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first thermal radiation constant
second thermal radiation constant
spectral intensity of blackbody radiation
measured spectral radiation intensity
index of refraction
roughness factor
surface temperature
spectral emissivity
directional emissivity
angle of energy
extinction coefficient
spectral wavelength
reflectance for rough surface
reflectance for polished surface
directional reflectance
bidirectional reflectance
root-mean-square (rms) surface roughness
solid angle
absorbance
spectral-bidirectional reflectance
transmittance
incident flux
absorption
reflection
transmission
constant
NDT non-destructive testing
MRT multispectral radiation thermometry
EDM electric discharge machining
SNR Signal-to-noise ratio
signal power
noise power
signal amplitude
noise amplitude
Angle is a projective angle in x-z plan composed by the centre line of the UV-light and the x-axis.
Angle is a projective angle on y-z plan composed by the centre line of the UV-light and y-axis.
1 Introduction
With science and technology developing, the required quality of products is higher and higher, especially in some special field, such as in the aerospace field, medical domain and so on. Therefore, the inspection of the surface is very important to ensure the product have good quality. One of the good ways to inspect the surface is to utilize a thermography measurement system, which includes, at least, one heating source (laser, infrared and so on), IR camera and PC. Thermography is a non-destructive testing (NDT) method that does not require contact with the surface of specimens.
1.1 Background
NDT was started during the 19th century [1]. It is widely used as an analysis technology to evaluate the properties of material in science and industry. One of the NDT-methods is thermography. The advantages with thermography have been recognized as fast, non-contact and providing full field information. Thermography inspection is based on studying the emitted thermal radiation from a surface. The thermal radiation is related to the emissivity of the surface. However, it is hard to define the relationship between surface roughness and the emissivity. Therefore, this report focuses on how the emissivity is related to the roughness of the surface.
1.2 Objective
The study focus on surface breaking cracks. The aims of this report are try to define how the heating angle influence the thermography measurement, define the relationship between surface roughness and emissivity, and define the influence of the angles of the heating source, the direction of crack and the direction of surface roughness on thermography measurement.
However, there still are some limitations, such as that it is impossible to ensure each crack, from different samples, has the same heating angle, surrounding temperature etc. Also the cracks which are used in the experiments are made by electric discharge machining (EDM). In the other words, no matter how similar between the artificial cracks and the real cracks, the artificial cracks still cannot replace the real cracks. Because of the real crack suffers from a more complicated set of variables compared to an artificial crack. It is simply impossible to avert this kind of noises one hundred percent.
2 Theory
Emissivity is a surface property that states the ability to emit thermal radiation. Each surface has a different emissivity. Emissivity is a unit-less quantity which can range from a theoretical 0.00 (completely not-emitting) to an equally theoretical 1.00 (completely emitting). The surface/object is called blackbody if the emissivity of the material equals to 1. However, it is important to point out that generally, emissivity is not constant, as it depends on several parameters, such as roughness of the surface, temperature, viewing angle, wavelength, and so on. Normally, emissivity increases with the increase of surface roughness and/or the increase of temperature [2-5].