Optics: measuring and testing – By light interference – Holography
Reexamination Certificate
2000-06-05
2003-09-30
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Holography
C356S035500
Reexamination Certificate
active
06628399
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a method and device for non-destructive testing of details, machine units and mechanisms, various materials, and in particular to a method and device for non-destructive determination of residual stresses which is based on optical holographic interferometry technique.
2. Background
Optical holographic interferometry technique is well suited for non-destructive testing of internal defects in blocks and units of machines and devices, welded seams, as well as measuring stresses of an object during the object's work load and residual stresses caused by technological processes of welding, forging, soldering etc. These applications are useful for fields such as offshore oil industry, shipping industry, process industry, air industry, and all types of constructions where strength is vital or fatigue may cause a problem.
An example of the state of the art for measuring residual stresses in an object by holographic interferometry is given in the journal: “Welding Engineering” 1983, vol. 12, p. 26-28. The article describes a typical device and method for measuring residual stresses which is based on drilling a small and shallow hole in the object for release of stresses as well as holographic interferometry technique for determination of surface displacements in the object at the edge of and in the vicinity of the drilled out hole. The principle of the method can be described as follows: First, a hologram of the investigation area of the object which is it's initial state is recorded and developed on a registering medium. Next, the residual stresses in a point of the investigation area of the object is released by drilling a small hole in the object. Then the registering medium with the developed image of the investigation area in the initial state and the investigation area of the object with the drilled out hole are simultaneously illuminated by the reference and object beams respectively. The components of the residual stresses is determined from the interference pattern which occurs in the hologram.
This device and its operation stages are shown schematically in
FIGS. 1-3
. The means for formation and registration of holograms from an area of the object (
10
), as well as for formation of an interferogram from this area after release of residual stresses at the investigation point (
14
) of the object, is given schematically as an optical block with reference number (
1
) in
FIGS. 1 and 3
. The block contains a coherent light source (
2
), a holographic interferometer with optical elements (
3
-
4
) forming a reference beam (
5
) and an object (
6
) beam, and a recording medium (
7
). All components are rigidly connected with regard to each other. The optical block contains also a response part (
8
) of a precision device for fine positioning of the optical block on the object (
10
) above the area which is to be investigated (a corresponding receiving part (
9
) of the precision device is fastened on the object (
10
)). Means for drilling a hole at the investigation point (
14
) is given schematically as a mechanical block (
11
) in FIG.
2
. Typical dimensions of the hole 1-3 mm in diameter and the depth are up to 1.5-2.0 times the diameter. In addition there are an apparatus for displaying and observation of the interferograms (in this case, a TV-camera (
12
) and a display screen (
13
)).
The operation of the device can be divided into three stages. The first stage is the registration of the hologram from the investigation area of the object; the second stage is the release of the residual stresses in the investigation point of the studied area of the object; the third stage is the formation of the interferogram from the studied object area and the determination of residual stresses in the point of the studied area. Let us consider the device operation stage by stage.
The First Stage
First, the receiving part (
9
) of the precision device is fixed on the investigation area of the object (
10
) (see FIG.
1
). Then, the optical block (
1
) is installed above the investigation area of the object (
10
) by attaching the response part (
8
) of the precision device into the receiving part (
9
), and a hologram from the investigation area is registered. This is made in the following way:
The beam from the coherent light source (
2
) is expanded by the micro-lens (
3
). One part of the expanded beam is reflected by the mirror (
4
) towards the recording medium (
7
), this part is usually named the reference beam (
5
). The other part of the expanded beam hits the investigation area (
14
) of the object and reflects therefrom towards the recording medium (
7
). This part is named the object beam (
6
). When the object beam meets the reference beam, an interference occurs and a holographic image of the studied area of the object is formed. This image is registered and developed by means of the recording medium (
7
).
After development of the holographic image, a hologram of the studied area can be restored (i.e. the light wave scattered from the investigation area of the object is restored behind the recording medium (
7
)). For this purpose, it is necessary to illuminate the registering medium (
7
) (which contains the developed holographic image) with the reference beam (
5
).
The optical scheme is designated in such a manner that it has maximal sensitivity towards normal displacements of the surface of the object.
After finishing the holographic image registration of the studied area, the optical block (
1
) is removed from the object surface.
The Second Stage
The mechanical block (
11
) is installed on the studied area of the object (see
FIG. 2
) and, with its use, a small and shallow hole is drilled at the investigation point (
14
) of the object (
10
). The surface of the studied area is deformed in the vicinity of the hole due to release of residual stresses nearby the hole edge, and the normal component of the surface displacement at the hole edge is measured.
The Third Stage
First, the optical block (
1
) is extremely precisely reinstalled in the original position which it had at the first stage of the measurements (see
FIG. 3
) by using the precision device (
8
,
9
). The error of positioning should be less than one wavelength. Then, the illumination of the recording medium (with the developed holographic image of the studied object area in its initial state) by the reference beam (
5
), and illumination of the studied area with the drilled out hole by the object beam (
6
) are performed simultaneously.
Thus, two light waves scattered from the investigation area of the object will simultaneously be behind the recording medium (
7
). One of which corresponds to the light wave scattered by studied area of the object in its initial state (before drilling the hole), and the other to the light wave scattered by the studied area of the object with the drilled out hole. As a result of the interference of these light waves, an interferogram (
15
) of the studied area is formed (see
FIG. 3
) which can be observed, for example, with a TV-camera (
12
) and displayed by suitable means (
13
). From the interferogram one can determine the normal components of the surface displacement at the hole edge. In any considered direction, for example, along the X-axis, the normal component of the surface displacement (W
x
) at the hole edge will be equal to the number of interference fringes (N) (observed in the chosen direction), multiplied by one half of the wavelength (L) and divided by the sine of the incidence angle of the object beam (
6
). The residual stresses can be calculated by using the measured values of the normal component of the displacement at the hole edge. This may be performed in the following way.
In the case of a welded seam, for instance of an aluminium plate, the main residual stresses Q
xx
, Q
yy
are directed in parallel and perpendicular to the welded seam, respectively, and the interference pattern consists of two pairs of mutually perpendicular lobes (
15
) which are schematically
Andrushchenko Sergey Gavrilovich
Fjeldstad Irina Evgenievna
Fjeldstad John Petter
Krotenko Peter Dmitrievich
Kushniruk Vladimir Petrovich
HoloTech a.s.
Lyons Michael A.
Turner Samuel A.
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