WO2004081491A1

WO2004081491A1 - Method and device for determination of residual stresses

Info

Publication number: WO2004081491A1
Application number: PCT/SE2004/000356
Authority: WO
Inventors: Nikolay Alexandrovich Davidenko; Irina Evgenievna Fjeldstad; Valeriy Aleksandrovich Pavlov; Nikolay Georgievich Kuvshinskiy
Original assignee: Agellis Ab
Priority date: 2003-03-10
Filing date: 2004-03-10
Publication date: 2004-09-23
Also published as: SE0300666D0

Abstract

A method and device for nondestructive testing of objects such as machine units and mechanisms, of various materials, and in particular, to a method and device for nondestructive determination of residual stresses which are based on an optical holographic interferometry technique. First, a hologram of the investigation area of the object in its initial state is registered. Then a release of the residual stresses in an investigation point in the investigation area is performed by exposing the surface of the object to a high-current electric pulse and applying a small mechanical pressure. Finally, an interferogram of exactly the same area of the object is made, and the residual stresses at the investigation area are determined from the shape and size of the fringes in the interferogram.

Description

METHOD AND DEVICE FOR DETERMINATION OF RESIDUAL STRESSES

FIELD OF INVENTION

[0001 ] This invention relates to a method and device for determination of residual stresses in an object, such as machine units and mechanisms of various materials, and in particular to a method and device for substantially non-destructive determination of residual stresses which is based on optical holographic interferometry technique.

BACKGROUND OF THE INVENTION

[0002] Optical holographic interferometry technique is well suited for substantially non-destructive testing of internal defects in blocks and units of machines and devices, welded seams, as well as measuring internal stresses of an object during the object's work load and residual stresses caused by such technological processes as 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. [0003] 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 hole before and after the drilling. The principle of the method can be described as follows: In a first stage, a hologram of the investigation area of the object in an initial state is recorded and developed on a registering medium. In a second stage, the residual stresses (RS) in a point of the investigation area of the object is released by drilling a small hole in the object. In a third stage, 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 hole are simultaneously illuminated by the reference and object beams respectively. The components of the residual stresses is determined from the interference pattern occuring.

[0004] As a result of the interference of these light waves, an interferogram of the studied area is formed which can be observed, for example, with an optical sensor and displayed by suitable means. From the interferogram, the normal components of the surface displacement at the hole edge can be determined. 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 f inges 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 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.

[0005] In the case of a welded seam, for instance of an aluminium plate, the main residual stresses Qxx, Qyy are determined from the simplified theoretical equations given below by using experimentally measured normal components of the surface displacements at the hole edge, W_x and W_y, and assuming that the depth of the drilled out hole (h_) is less or equal to its radius (r_s):

^W2x

sj where W_lx, W _x 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_lx) and, then in the Y-axis direction (when determining W_2x), and which are obtained from the theoretical dependencies of W_2x> W_lx on the ratio between r_s and h_s under unity stress for the studied material. E and EA are elasticity modules of the studied material and aluminium, respectively. [0007] However, the above-mentioned method and equipment for deterniining residual stresses have essential drawbacks:

[0008] 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. [0009] 2) It is necessary to remove the optical block with holographic interferometer from the studied area of the object before drilling out the hole, and to reinstall it with extreme precision in its original position. On one hand, this considerably increases the time consumption of measurements and, thus, the evaluation of residual stresses is not performed in a real-time scale. And on the other hand, this requires the use of extremely fine tuned precision devices for positioning of the optical block on the studied area of the object. [0010] US Patent No. 6,628,399 discloses a method, wherein the release of the residual stresses in an investigation point in the investigation area is performed by exposing the surface of the object to a high-current electric pulse.

DISCLOSURE OF THE INVENTION

[0011] The main object of the invention is to provide a device and method for performing non-destructive real-time determinations of residual stresses in materials by holographic interferometry which overcomes the above-mentioned drawbacks. [0012] It is a further object of the invention to provide a device and method for performing substantially non-destructive real-time determinations of residual stresses in materials by holographic interferometry which is able to release the residual stresses in a region with a sharp boundary of the object. [0013] It is moreover an object of the invention to provide a device and method for performing non-destructive real-time determinations of residual stresses in materials by holographic interferometry which makes it possible to employ the simple equations given in equations (1) and (2) to calculate the residual stresses.

[0014] The objects of the invention can be achieved by the device and method disclosed in the appended claims. [0015] The objects of the invention can be achieved by exposing a certain region (the investigation point) of the investigation area of the object to a "dislocation" release of the residual stresses. This is obtained by exposing the investigation point of the object to an electric high-current pulse and a small mechanical pressure since this enables a very fast and well defined release of the residual stresses. Moreover, the boundary of the stress release area will be well defined. During exposure to an electric pulse, an energy transfer from directionally traveling electrons to the dislocations occurs, and this phenomena as well as the magneto-dynamical effect of the mechanical compression of the investigation area in which the electron stream is passing leads to the directional movement of dislocations and to release of residual stresses. The mechanical pressure assists to this action. The release of residual stresses is thus carried out without causing a transition of the material into a plastic state, and it can be done in a region with a sharp boundary. Thus, by using an optical block which may be similar to the optical blocks described in the prior art, but which also includes a device for release of residual stresses by delivering an electric pulse and a mechanical pressure at an investigation point of the surface of the object, the drawbacks of the prior art devices and methods are avoided. It will also be possible to use the experience in calculating the residual stresses by using the analytical equations given in equations (1) and (2), as well as results on experimental determination of normal components of the surface displacement at the boundary of the region of stress release.

[0016] Further objects, features, advantages and scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention.

[0018] Figures 1 is an optical block scheme of the device according to the invention.

[0019] Figure 2(a) is a schematic representation of an interference picture schematically showing a hologram of the investigated area of the object. [0020] Figure 2(b) is a representation similar to Fig. 2(a) and shows the interference picture with the beam cut off.

[0021] Figure 3 is a representation similar to Fig. 2(a) and shows the interferogram.

[0022] Figure 4 is a schematic diagram of interference strips at a welded junction of a pipe. [0023] Figure 5 is a schematic diagram showing relaxation of residual stresses.

[0024] Figure 6 is a block diagram showing three steps of obtaining an interferogram. [0025] Figure 7 is a diagram showing the results of a computer processing of an interferogram as a three-dimensional picture of surface shifts close to an area of relaxation of residual stresses, wherein the surface shifts have been amplified.

[0026] Figure 8 is a diagram showing the theoretical dependencies of Wo on the distance to the center of gravity r for different ratios h/Rs.

[0027] Figure 9 is a schematic diagram showing an electrode pressed into the object and the application of an electric current and breastwork appearing around the electrode.

[0028] Figure 10 is a diagram showing an interferogram appearing as circles located around the print of the electrode. [0029] Figure 11 is a diagram showing an interferogram with a petalous character.

[0030] Figure 12 is a diagram of a plate being bent to cause stresses of loading.

[0031] Figure 13 is a diagram over measured dependency of Awr on Qx.

[0032] Figure 14 is a diagram of a general view of the laboratory prototype of the holographic probe. [0033] Figures 15 - 19 are schematic views showing the processing of preparation of the holographic recording medium for registration of holograms.

[0034] Figure 20 is a diagram showing the dependency of Wa on stress loading in sample 152.

[0035] Figure 21 is a diagram showing the dependencies of Wyo on stresses of loading obtained with help of experimentally measured values Wy and Wx in samples 152_s 044 , and

052.

[0036] Figure 22 is a diagram showing the measurement results of samples 044 and

152.

[0037] Figure 23 is a diagram showing the dependency of the value of residual stress on the distance to the sample surface.

[0038] Figure 24 is a diagram showing the measurements on the rail segment with polished surface being fulfilled in five spots located along the central rail line.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Below is given a description of a laboratory prototype of the holographic probe as well as the technique for measurements of residual stresses (RS) and stresses of loading of metal samples with flat and quasi-flat surface. The principle of operation of the holographic probe is based on holographic interferometry as well as on the methods of relaxation of residual stresses by means of pressing an electrode with hemispherical tip into a sample surface and input of high-current electric pulses. The method of holographic interferometry as a way for revealing geometrical shape changes occuring in the object as result either of loading or of relaxation of residual stresses and stresses of loading is first presented. Then, features of using the holographic interferometry method together with the methods of stresses relaxation for measurements of residual stresses and stresses of loading is described. Purposes, structures and peculiarities of operation of the holographic probe are then considered. Finally, there is described the outcomes of measurements of stresses of loading and residual stresses in aluminum and steel samples. The method of holographic interferometry according to the present invention consists of three stages. During the first stage, an area of the object is illuminated with coherent light and a wave front of reflected light wave is registered using the holographic method. During the second stage, an area of the object is loaded or relaxed of residual stresses. Surface shifts caused either by the process of loading or by relaxation of residual stresses is determined in the third stage with the help of an interferogram. Features of surface shifts of the investigated area of the object allow revealing of defects. Features of surface shifts at the edge of an area of stresses relaxation allow the determination of values of residual stresses or stresses of loading.

An optical scheme of the small-sized device 1 is shown in Fig. 1. Here a laser beam 2 is widened by a spherical mirror 3 and further it is divided by an optical cube 4 into two beams: a reference beam 5 and an object beam 6. The reference beam 5 is further widened by a spherical mirror 7 and falls on the surface of a recording medium 8. The object beam 6 falls on the surface of the investigated object 10 fixed in the device for loading 11. This beam also falls on the surface of recording medium 8 after reflection from the surface of investigated object 10. Reference and object beams interfere on the surface of recording medium 8. The interference picture schematically shown in Fig. 2(a) is a hologram of the investigated area of the object. It consists of information about the object beam 6. The hologram is registered by the recording medium 8 where the interference picture is transformed into a geometric shape of the surface of recording medium 8. Features of functioning of the recording medium and its properties will be described below with reference to the holographic probe. Thus, developed image of the hologram looks like the geometric shape of the surface of recording medium.

If the object beam 6 is cut off and the recording medium 8 with developed image 004/081491

ofaehologramisffl^mina.edwi.hreferance^'be^ S.tofte objec.wffl edisp.ayed

_me_m„d allows «o memorize *-« about Ma. ,a,e of the investigated area of ob^ject _{md fl}*her,o reconstruct* to show fte mvestigated area of fte objec. ^ήuts nutia. smte. After tire first hologram relation, Are object is changed in any way (in our case its surface ,s hifedbyan ar^e p wiftrespectto meinidal statej andnextfterecordnrgmedrum sw th

Λe changedmvesfig.ted areaofnre objectis fflumi tedwid_tobjec.beam β. nten. two

information about the surface of fte investigated area of object in the in^fra, state and ft other consists of information about the changed surface of the investigated area of ob^ject. Those beams interfere with each other and create the interference picture called an offteinvestigated area of object. At observation of the investigated arer . of

or using TV camera 14 and mirrors 12 and 13, the reconstructed imaginary unage of the

obiect is covered by interference stiips characterizing changes occuring m the mvestigated a of object_, m ease of nondestiuctive control, anomalous behavior of interference stπps on _me surface of the investigated area of object can characterize presence of defector example anomalous behavior of interference stiips located on wdded ^junctⁱon of prpes (F g. 4) characterises internal defect in tire welded junction (cavity). case of m— en* , _

residual stiesses me interference picture may have some peculiarities shown mBg. 5. The interference stiips are located a, the edge of me hole and consist of two pairs of mutuaUy ; perpendicularpetals mdicatogme direction ofmemamresidual stiesse,

The princips of measurement of residual stiesses is described wrth reference to _me block scheme shown inFig.6. The same reference numerals as in Fig. 1 has been used for similar components. This device consists of the holographic interferometer 1. a dev^tce for relaxationofresidual stresses l TVcamera W and a computer l Before the

objec.10 _MdfMyconnec.edmaewimwimftehelp ofpreds.onfixation system9. The operation of me device could be divided into three smges. Tbe first stage is relatⁱon ofa gram of an investigated area of the object. The laser beam is wrdened by a lens 23 dunng this process. Thus one part called the reference beam 5 falls on a flat mirror 24, reflects and then falls on the recording medium 8. Another part of the widened beam called the object beam 6 falls on the investigated object area, reflects from it and falls on the recording medium 8. The reference and object beams interfere on the surface of recording medium and create a hologram of the investigated object area. The hologram is registered and developed by the recording medium. Developed image of the hologram looks like the geometric shapes on the surface of recording medium. The second stage is the relaxation of residual stresses or stresses of loading in a spot of investigated object area. By means of the precision system of placement 28, 29, the holographic interferometer 1 could be removed from the investigated object area. In our case the relaxation of residual stresses is accomplished by using a device for relaxation of residual stresses 31, such as drilling of small and shallow hole, or by pressing an electrode with hemispherical tip into the surface of investigated object area and input ofa high-current electric pulses. Then, surface shifts appear around the area of relaxation of residual stresses independently of the method of relaxation of residual stresses. The value of normal components of these shifts or changes of this value at the edge of relaxation area are related to the value of residual stresses or stresses of loading. The value of normal component of surface shift at the edge of area of relaxation of residual stresses is determined at the third stage using the method of holographic interferometry with formation of the interferogram of the investigated object area. The holographic interferometer 1 is again placed on the investigated object area with the help of precision system 28,29. The recording medium 8 with developed image of the hologram of investigated object area is illuminated with the reference beam 5. The surface of investigated object area 10 with the area of relaxation of residual stresses is illuminated with the object beam 6. As result, two beams caπying information about the investigated object area in its initial state as well as in the state with area of relaxation of residual stresses appear behind the recording medium. These beams interfere. The interference picture is called the interferogram. The interferogram could be observed with the help of CCD sensor TV camera 14 on a screen ofa monitor 15. Decoding of the interferogram allows the determination of the value of normal component of surface shift at the edge of relaxation area or its changes responsible for the value of residual stresses. The interferograms have specific features for each method of relaxation of residual stresses and information about values of residual stresses is obtained differently from these interferograms.

Residual stresses or stresses of loading become uncompensated at drilling a hole

004/081491

- 9 - at the edge of the hole. This fact results in a deformation of a small area around the cavⁱty or hole. In case when, for example, stresses of stretching are uniaxial the surface shifts are positive (convex) in the direction of stress of stretching and negative (concave) in the perpendicular direction at both sides of hole. The ratio between the absolute values of normal component of surface shift at the edge of cavity in the direction of stresses effecting and in the perpendicular direction depends on the ratio between depth and radius of the cavity. For example, for the case when this ratio is equal to unity, ft^β ratio between above-mentioned values is about 7. This fact testifies that the main surface shifts appear in the direction of stresses. As result of theoretical calculations of deformed elastic solid with drilled small and shallow cavities and uniaxial stresses it was found that the values of stress and normal component of surface shift at the edge of cavity are linearly related. Due to that, processing of interferograms consists in determination of normal component of surface shift (Wr) at the edge of cavity in the direction of stress effecting (Qx). Generally the value Wr can be determined from the following equation:

Wr =* —-. — 00, 2sιn« where λ is the light wavelength, N is the number of interference strips, α is the angle of inclination of the object beam.

Usually, interferograms processing and calculation of Wr is accomplished using computer An example of an interferogram is shown in Fig. 5. Result of computer processing of interferogram as the three-dimensional picture of surface shifts close to area of relaxatⁱon of residual stresses is shown in Fig. 7. Hollow and hills in Fig. 7 correspond to the drilled hole and to the surface shifts caused by uniaxial stresses of stretching respectively. Measured values Wr are used for calculation of Qx with the help of following analytic equation (2) obtained theoretically:

where Wo is the theoretical value numerically equal to the normal component of surface shift at the edge of cavity for unit stress of loading 10 MPa, for Young modulus equal to Young modulus of aluminum Eal-70 GPa, for experimentally measured ratio between depth h and radius Rs and for unit radius Ro 1 mm; E is the Young modulus of the investigated object. The value Wo is determined from the theoretical dependencies of Wo on the distance to the center of cavity r for different ratios h/Rs. These theoretical dependencies are shown on Fig. 8. Analytical dependency determined by the equation (2) is well confirmed experimentally. The ratio h/Rs for the curves are: Curve 1 = 0.4, Curve 2 = 0.8, Curve 3 = 1.2, Curve 4 = 1.6, Curve 5 = 2, Curve 6 = 0.4, Curve 7 = 0.8, Curve 8 = 1.2, Curve 9 = 1.6 and Curve 10 = 2.

Pressing a steel electrode into the surface of investigated object area and application of an electric current is made with the result that a small area of the metal object under the electrode and close to electrode (2-4 radiuses of the print) transits into plastic state. Thus, the electrode is pressed into the metal and a breastwork appears around the print of the electrode, as is shown in Fig. 9. If residual stresses and stresses of loading are absent in the object, then breastwork is positioned homogeneously around the print of electrode and with radial symmetry. In this case the interferogram (see Fig. 10) looks like circles located around the print of electrode. The distribution of the surface shifts around the print of the electrode changes when e.g. uniaxial stresses of stretching exist in the investigated object area, hi the direction coinciding with the direction of uniaxial stress of loading the surface shifts slightly increase and significantly decrease in the perpendicular direction. Furthermore, for sufficiently high values of stress of stietchingthe surface shifts can become negative, i.e. hollow can appear. As it can be seen from the Fig. 11, the interferogram get a petalous character. The number of interference strips in the direction of effecting of stress of stretching is much higher, than in the perpendicular direction. This fact testifies that changes of the surface are more significant in the direction perpendicular to the direction of stress of loading.

For absence of stresses of loading the equations (3)-(5) determine respectively: 1) dependency of depth of print (d) on the force of pressing (Fo), on the radius of hemispherical tip of the electrode (Re), on the yield strength (Po) and Young modulus (G) for the values of yield strength 90 - 500 MPa and values of radius of the hemispherical tip of the electrode 0.4 - 3.0 mm:

d -- aFo

(2R_ef⁶⁶ - P₀ - (l + βP₀ +yG) (3),

a = 40, β = 0.001 (MPa)- γ = 10-*)

S'JBSI 11 UTE SHEET fRU E 2Bj 004/081491

- 11 -

2) Dependency of the height of the breastwork on above mentioned parameters:

Wr= 0.06d ^'

3) Dependency of fte distance L between fte prin. center and maximum of breastwork on above mentioned parameters:

_o - te______ (5). I = 1.3r ^R- - ^ P,(l ₊ β>,)

As it follows from fte equations (3>(5). fte breastwork is located at fte distance 1.3 of print radiuses, its heigh, depends weakly on fte Young modulus, increases with grow* of pressmg force and decreases at the constant pressing force with growth of print radius, yield strength and Young modulus. It should be notified additionally, ⁽bat fte length of the area of deformati_OT (distancebetweenfhebreastworkmaximum anditsmagnitude0.1 Wr)nses under grow* of pressing force and decreases under constant pressing force and growth of yield strengft. The profile of breastwork satisfies fte exponential law. Results of such theoretical investigations correlate well with experimental tests.

If stresses of loading (Qx) is applied to fte investigated object, ften estimation predicts small changes of surface shifts close to fte maximum of breastwork, obtained from electro* pressing, in fte direction of stiess ofloading as well as more significant then changes (2.2 times) in fte perpendicular direction. Thus, fte change of breastwork hergbt _ΔWr in fte direction perpendicular to fte direction of stiess ofloading is determined by fte equation (6): AWr = M6WrQx rø-

Asi. Mows fiomfte e,uation(6), change of fte breastwork height and vataeof stiess ^of tading areconnectedlineariy and it is necessary to measure change of fte breastwork hergh, in fte perpendicular direction for fte experimental determination of value of stress of ioading. Experimentallymeasured change of fte breastwork heigh, is less significant m the direction of stress of .oading, more significant in fte perpendicular direction and can exceed the value of the breastwork height

Now i, is considered fte results of experimental investigations of dependency of change of fte breastwork heigh, in fte direction perpendicular to fte stress of stietchmg on fte value of st_iess of stretching. An aluminum plate 6082 with lengft b=300 mm, w^tdft a=100 mm and thickness h=4 mm was employed as the object of investigation. The plate was bent as is shown in Fig. 12 to create stresses ofloading. Thus, the value of stress of stretching

(Qx) in different spots of the plate is determined by the equation (7):

Q = ^^ (7)

The measured dependency of ΔWr on Qx is plotted in Fig. 13. It could be seen that ΔWr increases with Qx growth. In the same figure dependency of Wr on Qx is also shown to compare with the case when relaxation of residual stresses is accomplished by means of drilling of cavities. This dependency is linear, thus confirming corresponding theoretical dependency determined by the equation (2). Now, peculiarities of surface shifts of the investigated object area at electrode pressing under presence of stresses of stietchmg will be considered. Presence of stresses of loading involves changes of the distribution of surface shifts around the electrode print as compare to the case of absence of stresses ofloading. The main changes of surface shifts happen in the direction perpendicular to the direction of stresses ofloading. The electrode is pressed into the sample surface with a small pressing force to demonstrate influence of high- current electric pulse on the process of relaxation of residual stresses. Then, surface shifts are much lower then one half of light wavelength and no interference strips occur. However, at the application ofa high-current electric pulse, surface shifts occur. Petals in the direction coinciding with the direction of stress ofloading become visible in the interferogram. hi the perpendicular direction there is no change. This fact testifies that at input of high-current electric pulse positive surface shifts increase in the direction of effecting of stress of stietching and surface shifts does not change in the perpendicular direction. We consider that such effecting of the high-current electric pulse is caused by the electro-plastic effect. This effect involves displacement of dislocations and decreasing of their concentration and, in turn, the relaxation of residual stresses or stresses ofloading. Thus, effect of stress ofloading in the vicinity of relaxation area becomes similar to the case of relaxation of stresses at drilling of small and shallow cavities. If we would estimate the changes of surface shifts in the maximum of breastwork as a difference between normal components of surface shifts in direction of stresses ofloading and in the perpendicular direction, then input of high-current electric pulse results in an increase of this change and consequently an increase of the signal consisting of information about the value of stress ofloading. Dependency of the difference 004/081491

between changes of normal component of surface shifts in fte direction of stress of loadmg and in fte perpendicular direction on fte value of stiess of stretching is plotted in Frg. 13. Tie same pressing force as in fte case ofrelaxation of residual stiesses by electiode pressing was used at measurements of mis dependency. As it could be seen from comparison of those dependencies, at high stresses of loadmg

(ISOMPa) input of high-cur_ient electric pulses involves increase of fte difference of surface shifts and, consequently, increase of sensitivity. Effect of input of high-current electric pulse vanishes with decreasing of the stiess ofloading and becomes negative a, stiess of loadmg 75 MPa Parasitic thermal effecting of electiic current causes vanishing of fte positive influence of high-curren. electric pulse at diminishing stress o loading. This conclusion is confirmed experimentally. So, e.g. three times deceasing of fte ampUtude of high-curren. dectiic pulse for stiess ofloading 75 MPa causes three times rising of fte signal marked on fte cons^tdered dependencies. I. should be stated, that current influence would not be observable, if we try to accomplish relaxation of stiess ofloading atl50 MPa for such ampUtude of the high-curren. electric pulse.

The laboratory prototype of the holographic probe is aimed at measurements of residual stiesses and stiesses ofloading in objects with quasi-flat surface. A general vrew of fte laboratory prototype of fte holographic probe is shown in Fig. 14. It consists of fte holographic interferometer and device for relaxation of residual stiesses. The holograph. probeis comp.e.edby: a source ofcoheren.ligh, - onemodeHe-Nelaserwiftpow r l7 mW

(light wavelengft is 0.63 μm) with fiber optics for input of laser irradiation mto fte holographic probe; an electronic control equipment; a computer with processor Intel 80386; a plated circuit for interferogram digitizing and control of piezo-mirror; and software for interferograms processing. The holographic interferometer comprises a steel case with diameter 60 mm and heigh.70 mm. It is placed on fte investigated object with fte help of steel legs. These legs provide hard connection between the holographic interferometer and investigated ob^ject The holographic interferometer consists of: a platform with connector for connection of fte optical tip of fiber cable; a platform with piezo-mirror; a cell with holographic recordmg medium and small-sized CCD sensor camera. Pie-o-m^ήτor is aimed a, change of fte optical lengft of fte reference beam and for formation of three interferograms of fte same investigated object area with region ofrelaxation of residual stresses. Determination of 004/081491

- 14 - surface shifts close to region ofrelaxation of residual stresses and their calculation ⁱs accomplished using the above mentioned interferograms and software.

Processes of preparation of the holographic recording medium for registration of holograms, their development and erasing as well as thermosetting of the recording medium are accomplished in the cell. The functioning of the holographic recording medium according to Fig. 15 is now considered. The holographic recording medium comprises three layers: a light-sensitive layer based on a film of amorphous molecular semiconductor with thickness about 1 um; a transparent conducting tin dioxide layer; and a glass substrate with thickness 3 mm. Functioning of the recording medium is guided and accomplished by electronic equipment. The mnctioning process of the recording medium consists of four stages.

Preparation of the holographic recording medium for holograms registration, i.e. charging of its surface in a corona discharge is realised at the first stage, Fig. 15. Registration of the hologram is accomplished during the second stage, Fig. 16. The distribution of light intensity in the hologram is shown in Fig. 16. During the recording, a modulation of surface density of charges happens due to the photoconductivity of the medium. In this way a latent image of the hologram is formed in a view of surface distribution of charge density accordingly to the distribution of light intensity in the hologram. Development of the latent image of hologram is performed in the third stage Fig. 17. For this purpose, a pulse of electric current is passed through the transparent conducting underlayer of the recording medium. The light-sensitive layer is heated and deformed by the field of ponderomotive forces of the latent image of the hologram. Thus, the latent image is transformed into the geometric shape of the surface of recording medium. Erasing of Die developed image of the hologram is accomplished during the fourth stage, Fig. 18, by means of once more passing of more powerful electric pulse through the conducting underlayer, whereby the surface and residues of surface charges relax. Preparation of the recording medium for the next hologram recording takes place at the fifth stage, Fig. 19. Next the process of hologram recording can be repeated.

' AL samples IN 0225143 - 152 (AL 7475-T761, Po=415 MPa), 1N9910000-044(AL 2024-T3, Po=290MPa) and IN9910002-025(AL 2024-T3plated) were investigated. These samples are in the following text marked as sample 152, sample 044 and sample 025, respectively. The samples have the shapes ofa flat plate with dimensions 300x100x5 mm. The stresses ofloading in these samples were created by bending and were determined from the equation (7). Investigations were carried out with the help of holographic probe using 004/081491

drilling cavities, diameter 1 mm and depft O^mm as weU as using a pressing eiectiode wift hemispherical tip into fte sample surface and apphcation of an electiic current.

Obtained interferograms differ from fte interferograms on the alununru samples ift non-treated surface by existence of an externa. ring. Usual interferogram contammg petals caused by stiesses was observed inside fte ring We attiibute presence of fte external

nnen. of the sample surface. One can suppose, fta. existence of fte external πng treatment v . . _h,_'ft= ,_m„n_d the drilled cavity. These shifts can be corresponds to additional posrtive surface shrfts around the αrmeα c y presented as a breastwork (parapet). Stinting from this supposition about exrstence of lastwork and using fteoretical predictions about fte vahre -4 of ratio between normal lmponen_B ofsmface shi_ftatfte edge ofcavi«yinftedtiection ofs»ss of.oaftn_g-

additional* supposed fta, results of measurement can be processed wrft fte help of following analytical equation: (Wy- Wa)/(Wx - Wa) = -4 (⁸) _whereWy and Wxare respectively fte experimental measured values ofnorma. component of surface shift a, fte edge of cavity in fte direction of stiess of loadmg and in perpendicular direction, Wa is fte normal component of shift of breastwork surface, differences Wyo-(Wy-Wa) and Wxo=(Wx-Wa) are respective* fte tiue magr_ήftdes ofnormal cc∞ponentsofs_tuface shiftatfte edge ofcavit causedby stress oftadingintwomu allyperpendicular directions. Equation (8) allows determining Wyo and Wa using fte experimentally measured values Wy and Wx:

Wyo=0.8(Wy-Wx) (9), and Wa=0.2( Wy + 4Wx) (10). Equation (4) allows to verify correction of our supposition about accounting of

-_-._-_{>• miilM} Accordingly to this supposition wa breastwork influence on the measurements results. Accorαrngry ^y shourd not depend on st_iess ofloading. The dependency of Wa on stⁱess of loadmg m fte

0 take place. Dependencies of Wyo on stiesses ofloading obtained wift fte help <* experimentaUy measured vatoes Wy and Wx and equation (9) m the samples 152, 044 and 052 are shown on Fig.21. h_t some approximation, results of ftese measurements can be O 2004/081491

16 -

* Wyo caused by stiess ofloading on fte value of stiess of loadmg (sohd fine). Ttas X s to fte theoretical predictions of equation (2). Besides, values of stⁱesses of ZTlulatedbyftesubstifttionofWyomftee,uation(2)werec,ose,o« of I sofloadrngmtioducedrnftesampIebybendinganddetermmedbyeuationf

ls„floadm_&andWx-mftedtiectionperpendicmatteftedirectionoffte stiess of

sampledirecedalongftesample: ^ _4e

1) dependency of the normal component of surface shut at 30 depftofdriUingwasmeasured; _{e ition>} ^ _resiαUal stresses

2) RS value was defined at mtervals of 0.25 mm wⁱthⁱn P are distributed homogeneous* until ftis depth; obtained value was 920 MPa, 004/081491

3) theoretical dependency of normal comport of surface shift on the depth of drilling for fte constant value of residual stress 920 MPa in depth of drilhng was calculated; . ,

4) fte first derivatives of fte experimental and theoretical dependences of normal components of surface shift on depft of drilling were determined dependent on fte depth of drilling;

5) dependency of the ratio between these derivatives on the depth of drilhng as determined;

6) dependency of residual stresses on fte sample depft was determined as a product of residual st_iess at fte depth 0.25 and ratio between above notified derivatives for dⁱfferent

^dePftS' This dependency of the value of residual stress on fte distance to fte sample surface is presented in Fig.23. Large stiesses of compression are observed close to fte sample surface along fte sample, furfter ftese stiesses αuick* decrease wift fte depft, become zero a. <he depftabout l mm, change sign and become fte stiesses of stietching, increase up to 130 MPa and next decease to zero at fte depft of 2 mm.

Residual stresses directed cosswise the sample are observed only in the near surface h er Those st_iesses are fte stiesses of compression and fteir value is about 250 MPa We have concluded ftai ftese stresses are independent of fte distance to the sample surface for depths larger ftan 0.25 mm proceeding from an independence of fte normal component of surfaee shift atfteedge ofca^onftedepft ofdrillingstartingfrom OJS mm.

Two segments ofa railway rail, lengths 150 mm and 100 mm served as fte ob^jects for investigations. The surface layer wift ftickness 2 mm was pohshed off fte rail wift lengft 150 mm Measurements were carried out using fte holographic interferometer wrth dev.ce for drilUn_gcavities. Measuremen_B onfte rail segmen.wiftpolished surftce were Mffl.ed m five spot, located along fte central line as it is shown on Fig. 24. Distance between fte spots was 20 mm. Residual st_i sses of stietching directed along fte rail as well as residual stⁱesses of compression directed crosswise the rail were revealed in ftese spot, The stiesses of stietchingmftemarked spots are 323, 314, 314, 310 and 289 MPa respectively. The largest deviation of fte measured value is 34 MPa It differs by no more tha^t.10%. The stⁱesses of compression in fte marked spots „ -51. -68, -76, -50 and -59 MPa respective*. The largest deviation is 25 MPa and it differs 33% from the largest value. Measurements of resrdual stresses in five spots located at the distance 5 mm from each other along the central line were also carried out. Values of stress of stretching differ by no more than 10% from the highest magnitude. Stresses of compression differ by no more than 18% from the highest magnitude. Besides, residual stresses have been measured on the rail segment with length 100 mm with non-polished surface. Residual stresses in this sample were revealed at a respectively large depths of drilling, such as 2 mm. The interferograms were, however, distorted by blisters. It was possible to decode these interferograms, to define surface shifts at the edge of cavity and to calculate residual stresses, appearing to be the stresses of stretching directed along the sample and the stresses of compression directed crosswise the sample. Although this invention has been described in terms of an example and by a schematically presentation in "blocks", one should understand that the main scope of this invention is a general idea to employ an electric pulse and a mechanical pressure in order to achieve a non-destructive release of residual stresses in an area with sharp boundaries of an object in such a manner that the need for removing and replacing the optical block is eliminated. One example of measurements is described below.

The residual stresses in a welded seam between flat aluminium plates were measured. A helium-neon laser (output power 5 mW) provided a coherent light beam, and it was employed a recording medium based on an amorphous molecular semiconductor AMS-film which is described in the Applicants' US Patent Application Serial No. 09/596,556, the contents of which is incorporated in the present specification by reference. The AMS-film was made up of about 92 wt% ofa co-polymer comprising N-epoxypropylcarbazole and about 5 wt% buthylglycedil ether doped with about 5 wt% of methyl-9-(4-dodecyl-oxyphenyl- l,3-selenathiol-2-ylidene)-2,5,7-trinitrofluorene-4-carboxylate and about 3-4 wt% of hexadecyl-2,7-dinitro-dicyanomethylenfluorene-4-carboxylate. After registration and development of the hologram of tiie investigation area of the welded seam, the release of residual stresses was performed. For this purpose, a pulse of electric current with a duration of 0.15 seconds and 2 kA amplitude was passed through the junction of the current supply electrode with the investigation area of the object. The force of application of the electrode was about 1 kg. The main residual stresses were calculated, which gave Q^ = -10,572, Q^ = 2,241 kP/mm².

These results were checked with measurements on the same weld by traditional techniques. Under repeated measurements of residual stresses in welded seams of flat aluminium plates by means of the claimed device and with prior art, the average difference

_SUBS H I U ΓE SHEET (RULE^"P^ between measured residual stresses was less than 20 %.

Hereinabove, the invention has been described with reference to an optical block for forming an interferogram by forming a first hologram of tiie surface and a second hologram after relief of the residual stresses in the surface, which holograms interfere to form said interferogram. It is pointed out that the formation of the interferogram can be performed in software by inputting said first and second hologram into a computer by for example an optical sensor (CCD probe) and calculating the interferogram by the computer. This is considered to be an equivalent method of forming the interferogram.

There are of course numerous ways of supplying and retrieving an electrode to the investigation point, and there are also several ways of arranging the different components in the optical block. There may also be several other ways to supply an electric pulse and mechanical pressure with the described parameters to the surface of the investigation area of the object. These alternatives occouring to a person a skilled person in the art reading the present specification should be considered as included in the scope of this invention. The invention is only limited by the appended patent claims.

Claims

PATENT CLAIMS

1. A method for the determination of residual stresses in an investigation area of an object by optical holographic interferometry, comprising: registering ofa first hologram of the investigation area of the object in an initial state of the object; releasing the residual stresses in a small region of said investigation area; forming an interferogram of the investigation area for determining normal components of surface displacement adjacent the boundary of the region with released residual stresses; calculating the released residual stresses; whereby the release of the residual stresses is obtained by exposing said small region of the investigation area of the object to a relatively small mechanical pressure and an electric high-current pulse.

2. The method as claimed in claim 1, wherein said electric high-current pulse has a rectangular shape and has pulse parameters in the range of; pulse amplitude 1.5-20 kA, pulse duration of 1 microsecond to 0.2 seconds, and a recurrence frequency 0-100 Hz.

3. The method as claimed in claim 2, wherein said object is of aluminum, the electric high-current pulse has an amplitude of 2 kA and duration of 0.15 seconds.

4. A device for determination of residual stresses in an investigation area of an object by optical holographic interferometry, comprising: optical means comprising a source of coherent light, a holographic interferometer and a registering medium, relief means for releasing the residual stresses in a small region of said investigation area, and means for attaching the optical means to said object, wherein said relief means is a tool means for application of a pressure and a high current electric pulse to said object upon establishment of electric contact with said investigation area.

5. The device as claimed in claim 4, wherein said tool means has a half-spheric shape with a radius in the range of 1.5-5 mm.

6. The device as claimed in claim 4, wherein the relief means comprises: a generator of high-current rectangular pulses; and an electric current supply electrode with a clamping device which is electrically connected to the generator.

SUBSTITUTE SHEETTRl πrg

7. The device as claimed in claim 4, wherein said electric current supply electrode connected with the generator is arranged to provide an electric high-current pulse with rectangular shape to the surface of the investigation area of the object with parameters in the range of; pulse amplitude 1.5-20 kA, pulse duration 1 microsecond to 0.2 seconds and a recurrence frequency 0-100 Hz.

SUBS IH U I h SHEET (RULE fij