Determination of temperature sequence length in pulse phase thermography

In pulse phase thermography (PPT), the temperature sequence length affects the efficiency of phase calculation and the ability to distinguish defects. To improve the efficiency and ability of detection, the influence of the temperature sequence length on the phase difference was analyzed. This was carried out using finite element simulations under the conditions of different substrate thicknesses, defect depths, defect diameters, heat power densities and material properties. The simulation results show that each defect at a different depth has a corresponding optimal temperature length topt, which can provide the maximal phase difference and the best ability to distinguish defects in materials of certain properties. topt varies with the material properties, the defect depth and diameter, and the substrate thickness. The numerical topt formula was fitted through simulation results, and the experiment was carried out on aluminum alloy specimens with flat-bottomed holes as artificial defects. Through the experimental results, the relationship between phase difference and temperature sequence length is shown similar to that in simulation. When topt is calculated using the fitting formula, we can approximately obtain the maximal phase difference and the best defect phase image. topt can be determined by its change rule and the numerical formula, which can provide better defect distinguishing ability, avoid redundant temperature sequence, and improve the detection efficiency.