Fretting Fatigue Crack Nucleation Research Articles

Fretting fatigue crack nucleation analysis under a variable coefficient of friction

Fretting fatigue is a phenomenon that occurs when two surfaces come into contact and experience oscillatory stresses and displacements of small amplitude. This can lead to a significant reduction in mechanical parts’ lifetime. The complexity of this type of problems relies on the many variables that can affect the results. One of its most important variables is the Coefficient of Friction (CoF), which has been observed to vary throughout a single fretting test. However, this variation is rarely considered in fretting fatigue finite element analyses. In order to analyse the effects of a Variable Coefficient of Friction (VCoF) on nucleation lifetime, a cylinder-on-plane finite element model is developed. To predict nucleation lifetime, two critical plane damage parameters, Fatemi-Socie (FS) and Findley parameter (FP), are used along with an evolving CoF during the first stress cycles. These parameters are evaluated by calculating initiation lifetime where the maximum value of the damage parameter occurs (hotspot method) and by the quadrant averaging method, because of its capability of predicting lifetime with relative accuracy and negative crack angles. To simulate the evolution of the CoF, a Python script that changes the value of CoF before each simulation is developed. This coefficient varies between 0.3633 and 0.65, following arbitrary points on the coefficient’s evolution curve. Once the simulation is concluded, the script evaluates the damage parameters and their equivalent lifetime throughout the contact surface and saves those values in an accumulation parameter that follows Miner’s rule. This process is repeated until CoF reaches its maximum value. The lifetime to nucleation is statistically analysed for all methodologies. The results suggest that, regarding the hotspot method, FP values remain within ± 3 N deviations from the experimental results, while FS results are nearer, within ± 2 N deviations. In addition, the quadrant method results remain at ± 3 N deviations, but providing more accurate results for the FP parameter. The application of a VCoF to these methods leads to less conservative and less deviated results, providing more accuracy for conservative approaches and more consistency for predictions in general. It is concluded that VCoF analysis is an interesting tool to determine more accurately nucleation lifetime for conservative approaches. Additionally, it reduces the predictions variation for differently loaded fretting setups regardless the desired approach.

A crystal plasticity based methodology for crack initiation lifetime prediction of fretting fatigue in AA2024-T351 alloy

AA2024-T351 alloy finds extensive use as a component material in the fields of civil engineering, machinery and aerospace, which are prone to long-term contact loading during service that may cause catastrophic failure such as fretting fatigue. In this work, a crystal plasticity finite element (CPFE) model has been established to predict the fretting fatigue crack initiation (FFCI) lifetime of AA2024-T351 alloy. Submodel methods for frictional contact parts are adopted, and crystal plasticity theory is utilized to obtain more accurate simulation results. The hotspots for fretting fatigue crack nucleation are determined by the difference of maximum value of maximum principal strain. The accumulated plastic slip and accumulated energy dissipation are utilized as fatigue indicator parameters (FIPs) to accurately predict FFCI lifetime. The results show that both FIPs predicted crack initiation lifetime are within twice the scatter band. Furthermore, the submodel is employed to generate diverse grain orientation sets while maintaining a constant total of 800 grains. The results indicate that different sets have little effect on the prediction of FFCI lifetime, but has a significant impact on the cumulative shear strain distribution between each individual grain at the mesoscale.

Effect of pad rotation angle and coefficient of friction on fretting fatigue crack nucleation behavior

Fretting fatigue may occur on surfaces that are in contact, subjected to a normal load, and where there is a cyclic small relative movement between them. It could occur in press-fitted shafts, bolted and dovetail joints, and many other engineering components. It is recognized that the coefficient of friction of the contacting bodies is a primary variable in the fretting process. Also, a recent study indicates that its behavior might be greatly affected by the possible rotation of the pad. Consequently, the aim of this investigation is to evaluate the impact of diverse angles of rotation of the pad and different coefficients of friction on crack initiation angle, location, and life. For this purpose, a numerical model based on a finite element model of a cylinder-plane test configuration is constructed. The Fatemi-Socie damage parameter for multiaxial fatigue is used to evaluate initiation behavior. Firstly, different angles of rotation are applied to the pad, in order to model a lack of stiffness of the test rig, while the coefficient of friction is kept constant and equal to 0.75. Secondly, several coefficients of friction between the pad and the specimen are utilized, while considering a cyclic rotation of the pad between 0.19° and – 0.19°. For the case studied, the results indicate that an increase of the coefficient of friction (μ) may cause a substantial reduction of crack initiation life. However, a variation of μ in the range 0.75 ≤ μ ≤ 0.85 does not produce a significant change in life estimation. This suggests that it cannot be stated that an increase in the coefficient of friction always leads to a reduction in nucleation life. Regarding the angle of initiation of the crack, the results show that an increase of μ causes an approximately linear reduction of the angle. In the case of an increase of the pad angle of rotation, this causes a longer nucleation life and a smaller crack nucleation angle. Even the rotation angle of 0.23° produces an increase of initiation life from 16.6 to 154 thousand cycles. It is concluded that there is a major impact on initiation behavior of the coefficient of friction and the rotation of the pad, which may be due to the lack of rigidity of the test setup. Therefore, controlling the stiffness of the test rig and measuring accurately the coefficient of friction are important factors to consider in fretting fatigue research.

On the fretting fatigue crack nucleation of complete, almost complete and incomplete contacts using an asymptotic method

In this work, the fretting crack nucleation of an aeronautical Ti-10V-2Fe-3Al titanium alloy is investigated. The main difficulty when designing complex industrial systems, especially helicopters rotor parts, lies in the prediction of the assembly interfaces behaviour. Those areas can be subjected to fretting loading characterized by a very confined multiaxial high stress gradient, leading to fatigue crack nucleation. This kind of failure reduces the parts potential lifetime and increases therefore the security margin that can be considered.The aim of this paper is to provide a criterion to prevent crack nucleation on industrial contacts configurations. To reach this objective, this study focuses on the geometrical conditions, loadings and material properties linked to crack initiation. Two dedicated fretting test campaigns are used to study crack initiation conditions with various contact shapes and loadings on the same material. The crack initiation conditions are compared with a contact mechanics representation based on asymptotic methods. This method allows the stress field evaluation at the hot spot location by using the problem’s eigenvalues.The combination of the asymptotic method with contact mechanics and the Goodman approach allows to represent the key parameters of the fretting fatigue crack initiation. They include the impacted material volume, the shear intensity and the bulk stress. A fretting fatigue crack initiation criterion is proposed. It is based on classical material properties and the bulk stress ratio Rσ0 only and is applicable to all sorts of contact shapes. The main interests of this prediction method are the absence of size effect parameters for industrial contacts and its efficiency, as it is only based on analytical formulations.

Crystal plasticity modeling of fretting fatigue behavior of an aluminum alloy

Aluminum alloy (AA)7075 is widely used to fabricate parts and components on aircrafts, which are subjected to contact loading that may induce fretting fatigue and catastrophic failure. In this work, a crystal plasticity finite element (CPFE) model accounting for the microstructural features is developed for simulating the fretting fatigue of AA7075-T651. A submodel technology is adopted to refine the contact region to obtain more accurate simulation data. An energy-based criterion is developed for prediction of crack initiation life. The hotspots for the fretting fatigue crack nucleation are identified by the maximum of plastic strain energy density. The proposed CPFE model achieves high accuracy on predicting the fretting fatigue crack initiation and validated by fretting fatigue experimental results.

A non-collinear fretting-fatigue experiment to compare multiaxial fatigue criteria: critical shear plane strategy is better than invariant formulations

Fretting-fatigue crack nucleation is usually predicted from multiaxial fatigue criteria combined with non-local approaches to take account of fretting stress gradient. Conventional collinear fretting-fatigue loadings, being effectively uniaxial near the hotspot surface contact border, lead to similar predictions whatever the non-local approach and fatigue criterion. To differentiate between approaches, a new non-collinear fretting-fatigue set-up was developed applying fatigue stress at a β-angle to the fretting direction. Using this device, the crack nucleation condition of 35NiCrMo16 steel was investigated as a function of the β-angle. 3D elastic simulations enabled, for the first time, performance to be compared between critical plane versus invariant formulations. Experiments and modeling suggested that the McDiarmid criterion provides the best crack nucleation predictions.

A review of the use of the asymptotic framework for quantification of fretting fatigue

We review the representation of fretting fatigue crack nucleation sites by simple asymptotic forms and show that the multipliers on these fields produce very satisfactory quantifiers of the tendency of any contact (but particularly incomplete ones) to nucleate a crack. A procedure is suggested to adopt simplified contact geometries, easily tested under carefully controlled conditions in a laboratory, to assess the likelihood of the avoidance of all failure or to estimate the time needed for crack initiation. The results found are true material properties which may then easily be used to solve a wide range of practical problems.

オートバイ用リアアームを想定したアルミニウム合金のフレッティング疲労強度改善のための表面処理に関する検討

Plain fatigue and fretting fatigue strength tests of aluminum alloy (JIS A7N01) specimens with surface treatments (fine particle peening, burnishing and solid lubricant film) were carried out using an electromagnetic fatigue testing machine. Fretting fatigue limit of untreated specimen was significantly low, which was about one fourth of the plain fatigue strength. Fretting fatigue strength of the specimen surface-treated with fine particle peening was slightly higher than that of the untreated specimen. The fretting fatigue strengths for solid lubricant film and burnishing have been remarkably increased compared to that of the untreated specimen. However, the fatigue limit for solid lubricant film was almost same as that of untreated specimen. From the in-situ SEM observation of fretting fatigue process, it was found that burnishing with smooth surface and high compressive residual stress effectively delayed the fretting fatigue crack nucleation and crack propagation. It was also found that solid lubricant film with low frictional coefficient effectively delayed them.

Fatigue Stress Ratio Effect on Fretting-Fatigue Crack Nucleation: Comparison between Multi-Axial and Uni-Axial Predictions

This research focuses on high cycle fretting fatigue crack nucleation prediction. A plastic steel/steel cylinder/plane contact was investigated keeping constant the normal force and the maximum fatigue stress ratio but varying the fatigue stress ratio (RF=0.6 to 1). The evolution of the crack length as function of the applied fretting tangential force amplitude at 106 cycles allows us to formalize the crack nucleation condition. It shows that the threshold tangential force marking the crack nucleation (i.e. bp_th=0μm) is not affected by the fatigue stress ratio. However an increase of the fatigue stress amplitude sharply increases the crack extension. To model the experiments, a 2D plastic plain strain FEM modeling is performed. As expected the computed stress field description is mesh dependent. However we demonstrate that a representative stress description is obtained at the 3rd node (i.e. 2nd node below the surface). By coupling this mesh condition and non-local critical distance approach, reliable prediction of the crack nucleation risk can be achieved either considering a Crossland multi-axial fatigue analysis or using a basic uni-axial Haighs description.

Finite element simulation of phase difference effects on fretting fatigue crack nucleation behaviour

Fretting fatigue is a mechanical failure process, which consists of two complex failure mechanisms. Fretting is caused by small relative movement between connected parts and fatigue is due to alternating bulk load. Fretting fatigue is highly non-linear and subjected to non-proportional loading condition. The multiaxial nature of stress at contact interface can be highly influenced by different phase difference between axial, tangential and contact load. In this study, the effect of different phase difference between axial and tangential load on fretting fatigue behaviour is investigated. Moreover, by applying cyclic contact load instead of constant contact load, its effect on fretting fatigue response and crack initiation lifetime is investigated. It was found that the phase difference affects the maximum location of dissipated energy along with fretting fatigue crack initiation lifetime.

Microstructure-sensitive prediction and experimental validation of fretting fatigue

This paper is concerned with the development of a micro-mechanical methodology for prediction of fretting fatigue crack nucleation life and short crack propagation. The methodology employs critical accumulated plastic slip as a fatigue indicator parameter for microstructure-sensitive crack nucleation. Crystal plasticity unit-cell models are employed for calibration of constitutive and crack nucleation parameters and a crystal plasticity frictional contact model of the fretting test rig is developed to predict the micro-scale surface fretting damage, crack nucleation and estimated micro-crack propagation. The methodology is validated against fretting fatigue tests using a fretting bridge arrangement for 316L stainless steel.

Micro-mechanical modelling of fretting fatigue crack initiation and wear in Ti–6Al–4V

A micro-mechanical modelling methodology is presented for predicting fretting fatigue crack nucleation in Ti–6Al–4V. The methodology is based on a combination of (i) a unit cell crystal plasticity model of Ti–6Al–4V, (ii) a frictional contact model of an experimental fretting wear configuration, and (iii) implementation of a microstructure-sensitive fatigue indication parameter for prediction of crack nucleation. A novel micro-mechanical method for wear prediction is presented. A key finding is the important role of cyclic micro-plasticity in ostensibly elastic loading regimes in fretting wear, resulting in significant effects on distributions of salient fretting contact variables such as contact pressure.

Prediction of the Fretting Fatigue Crack Nucleation Endurance of a Ti-6V-4Al/Ti- 6V-4Al Interface: Influence of Plasticity and Tensile/Shear Fatigue Properties

Fretting fatigue is a critical loading that appears on many structures such as the blade/disk contact of aircraft engines. It is characterized by small displacements between two bodies, one of which has an applied bulk stress. This phenomenon has been studied for a long time and induces damages which critically reduce the fatigue resistance. Fretting stressing are multi-axial and above all characterized by very severe stress gradient conditions. Besides, significant contact plastic deformations can be activated for LCF conditions which again complicate the predictions. To address this aspect a combined experimental – modeling analysis has been developed on a well known a Ti-6V-4Al alloy. Using an original double actuator fretting fatigue apparatus, the fretting fatigue cracking endurance of a cylinder/plane contact from 10E4 to 10E7 cycles was determined keeping constant the partial slip fretting loading (i.e. Q/P=0.32) and varying the fatigue stressing (700 MPa<σmax<50 MPa, R=0.01). This research shows that the fretting fatigue crack nucleation endurance (bCN=70 μm) can be predicted using a basic Crossland's stress invariant multi-axial fatigue description if the cyclic elasto-plastic (ep) response of the material is conveniently integrated in the FEM simulation, the stress gradient effect is taken into account by considering a stress averaging strategy defined on a ℓV=37μm cubic size previously calibrated from plain fretting experiments and finally if the ratio of tensile and shear fatigue limit is not assumed constant but expressed as a function of the loading cycles (σ-1(N)/τ-1(N)). Using this complete analysis the crack nucleation endurance from 10E4 - 10E7 cycle is predicted with an error smaller than 10%. A parametric study shows that if the σ-1/τ-1 ratio is assumed constant, defined at the fatigue limit conditions, the endurance predictions in LCF and most of the HCF domains are dangerously non conservative. Alternatively the application of plain elastic stress description leads to over conservative predictions when significant plastic accommodations are activated (N< 2 104 cycles).

Influence of relative slip on fretting fatigue behaviour of 2014 aluminium alloy with the age-hardened conditions T4 and T6

The present work deals with the effect of relative slip on fretting fatigue behaviour of 2014 Aluminium alloy with age-hardened conditions T4 and T6 in contact with a dissimilar mating material AISI 1045 steel, using a plane-on-plane configuration. There are a significant number of parameters influencing fretting fatigue behaviour. Relative slip value between the specimen and fretting pad is one of the most important factors. In the present study, the influence of relative slip was evaluated at different cyclic stress levels by using a fretting pad with a single pad span value of 10 mm. The results show the artificial temper T6 is more effective for improving the fretting fatigue life when compared with natural temper T4. This may be attributed to a higher microstructural barrier to the fretting fatigue crack nucleation and initiation, due to having lower values of relative slip with artificial temper T6.

Identification of fretting fatigue crack propagation mechanisms using acoustic emission

In this study, a fretting fatigue test has been equipped with an acoustic emission (AE) device in order to identify the successive crack propagation mechanisms. The fretting fatigue crack nucleation and propagation is a complicated process. Cracks initiate and propagate firstly due to shearing (mode II) and then by tension (mode I). The crack propagation generates mechanical energy emission. Elastic waves appear and can be detected through AE. A complete analysis of the AE signals (multi-parameter analysis, location of the AE in the loading cycle and a statistical analysis) led to an identification of three different steps in the crack propagation process. The evolution of the shearing and the tension influences in the crack propagation process is recognizable separately. Therefore, the three crack propagation steps have been identified as (a) crack propagation in mode II, (b) mixed mode crack propagation and (c) pure mode I crack propagation.

High temperature fretting fatigue behavior of IN100

Fretting fatigue behavior of a nickel-base superalloy, IN100, was investigated at 600 °C. Fretting fatigue tests were conducted at various stress levels using cylinder-on-flat contact configuration. Effects of microstructure were also investigated by varying the gamma grain size (3 μm versus 7 μm). Additionally, effects of contact load were studied. Furthermore, plain fatigue tests of IN100 with two microstructures were also conducted. Fretting reduced the strength/life in comparison to those of the plain fatigue almost equally in both microstructures. Increase of contact load reduced the fretting fatigue strength/life. An increase of the gamma grain size decreased the fretting fatigue strength/life of IN100. Fracture surfaces of the 3 μm grain microstructure showed intergranular and tortuous crack path while the 7 μm grain microstructure had transgranular and relatively smoother crack path. Fractographic analysis indicated that the 3 μm grain microstructure had a higher resistance to the fretting fatigue crack nucleation and initiation as well as to the crack growth in comparison to the 7 μm grain microstructure, which was in agreement with the observed plain and fretting fatigue behaviors. Furthermore, plain and fretting fatigue performances improved at 600 °C in comparison to room temperature with both microstructures.

Effects of microstructure on fretting fatigue behavior of IN100

Fretting fatigue behavior of a nickel-base superalloy, IN100, was investigated at room temperature. Two microstructures of IN100 were tested which varied primarily by the gamma grain size (3 μm versus 7 μm). Fretting fatigue tests were conducted at various stress levels using cylinder-on-flat contact configuration. An increase in the grain size was associated with decrease in the fretting fatigue strength/life of IN100. Microscopic analysis showed that the 3 μm grain microstructure provided a higher microstructural barrier to the fretting fatigue crack nucleation and initiation. On the other hand, the 7 μm grain microstructure had a higher intrinsic crack growth resistance due to the tortuous crack path requiring more energy. These features were in agreement with the plain fatigue where fine microstructures generally provide higher resistance to crack initiation but reduce crack propagation resistance while coarse microstructures have the opposite behavior.

Life estimation of Ti–6Al–4V specimens subjected to fretting fatigue and effect of surface treatments

Suitability of different multi-axial parameters in predicting fretting fatigue life of Ti–6Al–4V specimens has been investigated. Ameliorating effect of surface treatments on fretting fatigue has been studied. In simple uni-axial/multi-axial fatigue tests, nucleation as well as propagation of cracks occur under the influence of identical stresses. Hence nucleation accounts for most of the total life. Fretting fatigue crack nucleation occurs due to very large contact stresses, effect of which is felt only close to the surface (due to steep gradients). Propagation mostly occurs due to lower stresses in the bulk of the material (negligible influence of contact tractions) and forms a significant portion of total life. Total life has to be taken as sum of initiation life calculated from different multi-axial fatigue parameters and propagation life from conventional fracture mechanics approach. Steep stress gradients necessitate the adoption of a statistics based approach to predict the crack initiation life, based on an assumed distribution of flaws. The quality of comparison between predicted and experimentally observed failure lives provides confidence in the notion that conventional fatigue life prediction tools can be used to assess fretting fatigue failure. Effect of surface treatments like shot-peening with or without additional surface coatings on total life of the specimen and on friction coefficient has been studied.

真空中におけるアルミニウム合金(Al2024-T4)のフレッティング疲労特性(&lt;小特集&gt;締結・接合・接着部の評価技術)

Plain fatigue, fatigue crack growth and fretting fatigue tests of Al alloy (Al2024-T4) in air and in vacuum were carried out using a fatigue testing machine with SEM for in-situ observation. Fretting fatigue strength in vacuum was higher than that in air, while it was lower than plain fatigue strength in vacuum. High tangential force in vacuum would result in earlier fretting fatigue crack nucleation. However, high fatigue crack growth resistance would take dominant role for higher fretting fatigue strength in vacuum.

Influence of coefficient of friction on fretting fatigue crack nucleation prediction

The role of coefficient of friction (COF) on fretting fatigue damage prediction is studied using an elastic/plastic finite element analysis (FEA) of an experimental fretting fatigue test. At a scale on the order of tens of microns, the COF has a large impact on fretting fatigue damage prediction. By using a higher COF than that determined from conventional gross sliding tests, along with critical plane multiaxial fatigue damage models, the life and experimentally observed damage at this scale appear to be better predicted. Potential origins of this higher effective COF are discussed.