CA2000833A1 - Mechanical support for biaxial extensometers - Google Patents
Mechanical support for biaxial extensometersInfo
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- CA2000833A1 CA2000833A1 CA 2000833 CA2000833A CA2000833A1 CA 2000833 A1 CA2000833 A1 CA 2000833A1 CA 2000833 CA2000833 CA 2000833 CA 2000833 A CA2000833 A CA 2000833A CA 2000833 A1 CA2000833 A1 CA 2000833A1
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Abstract
ABSTRACT OF THE DISCLOSURE
A mechanical support is provided for mounting two axial strain sensing devices onto a plate test specimen subjected to a biaxial strain field. The preferred embodiment of the support comprises two sliding blocks mounted on guide rods in a generally rectangular frame. There are two of such frame assemblies. The two frames are attached together with the test specimen placed between them and with each pair of sliding blocks arranged in a direction perpendicular to the other. Each sliding block carries a cone-point screw for engaging the specimen and for establishing an initial gauge length. An axial extensometer mounted between each pair of blocks allows strain measurements along two orthogonal directions.
A mechanical support is provided for mounting two axial strain sensing devices onto a plate test specimen subjected to a biaxial strain field. The preferred embodiment of the support comprises two sliding blocks mounted on guide rods in a generally rectangular frame. There are two of such frame assemblies. The two frames are attached together with the test specimen placed between them and with each pair of sliding blocks arranged in a direction perpendicular to the other. Each sliding block carries a cone-point screw for engaging the specimen and for establishing an initial gauge length. An axial extensometer mounted between each pair of blocks allows strain measurements along two orthogonal directions.
Description
~)0(~833 MECHANICAL SUPPORT FOR BIAXIAL EXTENSOMETERS
BACKGROUND OF ~HE INVENTION
1. Fie~d of the invention:
The present invention relates to a mechanical support ~or mounting two axial strain sensors onto a material test specimen subjected to mechanical stresses. The support enables strains to be measured along two orthogonal directions of a plate test specimen. The strains measured along one direction are independent of those along the other direction.
BACKGROUND OF ~HE INVENTION
1. Fie~d of the invention:
The present invention relates to a mechanical support ~or mounting two axial strain sensors onto a material test specimen subjected to mechanical stresses. The support enables strains to be measured along two orthogonal directions of a plate test specimen. The strains measured along one direction are independent of those along the other direction.
2. Brief descri~tion of the prior art:
New structural materials such as composite materials are being increasingly developed for use in the aeronautical and the automobile industries for weight saving purposes. With this development has arisen the question of reliability of the final product. Ensuring reliable performance means being able to predict correctly whether a component will fail under typical operating conditions. This requires an accurate description of the stresses likely to be encountered in servic~, as well as a precise knowledge of the mechanical properties of the structural material of which the component is made.
Numerical analysis techniques enable a precise prediction of service stresses in structures even when the shape is complex. However, the use of ~0~0~3~3 numerical methods to predict the behaviour of structures in service or during forming operations necessitates a thorough knowledge of the mechanical behaviour of materials under multiaxial loading conditions which are closer to those encountered in practice. ~therwise, numerically exact calculations can result in erroneous predictions if the data concerning the mechanical properties of the material is incorrect.
The most common test method is the uniaxial tensile test. This consists of applying axial tensile loads along the axis of a specimen of circular or rectangular cross-section. The reliability of the data obtained from the uniaxial test for biaxial behaviour modelling depends on the nature of the material. For highly anisotropic materials, that is, materials whose mechanical properties depend on the direction of loading such as composite materials as well as metal alloys with hexagonal closed-pack structure, data from simple uniaxial tests cannot be used to model or predict the material's behaviour to service conditions. For such materials, biaxial testing is a necessity.
For instance, to model forming operations such as stamping, rolling, forging, etc, or to predict failure during forming operations, or to predict the mechanical behaviour of the structural body of airplanes and automobiles, biaxial stresses are involved. It is then only necessary to carry out biaxial tests on specimens in sheet`form.
Recently, several machines have been developed for conducting biaxial tests on flat cruciform specimens. With these machines, biaxial tensile or compressive loads are applied to the specimen along its two principal axes. During these tests, the significant parameters to be measured, at ambient temperature, are the stresses and strains in the central position of the cruciform specimen, that is, in its region of interest. Forces or stresses applied on the specimen are easily measured using load cells placed along the main axes of the specimen. On the other hand, measurement of strains in the central region of the specimen still presents important problems. The problems to be addressed are then the following:
- independent measurement of strains along two orthogonal directions;
- precise positioning of the axial strain sensors on the central portion of the specimen; and - the definition of an initial gauge length along each direction.
OBJECT OF THE INVENTION
An object of the present invention is therefore to provide a support for axial extensometers, as well as a multiaxial extensometer device which overcome the above discussed drawbacks of the prior art.
SUMMARY OF THE INVENTION
More specifically and in accordance with the present invention, there is provided a mechanical support for mounting axial extensometers onto a specimen made of a given material and subjected to a tensile and/or compressive test, to enable the extensometers to measure deformations of the specimen, comprising:
first and second frame means; 5 first guide means mounted on the first frame means, and second guide means mounted on the second frame means;
a first pair of specimen engaging units mounted onto the first guide means to slide along these first guide means, and a second pair of specimen engaging units mounted onto the second guide means to slide along these second guide means;
means for receiving a first axial extensometer measuring relative displacement between the units of the first pair, and a second axial extensometer measuring relative displacement between the units of the second pair; and means for attaching the first and second frame means together (a) with the test specimen placed between the first and second frame means, (b) ~(~0083~
with the first and the second guide means oriented along different, first and second directions, respectively, and (c) with the units of the first and second pairs all engaged with the specimen.
In operation, deformations of the specimen along the first and second directions slide the units of the first and second pairs on the first and the second guide means, respectively, to enable the first and second extensometers to measure independently the deformations along the first and second directions by measuring relative displacement between the units of the first and second pairs, respectively.
The present invention also relates to a multiaxial extensometer device for measuring deformations of a test specimen made of a given material and subjected to a tensile and/or compressive test, comprising: 0 first and second frame means;
first guide means mounted on the first frame means, and second guide means mounted on the second frame means;
a first pair of specimen engaging units mounted onto the first guide means, to slide along these first guide means, and a second pair of specimen engaging units mounted onto the second guide means to slide along these second guide means;
a first axial extensometer for measuring relative dislacement between the units of the first ;~000~:~3 pair, and a second axial extensometer for measuring relative dislacement between the units of the second pair; and - means for attaching the first and second frame means together (a) with the test specimen placed between said first and second frame means, (b) with the first and second guide means oriented along different, first and second directions, respectively, and (c) with the llnits of the first and second pairs all engaged with the specimen.
Again, in operation, deformations of the specimen along the first and second directions slide the units of the first and second pairs on the first and second guide means, respectively, to enable the first and second extensometers to measure independently the deformations along the first and second directions by measuring relative displacement between the units of the first and second pairs, respectively.
The present invention can be used to measure deformations of tsst specimens, in particular but not exclusively in sheet form, subjected to uniaxial and biaxial tensile tests, uniaxial and biaxial compressive tests, and fatigue tests, and to measure biaxial displacements of a general nature.
0~
The objects, advantages, and other features of the present invention will become more apparent upon reading the following non restrictive description of preferred embodiments thereof given in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Figure 1 shows a typical cruciform specimen which can be used with the support for extensometers in accordance with the invention;
Figure 2 is a perspective view of a support for extensometers in accordance with the invention;
Figure 3 illustrates a modification to the support of Figure 2 enabling the use of double-cantilever axial displacement gauges; and Figures 4a, 4b and 4c present the structureof ball bushings used in the manufacture of the support for extensometers as depicted in Figures 2 and 3.
A generally flat, cruciform specimen which can be tested in accordance with the invention, is identified in Figures la and lb by the reference numeral 1. The test specimen 1 is made of a given material, for example a composite material or a metal alloy. The central portion 2 of the specimen 1 is of t~ 3 reduced but even thickness. The specimen 1 is therefore a so called reduced-centre cruciform specimen. The thicker four arms such as 3 of specimen 1 are traversed by holes such as 4 through which the specimen 1 can be attached to a machine, for example, an electro-hydraulic machine, for testing cruciform specimens by applying axial loads thereon.
As can be appreciated, loads such as those identified by the arrows 5-8 can be applied to the specimen 1 along two orthogonal directions, namely along axes X-X and Y-Y.
Although the invention will be described hereinafter with reference to a biaxial, cruciform specimen stressed along two perpendicular directions, it should be remembered that the principle of the invention can also eventually be applied to other types of specimens.
As illustrated in Figure 2, a support for biaxial extensometers in accordance with the invention, generally identified by the reference numeral 10 comprises a pair of identical, generally square one-piece metal frames 11 and 12. A pair of parallel, cylindrical guide rods 13 and 14 oriented in the direction X-X are fixedly mounted between two parallel and opposite members of the frame 11, while another pair of parallel cylindrical guide rods 15 and 16, perpendicular to the rods 13 and 14, that is, oriented in the direction Y-Y, are fixedly mounted between two parallel and opposite members of the frame 1~. The rods 13-16 are advantageously made of 3.~
stainless steel, and fixedly mounted into holes such as 32, for example with set screws.
A pair of specimen engaging blocks 17 and 18, made of metal, slide along the rods 13 and 14, while another pair of specimen engaging blocks 19 and 20, also made of metal, slide along the guide rods 15 and 16. In order to mount the block 17 onto the guide rods 13 and 14, two straight and parallel bores 21 and 22, of larger diameter than the rods 13 and 14 are made through the block 17. A ball bushing such as 23 and 24 is pressure fitted at each end of the two bores 21 and 22. Four ball bushings are therefore associated with the block 17 to enable it to slide along the rods 13 and 14 with practically no friction. The three other blocks 18, 19 and 20 are mounted on their respective pair of guide rods in the same manner as the block 17.
The structure of each ball bushing such as 23 and 24 will now be described in detail with reference to Figures 4a, 4b and 4c.
More specifically, each ball bushing, for example bushing 23, comprises an outer steel sleeve 25 pressure fitted into thP bore 21. It also comprises at least three oblong circuits, such as 26, of freely revolving steel balls. Each oblong circuit is for~ed with a first straight side 27 in bearing contact with ~a) the inner surface of a longitudinal, inwardly embossed portion 25' of the sleeve 25, and (b) the guide rod 13. The block 17 is actually rolled freely along the rod 13 on the balls of the portion 27 of the circuit 26. Balls in the remainder ~0~08;~3 of the circuit are free to roll in clearance provided in the sleeve 25 (see channel 28 in Figure 4c). In the example of Figures 4a, 4b and 4c, five oblong circuits of steel balls, such as 26 are shown (see Figure 4c), which circuits are equally distributed along the periphery of the cross section of the bushing. This type of ball bushing is well known in the art and accordingly, it will not be further elaborated.
The function of the ball bushings such as 23 and 24 is obviously to enable sliding of the blocks 17 and 18 on the rods 13 and 14 and sliding of the blocks 19 and 20 on the rods 15 and 16 with practically no friction. Such a friction would of course influence the measurement of the deformations of the specimen 1.
Referring back to Figure 2 of the drawings, each block 17, 18, 19 or 20 has an inner surface formed with a groove rectangular in cross-section such as 29. The groove 29 receives an appropriately dimensioned, rectangular plate such as 30 which is fixedly screwed into the block such as 19. Each plate 30 bears cone-point screw such as 33 made of a hard material such as heat-treated steel and adapted to engage the specimen's central portion 2 of even thickness. The position of the two points 33 determines a reference length along a particular direction. This gauge length must of course be known to adequately determine the deformations in the specimen 1. This reference length can be adjusted differently in each of the two orthogonal directions X-X and Y-Y, in accordance with the requirements of ~()0(~3~
each given application. A plurality of holes such as 31 are provided in the plate 30 to enable selection between several, already adjusted reference lengths, by screwing the cone-points 33 on the blocks such as 19 into the appropriate threaded hole 31.
A set of four rods such as 34 have their two ends threaded. The lower, threaded end of each rod 34 engages a threaded bore such as 35 in the frame 12. The rod 34 also traverses a hole such as 36 in the frame 11. A knurled nut 37 engages the upper, threaded end of the rod 34 with a coil spring 38 between a shoulder 37' of the nut 37 and the frame 11. A flat washer 39 is interposed between the spring 38 and the frame 11. The specimen 1 outlined with dashed lines in Figure 2 is disposed and centered between the two frames 11 and 12 with the four rods 34 in the regions such as 40 defined by semicircular arcs such as 41 of the specimen 1 ~see Figure lb). The four nuts 37 are then screwed whereby the four springs 38 produce a force to apply the cone point screws 33 of blocks 17 and 18 on the upper face of the central region 2 of the specimen 1, and the cone-point screws 33 of blocks 19 and 20 on the underside of the region 2. As can be appreciated by one skilled in the art, the system 37, 38 and 39 of compression springs can be replaced by a system of tension springs interconnecting the two frames 11 and 12. The four springs 38 enable engaging the cone-point screws 33 onto the specimen 1 with a forceadjustable between a zero value and a maximum value depending on the design of the support (this maximum value depends in particular on the behaviour of the ~00~3 ball bushings when subjected to fatigue, that is, to prolonged cyclic stresses).
When the cone-point screws 33 are forced against both sides of the central portion 2 to thereby engage it, any deformation in this region 2 of the specimen 1 along the axis X-X will cause the blocks 17 and 18 to slide along the rods 13 and 14, while any deformation in the region 2 along the axis Y-Y will cause the blocks 19 and 20 to slide along the rods 15 and 16. The relative displacement of the blocks 17 and 18 is therefore directly representative of the deformation in the central portion 2 of the specimen 1 along the axis X-X, while the relative displacement of the blocks 19 and 20 is directly representative of the deformation along the axis Y-Y.
One can accordingly appreciate that, with the support for extensometers of Figure 2, relative displacement between the blocks 17 and 18 is completely independent from the relative displacement between the blocks 19 and 20, whereby measurement of the deformation in the specimen 1 along the axis X-X can be made independent from the measurement of the deformation along the axis Y-Y, by separately measuring the relative displacements between the blocks 17 and 18, and those between the blocks 19 and 20.
Before mounting the support of the invention on the specimen 1, L-shaped blocks such as 42 fixedly attached to the frame such as 11 through screws such as 43 are used in conjunction with removable pins such as 44 to prevent any relative displacement of the blocks 17-20 along the guide rods ~()008~
during mounting of the support for extensometers on the specimen 1. The L-shaped blocks also ensure that initial gauge-length in each frame is unaltered and accurate before the start of any test.
A~ter the support for extensometers is mounted onto the specimen 1, the so-obtained assembly is installed, by means of the holes such as 4 in the arms 3, onto a machine, for example of the electro-hydraulic type, to apply the forces 5-8 to the specimen l.
The pins 44 are then removed, the forces 5-8 (Figure lb) applied to the specimen 1, and the relative displacement between the blocks 17 and 18, and between the blocks 19 and 20 separately measured to thereby obtain the value of the deformations in the central region 2 of the specimen 1 along the axes X-X and Y-Y.
In accordance with a first alternative as depicted in Figure 2, LVDT's (Linear Variable Differential Transducer) such as 45 and 46 can be used as extensometers to measure the relative displacement between the blocks 17 and 18 and between the blocks l9 and 20, respectively. The LVDT 45 comprises a rod 47 with one end made of a magnetic material/ with another non-magnetic end fixedly attached to the block 18, and with its magnetic end sliding into a coil 48. Relative displacement between the blocks 17 and 18 causes sliding of the rod 47 into the coil 48 to generate a voltage across the latter coil. This voltage, which is representative of the relative displacement between ~0~ 33 the blocks 17 and 18, is accurately measured and processed to determine the amplitude of the relative displacement in question. Of course, the LVDT 46 is identical to the above described LVDT 45.
The miniature LVDTs 45 and 46 of Figure 2 are capable of measuring relative displacements with an infinite resolution in both the directions X-X and Y-Y. The maximum deformation which can be measured is limited by the LVTDs themselves and the initial reference length between the corresponding pairs of cone-point screws 33; the support of the invention can be appropriately dimension~d to receive LVDTs enabling measurements of larger amplitudes.
The operation and structure of LVTDs is believed to be otherwise well known in th~ art and accordingly will not be further elaborated.
In accordance with a second alternative as depicted in Figure 3, double-cantilever axial displacement gauges such as 49 are used as extensometers. The gauge 49 comprises two flat and flexible metallic arms 50 ad 51 of which the upper ends are respectively screwed at opposite ends of a bridging block 52 rectangular in cross section. At the lower end of the arms 50 and 51 are formed 90 V-shaped notches 52 and 53 on the outside surface of these flat arms, respectively.
In order to receive the double-cantilever gauge 49, each block 17 and 18 is modified as shown in Figure 3. It should be pointed out here that the blocks 19 and 20 ~Figure 2) are similarly modified to ~OOC3~33 receive another double cantilever axial displacement gauge.
As illustrated in Figure 3, the upper surfaces of the blocks 17 and 18 are respectively formed with a groove 54,54' rectangular in cross section. The blocks such as 42 (Figure 2) are also modified to adapt to these grooves 54 and 54'. The grooves 54,54' receive the blocks 55,55' notched to define a ridge 56,56' structured to receive the notch 52, 53 of the arm 50, 51. For that purpose, the ridges 56 and 56' of the two blocks 55 and 55' face each other, while the latter blocks 55 and 55' are mounted in the grooves 54 and 54' at the confronting ends of the blocks 17 and 18.
On the outside face of each flat arm 50 and 51 of the gauge 49 is conveniently bonded strain resistive gauges such as 57, of the foil type, while other strain resistive gauges such as 58, also of the foil type are conveniently bonded to the inside face of each flat arm 50 and 51 by means of an appropriate glue.
Any relative displacement between the blocks 17 and 18, and between the blocks 19 and 20, results in the bending of the arms 50 and 51 and variations in the resistance of the strain gauges such as 57 and 58 is measured to determine the amplitude of this relative displacement and accordingly of the deformations in the specimen 1 in the directions X-X and Y-Y.
~000~3;~
The alternative of Figure 3 allow the axial extensometers to disengage automatically from the blocks such as 55,55' without any damage upon rupture of the specimen 1. Also, the double-cantilever axial displacement gauges enable measurement of deformations of the order of the micron(10~5 m), as well as very large deformations under static and cyclic loading (forcas 5, 6, 7 and 8 in Figure 13.
o The support for biaxial extensometers in accordance with the invention presents the following advantages:
- the deformations measured along the two orthogonal axes X-X and Y-Y are completely independent from one another;
- its versatility enables measurement of biaxial deformations in a uniaxial specimen as well as in a cruciform specimen without any modification of the support to pass from one type of specimen to the other; and - the reference lengths, that is the distance between each pair of cone-point screws such as 33 can be easily selected by the user in accordance with the intended application and can even be different in the two directions X-X and Y-Y;
generally, the extensometer devices presently available on the market offer only one fixed reference length.
Although the present invention has been described hereinabove by means of prefer-red 200~33 embodiments thereof, such preferred embodiments can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.
New structural materials such as composite materials are being increasingly developed for use in the aeronautical and the automobile industries for weight saving purposes. With this development has arisen the question of reliability of the final product. Ensuring reliable performance means being able to predict correctly whether a component will fail under typical operating conditions. This requires an accurate description of the stresses likely to be encountered in servic~, as well as a precise knowledge of the mechanical properties of the structural material of which the component is made.
Numerical analysis techniques enable a precise prediction of service stresses in structures even when the shape is complex. However, the use of ~0~0~3~3 numerical methods to predict the behaviour of structures in service or during forming operations necessitates a thorough knowledge of the mechanical behaviour of materials under multiaxial loading conditions which are closer to those encountered in practice. ~therwise, numerically exact calculations can result in erroneous predictions if the data concerning the mechanical properties of the material is incorrect.
The most common test method is the uniaxial tensile test. This consists of applying axial tensile loads along the axis of a specimen of circular or rectangular cross-section. The reliability of the data obtained from the uniaxial test for biaxial behaviour modelling depends on the nature of the material. For highly anisotropic materials, that is, materials whose mechanical properties depend on the direction of loading such as composite materials as well as metal alloys with hexagonal closed-pack structure, data from simple uniaxial tests cannot be used to model or predict the material's behaviour to service conditions. For such materials, biaxial testing is a necessity.
For instance, to model forming operations such as stamping, rolling, forging, etc, or to predict failure during forming operations, or to predict the mechanical behaviour of the structural body of airplanes and automobiles, biaxial stresses are involved. It is then only necessary to carry out biaxial tests on specimens in sheet`form.
Recently, several machines have been developed for conducting biaxial tests on flat cruciform specimens. With these machines, biaxial tensile or compressive loads are applied to the specimen along its two principal axes. During these tests, the significant parameters to be measured, at ambient temperature, are the stresses and strains in the central position of the cruciform specimen, that is, in its region of interest. Forces or stresses applied on the specimen are easily measured using load cells placed along the main axes of the specimen. On the other hand, measurement of strains in the central region of the specimen still presents important problems. The problems to be addressed are then the following:
- independent measurement of strains along two orthogonal directions;
- precise positioning of the axial strain sensors on the central portion of the specimen; and - the definition of an initial gauge length along each direction.
OBJECT OF THE INVENTION
An object of the present invention is therefore to provide a support for axial extensometers, as well as a multiaxial extensometer device which overcome the above discussed drawbacks of the prior art.
SUMMARY OF THE INVENTION
More specifically and in accordance with the present invention, there is provided a mechanical support for mounting axial extensometers onto a specimen made of a given material and subjected to a tensile and/or compressive test, to enable the extensometers to measure deformations of the specimen, comprising:
first and second frame means; 5 first guide means mounted on the first frame means, and second guide means mounted on the second frame means;
a first pair of specimen engaging units mounted onto the first guide means to slide along these first guide means, and a second pair of specimen engaging units mounted onto the second guide means to slide along these second guide means;
means for receiving a first axial extensometer measuring relative displacement between the units of the first pair, and a second axial extensometer measuring relative displacement between the units of the second pair; and means for attaching the first and second frame means together (a) with the test specimen placed between the first and second frame means, (b) ~(~0083~
with the first and the second guide means oriented along different, first and second directions, respectively, and (c) with the units of the first and second pairs all engaged with the specimen.
In operation, deformations of the specimen along the first and second directions slide the units of the first and second pairs on the first and the second guide means, respectively, to enable the first and second extensometers to measure independently the deformations along the first and second directions by measuring relative displacement between the units of the first and second pairs, respectively.
The present invention also relates to a multiaxial extensometer device for measuring deformations of a test specimen made of a given material and subjected to a tensile and/or compressive test, comprising: 0 first and second frame means;
first guide means mounted on the first frame means, and second guide means mounted on the second frame means;
a first pair of specimen engaging units mounted onto the first guide means, to slide along these first guide means, and a second pair of specimen engaging units mounted onto the second guide means to slide along these second guide means;
a first axial extensometer for measuring relative dislacement between the units of the first ;~000~:~3 pair, and a second axial extensometer for measuring relative dislacement between the units of the second pair; and - means for attaching the first and second frame means together (a) with the test specimen placed between said first and second frame means, (b) with the first and second guide means oriented along different, first and second directions, respectively, and (c) with the llnits of the first and second pairs all engaged with the specimen.
Again, in operation, deformations of the specimen along the first and second directions slide the units of the first and second pairs on the first and second guide means, respectively, to enable the first and second extensometers to measure independently the deformations along the first and second directions by measuring relative displacement between the units of the first and second pairs, respectively.
The present invention can be used to measure deformations of tsst specimens, in particular but not exclusively in sheet form, subjected to uniaxial and biaxial tensile tests, uniaxial and biaxial compressive tests, and fatigue tests, and to measure biaxial displacements of a general nature.
0~
The objects, advantages, and other features of the present invention will become more apparent upon reading the following non restrictive description of preferred embodiments thereof given in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Figure 1 shows a typical cruciform specimen which can be used with the support for extensometers in accordance with the invention;
Figure 2 is a perspective view of a support for extensometers in accordance with the invention;
Figure 3 illustrates a modification to the support of Figure 2 enabling the use of double-cantilever axial displacement gauges; and Figures 4a, 4b and 4c present the structureof ball bushings used in the manufacture of the support for extensometers as depicted in Figures 2 and 3.
A generally flat, cruciform specimen which can be tested in accordance with the invention, is identified in Figures la and lb by the reference numeral 1. The test specimen 1 is made of a given material, for example a composite material or a metal alloy. The central portion 2 of the specimen 1 is of t~ 3 reduced but even thickness. The specimen 1 is therefore a so called reduced-centre cruciform specimen. The thicker four arms such as 3 of specimen 1 are traversed by holes such as 4 through which the specimen 1 can be attached to a machine, for example, an electro-hydraulic machine, for testing cruciform specimens by applying axial loads thereon.
As can be appreciated, loads such as those identified by the arrows 5-8 can be applied to the specimen 1 along two orthogonal directions, namely along axes X-X and Y-Y.
Although the invention will be described hereinafter with reference to a biaxial, cruciform specimen stressed along two perpendicular directions, it should be remembered that the principle of the invention can also eventually be applied to other types of specimens.
As illustrated in Figure 2, a support for biaxial extensometers in accordance with the invention, generally identified by the reference numeral 10 comprises a pair of identical, generally square one-piece metal frames 11 and 12. A pair of parallel, cylindrical guide rods 13 and 14 oriented in the direction X-X are fixedly mounted between two parallel and opposite members of the frame 11, while another pair of parallel cylindrical guide rods 15 and 16, perpendicular to the rods 13 and 14, that is, oriented in the direction Y-Y, are fixedly mounted between two parallel and opposite members of the frame 1~. The rods 13-16 are advantageously made of 3.~
stainless steel, and fixedly mounted into holes such as 32, for example with set screws.
A pair of specimen engaging blocks 17 and 18, made of metal, slide along the rods 13 and 14, while another pair of specimen engaging blocks 19 and 20, also made of metal, slide along the guide rods 15 and 16. In order to mount the block 17 onto the guide rods 13 and 14, two straight and parallel bores 21 and 22, of larger diameter than the rods 13 and 14 are made through the block 17. A ball bushing such as 23 and 24 is pressure fitted at each end of the two bores 21 and 22. Four ball bushings are therefore associated with the block 17 to enable it to slide along the rods 13 and 14 with practically no friction. The three other blocks 18, 19 and 20 are mounted on their respective pair of guide rods in the same manner as the block 17.
The structure of each ball bushing such as 23 and 24 will now be described in detail with reference to Figures 4a, 4b and 4c.
More specifically, each ball bushing, for example bushing 23, comprises an outer steel sleeve 25 pressure fitted into thP bore 21. It also comprises at least three oblong circuits, such as 26, of freely revolving steel balls. Each oblong circuit is for~ed with a first straight side 27 in bearing contact with ~a) the inner surface of a longitudinal, inwardly embossed portion 25' of the sleeve 25, and (b) the guide rod 13. The block 17 is actually rolled freely along the rod 13 on the balls of the portion 27 of the circuit 26. Balls in the remainder ~0~08;~3 of the circuit are free to roll in clearance provided in the sleeve 25 (see channel 28 in Figure 4c). In the example of Figures 4a, 4b and 4c, five oblong circuits of steel balls, such as 26 are shown (see Figure 4c), which circuits are equally distributed along the periphery of the cross section of the bushing. This type of ball bushing is well known in the art and accordingly, it will not be further elaborated.
The function of the ball bushings such as 23 and 24 is obviously to enable sliding of the blocks 17 and 18 on the rods 13 and 14 and sliding of the blocks 19 and 20 on the rods 15 and 16 with practically no friction. Such a friction would of course influence the measurement of the deformations of the specimen 1.
Referring back to Figure 2 of the drawings, each block 17, 18, 19 or 20 has an inner surface formed with a groove rectangular in cross-section such as 29. The groove 29 receives an appropriately dimensioned, rectangular plate such as 30 which is fixedly screwed into the block such as 19. Each plate 30 bears cone-point screw such as 33 made of a hard material such as heat-treated steel and adapted to engage the specimen's central portion 2 of even thickness. The position of the two points 33 determines a reference length along a particular direction. This gauge length must of course be known to adequately determine the deformations in the specimen 1. This reference length can be adjusted differently in each of the two orthogonal directions X-X and Y-Y, in accordance with the requirements of ~()0(~3~
each given application. A plurality of holes such as 31 are provided in the plate 30 to enable selection between several, already adjusted reference lengths, by screwing the cone-points 33 on the blocks such as 19 into the appropriate threaded hole 31.
A set of four rods such as 34 have their two ends threaded. The lower, threaded end of each rod 34 engages a threaded bore such as 35 in the frame 12. The rod 34 also traverses a hole such as 36 in the frame 11. A knurled nut 37 engages the upper, threaded end of the rod 34 with a coil spring 38 between a shoulder 37' of the nut 37 and the frame 11. A flat washer 39 is interposed between the spring 38 and the frame 11. The specimen 1 outlined with dashed lines in Figure 2 is disposed and centered between the two frames 11 and 12 with the four rods 34 in the regions such as 40 defined by semicircular arcs such as 41 of the specimen 1 ~see Figure lb). The four nuts 37 are then screwed whereby the four springs 38 produce a force to apply the cone point screws 33 of blocks 17 and 18 on the upper face of the central region 2 of the specimen 1, and the cone-point screws 33 of blocks 19 and 20 on the underside of the region 2. As can be appreciated by one skilled in the art, the system 37, 38 and 39 of compression springs can be replaced by a system of tension springs interconnecting the two frames 11 and 12. The four springs 38 enable engaging the cone-point screws 33 onto the specimen 1 with a forceadjustable between a zero value and a maximum value depending on the design of the support (this maximum value depends in particular on the behaviour of the ~00~3 ball bushings when subjected to fatigue, that is, to prolonged cyclic stresses).
When the cone-point screws 33 are forced against both sides of the central portion 2 to thereby engage it, any deformation in this region 2 of the specimen 1 along the axis X-X will cause the blocks 17 and 18 to slide along the rods 13 and 14, while any deformation in the region 2 along the axis Y-Y will cause the blocks 19 and 20 to slide along the rods 15 and 16. The relative displacement of the blocks 17 and 18 is therefore directly representative of the deformation in the central portion 2 of the specimen 1 along the axis X-X, while the relative displacement of the blocks 19 and 20 is directly representative of the deformation along the axis Y-Y.
One can accordingly appreciate that, with the support for extensometers of Figure 2, relative displacement between the blocks 17 and 18 is completely independent from the relative displacement between the blocks 19 and 20, whereby measurement of the deformation in the specimen 1 along the axis X-X can be made independent from the measurement of the deformation along the axis Y-Y, by separately measuring the relative displacements between the blocks 17 and 18, and those between the blocks 19 and 20.
Before mounting the support of the invention on the specimen 1, L-shaped blocks such as 42 fixedly attached to the frame such as 11 through screws such as 43 are used in conjunction with removable pins such as 44 to prevent any relative displacement of the blocks 17-20 along the guide rods ~()008~
during mounting of the support for extensometers on the specimen 1. The L-shaped blocks also ensure that initial gauge-length in each frame is unaltered and accurate before the start of any test.
A~ter the support for extensometers is mounted onto the specimen 1, the so-obtained assembly is installed, by means of the holes such as 4 in the arms 3, onto a machine, for example of the electro-hydraulic type, to apply the forces 5-8 to the specimen l.
The pins 44 are then removed, the forces 5-8 (Figure lb) applied to the specimen 1, and the relative displacement between the blocks 17 and 18, and between the blocks 19 and 20 separately measured to thereby obtain the value of the deformations in the central region 2 of the specimen 1 along the axes X-X and Y-Y.
In accordance with a first alternative as depicted in Figure 2, LVDT's (Linear Variable Differential Transducer) such as 45 and 46 can be used as extensometers to measure the relative displacement between the blocks 17 and 18 and between the blocks l9 and 20, respectively. The LVDT 45 comprises a rod 47 with one end made of a magnetic material/ with another non-magnetic end fixedly attached to the block 18, and with its magnetic end sliding into a coil 48. Relative displacement between the blocks 17 and 18 causes sliding of the rod 47 into the coil 48 to generate a voltage across the latter coil. This voltage, which is representative of the relative displacement between ~0~ 33 the blocks 17 and 18, is accurately measured and processed to determine the amplitude of the relative displacement in question. Of course, the LVDT 46 is identical to the above described LVDT 45.
The miniature LVDTs 45 and 46 of Figure 2 are capable of measuring relative displacements with an infinite resolution in both the directions X-X and Y-Y. The maximum deformation which can be measured is limited by the LVTDs themselves and the initial reference length between the corresponding pairs of cone-point screws 33; the support of the invention can be appropriately dimension~d to receive LVDTs enabling measurements of larger amplitudes.
The operation and structure of LVTDs is believed to be otherwise well known in th~ art and accordingly will not be further elaborated.
In accordance with a second alternative as depicted in Figure 3, double-cantilever axial displacement gauges such as 49 are used as extensometers. The gauge 49 comprises two flat and flexible metallic arms 50 ad 51 of which the upper ends are respectively screwed at opposite ends of a bridging block 52 rectangular in cross section. At the lower end of the arms 50 and 51 are formed 90 V-shaped notches 52 and 53 on the outside surface of these flat arms, respectively.
In order to receive the double-cantilever gauge 49, each block 17 and 18 is modified as shown in Figure 3. It should be pointed out here that the blocks 19 and 20 ~Figure 2) are similarly modified to ~OOC3~33 receive another double cantilever axial displacement gauge.
As illustrated in Figure 3, the upper surfaces of the blocks 17 and 18 are respectively formed with a groove 54,54' rectangular in cross section. The blocks such as 42 (Figure 2) are also modified to adapt to these grooves 54 and 54'. The grooves 54,54' receive the blocks 55,55' notched to define a ridge 56,56' structured to receive the notch 52, 53 of the arm 50, 51. For that purpose, the ridges 56 and 56' of the two blocks 55 and 55' face each other, while the latter blocks 55 and 55' are mounted in the grooves 54 and 54' at the confronting ends of the blocks 17 and 18.
On the outside face of each flat arm 50 and 51 of the gauge 49 is conveniently bonded strain resistive gauges such as 57, of the foil type, while other strain resistive gauges such as 58, also of the foil type are conveniently bonded to the inside face of each flat arm 50 and 51 by means of an appropriate glue.
Any relative displacement between the blocks 17 and 18, and between the blocks 19 and 20, results in the bending of the arms 50 and 51 and variations in the resistance of the strain gauges such as 57 and 58 is measured to determine the amplitude of this relative displacement and accordingly of the deformations in the specimen 1 in the directions X-X and Y-Y.
~000~3;~
The alternative of Figure 3 allow the axial extensometers to disengage automatically from the blocks such as 55,55' without any damage upon rupture of the specimen 1. Also, the double-cantilever axial displacement gauges enable measurement of deformations of the order of the micron(10~5 m), as well as very large deformations under static and cyclic loading (forcas 5, 6, 7 and 8 in Figure 13.
o The support for biaxial extensometers in accordance with the invention presents the following advantages:
- the deformations measured along the two orthogonal axes X-X and Y-Y are completely independent from one another;
- its versatility enables measurement of biaxial deformations in a uniaxial specimen as well as in a cruciform specimen without any modification of the support to pass from one type of specimen to the other; and - the reference lengths, that is the distance between each pair of cone-point screws such as 33 can be easily selected by the user in accordance with the intended application and can even be different in the two directions X-X and Y-Y;
generally, the extensometer devices presently available on the market offer only one fixed reference length.
Although the present invention has been described hereinabove by means of prefer-red 200~33 embodiments thereof, such preferred embodiments can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.
Claims (17)
1. A mechanical support for mounting axial extensometers onto a test specimen made of a given material, and subjected to a tensile and/or compressive test, to enable said extensometers to measure deformations of the specimen, comprising:
first and second frame means;
first guide means mounted on the first frame means, and second guide means mounted on the second frame means;
a first pair of specimen engaging units mounted onto the first guide means to slide along said first guide means, and a second pair of specimen engaging units mounted onto the second guide means to slide along said second guide means;
means for receiving a first axial extensometer measuring relative displacement between said units of the first pair, and a second axial extensometer measuring relative displacement between said units of the second pair; and means for attaching the first and second frame means together (a) with the test specimen placed between said first and second frame means, (b) with said first and second guide means oriented along different, first and second directions, respectively, and (c) with said units of the first and second pairs all engaged with the specimen;
whereby, in operation, deformations of the specimen along the first and second directions slide said units of the first and second pairs on the first and second guide means, respectively, to enable the first and second extensometers to measure independently said deformations along the first and second directions by measuring relative displacement between the units of the first and second pairs, respectively.
first and second frame means;
first guide means mounted on the first frame means, and second guide means mounted on the second frame means;
a first pair of specimen engaging units mounted onto the first guide means to slide along said first guide means, and a second pair of specimen engaging units mounted onto the second guide means to slide along said second guide means;
means for receiving a first axial extensometer measuring relative displacement between said units of the first pair, and a second axial extensometer measuring relative displacement between said units of the second pair; and means for attaching the first and second frame means together (a) with the test specimen placed between said first and second frame means, (b) with said first and second guide means oriented along different, first and second directions, respectively, and (c) with said units of the first and second pairs all engaged with the specimen;
whereby, in operation, deformations of the specimen along the first and second directions slide said units of the first and second pairs on the first and second guide means, respectively, to enable the first and second extensometers to measure independently said deformations along the first and second directions by measuring relative displacement between the units of the first and second pairs, respectively.
2. The support of claim 1, in which said extensometer receiving means comprises means for bridging the said units of the first pair with the first extensometer, and means for bridging the said units of the second pair with said second extensometer.
3. The support of claim 1, in which said first and second directions are orthogonal with respect to each other.
4. The support of claim 1, in which:
said first and second frame means each comprise two opposite and spaced-apart frame members;
said first guide means comprises two linear and parallel guide rods fixedly mounted between the spaced apart members of the first frame means; and said second guide means comprises two linear and parallel guide rods fixedly mounted between the spaced apart members of the second frame means.
said first and second frame means each comprise two opposite and spaced-apart frame members;
said first guide means comprises two linear and parallel guide rods fixedly mounted between the spaced apart members of the first frame means; and said second guide means comprises two linear and parallel guide rods fixedly mounted between the spaced apart members of the second frame means.
5. The support of claim 4, wherein:
all of said guide rods are cylindrical; and each of said specimen engaging units of the first and second pairs comprises a solid block formed with two cylindrical and parallel bores therein to receive two of said linear and parallel guide rods on which said block is mounted.
all of said guide rods are cylindrical; and each of said specimen engaging units of the first and second pairs comprises a solid block formed with two cylindrical and parallel bores therein to receive two of said linear and parallel guide rods on which said block is mounted.
6. The support of claim 5, in which said two cylindrical and parallel bores comprise respective inner surfaces, said support further comprising means interposed between the inner surfaces of said two bores and the two guide rods for reducing friction between the said inner surfaces and guide rods upon sliding of the block onto the latter rods.
7. The support of claim 6, in which said friction reducing means comprises freely revolving metal balls.
8. The support of claim 5, wherein each of said specimen engaging units of the first and second pairs further comprises a point made of hard material, fixedly secured to the corresponding solid block, and protruding from the latter block toward the specimen.
9. The support of claim 8, further comprising means for adjusting the position of the point on each solid block.
10. The support of claim 8, in which the specimen is a reduced-centre cruciform specimen with a central portion of even thickness, and in which said attaching means comprises means for forcing the points of the blocks against the central portion of the specimen to thereby engage said points with said central portion of the specimen.
11. The support of claim 1, further comprising means for blocking the specimen engaging units on the first and second guide means during attachment of the first and second frame means together.
12. The support of claim 1, in which:
each of said specimen engaging units comprises a point made of hard material and protruding from the unit in the direction of the said specimen; and said attaching means comprises means for forcing the points of said units against the specimen to thereby engage said points with the specimen.
each of said specimen engaging units comprises a point made of hard material and protruding from the unit in the direction of the said specimen; and said attaching means comprises means for forcing the points of said units against the specimen to thereby engage said points with the specimen.
13. The support of claim 12, wherein said forcing means comprises a plurality of rod, spring and nut assemblies each including:
a rod having a first end fixedly attached to one of the first and second frame means and a free threaded end, said rod passing through a bore in the other of said first and second frame means;
a nut engaged with the free threaded end of the rod; and a coil spring positioned on the rod between said nut and said other frame means.
a rod having a first end fixedly attached to one of the first and second frame means and a free threaded end, said rod passing through a bore in the other of said first and second frame means;
a nut engaged with the free threaded end of the rod; and a coil spring positioned on the rod between said nut and said other frame means.
14. The support of claim 2, wherein said first and second extensometers comprise, respectively, a first linear variable differential transducer bridging the two units of the first pair, and a second linear variable differential transducer bridging the two units of the second pair.
15. The support of claim 2, wherein said first and second extensometers comprise, respectively, a first double-cantilever axial displacement gauge bridging the two units of the first pair, and a second double-cantilever axial displacement gauge bridging the two units of the second pair.
16. The support of claim 15, in which each of said first and second displacement gauges comprises two generally flat arms each with a free end provided with a notch therein, and wherein said extensometer receiving means comprise each specimen engaging unit formed with a ridge structured to receive the notch in a corresponding one of the arms of the first and second displacement gauges.
17. A multiaxial extensometer device for measuring deformations of a test specimen made of a given material and subjected to a tensile and/or compressive test, comprising:
first and second frame means;
first guide means mounted on the first frame means, and second guide means mounted on the second frame means;
a first pair of specimen engaging units mounted onto the first guide means to slide along said first guide means, and a second pair of specimen engaging units mounted onto the second guide means to slide along said second guide means;
a first axial extensometer for measuring relative displacement between said units of the first pair, and a second axial extensometer for measuring relative displacement between said units of the second pair; and means for attaching the first and second frame means together (a) with the test specimen placed between said first and second frame means, (b) with said first and second guide means oriented along different, first and second directions, respectively, and (c) with said units of the first and second pairs all engaged with the specimen;
whereby, in operation, deformations of the specimen along the first and second directions slide said units of the first and second pairs on the first and second guide means, respectively, to enable the first and second extensometers to measure independently said deformations along the first and second directions by measuring relative displacement between the units of the first and second pairs, respectively.
first and second frame means;
first guide means mounted on the first frame means, and second guide means mounted on the second frame means;
a first pair of specimen engaging units mounted onto the first guide means to slide along said first guide means, and a second pair of specimen engaging units mounted onto the second guide means to slide along said second guide means;
a first axial extensometer for measuring relative displacement between said units of the first pair, and a second axial extensometer for measuring relative displacement between said units of the second pair; and means for attaching the first and second frame means together (a) with the test specimen placed between said first and second frame means, (b) with said first and second guide means oriented along different, first and second directions, respectively, and (c) with said units of the first and second pairs all engaged with the specimen;
whereby, in operation, deformations of the specimen along the first and second directions slide said units of the first and second pairs on the first and second guide means, respectively, to enable the first and second extensometers to measure independently said deformations along the first and second directions by measuring relative displacement between the units of the first and second pairs, respectively.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2000833 CA2000833A1 (en) | 1989-10-17 | 1989-10-17 | Mechanical support for biaxial extensometers |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2000833 CA2000833A1 (en) | 1989-10-17 | 1989-10-17 | Mechanical support for biaxial extensometers |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2000833A1 true CA2000833A1 (en) | 1991-04-17 |
Family
ID=4143337
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2000833 Abandoned CA2000833A1 (en) | 1989-10-17 | 1989-10-17 | Mechanical support for biaxial extensometers |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2000833A1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN104534969A (en) * | 2014-11-24 | 2015-04-22 | 重庆光荣摩托车配件有限公司 | Combined deformation detection tool for front mudguards of motorcycles |
| US20190072467A1 (en) * | 2016-03-28 | 2019-03-07 | Mitsubishi Heavy Industries, Ltd. | Biaxial load test specimen, biaxial load test apparatus, and biaxial load test method |
-
1989
- 1989-10-17 CA CA 2000833 patent/CA2000833A1/en not_active Abandoned
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN104534969A (en) * | 2014-11-24 | 2015-04-22 | 重庆光荣摩托车配件有限公司 | Combined deformation detection tool for front mudguards of motorcycles |
| US20190072467A1 (en) * | 2016-03-28 | 2019-03-07 | Mitsubishi Heavy Industries, Ltd. | Biaxial load test specimen, biaxial load test apparatus, and biaxial load test method |
| US10859478B2 (en) * | 2016-03-28 | 2020-12-08 | Mitsubishi Heavy Industries, Ltd. | Biaxial load test specimen, biaxial load test apparatus, and biaxial load test method |
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