Application of Holographic Interferometry in Nondestructive Testing of Materials and Products
Abstract Generated abstract
This paper discusses the use of holographic interferometry as a noncontact method for nondestructive testing of materials and products. It outlines the principle of coherent summation in multiple exposure holography, distinguishes real-time, double-exposure, and time-averaged methods, and considers forms of external excitation such as thermal pulses, microwave heating, vacuum, acoustic vibration, and mechanical loading. Experimental examples show that disruptions in interference fringes can indicate hidden defects, including unbonded regions in layered rubber and textolite structures and boundaries between alloys. The authors conclude that holographic interferometry is promising for inspecting porous, layered, and cellular materials, especially where defects are difficult to detect by other methods.
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APPLICATION OF HOLOGRAPHIC INTERFEROMETRY IN NONDESTRUCTIVE TESTING OF MATERIALS AND PRODUCTS
V. A. BERDONOSOV, V. I. GORBUNOV, G. I. SHTANICHEVA, L. K. YANISOVA
(Presented by the scientific seminar of the Scientific Research Institute of Electronic Introscopy)
Holography makes it possible to implement several rather unique methods of interferometry [1].
The basic principle of holographic interferometry consists in the fact that, by means of multiple exposure of holograms, one can carry out coherent summation of complex amplitudes. Let a photographic emulsion be exposed successively to \(N\) different intensity distributions \(I_1, I_2, \ldots, I_N\). Then the total exposure is written as
\[ E = \sum_{k=1}^{N} T_k \cdot I_k, \tag{1} \]
where \(T_k\) is the exposure time corresponding to the given distribution \(I_k\). The use of the principles of holography leads to the fact that, for each exposure interval, the incident radiation is the sum of a fixed reference wave \(\vec{A}(x,y)\) and a wave from the object \(\vec{a}_k(x,y)\), changing from interval to interval. Then the total exposure is
\[ E = \sum_{k=1}^{N} T_k \cdot |\vec{A}|^2 + \sum_{k=1}^{N} T_k |\vec{a}_k|^2 + \sum_{k=1}^{N} T_k \cdot |\vec{A}^{\,*}|a_k + \sum_{k=1}^{N} T_k \cdot \vec{A}\cdot \vec{a}_k^{\,*}. \tag{2} \]
Assuming that exposure and photographic processing are carried out in the linear region of the characteristic curve of the photographic emulsion, we obtain the transmission component
\[ t_{\alpha} = \beta' \sum_{k=1}^{N} T_k \cdot \vec{A}^{\,*}\cdot \vec{a}_k, \tag{3} \]
where \(\beta\) is the transmission coefficient, dependent on the parameters of the photographic emulsion. From (2) and (3) it is seen that, when the transparency is illuminated by the wavefront \(\vec{A}\), a component will appear in the transmitted field that is proportional to the sum (with weighting taken into account) of the complex wavefronts \(\vec{a}_1, \vec{a}_2, \ldots, \vec{a}_N\). As a result, \(N\) coherent virtual images of the original object, to which the set of waves \(\vec{a}_i\) corresponds, will undergo linear superposition and interfere.
Compared with ordinary interferometry, holographic interferometry has a number of advantages: it makes it possible to investigate transparent and reflecting objects of any complex shape, and even diffuse ones; it does not impose strict requirements on the quality of the optics, which for ordinary interferomet-
is very important; it makes it possible to study the interference of waves scattered by objects in different directions within the solid angle covered by the hologram; it is not associated with the requirement that the formation of the waves be simultaneous, which opens up a number of interesting possibilities.
By varying the combination of interfering wavefronts, three methods of holographic interferometry can be distinguished:
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Real-time interferometry, when the wavefront reconstructed from the hologram interferes with the actual wavefront reflected from the coherently illuminated object.
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The double-exposure method, when two holograms of an object in two different states are recorded successively on a single photographic plate and, upon reconstruction, interfere with one another.
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The time-averaged holography method, when the hologram is recorded continuously with an exposure time much longer than the period of oscillation of the object. Upon reconstruction, the two fronts corresponding to the extreme positions of the object interfere with one another.
Holographic interferometry is finding ever broader application for the study of deformations and vibrations, and for the investigation of plasmas, shock waves, and gas flows [2–6]. One of the most promising areas of application of holographic interferometry is holographic nondestructive testing of materials and products [7]. For the use of holographic interferometry in nondestructive testing, it is necessary that subsurface anomalies subjected to the action of energy manifest themselves as various displacements of the coherently illuminated surface. Among the types of energy that can be successfully used for testing objects and for detecting surface distortions corresponding to the location, dimensions, and shape of hidden defects are the following: thermal pulses; dielectric microwave heating; vacuum; acoustic vibrations; and mechanical loading.
The choice of the optimum action is determined by the properties of the object being tested. Holographic interferometry gives good results in the inspection of layered structures. Figure 1 presents a photograph of a typical interferogram in which the disruption of the interference fringes indicates the presence and localization of an unbonded area.
Fig. 1. Interferogram of an unbonded area
measuring 0.8–1.0 cm² between a rubber layer 1 mm thick and a textolite plate 3 mm thick.
The interferogram was obtained by the double-exposure method. Before the second exposure, the object was subjected to a mechanical load. Any of the above-mentioned holographic operations reveals defects over the entire inspected area simultaneously and with good sensitivity.
The application of the method of holographic interferometry to the problem of detecting the boundary between two alloys also gave positive results. The boundary is revealed in the form of disturbance and rupture of the interference fringes in the region where the alloys separate. A photograph of the corresponding interferogram is shown in Fig. 2. Experimental studies showed that the optimal action before the second exposure is thermal heating.
Fig. 2. Interferogram
of the boundary between alloys
Summarizing what has been said above, one may conclude that holographic interferometry, being a noncontact testing method, can be used for nondestructive testing of a broad class of porous, layered, and cellular structures which, by revealing defects, cannot be detected by other inspection methods.
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