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In-Situ Creep Monitoring Using Directional Potential Drop Sensors

Madhi, Elhoucine
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1288378941

Abstract Details

2010, PhD, University of Cincinnati, Engineering and Applied Science: Aerospace Engineering.
Recent research effort indicates that potential drop (PD) methods can be exploited for nondestructive evaluation (NDE) of creep degradation in low-alloy steels especially during early stages of creep which remains a big challenge for most other NDE methods. For this purpose, a highly directional low-frequency Alternating Current Potential Drop (ACPD) sensor was developed for in-situ monitoring of creep in metals. The sensor relies on a modified ACPD technique that measures both values of resistance in the axial and lateral directions using a square-electrode configuration. The technique essentially monitors the variation in the ratio of the measured axial and lateral resistances, therefore can efficiently separate the mostly isotropic common part of the resistivity variation caused by reversible temperature variations from the mostly anisotropic differential part caused by direct geometrical (size and shape) and indirect material (resistivity) effects of creep. As compared to the more commonly used in-line electrode configuration, the square arrangement used in this sensor has a much higher geometrical sensitivity and is much more directional and thus allows higher sensitivity to detect creep-induced anisotropy and texture. Similarly to ordinary strain gauges, the relative sensitivity of the sensor is defined as a gauge factor that can be approximated as a sum of geometrical and material parts. Initially, subtle material changes produce weak electric anisotropy via reversible and irreversible piezoresistivity due to elastic and plastic strains, respectively. So for an important part of creep life, the sensor is more sensitive to geometrical variations than material effects and acts as a strain gauge with a geometrical gauge factor as high as 4 to 5. At high temperature, much stronger irreversible resistivity changes also occur with time due to preferentially aligned clusters of cavities developing along grain boundaries approximately perpendicular to the applied stress and subsequent cracks forming between these cavities. The ensuing electric anisotropy is detected by the directional sensor. Results from a 2D Finite element analysis indicate however that the resulting anisotropy only starts to have a significant effect on the resistance ratio when the size of the creep-induced microcracks is comparable to the electrode separation (4 mm in this case), i.e., at later stages of creep degradation. A few thermo-mechanical creep tests were undertaken on stainless steel 304 and on 25% Cr - 1% Mo low-alloy steel typically used in the power plant industry. Despite the short time-span of most of these tests, the results show that the sensor is capable of monitoring material degradation from its onset in the elastic regime to ultimate rupture. These tests also show that, although the sensitivity to material effects remain smaller than to the geometrical ones up to the initiation of preferentially oriented microcracks, later the material gauge factor sharply increases and close to rupture can reach a value of more than 10.
Peter Nagy, PhD (Committee Chair)
Alberto Ruiz, PhD (Committee Member)
Vijay Vasudevan, PhD (Committee Member)
Shaaban Abdallah, PhD (Committee Member)
168 p.
Mechanical Engineering
creep monitoring; directional sensor; potential drop; ACPD; material gauge factor; geometrical gauge factor

Recommended Citations

Citations

  • Madhi, E. (2010). In-Situ Creep Monitoring Using Directional Potential Drop Sensors [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1288378941

    APA Style (7th edition)

  • Madhi, Elhoucine. In-Situ Creep Monitoring Using Directional Potential Drop Sensors. 2010. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1288378941.

    MLA Style (8th edition)

  • Madhi, Elhoucine. "In-Situ Creep Monitoring Using Directional Potential Drop Sensors." Doctoral dissertation, University of Cincinnati, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1288378941

    Chicago Manual of Style (17th edition)

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This open access ETD is published by University of Cincinnati and OhioLINK.