate if the same stress value being effective for fatigue appears at the weld toe. Thereby, the load-carrying grade of the weld, the weld flank angle and the geometrical configuration also have an impact on both the notch effect and the local stress concentration.     

Modified shear lag theory based fatigue crack growth life prediction model for short-fiber reinforced metal matrix composites

Closed form expressions for the low cycle and high cycle fatigue crack growth lives have been derived for the randomly-planar oriented short-fiber reinforced metal matrix composites under the total strain-controlled conditions. The modeling was based on fatigue-fracture mechanics theory under both the small scale and the large scale yielding conditions. The modified shear lag theory was considered to describe the effect of yielding strength. The present model is essentially a crack growth model because crack initiation period in short fiber reinforced metal matrix composite is much shorter; hence, not assumed to play a dominant role in the calculation of fatigue crack growth life. The effects of short-fiber volume fraction (V_f), cyclic strain hardening exponent (n′) and cyclic strain hardening coefficient (K′) on the fatigue crack propagation life are analyzed for aluminum based SFMMCs at different levels of cyclic plastic strain values. It is observed that the influence of fatigue crack growth resistance increases with increase in cyclic strain hardening exponent (n′) and decreases when volume fraction (V_f) or cyclic strain hardening coefficient (K′) increases. The present MSL theory based fatigue crack growth life prediction model is an alternative of modified rule of mixture and strengthening factor models. The predicted fatigue life for SFMMC shows good agreement with the experimental data for the low cycle and high cycle fatigue applications.     

Estimation of fatigue crack growth retardation due to crack branching

Quantitative analysis is provided to estimate the reduction of fatigue crack growth rate due to overload crack branching. A recent mixed-mode fatigue crack growth model based on the dilatational component of the accumulated strain energy density near the crack tip is modified to quantify the retardation factor of crack growth rate following an overload. It is found that crack branching due to an overload results in considerable reduction of fatigue crack growth rate. The retardation factor estimated by the proposed methodology is correlated with test results for the 2090-T8E41 aluminum–lithium alloy indicating encouraging agreement.     

Multiaxial fatigue and life prediction of elastomeric components

Elastomeric components have wide usage in many industries. The typical service loading for most of these components is variable amplitude and multiaxial. In this study a general methodology for life prediction of elastomeric components under these typical loading conditions was developed and illustrated for a passenger vehicle cradle mount. Crack initiation life prediction was performed using different damage criteria. The methodology was validated with component testing under different loading conditions including constant and variable amplitude in-phase and out-of-phase axial–torsion experiments. The optimum method for crack initiation life prediction for complex multiaxial variable amplitude loading was found to be a critical plane approach based on maximum normal strain plane and damage quantification by cracking energy density on that plane. Rainflow cycle counting method and Miner’s linear damage rule were used for predicting fatigue life under variable amplitude loadings. The fracture mechanics approach was used for total fatigue life prediction of the component based on specimen crack growth data and FE simulation results. Total fatigue life prediction results showed good agreement with experiments for all of the loading conditions considered.     

Recent developments in fatigue crack growth assessment

This paper examines the fatigue crack growth histories, at low to mid range ΔK’s, for a range of aircraft aluminium alloys, test specimens and service loaded components. It concludes that the crack growth history shows that, as a first approximation, there is a log–linear relationship between the apparent crack length or depth and the life (N), respectively. This leads to the further observation that, for the range of materials and spectra considered here, there is a linear relationship between the crack growth rate and the crack length when plotted on a log–log scale.     

Singular and non-singular approaches for predicting fatigue crack growth behavior

In this work, three classes of mechanisms that can cause load sequence effects on fatigue crack growth are discussed: mechanisms acting before, at or after the crack tip. After reviewing the crack closure idea, which is based on what happens behind the crack tip, quantitative models are proposed to predict the effects at the crack tip due to crack bifurcation. To predict the behavior ahead of the crack tip, a damage accumulation model is proposed. In this model, fatigue cracking is assumed caused by the sequential failure of volume elements or tiny εN specimens in front of the crack tip, calculated by damage accumulation concepts. The crack is treated as a sharp notch with a small, but not zero radius, avoiding the physically unrealistic singularity at its tip. The crack stress concentration factor and a strain concentration rule are used to calculate the notch root strain and to shift the origin of a modified HRR field, resulting in a non-singular model of the strain distribution ahead of the crack tip. In this way, the damage caused by each load cycle, including the effects of residual stresses, can be calculated at each element ahead of the crack tip using the correct hysteresis loops caused by the loading. The proposed approach is experimentally validated and extended to predict fatigue crack growth under variable amplitude loading, assuming that the width of the volume element broken at each cycle is equal to the region ahead of the crack tip that suffers damage beyond its critical value. The reasonable predictions of the measured fatigue crack growth behavior in steel specimens under service loads corroborate this simple and clear way to correlate da/dN and εN properties.     

Corrosion fatigue life prediction of a steel shaft material in seawater

Corrosion fatigue behaviour of a medium strength structural material was studied in air and in 3.5% NaCl solution. Emphasis was placed on the study of corrosion pit formation and the development of cracks from pits. Pitting and crack propagation were quantified throughout the fatigue loading thereby allowing a model to be developed that included the stages of pitting and the pit-to-crack transition in order to predict the fatigue life. The results showed that a large number of corrosion pits with small size form at a very early stage in the fatigue lifetime. The number of pits and subsequent cracks was found to be higher at higher stress levels leading to multiple crack development and coalescence. When compared to air, fatigue life in a corrosive environment was significantly reduced at low stress levels due to pitting damage, indicating a dominant role of corrosion over that of mechanical effects. The corrosion fatigue model proposed shows good agreement with the experimental test data at lower stress levels but predicts more conservative lifetimes as the stress increases. Kitagawa–Takahashi diagram was produced for both test environments where it is indicated that the fatigue limit can be eliminated in a corrosive environment.     

Calculating the fatigue life of smooth specimens of two-phase titanium alloys subject to symmetric uniaxial cyclic load of constant amplitude

A system of microstructure-dependent fatigue failure models that allows calculating the number of stress cycles to failure of smooth specimens in the absence of data on the fatigue resistance of the material is proposed. Fatigue life is represented as the sum of the number of stress cycles to the initiation of a microstructurally short crack with a depth equal to one grain size and the number of stress cycles to the instant the growing physically small and long crack reaches the depth taken as a fatigue failure criterion. To fill the models, it is necessary to test standard specimens under monotonic short-term tension to determine the elastic modulus, Poisson’s ratio, and the proportional limit and to analyze the microstructure and texture of the material to determine the Taylor factor, the magnitude of the Burgers vector, and the distance between neighboring parallel slip planes in the lattice, depending on which slip system is activated relative to the tension direction. The models are applicable to metals and alloys in which a fatigue crack is initiated along a persistent slip band in a surface grain under high-cycle loading. The models are validated against fatigue test data for flat specimens of different two-phase titanium alloys (VT3-1, IMI 834, Ti–6Al–4V, VT6, VT16, VT22, VT23, LCB) with different types of microstructure (bimodal, globular, fine-grained β-transformed) subject to symmetric in-plane bending. The plotted S–N curves are in a satisfactory agreement with experimental data.     

Fast multipole DBEM analysis of fatigue crack growth

A fast multipole method (FMM) based on complex Taylor series expansions is applied to the dual boundary element method (DBEM) for large-scale crack analysis in linear elastic fracture mechanics. Combining multipole expansions with local expansions, both the computational complexity and memory requirement are reduced to O(N), where N is the number of DOF. An incremental crack-extension analysis based on the maximum principal stress criterion and the Paris law is used to simulate the fatigue growth of numerous cracks in a 2D solid. Some examples are presented to validate the numerical scheme.     

Cumulative fatigue damage and life prediction of elastomeric components

Elastomeric components are widely used in many applications due to their good damping and energy absorption characteristics. The type of loading normally encountered by these components in service is variable amplitude cyclic loading. Therefore, fatigue failure is a major consideration in their design. In this work capabilities of Rainflow cycle counting procedure, maximum principal strain as a damage criterion, and Miner's linear cumulative damage rule are evaluated with both specimen and component tests. An automotive cradle mount is used as an illustrative component. Comparison of predicted and experimental fatigue lives in both specimen and cradle mount variable amplitude load tests indicate satisfactory predictions in both cases.     

A multifractal analysis of fatigue crack growth and its application to concrete

As is well-known, strength of materials is influenced by the specimen or structure size. In particular, several experimental campaigns have shown a decrease of the material strength under static or fatigue loading with increasing structure size, and some theoretical arguments have been proposed to interpret such a phenomenon. As far as fatigue crack growth is concerned, limited information on size effect is available in the literature, particularly for so-called quasi-brittle materials like concrete. In the present paper, by exploiting concepts of fractal geometry, some definitions of fracture energy and stress intensity factor based on physical dimensions different from the classical ones are discussed. A multifractal size-dependent fatigue crack growth law (expressing crack growth rate against stress intensity factor range) is proposed and used to interpret relevant experimental data related to concrete.     

Overload effects in fatigue crack growth by crack-tip blunting

A basic mechanisms for fatigue crack growth in ductile metals is that depending on crack-tip blunting under tensile loads and re-sharpening of the crack-tip during unloading. In the present paper, the effect of an overload in one of the cycles is studied based on this mechanism. In a standard numerical analysis accounting for finite strain, it is not possible to follow the blunting/re-sharpening process during many cycles, as severe mesh distortion at the crack-tip results from the huge geometry changes developing during the cyclic plastic straining. Here, based on an elastic-perfectly plastic material model, crack growth computations are continued up to 700 full cycles by using remeshing at several stages of the plastic deformation. Crack growth results for purely cyclic loading are compared with predictions for cases where an overload is applied, and it is shown how crack growth slows down after the overload. Different load amplitudes, and an overload at different cycle numbers are considered.     

Utilization of partial crack closure for fatigue crack growth modeling

Load ratio effects are of prime concern when modeling of fatigue crack growth (FCG) rate is required as a prerequisite for a reliable life prediction. The majority of research efforts regarding the load ratio effects are based on Elber's ΔK_eff approach. However, there are intrinsic difficulties encountered with its consistent application to FCG prediction. In this paper two popular crack-growth-life prediction codes FASTRAN and AFGROW are modified utilizing the enhanced partial crack closure model. The proposed utilization aggregates apparent closure mechanisms involved and demonstrates a better correlation and a significant scatter reduction of FCG data taken from literature, especially in the near-threshold region.     

The crack growth behavior after overloading on rotating bending fatigue

Most of the crack propagation behavior relating to the variable amplitude load is conducted in flat specimens such as centered-crack specimens or compact test specimens with various thicknesses. For that reason, it is also important to understand the crack growth behavior relating to the overload on other geometry which differs from the previous investigation. In the present study, the crack growth behavior after being overloaded was investigated on a solid round bar. It was found that the size of diameter influences the crack growth behavior. The retardation or acceleration depends on the residual stress state in front of the crack tip. If the residual stress state developing was compressive, retardation was observed, and if the tensile residual stress state developed, the acceleration of the crack occurred after overloading. In addition, the shape of the component has to be taken into consideration for evaluating fatigue life in association with the crack growth.     

Numerical study on fatigue crack growth in railway wheels under the influence of residual stresses

Accurate prediction of fatigue crack growth on railway wheels and the influence of residual stresses by finite element method (FEM) modeling can affect the maintenance planning. Therefore, investigation of rolling contact fatigue and its effect on rolling members life seem necessary. The objective of this paper is to provide a prediction of rolling contact fatigue crack growth in the rail wheel under the influence of stress field from mechanical loads and heat treatment process of a railway wheel. A 3D nonlinear stress analysis model has been applied to estimate stress fields of the railway mono-block wheel in heat treatment process. Finite element analysis model is presented applying the elastic–plastic finite element analysis for the rail wheel under variable thermal loads. The stress history is then used to calculate stress intensity factors (SIFs) and fatigue life of railway wheel. The effect of several parameters, vertical loads, initial crack length and friction coefficient between the wheel and rail, on the fatigue life in railway wheels is investigated using the suggested 3-D finite element model. Three-dimensional finite element analysis results obtained show good agreement with those achieved in field measurements.