Fracture Fatigue Life Research Articles

Damage evolution in sinter-hardening powder-metallurgy steels during tensile and fatigue loading

The damage approach was used to compare the tensile and push-pull fatigue behaviour of two high-strength sinter-hardened powder metallurgy (PM) steels, with a density of 6.7 and 7.2g/cm3. In both alloys, tensile damage was found to start when the ratio of the applied stress to UTS was greater than about 0.3. The tensile damage was due to localized yielding and, in a later stage, to the formation of several micro-cracks that joined to form more than one macrocrack. Fatigue damage was followed in the finite fatigue life regime and was found to develop through three stages. During the first one, damage increased with fatigue cycling up to the attainment of a plateau (second stage) that lasted to a fraction of about 0.9 of the fatigue life. The damage recorded during the second stage was very similar to that encountered in the tensile tests at the same stress to UTS ratio. The formation of microcracks was observed in the third stage only, when the fraction of fatigue life was greater than 0.9. During this stage damage increased very sharply and this was due to the growing and joining of the microcracks to form one long crack able to lead to final fracture.

On the NiTi wires in dampers for stayed cables

Recent studies were dedicated to the realization of measurements on stay-cable samples of different geometry and static conditions as available at several facilities. The elaboration of the acquired data showed a a satisfactory efficacy of the dampers made of NiTi wires in smoothing the cable oscillations. A further attempt to investigate the applicability of the achieved results beyond the specific case-studies represented by the tested cable-stayed samples is herein pursued. Comparative studies are carried out by varying the diameter of the NiTi wire so that similar measurements can be taken also from laboratory steel cables of reduced size. Details of the preparation of the Ni-Ti wires are discussed with particular attention being paid to the suppression of the creep phenomenon. The resulting shape of the hysteretic cycle differs according to the wire diameter, which affects the order of the fitting polynomial to be used when trying to retrieve the experimental results by numerical analyses. For a NiTi wire of given diameter, an estimate of the amount of dissipated energy per cycle is given at low levels of maximum strain, which correspond to a fatigue fracture life of the order of millions of cycles. The dissipative capability is affected by both the temperature and the cycling frequency at which the tests are performed. Such effects are quantified and an ageing process is proposed in order to extend the working temperature range of the damper to cold weathers typical of the winter season in Northern Europe and Canada. A procedure for the simulation of the shape memory alloy behavior in lengthy cables by finite element analysis is eventually outlined.

EFFECTS OF PRE-FATIGUE DEFORMATION ON THE UNIAXIAL TENSILE BEHAVIOR OF COARSEGRAINED PURE Al

The coarse-grained pure Al was first pre-fatigued to different fatigue life fractions D(D=2%—75%) at a constant stress amplitude,and then the effect of pre—fatigue deformation on its uniaxial tensile behavior,fracture surface deformation features and dislocation structures were investigated. The results show that with increasing D,the extrusion/intrusion phenomenon on the surfaces of the pre-fatigued coarse-grained pure Al becomes more serious,and the non-uniform deformation in grain interiors is also enhanced,leading to the nucleation of micro-cracks and micro-voids along slip bands(SBs) or at grain boundaries(GBs) as well as their subsequent propagation.As D is as high as 75%,the longer intergranular cracks are produced at triple grain boundary nodes.With increasing D, the fatigue dislocation structures transform from loose cellular structures under annealing state into regular cellular structures and sub-grains,but the size of sub-grains nearly does not change.After the pre-fatigued coarse-grained pure Al specimens were subjected to the uniaxial tension,the yield strengthσ_(YS) obviously increases,but the change inσ_(YS) is not so obvious as D increases.Meanwhile, the ultimate tensile strengthσ_(UTS) first decreases and then increases,and finally sharply re-decreases. However,the pre-fatigued coarse-grained pure Al has poor ability to work hardening.The tensile fracture surface consists of fibrous and shear lip zones,and the number of dimples in fibrous zones increases and the size reduces with increasing D;as D reaches 50%,the number of dimples re-reduces and the size raises,and the fracture surface exhibits tearing characteristics.The sub-structures after the uniaxial tension are mainly composed of sub-grains and cellular dislocation structures inside subgrains, and with increasing D,the size of sub-grains first reduces and then increases.The formation of fine sub-grains and cellular dislocation structures inside sub-grains results in the fact that the pre-fatigued coarse-grained pure Al has higher maximum uniform percent elongation.

A robust model for creep-fatigue life assessment

High temperature components subjected to long term cyclic operation will acquire life-limiting damage from both creep and fatigue. A new robust model for creep-fatigue life assessment is proposed with a minimal set of fitting constants, and without the need to separate creep and fatigue damage or life fractions. The model is based on the creep rupture behaviour of the material with a fatigue correction described by hold time (in tension) and total strain range at temperature. The model is shown to predict the observed creep-fatigue life of ferritic steel P91, austenitic steel 316FR, and Ni alloy A230 with a scatter band close to a factor of 2.

中炭素鋼高周波焼入れ材の捩り疲労強度におよぼすP, B, Mo, Siの影響

The purpose of this study is to clarify the effect of increase in the grain boundary strength on the torsional fatigue strength of induction hardened steels. The effect of lowering phosphorus content and addition of boron, molybdenum or silicon , as metallurgical factors of increasing the grain boundary strength, on torsional fatigue strength was investigated. Torsional fatigue fracture life in the test condition of high cycles fatigue is prolonged by lowering phosphorus content, or by adding boron, molybdenum or silicon, respectively. In these fatigue test, the crack propagation life against total fracture life is prolonged particularly. The fraction of intergranular fracture in the propagation region of mode I fracture is decreased by lowering phosphorus content or addition of boron etc. This is caused by increase in the grain boundary strength due to lowering phosphorus content or addition of boron etc. Consequently, improvement of torsional fatigue fracture life by lowering phosphorus content or addition of boron etc. is because that static final fracture in the process of fatigue crack propagation is disturbed due to improvement of fatigue fracture toughness caused by increase in the grain boundary strength.

Structure Fatigue Life and Reliability Analysis Based on Fatigue Crack Growth Data

The random formula on fatigue crack growth is deduced by the fatigue crack data and the improved Taguchi method, and the sample estimates of random variables are received by the least square method in the random formula. Fatigue fracture life and reliability of structure are analyzed by the random model. The result show the model is correct and practical, and get the same result with Monte Carlo simulation, moreover its calculation is very simple.

Fatigue Failure and Thermal Spalling Tests to Evaluate Dynamic Fatigue Fracture of MgO–C Bricks

The relationship between compressive stress and loading cycles in MgO–C bricks with various carbon contents was investigated at room temperature and high temperature. The relationship between fatigue failure and thermal spalling was investigated by estimating a characteristic material constant, which represents the degree of sensitivity of crack growth, in terms of fracture mechanics. The relationship between the ratio of compressive stress for compressive strength and loading cycles in MgO–C bricks is not affected by the temperature of the atmosphere. At the same acting stress ratio, fatigue fracture life decreased as the carbon content in the MgO–C bricks decreased. In the thermal spalling test, the number of heating cycles at which small cracks and large cracks were generated decreased as the carbon content decreased. In the fatigue failure test, the ratio of the dynamic elastic modulus to the initial elastic modulus decreased gradually. This ratio also showed a gradual decrease in the thermal spalling test as the number of heating cycles increased. A characteristic material constant was obtained and compared based on the results of a fracture mechanics discussion. The characteristic material constant obtained from the fatigue failure test was substantially the same as that obtained from the thermal spalling test. From this, a relationship in which crack growth behavior leading to fatigue failure is equivalent to that of thermal spalling in MgO–C brick was recognized.

Very early stage rolling contact fatigue crack growth in pearlitic rail steels

Abstract This paper reports on metallurgical characterisation of small cracks in pearlitic steels, using observations from both twin disc tests and rails removed from service, plus methods to determine three-dimensional (3D) aspects of such cracks. Tests to varying fractions of rolling contact fatigue (RCF) life have been assessed, based on twin disc tests where “average 100% RCF life” has been defined by eddy current detection of cracks between 200 and 800 μm in depth. Contact surface features and microstructural alterations have been mapped against near-surface strain and compared with worn rail examinations. Initial crack growth along favourably aligned and strained, prior-austenite (PA) grain boundaries, particularly if the boundary contains pro-eutectoid (PE) ferrite, was found to be restricted or halted at the PA grain limit or where the boundary altered to be less-favourably aligned. For samples with up to 55% RCF life, initial crack propagation length was still less than the average PA grain size. 3D aspects of this early growth and the deformed PA grain shapes were found to be linked to ratcheting flakes and flake cracks on test disc and rail contact surfaces. The results have shown how initial RCF crack propagation is very microstructurally influenced.

In situ characterization of fatigue damage evolution in a cast Al alloy via nonlinear ultrasonic measurements

A new methodology is described for in situ characterization of fatigue damage accumulation using nonlinear ultrasonic measurements via analysis of the feedback signal of a closed-loop ultrasonic fatigue system. In the very high-cycle fatigue regime, ultrasonic nonlinearity increases with initiation and growth of a dominant, life-limiting fatigue crack. Based on the increase in the ultrasonic nonlinearity with fatigue cycling, crack initiation, small fatigue crack growth and fast crack growth regimes have been distinguished during cycling in specimens with different pore sizes tested at various stress amplitudes. The fraction of fatigue life spent in initiation of a life-limiting fatigue crack decreases with increasing stress amplitude. For a constant stress amplitude, the initiation life also decreases with increasing pore size. The present study also demonstrates the applicability of the methodology for fatigue crack growth studies from natural defects located internally or at the surface in smooth specimens.

High-Cycle Fatigue Behavior of Microarc Oxidation Coatings Deposited on a 6061-T6 Al Alloy

The objective of this article is to evaluate the influence of microarc oxidation (MAO) coatings on the high-cycle rotating bending fatigue behavior of the 6061-T6 aluminum alloy. Toward this purpose, the influence of the MAO coating process parameter (current density) and coating thickness on the fatigue life of the 6061-T6 Al alloy has been evaluated in the present study. In addition, the influence of the coating roughness on the fatigue life of the MAO-coated 6061-T6 Al-alloy sample has also been investigated. The results indicate that the high-cycle fatigue life of the 6061-T6 Al alloy is substantially degraded due to the presence of MAO coatings, especially at lower alternating stress values and for thicker coatings. Surface roughness, altered by polishing, does not have any effect on fatigue life. An examination of coated samples interrupted at various fractions of fatigue life leads to the conclusion that the crack propagates from the coating surface to the coating-substrate interface very rapidly and thus fatigue life is largely controlled by the propagation of the crack into the substrate.

Effect of corrosion severity on fatigue evolution in Al–Zn–Mg–Cu

The effect of existing-localized corrosion on fatigue cracking of 7075-T6511 was established using crack surface marker-band analysis and a fracture mechanics model. The substantial reduction of fatigue life due to EXCO solution L–S surface pre-corrosion is nearly independent of exposure time after initial-sharp degradation, scaling with the evolution of pit-cluster size and initial stress intensity range with exposure time. Independent of exposure time, formation of a resolvable fatigue crack (∼10 μm) accounts for a similar-low (∼5%) fraction of total fatigue life at low stress range ( σ max = 150 MPa, R = 0.1). Crack formation occurs at microscopic protrusions into the corroded volume. A corrosion-modified-equivalent initial flaw size (CM-EIFS); predicted with the AFGROW tool using measured initial aspect ratio, initiation cycles, and total fatigue life inputs; accurately represents the corrosion damage effect on fatigue for a range of exposures. The similar deleterious effect of several corroding environments for various-exposed surfaces is described by a lower-bound CM-EIFS with a 300 μm depth and 1200 μm surface length suggesting fatigue is governed by a microscopic pit-based topography. Either an approximate lower-bound, or specific CM-EIFS calibrated by limited measurements of fatigue life for service-environment exposed specimens, can be used to assess the impact of corrosion in a damage tolerant framework. Complexities (e.g., local H embrittlement, 3D pit geometry, topography dependent initiation, and microstructure sensitive small-crack growth) do not compromise the CM-EIFS estimation, but must be better understood for refined modeling.

The Effects of Hot Corrosion Pits on the Fatigue Resistance of a Disk Superalloy

The effects of hot corrosion pits on low-cycle fatigue life and failure modes of the disk superalloy ME3 were investigated. Low-cycle fatigue specimens were subjected to hot corrosion exposures producing pits, then tested at low and high temperatures. Fatigue lives and failure initiation points were compared to those of specimens without corrosion pits. Several tests were interrupted to estimate the fraction of fatigue life that fatigue cracks initiated at pits. Corrosion pits significantly reduced fatigue life by 60 to 98%. Fatigue cracks initiated at a very small fraction of life for high-temperature tests, but initiated at higher fractions in tests at low temperature. Critical pit sizes required to promote fatigue cracking were estimated based on measurements of pits initiating cracks on fracture surfaces.

Fracture mechanism of full lamellar TiAl under high-temperature creep and fatigue conditions based on the in situ observational tests

In this paper, crack growth mechanism of full lamellar TiAl intermetallic compound under high temperature creep and fatigue conditions were investigated using in situ observational high temperature creep-fatigue testing machine. Under high temperature creep condition, inter-granular voids were found to be originated and to coalesce, which results in inter-granular cracking. Furthermore, since the grain size is large (1.5-3.0 mm), inter-granular cracks distribute not only around a notch but also the sites apart from the notch. Since the creep crack mechanism is coalescence of distributed inter-granular cracks, the resistance against creep crack growth becomes higher. Under high temperature fatigue condition, a trans-granular crack was found to initially grow along the direction of lamellar structure, which is different from that under creep condition. After that, the crack incubates and the crack opening displacement increases until it takes some critical value. After that, final unstable crack growth occurs along the perpendicular direction of applied load due to the delamination between matrix and lamellar structure. The characteristic of load frequency for fatigue fracture life is found to be dominated by cyclic dependent mechanism and it is not affected by time dependent mechanism even under low frequency.

Hydrogen-Induced Microstructural Change under Mode II Fatigue for a Tempered Bearing Steel

Slip band-like microstructural change formed under the cyclic shear stress coupled with hydrogen has a crucial importance to clarify the basic mechanism in the presence of hydrogen of Mode II fatigue failure. The authors' previous research works have indicated that the slip band-like microstructural changes are formed by the hydrogen enhanced slip deformation, and this microstructural change occurrence due to the hydrogen is detrimental when compared with a surface inclusion for the fatigue fracture life. The slip band-like microstructural change was observed by the electron backscattering diffraction (EBSD), scanning ion microscopy (SIM) and transmission electron microscopy (TEM). Numerous Mode II fatigue cracks and slip band-like microstructural changes were observed only in the hydrogen-precharged specimens. The EBSD analysis revealed that the blocks in the martensite structure in the vicinity of the slip band-like microstructural change were arranged along the slip band-like microstructural change. The SIM and TEM analyses revealed that the slip band-like microstructural change consists of band structures and that structure differed from base tempered martensite. Therefore, it is presumed that cyclic shear stresses act on the base tempered martensite and hydrogen enhanced slip deformation in the hydrogen-precharged specimens, as a result base tempered martensite structure is deformed, and eventually produces slip band-like microstructural change along the two shear stress directions.

A Practical Use of Simplified Fatigue Life Prediction Method at Stress Concentration of Metals

Fatigue crack initiation life (FCI life) of stress concentration can be predicted according to compare between elastic-plastic strain at there and total strain-fatigue life curve (Δet-Nf curve) of smooth specimen. Elastic-plastic strain is estimated by use of Neuber's rule etc., and Δet-Nf curve is estimated by use of Manson's universal slope method (US method) etc. At this point, cyclic stress-strain curve (CSS curve) is needed in order to estimate the elastic-plastic strain using Neuber's rule etc. However, CSS curve formulated by Ramberg-Osgood relation can be transform from Δet-Nf curve formulated by Coffin-Manson & Basquin relation. Consequently, simplified fatigue life prediction methods can be constructed by only mechanical properties (tensile strength, true fracture ductility, Young modulus, etc.), according to combine these estimation methods (Neuber's rule, US method, etc.).In this report, we compared actual fatigue fracture life (FF life) of some stress concentration specimens with predicted FCI life of them obtained by use of simplified methods, and then evaluated about a practical use of simplified methods. The conclusions are obtained as follows :(1) About the precision of predicted fatigue life, modified universal slope method (MUS method) was better for steels, and Median method was better for aluminum alloys and titanium alloys.(2) For steels, the predicted fatigue life using ESED rule was more effective than using Neuber's rule.(3) In case of using elastic stress concentration factor Kt, the ratio of predicted FCI life Np and actual FF life Nf (Np/Nf) was conspicuously decreasing with increasing of Kt.(4) Taking account of decrease of Np/Nf, simplified method can be employed for preliminary fatigue design.

Fatigue crack formation and growth from localized corrosion in Al–Zn–Mg–Cu

The effect of precorrosion on the fatigue life of aluminum alloy 7075-T6511 was measured, physical characteristics of corrosion topography plus fatigue damage were established by microscopy, and a corrosion modified equivalent initial flaw size (CM-EIFS) was established using fracture mechanics modeling. Fatigue life is reduced by clustered corrosion pits on the L–S surface from laboratory-EXCO exposure. Cracks initiate from pits clustered as a semi-elliptical surface micronotch rather than the deepest pits, consistent with shape-dependent stress intensity. Marker band analysis establishes that the number of cycles to form a crack about a pit cluster can be a significant fraction of total fatigue life. The CM-EIFS, back-calculated from fracture mechanics analysis of measured fatigue life, equals measured initiating-pit cluster size provided that important inputs are provided; such favorable comparison validates this approach to corrosion-fatigue interaction. Calculated CM-EIFS provides a metric to characterize alloy corrosion damage, and can be used to forward-model the effects of stress and loading environment on fatigue life distribution, critical for efficient alloy development. Use in prognosis of the fatigue performance of a service-corroded surface is hindered by uncertain non-destructive characterization of corrosion topography.

Characterization of creep–fatigue in ferritic 9Cr–1Mo–V–Nb steel using ultrasonic velocity

The microstructural evolution of ferritic 9Cr–1Mo–V–Nb steel, subjected to creep–fatigue at 550 °C, was evaluated nondestructively by measuring the ultrasonic velocity. The ultrasonic velocity was strongly depended on the microstructural changes during creep–fatigue. The variation in the ultrasonic velocity with the fatigue life fraction exhibited three regions. In the first region (within 0.2 N f), a significant increase in the velocity was observed, followed by a slight increase between the fatigue life fractions of 0.2 N f and 0.8 N f and a decrease in the final region. The change of the ultrasonic velocity during creep–fatigue was interpreted in relation to the microstructural properties. This study proposes an ultrasonic nondestructive evaluation method of quantifying the level of damage and microstructural change during the creep–fatigue of ferritic 9Cr–1Mo–V–Nb steel.

Fatigue life prediction for high-heat-load components made of GlidCop by elastic-plastic analysis

A procedure to predict the fatigue fracture life of high-heat-load components made of GlidCop has been successfully established. This method is based upon the Manson-Coffin equation with a cumulative linear damage law. This prediction was achieved by consolidating the results of experiments and analyses, and considered the effects of environment and creep. A low-cycle-fatigue test for GlidCop was conducted so that environment-dependent Delta(t)-N(f) diagrams for any temperature could be prepared. A special test piece was designed to concentrate the strain in a central area locally, resulting in the low-cycle-fatigue fracture. The experiments were carried out by repeatedly irradiating a test piece with an electron beam. The results of the experiment confirmed that the observed fatigue life was within a factor of two when compared with the predicted fatigue life, yet located on the safer side.

Microstructural Evolution and Ultrasonic Nondestructive Evaluation During Creep-Fatigue of 9Cr Ferritic Heat-Resisting Steel

The microstructural evolution of ferritic 9Cr-1Mo-V-Nb steel, subjected to creep-fatigue at <TEX>$550^{\circ}C$</TEX>, was evaluated nondestructively by measuring the ultrasonic velocity. The variation of the ultrasonic velocity with the fatigue life fraction exhibited three regions. In the first region (<TEX>$N/N_f$</TEX><0.2), a significant increase in the velocity was observed, followed by a slight increase between the fatigue life fractions of <TEX>$0.2N_f$</TEX> and <TEX>$0.8N_f$</TEX>, and then a decrease in the final region. The change of the ultrasonic velocity during creep-fatigue was interpreted in relation to the microstructural properties. This study proposes an ultrasonic nondestructive evaluation method of quantifying the level of damage and microstructural change during the creep-fatigue of ferritic 9Cr-1Mo-V-Nb steel.

Method to calculate fatigue fracture life of control fissure in perilous rock

Rupture and safety of perilous rock are dominated by control fissure behind perilous rock block. Based on model-I and model-II stress strength factors of control fissure under acting of weight of perilous rock, water pressure in control fissure and earthquake forces, method to calculate critical linking length of control fissure is established. Take water pressure in control fissure as a variable periodic load, and abide by P-M criterion, when control fissure is filled with water, establish the method to calculate fatigue fracture life of control fissure in critical status by contributing value of stress strength factor stemming from water pressure of control fissure in Paris’s fatigue equation. Further, parameters (C and m) of sandstone with quartz and feldspar in the area of the Three Gorges Reservoir of China are obtained by fatigue fracture testing.