Education

Research Description

Dr. Hassan is interested in experimental and analytical studies towards understanding and modeling low-cycle fatigue failures of steel structures and welded joints, seismic behavior of steel and concrete structures, sensor development, and constitutive modeling of various steels, high-temperature alloys, concrete and polymeric materials.

Publications

Initial residual stresses in hot-rolled wide-flange shapes: A computational technique and influence on structural performances

Quayyum, S., & Hassan, T. (2017), Journal of Structural Engineering (New York, N.Y.), 143(5).

Influence of notch geometry on the notch vicinity stress and strain responses

Robert, A., Morrison, M. L., & Hassan, T. (2017), (Proceedings of the ASME Pressure Vessels and Piping Conference, 2016, Vol 1A, ).

A novel hybrid heating method for mechanical testing of miniature specimens at elevated temperature

Li, L., Ngaile, G., & Hassan, T. (2017), Journal of Micro and Nano-Manufacturing, 5(2).

Thermomechanical fatigue response and constitutive modeling for Haynes 230

Morrison, M., Ahmed, R., & Hassan, T. (2017), (Proceedings of the ASME Pressure Vessels and piping conference, 2016, vol 5, ).

Improving simulations for low cycle fatigue and ratcheting responses of elbows

Islam, N., & Hassan, T. (2017), (Proceedings of the ASME Pressure Vessels and piping conference, 2016, vol 5, ).

Unified viscoplasticity modeling features needed for simulation of grade 91 creep and fatigue responses

Islam, N., Dewees, D., & Hassan, T. (2017), (Proceedings of the ASME Pressure Vessels and Piping Conference, 2016, Vol 6a, ).

Micromechanical modeling of the ratcheting behavior of 304 stainless steel

Ben Naceur, I., Sai, K., Hassan, T., & Cailletaud, G. (2016), Journal of Engineering Materials and Technology, 138(2).

A novel hybrid heating method for elevated temperature mechanical testing of miniature specimens

Li, L., Ngaile, G., & Hassan, T. (2016), (Proceedings of the ASME 11th International Manufacturing Science and Engineering Conference, 2016, vol 1, ).

Seismic enhancement of welded unreinforced flange-bolted web steel moment connections

Morrison, M. L., Schweizer, D. Q., & Hassan, T. (2016), Journal of Structural Engineering (New York, N.Y.), 142(11).

Resilient welded steel moment connections by enhanced beam buckling resistance

Morrison, M. L., & Hassan, T. (2016), Journal of Constructional Steel Research, 127, 77-91.

View all publications via NC State Libraries

Grants

ASME Code Application of the Compact Heat Exchanger for High Temperature Nuclear Service

US Dept. of Energy (DOE)(10/01/16 - 9/30/19)

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ASME Code Application of the Compact Heat Exchanger for High Temperature Nuclear Service

Sponsor: US Dept. of Energy (DOE)

Start Date: 10/01/16

End Date: 9/30/19

Abstract

The proposed project will implement the recently developed elastic-perfectly plastic (EPP) analysis methodologies in accordance with the ASME Code, Section III, Division 5 for diffusion welded compact heat exchangers (CHXs) in high temperature nuclear service. Burst, and cyclic pressure and thermal experiments on diffusion bonded CHX specimens of SS316L and Alloy 617 will be performed for investigating stress concentrations at sharp channel corners and determining possible failure modes. Hybrid and printed circuit heat exchangers (H2X and PCHE) meet the requirements of space and weight savings, high thermal effectiveness, low pressure drop and high design pressure capability. These attributes improve cost and efficiency of advanced reactors and thereby advances DOE’s goal of carbon-free energy production. Currently, CHXs are covered by design rules in the ASME Code, Section VIII, Division 1 along with Section IX procedures for diffusion welding. But the Section VIII rules are limited to the maximum temperature of 427oC, hence cannot be applied to the intermediate and secondary heat exchangers (IHXs and SHXs) in Sodium Fast Reactors (SFRs) at 550oC and in high temperature gas-cooled reactors (HTGRs) at 950oC). No detailed design strategies for the CHX at the temperature range (550-950oC) have been published. For the IHXs in SFR and HTGR, thermal stresses, fatigue and creep deformation and rupture life limits must be considered for Section III, Division 5, Class A applications, and these are not covered in the Section VIII design methodology. Some IHX and SHX applications are anticipated to operate at significant pressure differentials in addition to cyclic thermal conditions. Hence, determination of thermal stresses in addition to primary stresses is essential for the calculation of creep strains, thermal deformations and peak stresses for fatigue and creep/fatigue usage estimates. It is anticipated that the thermal stresses in the diffusion bonded core will be large, especially during transients. Also high thermal strain concentrations are expected in the core near sidewalls, as well as at header attachment welds. On some scales, these concerns may be best addressed by the recently introduced EPP analysis methodology proposed for the evaluation of primary loads, strain limits and creep-fatigue in Division 5 of the ASME Code. Current Division 5 rules using simplified elastic, and decoupled creep and plasticity analyses have been deemed inappropriate for elevated temperature applications. Hence, EPP methodologies which considers creep-plasticity interactions will allow limit to various stress measures and strain limits. This project will perform a systematic set experiments on stainless 316L and Alloy 617 ASTM coupons and CHX specimens to develop structural design methodology based on EPP analysis in order to provide assessment of the elevated temperature failure modes of two types of CHXs under sustained and cyclic thermal and pressure loads. The research will be performed in consultation with the industry and ASME Code experts such that the outcomes can be used as technical basis for Section III, Division 5 ASME Code Case for CHXs in high temperature nuclear service.

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Advanced Constitutive Modeling of Grade 91 Coupled with Tertiary Creep Behavior and Damage

Babcock & Wilcox Power Generation Group, Inc.(3/01/15 - 2/28/18)

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Advanced Constitutive Modeling of Grade 91 Coupled with Tertiary Creep Behavior and Damage

Sponsor: Babcock & Wilcox Power Generation Group, Inc.

Start Date: 3/01/15

End Date: 2/28/18

Abstract

Grade 91 (modified 9Cr-1Mo, or 9Cr-1Mo-V) martensitic steel is an important material in the domestic and international power generation industries. It is the default material for high temperature steam outlet headers and main steam piping around the world. The material relies on a specific state of microstructure to ensure its excellent high temperature strength, but changes to this microstructure, and a subsequent loss of strength, often occur in operation due to strain cycling and aging. There is a very large class of pressure equipment that sees only a few hundred cycles over its lifetime, although of typically significant strain amplitude. This equipment usually sees creep and plasticity occurring independently (time-independent plastic straining followed by high temperature relaxation, or vice-versa). For this class of problem, creep damage is the focus and fatigue (or plastic strain) is the quantity lumped into the damage properties, margin or calculation procedure. This is the type of equipment and the operation that is of concern in this project. Hence, for feasible design development for components of such pressure equipment, it is essential to develop advanced constitutive models to simulate the tertiary creep, low-cycle fatigue and creep-fatigue responses of Grade 91 steels. As such, meaningful predictions of time-independent strain, short term relaxation and long term creep and failure can be predicted with a single model. Emphasis will be given on the actual damage and failure prediction rather than extremely exact strain responses. Hence, the project is envisioned to be executed in two phases, with the first phase dedicated to the development of a one-way coupled viscoplasticity-Omega model, and the second phase dedicated to the development of a two-way coupled viscoplasticity-Omega model, and subsequently a robust unified viscoplasticty model. All models to be developed will be validated against a set of uniaxial experimental responses to be acquired from published literature.

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MRI: Development of a Miniature, High Temperature, Multiaxial Testing System for Advanced Materials and Engineering Research

National Science Foundation (NSF)(8/15/13 - 7/31/17)

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MRI: Development of a Miniature, High Temperature, Multiaxial Testing System for Advanced Materials and Engineering Research

Sponsor: National Science Foundation (NSF)

Start Date: 8/15/13

End Date: 7/31/17

Abstract

A novel testing system will be designed and developed for mechanical testing of miniature tubular specimens under any combination of axial, torsional, and internal pressure monotonic and cyclic loading in the room to 1000oC temperature. The proposed test system size and orientation will be designed such that it can be set under an optical microscope (OM) and scanning electron microscope (SEM) for in-situ microstructural studies. With the emerging research in materials genome and integrated computational materials engineering, importance and significance in performing materials testing under realistic multiaxial loading and environmental (temperature, chemical and gas) conditions are in urgent need. For enhancing resilience of manufactured components, design of new materials should consider failure mechanisms at critical points (weld toe, stress concentration and hot spot) through testing miniature specimen under multiaxial loading. Such tests need to be performed at high temperatures for understanding premature failures of components in energy and aerospace industries. Manufacturing processes, welding, cold drawing, quenching, surface finishing etc. induce localized material heterogeneity whose influence on failure mechanism can only be studied through testing miniature coupons. Extreme service loading and environmental conditions alter local material properties, which can only be studied through miniature specimens in determining remaining life of critical components. Development of micro-fabrication techniques requires material properties and their evolutions which can only be determined accurately through miniature specimen testing under realistic loading. According to the investigators? knowledge, the proposed testing system currently is not commercially available anywhere in the world. Moreover, even a standard size axial-torsion mechanical testing machine with or without high temperature capability is not available at any universities in North Carolina or its neighboring states. The proposed miniature testing system will be shared by a large group of inter and intra university researchers in performing fundamental research on advanced material design and characterization, micro-forming, constitutive modeling, computational modeling, and failure-life prediction for the energy, aerospace, automobile, electronics, communication, biomedical, sensor and infrastructure industries. The proposed testing system will create research opportunities in material design and component manufacturing, integrated through constitutive and computational modeling research, as well as, meeting the research needs of material characterization usually obtained through standard testing systems. Development of the proposed testing system will performed by detailed thermo-mechanical analysis. Technologies of actuators, sensors, heating and pressurization, digital image correlation, water cooled grips and their controls will be critically evaluated in developing and integrating technologies needed for developing the proposed material testing system. Cooperation with the U.S. commercial partners will facilitate this process. The proposed system will impact design and development of new materials, high performance components, and micro-forming processes. Other important outcomes will be eliminating the long (10-20 years) trial & error methods of designing new materials for enhancing component life, and predicting failure life of components with much less uncertainty than today. Two Graduate and one undergraduate student will be trained on the novel testing system through designing and developing the proposed testing system. Many other students will be trained for use of the testing system in performing their research towards MS and PhD degrees. Undergraduate students through their research projects and K-12 teachers through The Engineering Place and Women in Engineering will perform experiments with the proposed system. Partnership with an US commercial testing system manufacturer will allow quick commercialization of the new system to other r

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GOALI: Continuous-Bending-Under-Tension Studies to Enhance the Formability of Advanced Steels and Aluminum Alloys

National Science Foundation (NSF)(7/01/14 - 5/15/16)

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GOALI: Continuous-Bending-Under-Tension Studies to Enhance the Formability of Advanced Steels and Aluminum Alloys

Sponsor: National Science Foundation (NSF)

Start Date: 7/01/14

End Date: 5/15/16

Abstract

The objectives of this Grant Opportunity for Academic Liaison with Industry (GOALI) research project are to, a) create a fundamental understanding of the mechanism which enhances formability during continuous-bending-under-tension (CBT) and b) to exploit this behavior through innovative manufacturing processes. If less formable materials such as advanced high strength steels (AHSS) and Aluminum alloys are able to be processed more easily using this concept, significant improvements in product performance, environmental friendliness, and productivity of sheet metal components will be achieved. The PIs and senior personnel have significant and relevant expertise in experimental mechanics; material characterization; microstructural and continuum modeling; numerical simulations; and sheet metal forming processes, to assure the success of this research. Background: A ubiquitous experiment to characterize the formability of sheet metal is the standard uniaxial tension test. Past research has shown that if the material is repeatedly bent and unbent during this test (i.e., CBT), the percent elongation at failure increases significantly (e.g., from 22% to 290% for an AISI 1006 steel). This phenomenon has also been empirically observed in industry; the failure strains of a sheet which is passed through a drawbead (i.e., has been bent and unbent three times before entering the die) are considerably higher than those of the original sheet. The phenomenological rationale for this behavior is that the compressive stresses due to bending lower the net axial force on a cross-section, thus delaying necking. However, the exact microstructural cause of this ductility enhancement and the effect of precipitants, alloying elements, grain size, loading paths, specimen geometry, etc., are not known. Because of this lack of fundamental understanding, failure cannot be accurately predicted, restricting applications. Intellectual merit: In order for the industry to take advantage of CBT to achieve formability improvements, fundamental knowledge of the underlying phenomena, accurate models to characterize the effect, and innovative processes to exploit this behavior are required. The tasks proposed here, which will all benefit from the industrial collaboration, are, a) conducting strip- and sheet-CBT tests with real-time variation of process parameters to determine the material behavior during CBT, b) assessing the remaining formability after a specified number of CBT cycles, c) characterizing the material microstructure after CBT processing, d) developing material and numerical models (including experiments to assess the kinematic hardening behavior of the material) to accurately capture the observed CBT effects, e) devising and implementing process innovations to exploit this behavior, and f) disseminating the results to industry through workshops, presentations, and publications. Two specific materials, DP780 and AA6022, will be investigated in this research. Both of these are of interest for various weight-sensitive applications, but have very different microstructures and are expected to behave differently during CBT. In addition to enlarging and enhancing the scientific understanding of the CBT process, the knowledge gained from this research will also benefit other non-conventional forming processes, e.g., incremental sheet forming, and will provide insight into the failure modes during bending of AHSS over small radii. Broader impacts: The CBT process and models developed in this research will benefit society at large by reducing the environmental impact of products such as automobiles and by enabling more aggressive product designs, which will further decrease the weight and cost of components. Graduate and undergraduate students will benefit from the industrial focus of this project and the fact that the results will be directly integrated into the curriculum. In addition, the PIs will build on their past success in recruiting students from underrepresented groups in engineering to participate in the research. Research Experience for

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Monitoring microstructural evolution of Alloy 617 with nonlinear acoustics for remaining useful life prediction: multiaxial creep-fatigue and creep-ratcheting

US Dept. of Energy (DOE)(8/04/10 - 12/31/13)

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Monitoring microstructural evolution of Alloy 617 with nonlinear acoustics for remaining useful life prediction: multiaxial creep-fatigue and creep-ratcheting

Sponsor: US Dept. of Energy (DOE)

Start Date: 8/04/10

End Date: 12/31/13

Abstract

The proposed research will build upon a current investigation to develop a unified constitutive model intended for design-by-analysis of the intermediate heat exchanger (IHX) for a very high temperature reactor (VHTR) design of next generation nuclear plants (NGNPs). Model development requires a set of failure data from complex mechanical experiments to characterize the material behavior. Therefore uniaxial and multiaxial creep-fatigue and creep-ratcheting are being conducted on the nickel-base Alloy 617 at temperatures of 850 and 950 °C. The time dependence of material behavior, and the interaction of time dependent behavior (e.g., creep) with ratcheting, which is an increase in the cyclic mean strain under load control cycling, are major concerns for NGNP design. The proposed research is aimed at characterizing the microstructure mechanisms activated in Alloy 617 by mechanical loading and dwell times at elevated temperature. The acoustic harmonic generation method will be researched for microstructural characterization. It is a nonlinear acoustics method with excellent potential for nondestructive evaluation, and even online continuous monitoring once high temperature sensors become available. It is unique because it has the ability to quantitatively characterize microstructural features well before macroscale defects (e.g., cracks) form. The nonlinear acoustics beta parameter will be correlated to microstructural evolution using a systematic approach to handle the complexity of multiaxial creep-fatigue and creep-ratcheting deformation. Limited mechanical testing is proposed to supplement the current investigation and provide a full spectrum of data for: thermal aging, tensile creep, uniaxial fatigue, uniaxial creep-fatigue, uniaxial creep-ratcheting, multiaxial creep-fatigue, and multiaxial creep-ratcheting. Thorough microscopy, including High Resolution Transmission Electron Microscopy (HR-TEM), Scanning Transmission Electron Microscopy (STEM), Energy Dispersive Spectroscopy (EDS), and Scanning Electron Microscopy (SEM), is required and will be conducted to correlate the beta parameter with individual microstructure mechanisms. The acoustic harmonic generation method is well developed for longitudinal waves in large specimens. Higher harmonic generation in thin plates is becoming better understood and some success has been achieved. We propose to research applying the method to tubular mechanical test specimens, and eventually pipes for nondestructive evaluation. Tubular specimens, samples sectioned from these specimens, and pipes will all act as waveguides, thus we will apply the acoustic harmonic generation method to guided waves in shells. Coupling ultrasonic transducers to the shell sample will be achieved to generate a pure mode and receive low amplitude harmonics. The diverse project team, which includes a minority institution, enables the results of the microstructural evolution characterization to be employed for remaining useful life prediction. The unified constitutive model currently being developed for design purposes will be adapted for life prediction by correlating the beta parameter evolution obtained from nondestructive evaluation (or eventually online monitoring) to the model?s internal state variables. Such a microstructure based constitutive model will enable improving the ASME-NH code for Alloy 617.

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Development of a Constitutive Model for Simulation of Stress-Strain Responses of HA230 under Thermo-Mechanical Cyclic Loading

Honeywell International, Inc.(1/01/10 - 8/15/13)

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Development of a Constitutive Model for Simulation of Stress-Strain Responses of HA230 under Thermo-Mechanical Cyclic Loading

Sponsor: Honeywell International, Inc.

Start Date: 1/01/10

End Date: 8/15/13

Abstract

Honeywell Aerospace is seeking the development of time independent and time dependent plasticity models for analyzing the constitutive response of metallic alloys which are employed in gas turbine engines. The alloys range from low strength materials such as HA-230 and Hast-X to intermediate strength alloys such as Ti-6-4 and Ti-6-2-4-2 to high strength alloys such as U720LI, DP718. This study will develop constitutive model for simulating thermo-mechanical responses of HA-230. The temperatures range will vary from room temperature to as high as 1800F. At elevated temperatures, time dependence of deformation is expected to play a significant part. The phenomena that have to be included in an all encompassing constitutive model are (i) the hysteresis and Bauschinger effect, and strain hardening and softening and (ii) stress relaxation and time dependent creep at peak stresses. Development of advanced constitutive models helps in improving accuracy and fidelity in prediction of stress and strain redistribution in engine components during service. The redistributed stresses and strains are parameters that are used to assess the useful life of a component. Models for predicting component lives are developed from specimen coupon tests, the results of which embody stress redistribution according to at least one of the phenomena mentioned above.

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BEHAVIOR OF HOLLOW STEEL PIPE PILES IN COASTAL REGIONS

US Army - Corps of Engineers(6/01/09 - 12/31/10)

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BEHAVIOR OF HOLLOW STEEL PIPE PILES IN COASTAL REGIONS

Sponsor: US Army - Corps of Engineers

Start Date: 6/01/09

End Date: 12/31/10

Abstract

Past performance of spirally welded pipe piles has been questionable, however, advantages related to quick delivery and cost make their use attractive. As a result, the project described in this proposal has been developed to identify the required information needed by structural engineers to make a proper judgment on the use of these piles in coastal applications subjected to storm surge and wave action. Spirally welded pipe piles are manufactured in a manner that may result in various categories of welded joints (examples of three are shown in Fig. 1), any of which at or near the plastic hinge zone of a pile may impact its performance. The location of the plastic hinge zone in the ground will depend on the stiffness of the soil and pile diameter. The three scenarios of weld categories shown in Fig, 1 represent the simplest, which is spiral only (Fig. 1a) to most complex, which includes a spiral, skelp and splice weld (Fig. 1c) . This project will conduct experiments to determine the load-displacement responses under a load history that would mimic the effects of surge loading as well as to determine failure mechanisms for these three welds located at the plastic hinge of the pile.

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NEESR CR, An Innovative Seismic Performance Enhancement Technique For Steel Building Beam-Column Connections

National Science Foundation (NSF)(1/01/10 - 9/30/13)

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NEESR CR, An Innovative Seismic Performance Enhancement Technique For Steel Building Beam-Column Connections

Sponsor: National Science Foundation (NSF)

Start Date: 1/01/10

End Date: 9/30/13

Abstract

An innovative technique for improving seismic performance of steel beam-column connection with the advantages of the reduced beam section (RBS) connection, but with improved energy dissipation. Through a pilot study the concept is validated numerically at NC State University. This proposed research will conduct a systematic set of beam-column connection experiments for evaluating the novel seismic performance enhancement concept. The novel concept involves heat treating sections of beam flanges by exposing these sections to very high temperatures for certain amount of time before slow air cooling. Such heat treatment reduces the strength of steel in the heat treated areas of the flange. Consequently, under seismic loading plastic hinge develops at the heat treated beam section (HBS). Among the three moment resisting connections prequalified for special and intermediate moment frames by the 2005 AISC 358 standards, the reduced beam section (RBS) is the most popular because of its seismic performance and cost effectiveness. In the RBS connections, ?weakening? of the beam flanges induces plastic hinge away from the welds. In case of the HBS connections, plastic hinge develops at the heat treated beam section where steel strength is reduced by the heat treatment. As the beam flange dimensions, and the elastic and plastic moduli of steel are not altered in the HBS connection, the lateral and torsional buckling resistance of the HBS connection is higher than the RBS connection. Consequently, the HBS connections will dissipate a larger amount of energy with a minimum loss of strength or stiffness than the RBS connections. The proposed research will experimentally validate this novel seismic performance enhancement concept, and develop the design methodology for implementing the concept. The proposed research will conduct seven seismic experiments on the HBS beam-column connections to evaluate their seismic performance. The novel seismic performance enhancement technique will be validated on the welded unreinforced flange-bolted web (WUF-B) connection and hence an additional experiment on WUF-B connection (without heat treatment) will be conducted for reference. Strains, displacements and rotations at various locations will be recorded for investigating both the local and global failure mechanisms of the HBS connections. In the first step, a set of monotonic and cyclic material experiments on heat treated coupons will be conducted. The material coupons will be heat treated by exposing to various peak temperatures in the range of 800-1150oC and two hold times at peak temperatures. This set of material data will be analyzed to quantify the influence of heat treatment on material responses and used to determine the constitutive model parameters of the Chaboche model in ANSYS. A series of finite element analyses of the HBS connection using the Chaboche nonlinear kinematic hardening model will be performed to determine the optimum heat treatment parameters: i) maximum temperature and hold time, and ii) length of the heat treated flange section and, and iii) offset of the heat treated section from the column flange. The seismic performance of the HBS connection with i) both the top and bottom flanges are heat treated (new construction), and ii) only the bottom flange is heat treated (existing building) will be evaluated experimentally. Finally, based on the experimental and finite element analysis data a design methodology for the novel HBS connection will be developed.

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US-Tunisia Cooperation For Next Generation Material Model Development

National Science Foundation (NSF)(9/15/09 - 8/31/10)

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US-Tunisia Cooperation For Next Generation Material Model Development

Sponsor: National Science Foundation (NSF)

Start Date: 9/15/09

End Date: 8/31/10

Abstract

The main objective of this proposed visit is to discuss and finalize research cooperation between Profs. Hassan and Sai?s group on developing material models for simulating a broad set of low-cycle fatigue (LCF) failure responses of stainless steel (SS) 304. Using the proposed travel grant, Hassan from North Carolina State University will visit Sai at Ecole Nationale d?Ingénieurs de Sfax, Tunisia (National Engineering School of Sfax, Tunisia). Their past two years of research cooperation over email communications have evaluated two advanced constitutive models against a set of LCF failure responses of stainless steel (SS) 304. One of the constitutive models is the nonlinear kinematic hardening rule (macroscopic) model, developed by Hassan?s group over the last eight years that demonstrated a reasonable success in simulating the SS304 responses. The limitation identified for this model was that it can only simulate the class of material responses which were used in the parameter determination. The other constitutive model evaluated was the multiscale model developed by Sai?s group for simulating the same set of fatigue responses. This model was deficient in simulating the fatigue responses. Further improvement of these models is warranted to meet the need of a robust material model for designing economical and sustainable power generation and chemical plants, offshore structures, buildings and bridges. Towards that direction, Hassan and Sai?s forty years of material modeling experience will be combined and their modeling ideas will be exchanged for developing the next generation material models. Hassan?s group will provide the material responses for model development and validation.

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Multiaxial creep-fatigue and creep-ratcheting failures of Grade 91 steel for addressing design issues of next generation reactor pressure vessel

US Dept. of Energy (DOE)(10/01/09 - 9/30/14)

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Multiaxial creep-fatigue and creep-ratcheting failures of Grade 91 steel for addressing design issues of next generation reactor pressure vessel

Sponsor: US Dept. of Energy (DOE)

Start Date: 10/01/09

End Date: 9/30/14

Abstract

Feasibility and sustainability of the Gen IV nuclear power plants (NPPs) require operation at high temperature around 900oC for a design life of 60 years. For design code development and structural integrity evaluation of the Gen IV NPP components, such as, reactor pressure vessels (RPVs), heat exchanger and steam generator, one of the major missions of the DOE is to develop mechanistic understanding of creep, creep-fatigue and creep-ratcheting failures of their candidate steels. Towards reaching the goals of the DOE mission, this proposed project will conduct a systematic set of experiments to understand multiaxial, low-cycle, creep-fatigue and creep-ratcheting failures of Grade 91 steel. The experimental loading parameters will represent the Gen IV RPV stress and temperature histories at critical locations. Detailed finite element analysis of the Gen IV RPV will be performed to determine these experimental loading histories. The multiaxial cyclic loading to be prescribed to the Grade 91 steel tubular coupons will involve axial, torsion and internal pressure cycles. Various temperature levels, and stress and strain rates will be prescribed to quantify the influence of ?dynamic strain ageing? on the creep-fatigue and creep-ratcheting failure responses. TEM study will be conducted to understand the dislocation substructure evolution and failure mechanisms of Grade 91 steel at various stages of the fatigue loading. An experimentally validated thermo-mechanical constitutive model will be developed for simulating the elevated temperature, multiaxial failure responses of Grade 91 RPV steel and its weld joints. A unified, non-linear kinematic hardening model involving various internal state variables to simulate rate and temperature dependent material responses will be developed. This advanced constitutive model will be implemented into the widely used finite element software package ANSYS so that realistic simulations of Gen IV RPV can be performed for developing ASME code (ASME-NH) design methodologies and addressing licensing issues.

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