bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Long-Time Aging on Fatigue Crack Propagation Behaviors of a Hardworking Ni-Based Superalloy
Authors: Wang Lei , Liu Yang , Song Xiu , Xu Guo Hua , Zhao Guang Pu
Published: March 2014

Fatigue crack propagation behaviors coupled with the microstructure evolution of a hardworking Ni-based superalloy after long-time aging at 1023K were studied, to provide fundamental data for the fatigue life prediction of the superalloy. The results show that the microstructures remarkably change and the fatigue crack propagation resistance decreases with the aging time prolongs. It is found that the precipitation and the growth of topologically close packed (TCP) phases as well as the coarsening of γ' phase and carbides on grain boundaries can significantly affect the fatigue crack growth rate. On one hand, coarsened γ' phase and carbides at grain boundaries block dislocation movements near the crack tip, thus the fatigue crack propagation is hindered in near-threshold region and Paris region. On the other hand, the stress concentration accumulates continually with carbides precipitation increases, so that the grain boundaries become the main fatigue crack propagation rout. As well as, the effect of the TCP phases on the fatigue crack propagation behavior ascribes to the size and the distribution of TCP phases. Very small quantity of TCP phases contribute to pinning dislocation and enlarging fatigue crack propagation absorption energy, but high quantity of TCP phases with short rod shape changed to the needle, which gradually precipitate uniformly within the grain after 1000h besides on grain boundaries in the earlier aging, leads to much higher stress concentration degree. Those discussed above are the most important reasons why the fatigue crack growth rate increases after long-time aging.
Non-Classical Solutions of Critical Rock Behavior
Authors: Guzev Mikhail A.
Published: March 2014

Strain-gradient and non-Euclidean continuum theories are used for the construction of non-classical solutions of continuum models. Both models result in identical structures in terms of their kinematic and force characteristics. The obtained solutions exhibit a critical behavior with respect to the external loading parameter. The proposed analysis allows us to use different theoretical approaches for descriptions of rock critical behavior under different loading conditions.
Fatigue and Damage Tolerance of Aging Airplane Structures
Authors: Nesterenko Boris G. , Nesterenko Grigory I.
Published: March 2014

The paper highlights key questions in ensuring safe operation of aging civil\transport aircraft in Russia. Presented is the analysis of current requirements to fatigue, fail-safe and damage tolerance for transport aircraft structures stated in FARs, Advisory Circulars (UAS) and Russian Airworthiness Regulations\ AR IAC Aviation Regulations. The paper gives the design goals and actual service life values\ service years of aging aircraft fleet and data on full-scaled fatigue tests, together with methods and approaches to ensure safe operation of aircraft structures in case of multiple site damages, corrosion and materials degradation for Russian aging fleet.
Fatigue and Damage Tolerance Substantiation Approach for a Regional Jet
Authors: Fukuoka Toshiyasu , Tsukigase Kaoru , Kumagai Keisuke
Published: March 2014

The overview of the fatigue and damage tolerance substantiation approach for a 70-90 seat class regional aircraft named Mitsubishi Regional Jet, which is now under development by Mitsubishi Aircraft Corporation, is presented. To comply with the new regulatory requirement for prevention from widespread fatigue damage, full-scale airplane fatigue test will be performed with simulating actual airplane structural configuration and typical loading spectra expected in service. In addition to widespread fatigue damage, damage tolerance evaluations assuming manufacturing defect or in-service damage are being performed. Hundreds of structural tests from coupon level to sub-component level are to be used to verify the material properties and analysis methodology.
Evaluation of the Serviceability of the Transverse Joint of a Modular Bridge under a Cyclic Load
Authors: Hong Sung Nam , Kim Sang Hoon , Park Sun Kyu , Choi Jin Woong
Published: March 2014

Modular bridges have structurally weak joints that require a structural safety review. In this study, a static bending test was conducted on a specimen that was prestressed after grouting of its joints with shear keys. The results were compared with those of integral specimens to analyze the structural behavior characteristics of the specimen with grouted joints. Through cyclic loading tests (200,000 and 2,000,000 times), the long-term serviceability was examined in terms of deflections and cracks.
Plastic Slip and Early Stage of Fatigue Damage in Polycrystals: Comparison between Experiments and Crystal Plasticity Finite Elements Simulations
Authors: Signor Loic , Lacoste Emmanuel , Villechaise Patrick , Ghidossi Thomas , Courtin Stephan
Published: March 2014

For conventional materials with solid solution, fatigue damage is often related to microplasticity and is largely sensitive to microstructure at different scales concerning dislocations, grains and textures. The present study focuses on slip bands activity and fatigue crack initiation with special attention on the influence of the size, the morphology and the crystal orientation of grains and their neighbours. The local configurations which favour - or prevent - crack initiation are not completely identified. In this work, the identification and the analysis of several crack initiation sites are performed using Scanning Electron Microscopy and Electron Back-Scattered Diffraction. Crystal plasticity finite elements simulation is employed to evaluate local microplasticity at the scale of the grains. One of the originality of this work is the creation of 3D meshes of polycrystalline aggregates corresponding to zones where fatigue cracks have been observed. 3D data obtained by serial-sectioning are used to reconstruct actual microstructure. The role of the plastic slip activity as a driving force for fatigue crack initiation is discussed according to the comparison between experimental observations and simulations. The approach is applied to 316L type austenitic stainless steels under low-cycle fatigue loading.
Fatigue Behavior and Surface Sensitivity of Board-Shaped Sample of Powder Metallurgy FGH 96
Authors: Liu Xin Ling , Zhao Kai , Liu Chun Jiang
Published: March 2014

Powder metallurgy superalloy (P/M superalloy) was developed following the development of turbine engine disc, and it was usually characterized the mechanical behavior using rod specimen . Up to mow, P/M superalloy has not only been the primary material of turbine disc but also the main material of thin plate components. It is insufficient only to carry out mechanical property of rod-shaped samples. In this paper, the fatigue performance and surface sensitivity of powder metallurgy alloys of two kinds of powder (AA powder and PREP powder) were studied. Tensile fatigue tests were carried out for studying the changing law of tensile fatigue behaviors using plate-shaped and rod-shaped samples with shot peening and without shot peening. The surface sensibility of plate specimens was also analyzed according to the fatigue behaviors and crack initiation sites.
Screw Dislocations Emitted from a Blunt Crack in a Piezoelectric Material
Authors: Song Hao Peng , Gao Cun Fa
Published: March 2014

The problem of a piezoelectric screw dislocation emitted from a blunt crack is dealt with in this paper. For an arbitrary distribution of the residual dislocation, the series-form solutions are derived. The results show that the force acting on the dislocation decreases with the value of the dielectric constant within the crack increasing. And the increase of the dielectric constant within the crack helps dislocation emission effectively.
Measurement and Simulation of Random Vibro-Impact Responses at Brake Guide Pins
Authors: Mroueh Jawad M. , Kang Hong Tae
Published: March 2014

This study firstly conducted measurement of load history profiles at brake guide pins using strain gages subject to random road load history obtained from vehicle durability tests. Then, this study developed a simple mathematical model capable of estimating guide pin load history using the principle of energy conservation for a given road load profile. The model took into account the effect of impact that occurs from a range of acceleration levels and frequency content of the road input. The mathematical model also utilized the measured stiffness of the guide pin specimens and strain versus load relationship to increase the accuracy of the predictions. The estimated loads at a brake guide pin subject to a given random road load history were then computed using the mathematical model and compared to the load history obtained from the measurement. This study also compared cumulative fatigue damages calculated using Smith-Watson-Topper equation with the measured and predicted load profiles at the guide pin. The comparison study showed that the developed simple model is applicable to predict load profiles for fatigue life calculation at brake guide pins.
Influence of Milling and Abrasive Waterjet Cutting on the Fatigue Behaviour of DP600 Steel Sheet
Authors: Duarte Pedro , Infante Virgínia , de Freitas Manuel
Published: March 2014

Theobservation of cracks in mechanical parts shows that cracks often initiate oncutting edges. A lot of effort has been done to developing theories to predictfatigue behaviour of welds. However for the cut-edges available data is veryscarce on the fatigue behaviour. Thispaper presents theresults obtained in fatigue tests on DP600 dual-phase steel sheet specimens, underthree types of cutting edges processes: milling with two cutting parameters andabrasive waterjet cutting. The tests were carried out using smooth specimens (Kt=1)and with a fatigue constant amplitude loading with R=0.1. Surface roughness andresidual stresses induced by these different cutting conditions were measuredand analysed. It was found that the fatigue strength of the abrasive waterjetcutting specimens was smaller than the predicted fatigue strength of themilling specimens and these may be attributed to the surface roughness inducedby the cutting process. Finally failure mechanisms were studied with the scanningelectron microscope (SEM) at fracture surfaces, including the identification ofthe fatigue crack initiation region. It was also observed that fatigue crackinitiation takes place on the cut-edges.
Low Cycle Fatigue Estimation Based on the Critical Distance Stress Theory
Authors: Hattori Toshio , Bin Ab Wahab Muhammad Amiruddin , Yamashita Minoru
Published: March 2014

Generally the critical distance stress theory was applied for the fatigue limit estimation of general structures using fatigue limit of smooth specimen (σw0), and threshold stress intensity factor range (Kth). In this paper we extended this method for the estimation of low cycle fatigue life too. In this method we define the critical distance (rc) on static strength conditions, which is calculated using ultimate tensile strength (σB) and fracture toughness (KIC), in addition to the critical distance on fatigue limit condition (rc). Then the critical distances of any low cycle fatigue conditions can be calculated by interpolation of critical distance on fatigue limit (rc) with critical distance on static strength (rc). By unifying these low cycle fatigue life estimation method with high cycle fatigue limit estimation method we can estimate the full range fatigue life easily. And to confirm the availability of this estimation method we perform the fatigue test for any stress concentration specimens.s
Peculiarities of Elastic Waves Generated by Fatigue Tensile Fractures
Authors: Turuntaev Sergey
Published: March 2014

In the case of fracturing of rocks in subcritical stress state, the stress release due to fracturing could be accompanied by stress increase near the fracture tips, so the rock deformation near the tips could also generate elastic waves (so called "stopping-phase"). Results of experimental modeling of elastic wave generations by fatigue tensile fractures are considered. The model sample consisted of elastic layer made of rubber and fragile layer made of paraffin, the layers were bounded. The elastic layer was stretched and fixed, so the fragile layer was under static tension and started fracturing by tensile fractures. First fractures appeared in visually intact material, later fractures were preceded by a cloud of small "micro" fractures. The fracturing generated elastic waves, which had two components: one corresponded to fracturing of the fragile layer and had characteristic frequency 5-10 kHz; another one had frequency 100-300 Hz, opposite onset and corresponded to tension of elastic layer. It was concluded that tensile fractures in stressed rocks could be considered as a kind of a double-source of elastic waves: one source is the fracture itself, another source is an area of deformations due to stress increase in the vicinity of the fracture tips.
Study of Fatigue Crack Growth in Al-Mg-Si Alloys Using a Predictive Model under Positive and Negative Load Ratios
Authors: Espezua Sandro V.P. , Reis Pereira Baptista Carlos Antonio , da Silva Antunes Ana Márcia Barbosa , Pastoukhov Viktor , Torres Marcelo A.S.
Published: March 2014

The study of fatigue crack growth (FCG) is aimed at residual life estimations in order to apply the damage tolerant criterion. Usual approaches are based on semi-empirical models that consider the stress intensity factor range of fracture mechanics, ΔK, as the governing driving force for crack propagation. An alternative approach is the use of predictive theoretical schemes arising from damage mechanics. Although they havent achieved a reliability level high enough to be used in design, predictive models may be important in some situations like material selection. In the present work, a predictive FCG method based on the cumulative damage of volume elements along the crack path is employed. The development of the work includes considerations about the stress distribution in the cracked body and the stress-life and strain-life relations used in the computational procedure. A previously developed analytical expression for the stress distribution ahead of the crack in a finite width plate, based on the numerical analysis performed by the Finite Element Method, is used in the predictive method. The stress field is determined for both upper and lower limits of cyclic loadings. The fatigue crack growth behavior of three Al-Mg-Si alloys: AA 6005, AA 6351 and AA 6063, tempered and aged for the T6 condition, was analysed for positive and negative R-ratios. In order to check the model results, constant amplitude FCG tests with load ratios ±0,5 were carried out in M(T) specimens. The experimental results, compared to the computational simulations, show that it is possible to obtain predictions of FCG behaviour for both positive and negative load ratios.
Effect of Disbonds on the Fatigue Endurance of Composite Scarf Joints
Authors: Goh J.Y. , Georgiadis S. , Orifici Adrian C. , Wang Chun H.
Published: March 2014

The certification of scarf repairs requires that the repair is capable of handling flight loads in the presence of disbonds. This paper presents a study of the fatigue disbond growth behaviour of scarf joints. By determining the strain energy release rates of a disbond in a scarf joint subjected to a unit load, a predictive model based on linear elastic fracture mechanics is presented, which is shown to correlate well with experimental results. This method offers a promising technique of predicting the fatigue life of composite scarf joints with disbonds.
Delamination Fatigue Properties of Z-Pinned Composites
Authors: Pingkarawat Khomkrit , Mouritz Adrian P.
Published: March 2014

This paper presents an experimental investigation into the mode I interlaminar fatigue resistance of carbon fibre-epoxy laminate reinforced in the through-thickness direction with z-pins. The effects of the volume content, diameter and length of z-pins on the interlaminar toughness, fatigue resistance and crack bridging toughening mechanisms are determined. The delamination growth rate also slowed when the volume content or length of the z-pins was increased or the z-pin diameter was reduced.
Influence of Load Signal Form and Variable Amplitude Loading on the Corrosion Fatigue Behaviour of Aluminium Alloys
Authors: Bauer-Trossmann Kathrin , Yu Liu Hao , Melz Tobias , Oechsner Matthias , Kaufmann Heinz
Published: March 2014

Aluminium alloys are used for a large variety of safety relevant applications for example in the automotive and aviation industries. With the introduction of high strength but possibly less corrosion resistant alloys it is essential to determine if an enhanced sensitisation against corrosion comes into effect under simultaneous mechanical and corrosive loading. Within this work corrosion fatigue tests under constant and variable amplitude loading were carried out on aluminium alloys established for chassis applications such as EN AW-5018 with slightly elevated magnesium content (AlMg3.5Mn), EN AW-6082-T6, EN AW-6110A-T6 and EN AC‑42100-T6 as well as alloys sensitised to corrosion. Sensitisation was obtained by a borderline (17 h at 130 °C) and an excessive (500 h at 130 °C) thermal ageing treatment and elevated copper contents for the forged and cast alloys. Aforementioned alloys and material conditions were assessed concerning the impact of mechanical loading conditions such as load signal type (sinusoidal and square-wave signal), strain rate and load spectra on the damaging process and on corrosion fatigue life. Fatigue tests were complemented by simultaneous determination of electrochemical characteristics as well as the type of corrosion by metallographic investigations.
The Effect of Corrosion Inhibitors on Environmental Fatigue Crack Growth in Al-Zn-Mg-Cu
Authors: Galyon Dorman Sarah E.
Published: March 2014

Corrosion fatigue is an area of concern for the United States Air Force (USAF) and other Department of Defense (DoD) organizations. Often DoD corrosion prevention systems include chromate containing coatings, typically in the form of chromate conversion coatings and polymer primers. Chromate has been used successfully for many years within the DoD to prevent corrosion damage. However the environmental and personnel risks associated with chromate coatings have caused the USAF to pursue non-chromate containing corrosion prevention coatings [1]. To fully quantify chromate replacement coatings, an understanding of the effects that chromate has on corrosion fatigue crack growth rates must be fully characterized. Some researchers have shown that high levels of chromate added to 0.6 M NaCl full immersion corrosion fatigue tests on 7xxx series aluminum alloys slow the fatigue crack growth rate substantially [2,3]. The limitation of that research was that the amount of chromate present in the test solution environment was not connected to expected leach rates of chromate from polymeric coatings and a high solubility salt was used.
The majority of DoD assets are protected from corrosion by polymer coatings loaded with corrosion inhibitors. For these coatings to slow fatigue crack propagation the corrosion inhibitors must become mobile as a consequence of hydration of the polymer coating matrix. Based on this mechanism of corrosion inhibitor release, the examination of atmospheric corrosion fatigue becomes important to help understand how inhibitors work in real world situations with hydrated salt layers rather than only fully immersed solutions.
A Model for Predicting the Stress Concentration of Intergranular Corrosion around a Fastener Hole
Authors: Harrison Timothy J. , Crawford Bruce R. , Clark Graham , Brandt Milan
Published: March 2014

This paper presents a model that predicts the stress field around intergranular corrosion. The stress analysis is conducted in ABAQUS via a Python input script, which is written in Igor Pro. The intergranular corrosion path is described using a Monte-Carlo Markov Chain based on the materials grain size distribution and probability that the corrosion will turn at a grain boundary junction.
The model allows a complete analysis of the stresses resulting from intergranular corrosion around a fastener hole of any size. As fatigue initiation is most likely to occur at the highest stress concentration, this model gives an understanding of which of the features of intergranular corrosion are most critical and can allow for the development of beta solutions for crack growth. This model has been applied to 7075-T651 extruded aluminum alloy from a legacy era aircraft but can be readily applied to any material where the microstructure is known and can be described using a statistical distribution.
Corrosion-Fatigue Crack Growth in Age-Hardened Al Alloys: Examples of Failures and Explanations for Fractographic Observations
Authors: Byrnes Rohan , Goldsmith Noel , Knop Mark , Lynch Stan
Published: March 2014

The characteristics of corrosion-fatigue in age-hardened Al alloys, e.g. brittle striations on cleavage-like facets, are described, with reference to two examples of component failure. Mechanisms of corrosion fatigue (and explanations for fracture-surface features) are then reviewed. New observations of corrosion-fatigue crack growth for 7050-T7451 alloy compact-tension specimens tested in aqueous environments using a constant (intermediate) ΔK value but different cycle frequencies are then described and discussed. These observations provide additional support for a hydrogen-embrittlement process involving adsorption-induced dislocation-emission from crack tips.
An Extended Finite Element Model Coupled with Level Set Method for Stable Pitting Corrosion
Authors: Vagbharathi A.S. , Gopalakrishnan Srinivasan
Published: March 2014

Mass balance between metal and electrolytic solution, separated by a moving interface, in stable pit growth results in a set of governing equations which are solved for concentration field and interface position (pit boundary evolution), which requires only three inputs, namely the solid metal concentration, saturation concentration of the dissolved metal ions and diffusion coefficient. A combined eXtended Finite Element Model (XFEM) and level set method is developed in this paper. The extended finite element model handles the jump discontinuity in the metal concentrations at the interface, by using discontinuous-derivative enrichment formulation for concentration discontinuity at the interface. This eliminates the requirement of using front conforming mesh and re-meshing after each time step as in conventional finite element method. A numerical technique known as level set method tracks the position of the moving interface and updates it over time. Numerical analysis for pitting corrosion of stainless steel 304 is presented. The above proposed method is validated by comparing the numerical results with experimental results, exact solutions and some other approximate solutions.
Managing Fatigue from Corrosion Pits in Aircraft Structures
Authors: Molent Lorrie
Published: March 2014

Despite corrosion prevention or protection schemes/treatments and corrosion prevention and control plans, in-service corrosion does occur and has the potential to impact the structural integrity of aircraft. Whilst the fatigue management of the aircraft is generally well understood as reflected in typical Aircraft Structural Integrity Management Plans (ASIMP), which in some cases contain environmental degradation plans, limited provision beyond find and fix exists for corrosion repair. Thus the repair of corrosion can be a major through life cost driver as well as an aircraft availability degrader. This find and fix policy exists largely because tools are currently considered too immature to accurately assess the structural significance of corrosion when it is detected.
In this paper a process is described which should allow an alternative to the current find (corrosion) and fix philosophy for pitting corrosion. The method is intended to maintain a probability of failure consistent with ASIMP structural certification requirements for fatigue cracks initiating from corrosion pits for a specific period. Unanticipated maintenance costs significantly more than planned maintenance. Thus delaying the repair of pitting corrosion until the next scheduled maintenance, should save considerable resources and improve aircraft availability. The development of analytical tools capable of accurately assessing the effect of corrosion on the durability of a structure would be considered a major advance for the ASIMP.
Three Ways of Sampling and Fatigue Test Results of Steel P92
Authors: Volák Josef , Bunda Zbynek
Published: March 2014

This paper describes the fatigue properties of the steel P92. This material is widely used in the energy industry, especially for pipes and pipe bends of supercritical steam turbines. Steel P92 is alloyed with 2 % of tungsten compared to steel P91. This increases a creep strenght of the material. It is possible to reduce wall thickness of the P92 pipe up to about 20%. Fatigue tests were carried out on standard samples and compared with SFT samples (Small Fatigue Test). Using the device SSam 2 made by company Rolce Royce, it is possible to gently remove a samples from energy component without power plant shutdowns. Consider these correlations, i tis possible to determine mechanical properties of the material from small amount of removed experimental material.
Effect of Environmental Corrosion on Fatigue Crack Growth Morphology: A Detailed Investigation of the Boundary between Fatigue and Corrosion Control Regimes
Authors: Shah Sandeep R. , Pryce Ian L. , St John Todd B. , Greer James M.
Published: March 2014

Legacy 7XXX series aluminum alloys were developed primarily for their high strength with less regard for their fatigue properties, corrosion resistance and fracture toughness. The constituent alloying elements in these materials (used to achieve high strengths) markedly increased their corrosion susceptibility. Consequently, aircraft structures made from these alloys have exhibited fatigue and corrosion damage. In the present work, we have investigated a crack finding in a fuselage skin of AA7XXX series alloy. This investigation revealed the crack propagated by a combination of fatigue and corrosion. Through the use of extensive metallography, mechanical analysis and laboratory experiments, we have separated the contributions to the damage growth due to corrosion and fatigue. We have also confirmed that in-service mixed-mode failures like this, observed in these alloys, can be reproduced reliably in the laboratory. Furthermore, it was observed that the presence of corrosion can actually change the propagation of a fatigue crack from mode I, the preferable orientation for fatigue crack propagation, to mode II, the preferable orientation for corrosion propagation. Even though the mechanical driving force is enough to grow the crack in mode I, the presence of corrosion can change it to mode II by electrochemical degradation of the material. Using electrochemical measurements, we relate the change in failure mode to the frequency of cyclic loading. At slow enough cyclic frequency the electrochemical energy released due to galvanic corrosion degrades the material such that the crack turns and propagates in the orientation which has only one third the mechanical driving force as compared to the original crack propagation path. This is the first time such phenomenon has been successfully replicated in the laboratory and modeled with finite element analysis.
Micro Fatigue Crack Propagation Behavior in a Duplex Stainless Steel Studied Using In Situ SEM/EBSD Method
Authors: Chai Guo Cai , Peng Ru Lin , Johansson Sten
Published: March 2014

Fatigue crack propagation behaviors in a duplex stainless steel have been studied using an in-situ SEM/EBSD fatigue test and a conventional da/dN test. Crack propagation behaviors in grain, effect of Schmid factor, propagation cross the grain or phase boundaries have been discussed. Crack propagation occurs mainly in the grains with a high Schmid factor, but the crack can also propagate in the grains with very small Schmid factor. Crack deflection occurs mainly at the phase boundaries, but crack branching occurs mainly in the grains due to the dislocation slip. In-situ SEM/EBSD fatigue test confirms that crack propagation deflection can lead to a decrease in crack propagation rate. Formation of crack branches can significantly reduce the crack propagation rate, which can cause crack growth retardation in the main crack path in the worst case. The crack branches formed are usually not ideal. They can propagate almost transversely to the main crack direction with a mode II stress intensity factor, SIF, and a rate that is much higher than that of the main crack.
Crack Growth Threshold Criterion Based on Calculation of Dynamic Stress Intensity Factors in the Mixed Mode (Complex Functions Theory Approach)
Authors: Eslami Seyed Hashem
Published: March 2014

In this paper, an analytical method is presented for crack growth threshold criterion based on calculation of dynamic stress intensity factors (DSIF) in the mixed mode. The basis of the method is comparison of DSIF and fracture toughness. The analytical method is grounded on solving two-dimensional wave equations in the frequency domain, using complex functions and mapping technique. Using this method, the J-integrals are calculated and based on relation between the J-integrals and DSIF, these factors are derived. The proposed criterion for crack growth threshold is defined as a comparing calculated DSIF (KI, KII) with fracture toughness (Kc), using complex functions and mapping technique. The mapping technique helps to convert the complicated boundaries geometry into a simple one (unit circle). Finally, since using complex functions theory is an analytical method, the results have a high precision compared with numerical methods.
Influence of the Inclusion Type on the Threshold Value of Failure in the VHCF-Regime of High-Strength Steels
Authors: Grad Patrick , Spriestersbach Daniel , Kerscher Eberhard
Published: March 2014

The very high cycle fatigue behaviour of high-strength steels is mostly affected by cracks which were initiated at non-metallic inclusions inside the fatigue specimens and hence under the surface. By separating the data points in the S-N-diagram by the chemical composition of the inclusion at which a crack is initiated the authors recognized that the lifetime depends strongly on the inclusion type. The authors further deduce that the chemical composition of an inclusion as well as its geometry influences the mechanism by which such an inclusion triggers crack initiation. Thus, titanium nitride and homogeneous calcium oxide inclusions have been observed to show fractures caused by the stress concentration in the inclusion. The broken inclusions then present sharp notches in the steel matrix and cause further damage from there. In contrast, aluminium calcium oxide inclusions decay or detach from the steel matrix during loading on account of a rather low interface stress. This detachment process results in holes in the steel matrix, which act as logical starting points of fatigue cracks. Both processes described above occur at different stress intensity factors and lead to failure before an ultimate number of cycles of 10^9. Furthermore, it was possible to determine threshold values of failure for each inclusion type by stressing run out specimens on a higher stress level. These threshold values are in accordance with those of specimens which failed during one-level stressing. The fatigue tests were performed with 100Cr6 in martensitic and bainitic condition. Tension/compression tests at a load ratio of R = -1 were conducted on an ultrasonic fatigue testing facility. Failures were only initiated at non-metallic inclusions. The fracture surfaces were analyzed by scanning electron microscopy and by energy dispersive X-ray spectroscopy.
Comparison of SGBEM-FEM Alternating Method and XFEM Method for Determining Stress Intensity Factor for 2D Crack Problems
Authors: Sarkar Subhasis , Apetre Nicole , Iyyer Nagaraja , Phan Nam , Goel Kishan , Atluri Satya
Published: March 2014

The two most promising approaches to determine Stress Intensity Factor (SIF) developedover the past decade are the Symmetric Galerkin Boundary Element Method - Finite Element Method(SGBEM-FEM) based alternating method and the Extended Finite Element (XFEM) method. Thepurpose of this paper is to determine the SIFs for a number of 2-D crack problems by the two ap-proaches and measure their relative effectiveness in terms of accuracy, speed and computational re-sources.In the SGBEM-FEM alternating method, a finite element analysis is carried out on the un-crackedbody using the externally applied loading and next a boundary element analysis is performed byreversing the stresses found on the crack location from the finite element analysis, and the residualstresses on the boundary of the finite body are determined. The steps are repeated until convergenceis achieved where the residual stresses on the boundaries and traction on the crack surfaces are closeto zero.In the XFEM method, the mesh is created without considering the topology of the crack configura-tion and the discontinuities are handled by special discontinuity enrichment functions. The enrichmentfunctions increase the degrees of freedom and the regular stiffness matrix is augmented by additionalterms corresponding to the extra degrees of freedom but the increase in computational requirement isoffset by not having the burden of remeshing the finite elements.Both SGBEM-FEM alternating method and XFEM method are used to solve a number of crackproblems and the example cases clearly show the computational efficiency of the SGBEM-FEM al-ternating method over the XFEM method.
Influence of Crack Retardation on Fatigue Crack Propagation in Steels for Railway Axles
Authors: Náhlík Luboš , Pokorný Pavel , Hutař Pavel
Published: March 2014

The railway axles are subjected to cyclic loading, therefore there is a risk of fatigue failure. For reason that possible crack could not be detected by non-destructive testing method an existing crack in the railway axle must be considered. This is conservative approach commonly used in applications where potential fatigue failure has unaccepted consequences. This paper deals with retardation effect caused by overload cycles and compares results obtained by no retardation approach and results obtained by generalized Willenborg model, which takes into account the retardation effects due to plastic zone around the crack tip. Results obtained can contribute to the better understanding of fatigue crack behavior in railway axles.
Determination of Creep Crack Growth Threshold by Experiments under Elevated Temperature with Pre-Stressed Specimens
Authors: Liu Hao , Bao Rui , Fei Bin Jun
Published: March 2014

A novel test method is presented in this paper for determination of the creep crack growth threshold. In this method, a wedge is placed in the notch of a standard compact tension (CT) specimen with a pre-crack, which provides an evaluable stress fields characterized with stress intensity factor (K) at the crack tip. Then the specimen is kept in the test temperature for a certain time. The creep crack growth threshold of the material at a given temperature can be obtained by extrapolating the curves of the crack growth rate vs. K in the double logarithmic coordinates. Since it does not need mechanical testing machine and several specimens can be tested simultaneously, the proposed method is economical in both costs and time. Tests on a nickel-based powder metallurgy superalloy FGH97 are conducted using this method at 600°C, 650°C and 750°C, which indicate its feasibility.
Low Cycle Fatigue Behavior and Mechanism of Newly Developed Advanced Heat Resistant Austenitic Stainless Steels at High Temperature
Authors: Chai Guo Cai
Published: March 2014

Austenitic stainless steel grade UNS S31035 (Sandvik Sanicro® 25) has been developed for the next generation of 700°C A-USC power plant. This paper will mainly focus on the study of low cycle fatigue behavior and damage mechanisms of the material at room temperature, 600C to 700C by using electron back scatter diffraction and electron channeling contrast image techniques. At room temperature, the material shows a hardening and softening behavior as usual. At high temperature, however, it shows only a cyclic hardening behavior. Dynamic strain ageing can be one of the mechanisms. The damage and fatigue crack initiation mechanisms due to cyclic loading at different temperatures and loading conditions have been identified. The interactions between dislocations or slip bands with grain boundary or twin boundary are the main damage mechanism at low temperature or at high temperature with large strain amplitudes. Strain localization due to dislocation slipping is the main mechanism for the damage in grain.
Low Cycle Fatigue and Creep-Fatigue Interaction Behaviour of Reduced Activation Ferritic Martensitic (RAFM) Steels with Varying W and Ta Contents
Authors: Sandhya R. , Shankar Vani , Mariappan K. , Mathew M.D. , Jayakumar Tammana , Kumar Ellappan Rajendra
Published: March 2014

Reduced activation ferritic/martensitic (RAFM) steels are candidate materials for the test blanket modules of ITER. Several degradation mechanisms such as thermal fatigue, low cycle fatigue, creep fatigue interaction, creep, irradiation hardening, swelling and phase instability associated irradiation embrittlement must be understood to estimate the component lifetime. The current work focuses on the effect of tungsten and tantalum on low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behavior of four RAFM steels with varying W and Ta contents. Total strain controlled LCF experiments were performed under various strain amplitudes in the range +0.25% to +1% and temperatures (300 K to 873 K) in air at a constant strain rate of 3×10-3s-1 using a servo hydraulic fatigue testing system. CFI experiments were carried out at total strain amplitude of +0.6% and by applying strain hold of different durations (10 min and 30 min) in peak tension and peak compression. Both LCF and CFI life of the RAFM steels improved with the increase in tungsten and tantalum contents. Based on the amount of softening during continuous cycling, tungsten content was optimized at 1.4 wt. % and the tantalum content at 0.06 wt%. Stress relaxation obtained during creep-fatigue interaction studies showed close relation with the chemical composition of the RAFM steels. Other damaging parameters influencing fatigue life were dynamic strain ageing (DSA) occurring in the intermediate temperature regime and oxidation at elevated temperatures. Keywords: RAFM steel, low cycle fatigue, dynamic strain ageing, creep-fatigue interaction, oxidation
Cyclic Deformation of Rare-Earth Containing Magnesium Alloys
Authors: Mirza F.A. , Chen Dao Lun , Li De Jiang , Zeng Xiao Qin
Published: March 2014

Cyclic deformation characteristics of a rare-earth (RE) element containing extruded Mg-10Gd-3Y-0.5Zr (GW103K) magnesium alloy were evaluated via strain-controlled low-cycle fatigue tests under varying strain amplitudes. Microstructural observations revealed that this alloy consisted of fine equiaxed grains and a large number of RE-containing precipitates. Unlike the RE-free extruded magnesium alloys, this alloy exhibited essentially cyclic stabilization and symmetrical hysteresis loop due to relatively weak crystallographic textures and reduced twinning-detwinning activities. The fatigue life of the present alloy was observed to be longer than that of the RE-free extruded magnesium alloys, which could also be described by the Coffin-Manson law and Basquins equation. Fatigue crack was observed to initiate from the specimen surface and crack propagation was basically characterized by fatigue striations.
Fatigue of Austenitic High Interstitial Steels - The Role of N and C
Authors: Schymura Michael , Fischer Alfons
Published: March 2014

In order to increase the strength and maintain the ductility of austenitic steels high Nitrogen austenitic steels (AHNS) emerged of which Ni was substituted by Mn so that up to 1 w% N could be alloyed and kept in solid solution. Cold working was added to gain strength values up to 3000 MPa. Still the endurance limit did not follow this trend. The low stacking fault energy was thought being the main reason for the solely planar slip but it became clear that other near-field effects might govern this behaviour as well. Thus the density of free electrons could be identified as being one for CrMn-steels being mainly influenced by the sum and the ratio of C and N. In order to investigate this strain-controlled fatigue tests are carried out. This contribution presents the results of strain-controlled fatigue tests and discusses them on the basis of SEM-EBSD and TEM investigations in relation to the microstructural characteristics.
Low-Cycle Fatigue Crack Growth in Ti-6242 at Elevated Temperature
Authors: Brommesson Rebecka , Hörnqvist Magnus , Ekh Magnus
Published: March 2014

During low-cycle fatigue test with smooth bars the number of cycles to initiation is commonly defined from a measured relative drop in aximum load. This criterion cannot be directly related to the actual measure of interest - the crack length. By relating data from controlled crack growth tests under low-cycle fatigue conditions of a high strength Titanium alloy at 350°C and numerical simulation of these tests, it is shown that it is possible to determine the relationship between load drop and crack length, provided that care is taken to consider all relevant aspects of the materials stress-strain response.
Period of Fine Granular Area Formation of Bearing Steel in Very High Cycle Fatigue Regime
Authors: Oguma Noriyasu , Sekisugi Naoya , Kida Katsuyuki , Odake Yasuhiro , Sakai Tatsuo
Published: March 2014

In order to examine the period of fine granular area (FGA) formation of bearing steel in very high cycle fatigue regime, rotating bending fatigue tests were carried out at the stress amplitude 1100 MPa below the fatigue limit. The tests were interrupted at the cumulative damage values ranging from 0.1 to 0.5 with an increment of 0.1 to charge hydrogen to the specimens. After the charge, the rotating bending tests were continuously carried out. The crack origin areas on all fracture surfaces were checked by a scanning electron microscope (SEM), and it was discovered that FGA was not formed in some of them. From a view point of fracture mechanics, the stress intensity factor ranges of FGA areas, ΔKFGA, were calculated by using Murakamis area model. The ΔKFGA values increase with the increase of the cumulative damage values. Furthermore, ΔKFGA values in this study were smaller than 5 MPam which was obtained from usual fatigue testing. Therefore, we conclude that the stable crack growth stage starts when the threshold stress intensity factor range decreases due to hydrogen embrittlement in the middle of formation of FGA.
Experimental Investigations and Damage Calculations of a Load Time History in the Very High Cycle Fatigue
Authors: Müller Thomas , Sander Manuela
Published: March 2014

The main focus of this investigation is to clarify the influence of variable amplitude loadings on subsurface crack initiation and crack growth. Therefore, differently reconstructed load sequences on the basis of a standardized load time history called FELIX are investigated with an R-ratio of -1. The major amount of cycles is situated beneath the fatigue strength. A new damage calculation approach considering inclusion sizes is presented. Thus, the stress amplitude in the S-N curve was normalized with a calculated fatigue limit σw(area), which is defined by Murakami. Afterward, the fatigue life depending on the inclusion size is calculated using a Palmgren/Miner rule. The largest inclusion in the measurement volume was determined using extreme value statistics. Fatigue lives for each investigated load sequence were calculated taking the scatter of inclusion sizes into account.
Identifying the Effect of 475°C Embrittlement on the Cyclic Stress-Strain Response of Duplex Stainless Steel by Means of the Change in the Yield Stress Distribution
Authors: Wackermann Ken , Christ Hans-Jürgen
Published: March 2014

Duplex Stainless Steel is prone to the 475°C embrittlement, which leads to a change of the material properties. In this study the change in the cyclic stress-strain behavior was analyzed in room temperature fatigue tests in a condition without embrittlement and after annealing at 475°C for various times (different degrees of embrittlement), giving different degrees of pre-embrittlement. In the cyclic deformation curves a stronger softening with increasing degree of the embrittlement can be seen after an initial stage of cyclic hardening. In order to characterize the separation of the cyclic plastic deformation between the ferritic and austenitic phase of Duplex Stainless Steel the yield stress distribution function of the statistical treatment of the Masing model was used. For an automatic calculation of these individual yield stress distribution functions suitable mathematical distributions were selected and applied. The development of the distributions with the degree of embrittlement and the number of loading cycles reflect the microstructural changes in both phases.
Influence of Surface Morphology on the Fatigue Behavior of Metastable Austenitic Steel
Authors: Skorupski Robert , Smaga Marek , Eifler Dietmar
Published: March 2014

Using a low temperature turning process with carbon dioxide cooling in the cutting zone a variation of the morphology at the specimen surfaces of the metastable austenitic steel AISI 347 was realized. In LCF and HCF fatigue tests at ambient temperature and 300 °C the influence of the surface morphology on the cyclic deformation behavior and fatigue life was investigated by the measurement of stress-strain hysteresis. An additional magnetic measurement allows the characterization of the phase transformation from paramagnetic austenite in ferromagnetic α´-martensite during the turning processes and during cyclic loading. The surface morphology was studied in detail by SEM and x-ray investigations.
Fatigue Behavior of Precipitation Hardening Alloys in the LCF and VHCF Regime
Authors: Zimmermann Martina , Stoecker Christian , Cremer Martin , Kolyshkin Anton , Christ Hans-Jürgen
Published: March 2014

LCF/HCF strength of precipitation hardening alloys is primarily controlled by its heat treatment condition. However, for a nickel-based superalloy and a wrought aluminium alloy it will be shown, that the VHCF behavior cannot solely be explained by the precipitation morphology. Damage accumulation is dominated by microstructure related slip localization, grain morphology and microstructural flaws. In contrast to the LCF behavior, the prediction of cyclic strength in the VHCF regime requires a detailed analysis of the competing microstructural crack initiating characteristics. Hence, new fatigue life prediction models have to be developed, which consider a statistical analysis of the failure-relevant inhomogeneities. In the case of the two materials studied, VHCF behavior is dominated by isolated and inhomogeneously distributed irreversible slip accompanied by a low dislocation density. The formation of single slip bands in favorably oriented grains in Nimonic 80A results in a decrease of the VHCF strength for the peak-aged condition. The overaged condition shows better VHCF strength due to a more homogeneous distribution of slip bands, as dislocations pile up at the overaged precipitates due to the very low strain amplitudes, while in the LCF regime the Orowan mechanism results in a weaker cyclic strength compared to the peak-aged condition. Crack initiation of the aluminium alloy EN AW-6082 on the one hand depends on the size and distribution of primary intermetallic particles of the Al-Fe-type acting as local stress raiser embedded in a strong matrix in case of the peak-aged condition. In contrast, such stress peaks are less failure-relevant due to the softer solid solution depleted matrix. Hence, the heat treatment alone does not define VHCF behavior.