Fatigue Damage Accumulation Process Research Articles

Numerical modeling of elastoplastic deformation and damage accumulation in metals under low-cycle fatigue conditions

A mathematical model that describes the processes of fatigue damage accumulation in structural materials (metals and alloys) under multiaxial disproportionate combined thermomechanical loading is advanced from the standpoint of the damaged medium mechanics. Based on the results of basic experiments performed in a special way, an experimental-theoretical procedure for finding material parameters of the advanced constitutive relations of the damaged medium mechanics is put forward. The advanced version of the constitutive relations of the damaged medium mechanics is shown to adequately (qualitatively and quantitatively) reflect the main effects of elastoplastic deformation and damage accumulation in metals under low-cycle fatigue.

A Probabilistic Mechanical Model for Simulating the Fatigue Failure Process in Metals

A model for simulating fatigue damage accumulation and the fatigue failure process in metals is presented. The simulation is achieved by modeling material behavior with a series of nonlinear mechanical springs with randomized behavior. With each applied stress, a certain number of springs rupture. The damage accumulation process is modeled by the number of springs that have ruptured during the entire stress application cycle. When a sufficiently large number of springs rupture, the entire system is considered to have failed. This constitutes fatigue failure. This article follows two previous publications by the first two authors and extends the model further by incorporating additional random variables, investigating the significance of uncertainty in the spring behavior and simulation of the hysteresis behavior of metals during the fatigue damage accumulation process. Results similar to (1) the Wöhler S—N curve and (2) the hysteresis loss versus the number of stress cycle relationship, observed in laboratory testing of metal specimens, are presented.

A high‐cycle fatigue accumulation model based on electrical resistance for structural steels

ABSTRACTFor high‐cycle fatigue of metals, the DC electrical resistance is a more sensitive parameter to the initiation of micro‐cracks during the irreversible fatigue damage accumulation process. This implies that the electrical resistance is a suitable parameter that can be consistent with the fatigue damage physical mechanism. The relation between the ratio of electrical resistance changes and the cyclic fraction of the fatigue specimen may reasonably represent deterioration in mechanical properties of structural steels during the high‐cycle fatigue process. The high‐cycle fatigue damage accumulation model based on electrical resistance for structural steels was proposed. The model was verified by some experimental data for three structural steels; normalized 45C steel, 20 Mn steel and 16 Mn steel, and good agreement was obtained. The corresponding fatigue lifetime on the basis of the electrical resistance model was also performed. The results show that the approach to fatigue lifetime prediction and failure based on the electrical resistance is a good non‐destructive technique.

Durability of Soil–Cements against Fatigue Fracture

Cementitious stabilization is a common method of ground improvement when weak foundation soils are encountered in practice, and marginal or nonstandard materials such as recycled aggregates are used in civil engineering construction. Long-term durability of stabilized materials becomes an issue, especially when it involves one or more recycled materials with unknown and often questionable properties. The study presented herein investigates the fatigue durability, endurance limit, and damage accumulation process in recycled crushed concrete aggregate stabilized with cement-fly ash mixtures. Results show that the 2 million cycles fatigue endurance limit for the stabilized recycled aggregate was nearly 53% of the static modulus of rupture, indicating that the fatigue strength of this material is quite similar to or better than other traditional cementitious composites. It was also found that the accumulated permanent deformation and the expended fatigue life can be related by a nonlinear power law, and the fatigue damage in this material approximately follows Miner's rule for cumulative damage. Finally, the importance of developing innovative testing methods for durability assessment are highlighted, which include coupled mechanical and environmental loadings to simulate the most damaging field conditions, and accelerated aging and life prediction processes using the Arrhenius time-temperature superposition methods.

Dynamical systems approach to fatigue damage identification

In this paper, we present a dynamical systems approach to material damage identification. This methodology does not depend on knowledge of the particular damage physics of material fatigue. Instead, it provides experimental means to determine what are practically observable and observed facts of damage accumulation, thus making it possible to develop or experimentally verify appropriate damage evolution laws. A concept of phase space warping is used to develop new damage tracking feature vectors from measured time series through phase space reconstruction. These feature vectors provide critical information about the dimensionality of a damage process that was missing from an old scalar metric described in previous work. Damage identification is achieved by applying either proper orthogonal decomposition or smooth orthogonal decomposition to these vectors. The method is experimentally validated using an elasto-magnetic system, in which a harmonically forced cantilever beam in a non-linear magnetic field accumulates fatigue damage. Both damage identification methods yield a single active damage mode that shows power-law-type monotonic trends, which is also consistent with a result obtained using the old scalar metric. These results are shown to be in good agreement with the Paris fatigue crack growth model during the time period of macroscopic crack growth. Using this simple model, accurate time-to-failure predictions are shown well ahead of actual beam fracture. In addition, this identification scheme provides a much-needed insight into the dimensionality of incipient or early fatigue damage accumulation process, which is shown to be describable by only one scalar damage variable for this specific experiment.

Nondestructive evaluation of the fatigue damage accumulation process around a notch using a digital image measurement system

The fatigue damage accumulation process around a notch is studied using a noncontact digital image measurement system. This system incorporates a contrast correlation method to evaluate the level of plastic damage at each point of the studied area of the specimen from two images acquired before and after the introduction of fatigue deformation. A compact tension specimen of 304 stainless steel with a notch radius of 1 mm is analyzed during the first 1000 cycles of the crack growth stage. During this period, the externally given work not only impels the crack growth and its local plastic zone but also generates plastic damage around the notch. The obtained results are used to explain the behavior of cracks emanating from notches.

Cumulative fatigue damage assessment and life predictions of as-forged vs QT V-based MA steels using two-step loading experiments

This study examines the fatigue damage accumulation process associated with a commonly produced forged vanadium-based microalloyed (MA) steel and its comparison with its quenched and tempered (Q&T) counterpart at the same hardness level. The advantage of MA steels compared to the traditional Q&T steels is the elimination of the costly quenching and tempering processes. Completely reversed strain-controlled two-level block loading tests were conducted on smooth axial specimens at room temperature. Under multi-level block cycling, the two steels displayed different characteristics, though they showed similar behaviour in constant amplitude fatigue. Therefore, a key to successful assessment of fatigue damage accumulation under variable amplitude service loading is selection of an appropriate cumulative fatigue life prediction model which reflects the material's damage characteristics. The effectiveness of several cumulative fatigue damage models and their life prediction capabilities are evaluated using the experimental data.

Numerical Investigation of the Effect of Loading Multiaxiality and Deformation Process Parameters on the Fatigue Life of Metals

Known experimental data on the effect of loading multiaxiality and the type of deformation path on the fatigue damage accumulation process have been analyzed. In terms of the mechanics of a damaged medium, a variant of the defining relations has been developed that describes the processes of elastoplastic deformation and damage accumulation in structural materials (metals and their alloys) in the case of multiaxial nonproportional low-cycle loading paths. The damaged-medium model consists of three interrelated components: relations defining elastoplastic behavior of the material with allowance for the influence of the fracture process, equations describing the damage accumulation kinetics, and a strength criterion for damaged material. To qualitatively assess the defining relations, a numerical experiment to construct equal-damage surfaces has been carried out. The calculated data are compared with the experimental ones. The effect of the stress-state multiaxiality and the type of the deformation path on the low-cycle fatigue life of metals has been investigated. It has been shown that the developed variant of defining relations for a damaged medium reflects correctly the main effects of cyclic elastoplastic deformation and damage accumulation at a multiaxial stress-state and arbitrary material deformation paths.

Fatigue characterization of a hydroxyapatite‐reinforced polyethylene composite. I. Uniaxial fatigue

The fatigue behavior of 40 volume % hydroxyapatite-reinforced polyethylene composite (HAPEX™) at 37°C in saline was determined. S-N curves for this material, both in fully reversed axial tension–compression and fully reversed torsion, have been established. In tension, the cycles to failure ranged from 1000 cycles at 13 MPa to more than 1 million cycles at 4.4 MPa while in torsion they ranged from 100 cycles at 14.4 MPa to more than 1 million cycles at 4.8 MPa. Changes in strain range, tangent modulus, and cyclic energy dissipated during fatigue loading also were examined. Torsional fatigue was found to be considerably more damaging than axial fatigue. The fatigue damage accumulation process occurring in the composite was monitored by observing the reduction in modulus and the increase in energy dissipated. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res, 51, 453–460, 2000.

Fatigue characterization of a hydroxyapatite-reinforced polyethylene composite. I. Uniaxial fatigue.

The fatigue behavior of 40 volume % hydroxyapatite-reinforced polyethylene composite (HAPEXtrade mark) at 37 degrees C in saline was determined. S-N curves for this material, both in fully reversed axial tension-compression and fully reversed torsion, have been established. In tension, the cycles to failure ranged from 1000 cycles at 13 MPa to more than 1 million cycles at 4.4 MPa while in torsion they ranged from 100 cycles at 14.4 MPa to more than 1 million cycles at 4.8 MPa. Changes in strain range, tangent modulus, and cyclic energy dissipated during fatigue loading also were examined. Torsional fatigue was found to be considerably more damaging than axial fatigue. The fatigue damage accumulation process occurring in the composite was monitored by observing the reduction in modulus and the increase in energy dissipated.

Nondestructive Evaluation of Fatigue Damage by Induced Current Focusing Potential Drop.

Quantitative evaluation of fatigue damage accumulation and resultant crack initiation and propagation is one of the greatest concerns for plant life time management and license renewal in various industries. To develop a novel nondestructive evaluation method for fatigue damage evaluation, we applied Induced Current Focusing Potential Drop (ICFPD), which was developed for an inspection of defects, to processes of fatigue damage accumulation, small crack initiation and coalescence, and macroscopic crack growth. Measurements were performed on specimens fatigued under an axial strain controlled tension/compression load with various degrees of fatigue damage defined as a cycle ratio, N/Nc where Nc is a cycle giving a maximum crack size of 3 mm at surface. ICFPD can be used to clearly detect and differentiate all of these fatigue processes of damage accumalution without crack initiation and subsequent small crack initiation and macroscopic crack propagation.

A RANDOMIZED ENDURANCE LIMIT IN FATIGUE DAMAGE ACCUMULATION MODELS

AbstractThe use of random parameters for describing S‐N curves requires special caution. A Peculiar Fatigue Curves (PFC) method based on a linear damage accumulation rule is proposed for modelling the fatigue damage accumulation process. The fatigue properties of a component are presented in the PFC form by randomizing its endurance limit which facilitates a simple technique to predict fatigue life and residual life distributions. Possible usages of the models and their restrictions are illustrated for some common load histories of a component. Particular attention is paid to the stochastic relations between random variables having in some cases improper distributions. The problem of experimental testing of the models is also discussed.

低炭素鋼の二段二重々複応力疲れ寿命に及ぼす時効の影響

In the case of two-level cyclic stress loading, it is generally recognized in steels that the cumulative cycle ratio (ΣN/Nf) is greater than unity for a low to high stress sequence (σL→σH) but less than unity for a high to low stress sequence (σH→σL). This kind of deviation from the Palmgren-Miner's rule (linear damage rule) is expected to be related to the fatigue damage accumulation process and age-hardening ability.Two kinds of low carbon steels were prepared; specimen A, in which the age-hardening ability was attained through low temperature quenching, and specimen B, in which the ageing process in specimen A was completed by leaving them at room temperature. In specimen B, the deviation from Palmgren-Miner's rule was a little for both σL→σH and σH→σL sequences. On the other hand, in specimen A which had age-hardening ability, the deviation from Palmgren-Miner's rule for σL→σH sequence was greater than that in specimen B.The change of internal friction energy and propagation of fatigue crack were observed during cyclic loading. Based on the experimental results of accumulation process of internal friction energy, the following has been assumed.(1) A part of the internal friction energy is dissipated for the accumulation of fatigue damage.(2) Crack growth corresponds to fatigue damage.(3) The accmulation process of fatigue damage is expressed by the Marco-Starkey and New-mark models in which an exponent depends on stress amplitude and increases with progress of ageing.Based on the above assumptions, the cumulative cycle ratio was calculated, and the result showed a similar tendency as the experimental one.

Improved fatigue-life prediction by acoustic emission

Low cycle fatigue tests were conducted by tension-tension until rupture, on an Alclad 7075-T6 aluminum sheet alloy. Extremal peak Acoustic Emission amplitudes were continuously monitored. They were shown to be extremally distributed. The prediction of the number of cycles left until failure is made possible, and improved within a fair practical interval and at a good confidence level, very early in the fatigue damage accumulation process. It has been shown that only a few percent of the elastic energy associated with the crack propagation phenomena is converted into detectable acoustic energy.