GB2158242A - Method and apparatus for determining applied stress - Google Patents
Method and apparatus for determining applied stress Download PDFInfo
- Publication number
- GB2158242A GB2158242A GB08411313A GB8411313A GB2158242A GB 2158242 A GB2158242 A GB 2158242A GB 08411313 A GB08411313 A GB 08411313A GB 8411313 A GB8411313 A GB 8411313A GB 2158242 A GB2158242 A GB 2158242A
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- GB
- United Kingdom
- Prior art keywords
- sequences
- input signals
- strain gauge
- electrical
- principal stress
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/20—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress
- G01L1/22—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress using resistance strain gauges
- G01L1/225—Measuring circuits therefor
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Investigating Strength Of Materials By Application Of Mechanical Stress (AREA)
Abstract
A method and apparatus are provided for continuously determining and monitoring an induced dynamic stress field at a location on or in a structure, in which multiple strain gauge elements, suitably in the form of a rosette, are provided at the location. Sequences of electrical input signals (e.g. voltages or currents) are received at input means, these signals being derived from the elements and proportional to induced strains in the structure. Arithmetic and logic means are connected to the input means and operate on the sequences of input signals to derive sequences of electrical output signals (voltages or currents) proportional to stress field parameters at the location, e.g. maximum and minimum principal stresses and an angle identifying the direction of one of the principal stresses with respect to an axis of one of the strain gauge elements.
Description
SPECIFICATION
Method and apparatus for determining applies stress
This invention relates to the determination of a state of applied stress at a location on or in a structure.
It is known to bond a multiple element strain gauge rosette to a structure and to obtain static readings of the stress state of the structure by substituting individual values of mechanical strain derived from the gauge elements into well-known standard equations.
Problems arise, however, when it is required to obtain dynamic measurements of the stress state. When a three-element strain gauge rosette is employed, it is extremely time-consuming to convert continuous recordings of strain amplitude with time, obtained from each of the three elements, into the corresponding variation with time of the two principal stresses, e.g. maximum and minimum principal stresses, and of the direction of one of the principal stresses. Even for relatively short recordings of these mechanical strain values, complete and rapid processing can require the resources of a computer installation and this has meant that, in practice, only a relatively small number of points of maximum strain are generally analysed to provide an estimate of the range of the stress state of the structure.
it is often required to determine for a structure a parameter known in the art as fatigue life. For example, in the case of a welded structure and established method of determining this has involved counting strain cycles acting normal to the weld at a region known as the toe of the weld. However, recent indications are that fatigue life for such a structure is better estimated on the basis of counting cycles in respect of the maximum principal stress adjacent to the weld. Hitherto it has not been possible to carry out this test dynamically, but rather a method involving the postprocessing of total collected data has been used.
From one aspect the present invention provides apparatus for continuously determining and monitoring an induced dynamic stress field at a location on or in a structure, in which multiple strain gauge elements, eg in the form of a rosette, are provided at said location, said apparatus comprising: input means for receiving sequences of electrical input signals derived from said elements of said rosette, said input signals being proportional to induced strains in said structure as sensed by said elements; arithmetic means and, optionally, logic means, connected to said input means and adapted to operate on said sequences of electrical input signals to derive one or more sequences of electrical output signals proportional to one or more stress field parameters at said location; and output means for said one or more sequences of electrical output signals.
From another aspect the present invention provides a method for continuously determining and monitoring an induced dynamic stress field at a location on or in a structure, in which multiple strain gauge elements, eg in the form of a rosette, are provided at said location, said method comprising: receiving sequences of electrical input signals derived from said elements and proportional to induced strains in said structure as sensed by said elements; operating on said sequences of electrical input signals with arithmetic means and, optionally, logic means, to derive one or more sequences of electrical output signals proportional to one or more stress field parameters at said location; and providing an output of said one or more sequences of electrical output signals.
The said one or more stress fields parameters may suitably comprise one or more of: principal stress in a first direction; principal stress in a second direction normal to said first direction; an angle identifying said first or said second direction with respect to an axis of one of said strain gauge elements.
The said principal stress in a first direction may, for example, comprise maximum principal stress, with said principal stress in a second direction comprising minimum principal stress.
The said arithmetic means suitably operates to solve one or more equations relating principal stress to said induced strains and mechanical properties of material of said structure, e.g. Young's Modulus of Elasticity and Poisson's ratio. The said arithmetic means also suitably operates, in association with said logic means, to solve one or more equations relating the said angle to said induced strains.
The equations referred to above are wellknown in the art (see for example page 147 of the Strain Gauge Primer, by Perry and Lissner-Second Edition.)
In a particular embodiment, three said sequences of electrical input signals are derived from the said strain gauge elements and three said sequences of electrical output signals are provided at said output means. The three sequences of input signals may be derived from three strain gauge elements, eg the elements of a three element rosette, such as a rectangular rosette or a delta rosette. Alternatively, two of the three sequences of input signals may be derived from two strain gauge elements, eg the elements of a two-element rosette, suitably a tee rosette, with the third of said three sequences being derived by averaging the said two sequences of input signals.
Suitably the said electrical input signals comprise electrical voltages or currents proportional to said induced strains.
The said electrical output signals also suitably comprise electrical voltages or currents.
The said sequences of electrical output signals proportional to the one or more stress field parameters may be processed further, e.g. using well known rainflow counting or time-at-a-level techniques, and/or displayed, e.g. on a chart recorder, as required.
The invention is now described by way of example with reference to the accompanying drawings in which Figs. 1 and 2 are diagrammatic representations of two interconnected circuit parts of apparatus according to the invention for use in processing signals obtained from three strain gauge elements, eg a rectangular rosette of strain gauge elements.
In Figs. 1 and 2, reference numerals 1, 2, 3, 4 and 5 represents points of interconnection of the two circuit parts.
A three-element strain gauge rectangular rosette (not shown) of well known form is provided at a location on a structure (not shown) where it is required to continuously determine and monitor an induced dynamic stress field. Three sequences of electrical input signals E1, E2 and E3 are derived from the three elements of the rosette, the signals being proportional to induced strains in the structure as sensed by the elements. The three sequences of signals E" E2 and E3 are suitably in the form of sequences of electrical voltages and are derived and conditioned using well-known standard procedures.The three sequences of signals E,, E2 and E3 are applied to respective input terminals 6, 7 and 8 (Fig. 1) of apparatus according to the invention and are processed by operational amplifiers 9 and 10, unity gain differential amplifiers 11, 12, 1 3 and 14, and precision active bridge rectifier circuits 15, 16, 1 7 and 18, in association with electrical resistors denoted by
R, R1, R2 R/3 and R1 + R2
R,R2
At the point of interconnection 1 between the two circuit parts the resultant sequence of signals is proportional to the sum of the input sequences of signals E, and E3. The input signals E" E2 and E3 are bipolar, but unipolar signals are obtained at the points of interconnection 2, 3, 4 and 5 between the two circuit parts.At the point of interconnection 2, a sequence of signals is obtained representing
E2-E3. At point 3, a sequence of signals is obtained representing 2E2-E3-E1. At point 4, a sequence of signals is obtained representing
E,-E2. At point 5, a sequence of signals is obtained representing E1-ES. The sequences of signals at points 3 and 5 are then applied to a commercially available converter arrangement 19, (Fig. 2) such as Model 4302 Multifunction Converter manufactured by Burr
Brown Research Corporation, appropriately connected according to the the manufacturer's recommendation, such that it operates to provide, at a first output terminal 20, a sequence of voltage signals representing one half of the inverse tangent of the ratio of the signals at points 3 and 5.This output provides information of the angle between the direction of maximum principal stress at the selected location on or in the structure and an axis of an element of the rosette. For the latter, reference is generally made to the axis of one of the two orthogonally arranged elements in the rosette. In determining the angle, attention must be paid to the relative magnitudes of the input signals E" E2 and E3. An angle a calculated directly from the output at terminal 20 represents the true angle between the direction of maximum principal stress and the axis of the selected element provided that E1 is greater than E3. If E3 is greater than E" 90 degrees must be added to a. If, however,
E, = E3, a will be + 45 degrees, the sign (i.e.
positive or negative) being determined according to whether E2 is positive or negative respectively.
Logic circuit means can be readily applied to determine the angle on the basis of E" E2,
E3 and the output signals at terminal 20.
The sequences of signals at points 1, 2 and 4 are processed by: a commercially available converter circuit arrangement 21, e.g. Model 4302 Multifunction Converter manufactured by Burr-Brown Research Corporation, connected according to the manufacturer's recommendation and operating to produce the square root of the sum of the squares of two inputs thereto; operational amplifiers 22 and 23; a differential amplifier 24 whose gain is determined by a resistor denoted by R3; and resistors denoted by R, R4, R5, R/3 and
R4 + R5 R4Rs A sequence of voltage signals is produced at a second output terminal 25, representing a function (which takes into account Young's
Modulus of Elasticity and Poisson's Ratio for the material of the structure) of minimum principal stress at the selected location on the structure. A further sequence of voltage signals is produced at a third output terminal 26, representing a function (which takes into account Young's Modulus of Elasticity and Poisson's Ratio for the material of the structure) of maximum principal stress at the selected location on the structure. The resulting sequences of output signals at the terminals 20, 25 and 26 may, if desired, be displayed on a chart recorder to provide a record of the variation with time of the maximum and minimum principal stress and the determined angle. The sequences of output signals may also be fur ther processed using, for example, well-known rainflow counting techniques, suitable equipment for this purpose being described in this specification of published British Patent Application No. GB2110421A.
Claims (28)
1. Apparatus for continuously determining and monitoring an induced dynamic stress field at a location on or in a structure, in which multiple strain gauge elements are provided at said location, said apparatus comprising: input means for receiving sequences of electrical input signals derived from said elements, said input signals being proportiinal to induced strains in said structure as sensed by said elements; arithmetic means connected to said input means and adapted to operate on said sequences of electrical input signals to derive one or more sequence of electrical output signals proportional to one or more stress field parameters at said location; and output means for said one or more sequences of electrical output signals.
2. Apparatus according to claim 1 in which logic means are provided in addition to said arithmetic means,
3. Apparatus according to claim 1 or 2 in which said strain gauge elements are provided in the form of a rosette.
4. Apparatus according to claim 1, 2 or 3 in which the said one or more stress field parameters comprise one or more of: principal stress in a first direction; principal stress in a second direction normal to said first direction; an angle identifying said first or said second direction with respect to an axis of one of said strain gauge elements.
5. Apparatus according to claim 4 in which the said principal stress in a first direction comprises maximum principal stress, with said principal stress in a second direction comprising minimum principal stress.
6. Apparatus according to claim 4 or 5'in which the said arithemetic means operates to solve one or more equations relating principal stress to said induced strains and mechanical properties of material said structure.
7. Apparatus according to claim 6 in which said mechanical properties of material of said structure comprise Young's Modulus of Elasticity and Poisson's ratio.
8. Apparatus according to any of claims 4 to 7 in which the said arithmetic means operates, in association with said logic means, to solve one or more equations relating the said angle to said induced strains.
9. Apparatus according to any preceding claim in which three said sequences of electrical input signals are derived from the said strain gauge elements and three said sequences of electrical output signals are provided at said output means.
10. Apparatus according to claim 9 in which the three sequences of input signals are derived from three strain gauge elements.
11. Apparatus according to claim 9 in which two of said three sequences of input signals are derived from two strain gauge elements, with the third of said three sequences being derived by averaging the said two sequences of input signals.
12. Apparatus according to any preceding claim in which the said electrical input signals comprise electrical voltages or currents proportional to said induced strains.
1 3. Apparatus according to any preceding claim in which the said electrical output signals comprise electrical voltages or currents.
14. Apparatus for continuously determining and monitoring an induced dynamic stress field, constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.
1 5. A method for continuously determining and monitoring an induced dynamic stress field at a location on or in a structure, in which multiple strain gauge elements are provided at said location, said method comprising: receiving sequences of electrical input signals derived from said elements and proportional to induced strains in said structure as sensed by said elements; operating on said sequences of electrical input signals with arithmetic means to derive one or more sequences of electrical output signals proportional to one or more stress field parameters at said location; and providing an output of said one or more sequences of electrical output signals.
1 6. A method according to claim 1 5 in which logic means are provided in addition to said arithmetic means.
1 7. A method according to claim 1 5 or 1 6 in which said strain gauge elements are provided in the form of a rosette.
18. A method according to claim 1 5, 16 or 1 7 in which the said one or more stress field parameters comprise one or more of: principal stress in a first direction; principal stress in a second direction normal to said first direction; an angle identifying said first or said second direction with respect to an axis of one of said strain gauge elements.
19. A method according to claim 18 in which the said principal stress in a first direction comprises maximum principal stress, with said principal stress in a second direction comprising minimum principal stress.
20. A method according to claim 1 8 or 1 9 in which the said arithmetic means operates to solve one or more equations relating principal stress to said induced strains and mechanical properties of material of said structure.
21. A method according to claim 20 in which said mechanical properties of material of said structure comprise Young's Modulus of Elasticity and Poisson's ratio.
22. A method according to any of claims 18 to 21 in which the said arithmetic means operates, in association with said logic means, to solve one or more equations relating the said angle to said induced strains.
23. A method according to any of claims 1 5 to 22 in which three said sequences of electrical input signals are derived from the said strain gauge elements and three said sequences of electrical output signals are provided as said output.
24. A method according to claim 23 in which the three sequences of input signals are derived from three strain gauge elements.
25. A method according to claim 23 in which two of said three sequences of input signals are derived from two strain gauge elements, with the third of said three sequences being derived by averaging the said two sequences of input signals.
26. A method according to any preceding claim in which the said electrical input signals comprise electrical voltages or currents proportional to said induced strains.
27. A method according to any preceding claim in which said output signals comprise electrical voltages or currents.
28. A method for continuously determining and monitoring an induced dynamic stress field substantially as hereinbefore described with reference to the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08411313A GB2158242B (en) | 1984-05-03 | 1984-05-03 | Method and apparatus for determining applied stress |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08411313A GB2158242B (en) | 1984-05-03 | 1984-05-03 | Method and apparatus for determining applied stress |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8411313D0 GB8411313D0 (en) | 1984-06-06 |
| GB2158242A true GB2158242A (en) | 1985-11-06 |
| GB2158242B GB2158242B (en) | 1987-10-21 |
Family
ID=10560427
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08411313A Expired GB2158242B (en) | 1984-05-03 | 1984-05-03 | Method and apparatus for determining applied stress |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2158242B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2364127A (en) * | 2000-06-29 | 2002-01-16 | Univ London | Method and apparatus for monitoring structural fatigue |
-
1984
- 1984-05-03 GB GB08411313A patent/GB2158242B/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2364127A (en) * | 2000-06-29 | 2002-01-16 | Univ London | Method and apparatus for monitoring structural fatigue |
| GB2364127B (en) * | 2000-06-29 | 2004-08-25 | Univ London | Method and apparatus for monitoring structural fatigue and use |
| US6928881B2 (en) | 2000-06-29 | 2005-08-16 | University College London | Method and apparatus for monitoring structural fatigue and use |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2158242B (en) | 1987-10-21 |
| GB8411313D0 (en) | 1984-06-06 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 732 | Registration of transactions, instruments or events in the register (sect. 32/1977) | ||
| PCNP | Patent ceased through non-payment of renewal fee |