Education

Research Description

Dr. Hassan is interested in experimental, constitutive modeling and simulation modeling research towards understanding and modeling failures of steel structures and welded joints, concrete structures, and high temperature components in nuclear and fossil power systems and jet engines.

Publications

Critical Evaluation of a Novel Analysis Technique for Assessment of Printed Circuit Heat Exchangers in High-Temperature Nuclear Service

Shaw, A., Mahajan, H. P., & Hassan, T. (2023), JOURNAL OF PRESSURE VESSEL TECHNOLOGY-TRANSACTIONS OF THE ASME, 145(3). https://doi.org/10.1115/1.4057061

A Practical Analysis Framework for Assessment of Printed Circuit Heat Exchangers in High-Temperature Nuclear Service

Shaw, A., Mahajan, H. P., & Hassan, T. (2022), JOURNAL OF PRESSURE VESSEL TECHNOLOGY-TRANSACTIONS OF THE ASME, 144(4). https://doi.org/10.1115/1.4052697

Development of a unified constitutive model coupled with a continuum damage model for design and evaluation of high-temperature components

Islam, N., Dewees, D. J., & Hassan, T. (2022), INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, 257. https://doi.org/10.1016/j.ijsolstr.2022.111935

Influence of Weld Sequence on the Low-Cycle Fatigue Failure of WUF-B Connections

Hassan, T., & Quayyum, S. (2022), JOURNAL OF STRUCTURAL ENGINEERING, 148(7). https://doi.org/10.1061/(ASCE)ST.1943-541X.0003350

Mechanical and Microstructural Performance Evaluation of Diffusion Bonded Alloy 800H for Very High Temperature Nuclear Service

Mahajan, H. P., Lima, L. M. A., & Hassan, T. (2022), JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY-TRANSACTIONS OF THE ASME, 144(2). https://doi.org/10.1115/1.4052825

Proposed Material Properties, Allowable Stresses, and Design Curves of Diffusion Bonded Alloy 800H for the ASME Code Section III Division 5

Mahajan, H. P., McKillop, S., Keating, R., & Hassan, T. (2022), JOURNAL OF PRESSURE VESSEL TECHNOLOGY-TRANSACTIONS OF THE ASME, 144(6). https://doi.org/10.1115/1.4054073

Material modeling and simulation of continuous-bending-under-tension of AA6022-T4

Barrett, T. J., Takagi, S., Islam, N., Kuwabara, T., Hassan, T., Kinsey, B. L., … Korkolis, Y. P. (2021), JOURNAL OF MATERIALS PROCESSING TECHNOLOGY, 287. https://doi.org/10.1016/j.jmatprotec.2020.116658

Non-contact temperature control and stereo digital image correlation for high-temperature testing of miniature tubular specimens

Rahman, F., Ngaile, G., & Hassan, T. (2021), REVIEW OF SCIENTIFIC INSTRUMENTS, 92(11). https://doi.org/10.1063/5.0055718

Optimized stereo digital image correlation setup for miniature round specimen: framework development and implementation

Rahman, F., Ngaile, G., & Hassan, T. (2021), OPTICS AND LASERS IN ENGINEERING, 144. https://doi.org/10.1016/j.optlaseng.2021.106555

Ratcheting and stress relaxation of SA333 Gr. 6 carbon steel samples under uniaxial multistep strain-controlled condition

Das, P., Khutia, N., Dey, P. P., Arora, P., & Gupta, S. K. (2021), JOURNAL OF THE BRAZILIAN SOCIETY OF MECHANICAL SCIENCES AND ENGINEERING, 43(6). https://doi.org/10.1016/j.mechmat.2015.05.011

View all publications via NC State Libraries

Grants

Advancing the Technical Readiness of FeCrAl alloys and ODS Steels under Extreme Conditions for Fast Reactor Fuel Cladding

US Dept. of Energy (DOE)(10/01/22 - 9/30/25)

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Advancing the Technical Readiness of FeCrAl alloys and ODS Steels under Extreme Conditions for Fast Reactor Fuel Cladding

Sponsor: US Dept. of Energy (DOE)

Start Date: 10/01/22

End Date: 9/30/25

Abstract

The project team will perform several closely-knit tasks to probe the microstructural behavior and to evaluate their effects on the mechanical properties of two candidate cladding alloys – FeCrAl and ODS-14YWT. These alloys have been investigated for reactor use for a number of years and are widely considered to be radiation-tolerant materials that can withstand the extreme environment of a nuclear reactor. As mentioned previously, the behavior at extremely large doses and high temperatures is largely unknown. The project team, therefore, proposes ion irradiation with doses reaching to 400 dpa for temperatures ranging from 300 to 700°C on two alloys FeCrAl and ODS-14YWT. At the University of Tennessee-Knoxville (UTK) – Ion Beam Materials Laboratory, co-PI Weber will conduct the irradiation tests with several types of ions. These experiments will generate a database on microstructure evolution and material degradation, with irradiation temperature and high dose as key variables. Separate gas implantation effects will be probed to investigate the effects of He concentration at high irradiation doses. Thus, the effects of void interactions and void swelling, which are critical to the technical readiness of these alloys, will be evaluated at high dpas and temperatures. Leveraging the ongoing NEUP projects on miniature specimen testing, the project team will then perform in-situ thermo-mechanical experiments (tension, torsion, creep, and creep-fatigue) on the ion-irradiated samples up to a temperature of 700°C. The primary objective is to probe and characterize the microstructural changes in-situ using a scanning electron microscope (SEM). The test rig is currently being installed as a user facility by PI/PD, and co-PIs Hassan and Eapen.

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Development of an In-Situ Testing Laboratory for Research and Education of Very High Temperature Reactor Materials

US Dept. of Energy (DOE)(10/01/20 - 9/30/22)

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Development of an In-Situ Testing Laboratory for Research and Education of Very High Temperature Reactor Materials

Sponsor: US Dept. of Energy (DOE)

Start Date: 10/01/20

End Date: 9/30/22

Abstract

A novel thermo-mechanical fatigue (TMF) testing system, referred by miniature TMF (MTMF) system has been developed at NCSU for in-situ testing of miniature specimens within Scanning Electron Microscopes (SEM). The MTMF is capable of prescribing axial-torsional loading to solid specimen and axial-torsional-internal pressure loading to tubular specimen of 1 mm diameter at elevated temperatures (up to 1000oC) to investigate deformation of microstructure and failure mechanism in real time. Currently, in-situ SEM testing with the MTMF is performed at the Analytical Instrumentation Facility (AIF) at NCSU. This poses a serious restriction to investigate failure mechanisms of very high temperature reactor (VHTRs) materials primarily because with a user facility, such as AIF, we can only perform short-term tests that span over few days. However, fatigue, creep and creep-fatigue tests for VHTR materials may span from few days to several weeks. Hence, existing SEMs on campus are not available for long-term in-situ testing of VHTR materials. Currently, fatigue, creep and creep-fatigue failure mechanisms of new and existing alloys are mostly investigated through ex-situ testing or short duration in-situ uniaxial testing within SEM. Consequently, initiation and propagation of many failure mechanisms, especially interactions between creep and fatigue mechanisms in reducing high temperature component lives remain unknown. Hence, developing a shared in-situ testing laboratory (ISTL) is essential to allow NCSU researchers to perform novel research on nuclear materials addressing issues of fatigue, creep and creep-fatigue failure mechanisms. The proposed ISTL dedicated to performing long-term fatigue, creep and creep-fatigue tests is in critical need to develop design criteria of VHTR materials for ASME Code Sec III Div 5. However, existing facilities at NCSU or any other universities or national labs in the nation do not have a facility dedicated to perform long term tests representing realistic loading conditions of VHTR. Therefore, a suitable SEM compatible with the MTMF system at NCSU is proposed to be acquired to develop an ISTL to address high temperature nuclear materials and ASME Code issues. With the availability of such a ISTL, uniaxial and multiaxial cyclic experiments prescribing realistic thermo-mechanical fatigue (TMF), creep and creep-fatigue loading can be performed on specimens of VHTR materials, such as Alloy 617, 316H, 800H, Grade 91 steel, for addressing the high temperature component design and development issues. Finally, because of the size of commercially available TMF systems, these cannot be used for in-situ SEM testing, which is essential for investigating existing alloys and developing new alloy for VHTRs. Hence, acquisition of a SEM will give the NCSU research community unprecedented capability to perform fundamental research and educate next generation scientists in studying real-time long-term microstructure evolution of nuclear materials under uniaxial and multiaxial loading. In addition, the proposed equipment will allow training undergraduate and graduate students and postdocs in performing material characterization using advanced techniques and provide hands on experiences to students in various undergraduate and graduate courses.

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Novel Miniature Creep Tester for Virgin and Neutron Irradiated Clad Alloys with Benchmarked Multiscale Modeling and Simulations

US Dept. of Energy (DOE)(10/01/19 - 9/30/23)

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Novel Miniature Creep Tester for Virgin and Neutron Irradiated Clad Alloys with Benchmarked Multiscale Modeling and Simulations

Sponsor: US Dept. of Energy (DOE)

Start Date: 10/01/19

End Date: 9/30/23

Abstract

Fast and accurate measurements of creep are needed for qualifying new alloys for current and next generation reactors. For recently developed ferritic alloys such as FeCrAl, the lack of creep/fatigue data is more acute. To address this concern, this project will design and develop a novel miniature creep testing system for performing creep and load relaxation tests at multiple scales inside a scanning electron microscope (SEM). The primary objectives of the proposal are: (i) Collect rapid thermal creep and load relaxation data for two selected ferritic alloys: FeCrAl and oxide dispersed strengthened (ODS-14YWT) alloy at accelerated test conditions using solid, thin-walled and flat specimens under biaxial and uniaxial loading conditions across a temperature range of 500 ºC to 1000 ºC, (ii) Benchmark select data from miniature specimens against data from conventional creep tests with larger samples, (iii) Extract deformation mechanisms using in-situ SEM for virgin and neutron irradiated samples using the miniature tester, which otherwise is onerous with macroscopic creep equipment, and (iv) Perform mesoscale discrete dislocation dynamics (DD) simulations using information derived from SEM, and macroscopic constitutive modeling for predicting long-time behavior.

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Development of a Viscoplastic Constitutive Model for Wrought Alloy Haynes 282 in the temperature range 75F – 1800F for use with ANSYS finite element code for analyzing combustor liners

Honeywell International, Inc.(7/11/18 - 7/10/19)

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Development of a Viscoplastic Constitutive Model for Wrought Alloy Haynes 282 in the temperature range 75F – 1800F for use with ANSYS finite element code for analyzing combustor liners

Sponsor: Honeywell International, Inc.

Start Date: 7/11/18

End Date: 7/10/19

Abstract

Honeywell Aerospace is interested to develop a USERMAT subroutine in ANSYS to analyze the deformation behavior of wrought Haynes 282 (HA-282) combustor liners. The commercially available nonlinear finite element codes such as ABAQUS and ANSYS, which are used for the analysis of such components, do not have the proper constitutive model in their material libraries. Hence, it will be necessary to develop an experimentally validated advanced constitutive model and develop necessary numerical scheme to incorporate the model into the ANSYS USERMAT subroutine in order to perform combustor liner and other high temperature structural analyses. Honeywell Aerospace will develop the needed set of high temperature experimental data through consultation with the NCSU investigators.

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Advancements towards ASME nuclear code case for compact heat exchangers

US Dept. of Energy (DOE)(10/01/17 - 12/31/20)

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Advancements towards ASME nuclear code case for compact heat exchangers

Sponsor: US Dept. of Energy (DOE)

Start Date: 10/01/17

End Date: 12/31/20

Abstract

A group of investigators from several universities and industrial organizations (with University of Wisconsin as the lead) proposes to advance the state of the ASME section III code (nuclear service) for compact heat exchangers (CHXs). This proposed project will advance the technical knowledge of CHXs and lay the foundation necessary for of CHXs to be certified for use in nuclear service. During the course of this project the investigators will advance the understanding of the performance, integrity and lifetime of the CHXs for use in any industrial application making their use more attractive and accessible to industry. This will be achieved by developing qualification and inspection procedures that utilize non-destructive evaluation (NDE) and advanced in service inspection techniques, with insight from an industrial utility leader, EPRI. ASME Code experts on section III from MPR associates will direct the testing and help to develop a series of documents that define the rules and regulations for use of the CHX with input from members of the ASME section III committee. Currently, CHXs are covered by design rules in the ASME Code, Section VIII, Division 1, which is limited to the maximum temperature of 427oC, hence cannot be applied to the intermediate and secondary heat exchangers in Sodium Fast Reactors (SFRs) and High Temperature Gas-Cooled Reactors (HTGRs) with maximum outlet temperatures 550oC and 950oC, respectively. No detailed design strategies for the CHXs in the temperature range 550-950oC have been published. Through this IRP and earlier CHX projects sponsored by the US DOE, the investigators at NC State University (NCSU) will develop high temperature material properties of diffusion welded laminated structures for Alloys 617 and 800H, and Stainless Steel (SS) 316H. A set of isothermal tension, creep, fatigue and creep-fatigue tests on diffusion welded Alloy 800H will be performed and combined with the diffusion welded Alloy 617 and SS316H data from earlier CHX projects to determine the elevated temperature material properties of these ASME Code approved materials. NCSU will perform isothermal burst, and steady and cyclic pressure experiments on diffusion bonded small CHX cores of Alloy 800H, and again will combine with the earlier CHX test results on Alloy 617 and SS316H to explore the influence of sharp channel corners and thermal stresses on the failure modes. NCSU will implement a recently developed advanced unified constitutive model (UCM) in performing full inelastic analyses of CHX to provide insight on the failure responses observed in the CHX experiments to be performed through this proposed IRP. The primary outcomes of the NCSU tasks will include, i) a set of high temperature fatigue, creep, and fatigue-creep properties of three ASME Code approved materials, ii) a set of fatigue, creep and fatigue-creep test data of diffusion welded CHX cores of these materials, iii) experimentally validated UCMs and corresponding model parameters of the ASME Code approved materials, and finally iv) insight on the influence of sharp channel corners and thermal stresses on the failure modes of CHXx. These outcomes will facilitate the development of an elastic perfectly plastic (EPP) analysis based design methodologies for CHXs, background document for incorporating the EPP based structural design methodologies as an ASME Code case in Section III, Division 5, and NDE and service inspection methodologies. The project tasks will be accomplished through integrated efforts of one PhD and one undergraduate students, and two NCSU faculty members. The PhD and undergraduate students will perform the analysis and experimental tasks under the supervision of the faculty members. Through performing the research tasks and interacting with other university researchers and industry experts, graduate and undergraduate students will be trained for the future work force of the nuclear power industry.

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

US Dept. of Energy (DOE)(10/01/16 - 9/30/21)

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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: 9/30/21

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 - 7/31/18)

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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: 7/31/18

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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