Papers by Bahram Farahmand
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Theoretical and Applied Fracture Mechanics, 2009
This paper examines cracking in D6ac and 4340 steel along with Mil Annealed and STOA Ti-6Al-4V an... more This paper examines cracking in D6ac and 4340 steel along with Mil Annealed and STOA Ti-6Al-4V and finds that the data implies that in the Paris Region (Region II) of the crack growth curve there is only a minimal R ratio dependency. Presented is a theoretical basis for explaining this behaviour and suggest alternative ways for characterising crack growth prediction through the use of the Generalised Frost-Dugdale crack growth law. The Fatigue Damage Map method is then used to explain the physics behind this behaviour.
Submicron and …, 1992
The Submicron Structures Laboratory at MIT develops techniques for fabricating surface structures... more The Submicron Structures Laboratory at MIT develops techniques for fabricating surface structures with feature sizes in the range from nanometers to micrometers and uses these structures in a variety of research projects. These projects, described briefly below, fall into four major categories: (1) development of submicron and nanometer fabrication technology; (2) nanometer and quantumeffect electronics; (3) crystalline films on non-latticematching substrates; and (4) periodic structures for x-ray optics, spectroscopy and atomic interferometry.
Fracture critical parts of aerospace structures must have adequate life to withstand the cyclic l... more Fracture critical parts of aerospace structures must have adequate life to withstand the cyclic load environment throughout their service usage. The safe-life analysis of these parts requires having comprehensive fracture mechanics data that are obtainable through the ASTM testing standards. These tests are costly and time consuming and in many cases can have impact on the program when needed data for life analysis are not available to designers. Any method that can be used to minimize testing can be of great help to the program. The proposed virtual testing technique can be extremely helpful to generate fracture toughness and fatigue crack growth data by using only the full stress-strain curve for the material under consideration. Results of fracture data generated by this method have been verified by test results for numerous aerospace alloys [1,2]. Excellent agreement between analysis and test data were obtained for both fracture toughness and fatigue crack growth rate allowable....
Virtual Testing and Its Application in Aerospace Structural Parts.- Tools for Assessing the Damag... more Virtual Testing and Its Application in Aerospace Structural Parts.- Tools for Assessing the Damage Tolerance of Primary Structural Components.- Cohesive Technology Applied to the Modeling and Simulation of Fatigue Failure.- Fatigue Damage Map as a Virtual Tool for Fatigue Damage Tolerance.- Predicting Creep and Creep/Fatigue Crack Initiation and Growth for Virtual Testing and Life Assessment of Components.- Computational Approach Toward Advanced Composite Material Qualification and Structural Certification.- Modeling of Multiscale Fatigue Crack Growth: Nano/Micro and Micro/Macro Transitions.- Multiscale Modeling of Nanocomposite Materials.- Predictive Modeling.- Multiscale Approach to Predicting the Mechanical Behavior of Polymeric Melts.- Prediction of Damage Propagation and Failure of Composite Structures (Without Testing).- Functional Nanostructured Polymer-Metal Interfaces.- Advanced Experimental Techniques for Multiscale Modeling of Materials.
Aluminum-Lithium (Al-Li) alloys offer significant performance benefits for aerospace structural a... more Aluminum-Lithium (Al-Li) alloys offer significant performance benefits for aerospace structural applications due to their higher specific properties compared with conventional Al alloys. For example, the application of Al-Li alloy 2195 to the space shuffle external cryogenic fuel tank resulted in weight savings of over 7,000 lb, enabling successful deployment of International Space Station components. The composition and heat treatment of 2195 were optimized specifically for strength-toughness considerations for an expendable cryogenic tank. Time-dependent properties related to reliability, such as thermal stability, fatigue, and corrosion, will be of significant interest when materials are evaluated for a reusable cryotank structure. Literature surveys have indicated that there is limited thermal exposure data on Al-Li alloys. The effort reported here was designed to establish the effects of thermal exposure on the mechanical properties and microstructure of Al-Li alloys C458, L277...
Designers are always looking for ways to reduce the weight and prolong the fatigue life of struct... more Designers are always looking for ways to reduce the weight and prolong the fatigue life of structural parts. This paper will discuss the benefit of self-healing materials. Selfhealing materials possess the novel property of diagnosing any damage occurring to the structure, and repairing it autonomically like a living system. Various methodologies have been reported in the literature to impart self-healing properties in a synthetic materials system. One of the earliest and most successful methods for self healing deals with the release of liquid monomer in the wake of a propagating crack that eventually retards the crack growth [1]. The physics behind the crack retardation phenomenon in this situation has been variously attributed to the functionalization and eventual adhesion of the crack flanks due to curing polymer, and the crack closure effect due to the solidified polymer wedge behind the crack tip. Moreover, a competition between the time scales associated with healing chemistr...
Materials & Design
Variants of the Frost-Dugdale crack growth law now form the basis of one of the fleet management ... more Variants of the Frost-Dugdale crack growth law now form the basis of one of the fleet management tools in use within the Royal Australian Air Force (RAAF) Directorate General for Technical Airworthiness (DGTA) for the management of cracking in RAAF Combat Aircraft. To help understand the basis for this methodology the present paper examines the crack length versus cycles histories associated with a range of materials, viz: Mil Annealed Ti-6Al-4V, a high strength aerospace steel, several aerospace quality aluminium alloys and several rail wheel steels, subjected to constant amplitude loading. This paper also examines the crack length histories obtained in surface flawed 7050-T7451 and Ti-6Al-4V specimens under operational flight load spectra, in the Boeing 767 and 757 Material Characterisation test program, in a Northrop-Grumman study into crack growth under a representative fighter spectrum, in the Aloha Airlines accident, in several full scale fatigue tests and in-service cracking in rail wheels. In each case it is shown that crack growth conforms to the Generalised Frost-Dugdale crack growth law and that for spectra that consist of a number of repeated block loads the crack length history follows a form similar to that used to manage the RAAF Combat Aircraft fleet.
47th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference<BR> 14th AIAA/ASME/AHS Adaptive Structures Conference<BR> 7th, 2006
45th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics & Materials Conference, 2004
Fracture Mechanics of Metals, Composites, Welds, and Bolted Joints, 2001
Between 1930 and 1950, a series of failures of several large structures, including pressure vesse... more Between 1930 and 1950, a series of failures of several large structures, including pressure vessels, storage tanks, ships, gas pipe lines, bridges, dams and many welded parts alarmed government regulators search for more effective ways to prevent structural failures [1,2]. Most of the observed failures occurred under operating cyclic stress well below the yield value of the material, in a catastrophic manner, with high velocities and little or no plastic deformation. In-depth scientific investigation into the nature of these failures indicated that poor structural design practices (the presence of stress concentrations), insufficient material fracture toughness, residual stresses, lack of inspection, unaccounted variation in load spectrum and presence of corrosive environment, can each contribute to an accelerated crack growth that may result in catastrophic failure and possible loss of life. Structural failure prevention and potential savings can be obtained by focusing attention on a few major areas which have material and structural dependency. Tighter control over material properties (such as static strength and fracture toughness) throughout the manufacturing and assembly phases of the hardware, is a major factor, which contributes to prevention of structural failures
Fracture Mechanics of Metals, Composites, Welds, and Bolted Joints, 2001
As discussed in Chapters 2 and 3, the application of the LEFM approach to structural life assessm... more As discussed in Chapters 2 and 3, the application of the LEFM approach to structural life assessment is limited to a low load environment where the bulk of the structure is elastic and crack tip plastic deformation is highly localized. In many structural parts that are made of low-strength, tough material, however, an appreciable amount of crack tip plastic deformation and stable crack growth (also called stable tearing or simply tearing) can occur prior to instability. Application of the LEFM theory, using the stress intensity factor, K, is not adequate to characterize the crack tip field behavior in the presence of large yielding and extensive stable crack growth. Fracture mechanics concepts other than the LEFM approach are therefore necessary to address structural integrity analysis of components that are ductile. Two fracture mechanics approaches are discussed in this book for the analysis of tough metals used in building aerospace, aircraft, and nuclear structures, where fracture behavior often extends beyond the elastic dominant regime. The first approach is called the Elastic-Plastic Fracture Mechanics (EPFM) theory and it uses the J-Integral concept first proposed by Rice (1968) as a path independent integral for characterizing crack tip stresses and strains [1].
In the field of linear elastic fracture mechanics, the fracture toughness is an important paramet... more In the field of linear elastic fracture mechanics, the fracture toughness is an important parameter for safe-life estimation of structural parts. The plane strain fracture toughness, KIc, is thickness independent and it can be obtained through the ASTM-E399 testing standards. The ASTM standards and procedures require detailed specimen preparation, pre-fatiguing the notch, and fatigue crack growth rate measurements. These tests are expensive and time consuming. Progressive failure analysis is therefore used to estimate material fracture toughness. This approach will combine finite element analysis (FEA) with a General Optimization Analysis (GENOA) software [1] to update material properties as damage occurs. The proposed approach will consider the static properties obtained from the stress-strain curve as an input to assess the crack tip stress and strain fields. The GENOA Progressive Failure Analysis (PFA) is based on material degradation concept throughout the FEA model as the load magnitude increases. The structural stiffness will decrease as the load increases, new properties will be assigned to the FEA elements, and the process will continue until structural stiffness is depleted. The plane strain fracture toughness is calculated by establishing the load versus displacement. The maximum load is used to calculate the fracture toughness through the stress intensity factor equation. To verify the validity of this approach several aluminum alloys were selected and their static properties were obtained through MIL-HDBK5. The mode I load versus displacement for each specimen was extracted from the progressive failure analysis and the maximum load was captured. The estimated critical stress intensity factors were in good agreement with the test data obtained through the NASGRO [2] database. The application of this work on aircraft and aerospace metallic alloys is in progress. Moreover, the extension of this concept to non-metallic materials (composite and foam) is promising. Currently, a preliminary progressive failure analysis task for the Shuttle Columbia external tank foam failure is underway.
Compared with aluminum alloys such as 2219, which is widely used in space vehicle for cryogenic t... more Compared with aluminum alloys such as 2219, which is widely used in space vehicle for cryogenic tanks and unpressurized structures, aluminum-lithium alloys possess attractive combinations of lower density and higher modulus along with comparable mechanical properties. These characteristics have resulted in the successful use of the aluminum-lithium alloy 2195 (Al-1.0 Li-4.0 Cu-0.4 Mg-0.4 Ag-0.12 Zr) for the Space Shuttle External Tank, and the consideration of newer U.S. aluminum-lithium alloys such as L277 and C458 for future space vehicles. These newer alloys generally have lithium content less than 2 wt. % and their composition and processing have been carefully tailored to increase the toughness and reduce the mechanical property anisotropy of the earlier generation alloys such 2090 and 8090. Alloy processing, particularly the aging treatment, has a significant influence on the strength-toughness combinations and their dependence on service environments for aluminum-lithium allo...
Fracture Mechanics of Metals, Composites, Welds, and Bolted Joints, 2001
In brittle fracture it is assumed that failure in an elastic material takes place when the availa... more In brittle fracture it is assumed that failure in an elastic material takes place when the available elastic energy is adequate to overcome the energy necessary to propagate a crack and to create new crack surfaces. At the instant of instability, the stresses at the apex of an elastic crack must have sufficient magnitude, capable of driving the crack to failure. In other words, one must assume the presence of either one single crack of sufficient length, or a group of smaller cracks in the material, that will eventually join together to form one large crack, capable of creating brittle fracture. The fundamental mechanism by which pre-existing flaws are formed, which ultimately grow and become critical upon application of increasing monotonic load, is not well defined. An understanding of elastic fracture behavior in material may be reached through a microscopic approach to fracture mechanics [1], where the formation of microcracks within grains of the material is assumed upon application of tensile load. The formation of microcracks within the grain, having length much smaller than the grain diameter, is a result of microscopic stress risers, which are created due to the presence of pile up dislocations forming cavities. These cavities are suitable locations for crack initiation [2].
Key Engineering Materials, 2013
Evaluating fracture and fatigue life properties of structural components involves tests that are ... more Evaluating fracture and fatigue life properties of structural components involves tests that are costly and time consuming. To estimate total life of engineering parts, high cycle fatigue data (S-N) for the material under study is needed. In many cases the S-N data is not available to the analyst and both the time and budget required for testing prevent engineers to meet the deadline imposed on the program. An analytical combined Progressive Damage and Fracture Mechanics based approach is proposed that estimates the S-N data for components that have stress concentrations. The proposed methodology starts from a full engineering tensile stress-strain curve of the material under study and ends up with the estimation of fracture toughness, fatigue crack growth and fatigue S-N curves.
Fatigue and Fracture Mechanics of High Risk Parts, 1997
ABSTRACT During the past few decades fracture mechanics has become a necessary discipline for the... more ABSTRACT During the past few decades fracture mechanics has become a necessary discipline for the prevention of many structural failures. These are problems resulting primarily from structures containing part through or through cracks. Such cracks can originate in many ways. For example, they may be introduced as cracks or as incipient cracks during manufacture of structural parts; they may grow from defects in the parent metal, from incomplete welds, from shrink cracks or other imperfections in weldments; or they may nucleate and grow in structure under fatigue loading. It is the role of fracture mechanics to determine when these cracks become critical, that is, when they will reach a size at which the crack will grow catastrophically at an operational stress well below the yield strength.
Fatigue and Fracture Mechanics of High Risk Parts, 1997
Almost all metallic materials manifest some plastic deformation in the region at the crack tip be... more Almost all metallic materials manifest some plastic deformation in the region at the crack tip before catastrophic crack propagation. The general principle from which the Griffith theory [1] (see Section 3.1 of Chapter 3) is derived is not limited to materials that obey Hooke's law. The principle applies as well when dissipative mechanisms, such as plastic deformation, are present. Irwin and Orowan [2,3] showed that Griffith's principle can also be applied to materials that manifest ductile behavior, that is: (6.l) where U E is the stored energy, and Up is the energy consumed per unit thickness in plastic straining in the region at the crack tip. For ductile metals, where Up » Us' the expression for surface energy, Us' was omitted from Eq.
Fatigue and Fracture Mechanics of High Risk Parts, 1997
structural failure modes are "discovered" by disasters • Examples? • Failure involving instabilit... more structural failure modes are "discovered" by disasters • Examples? • Failure involving instability can be particularly surprising • Fatigue is a failure mode that can be characterized experimentally without the need for fracture mechanics. • Fracture mechanics provides a means for analytical predictions, but still experiments are paramount
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Papers by Bahram Farahmand