ES2364212A1 - Apparatus and procedure for measuring deformation in a traction test. (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Apparatus for measuring the tensile deformation of a test piece, comprising: an articulated arm configured to be attached to a testing machine, wherein said articulated arm comprises at least two rigid sections (1, 2), and anchoring means (8); a guide (10) mounted at one end of the second section (2) of the articulated arm. On said guide (10) two frames (11, 12) are mounted in the shape of a "U", where each one of said frames (11, 12) carries a respective optical sensor (15, 13), each of which (15, 13) has the capacity to move along said guide (10); where inside the "U" of said supports (11, 12) a test tube (17) is housed, including two targets (16, 14) attachable to said test tube (17), said targets (16, 14) being configured to measure the deformation of said specimen (17) by measuring by said optical sensors (15, 13) the displacement of said targets (16, 14).

Description

Apparatus and procedure for measuring deformation in a tensile test.

Field of the Invention

The present invention relates to procedures and equipment (devices or extensometers) to measure the deformation in a tensile test.

Background of the invention

For measuring the deformation of a Tensile specimen currently used extensometers that can be grouped into four types:

one.

Contact Extensometry of resistive character . Its operation is based on the use of strain gauges. The main drawbacks are those listed below:

\ circ

The measurement bases are relatively small

\ circ

The tours are also relatively small

\ circ

Nominal bases can be modified during mounting of the extensometer so it will be A subsequent correction is necessary.

\ circ

They can't work at high temperatures if they are not equipped with special equipment refrigeration.

\ circ

The extensometer comes into contact with the specimen so it can suffer damage at break if this is violent

\ circ

The contact can cause concentration of tensions can precipitate the appearance of break.

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2.

Inductive contact extensometry . Its operation is based on the use of LVDT comparators. The main drawbacks are those listed below:

\ circ

Nominal bases can be modified during mounting of the extensometer so it will be A subsequent correction is necessary.

\ circ

They can't work at temperatures high.

\ circ

The extensometer comes into contact with the specimen so it can suffer damage at break if this is violent

\ circ

The contact can cause concentration of tensions can precipitate the appearance of break.

\ circ

The weight of the set is relatively high, so the pressure on the supports can accentuate the notch effect.

\ circ

Precision for small deformations decreases compared to previous models.

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3.

Video-extensometer : extensometry based on images with one or several fixed or mobile cameras. The main drawbacks are those listed below:

\ circ

On some computers it can be A previous calibration is necessary.

\ circ

Optimal conditions are needed of lighting.

\ circ

The device is very bulky and It makes the task of exchanging between several testing machines difficult. Some models are associated with a specific one and cannot dismount

\ circ

The complexity of use is greater, since which usually has its own control software associated.

\ circ

Some models have not resolved the measure of the measurement basis so this may be necessary pre-trial operation.

\ circ

They can't work at high temperatures

\ circ

Accuracies for deformations Small decrease.

\ circ

It is very expensive.

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Four.

Laser extensometer: extensometry based on the use of reflection lasers. The main drawbacks are those listed below:

\ circ

The device is very bulky and hinders the task of exchanging between several machines test.

\ circ

The complexity of use is great, since it usually has its own control software associated.

\ circ

Some models have not resolved the measure of the measurement basis so this may be necessary pre-trial operation.

\ circ

The identification of the basis of measurement is made based on reflective elements that stick on the specimen

\ circ

They can't work at high temperatures

\ circ

Accuracies for deformations Small decrease.

\ circ

It is very expensive.

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Spanish patent ES 2324696 B2 describes a apparatus and a procedure for measuring deformation in a test of traction in a test tube. The device is formed by an arm articulated, at one of whose ends is a guide on the that two laser sensors are mounted, with the ability to move to guide length

However, the apparatus described in ES 2324696 B2 have the disadvantages that: laser sensors are placed relatively close to the specimen, so they may suffer some physical damage in case of accidental breakage; can't measure the deformation in test tubes submerged in a liquid; can't measure the deformation at elevated temperatures; its use with specimens of Small size is cumbersome.

Summary of the Invention

The present invention tries to solve the aforementioned drawbacks by means of a method and apparatus that allow to determine the deformation in uniaxial tensile tests, in which the measurement is carried out without contact with the specimen, so that possible damage to the equipment can be prevented in the breakage of the
same.

Specifically, in a first aspect of the In the present invention, an apparatus for measuring the tensile deformation of a test tube, comprising: an arm articulated configured to be fixed to a test machine, where said articulated arm comprises at least two rigid sections related by means of an intermediate articulation axis, and means of anchor connected to one end of a first arm section articulated and configured to act as a means of fixing the articulated arm to a pillar of the test machine; a guide mounted on one end of the second section of the articulated arm. On the guide are mounted two "U" shaped racks that are located parallel to each other and perpendicular to the guide. Each one of the racks carries a respective optical sensor, each of the which has the ability to move along the guide by moving the respective support; inside of the "U" of the supports houses a test tube whose axis coincides with that of the guide, including two objectives that can be fixed to said specimen in two points of it. These goals are set to measure the deformation of the specimen by measuring of the optical sensors of the displacement of the objectives.

In a preferred embodiment, the two sensors Optics are two digital optical micrometers. In this case, each digital optical micrometer preferably comprises a transmitter configured to emit a beam of light and a receiver configured to receive said beam of light and stop, when said target is interposed in the trajectory of said beam of light and generates a shadow on said do, detect said shadow and translate it to a displacement of said objective.

Alternatively, the two optical sensors are two laser micrometers

Preferably, the two objectives are set to said test tube by elastic bands.

Preferably, each of said two objectives consists of an elongated element. In this case, preferably elongated elements have a cylindrical shape.

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In another aspect of the present invention, provides a procedure to measure deformation in an assay of traction by means of an extensometer. The procedure includes the following stages:

to)

signal two points P_ {p} and P_ {a} in a test tube, separated along it a distance L_ {0} by means of two cylindrical objectives of diameter diameter fixed to the specimen by means of an element elastic;

b)

calculate using optical sensors such as those described above the distance L 0, by means of the expression:

one

in which R is the length of a reference pattern, R_ {p} and R_ {a} are the sensor reading when the strain gauge is introduced on the reference standard and p_ {0} and a_ {0} are the initial reading of the sensors when the lenses are placed;

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C)

to size during the application of increasing tensile stress the relative displacements \ DeltaL_ {a} and \ DeltaL_ {p} of the points P_ {a} and P_ {p} of the specimen, using the same sensors so that you can determine the variation of the separation between the points through the expression:

2

d)

calculate the deformation produced by expression

3

in which ΔL is the variation in length suffered by the measurement base, L_ {0}.

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The method and apparatus of the invention also allow the measurement of the deformation of the test tube directly, without prior calibration.

Another advantage of the procedure and apparatus of the invention is that they allow automatic calculation of the measuring base of the device without initial measurements.

Finally, the procedure and apparatus of the invention allows measuring deformations on a specimen within a transparent container in which the temperature can be modified, and even submerged in a liquid, if it is also transparent.

Other advantages of the invention will be made evident in the following description.

Brief description of the figures

In order to help a better understanding of the characteristics of the invention, according to an example preferred practical implementation thereof, and to complement This description is accompanied as an integral part of it, a game of drawings, whose character is illustrative and not limiting. In these drawings:

Figure 1 shows a perspective of a apparatus for measuring deformation in a tensile test of a test tube, according to an embodiment of the invention.

Figure 2 is a diagram showing the detail of union of the objective to the test piece by means of an elastic rubber, according to an embodiment of the invention.

Figure 3 shows, schematically, the variables to be measured during the tensile test. The team during The test continuously records the distance between Two points of the test tube. Two points are identified on the specimen P_ {p} P_ {a}: P_ {p} is the point near the passive side of the test machine and P_ {a} is the one near the active side. The Variables to measure are the displacements of these two points, ΔL_ {p} and ΔL_ {a}.

Figure 4 indicates the measured "a" dimension by the sensor next to the active side, Sa.

Figure 5 indicates the measured "p" dimension by the sensor next to the passive side, Sp.

Figure 6 represents the measure of variation of displacement, \ DeltaL_ {a}, suffered by the point P_ {a}.

Figure 7 represents the measure of variation of displacement, \ DeltaL_ {p}, suffered by the point P_ {p}.

Figure 8 represents the measurement of the base of measurement of the extensometer, L_ {0}, from the previous measurement with a reference pattern of length L.

Detailed description of the invention

In this text, the term "comprises" and its variants should not be understood in an exclusive sense, that is, These terms are not intended to exclude other technical characteristics, additives, components or steps.

In addition, the terms "approximately", "substantially", "around", "ones", etc. must understood as indicating values close to which said terms accompany, since due to calculation or measurement errors, it results impossible to get those values with total accuracy.

The characteristics of the procedure and apparatus of the invention, as well as the advantages derived therefrom, may be better understood with the following description, made with Reference to the drawings listed above.

The following preferred embodiments will be provided by way of illustration, and are not intended to be Limitations of the present invention. In addition, the present invention covers all possible combinations of particular embodiments and preferred here indicated. For experts in the field, others objects, advantages and features of the invention will come off partly from the description and partly from the practice of invention.

The measuring device is described below. the tensile deformation of a test piece of the invention of according to the scheme thereof in figure 1.

As detailed below, the device or extensometer to measure the deformation of the specimen has an arm articulated that is fixed to the test machine and is a carrier of two optical sensors, and two targets or targets, preferably small diameter cylinders, attachable to the test tube in as many reference points of the same, for measuring its deformation.

The articulated arm preferably comprises, at least two rigid sections 1, 2 that are connected or related each other by an intermediate joint 3, whose axis is indicated with reference 3. At the free end of one of the sections of the articulated arm (the one referenced with the number 1 in figure 1), an anchor base 8 is connected which serves as a fixing means from the arm to a pillar of the test machine. At the free end of the other section of the articulated arm (the one referenced with the number 2 in figure 1), they are mounted, through another joint 4, a guide 10, on which two racks are moved in the form of "U" 11, 12, whose mission is to support two optical sensors 13, 15. The frames are also linearly movable on the guide 10 to adjust the measuring base of the extensometer to Different lengths In figure 1, the assembly formed by the guide 10 and the two "U" shaped racks 11, 12 has referenced as 26.

The anchor base 8 is connected to section 1 of the articulated arm by means of an end joint 5 of parallel axis to the intermediate articulation axis 3 between sections 1 and 2 of said arm.

In a particular embodiment, the basis of anchor 8 discussed above may consist of a flange adjustable axis parallel to articulation 3 between the sections of the articulated arm 1 and 2. The adjustable flange is constituted preferably by two halves 6 and 7 of adjustable separation and preferably equipped with a radial pressure screw 9.

As already advanced, the device of the The invention also includes two fixed targets 14 (O_a) and 16 (O_ {p}), in as many reference points of a test tube 17, P_ {a} P_ {p}, for measuring its deformation.

Objectives 14, 16 set to test tube 17 they consist of an elongated element, preferably a cylinder, of a light material and of a certain diameter \ diameter. Its length must be adequate to cause arousal in the sensors 13, 15, remaining attached to the test specimen. He diameter of the cylinders is preferably between 2 and 4 mm, approximately. Non-limiting examples of lightweight material they are: all types of plastic, aluminum, ... These objectives 14, 16 are they attach to the test piece 17 by means of preferably a rubber elastic, 21.

The test specimen came to the test machine by means of the two anchors or jaws, one passive 18 and one active 19, which are outlined in Figure 3.

The sensors used to measure the displacement are preferably two optical micrometers of precision 13, 15 (one 13 located on the active side and another 15 located on the passive side). The precision optical micrometer 13, whose scheme is shown in figure 4, it is constituted by two elements, a transmitter 13 'and a receiver 13' '. The element 13 'transmitter emits a light through a light source, preferably a led, and more preferably gallium nitrite, It provides a green light. This light is transmitted to through a collimator lens that originates a continuous beam of light and 13 '' parallel. The receiving element 13 '' picks up the light beam by medium of a telecentric optical system that allows only light parallel forms the images on a CCD and a CMOS camera that capture the image of the measurement point. When an object intercepts the light beam 13 '' 'between transmitter 13' and receiver 13 '', or what is the same, generates a shadow in the receiver (in color white on the beam 13 '' 'in figures 4), the CCD and the camera detect in a certain position of the measurement field the lack of light and internally translate it to a length. Other details of precision optical micrometer are out of reach of the present invention

Figure 5 illustrates the other optical micrometer 15, with its 15 'transmitter, 15' 'receiver, 15' 'light beam and shadow (blank) generated by an object on the 15 '' beam.

These sensors can be replaced by laser micrometers that also work according to the same principle of Scan or shadow.

To carry out the procedure of the invention, it is necessary to know the separation between two test tube points 17, L (t), at all times, while Increasing efforts are applied. This separation is illustrated in the Figure 3. The initial distance between these two points, before begin the test, it is called the measurement base of the extensometer, L_ {0}. From these parameters you can determine the deformation, ε, of test tube 17 tested in each instant, from the expression:

7

where \ DeltaL is the increase of length suffered by the measurement base of the extensometer in each instant.

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For measuring the separation of the points mentioned, the process of the invention uses two High precision digital optical micrometers, which follow throughout moment the displacement of two objectives 14, 16 together with said points.

The procedure for measuring with the device described can be checked in figure 3, where it is represented schematically a tensile test, with test tube 17 and both Anchors or jaws of the test piece to the test machine, one passive 18 and the other asset 19.

Two points, P_ {p}, near the passive side, and P_ {a}, close to the active side, located, initially at a distance L_ {0}, for a certain moment of time they pass to be located at a distance L (t). The two points move, ΔL_ {p} and ΔL_ {a}, respectively, the displacement closer to the active jaw.

According to the process of the invention, the sensor near the active jaw, Sa, 13 measures the distance "a" between the shadow generated by the target, Oa 14, and the end of the beam of light near the active jaw (figure 4), while the sensor near the passive jaw, Sp, 15 measures the distance "p" between the shadow generated by the passive objective, Op 16, and the end of the light beam near the active jaw (figure 5). In Figure 4, the measured "a" dimension is shown by the sensor next to the active side, Sa. In figure 5, it is shown the dimension "p" measured by the sensor next to the passive side, Sp.

The first step, before placing the objectives on the specimens, is the initial measurement of the measurement base of the extensometer For this, a reference standard of length R, which has different lengths depending on the test requirements in terms of length of the measurement base. This is outlined in Figure 8. The length of the pattern is such. which generates a reading within the range of the two sensors at the same time. The measurements obtained with the pattern, R_ {a} R_ {p}, active side and passive, respectively, are maintained while the sensors are not Change places If there is any change in the position of the sensors it is necessary to repeat the initial measurement operation of the measurement base.

Subsequently, the objectives on the randomly selected reference points along the shaft of the test tube so that each of them matches inside of the measurement range of the corresponding sensor. The two points of reference are called P_ {a} (closest to the active side) and P_ {p} (closest to the passive side) and are separated a distance L_ {0}. The initial reading of the sensors after placing the targets is a_ {0} for the Sa sensor and p_ {0} for the Sp sensor.

The measurement basis, L_ {0}, is determined at from the expression:

4

where \ diameter is the diameter of the objective.

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The deformation of the test tube as an increasing tensile stress is applied, through the Sa and Sp sensors discussed above. Displacement ΔL_ {a} and ΔL_ {p} of points P_ {a} and P_ {p}, respectively, are determined from the displacement measured in every moment through the sensors, a_ {i} and p_ {i} and the initial displacement, a_ {0} and p_ {0} (figures 6 and 7).

5

The length increase suffered by the basis of measurement by the expression:

6

Then, the principle of operation of the apparatus of the invention, knowing what has been separated each point of the bar individually, measured by each of the optical sensors, the length increase can be obtained, and therefore, the deformation of the bar.

Finally, the deformation produced is obtained by expression

8

where \ DeltaL is the difference between the displacements of points P_ {a} and P_ {p} of the specimen at each moment and L_ {0} the measurement base initial.

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The measuring ranges of the sensors selected depend on the magnitude to be measured: if you want to measure very accurately, small ranges are used, while yes not much precision is required you can use strain gauges greater range that allow the measurement of larger elongations.

If you need both properties, it is that is, precision for small elongations and measuring ranges large with little precision, sensors can be mounted in parallel with different properties so that both are covered needs

Depending on the type of specimen, its length, of the machine used and the space available between anchors 18 and 19, the sensors will be positioned differently way, being able to move vertically.

To be able to measure the measurement base or distance between the contact points of the objectives 14, 16, with the test tube, 17, it is necessary to know the measure of a pattern of reference 20.

As already mentioned, the measurement basis, L_ {0}, is determined from Figure 8 and the expression:

9

In short, with the procedure and apparatus of the invention can be carried out uniaxial tensile tests with the following properties:

* Measurement of deformation in a test of uniaxial traction without contact with the specimen, to prevent possible damage to the breakage of the same.

* Measurement of the test tube deformation of directly, without prior calibration.

* Automatic calculation of the measurement base of the extensometer, without initial calibrations.

* Possibility to measure very accurately in short runs or large elongations with less precision.

* Possibility of measuring deformations on a test tube inside a transparent container in which you can modify the temperature

* Possibility of measuring deformations in specimens submerged in transparent liquids.

Claims (8)

1. Apparatus for measuring deformation by traction of a test tube, comprising: - an articulated arm configured to be fixed to a test machine, wherein said articulated arm comprises the minus two rigid sections (1, 2) related by an axis of intermediate joint (3), and anchoring means (8) connected to a end of a first section (1) of the articulated and configured arm to act as a means of fixing the articulated arm to a pillar of the testing machine; - a guide (10) mounted at one end of the second section (2) of the articulated arm; the apparatus being characterized by that on said guide (10) two are mounted U-shaped frames (11, 12), where said two Racks (11, 12) are located parallel to each other and perpendicular to said guide (10), where each of said racks (11, 12) carries a respective optical sensor (15, 13), each one of which (15, 13) has the capacity to move to along said guide (10) by moving the support respective (11, 12); where inside the "U" of sayings supports (11, 12) a test tube (17) is housed whose axis coincides with the of said guide (10), including two objectives (16, 14) fixed to said test tube (17) at two points thereof, said objectives (16, 14) configured to mediate the deformation of said test tube (17) by measurement by said sensors optical (15, 13) of the displacement of said objectives (16, 14).

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2. The apparatus of claim 1, wherein said optical sensors (13, 15) are two optical micrometers digital 3. The apparatus of claim 2, wherein each digital optical micrometer (13, 15) comprises a transmitter (13 ', 15 ') configured to emit a beam of light and a receiver (13' ', 15 '') configured to receive said beam of light and to, when said objective (14, 16) gets in the way of said beam of light and generates a shadow on said beam, detect that shadow and translate it to a displacement of said objective (14, 16). 4. The apparatus of claim 1, wherein said optical sensors (13, 15) are two laser micrometers. 5. The apparatus of any of the previous claims, wherein said two objectives (14, 16) are fixed to said test tube (17) by elastic bands (21). 6. The device of any of the previous claims, where each of said two objectives (14, 16) are formed by an elongated element. 7. The apparatus of claim 6, wherein said elongated elements have a cylindrical shape. 8. Procedure for measuring deformation in a tensile test using an extensometer, characterized in that It comprises the following stages:

to)

signal two points P_ {p} and P_ {a} in a test piece (17), separated along it a distance L_ {0} by means of two cylindrical objectives (14, 16) of diameter diameter fixed to the test piece (17) by means of a elastic element;

b)

calculate using optical sensors (14, 16) according to any of the preceding claims the distance L_ {0}, by means of the expression:

10

in which R is the length of a reference pattern, R_ {p} and R_ {a} are the sensor reading (14, 16) when the extensometer is enter the reference pattern and p_ {0} and a_ {0} are the reading initial sensor (14, 16) when the lenses are placed (14, 16).

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C)

to size during the application of increasing tensile stress the relative displacements \ DeltaL_ {a} and \ DeltaL_ {p} of the points P_ {a} and P_ {p} of the specimen (17), by means of them sensors (14, 16) so that the variation of the separation between the points by means of the expression:

eleven

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d)

calculate the deformation produced by expression

12

in which ΔL is the variation in length suffered by the measurement base, L_ {0}.