A device for the determination of residual stress in a material sample consisting of a sensor coil, adjacent to the material sample, whose resistance varies according to the amount of stress within the material sample, a mechanical push-pull machine for imparting a gradually increasing compressional and tensional force on the material sample, and an impedance gain/phase analyzer and personal computer (PC) for sending an input signal to and receiving an input signal from the sensor coil is presented. The PC will measure and record the change in resistance of the sensor coil and the corresponding amount of strain of the sample. The PC will then determine, from the measurements of change of resistance and corresponding strain of the sample, the point at which the resistance of the sensor coil is at a minimum and the corresponding value and type of strain of the sample at that minimum resistance point, thereby, enabling a calculation of the residual stress in the sample

Chern, Engmin J.

Flom, Yury

English

NASA Technical Reports Server

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United States Patent 1191 
Chern et al. 
[54] METHOD AND APPARATUS FOR 
DETERMINATION OF MATERIAL 
RESIDUAL STRESS 
[75] Inventors: Engmin J. Chern, Columbia; Yury 
Nom, Silver Spring, both of Md. 
[73] Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 801,141 
[22] Filed: Dec. 2,1991 
[51] Int. Cl .5  ............................................... GOlN 3/32 
1521 U.S. a. ........................................ 73/779; 73/797; - -  
324/222 
[58] Field of Search ................... 324/222, 209, 207.15, 
324/654, 656; 73/187,779, 797 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,969,928 7/1976 Zarka .................................. 73/88 R 
4,249,423 2/1981 Viertl et al. ........................... 73/783 
4,528,856 7/1985 Junker et al. ......................... 73/779 
5,055,784 10/1991 Jaeger et al. .................... 324/654 X 
4,852,397 8/1989 Haggag ................................... 73/82 
FOREIGN PATENT DOCUMENTS 
2194336 3/1988 United Kingdom . 
US005193395A 
[11] Patent Number: 5,193,395 
[45] Date of Patent: Mar. 16, 1993 
OTHER PUBLICATIONS 
Determination of Residual Stresses in Sheet Metal Mar. 
1981. 
A Technique for Measuring Residual Stresses Jul. 1981. 
Primav Examiner-Jerry W. Myrade 
Attorney, Agent, or Firm-R. Dennis Marchant; Guy M. 
Miller; Paul S. Clohan, Jr. 
A device for the determination of residual stress in a 
material sample consisting of a sensor coil, adjacent the 
material sample, whose resistance varies according to 
the amount of stress within the material sample, a me- 
chanical push-pull machine for imparting a gradually 
increasing compressional and tensional force on the 
material sample, and an impedance gain/phase analyzer 
and PC for sending an input signal to and receiving an 
input signal from the sensor coil. The PC will measure 
and record the change in resistance of the sensor coil 
and the corresponding amount of strain of the sample. 
The PC will then determine from the measurements of 
change of resistance and corresponding strain of the 
sample the point at which the resistance of the sensor 
coil is at a minimum and the corresponding value and 
type of strain of the sample at that minimum resistance 
point thereby enabling a calculation of the residual 
stress in the sample. 
1571 ABsTRAcr 
6 Claims, 3 Drawing Sheets 
4 
6 
2 
https://ntrs.nasa.gov/search.jsp?R=19930010929 2020-03-24T07:29:49+00:00Z
U.S. Patent Mar. 16,1993 Sheet 1 of 3 5,193,395 
US, Patent Mar. 16,1993 Sheet 2 of 3 5,193,395 
U,S, Patent Mar. 16,1993 Sheet 3 of 3 5,193,395 
5,193,395 
1 2 
phase analyzer is also connected to the mechanical test 
machine through the data acquisition/control device. 
The data acquisitiodcontrol device acts as the system 
controller and is used to control the mechanical stress/- 
5 strain machine and to process data from the impedance 
gain/phase analyzer. The data acquisition device/sys- 
METHOD AND APPARATUS FOR 
DETERMINATION OF MATERIAL RESIDUAL 
STRESS 
ORIGIN OF THE INVENTION 
The invention described herein was made by employ- 
e e ~  of the US. Government, and may be manufactured 
and used by or for the Government for governmental 
purposes without the payment of any royalties thereon 10 
or therefor. 
controller be an IBM pc Or the equivalent. 
BRIEF DESCmPT.ON OF THE DRAWINGS 
FIG. 1 is a graph of the sensor coil normalized resis- 
tance vs. strain when compression and tension is applied 
to a material sample made from nickel. 
FIG. 2 is a block diagram of a device according to the 
present invention. 
FIG. 3 is a schematic diagram of a device according 
to the present invention. 
TECHNICAL FIELD 
This invention relates to Non Destructive hpection 
techniques and more particularly to a method and de- l5 
vice for the determination of the initial stress state of a 
material specimen. 
BACKGROUND ART 
DETAILED DESCRIPTION OF THE 
INVENTION 
Methods of measuring residual stresses have been in 
Use for some time, and fall generally into two catego- 
ries: destructive and non-destructive. Destructive meth- 
ods required that the specimen was cut-up by hole dril- 
The invention consists of a method and apparatus for 
determination of material residual stress utilizing three 
major hardware components as shorn in FIG. 2, 
which is a block diagram of the present inventive sys- 
or trepanning. Hole drilling Was a common method 25 major components are: a mechanical tester 6, a data 
lyzer 2. Other sub-components, shown schematically in 
FIG. 3 are sensor coil 10, mounted on the surface of 
ling, electrical discharge machining (EDM), sand blast- 
but had the undesirable feature that the drill worked the 
tem for material residual stress determination. The three 
acquisition device 4, and an impedance gain/phase ana- 
surface locally and built-in its own stresses. The EDM 
method good was awkward to 
destructive methods have relied on some change III material 
physical Property when the material is stressed, e.& the 30 ne novelty of the present invention lies in the dis- velocity of ultrasonic waves is affected. Unfortunately covery that sensor coil resistance and reactance the velocity is affected far more by small variations in monotonically increases/decreases with respect to ex- 
ternally applied stresses regardless of the direction, Le., materials properties which makes the system useless for stress measurement on anything but pure metals. X-rays have also been used but can only measure crystals near 35 tension or compression. This relationship is shown in 
the surface and this is a complex method which can be FIG. 1 for a material sample 8 made from nickel where 
handled well only in the laboratory while the real re- the sensor coil 10 normalized resistance measurements, 
namely those which would influence the initiation and against the applied compressional and tensional forces 
propagation of cracks. 40 exerted on specimen 8 by the mechanical tester 6. Start- 
ing from point “A”, the point at which tester 6 is apply- 
STATEMENT OF THE INVENTION ing no stress to sample 8 (zero applied stress point), it 
It is therefore an object of the present invention to can be seen that the normalized resistance of sensor coil 
provide a method and an apparatus for the dete&- 10 has some positive Value. AS tension iS applied to 
tion of the residual stress in materials that overcomes 45 sample 8, the re&ance of Coil 10 increases to a maxi- 
the limitations found in prior art methods and devi-. mum Value at point “c”. AS the tension iS released, the 
The foregoing objects and others are achieved by resistance Of Coil 10 retUll.S to its former Value at Zero 
providing a method and apparatus for the determination applied stress Point “A’’. As compression is applied to 
of material residual stress utilizing three major hard- sample 8, the resistance of coil 10 decreases from its 
ware components; a mechanical compression/tension 50 value at point “A’’ until reaching Some rninirnum value 
tester, a data acquisition/control device, and an imp-  at the zero stress point “B”. AS compression continues 
dance gain/phase analyzer. Other sub-components are a to be applied to sample 8, the resistance of coil 10 will 
sensor coil mounted on the surface of the material speci- then S t a r t  to increase again from point ‘3‘’ until it 
men, and cabling to interconnect the components. reaches some maximum value at point “D’. The resid- 
stress in sample 8 is thus equal to the compressional 
covery that the sensor coil resistance and reactance strain imparted by tester 6 on sample 8 at point “B, the 
monotonically increases and decreases with respect to point at which the resistance of coil 10 is at a minimum. 
externally applied stresses regardless of direction, Le., This p i n t  could just as easily occur at a tensional value 
tension or compression. If plotted on a graph of resis- in other samples. 
tance vs. strain, the point at which the resistance of the 60 In the present invention, an impedance gain/phase 
sensor coil is at a minimum indicates the zero stress state analyzer 2, such as is well known in the art, is connected 
of the specimen. The residual stress is then the strain to sensor coil 10, via cable 12, to excite sensor coil 10 
(cornpression or tension) indicated on the graph at this and to monitor its resistance, and to data acquisition 
minimum resistance point. device 4 via cable 14. Impedance gaidphase analyzer 2 
In the present invention, the impedance gain/phase 65 k also connected indirectly to mechanical tester 6 
analyzer, well known in the art, is connected to the through data acquisition device 4. Data acquisition 
sensor coil via a cable and to the data acquisition/con- device 4 not only excites coil 10 but also acts as the 
trol device via another cable. The impedance gain/- system controller and is used to control mechanical 
8, and cabling 12, 14, 16. 
quirement is to know the stresses Over a greater depth, Obtained from the data device 4, are plotted 
The novelty of the present invention lies in the dk- 55 
3 
5,193,395 
4 
tester 6 and to process datafrom impedance gain/phase strain of said sample is implemented by an impedance 
analyzer 2. Data acquisition device/system controller 4 gain/phase analyzer in conjunction with a data acquisi- 
can be an IBM series 386 PC or the equivalent. An tion and analysis computer. 
example of a suitable impedance gaidphase analyzer 2 3. The method of claim 2 wherein said steps of im- 
is an HP 4194A analyzer connected by an IEEE488 bus 5 parting a gradually increasing compressional force and 
to data acquisition device/system controller 4. A typi- a gradually increasing tensional force on said materid 
cal sensor coil 10 is a Staveley Instruments, Inc. NOR- sample is implemented by a mechanical push-pull ma- 
TEC-3551 100 kI-h pencil probe. Mechanical tester 6 chine. 
can be an Instron, an MTS, or equivalent universal 4. A device for the determination of residual stress in 
tester. Cables 12, 14, 16 are well known in this art. 
To those skilled in the art, many modifications and sensing means adjacent said material sample whose 
variations of the present invention are possible in light resistance varies according to the amount of stress 
of the above teachings. It is therefore to be understood within said material sample; 
that the present invention can be practiced otherwise means for imparting a gradually increasing comprcs- 
than as specifically described herein and still will be 15 sional force on said material sample; 
within the spirit and scope of the appended claims. means for sending an input signal to and receiving an 
I claim: input signal from said sensing means; 
1. A method for the determination of residual stress in means for measuring and recording the change in 
resistance of said sensing means and the corre- 
placing a sensing means whose resistance varies ac- 20 sponding amount of compressional strain of said 
cording to the amount of stress within said material sample; 
sample adjacent said material sample; means for imparting a gradually increasing tensional 
imparting a gradually increasing compressional force force on said material sample; 
on said material sample; means for measuring and recording the change in 
sending an input signal to and receiving an input 25 resistance of said sensing means and the corre- 
signal from said sensing means; sponding amount of tensional strain of said sample; 
measuring and recording the change in resistance of means for determining from said measurements of 
said sensing means and the corresponding amount change of resistance and corresponding strain of 
of compressional strain of said sample; said sample the point at which the resistance of said 
imparting a gradually increasing tensional force on 30 sensing means is at a minimum and the correspond- 
said material sample; ing value and type of strain of said sample at that 
measuring and recording the change in resistance of minimum resistance point thereby enabling a calcu- 
said sensing means and the corresponding amount lation of the residual stress in said sample. 
of tensional strain of said sample; 5. The device of claim 4 wherein said means for send- 
determining from said measurements of change of 35 ing an input signal to and receiving an input signal from 
resistance and corresponding strain of said sample said sensing means and measuring and recording the 
the point at which the resistance of said sensing change in resistance of said sensing means and the cor- 
means is at a minimum and the corresponding value responding amount of compressional and tensional 
and type of strain of said sample at that minimum strain of said sample comprises an impedance gain/- 
resistance point thereby enabling a calculation of 40 phase analyzer in conjunction with a data acquisition 
the residual stress in said sample. and analysis computer. 
6. The device of claim 5 wherein said means for im- 
parting a gradually increasing compressional force and 
a gradually increasing tensional force on said material 
10 a material sample comprising: 
a material sample comprising the steps of: 
2. The method of claim 1 wherein said steps of send- 
ing an input signal to and receiving an input signal from 
said sensing means and measuring and recording the 
change in resistance of said sensing means and the cor- 45 sample comprises a mechanical push-pull machine. 
responding amount of compressional and tensional + + . . .  
50 
55 
60 
65 


Method and apparatus for determination of material residual stress

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