Optics: measuring and testing – By light interference – Holography
Reexamination Certificate
2000-11-22
2004-07-20
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Holography
C356S035500
Reexamination Certificate
active
06765677
ABSTRACT:
FIELD OF INVENTION
This invention relates to a method and device for non-destructive measurements of residual stresses and loading stresses which is based on optical holographic interferometry technique, where the holographic interferometer is divided into a hand-held holographic probe which is being installed on the object that is to be investigated and a holographic camera which may be situated in a protected in-door environment. The hand-held holographic probe allows to measure residual stresses on surfaces of an object with high curvatures, in places where access is difficult, and under many weather conditions by a simple hand-held manual positioning of the holographic probe during the measurements.
BACKGROUND
Optical holographic interferometry technique is well suited for measuring residual stresses caused by technological processes of welding, forging, soldering etc. as well as stresses in an object during the object's work load.
These applications are useful for fields such as offshore oil industry, shipping industry, air industry, process industry, and all types of constructions where loading stresses and residual stresses are 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 for measuring residual stresses which elements, including a laser, optical elements of a holographic interferometer, and a registering medium are rigidly connected between each other by a common metallic basis for protection against vibrations. Also, the operation of the device is based on optical holographic interferometry technique. The device should be installed onto an investigation object during measurements.
The principle of the residual stress measurements by means of this device can be described as follows: First, a hologram of the investigation area of the object is recorded and developed on a registering medium. Further, the residual stresses in a point of the investigation area of the object is released by drilling a small and shallow hole in the object. Then the registering medium with the developed holographic image and the investigation area of the object with the drilled out hole are simultaneously illuminated by the reference and object beams respectively. An interferogram of the investigation area of the object is formed as a result of interference of the two light waves scattered by the object under its illumination with an object beam before and after drilling the hole.
In the case of a welded seam, for instance of an aluminum plate, the interference pattern consists of two pairs of mutually perpendicular lobes which indicate directions of the main residual stresses, namely in longitudinal (Q
zx
) and in transverse (Q
yy
) direction of the welded seam. From the interferogram one can determine the normal components of the surface displacement at the hole edge (W
x
and W
y
), which are equal to the respective number of interference fringes observed in the chosen direction multiplied by one half of the wavelength and divided by the sine of the incidence angle of the object beam. The main stresses are determined by using the above values of W, and Wy from simplified theoretical expressions which presume that the depth of the drilled out hole (h
s
) is less or equal to its radius (r
s
):
Q
x
=
W
x
W
1
x
[
r
1
/
r
S
]
{
E
/
E
AL
}
(
1
)
Q
y
=
W
y
W
2
x
[
r
1
/
r
S
]
{
E
/
E
AL
}
(
1
)
where W
1x
, W
2x
are parameters equal to the normal components of the surface displacement at the hole edge along the X-axis for unity values of stresses applied first in the X-axis direction (when determining W
1x
) and, then in the Y-axis direction (when determining W
2x
), and which is obtained from the theoretical dependencies of W
1x
, W
2x
on the co-ordinate from the center of the hole for different ratios between r
s
and h
s
, under unity stress for the studied material. E, E
AL
, and r
1
, are elasticity modules of the studied material and aluminum and the unity radius, respectively.
However, the above mentioned device has essential drawbacks:
1) It is necessary to drill holes in the object that is to be investigated for residual stresses. Thus the method is a destructive test, and is obviously not acceptable for a variety of objects and applications.
2) The device does not allow the evaluation of residual stresses in the real-time scale due to use of silver-halide-based photographic emulsions as the registering media. These requires large development times.
3) The device enables only indoor measurements, c.f. under workshop comfortable conditions.
4) The device enables measurements only on horizontal surfaces with a weak curvature, and it does not allow to perform measurements on inclined and vertical surfaces and on hardly accessible places of the investigation object.
An attempt to eliminate the mentioned drawbacks was made in the device for measuring residual stress, described in U.S. Pat. No. 5,432,595 to Pechersky. This device is similar to the device described above, but the release of the residual stresses is achieved by heating the investigation point by radiating it with the infrared (IR)-pulse.
However, this device does also suffer from considerable drawbacks which can be summarized as follows:
1) Deviation of the energy distribution over the IR-pulse cross-section from a rectangular shape as well as the heat dissipation from the investigation point of the object irradiated with the IR-pulse results in a blurring out of the boundaries of the spot where the release of residual stresses occurs. This excludes the use of expression (1) and (2) for quantitative evaluation of residual stresses from the measurements of normal components of the surface displacement. It also makes it difficult to obtain analytical expressions for subsequent quantitative determinations of residual stresses from the measurements of normal components of the surface displacement, and makes the assignment of the determined residual stress of a particular point of the object difficult.
2) Due to the heating of the investigation point up to the transition temperature into the plastic state where the residual stresses are released, the action of residual stresses localized outside of the heated spot will deform the surface of the object, not only in the vicinity of the heated spot but also within the spot itself. This is an additional confirmation for the above given conclusion that this device does not allow the use of the analytical expressions given in equation (1) and (2), since these assume that the stress release occurs in a spot with sharp boundaries and no deformation within the region with released stresses. Further, the problem of obtaining new analytical expressions for quantitative determinations of residual stresses is very complicated due to the uncertainty in the determination of the boundaries of the region of stress release and the deformation of the region of stress release. This allows one to assume that the considered device can only be used, at best, to reveal residual stresses.
3) New stresses are created by structural changes in the irradiated spot which occurs during heating up to the transition temperature by the IR-pulse. These new stresses together with residual stresses localized outside the region of residual stress release, should deform the irradiated region and its surroundings as well.
Therefore it becomes impossible, from the distribution of normal displacement components outside the irradiated spot, not only to quantitatively determine the residual stresses, but even to determine the directions of the main residual stresses.
Thus, none of the considered drawbacks, also including the fist one has not been overcome in the device described above. Thus, the device cannot be considered as a non-destructive device.
The first mentioned drawback has been overcome in a device for measuring residual stresses, where a “dislocation” release of the re
Andrushchenko Sergey G.
Davidenko Nikolay A.
Fjeldstad Irina Evgenievna
Fjeldstad John Petter
Kijanets Irina V.
Birch & Stewart Kolasch & Birch, LLP
Font Frank G.
Holotech A.S.
Lyons Michael A.
LandOfFree
Method and device for non-destructive real-time measurements... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and device for non-destructive real-time measurements..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and device for non-destructive real-time measurements... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3207787