GB2264177A

GB2264177A - Material testing apparatus.

Info

Publication number: GB2264177A
Application number: GB9302884A
Authority: GB
Inventors: Karl Lechner
Original assignee: MTU Motoren und Turbinen Union Muenchen GmbH
Current assignee: MTU Aero Engines GmbH
Priority date: 1992-02-15
Filing date: 1993-02-12
Publication date: 1993-08-18
Also published as: GB9302884D0; FR2687474A1; DE4204589A1; FR2687474B1

Description

2264177 Material-Testing Apparatus The invention relates to a mount for a

test piece, in particular for use in a material-testing apparatus for tension or compression tests in which unintentional bending moments in the test piece can be compensated for or eliminated. In such testing apparatuses the test piece is aligned substantially in the testing force direction and clamped between two opposing specimen holders, each specimen holder being connected to an armature generating compressive or tensile force.

During static or dynamic testing of material samples any misalignment, inaccuracy in the manufacture of the test piece or inaccuracy in the testing machine can give rise to an undesirable bending moment stress in the test piece. These additional bending moments overlap the testing stress proper and lead to adulteration of the test results. Small centring errors, particularly with very rigid samples and brittle materials (e.g. ceramics) have extreme effects on the increase in the bending moment. If samples are subjected to a compression test, then with very long samples and with misalignment of the central point of the clamping apparatus an increasing danger of buckling exists. For maximum accuracy therefore it is of great importance that these bending moments are substantially reduced or compensated for. To do this the bending moment occurring in the test piece must be ascertained.

Methods used hitherto measure the bending moment by means of strain gauges applied to the test piece, with an associated amplifier. For this purpose the gauges are initially adjusted with the aid of the amplifier when the test piece is not yet clamped, i.e. in the stress-free state of the test piece. Subsequently the test piece is clamped in the testing appar- atus, but without applying a test load. In the clamped state the stresses now occurring are measured and efforts are made with corresponding adjusting procedures of the upper and lower specimen holder to align the test piece so that it is free of bending moment. This process is extremely time-consuming and cost-in- tensive, in particular if, through temperature-induced expansion, the adjusting processes have to be repeated. If the test piece should in addition be exposed to extreme temperatures then errors of indeterminable size can occur in this arrangement since a sufficiently accurate bending moment compensation is no longer possible. Moreover the application of strain gauges to test pieces in large quantities is expensive.

It is therefore desirable to provide a materialtesting apparatus in which even under extreme test temperatures an accurate and quick compensation of undesirable bending moments in the test piece is possible.

According to the invention there is therefore provided an apparatus for applying a force to a test piece, comprising two opposed test piece holders each connected to a corresponding armature for applying the force, in which two or more adjusting elements are arranged between at least one of the holders and its armature in such a way as to make possible the application of a desired bending moment to the test piece.

Preferably the test mount comprises two holders for holding a specimen along an axis between the holders, at least two adjusting elements arranged off the test axis in such a way as to act on the holders, a force-sensing device allocated to each adjusting element and a control means for operating the adjusting elements on the basis of the output of the forcesensing devices, in order to set the relative orientation of the holders. Having sensors associated with the adjusting elements means that it is not necessary to apply sensors such as strain gauges to the test piece; however, the use of controllable actuators in principle enables automatic compensation of bending stresses even if gauges are applied to the test piece. 5 The adjusting elements are preferably linear and arranged in a plane substantially perpendicular to the test axis. Generally they will extend and operate parallel to the longitudinal axis of the specimen holders. In advantageous embodiments of the invention each holder, i.e. the holder at each end of the specimen, has its own set of preferably three adjusting elements. Test apparatus in accordance with the invention has the advantage that any bending moments in the test piece can be compensated automatically and quickly by adjustment of the adjusting elements, corresponding to the forces measured by the sensors, as a result of which a time-consuming manual adjustment of the specimen holder is unnecessary. Because of these properties and features compensation can be made not only for static bending moments but also continuously with dynamic processes of high frequency. When testing large quantities of samples the automation of the testing process is of great help in increasing efficiency. Embodiments of the invention can deal with this aspect by connecting the force-sensing devices of each specimen holder to a controller for force balancing control of the linear adjusting elements.

It is a further advantage that temperature sensitive measuring elements do not need to be applied directly to the test piece, since this means that material testing can be carried out over a wide range of temperatures. The simple construction of the mount permits its subsequent fitting to existing materialtesting apparatuses without trouble.

It is sufficient for the determination or the balancing of the bending moment that three force-sensing 4- devices are provided. In special cases where the specimen bends in only one plane two may be adequate. The invention can, however, also be designed with more than three force-sensing devices, even though this normally means unnecessary expense. The method according to the invention, while generally involving the cancellation or balancing of unwanted bending of the specimen, could also be applied to setting a predetermined bending moment where this is of interest.

The adjusting elements can be arranged in a tri- angle, preferably an equilateral triangle with the axis in the centre. This entails a substantially even load of the balancing elements when in operation, as a result of which equally dimensioned adjusting elements and force-sensing devices can be used. The construction of the test apparatus is considerably simplified because of this.

A further simplification of the apparatus is brought about by designing one of the adjusting elements to have a constant length. Since only relative movements of the linear adjusting elements are necessary for the bending moment adjustment, one linear adjusting element can have a constant length whilst the two remaining linear adjusting elements execute the movements required for the adjustment. For this purpose it is useful to save costs if the constant linear "adjusting elemenC can be constructed as a simple connecting element of a constant length.

An economical embodiment of the invention is made possible by mounting the adjusting elements on the arms of a three-armed star member via which the force is applied to the specimen holders. This enables the use of cheapdr force sensors in the form of strain gauges which can be mounted on the arms and detect bending of the arms. A high sensitivity and accuracy can be guaranteed by making the arms more flexible in the region of the strain gauges, for instance by local thinning or removal of material.

In an alternative design each linear adjusting element is arranged, as before, so that its direction of adjustment is coaxial to the measuring direction of the force-sensing device, but here the adjusting element and force-sensing device are connected to each other in such a manner as to form a unit transferring force to the specimen. This enables the use of common force gauges for the force-sensing devices, as a result of which existing material testing apparatus can be adapted in a particularly simple manner.

In a preferred embodiment of the invention the linear adjusting elements are piezoelectric trans- lators, allowing adjusting movements which are very sensitive, quick and free of play. Piezoelectric translators execute positioning in the nanometer to millimetre range with extremely high accuracy and are thus particularly suitable for an accurate compensation of bending moments and thus for high accuracy of the material-testing apparatus. In this way forces in the milli-newton to kilo-newton range can be transferred.

In a further embodiment the adjusting elements are prestressed by compression in the adjusting direction; the result is an enlarged tension capacity of the piezoelectric translators, in particular in static tension tests. In this way the fatigue limit of the adjusting elements is simultaneously increased. One embodiment which has proved particularly advantageous has the force- sensing devices screwed on the one hand to the specimen holder and, on the other hand, to the armature, clamping the adjusting element in a flexible manner.

The early recognition of cracks in the test piece during the testing process is possible with a preferred embodiment of the invention in which one or more force- sensing devices of at least one holder are connected to an apparatus for recognising cracks in the sample. Since a crack in the test piece during the testing process results in a shift of the flux of force, the shift in the forces measured results in a differing stress of the linear adjusting elements; this change is registered by the force-sensing devices and can be indicated as a warning of a crack.

Since with the invention it is not necessary to apply strain gauges directly to the specimen it is possible to conduct tests with the specimen at an extreme temperature; the generally sensitive forcesensing devices are arranged outside the heating or cooling device and therefore temperature-induced measuring errors can be largely avoided. In addition when selecting the test temperature no regard need be had for the permissible operating temperatures of the force-sensing devices or adjusting elements.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a side view of a testing apparatus with a test piece clamped in it; Figure 2 shows an enlarged cutaway section of the upper specimen holder of Fig. 1 with a heating device; Figure 3 is a section from Figure 2 showing the triangular arrangement of the linear adjusting elements and force-sensing devices; Figure 4 shows a folded cutaway section of a second embodiment of the upper sample holder with a force-measuring clamp and armature; and Figure 5 shows a plan view of the force measuring clamp of Fig. 4.

Figure 1 shows a material-testing apparatus 1, consisting of a frame standing on a machine base 2, with a clamped test piece 3.

The testing apparatus 1 has two armatures 4 and -7 5, one set on the base 2 and the other over it, suspended from the frame, the two armatures being displaceable relative to each other in the vertical direction so as to apply a testing force to the specimen. Upper and lower holders 6 and 7 are mounted respectively on the armatures, each by way of three linear adjusting elements 8 and associated force-sensing devices 9 transferring the testing force. The facing ends of the two holders 6 and 7 hold the test piece 3 clamped between them and as nearly as possible aligned in the testing force direction P. In the ideal case, i.e. with error-free clamping of the test piece 3 in the holders 6 and 7, the testing force direction P, the longitudinal axis L of the test piece 3, the longitudinal axes E of the specimen holders 6 and 7 and the adjusting direction A of the armatures 4 and 5 extend coaxially relative to each other.

Longitudinal movement of the upper or the lower armature 4 or 5, or both, applies a testing force, which depending on the direction of the relative movement of the armatures 4 and 5 is tensile or compressive, by way of the linear adjusting elements 8, the pressure gauges 9 and the holders 6 and 7 to the clamped test piece 3. Static or dynamic tensile or compressive tests can therefore be carried out, depending on the testing program.

In practice, because of misalignment, inaccuracies in the manufacture of the test piece 3, inaccuracy in the test mount 1, and other such reasons, the longi- tudinal axis L of the test piece 3 does is not exactly aligned with the longitudinal axes E of the specimen holders 6 and 7 or with the adjusting direction A of the armatures 4 and 5. As a result when the test piece 3 is rigidly clamped, bending stresses occur which lead to unpredictable measuring errors or to immediate fracture. In Figure 1 the alignment error is shown as an -8 angle a between the longitudinal axis of the test piece 3 and the longitudinal axis E of the test specimen holder 6 or 7 and represented in an exaggeratedly large form for the sake of clarity. The error might be the result, for example, of a misaligned thread cut on the test piece 3.

In order largely to avoid undesirable bending stress in the test piece 3 there are arranged between each specimen holder 6 and 7 and its corresponding armature 4 or 5 three actuators in the form of linear adjusting elements 8 and three force-sensing devices 9. Figures 2 and 3 show an example of the coaxial arrangement of the linear adjusting elements 8 and force-sensing devices 9. For this purpose each linear adjusting element 8 is combined with a corresponding forcesensing device so as to form a balancing element 10 transferring force from the armature to the holder. The force-measuring direction K of the force-sensing devices 9 extends coaxially to the adjusting direction V of the corresponding linear adjusting element 8 and forms the longitudinal axis of the balancing element 10. In order to transfer the testing force and the strain-applying movements of the armature 4, 5 the balancing elements 10 are connected on the one hand to the armature 4 or 5 and on the other hand to the corresponding holder 6 or 7. The balancing elements 10 are arranged parallel to the longitudinal axis E of the associated holder, and in the embodiment shown are screwed to the holder 6 or 7 at the corners of an imaginary equilateral triangle whose plane is perpendicular to the longitudinal axis E of the holder (see Figure 3). The central point M of the triangle lies on the longitudinal axis of the holder 6 or 7.

Compressive prestress can be applied to the piezoelectric translators 8 by clamping them between the force-sensing devices 9 and the armature 4 or 5, for instance as shown here by means of bolts 16 and spring washers.

In operation any unwanted bending moment applied by the armatures 4, 5 is detected by the torque- measuring means constituted by the force sensors 9. The adjusting elements 8 are driven in or out in the longitudinal direction as required until each balancing element 10 is stressed with the same compressive force or tensile force; since the sensors 9 are equidistant from the axis E this results in the desired zero moment. The addition of the forces of the three balancing elements 10 of a holder 6 or 7 produces the testing force. The balancing elements 10 can be arranged on a triangle which is not equilateral; in this case the forces to be transferred through the balancing elements 10 are to be determined relative to each other according to the geometric ratios of the balancing elements 10 and the holders 6 and 7 for balancing or setting the bending moment.

The longitudinal adjustment, i.e. the driving in and out of the balance elements 10, is executed in each case by a linear adjusting element 8 and the measure ment of the force to be transferred by the balancing elements 10 occurs through the force-sensing devices 9.

In order to control the linear adjusting elements 8 for an automatic bending moment compensation, the linear adjusting elements 8 of each holder 6 and 7 are connected to a controller (not shown) with a closed triplicate loop circuit. The movement required is determined in the controller from the force differences measured between the three force-sensing devices 9 of a holder, which represent a gauge for the bending moment.

A heating device 15 surrounding the test piece serves to heat the test piece 3 to varying temperat- ures. The sensitive elements 8 and 9 are sufficiently remote from this heater to be unaffected by it.

10- Figures 4 and 5 show a second example of the arrangement and formation of the adjusting elements 8 and of the force-sensing devices 9 between the armatures 4 and 5 and the holders 6 and 7. A three-armed- force-measuring member 11 carries the linear adjusting elements 8 and the force-sensing devices. The forcesensing devices in this embodiment are in the form of strain gauges 14 mounted on each arm 12 of the star member 11. The arms 12 also transfer the testing force from the armatures 4 and 5 to the linear adjusting elements 8, being arranged in a star configuration with an angle of 1200 between them, in a plane extending perpendicular to the armature-adjusting direction A. The centre 13 of the forcemeasuring member 11 is screwed into the respective armature 4, 5. The linear adjusting elements 8 are screwed into the ends of the arms 12, their adjusting axes V extending substantially perpendicularly to the plane of the forcemeasuring member 11. Upon longitudinal adjustment of the linear adjusting elements 8 in order to balance out the bending moment slightly inclined positions of the linear adjusting elements 8 relative to the plane of the force-measur ng member 11 are produced. At their ends opposite the force-measur ng member 11 the linear ad-justing elements 8 are screwed to the specimen holders 6 and 7, the adjusting axes V of the linear adjusting elements 8 extending parallel to the longitudinal axis E of the holder 6 or 7.

The arms 12 are designed in a flexible manner, for instance as shown by local removal of material, in a region between the centre 13 and the ends carrying the adjusting elements 8, in the adjusting direction of the linear adjusting elements 8. The strain gauges 14 are fitted in these regions and serve in this way as force- sensing devices comparable to the sensors 9 in the first embodiment. Again, one force sensor is associated with each adjusting element, as in the first embodiment, and operation of the testing apparatus takes place in an analogous way.

While the apparatus has been described as compensating and cancelling out unwanted bending moments in test pieces, it is clear that it can also be used within its limits for applying a predetermined bending stress. The invention is therefore also directed to methods of using such an apparatus for applying prede termined static or dynamic stresses using such an apparatus with off-axis positioning elements and sensors. Also, while in general both ends of the apparatus will have a holder and armature as described, for some test pieces an adjustable device at only one end may be adequate

Claims

Claims:

1. An apparatus for applying a force to a test piece, comprising two opposed test piece holders each connected to a corresponding armature for applying the force, in which two or more adjusting elements are arranged between at least one of the holders and its armature in such a way as to make possible the application of a desired bending moment to the test piece.

2. An apparatus as claimed in claim 1, in which the adjusting direction of the adjusting elements is substantially parallel to the axis of the test piece holders.

3. An apparatus as claimed in claim 1 or 2, in which a force-sensing device is associated with each adjusting element.

4. An apparatus as claimed in claim 3, and further including a controller to which the force-sensing devices and the adjusting devices are connected for automatic force-balancing control of the adjusting elements.

5. An apparatus as claimed in any preceding claim, in which the adjusting elements of the or each holder are arranged in a plane substantially perpen- dicular to the testing force direction.

6. An apparatus according to claim 5, in which there are three adjusting elements, each arranged at the corner of an imaginary triangle lying on the plane.

7. An apparatus according to claim 6, in which the imaginary triangle is equilateral and the central point of the triangle lies on the axis of the test piece holders.

8. An apparatus according to any preceding claim, in which one of the adjusting elements of each specimen holder is designed as a connecting element of a constant length.

9. An apparatus according to any preceding claim and further including a star-like intermediate member attached to the or each armature, or the or each holder, the adjusting elements each being secured between on the one hand the end of one arm and on the other hand the corresponding holder or armature.

10. An apparatus as claimed in claim 9 when appendant to claim 3, in which the force-sensing devices are formed in each case between the centre of the star member and the corresponding adjusting element.

11. An apparatus according to claim 10, in which the force-sensing devices are formed by strain gauges applied to the arms.

is

12. An apparatus according to claim 11, in which the arms are constructed in the region of the strain gauges in such a way as to flex in the testing force direction.

13. An apparatus according to any of claims 3 to 8, in which each adjusting element is arranged with its adjusting direction coaxial to the force-sensing direction of the corresponding force-sensing device and the adjusting element and the force-sensing device are connected in series for the transfer of force. 25

14. An apparatus according to any preceding claim, in which the adjusting elements are piezo-electric translators.

15. An apparatus according to claim 14, in which the translators are prestressed under compression in the adjusting direction.

16. An apparatus according to any preceding claim, in which the force-sensing devices of at least one test piece holder are connected to a device for detecting cracks in the test piece.

17. An apparatus according to any preceding claim, in which the test piece is surrounded by a heat- ing or cooling device and the adjusting elements and force-sensing devices are arranged outside this device.

18. A testing apparatus substantially as described herein with reference to any of the embodi 5 ments shown in the accompanying drawings.

19. A method for operating a materialtesting apparatus according to any preceding claim, in which measurements of the forces at the force-sensing devices of each specimen holder are used to control the long10 itudinal adjustment of the adjusting elements.

20. A method according to claim 19, in which the adjusting elements are controlled in such a way as to apply a substantially zero bending moment to the test piece. 15