Note: Descriptions are shown in the official language in which they were submitted.
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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.
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
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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.
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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
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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)
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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
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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.
DETAILED DESCRIPTI4N OF THE PREFERRED EMBODIMENTS
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
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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
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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
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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
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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
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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
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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.
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16
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
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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.
