Falls Church, VA, United States
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Moon S.,Technical Data Analysis | Phan N.,U.S. Navy | Stull M.,Technical Data Analysis
5th Asian-Australian Rotorcraft Forum, ARF 2016 | Year: 2016

The material S.N. curve shape is essential to determine the rotorcraft dynamic components and structures fatigue life. The equations to S.N. curves are generally valid from 104 to 107 number of cycles to failures and are extended to 1 cycle failure at yield strength or ultimate strength. The yield strength and ultimate strength vary from sample to sample and they shall be associated with failure probability. The endurance limits are assumed above 106 and 108 cycles for steel and aluminum alloys respectively. But, fatigue tests at high cycles reveal that fatigue strength deteriorates beyond 108 cycles, Ref. 1-3. Thus it is necessary to extend S.N. curve beyond 108 cycles. Sendeckyj developed the Wear Out Model (WOM) to analyze composite material fatigue failures, Ref. 4. Aluminum alloys 7075 T-6 coupon test data have been analyzed to evaluate WOM curve parameters. The WOM applied to aluminum alloy data do not show any endurance limit but fatigue strength deteriorates as the number of cycles to failure increases. WOM provides the survival probability of the equivalent static strength data corresponding to fatigue test data. To study the effect of the WOM curve on rotorcraft dynamic component fatigue life, the usage spectrum is developed using 121,334 hours of regime recognized data from 170 H-60 rotorcraft. The Pitch Horn fatigue life is computed using WOM and design S.N curves. The computed fatigue life do not differ significantly with these two approaches. With an increase spectrum severity from 50th to 99.9th fatigue life reduces significantly and the difference in the two approaches converge. © 2016, American Helicopter Society International. All rights reserved.

Sadananda K.,Technical Data Analysis | Vasudevan A.K.,Technical Data Analysis
Transactions of the Indian Institute of Metals | Year: 2016

Component failures occur by crack initiation, growth and fast or overload fracture. The energetics demands that the crack tip driving forces must exceed the material resistance to crack growth. Crack being a high energy defect cannot nucleate spontaneously. However, it can nucleate in the presence of applied and localized internal stresses. Internal stresses can be of mechanical, chemical, electrochemical in origin. We show that crack nucleation and its continuous growth can occur only if the applied and internal stresses, and its gradients exceed some minimum values. Otherwise, cracks do not nucleate or nucleated cracks get arrested resulting in non-propagating cracks. Crack nucleated at sharp notches can get arrested if the notch-internal stresses decrease sharply with distance from the notch tip. Failure diagrams have been developed defining the mechanical equivalent of chemical driving forces by extending Kitagawa–Takahashi diagram developed for simple fatigue. We show the applicability of these concepts to various cases of subcritical crack growth. The available experimental data on the growth kinetics of incipient cracks nucleated at stress concentrations are limited in the literature for validation. Nevertheless, these concepts are based on physical principles and should be valid for all subcritical crack nucleation and growth, and should provide guidelines for design. © 2015, The Indian Institute of Metals - IIM.

Singh A.,Technical Data Analysis | Iyyer N.,Technical Data Analysis | Phan N.,U.S. Navy | Semidey R.,U.S. Navy | And 2 more authors.
Proceedings of the 6th European Workshop - Structural Health Monitoring 2012, EWSHM 2012 | Year: 2012

Over the last two years, TDA developed a framework for the rotorcraft dynamic component structural life tracking. The framework addressed three key important areas in dynamic component life tracking: accurate component tracking, reliable fatigue life assessment using FlUMS and other sensor data, and dissemination of required information to stakeholders for decision making via an enterprise Web application. This paper discusses the key areas of framework and lessons learned for future implementation and adaptation.

Chierichetti M.,Georgia Institute of Technology | McColl C.,Technical Data Analysis | Palmer D.,Technical Data Analysis | Ruzzene M.,Georgia Institute of Technology | Bauchau O.,Georgia Institute of Technology
Proceedings of the Institution of Mechanical Engineers, Part K: Journal of Multi-body Dynamics | Year: 2011

A combined analytical and experimental approach is introduced to estimate the dynamic response of complex systems from a limited number of measurements. The method is based on the concept that modal information is sufficient to extrapolate the complete map of the response from experimental data through the reconstruction of modal loads. The capabilities of the algorithm are first verified via well-controlled lab experiments on a thin-walled aluminium-rotor blade. Numerical results from a comprehensive UH-60 multibody model are then compared with available experimental data. Significant improvements in the accuracy of the predicted results are achieved when simple airloads models are employed as inputs. © Authors 2011.

Sadananda K.,Technical Data Analysis | Vasudevan A.K.,U.S. Navy
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2011

A failure diagram that combines the thresholds for failure of a smooth specimen to that of a fracture mechanics specimen, similar to the modified Kitagawa diagram in fatigue, is presented. For a given material/environment system, the diagram defines conditions under which a crack initiated at the threshold stress in a smooth specimen becomes a propagating crack, by satisfying the threshold stress intensity of a long crack. In analogy with fatigue, it is shown that internal stresses or local stress concentrations are required to provide the necessary mechanical crack tip driving forces, on one hand, and reaction/transportation kinetics to provide the chemical potential gradients, on the other. Together, they help in the initiation and propagation of the cracks. The chemical driving forces can be expressed as equivalent mechanical stresses using the failure diagram. Both internal stresses and their gradients, in conjunction with the chemical driving forces, have to meet the minimum magnitude and the minimum gradients to sustain the growth of a microcrack formed. Otherwise, nonpropagating conditions will prevail or a crack formed will remain dormant. It is shown that the processes underlying the crack nucleation in a smooth specimen and the crack growth of a fracture mechanics specimen are essentially the same. Both require building up of internal stresses by local plasticity. The process involves intermittent crack tip blunting and microcrack nucleation until the crack becomes unstable under the applied stress. © 2010 The Minerals, Metals & Materials Society and ASM International.

Sadananda K.,Technical Data Analysis | Vasudevan A.K.,U.S. Navy
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2011

Many efforts have been made in the past by several researchers to arrive at some unifying principles governing the embrittlement phenomena. An inescapable conclusion reached by all these efforts was that the behavior is very complex. Hence, recognizing the complexity of material/environment behavior, we focus our attention here only in extracting some similarities in the experimental trends to arrive at some generic principles of behavior. Crack nucleation and growth are examined under static load in the presence of internal and external environments. Stress concentration, either pre-existing or in-situ generated, appears to be a requirement for embrittlement. A chemical stress concentration factor is defined for a given material/environment system as the ratio of failure stress with and without the damaging chemical environment. All factors that affect the buildup of the required stress concentration, such as planarity of slip, stacking fault energy, etc., also affect the stress-corrosion behavior. The chemical stress concentration factor is coupled with the mechanical stress concentration factor. In addition, generic features for all systems appear to be (a) an existence of a threshold stress as a function of concentration of the damaging environment and flow properties of the material, and (b) an existence of a limiting threshold as a function of concentration, indicative of a damage saturation for that environment. Kinetics of crack growth also depends on concentration and the mode of crack growth. In general, environment appears to enhance crack tip ductility on one side by the reduction of energy for dislocation nucleation and glide, and to reduce cohesive energy for cleavage, on the other. These two opposing factors are coupled to provide environmentally induced crack nucleation and growth. The relative ratio of these two opposing factors depends on concentration and flow properties, thereby affecting limiting thresholds. The limiting concentration or saturation depends on the relative chemistry of environment and material. A dynamic dislocation model is suggested to account for crack growth. © 2010 The Minerals, Metals & Materials Society and ASM International.

Sadananda K.,Technical Data Analysis | Vasudevan A.K.,U.S. Navy
Fatigue of Materials: Advances and Emergences in Understanding, Held During Materials Science and Technology 2010, MS and T'10 | Year: 2010

Fatigue requires two load parameters and this requirement manifests in crack growth as two thresholds stress intensities, K max.th and ΔK th, and in terms of peak stress, σ max.e and amplitude, Δσ e. The endurance of a smooth specimen and the crack growth threshold of fracture mechanics specimen are combined in the Kitagawa diagram. This diagram is modified considering peak endurance, σ max.e and K max threshold, and by defining a region of internal stress where transition from no crack to short crack to propagating crack can be correctly accounted. This modification helps to combine safe-life and damage control approaches into one unified approach.

Sadananda K.,Technical Data Analysis | Sarkar S.,Technical Data Analysis
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2013

Kitagawa-Takahashi diagram combines the endurance limit of a smooth specimen and the crack propagation threshold in a fracture mechanics specimen into single diagram thus providing the connection between the stress or strain-life and damage tolerance approaches. The diagram is modified by considering that (a) fatigue requires two independent load-parameters for unambiguous description, (b) long crack growth behavior defines the material resistance under constant stress amplitudes along with the associated R-ratio effects, (c) remote applied stresses and localized plasticity-effects can be combined to provide the total mechanical force opposing the material resistance leading to crack initiation, growth and failure describable in the diagram, (d) localized plasticity contributes to internal stresses that either augment or retard the remote stresses, and finally (e) the magnitude and gradient of these internal stresses determine the condition for propagation and/or non-propagation of the incipient cracks that form either at pre-existing stress concentrations or in situ formed stress concentrations due to localized plasticity. Localized plasticity forms the basis for the additional crack tip driving forces in either accelerating or decelerating crack growth kinetics thereby providing conditions for either crack arrest with resulting non-propagating cracks or for continuous uninterrupted crack growth. Internal stresses are generated during fatigue damage in the form of dislocation pile-ups, intrusions and extrusions. The analysis shows that critical magnitude and gradient of the internal stresses are required for an incipient crack to grow continuously, failing which crack arrest can occur. The methodology is based on separating the mechanically introduced crack tip driving forces vs the material resistance; the later can be extracted from long-crack growth data under constant amplitudes. Analysis of incipient short cracks growing under the elastic-plastic notch tip stress fields are analyzed systematically for various elastic stress concentrations, K t, and notch-tip radii, ρ. A general formulation is developed based on the calculations that can be incorporated into the unified life predication model that is being developed. © 2012 The Minerals, Metals & Materials Society and ASM International.

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