Research Papers: Heat and Mass Transfer

Impact of Pulse Length on the Accuracy of Defect Depth Measurements in Pulse Thermography

[+] Author and Article Information
James Pierce

Mechanical Engineering Department,
University of South Florida,
Tampa, FL 33620

Nathan B. Crane

Mechanical Engineering Department,
Brigham Young University,
Provo, UT 84602
e-mail: nbcrane@byu.edu

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received July 9, 2018; final manuscript received January 18, 2019; published online February 25, 2019. Assoc. Editor: Ali Khounsary.

J. Heat Transfer 141(4), 042002 (Feb 25, 2019) (6 pages) Paper No: HT-18-1442; doi: 10.1115/1.4042785 History: Received July 09, 2018; Revised January 18, 2019

Pulse thermography (PT) is a nondestructive testing method in which an energy pulse is applied to a surface while the surface temperature evolution is measured to detect sub surface defects and estimate their depth. This nondestructive test method was developed on the assumption of instantaneous surface heating, but recent work has shown that relatively long pulses can be used to accurately determine defect depth in polymers. This paper examines the impact of varying input pulse length on the accuracy of defect depth quantification as a function of the material properties. Simulations using both thermoplastics and metals show that measurement error is dependent on a nondimensionalized pulse length. The simulation results agree with experimental results for three-dimensional (3D) printed acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) components. Analysis and experiments show that defects can be accurately detected with minor modification to the standard methods as long as the pulse ends before the characteristic defect signal is detected.

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Fig. 1

Schematic representation of the 1D boundary condition for PT. The illustrated cross section represents the simplified 1D heat transfer representation of the three-dimensional (3D) specimen.

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Fig. 2

Schematic representation of time measurements in defect-depth calculation. ΔT represents the temperature contrast between defective and sound regions. The peak slope is calculated from a polynomial fit of the data after the end of the pulse, but the peak slope time (ts) used for defect-depth calculation includes half the pulse length (tp/2).

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Fig. 3

Schematic representation of the simulation boundary conditions used for 1D simulation of pulse thermographic method. (Left) A 3D case is simplified to two 1D models to represent a defect free region (middle) and defect region (right). Convection with a coefficient of h = 10 W m−2 K and radiation with emissivity (ɛ = 0.9) wereapplied at the top of both and backside of the defect. The initial and atmospheric temperature (T0, T) = 298 K. Ld is the defect depth and Ls is the depth of the sound region of the specimen.

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Fig. 4

Diagram of experimental measurement system

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Fig. 5

Comparison of defect depth calculations in simulation and experimental results for a range of materials

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Fig. 6

Experimental evaluations of the constants required to accurately calculate the exact defect depths of the ABS and PLA parts. The black line is the least squares fit of the experimental results.

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Fig. 7

Comparison of defect depth calculations in simulation and experimental results after modifying the constants in Eq.(6) based on the normalized pulse length as described in Fig. 5

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Fig. 8

Simulation of thermal response to different pulse lengths but with constant energy. (Left) comparison of temperature contrast produced by a 0.3 mm defect in ABS as a function of pulse length. (Right) Comparison of the surface temperature increases with varying pulse lengths producing the same amount of energy.



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