Fatigue Of Metallic Materials Research Articles

Historical review and future prospect for researches on very high cycle fatigue of metallic materials

AbstractSince the first paper on the fatigue of metallic materials by J. Albert in 1837, tremendous numbers of papers have been published in various journals by many researchers all over the world. Based on such a long history, several papers on the very high cycle fatigue (VHCF) in the life‐time longer than 107 cycles had appeared in some journals during the period of 1980's. One characteristic finding in these works is the fact that the metallic material can fail even at the stress level lower than the conventional fatigue limit. This fact means that the conventional fatigue design of mechanical structures cannot give the safety of the practical products in the very high cycle regime. Due to this fact, fatigue properties of structural materials in very long life regime has become an important subject; and a lot of studies have been carried out, and many important results have been accumulated until now. Typical aspects on VHCF property are summarized as follows: (1) the fatigue crack tends to occur around the interior inclusion, (2) fine granular area (FGA) is formed around such an inclusion, and (3) duplex S–N characteristics appear in the VHCF regime. In this paper, a brief historical review together with the future prospect on the very high cycle fatigue of metallic materials is attempted for the sake of reference to facilitate the research in this area.

FATIGUE PROCESS OF AUTOMOBILE MATERIALS

During operation, the structural elements of cars are exposed to temperatures and vibrations. Overwhelming majority of the destruction of metal structures is caused by their fatigue. It causes economic losses and often human casualties from accidents. Therefore, the task of ensuring the operability of parts and components of automobiles is one of the most relevant in the modern automotive industry. So it is necessary to know the patterns of behavior of metallic materials, obtained by different technologies, when they are exposed to vibration. Destruction of the metal structure directly affects the behavior of the samples deflection, reflecting the competition of two mutually oppositephenomena – hardening and softening. It directly influences structural damageability of the metal. The article is devoted to the study of kinetics of fatigue failure of automotive materials using the calibration of structural damage to their surface with behavior of the curves of changes in current deflection under alternating loading. The paper considers automotive materials (steel grades 20KhI3, 14Kh17N2, 35KhGSА) and model metals and alloys (Copper M1, Brass L63T, aluminum alloy V95pchT2) in different structural state under cyclic loading for low, room and high temperatures with fixation of the sample deflection and structural damage corresponding to it. It is possible to study kinetics of fatigue destruction of the sample material by the deflection curves, which is an integral characteristic of destructive processes occurring under alternating loading. Using these processes, one can track the stages of damage during fatigue of metallic materials – damage to the structure at the initial stage, moment of the macroscopic crack appearance, its subsequent advancement up to complete separation of the structural material. It is probable to identify ratio of the period duration before the appearance of a fatigue crack and its subsequent growth, as well as to determine the average rate at which the fatigue crack moves through the body of the metal sample. It is important that it is also possible to estimate the kinetics of materials destruction under the conditions when direct study of the structural state of the sample surface is impossible, for example, in conditions of cryogenic and high temperatures, and also, for example, in the presence of corrosive media. In combination with fractographic and metallographic analysis of the fatigue process, the deflection curves allow, based on the evaluation of the stages of materials destruction, to carry out selection of the latter for the structural elements of a car taking into account its operating conditions and optimizing the technology of parts manufacturing to increase serviceability and maintainability.

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.

A review on mechanisms and models for very-high-cycle fatigue of metallic materials

The process of fatigue failure beyond 107 cycles for a metallic material subjected to cyclic load is called very-high-cycle fatigue (VHCF). This review will summarize the research progress of VHCF starting with its origination in early 1980s, until the cutting-edge development in recent years. After Introduction, this review contains following parts: Origination of VHCF research, Main characteristics of VHCF, Characteristic region of crack initiation and related parameters for VHCF, Formation mechanisms and models for characteristic region of crack initiation, and Prediction models of VHCF properties. The relevant descriptions attempt to answer the following questions: What is VHCF? Why VHCF should be investigated? What are the essential scientific issues for VHCF? Why the tendency of S-N curve for VHCF is changed? Why crack initiates from the interior of material (specimen) for VHCF? What are the process and the mechanism of interior crack initiation? Some of the questions will be clearly answered, but some of them be just addressed by newly results, which require further exploration.

High pulse number thermal shock tests on tungsten with steady state particle background

Thermal fatigue of metallic materials, which will be exposed to severe environmental conditions e.g. plasma facing materials in future fusion reactors, is an important issue in order to predict the life time of complete wall components. Therefore experiments in the linear plasma device PSI-2 were performed to investigate the synergistic effects of high pulse number thermal shock events (L = 0.38 GW m−2, Δt = 0.5 ms) and stationary D/He (6%) plasma particle background on the thermal fatigue behavior of tungsten. Similar to experiments with pure thermal loads, the induced microstructural and surface modifications such as recrystallization and roughening as well as crack formation become more pronounced with increasing number of thermal shock events. However, the amount of damage significantly increases for synergistic loads showing severe surface roughening, plastic deformation and erosion resulting from the degradation of the mechanical properties caused by bombardment and diffusion of D/He to the surface and the bulk of the material. Additionally, D/He induced blistering and bubble formation were observed for all tested samples, which could change the thermal and mechanical properties of near surface regions.

The nature and the mechanism of crack initiation and early growth for very-high-cycle fatigue of metallic materials – An overview

Very-high-cycle fatigue (VHCF) is the fatigue damage and failure beyond 107 loading cycles. Crack initiation of VHCF for metallic materials is commonly from the interior of specimens and possesses a unique feature of fish-eye (FiE) containing a fine-granular-area (FGA). It is regarded that FGA together with FiE is the characteristic region of crack initiation for VHCF, because it consumes more than 95% of total fatigue life and the value of stress intensity factor range for this region keeps constant for a given material. The aim of this overview is to summarize the research progress on the crack initiation characteristics and the related mechanisms for VHCF of metallic materials. After a brief introduction to the origination of VHCF research in 1980s, this overview contains the following parts: Characteristics of crack initiation region for VHCF, Characteristic region and intrinsic dimensions of crack initiation for VHCF, The formation mechanism and the model of crack initiation region, The new model of Numerous Cyclic Pressing to explain the formation mechanism of FGA, and Verification of proposed NCP model. In the descriptions, the two essential aspects are especially focused: the nature of crack initiation region for VHCF, and the mechanism for the formation of crack initiation region.

Effects of stress ratio on crack growth rate and fatigue strength for high cycle and very-high-cycle fatigue of metallic materials

This paper studies the effects of stress ratio R on crack growth rate and fatigue strength for high cycle and very-high-cycle fatigue of metallic materials. First, the effect of R on fatigue crack growth rate is analyzed, and the result shows that the effect of R on crack growth rate at near-threshold region and on Paris region is well expressed by a unified relation. Then, a model is developed for describing the fatigue crack growth rate with the effect of R in near-threshold region and Paris region. The model is verified by experimental data of metallic materials in literature. Finally, a formula is derived for the effect of R on fatigue strength. The estimated results are in good agreement with the experimental data for high cycle and very-high-cycle fatigue regime in literature. The comparison of the present result with Goodman diagram is also discussed.

Cyclic Capacity Fade Plots for aging studies of Li-ion cells

The article introduces a Cyclic Capacity Fade Plot, to depict the deterioration of a lithium-ion cell material, as a graph between a factor based on State of Charge (ΔSOC*SOCmean) and the % capacity loss per cycle. The plots are generated entirely using experimental data. A variant of this plot is presented as Log(cycle life) vs. ΔSOC*SOCmean, that is analogous to Stress–Number of Cycles (S–N) type limit curves used in the study of cyclic fatigue of metallic materials. The monotonic variation and in some cases near linear nature of these plots, valid for different temperatures, is noteworthy. These plots provide a wealth of information at a quick glance and are shown to be useful in comparing the performance of different cell materials. The utility of the Cyclic Capacity Fade Plots is demonstrated illustratively for two (Li Manganese Oxide) LMO-graphite cells studied in-house and for an olivine system reported in the literature. These plots are expected to be useful in electrode material development, accelerated testing, battery health management etc.

A model of continuum damage mechanics for high cycle fatigue of metallic materials

A non-linear continuum damage model was presented based on the irreversible thermodynamics framework developed by LEMAITRE and CHABOCHE. The proposed model was formulated by taking into account the influence of loading frequency on fatigue life. The parameters H and c are constants for frequency-independent materials, but functions of cyclic frequency for frequency-dependent materials. In addition, the expression of the model was discussed in detail at different stress ratios (R). Fatigue test data of AlZnMgCu1.5 aluminium alloy and AMg6N alloy were used to verify the proposed model. The results showed that the model possesses a good ability of predicting fatigue life at different loading frequencies and stress ratios.

Numerical study of the thickness transition in bi-dimensional specimen cracks

Concerning bi-dimensional crack configurations, classical linear elastic fracture mechanic theory ignores the three-dimensional effects of crack propagation. Issues as the influence of the specimen thickness are not considered. Nevertheless, real cracks show some effects difficult to explain. It is the case of the load effect on fatigue crack closure. The yielded zone size close to the crack front is an important factor in fracture and fatigue of metallic materials. They are related to the two extreme stress states: plane stress or plane strain, but no clear information about out-of-plane plastic zone development or stress state has been provided. In the present work, a CT aluminium specimen has been modelled three-dimensionally and several calculations have been made considering a huge combination of different single load levels and specimen thickness. Due to the abrupt transition from plane strain to plane stress, an ultrafine mesh along the thickness has been applied. The analysis of the evolution of the plastic zone and the stress state along the thickness provides information about the combined influence of these parameters on fracture mechanics.

Mechanical properties distributions of PM manganese steels analysed by Gaussian and Weibull statistics

Powder metallurgy (PM) parts acceptance is determined by compositional and processing parameters and their controls. Statistical procedures are used for assessment; normal distribution and 'six sigma' appear to predominate. For fatigue of metallic materials and strengths of ceramics, fibres and composites, however, Weibull probability of survival analyses are widely used. The original analysis considers a threshold stress at which the probability of failure is zero. This stress is frequently taken to be zero, simplifying the analysis to two parameters. The yield stress has been suggested for the threshold stress, probably not sufficiently conservative for less ductile PM materials. A new three-parameter Weibull analysis, in which it is taken to be the fracture strength minus six standard deviations, is presented. Powder metallurgy manganese steels are under commercial consideration and this approach is applied to 12 variants of such laboratory processed specimens. It is compared with the two-parameter Weibull and Gaussian, the least conservative, analyses.

A simulation on microstructure sensitivity to very-high-cycle fatigue behavior of metallic materials

This paper studied the microstructure sensitivity to very-high-cycle fatigue of metallic materials based on Tanaka and Mura model by using Monte-Carlo method. In this simulation, local stress, frictional stress and inclusion size are assumed to be random variables, and two sensitivity indicators are proposed: sensitivity of mean value and sensitivity of standard deviation. Results indicate that fatigue failure is most sensitive to frictional stress, and the dispersity of variables also influences the fatigue life.

Review and Prospects for Current Studies on Very High Cycle Fatigue of Metallic Materials for Machine Structural Use

In recent years, mechanical structures such as railway wheels, rails, offshore structures, bridges, engine components, load bearing parts of automobiles, etc. have to endure for a long term up to 108-1010 loading cycles in order to save resources and to reduce the cost together with the environmental load to the globe. Thus, the fatigue behavior of structural materials in the very high cycle regime of 108-1010 cycles has become an important subject of the research. In this paper, a review of the current studies in this area performed by many researchers is described in order to provide a certain milestone in the history of the research on fatigue behavior of the metallic materials in the very high cycle regime.

A new look at the atomic level virial stress: on continuum-molecular system equivalence

The virial stress is the most commonly used definition of stress in discrete particle systems. This quantity includes two parts. The first part depends on the mass and velocity (or, in some versions, the fluctuation part of the velocity) of atomic particles, reflecting an assertion that mass transfer causes mechanical stress to be applied on stationary spatial surfaces external to an atomic‐particle system. The second part depends on interatomic forces and atomic positions, providing a continuum measure for the internal mechanical interactions between particles. Historic derivations of the virial stress include generalization from the virial theorem of Clausius (1870) for gas pressure and solution of the spatial equation of balance of momentum. The virial stress is stress‐like a measure for momentum change in space. This paper shows that, contrary to the generally accepted view, the virial stress is not a measure for mechanical force between material points and cannot be regarded as a measure for mechanical stress in any sense. The lack of physical significance is both at the individual atom level in a time‐resolved sense and at the system level in a statistical sense. It is demonstrated that the interatomic force term alone is a valid stress measure and can be identified with the Cauchy stress. The proof in this paper consists of two parts. First, for the simple conditions of rigid translation, uniform tension and tension with thermal oscillations, the virial stress yields clearly erroneous interpretations of stress. Second, the conceptual flaw in the generalization from the virial theorem for gas pressure to stress and the confusion over spatial and material equations of balance of momentum in theoretical derivations of the virial stress that led to its erroneous acceptance as the Cauchy stress are pointed out. Interpretation of the virial stress as a measure for mechanical force violates balance of momentum and is inconsistent with the basic definition of stress. The versions of the virial‐stress formula that involve total particle velocity and the thermal fluctuation part of the velocity are demonstrated to be measures of spatial momentum flow relative to, respectively, a fixed reference frame and a moving frame with a velocity equal to the part of particle velocity not included in the virial formula. To further illustrate the irrelevance of mass transfer to the evaluation of stress, an equivalent continuum (EC) for dynamically deforming atomistic particle systems is defined. The equivalence of the continuum to discrete atomic systems includes (i) preservation of linear and angular momenta, (ii) conservation of internal, external and inertial work rates, and (iii) conservation of mass. This equivalence allows fields of work‐ and momentum‐preserving Cauchy stress, surface traction, body force and deformation to be determined. The resulting stress field depends only on interatomic forces, providing an independent proof that as a measure for internal material interaction stress is independent of kinetic energy or mass transfer.

The influence of defects on high cycle fatigue of metallic materials

Structural ceramics are known to be particularly sensitive to singular defects such as inclusions, secondary pores, pore clusters, etc. Within this work, it is shown that similar behavior can be found also with high strength/low ductility metallic materials in the case of high cycle fatigue loading. Various types of such defects are described in different metallic materials, those produced by powder metallurgy being particularly suited for these investigations. It is shown that the influence of the singular defects can be described by a modified Kitagawa diagram based on the fatigue limit of the defect-free material and the threshold stress intensity factor. Realistic predictions are however only possible if both the stress field around the defects and possible interaction between adjacent defects are taken into account.

On cyclic stress–strain behaviour and low cycle fatigue life

In low cycle fatigue of metallic materials, the Manson–Coffin and Basquin relations are considered to be in conjunction with the cyclic stress–strain curve. Design considerations under constant load range or constant total displacement range are discussed based on the above relations.

The validation of some methods of notch fatigue analysis

This paper is concerned with the testing and validation of certain methods of notch analysis which the authors have developed theoretically in earlier publications. These methods were developed for use with finite element (FE) analysis in order to predict the fatigue limits of components containing stress concentrations. In the present work we tested and compared these methods using data from standard notches taken from the literature, covering a range of notch geometries, loading types, R‐ratios and materials: a total of 47 different data sets were analysed. The greatest predictive success was achieved with critical‐distance methods known as the point, line and area methods: 94% of these predictions fell within 20% of the experimental fatigue limits. This was a significant improvement on previous methods of this kind, e.g. that of Klesnil and Lucas [(1980) Fatigue of Metallic Materials, Elsevier Science]. Methods based on the Smith and Miller [(1978) Int. J. Mech. Sci. 20, 201–206] concept of crack‐like notches were successful in 42% of cases; they experienced difficulties dealing with very small notches, and could be improved by using an ElHaddad‐type correction factor, giving 87% success. An approach known as ‘crack modelling’ allowed the Smith and Miller method to be used with non‐standard stress concentrations, where notch geometry is ill defined; this modification, with the same short‐crack correction, had 68% success. It was concluded that the critical‐distance approach is more accurate and can be more easily used to analyse components of complex shape, however, the crack modelling approach is sometimes preferable because it can be used with less mesh refinement.

A critical plane approach for life prediction of high cycle fatigue under multiaxial variable amplitude loading

In this paper, a new life prediction method for high cycle multiaxial fatigue of metallic materials is presented. It is based on a local approach introduced by Dang Van and developed by Papadopoulos, where accumulated plastic strain due to external loading is estimated at a scale of the order of a few grains. Its evaluation requires the use of a critical plane type fatigue criterion. As soon as the accumulated plastic mesostrain, considered as the damage variable, reaches a critical value, a crack is considered to be initiated. The complex and combined cases of loading (multiaxial and variable amplitude loading) can be analysed with this new method. In particular, attention is given to a description of the damaging effect of the loading path shape on fatigue strength. The application of hardening plasticity laws at the local scale allows one to estimate and sum the damage caused by elementary loading events like transitions.

Compression fatigue crack growth behavior of metallic alloys: Effect of environment

Many structures are subjected to cyclic compressive loading and corrosive atmosphere. However, research on environmentally-assisted compression fatigue of metallic materials has not yet been undertaken. In the present paper, the behavior of fatigue cracks under compressive cycling in laboratory air, mineral oil and 3.5% NaCl was studied on circumferentially notched cylindrical specimens of an Al-alloy and CrNi stainless steel. Fatigue tests were supplemented by fractographic observations. In oil and air, the non-catastrophic character of the compression fatigue crack growth process was confirmed for both materials. The saturation depth of the crack was found to depend on the material as well as on the nominal stresses, but was insensitive to the load frequency in a range of 2–110 Hz. In the case of the aluminum alloy tested at low load frequency in 3.5% NaCl, continuous growth of fatigue crack was observed. The role of hydrogen embrittlement and anodic dissolution in promoting fatigue damage under compressive cycling is discussed and the mechanism rationalizing observed behavior is proposed. The detrimental influence of corrosive environment is attributed to the crack tip blunting via anodic dissolution and concomitant development of the tensile residual stresses ahead of the crack growing under compressive cycling. If operative in service, the above mechanism may cause a catastrophic failure even under fully compressive cyclic loading.

A review of: “Fatigue of Metallic Materials” (Second Revised Edition) Edited by M. Klesnil and P. Lukas

A review of: “<i>Fatigue of Metallic Materials</i>” (Second Revised Edition) Edited by M. Klesnil and P. Lukas