WO2006128445A1

WO2006128445A1 - Extensometer for high temperatures

Info

Publication number: WO2006128445A1
Application number: PCT/DE2006/000952
Authority: WO
Inventors: Ralf Westerheide; Erhard Haas; Walter Weinschenk; André Schütz
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2005-06-03
Filing date: 2006-06-02
Publication date: 2006-12-07
Also published as: DE102005025608B3

Abstract

The invention relates to a device for measuring a change in length of a sample, i.e. to an extensometer. The device comprises at least two measuring rods (1) disposed next to each other in a plane of measurement and received in separate first bearings (3) in a bearing block (5) so as to be rotatable about parallel first axes of rotation that extend at a right angle to the plane of measurement. The bearing block (5) is received in a second bearing (6) in a suspension (7) so as to be rotatable about a second axis of rotation that extends in parallel to the first axes of rotation. The device also comprises at least one measuring sensor for detecting a change in a distance between the measuring rods (1). The inventive device is characterized in that the measuring rods (1) are configured from a high-temperature resistant material having a low thermal expansion coefficient. The first bearings (3) and the second bearing (6) are configured as prismatic bearings. The inventive device allows to carry out measurements at high temperatures and high frequencies.

Description

Extensometer for high temperatures

Technical application

The present invention relates to a device for measuring a change in length of a sample, the device having at least two measuring rods arranged side by side in a measuring plane, a bearing block in which the measuring rods are rotatably mounted in separate first bearings about parallel first axes of rotation perpendicular to the measuring plane. a suspension for the storage block in which the

Bearing block is rotatably mounted in a second bearing about a second axis of rotation which is parallel to the first axes of rotation, and comprises a measuring sensor for detecting a change in a distance between the measuring rods. Such devices are also known by the term extensometer.

In material testing, the measurement of the force-deformation behavior is required to determine the elongation behavior, the modulus of elasticity, the

Hystereseverhalten to obtain the elongation at break and the life of a material under thermomechanical cyclic or static stress. Many materials are loaded with high temperatures, changing temperatures and / or high frequencies. Accordingly, in material testing, measuring systems must be available which also determine a change in length of a sample as a basis for the calculation of the abovementioned material properties allow for changing temperatures, high temperatures and / or high load frequencies.

State of the art

In addition to measuring systems based on strain gauges and non-contact optical measuring systems, extensometers are also used to measure changes in length, which require direct contact with the sample. In the case of these contacting extensometers, two measuring rods which abut directly against the sample to be measured via a frictional connection transmit the change in length directly to a sensor arranged further away from the sample, which measures a change in the distance of the measuring rods.

Basically, there is the problem with known extensometers that they are designed either for the measurement at high frequencies or at high temperatures. Measurements at high temperatures require in the

Usually long dipsticks to reduce heat transfer to the bearings and the sensor. However, longer rods increase the masses to be moved, so that use of such extensometers is not suitable for measurements at high load frequencies.

EP 0697095 B1 describes an extensometer suitable for high temperatures, in which the two measuring rods are gimbal-mounted. In order to meet the requirements of high temperatures, the dipsticks in this extensometer are preferably made of ceramic or fiber-ceramic materials. Furthermore, the extensometer with a cooling provided direction to cool controlled parts of the extensometer. Such cooling is relatively expensive.

From US 4823473 is an extensometer for

Measurement of the change in length of a sample is known, which has two measuring rods arranged side by side in a measuring plane, which are rotatably mounted in a bearing block in separate bearings about parallel first axes of rotation which are perpendicular to the measuring plane. Furthermore, a suspension for the bearing block is provided, in which the bearing block is rotatably mounted in a bearing about a second axis of rotation which is parallel to the first axes of rotation. The dipsticks are made of a high-temperature-resistant material with a thermal expansion coefficient of <10 ^"5 K ^{" 1} . The bearings for the measuring rods and the bearing block are formed in this document as journal bearings.

DE 3714185 C3 also relates to a device for measuring the axial elongation of a sample bar with two spaced apart and approximately parallel to each other and against the sample rod by means of spring force pressure rods, which are pivotally mounted on a respective component of the measuring device. The measuring rods are in this case connected in a simple manner by means of pivot bearings, which may consist of rivets, with the frame.

In the extensometer of US 5083465 A, the two measuring rods are also simple pivot bearings connected to a bearing block, which in turn is mounted on a spring suspension.

No. 4,939,445 shows an extensometer in which two arms whose ends are designed to contact the sample are connected to one another via a rotary joint.

No. 4,884,456 describes a complex extensometer in which a parallelogram-type bearing frame is provided for the two measuring rods. The frame parts are connected to one another by means of bending connections, in order to allow unimpeded movement of the two measuring rods along their longitudinal axis. A pivoting movement of the measuring rods is made possible by bending connections.

The object of the present invention is to provide a contacting extensometer which is simple in construction and suitable for use at high temperatures. The extensometer should continue to be suitable for a measurement at high frequencies or at least be easily adapted.

Presentation of the invention

The object is achieved with the device according to claim 1. Advantageous embodiments of the device are the subject of the dependent claims or can be found in the following description and the embodiments. The proposed device comprises at least two measuring rods arranged side by side in a measuring plane, which are rotatably mounted in separate first bearings in a bearing block about parallel first axes of rotation, which run perpendicular to the measuring plane. The bearing block is rotatably mounted in a second bearing in a suspension about a second axis of rotation which is parallel to the first axes of rotation. The apparatus further comprises at least one measuring sensor for detecting a change in a distance between the measuring rods, which are in frictional contact with the sample during the measurement. The two measuring rods are formed of a hochtemperaturbestän- ended material, preferably for temperatures> 1000 ° C, with a low thermal expansion coefficient of ≤ 10 * 10 ^-6 K ^'1. The device is characterized primarily by the fact that the first bearings and the second bearing are designed as prism bearings.

Due to the choice of prism bearings for the mounting of the measuring rods and the bearing block, there is only a point contact with the measuring rods. Independently of the temperature, the two measuring rods each have a constant spacing at the positions of the bearings, since the heat transfer is minimized by the only selective material contact in the prism bearing, so that heating of the bearing block by the heat conduction in the rods is virtually ruled out can. A jamming or change of position due to increased heat conduction through the rods in the storage and thus a falsification of the measurement result can therefore not occur. This also applies to influences of changing temperatures. The measuring rods themselves are stored without play due to the prism bearing. The play-free storage offers particular advantages in a load reversal, ie in measurements on samples under cyclic load. On the other hand, the choice of a prism bearing offers the advantage of a simple, robust and thus interference-resistant construction of the extensometer.

The present extensometer is insensitive to changes in the sample surface due to high temperatures, for example by scaling, and can also be used for measurements at high frequencies due to the compact design with a suitable choice of the dimensions of the measuring rods. For this purpose, measuring rods made of lightweight construction material, ie preferably with a density of ≦ 4 g / cm ³ , are selected and / or the measuring rods are made of lightweight construction. Under the

Execution in lightweight construction is to be understood here as the geometric design of the rods, which are then not formed from a solid material, but have a corresponding lightweight profile, for example. Are in the form of a thin-walled tube or formed with an interrupted support structure. For a measurement up to a frequency of 100 Hz, the measuring rods are designed so that their natural frequency is well above 100 Hz. Preferably, the length of the bars is chosen to be very short, for example between 100 mm and 180 mm. Due to the choice of a material with a low thermal expansion coefficient in In spite of these short rods, a combination with the prism bearings can be measured at temperatures above 1000 ° C. Of course, the choice of the measuring sensor must also be such that it withstands the temperatures to which it is exposed during the measurement. Preferably, in this case a capacitive measuring sensor is used.

As suitable materials for the measuring rods are, for example, ceramic materials, alumina or

Silicon carbide suitable. Preferably, the bearing block consists of such a material with a low thermal expansion coefficient, such as, for example, a ceramic material.

By choosing a self-adjusting double play-free prism bearing in combination with the materials mentioned a universal use of the present extensometer is possible. The extensometer is very robust and inexpensive to produce due to its simple construction. It is suitable with a suitable choice of materials for measuring the deformation of a sample under mechanical stress at high temperatures of up to 1600 ° C and high frequencies to above 100 Hz. Optionally, a heating of the bearing block by radiation and convection by the additional heat Installation of heat shields can be reduced. The extensometer is particularly suitable for the measurement of samples under changing conditions, such as those in thermomechanical fatigue tests (cyclization of the temperature and / or the mechanical load) or given load cycles, eg. The superposition of Low-cycle-fatigue (Icf) and high-cycle-fatigue (ran) are present. The play-free storage, the accuracy of the measuring signals is high. Thus, the extensometer can also be used to control testing machines, for example by specifying a length change to be achieved. In non-clearance storage this would be made difficult or impossible by small inaccuracies in the direction reversal (from compression to elongation or vice versa). The backlash of the bearing is determined by the present

Structure of the extensometer maintained even with temperature changes.

In preferred embodiments of the present device, the sliding surfaces of the prism bearing are additionally provided with a friction-reducing coating in order to be able to realize a lower contact pressure of the measuring rods on the sample. Furthermore, the bearing block can have further prism bearings, in which the measuring rods can also be stored. By suitable arrangement of these further prism bearings, the distance of the measuring rods can then be adjusted stepwise in order to be able to select different measuring lengths.

Brief description of the drawings

The present device is described below by way of an embodiment in conjunction with the drawings without limitation by the

Claims specified protection range again exemplified. Hereby show: 1 shows an example of the configuration of the prism support for the measuring rods in the bearing block in two sectional views;

Fig. 2 shows an example of the configuration of the prism support of the bearing block in the

Suspension in two sectional views;

3 shows an overall view of the extensometer without measuring sensor; and

Fig. 4 shows an example of a bearing block with additional prism bearings for stepped

Setting of scanning distances.

MODES FOR CARRYING OUT THE INVENTION In the present example, an embodiment of the proposed extensometer is described which enables measurement at high temperatures of up to 1650 ° C. and high frequencies of at least 100 Hz.

Figure 3 shows an overall view of this extensometer, in which, however, the capacitive measuring sensor used in this case is not shown. The extensometer has the two measuring rods 1, which are applied with their tips visible in the figure to a sample to be measured. At the opposite ends of the measuring rods 1, these are pierced with thin pins 2. These pins 2 are firmly connected to the measuring rods 1, for example. Glued into corresponding holes of the measuring rods 1. The pins 2 form the axes of rotation about which the measuring rods 1 are rotatable. The thin pins 2 are held by springs 4 in a prism holder 3 of the bearing block 5. Thus, these pins 2 and indirectly also the measuring rods 1 are mounted without clearance in the bearing block 5. FIG. 1 shows a detailed representation of such a prism mounting in two sectional views from the front and from the side.

On each of the two measuring rods 1, part of a capacitive measuring sensor is mounted in the region between the bearing block 5 and the tips of the measuring rods 1 adjacent to the sample. The position of the measuring sensor on the measuring rods 1 is freely selectable depending on the desired measuring accuracy and required temperature resistance.

The bearing block 5, in which the prism support is located with the prism holder 3 for the pins 2 and thus for the measuring rods 1, is rotatably mounted in a manner similar to the measuring rods 1 in a prism mounting 6 of a rocker 7. As a result, a length compensation of the measuring rods 1 is made possible. FIG. 2 shows this prism support 6 of the bearing block 5 in two sectional views from the front and from the side.

The rocker 7 is rotatably mounted on one side of a holder 8 and is pressed by springs 9 against a stop. Depending on the deflection, the springs 9 determine the contact pressure of the measuring rods 1 against the sample. Furthermore, an adjustment unit 10 can still be seen in FIG. 3, which serves for the adjustment of the measuring rods 1 before the beginning of the measurement. This adjustment unit 10 can be removed in the present example. In an alternative embodiment, the contact pressure of the measuring rods 1 is set to the sample not defined by springs but via one or more weights. For this purpose, the retaining pins shown in Figure 3 for the springs 9 can be used. The weight is attached to a thin rope, which is guided around one of these retaining pins and attached to the other retaining pin.

In order to meet the requirements of the high temperatures and high frequencies, in the present example, measuring rods 1 made of aluminum oxide are designed as thin-walled tubes with a length in the range between 100 and 180 mm. The bearing block 5 was made of a ceramic material, which also has a very low coefficient of thermal expansion. By choosing the bearing block 5 from such a material, the influence of temperature changes on the measurement is additionally reduced. With the illustrated extensometer, a measurement accuracy of <0.1 μm could be achieved.

The entire extensometer can also be used in vacuum, in protective gas, under pressure and in chambers that are at elevated temperature. The maximum temperature is determined by the choice of materials of construction and the electrical leads for the sensor (s). The further processing of the measurement signals in a known manner as in Extensometers according to the prior

Technology. While in the example of FIGS. 1 to 3 the bearing block 5 has a double (ie bilateral) prism bearing for each of the measuring rods 1, such a bearing block can also be provided with further prism bearings. This is schematically indicated in Figure 4, which shows the parallel staggered prism shots 3 of the prism bearings with such a bearing block 5. Such an arrangement of the prism bearings allows the measuring rods 1 to be used at different distances from one another, so that the measuring length can be adjusted in steps, for example in steps of 5 mm, up to a maximum measuring length of 50 mm. The term "measuring length" is understood to mean the distance between the two tips of the measuring rods on the sample.

LIST OF REFERENCE NUMBERS

dipsticks

pencils

Prismenaufnähme

feather

bearing block

prism storage

seesaw

bracket

feather

adjustment unit

Claims

claims

1. Apparatus for measuring the change in length of a sample with

at least two measuring rods (1) arranged next to each other in a measuring plane,

- A bearing block (5) in which the measuring rods (1) are rotatably mounted in separate first bearings (3) about parallel first axes of rotation which are perpendicular to the measuring plane, - a suspension (7) for the bearing block (5), in the the bearing block is rotatably mounted in a second bearing (6) about a second axis of rotation, which runs parallel to the first axes of rotation, and

a measuring sensor for detecting a change in a distance between the measuring rods (1),

- wherein the measuring rods (1) made of a high temperature resistant material having a coefficient of thermal expansion of ≤ 10 * 10 ^"6 K ^{" 1} and - the first bearings (3) and the second storage

(6) are designed as prism bearings.

2. Apparatus according to claim 1, characterized in that the measuring rods (1) consist of a lightweight construction material and / or are designed in lightweight construction.

3. Apparatus according to claim 1 or 2, characterized in that in that the measuring rods (1) have a natural frequency which is at least a factor of 1.2 above 100 Hz.

4. Device according to one of claims 1 to 3, characterized in that the measuring rods (1) have a length of ≤ 200 mm.

5. Device according to one of claims 1 to 4, characterized in that the bearing block (5) made of a material having a thermal expansion coefficient of ≤

10 * 10 ^"6 K ^{" 1} exists.

6. Device according to one of claims 1 to 5, characterized in that the bearing block (5) consists of a lightweight material.

7. Device according to one of claims 1 to 6, characterized in that sliding surfaces of the prism bearings (3, 6) have a coating for reducing friction.

8. Device according to one of claims 1 to 7, characterized in that the bearing block (5) further prism bearings (3), which allow a gradual adjustment of a distance between the measuring rods (1).

9. Device according to one of claims 1 to 8, characterized in that the prism bearings (3, 6) respectively on both sides of the measuring rods (1) or the bearing block (5) arranged receptacles (3) for rod-shaped

Axes (2), which are connected to the measuring rods (1) or the bearing block (5) and form the axes of rotation.

10. Device according to one of claims 1 to 9, characterized in that the measuring sensor is a capacitive sensor which is fixed to the measuring rods (1).

11. Device according to one of claims 1 to 10, characterized in that the high-temperature-stable material of the measuring rods (1) for temperatures of ≥ 1000 ⁰ C is suitable.

12. Device according to one of claims 1 to 11, characterized in that a contact pressure of the measuring rods (1) to the sample via spring elements (9) is adjustable, which connect the suspension (7) with a fixed support (8).

13. Device according to one of claims 1 to 11, characterized in that a contact pressure of the measuring rods (1) to the sample via one or more weights is set adjustable, the pressing force over at least one deflecting element on the suspension (7) or a fixed support (8) transferred to the measuring rods (1).