Thermal measuring and testing – Leak or flaw detection – With heating or cooling of specimen for test
Patent
1983-11-03
1985-11-05
Frankfort, Charles
Thermal measuring and testing
Leak or flaw detection
With heating or cooling of specimen for test
374 7, 374129, 250338, 356237, G01N 2572
Patent
active
045510300
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a procedure and means for examining the surface qualities of materials in solid state. More particularly, the invention relates to a procedure and means for examining surface qualities and defects of solid state materials such as metals, or the equivalent, and the properties of their coatings, such as adhesion. This is accomplished by using a light beam and without destruction of the material. The procedure and means may also be used for measuring other types of coating, such as, for example, plasma coatings. In the interest of perspicuity, however, the following disclosure is concentrated specifically on coatings on metals and particularly on surface hardening.
Examination of coatings and of the surface hardening of steel is usually carried out by cutting a piece from the object to be examined and examining the structure of the surface, or the thickness of the coating, from one side with an optical microscope. If it is desired to examine the distribution of hardness in the surface, the usual procedure is to press a pointed diamond stylus into the surface of the specimen with constant force and to measure the depth of the resulting depression. The greater the hardness, the shallower the depression. This type of instrument is called a Vickers hardness tester. Similarly, it is also possible to measure the thickness and other characteristics of paint coat, for example, from the side. In case the object to be examined is precious, it is most awkward to cut off specimens.
SUMMARY OF THE INVENTION
The principal object of the invention is to measure the surface characteristics of solid materials without contacting the specimen.
An object of the invention is to measure the thickness of a surface-hardened layer of steel without contacting the specimen.
Another object of the invention is to provide a device and procedure for measuring the surface characteristics of solid material without contacting the specimen.
Still another object of the invention is to provide a device and procedure for measuring the surface characteristics of solid material which provides accurate measurements.
Yet another object of the invention is to provide a device and procedure for measuring the surface characteristics of solid state material with accuracy, facility and ease.
The objects of the invention are achieved, for example, by making use of the so-called photothermic effect. If a light beam is directed on the surface of a specimen, part of the light energy is absorbed by the specimen surface and converted into heat. If the light is intensity-modulated at a given frequency, the heating will also be periodic with the same frequency. If the intensity of the incident light is represented by the equation I=I.sub.o /2 (1+cos .omega.t), where I.sub.o /2 cos .omega.t represents the alternating component of the light intensity, then also the temperature at the light spot conforms to the equation and T.sub.o /2 A(.omega.) cos (.omega.t+.phi.), the alternating component, which displays a certain phase shift .phi. with reference to the light. A(.omega.) is the amplitude factor of the temperature, dependent on frequency. This periodic heating of the surface is propagated along the surface of the specimen, and into it, as a kind of "heat wave" or "thermal wave". This thermal has a given wavelength and phase angle, the phase changing as a function of time and location. The depth of penetration of the thermal wave is the so-called thermal diffusion length ##EQU1## wherein .omega. the angular frequency of modulation of the light and .alpha. is the thermal diffusivity of the material in question. The penetration of the thermal wave into the material may be expressed by the equation ##EQU2## where x is the distance which the wave has travelled. It is seen that the thermal wave is exponentially attenuated and the associated phase angle .phi. has the form ##EQU3## It is important to observe that although the thermal wave penetrates into the material, part of it still proceeds along the surface of the object in the plan
REFERENCES:
patent: 3043956 (1962-07-01), Cohen
patent: 3222917 (1965-01-01), Roth
patent: 4255971 (1981-03-01), Rosencwaig
patent: 4430897 (1984-02-01), Quate
patent: 4513384 (1985-04-01), Rosencwaig
"Thermal Wave Electron Microscopy of Metals", A. Rosencwaig, Sep. 8, 1980, Thin Solid Films, 77 (1981) pp. L43-L47.
"Alternating Beam Method in Photothermal Microscopy and Photoacoustic Microscopy", A. Lehto et al, Electronics Letters, Jul. 23, 1981, vol. 17, No. 15, pp. 540-541.
Lehto Ari
Luukkala Mauri
Frankfort Charles
Will Thomas B.
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