Fatigue Life Of Metallic Materials Research Articles

Ultra-High-Cycle Fatigue Life Prediction of Metallic Materials Based on Machine Learning

The fatigue life evaluation of metallic materials plays an important role in ensuring the safety and long service life of metal structures. To further improve the accuracy and efficiency of the ultra-high-cycle fatigue life prediction of metallic materials, a new prediction method using machine learning was proposed. The training database contained the ultra-high-cycle fatigue life of different metallic materials obtained from fatigue tests, and two fatigue life prediction models were constructed based on the gradient boosting (GB) and random forest (RF) algorithms. The mean square error and the coefficient of determination were applied to evaluate the performance of the two models, and their advantages and application scenarios were also discussed. The ultra-high-cycle fatigue life of GCr15 bearing steel was predicted by the constructed models. It was found that only one datapoint of the GB model exceeded the triple error band, and the RF model had higher stability. The network model coefficient of determination and mean square error for the GB and RF models were 0.78, 0.79 and 0.69, 3.79, respectively. Both models could predict the ultra-high-cycle fatigue life of metallic materials quickly and effectively.

Residual stress distribution and surface geometry of medical Ti13Nb13Zr alloy treated by laser shock peening with flat-top laser beam

The metallic implants made of Ti13Nb13Zr alloy are usually used under the action of the cyclic loads conditions and fatigue failure occurs occasionally. Laser shock peening (LSP) is an innovative surface treatment process, which has been used to improve the fatigue life of metallic materials. The present research was performed on the Ti13Nb13Zr alloy subject to laser modification in order to determine the effects on the residual stress distribution and surface geometry. A neodymium doped yttrium lithium fluoride (Nd: YLF) pulse laser with a flat-top beam was applied at three different overlapping rates (10%, 30%, 50%), laser pulse energies (5J, 6J, 7J), and impact times (1 impact, 2 impacts, 3 impacts), respectively. The residual stresses were determined by X-ray diffractometer with sin 2ψ method. Firstly, the 30% overlap was chosen by comparing the residual stresses induced by LSP with different overlapping rates. Then, the effects of laser energies and impact times on surface residual stresses, in-depth residual stresses, surface deformation, and surface roughness were investigated and compared. The results showed that the residual stresses, surface deformation, and surface roughness increase with the increasing laser energies and impact times. The effected layer depth of residual stress is about 1058 μm with the 6J laser energy.

Surface residual stress, micro-hardness and geometry of TC6 titanium alloy thin-wall parts processed by multiple oblique laser shock peening

The titanium alloy thin-walled part with light in weight and high specific strength is widely used in the aviation engine fan, turbine and compressor. The fatigue damage of the thin-walled part often happens under the action of the cyclic loads and high temperatures. Laser shock peening(LSP) is an innovative surface treatment process, which has been used to improve the fatigue life of metallic materials. Considering the shape and size of the thin-walled parts, the TC6 titanium alloy thin-wall parts were treated by the oblique incident laser beam in this work. The purpose of this study is to investigate the effects of multiple oblique LSP on TC6 titanium alloy thin-wall parts. Nd: YLF laser with a wavelength of 1053 nm was applied. The laser incidence angle was determined first by experiment. The effects of laser energies, impact times, pulse durations and pulse frequencies on the surface residual stress, micro-hardness and geometry of TC6 titanium alloy thin-wall parts were investigated. The results showed that by the use of oblique LSP, the higher surface residual stress and surface micro-hardness could be induced on the surface of treated specimens. However, surface deformation and surface roughness of treated specimens can also increase.

A Walker exponent corrected model for estimating fatigue life of metallic materials in loading with mean stress

AbstractMean stress significantly influence the fatigue life predictions of metallic materials. The Walker mean stress equation with its additional material parameter w provides good predictions for a wide range of materials. Unfortunately, additional tests are necessary to determine the Walker exponent w. In order to overcome this shortcoming, for aluminum alloys, the Walker exponent w was correlated linearly with the sum of ultimate tensile strength and true fracture strength. Then, a Walker exponent corrected effective strain energy density criterion was developed by incorporating the Walker mean stress equation into the strain life curve. The capability of fatigue life prediction for the developed model was checked against the tested data of 304 L stainless steel, SAE 1045 steel, 7075‐T651 aluminum alloy, and Incoloy 901 superalloy, and comparisons were also performed by using the Lv's Walker exponent corrected model. The developed model provides more satisfactory results, especially for the considered materials in loading with mean stress.

StressLifetc – NDT-related assessment of the fatigue life of metallic materials

Abstract Weight-optimized component design as well as a reliable estimation of the lifetime of metallic materials and components requires a comprehensive understanding of fatigue processes and a systematic investigation of the underlying fatigue behavior. Therefore, nondestructive testing methods, digitalization of measurement techniques as well as signal processing can be combined with a short-term procedure in order to acquire potentially more information about fatigue processes, while experimental effort and costs are reduced significantly. This leads not only to considerable advantages over conventional methods for determining S-N curves, but also over established short-term procedures, due to the possibility of applying this information from just a few specimens to attain fatigue life calculations. The StressLifetc approach is a new short-term calculation method which considers the nonlinear relation between the elastic, elastic-plastic and plastic portion of the material response in the deformation process. Within the scope of the present work, the change in temperature of SAE 1045 (C45E) specimens was measured during fatigue tests via an infrared camera in order to feed the thermal response back into the new StressLifetc approach for a reliable fatigue life calculation.

A method for evaluating the effects of specimen geometry and loading condition on fatigue life of metallic materials

In this paper, the effects of specimen geometry and loading condition on fatigue life are investigated based on statistical analysis and control volume concept. A probabilistic control volume method is developed for correlating the fatigue life of specimens with different control volumes. The predicted P-S-N curves accord with the experimental data for the titanium alloy Ti-6Al-2Sn-2Zr-3Mo-X and the high strength steel JIS SUJ2 in literature. The relative error of the predicted fatigue life at 50% survival probability to that directly analyzed from the conventional testing method is –18.7% for the specimens of titanium alloy. The paper also indicates that the determination of the control volume should be based on the fatigue failure mechanism of specimens, which is appropriate to use the control volume for the interior induced fatigue failure and the control surface with a certain thickness for the surface induced fatigue failure.

Low cycle fatigue of metallic materials under uniaxialloading at elevated temperature

This paper reviews experimental tests of the fatigue life of metallic materials under uniaxial fatigue loading at elevated temperature. The influence of elevated temperature on the properties of different materials were investigated for monotonic and cyclic loads with high loading amplitude values. Models most commonly used to estimate the fatigue life of materials under these conditions are presented. Based on compiled results of experimental tests, an empirical model of fatigue damage accumulation was developed, making it possible to predict fatigue life under uniaxial, low-cycle loading conditions at elevated temperature. The model's structure was based on fatigue deformation curves obtained by way of experimental tests. The low number of material constants, which can be determined from fatigue tests at ambient temperature, is an advantage of the proposed model. The model was verified experimentally, and compiled test results were used for this purpose.

On the transition from plastic deformation to crack initiation in the high- and very high-cycle fatigue regimes in plain carbon steels

It is well known that fatigue life of metallic materials involves processes at different stages. Two main regimes can be distinguished: The first is governed by plastic deformation and related dislocation processes like cyclic saturation and strain localization. The second regime is mainly dominated by crack initiation and propagation. Whether an infinite fatigue life is obtained or not is therefore ultimately determined by the complex interplay of plastic deformation processes, the formation of small crack nuclei and their subsequent interaction with the surrounding material’s microstructure. Thus, it is necessary to investigate the transition from plastic deformation to crack initiation in the high- and very high-cycle fatigue regime in more detail. Three plain carbon steels with different ferrite to pearlite ratios were investigated in order to determine the influence of different microstructures on the transition from cyclic deformation to crack initiation in the high and very high cycle fatigue (VHCF) regime. A new method was developed where the changes in the dissipated energy are used to account for irreversible plastic deformation during ultrasonic fatigue loading. At very small fatigue amplitudes, which still lead to VHCF-failure, the cyclic deformation regime covers the main part of the fatigue life. In this regime dislocation interactions lead to the accumulation of irreversible plastic deformation followed by small crack nuclei that initiate at phase or grain boundaries (GBs), marking the transition to the damage regime. In order to better understand the fundamental mechanisms at the transition between both regimes, idealized atomistic simulations were performed. To model fatigue-induced plastic deformation various defects, i.e. dislocations and vacancies, were placed in the vicinity of a GB. These defects generally decrease the globally applied critical stress for crack initiation whilst the local critical stress stays mainly unchanged in the same range as the theoretical GB strength. Based on the experimental results and on the general findings from the atomistic simulations it is concluded that an infinite fatigue life is obtained either when cyclic irreversible deformation is too small to build up the necessary stresses for crack initiation or when the formed crack nuclei cannot overcome the next microstructural barriers.

Experimental investigation and analytical description of the damage evolution in a Ni-based superalloy beyond 106 loading cycles

The fatigue life of metallic materials in the very high cycle fatigue (VHCF) regime is characterized by a large scatter of experimental results. The aim of the present work is to investigate the reasons for the scattering of the fatigue life as well as to model its impact statistically. For this purpose crack initiation in the Ni-base superalloy Nimonic 80A was investigated as a function of the number of cycles to failure at different stress amplitudes both in the HCF and the transitional HCF–VHCF regime. According to the experimental observations the fatigue crack initiation depends on the stress amplitude and occurs at twin boundaries and grain boundaries with a large misorientation angle. The application of two analytical approaches for the assessment of the local stress concentration at different grain boundaries is discussed. In particular in the VHCF regime, the misorientation factor developed by Blochwitz et al. (1997), which depends on the misorientation angle between two adjacent grains as well as on the orientation of their boundary with respect to the external load, was used to estimate the stress concentration at the grain boundaries. It was revealed, that the cracks initiate at the grain boundary of maximum misorientation factor calculated for each specimen. Furthermore, the calculated maximum misorientation factor per specimen show a direct relation to the number of cycles to failure and can be used as an additional microstructural information for statistically based fatigue life prediction models.

Effects of specimen size on fatigue life of metallic materials in high‐cycle and very‐high‐cycle fatigue regimes

AbstractThis paper proposes a new method to investigate the effect of specimen size on fatigue life of metallic materials in high‐cycle fatigue and very‐high‐cycle fatigue regimes in the light of statistical analysis. The performance of large specimen is correlated with that of small specimen via control volume (a high stress domain where potential fatigue crack initiation sites are located). The simulation results are in good agreement with a series of experimental data in high‐cycle fatigue and very‐high‐cycle fatigue regimes from literature. The method is helpful for a quantitative estimation of fatigue life of large specimens from the experimental data of small specimens.

Fatigue Life Improvement of Laser Clad 7075 Aluminium Alloy by Deep Surface Rolling Technique

The Deep Surface Rolling (DSR) technology can substantially increase the fatigue life of metallic materials due to the introduction of deep compressive residual stresses in the material surface. These beneficial compressive residual stresses can be achieved up to a depth of 1 mm. The DSR technology also produces a good surface finish unlike bead peening technology. In this study, the main objectives were: (1) to study the feasibility of DSR for fatigue life improvement of high strength aluminium alloy (7075-T651) repaired with laser cladding technology, and (2) to investigate the effect of thermal stressing on the fatigue life improvement of DSR. Previously published results have shown that post-heat treatment of laser clad high strength Al alloy coupons improved their fatigue life. The experimental results in this paper show that the fatigue life was substantially increased using the DSR technique on laser clad 7075-T6 aluminium alloy compared to laser cladding alone and laser cladding followed by post heat treatment.

Influences of surface grain size and gradient variation along depth on fatigue life of metallic materials

The Navarro–Rios (N–R) model was used to investigate the effects of surface grain size and the grain size gradient along depth (characterized as parameter g) induced by surface nanocrystallization (SNC) on the fatigue damage of the metallic materials in this paper. Two equations were deduced and the results showed that the crack arrest of metallic materials decreased when the surface grain became nanocrystalline. Nevertheless, the crack arrest of the SNCed materials increased as the top layer grain size decreased and it rose with the increase of the value of g when the surface grain size was smaller than 100nm. The transition stress diminished with the increase of the surface grain size and the decrease of the value of g. The short crack growth rate descended with the decrease of surface grain size and grain size gradient. It explained why the SNC could improve the fatigue life of the metallic materials effectively.

The effect of specimen geometry on the low cycle fatigue life of metallic materials

The effect of the specimen geometry on the measured fatigue life was investigated in this study. Low-cycle fatigue tests with hollow and solid specimens were performed and the differences in the two different specimen geometries were observed. A difference in the fatigue life between the hollow and solid specimens exists when the strain is large and diminishes as the strain decreases, which indicates that the difference stems from the crack propagation stage rather than the crack initiation stage. Two-dimensional finite element analysis was performed to quantify the effects of the specimen geometry in terms of the stress and strain near the crack tip. Larger internal plastic strain in the hollow specimen was found. This led to a higher plastic strain energy density and could explain the difference in the lifespan using the two specimen geometries.

The effect of specimen geometry on the low cycle fatigue life of metallic materials

The effect of the specimen geometry on the measured fatigue life was investigated in this study. Low-cycle fatigue tests with hollow and solid specimens were performed and the differences in the two different specimen geometries were observed. A difference in the fatigue life between the hollow and solid specimens exists when the strain is large and diminishes as the strain decreases, which indicates that the difference stems from the crack propagation stage rather than the crack initiation stage. Two-dimensional finite element analysis was performed to quantify the effects of the specimen geometry in terms of the stress and strain near the crack tip. Larger internal plastic strain in the hollow specimen was found. This led to a higher plastic strain energy density and could explain the difference in the lifespan using the two specimen geometries.

Non-isothermal low-cycle fatigue analysis of elasto-viscoplastic materials

A non-isothermal elasto-viscoplastic model, developed within the framework of continuum damage mechanics, is proposed to investigate the influence of thermomechanical coupling on fatigue life of metallic materials. The numerical simulation of constant-amplitude low-cycle fatigue testing of a 316L stainless steel at room temperature shows that the hypothesis of isothermal processes may be inadequate when cyclic inelastic deformations are involved. The results show that the specific heat can decrease when the fatigue damage is high, causing a localized increase of the temperature even if the amount of heat generated is not so high and, hence, promoting a reduction in lifetime.

Numerical and Experimental Analysis of Crack Closure

The fatigue life of metallic materials is strongly influenced by crack closure effects. Finite element (FE) methods allow the study of crack closure with great detail and can provide valuable information about phenomena occurring in the bulk of the material. In this work the distribution of stresses through the thickness of a cracked specimen has been studied using 3D FE simulations. It was found that the transition between the interior of the specimen (plane strain) and the surface (plane stress) differs from that predicted by 2D plane stress models. In addition, an attempt is presented to experimentally validate the results at the surface level. For this purpose full-field image correlation technique was utilized. This allowed direct comparison between the displacement field predicted by the numerical simulations and the experimental results measured by digital image correlation.

INTRODUCTION OF MACRO-STRAIN WITH RELEASING MICRO-STRAIN BY CAVITATION SHOTLESS PEENING

When cavitation bubble is collapsed, shock wave which can deform metallic materials is produced. Cavitation impacts can be used for surface modification to enhance fatigue life of metallic materials in the same way as shot peening. As a peening method using cavitation impact does not require shots in shot peening, it is called “cavitation shotless peening CSP”. Although CSP can introduce compressive residual stress, i.e., macro-strain into metallic materials, full width at half maximum of diffractive X-ray profile was decreased by CSP. In the present paper, tool alloy steel for forging die was chosen as tested material to investigate mechanism of improvement of fatigue life, as CSP improved the life time of the forging die. The introduction of macro-strain was revealed by measuring residual stress, which was evaluated by X-ray diffraction method. The fatigue life was investigated by using a plate bending fatigue test changing with processing time of CSP. The micro-strain was evaluated by a fundamental parameter approach, which is one of X-ray diffraction method. It was concluded that the fatigue life of the steel was improved about 90 times by CSP and CSP can introduce macro-strain, i.e., compressive residual stress and releasing micro-strain. The micro-strain becomes about 1/20 of heat treated specimen by CSP.

Residual fatigue life of metallic materials with a long service life

The concept of residual life is discussed as most widely used in assessing the additional resource of materials of power station structural elements after they have exceeded their calculated life. No methods have been developed which provide a highly reliable solution of numerical problems involved in determining the residual life of materials damaged in operation of steam-piping straight sections or their bends. Stuctural state of a material can not be predicted in time. The possibility of the use of the equation of linear summation for the evaluation of residual life is considered and presented.

Evaluation methods of residual fatigue life of metallic materials

Evaluation methods of residual fatigue life of metallic materials

Plastic Fatigue Life of Steel under Periodical Multi-Stage Stress Amplitudes

In the calculation of the fatigue life of metallic materials under the varying stress amplitude, it is common to apply the linear damage criterion, Σ(n/N)=1, based on the stress amplitude (i.e. S-N curve). In the present, the author proposes the fatigue fracture criterion derived from the damage function on the basis of the plastic strain in stead of the customary Σ(n/N)=1. The fatigue fracture criterion is represented as1/C0∫N0epaadn=1for the varying plastic strain amplitude.The medium carbon steel S35C (normalized) is tested under the condition of the rotating bending and the repeated tension-compression with constant- and varying-stress amplitude to prove the criterion described above.As a result, the experimental values of the cumulative damage based on this criterionDf=1/C0m∫N0epaadnare scattered closely about unity, although Σ(n/N)=1 is also established in this case of low-cycle fatigue: the scatter of Df-value being almost the same as in the value of Σ(n/N). In all the tests performed here, neither effect of the frequency distribution of stress (or strain) nor of their sequence of application on the fatigue life under the varying stress amplitude has been recognized.