Mr. Nimun Nak Khun Profile

Nimun Nak Khun

at Univ. of Massachusetts Lowell

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Proceedings Article | 18 April 2022 Paper

Investigating the effect of mass redistribution on damping of building models using a shake table and a laser Doppler vibrometer

Nimun Nak Khun, Tiana Robinson, Tzuyang Yu

Proceedings Volume 12047, 120470O (2022) https://doi.org/10.1117/12.2612706

KEYWORDS: Laser Doppler velocimetry, Motion models, Structural health monitoring, Doppler effect, Data modeling, Signal to noise ratio, Inspection

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Remote sensing is one of the promising technologies in nondestructive evaluation (NDE) and structural health monitoring (SHM) for civil infrastructures, in view of its convenience and flexibility in field data collection. Among existing remote sensing techniques, laser doppler vibrometer (LDV) is an ideal tool for periodic inspection of building-type and bridge-type structures. In this paper, application of LDV on extracting modal frequencies and damping of a building model on a shake table is investigated to study the effects of mass distribution on characteristic damping of the building model. A commercially available LDV (Short Wave InfraRed (SWIR) laser, range = 1.7m 300m, OptoMET) was used on a building model excited by ground motion generated by a shake table in the frequency range of 1Hz and 10Hz. Sinusoidal ground motion was adapted to produce non-trivial initial conditions (displacement and velocity) in the vibration of the building model. Free vibration response of the building model with different configurations (e.g., location of introduced mass) was measured by the LDV. A metal block was used as introduced mass and a target reflector for the LDV. Effects of excitation frequency on the measurement accuracy of modal frequencies and characteristic damping were also studied. From our experimental result, it is found that the measurement accuracy of LDV is not vulnerable to the change in excitation frequency. In-plane displacement appears to be more reliable than out-of-plane displacement for extracting modal frequencies and characteristic damping of the building model.

Proceedings Article | 18 April 2022 Paper

Application of dual-frequency GPR for subsurface void detection in culverts

Koosha Raisi, Nimun Nak Khun, Tzuyang Yu

Proceedings Volume 12047, 120470Q (2022) https://doi.org/10.1117/12.2613083

KEYWORDS: Dielectrics, Radar, Reflection, Metals, Inspection, Roads, Image processing, Signal detection

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Deterioration of underground structures such as pipelines and culverts are very difficult to be detected in the early stage. Yet, their failures can cause significant damages to the above roadway and adjacent superstructures. However, detecting the development of deterioration in underground culverts is a challenging task. For example, development of an underground void or sink hole can cause stress redistribution in the vicinity of a culvert and potentially damage the culvert. In this paper, we present our research work on the use of a dual-frequency (300MHZ and 800MHZ) ground penetrating radar (GPR) system (UtilityScan, GSSI) for detecting the growth of an artificial void in a layered culvert structure. A laboratory three-layer (asphalt layer, gravel sub-base layer, and subgrade layer) setup with an underground air void and a 30” diameter culvert was manufactured as a testbed. Three different levels of damage were artificially created inside the laboratory setup and inspected by the GPR system on surface layer. From our experimental result, it is found that the wavelength of 300MHz GPR signal is not sensitive enough to image the growth of an underground void inside the experimental setup. On the other hand, the wavelength of 800 MHz GPR signal can reconstruct the growth of the underground void. This research work indicates the advantage of multiple frequencies for GPR subsurface sensing.

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