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Über dieses Buch

The papers in this collection cover a diverse range of topics on the topic of fatigue of materials. The editors have grouped the papers into five sections. Sections 1 and 2 contain papers that (i) review the current state of knowledge both related and relevant to the subject of fatigue behavior of materials, and (ii) present new, innovative, and emerging techniques for experimental evaluation of the fatigue behavior. Sections 3 and 4 focus on advanced materials that are used in performance-critical applications in the aerospace and automotive industries, such as the alloys of titanium, nickel, aluminum, and magnesium. Section 5 presents papers relating to other materials of engineering interest, such as iron and steel, polymer, rubber, and composites.



Overview I


Fundamentals of Fatigue Crack Initiation and Propagation: Some Thoughts

The prediction of fatigue properties of structural materials is rightly recognized as one of the most important engineering problem. Therefore a basic understanding of the fundamental nature of fatigue crack initiation and growth in metals has long been a major scientific challenge starting with the first dislocation model of fatigue crack growth of Bilby et al. in 1963. For this purpose understanding the process of emission of dislocations from cracks, and determining precise expressions for the size of the plastic zone size, the crack-tip opening displacement and the energy release rate of the cracks are some of the major technical challenges. In this short paper we comment briefly on some of our important recent results obtained theoretically and by in-situ TEM studies and discuss how they may contribute to the understanding of the phenomena
C. S. Pande

In Situ Three Dimentional (3D) X-Ray Synchrotron Tomography of Corrosion Fatigue in Al7075 Alloy

X-ray synchrotron tomography was used to investigate the fatigue corrosion behavior of Al7075-T651 alloys in a corrosive solution. Single edge- notch specimens machined along the L-T orientation were used for corrosion-fatigue testing, under a load ratio of R = 0.1. The evolution of reaction between the corrosive fluid and the metal was observed, as well as the formation of hydrogen bubbles. The shape of these bubble also changed significantly during the fatigue cycle. The fatigue crack growth rate was found to be significantly higher in the corrosive solution, compared to crack growth in air, for stress intensity levels between 5 and 10 MPa√m.
Sudhanshu S. Singh, Jason J. Williams, X. Xiao, F. De Carlo, N. Chawla

Variable Amplitude Fatigue

Fatigue crack growth behavior of selected aluminum alloys under variable amplitude loading is discussed in this study, based principally on experimental observations. The tests include single overloads tests in different environments, block load tests and tests using an aircraft wing loading spectrum. It is shown that conditions favoring a planar slip behavior lead to very high delays as opposed to conditions leading to multiple slip behavior. The Aluminium Liithium alloy studied here, has the best fatigue crack growth resistance in almost all test conditions studied here as compared to other conventional alloys. Under the spectrum loading studied here, the same alloy exhibits a change in micromechanism leading to a four fould acceleration of growth rates. Acceptable life predictions can be made, by taking into account this crack acceleration effect.
Narayanaswami Ranganathan, Damien Joly, René Leroy

High Frequency Vibration Based Fatigue Testing of Developmental Alloys

Many fatigue test methods have been previously developed to rapidly evaluate fatigue behavior. This increased test speed can come at some expense, since these methods may require non-standard specimen geometry or increased facility and equipment capability. One such method, developed by George et al, involves a base-excited plate specimen driven into a high frequency bending resonant mode. This resonant mode is of sufficient frequency (typically 1200 to 1700 Hertz) to accumulate 107 cycles in a few hours. One of the main limitations of this test method is that fatigue cracking is almost certainly guaranteed to be surface initiated at regions of high stress. This brings into question the validity of the fatigue test results, as compared to more traditional uniaxial, smooth-bar testing, since high stresses are subjecting only a small volume to fatigue damage. This limitation also brings into question the suitability of this method to screen developmental alloys, should their initiation life be governed by subsurface flaws. However, if applicable, the rapid generation of fatigue data using this method would facilitate faster design iterations, identifying more quickly, material and manufacturing process deficiencies. The developmental alloy used in this study was a powder metallurgy boron-modified Ti-6Al-4V, a new alloy currently being considered for gas turbine engine fan blades. Plate specimens were subjected to fully reversed bending fatigue. Results are compared with existing data from commercially available Ti-6Al-4V using both vibration based and more traditional fatigue test methods.
Casey M. Holycross, Raghavan Srinivasan, Tommy J. George, Seshacharyulu Tamirisakandala, Stephan M. Russ

Overview II


Dwell Fatigue Design Criteria

Many engineering components operate under steady state conditions for a period of time after the peak loads had been achieved. The time during which the loads are near constant is considered dwell time. Materials exhibit sensitivity to dwell times differently to the tension and compression loads and continue to challenge engineers designing such systems. New dwell sensitivity maps proposed in this paper showing the dwell sensitivity behavior of materials in low cycle fatigue (LCF) where life is determined by the development of plastic/inelastic strains and long crack growth (LCG) which is also known as high cycle fatigue life of components. While dwell sensitivity in LCF was significant in lower ranges of strains, higher strains and dwell time did not lower fatigue life as highly as it was under lower strains. LCF dwell sensitivity was predicted with the use of strength ratio, dynamic balance between the hardening and softening, and dwell time damage parameter. Each of these characteristic features predicted dwell sensitivity.
Dwell sensitivity in LCG has not been investigated as widely as in LCF. A new parameter proposed which measures the sensitivity in LCG called, normalized crack growth ratio (NCGR). It was a ratio of crack growth rates under continuous fatigue and dwell fatigue at the same mode I stress intensity factor range. The LCG dwell sensitivity was found from an order-of-magnitude to four orders-of-magnitude. Therefore, the dwell sensitivity was more significant in LCG. Dwell fatigue crack growth mechanism map was generated for Inconel 718 charting various zones in which a particular damage mode was observed. Remaining life assessment performed for a disk component with dwell fatigue crack growth models showed negligible life extension potential under the assumed conditions. There is a need for further collaboration and consensus development defining and/or refining the input parameters.
Tarun Goswami

Probabalistic Risk Assessment of a Turbine Disk

Current Federal Aviation Administration (FAA) rotor design certification practices risk assessment using a probabilistic framework focused on only the life-limiting defect location of a component. This method generates conservative approximations of the operational risk. The first section of this article covers a discretization method, which allows for a transition from this relative risk to an absolute risk where the component is discretized into regions called zones. General guidelines were established for the zone-refinement process based on the stress gradient topology in order to reach risk convergence. The second section covers a risk assessment method for predicting the total fatigue life due to fatigue induced damage. The total fatigue life incorporates a dual mechanism approach including the crack initiation life and propagation life while simultaneously determining the associated initial flaw sizes. A microstructure-based model was employed to address uncertainties in material response and relate crack initiation life with crack size, while propagation life was characterized large crack growth laws. The two proposed methods were applied to a representative Inconel 718 turbine disk. The zone-based method reduces the conservative approaches, while showing effects of feature-based inspection on the risk assessment. In the fatigue damage assessment, the predicted initial crack distribution was found to be the most sensitive probabilistic parameter and can be used to establish an enhanced inspection planning.
Jace A. Carter, Michael Thomas, Tarun Goswami, Ted Fecke

Simulating Fatigue Cracks in Healthy Beam Models for Improved Identification

There is need for an automated Structural Health Monitoring (SHM) system capable of fatigue crack detection in bladed disks as current methods are slow, costly and imperfect. Prerequisite for such a system is a fast method for producing the necessary data libraries. In an effort to develop such a method for simulating nonlinear structural response, fatigue cracks in beams have been modeled by modifying the inputs to the structure rather than the structure model itself to produce a closed-form solution for the total response. Although the time savings are enormous, and the method has proven capable of correctly identifying fatigue cracks over an effective region in data generated by a more traditional bilinear model, additional refinement is needed. The method for calibrating the signature profiles used to identify fatigue cracks between methods is revisited and successfully improved. Existing signature profiles for the bilinear model are reproduced with higher resolution, and new features are observed. Different boundary conditions are evaluated with the new method and compared to published results. Although similar, there are still discrepancies that remain and will need to be investigated. Overall, the proposed method for modeling and identifying fatigue cracks in beams has been improved, but will require validation against physical experiments before being used on more complicated structures such as bladed disks.
Phillip E. Cooley, Joseph C. Slater, Oleg V. Shiryayev

Advanced Materials I


Stress-Corrosion Cracking and Fatigue Crack Growth Behavior of Ti-6Al-4V Plates Consolidated from Low Cost Powders

Titanium is highly desirable for a wide range of applications because of its combination of high strength, low density and outstanding corrosion characteristics. However, the cost of titanium, produced by conventional technology, is high compared to steel and aluminum, which is a result of high extraction and processing costs. New approaches are being investigated maintaining required quality while lowering the cost of finished products. Ti alloy powder, Ti-6Al-4V, manufactured by a low cost hydride-process and consolidated into flat products (sheet, plate), were studied. The results of the study were compared with the properties obtained from plates of Armstrong Titanium consolidated powder. To remove the prior history of consolidation, the plates are beta annealed and the test results are compared with “as received” condition. The mechanism of the fatigue crack growth rate difference, fracture toughness, and stress-corrosion cracking resistance in terms of the respective Ti-6Al-4V microstructure differences will be discussed.
M. Ashraf Imam, Peter S. Pao, Robert A. Bayles

Advanced Materials II


The High Cycle Fatigue, Damage Initiation, Damage Propagation and Final Fracture Behavior of Aluminum Alloy 2024

In this technical paper the results of a study aimed at understanding the high cycle fatigue properties and fracture characteristics of aluminum alloy 2024 is presented and discussed. Specimens of the alloy in the T-8 temper were cyclically deformed over a range of stress amplitudes at ambient temperature and at a stress ratio of 0.1. Specimens of the alloy were taken from the longitudinal orientation of the as-provided plate and cyclically deformed. The influence of alloy temper (T8 versus T3) on cyclic fatigue life under stress amplitude control is briefly discussed. At the ambient test temperature, the macroscopic fracture mode was essentially identical with specific reference to the magnitude of cyclic stress amplitude. The microscopic mechanisms governing cyclic deformation, fatigue life and final fracture behavior are discussed in light of the mutually interactive influences of magnitude of applied stress, intrinsic microstructural effects, deformation characteristics of the alloy microstructure and macroscopic fracture mode.
T. S. Srivatsan, Satish Vasudevan, K. Manigandan

Fractographic Observations on the Mechanism of Fatigue Crack Growth in Aluminium Alloys

Special load histories are adopted to obtain information about the behavior of the moving crack tip during the increasing and decreasing part of a load cycle. It is associated with the crack opening and closure of the crack tip. Secondly, modern SEM techniques are applied for observations on the morphology of the fractures surfaces of a fatigue crack. Information about the cross section profiles of striations are obtained. Corresponding locations of the upper and the lower fracture surface are also explored in view of the crack extension mechanism. Most experiments are carried out on sheet specimens of aluminum alloys 2024-T3, but 7050-T7451 specimens are also tested in view of a different ductility of the two alloys.
R. C. Alderliesten, J. Schijve, M. Krkoska

Fatigue Predictions of Various Joints of Magnesium Alloys

In this project, a front shock tower of a passenger vehicle is developed with various magnesium alloys and joining methods. To predict the fatigue life of the joints in the structure, fatigue tests of various joint specimens including friction stir linear welding, self-piecing rivet joint with and without adhesive, and friction stir spot welding were conducted. The magnesium alloys used for the specimens are AM60 (cast), AM30 (extrusion), and AZ31 (sheet). Various finite element modeling techniques were attempted for simulating the various joints. Fatigue life prediction method for the joints was performed using the stress-life curve approach. The finite element modeling technique and the fatigue prediction method will be verified with fatigue tests of the actual front shock tower structure subjected to variable amplitude loadings in near future.
H. Kang, K. Kari, A. Getti, A. K. Khosrovaneh, X. Su, L. Zhang, Y.-L. Lee

Fatigue Behavior of AM60B Subjected to Variable Amplitude Loading

Magnesium alloys are considered as an alternative material to reduce vehicle weight due to their weight which are 33% lighter than aluminum alloys. There has been a significant expansion in the applications of magnesium alloys in automotives components in an effort to improve fuel efficiency through vehicle mass reduction. In this project, a simple front shock tower of passenger vehicle is constructed with various magnesium alloys. To predict the fatigue behavior of the structure, fatigue properties of the magnesium alloy (AM60B) were determined from strain controlled fatigue tests. Notched specimens were also tested with three different variable amplitude loading profiles obtained from the shock tower of the similar size of vehicle. The test results were compared with various fatigue prediction results. The effect of mean stress and fatigue prediction method were discussed.
H. Kang, K. Kari, A. K. Khosrovaneh, R. Nayaki, X. Su, L. Zhang, Y.-L Lee

Other Materials


Rubber fatigue — the intrinsic Intricacies

This paper brings out the salient fractographic features that can be found on the fracture surface of some selected rubbers. It is shown that the presence of a specific feature — called the tongues can be correlated to the effect of load ratio and hence the fatigue lives in a synthetic rubber.
Jean-Louis Poisson, Florian Lacroix, Stéphane Méo, Gaelle Berton, Narayanaswami Ranganathan

Mechanistic Approach towards Fatigue Initiation and Damage Propagation in Fiber Metal Laminates & Hybrid Materials

This paper provides an overview of the research performed on fatigue in fiber metal laminates, emphasizing the knowledge and understanding gained with respect to fatigue phenomena in hybrid laminated materials, illustrating major observations, and presenting the generalized mechanistic approach towards both the initiation and the propagation phase of fatigue damage evolution.
With the mechanistic approach it is explained how correct similitude principles for both initiation and damage growth will link the constituent properties in formulating the fatigue behavior of these hybrid material systems. In relation to that, it will be explained how fatigue can be characterized at constituent level to provide input to prediction methodologies
R. C. Alderliesten

Influence of Austenite Stability on Steel Low Cycle Fatigue Response

Austenitic steels were subjected to tensile and total strain controlled, fully reversed axial low cycle fatigue (LCF) testing to determine the influence of stacking fault energy on austenite stability, or resistance to strain induced martensitic transformation during tensile and fatigue deformation. Expected differences in stacking fault energy were achieved by modifying alloys with different amounts of silicon and aluminum. Al alloying was found to promote martensite formation during both tensile and LCF loading, while Si was found to stabilize austenite. Martensite formation increases tensile work hardening rates, though Si additions also increase the work hardening rate without martensite transformation. Similarly, secondary cyclic strain hardening during LCF is attributed to strain induced martensite formation, but Si alloying resulted in less secondary cyclic strain hardening. The amount of secondary cyclic hardening scales linearly with martensite fraction and depends only on the martensite fraction achieved and not on the martensite (i.e. parent austenite) chemistry. Martensite formation was detrimental to LCF lives at all strain amplitudes tested, although the total amount of martensitic transformation during LCF did not always monotonically increase with strain amplitude nor correlate to the amount of tensile transformation.
G. R. Lehnhoff, K. O. Findley

The High Cycle Fatigue and Final Fracture Behavior of Alloy Steel 9310 for Use in Performance-Sensitive Applications

In this technical paper the results of a recent study aimed at understanding the high cycle fatigue properties and fracture behavior of an alloy steel, a viable candidate for use in performance-critical applications, is presented and briefly discussed. The alloy steel investigated was 9310. The material was evaluated in the as-forged (wrought) and normalized condition. Test specimens of this alloy steel were precision machined and conformed to specifications delineated in the ASTM E8. The as-machined and subsequently polished test samples were cyclically deformed over a range of maximum stress, in the room temperature (T = 25 C), laboratory air environment (Relative Humidity 55 pct), at the load ratios of 0.1 and -1.0. The number of cycles-to-failure was recorded. The specific significance of load ratio on cyclic fatigue life of alloy steel 9310 is presented and differences discussed based on a synergism of the nature of loading, intrinsic microstructural effects, and macroscopic fracture behavior. The fatigue fracture surfaces were examined in a scanning electron microscope to determine the macroscopic fracture mode and to concurrently characterize the intrinsic features on the fatigue fracture surfaces and thus establish the microscopic mechanisms governing failure. The conjoint influence of microstructure, maximum stress and load ratio on cyclic fatigue life and fracture behavior is highlighted.
K. Manigandan, T. S. Srivatsan, T. Quick, A. M. Freborg

Ultrasonic Corrosion Fatigue Behavior of High Strength Austenitic Stainless Steels

Ultrasonic corrosion fatigue tests were conducted for high strength austenitic stainless steels such as YUS270 and SUS304N2 in 3%NaCl aqueous solution. The reduction of giga-cycle corrosion fatigue strength of YUS270 and SUS304N2 was not observed at all, while the reduction of corrosion fatigue life was observed at higher stress amplitude. Corrosion pit was observed on corrosion fatigue crack initiation area. Striation was predominantly observed on crack propagation area in air and in 3% NaCl aqueous solution. The reduction of corrosion fatigue strength of high strength austenitic stainless steels such as YUS270 and SUS304N2 is due to the corrosion pit formation at corrosion fatigue crack initiation area. It can be concluded that the higher the ultimate tensile strength of austenitic stainless steels the higher the giga-cycle corrosion fatigue strength in 3%NaCl aqueous solution is.
R. Ebara, Y. Yamaguchi, D. Kanei, Y. Yamamoto

Influence of Microstructural Features on the Propagation of Microstructurally Short Fatigue Cracks in Structural Steels

Cyclically loaded structural steel components are usually designed to endure macroscopic stress amplitudes close to the material’s endurance strength where microcracks initiate due to microstructural inhomogeneities and exhibit strong interactions with the various microstructural features in their neighborhood upon propagating. The current study presents a microstructural model with a capability to quantitatively describe the influence of microstructural features on the growth of cyclic cracks in the decisive, very early fatigue behavior stage. The FE model is based on the crystal plasticity theory and accounts for relative grain orientations. Both the extended finite element method (XFEM) and a coupled damage mechanics approach are used to describe crack opening behavior. The model is implemented to simulate real microcracking events produced in interrupted cyclic multiple-step tests under metallographic observation with temperature change measurements. Furthermore, the model is implemented on virtually created microstructures with altered grain sizes and orientations based on statistical EBSD analysis.
M. Sharaf, J. Lian, N. Vajragupta, S. Münstermann, W. Bleck, B. Schmaling, A. Ma, A. Hartmaier


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