DE102007058702A1 - Damping element for a test device - Google Patents

Abstract

The invention relates to a test device for determining speed-dependent connection characteristics of a sample under impact load, in which the sample is gripped in a clamping section and in which an accelerated in an accelerator (4) over a delay line piston (18) the load in the form of a substantially elastic shock to the sample (2), wherein by a certain integration of a plastic impact component by means of a in the accelerator (4) mounted damping element (21) whose damping behavior during the impact load increases exponentially, and by a defined ratio of the energy required to accelerate the piston mass to the fracture energy of the sample, the determination of force-displacement or force-time curves at an approximately constant deformation rate during the sample failure and is achieved without significant superposition of vibrations from bounce, dad urch characterized in that the damping element only at load start up to a defined force (Fu) affects the force-time curve at the sample.

Description

The invention relates to a testing device for determining speed-dependent connection characteristics of a sample under impact load, in which the sample is gripped in a clamping section and in which a device is mounted in an acceleration device. 4 ) via a flow path accelerated piston ( 18 ) the load in the form of a substantially elastic impact on the sample ( 2 ), whereby by a specific integration of a plastic impact component by means of a in the accelerating device ( 4 ) mounted damping element ( 21 ), whose damping behavior increases exponentially during the impact load, and by a defined ratio of the energy required to accelerate the piston mass to the fracture energy of the sample, determining force-displacement or force-time characteristics at approximately constant strain rate during sample failure, and without appreciable Overlapping of vibrations from bounce processes is achieved.

The Knowledge about the resistance, the deformability and the Labor absorption capacity of materials and material connections At high speed loads is for many industrial areas such as automotive engineering, ballistics, manufacturing technology, Construction and aerospace in terms of material strength and safety of the utmost importance.

at Traffic engineering products in general and in particular for products of the automotive industry wins the crash safety assessment of components against the background of increased demands on occupant protection increasingly central. To have such a crash safety rating in the development phase of new vehicle structures with the required to perform high prediction accuracy is a knowledge of the exact strength and deformation behavior of materials and joined joints under high loading speeds necessary. For experimental determination of this strength and Deformation behavior becomes a testing device appropriate speed profile used. At such a Testing device for determining characteristic values under impact load are very high demands. Dynamic effects during The high-speed test can record the recorded Superimpose measurement signals so significantly that they overlap Measuring signals no longer provide information about the actual load allow the sample. The requirements for test setups for high load speeds are therefore significantly higher those for qunsistatic experiments, and the use of commercial ones Testing machines is only in rare cases and with Restrictions possible.

in the Over time, different concepts have become available for the exam of materials, joints and components with high Test speeds developed. For the determination the strength and deformation behavior of the for the Automotive engineering relevant strain rate of max. 500 / sec are here in particular the servohydraulic quick tearing machine, the rotary impact mechanism and to call the Pendulum Impact.

in the Below, the relevant experimental plants are in their operative principle briefly explained.

The Servohydraulische Schnellzerreißmaschine is with a Pressure oil storage equipped, the hydraulic fluid over emits a control valve and thus a piston in an accelerator brings to the test speed. At the end of the delay line the cylinder of the accelerator couples to the end of the inner piston, the opposite side is connected to the sample, and then accelerates the sample up to failure. The force comes with a at the upper crosshead of the testing machine measured above the upper clamping force-measuring ring, which should have a corresponding natural frequency. The way of Piston rod is driven by an inductive long-distance transducer (LVDT = Longitudinal Velocity Differential Transformer).

A Determination of sample deformation via the measurement of displacement the test cylinder delivers especially in experiments under often no sufficient information about the actual deformation behavior of the samples.

in the Difference to the experiments under quasi-static and swinging Strain difficult at higher test speeds additionally dynamic influences during the experiment Interpretation and evaluation of the force signals. So it can at the Acquisition of measured values while trying to overlay the force signals come through vibrations, which are not the burden correspond to the sample. These disturbing influences arise depending on the strain rate from the overlay the actual measurement results with vibration influences from natural vibrations, inertial effects and wave phenomena and can falsify the measuring signals in extreme cases, that an evaluation is no longer possible.

Against this background, the force measurement is generally carried out not only on the test device, z. B. on the crosshead in conventional servo-hydraulic Schnellzerreißmaschinen, but in addition, by evaluating the measurement signals of strain gauges applied to the sample. Such local measured value recording on the sample is state-of-the-art in material testing. However, with joined samples to determine characteristic values as a function of the test speed and the load introduction angle, it is not possible to record measured values directly at the joining point. Here, a complex test arrangement with many interfaces and comparatively large masses is necessary. These properties are in contradiction to the low masses and high stiffnesses required for abrupt investigations in the clamping strand. In addition to the problem area described, it is generally not possible to achieve a constant strain rate or local strain rate.

A test device for the measurement recording of connection characteristics of samples under sudden load is in the German patent application DE 10 2007 010 426 been revealed. This test device is characterized by a damping element in the clamping.

It Object of the invention, the damping element in the Clamping distance of the tester mentioned so on to improve that is a largely constant deformation rate-time course at the sample.

The Solution of this problem is achieved in that the damping element only at the beginning of stress up to a defined force (Fu) the force-time course influenced at the sample.

The impact load on the sample in the chucking path is generated by a high-speed servohydraulic cylinder. In the clamping section of the test apparatus, an acceleration device is included. This consists of an internal piston including an outer, cylindrical receptacle - the test cylinder - and is to ensure that the sample is not loaded during the acceleration of the test cylinder to the test speed. Immediately before the start of a test cycle, the piston is completely in the test cylinder. The test cylinder is still in the rest position, so both the speed of the test cylinder v _P and its acceleration a _{P are} equal to zero. In the preliminary phase, the test cylinder is now accelerated, the speed v _P increases from zero starting. After reaching the set test speed of the accelerated test cylinder tears the piston with the clamping with by the test cylinder positively engages around the piston. The piston speed v _K and the piston acceleration a _K abruptly increase from zero at this time. As a result of the sudden entrainment of the piston by the test cylinder, which has already been accelerated to test speed, the piston reaches the speed v _K almost immediately without the sample body being previously loaded. Due to the high force introduced by the test cylinder (very high acceleration a _P with simultaneously high mass), it is possible to keep the velocity of the piston v _K constant over the entire test period. There is no drop in the test speed, so that the piston acceleration a _K takes the value zero over the measurement period. When the piston engages, the steel mount of the already accelerated test cylinder beats undamped onto the still stationary steel piston. This results in considerable bouncing effects, which are reflected in the form of vibrations on the force signals recorded on the sample by means of strain gauges (DMS).

One decisive approach for reducing dynamic disruptive effects during the high-speed tensile tests is the minor Increase of the plastic impact component during sudden engagement of the piston. Without this central optimization, the snap-in the piston into the receptacle - here beats undamped Steel on steel - in first approximation as elastic Shock are considered. At this elastic shock arise However, significant bounce effects, resulting in the significant vibrations knock down in the force measurement, so that the evaluation of the Measuring signals do not reveal the actual Stress of the sample allows. This succeeds only by the targeted integration of a minor plastic Shock fraction in the operating principle of the test device. This is a thin damping element directly mounted in the accelerator device.

On the cone-shaped piston is the damping element non-slip applied. By the damping element is a targeted and defined plastic content on the first Approximation as elastically designated impact mechanism integrated into the test bench.

The wear-resistant damping element applied to the piston, which is made of rubber or elastic plastic, is in his Thickness depends on the moving mass in the clamping section. At a foreseeable fracture energy of the sample of about 40 J, one set test speed of 2 m / s and a mass of the piston of 7 kg is the thickness of the damping element about 1.5 mm.

One such damping element has a non-constant damping on. The damping increases with increasing pressure load through the test cylinder exponentially too. Here is the damping element deformable up to a maximum force Fu.

It has now shown that a low - vibration course of Sets measured values, if the force-displacement curve only up to the specified maximum force Fu is affected. This will cause a drop in the load rise speed limited to the lower part of the force increase.

Of the Force value Fu marks the upper limit of the flatter, through the Damping element conditional increase in force. This force value Fu should not exceed one quarter of the maximum force Fo, which denotes the limit of the elastic range of the sample. Above This force Fo is the sample plastically deformed.

It So the formula applies: Fu ≤ 0.25 Fo.

The Damping element distributes the acceleration phase over a longer period. It is characterized by the exponential Spring characteristic of the damping element reaches that this acts only in the lower power range and the further increase in power unaffected. Due to the plastic impact component becomes a sudden increase in acceleration at the beginning of the acceleration phase avoided and thus high vibration-inducing inertia effects suppressed.

On this way, the determination of force-distance / force-time progressions when testing samples at approximately constant Stress rate during sample failure and without significant superimposition of vibrations in the Force signal from bounce operations possible.

The Maximum force Fo is the one of the material parameters to be measured Sample dependent. The sample thickness, the sample material, plays a role here and joining technology plays an essential role. Based on Maximum force Fo is the thickness of the damping element and to determine its spring characteristic.

In the figures, the invention is exemplified.

1 schematically shows a test system with servohydraulischer tensile machine.

2 shows diagrams of vibration curves.

3 shows an accelerating device in section before the engagement of the piston.

4 schematically shows the operation of the accelerator.

5 shows the course of the sample deformation with damping element

In 1 is schematically a testing device 1 for determining connection parameters on a sample 2 shown. At the testing machine 9 is the high-speed cylinder 3 attached, its energy from the high-pressure oil storage 11 receives. The high-speed cylinder 3 puts the load on the chucking line coming out of the accelerator device 4 , the joints 6 , the holder 5 and the clamping block 7 , in which also a force measuring device may be included exists. About the accelerator device 3 is the impact load on the sample 2 generated. The deformation of the sample 2 is via the high speed camera 8th measured directly locally on the sample. The force is measured on the sample or in close proximity to the sample "quasi-lokally" via applied strain gauges 10 fed and evaluated. For this purpose, a special software tool is available, which enables the evaluation of the connection characteristics (breaking load, fracture energy, rigidity, fracture path) automatically.

In 2 FIG. 2 shows typical measurement curves A and A 'which show the oscillations in the force values F on the ordinate axis and the sample deformation s determined via the measurement of the displacement of the test cylinder or the sample deformation s determined directly on the sample by means of optical deformation measurement and gray value correlation methods on the abscissa axis ,

In 3 is a schematic section through the high pressure cylinder 3 and the accelerator 4 shown. About the cylinder piston 15 and his piston rod 16 becomes the test cylinder 20 accelerated in the direction of the arrow. In the state shown here, the velocity v _{k of} the piston 18 and its acceleration a _{k is} still zero

After that, the piston hits 18 on the cone 19 and tear the piston rod 17 with, where the sample is attached. The piston 18 is the damping element 21 attached, which causes the plastic impact component.

In 4 is shown schematically in the sub-figures a, b, c, d, the operation of the accelerator device.

In the partial figure a, the entire accelerator device is at rest. The speeds v _{P of} the test cylinder 20 and the piston 18 v _k are zero.

In the subfigure b, the accelerated test cylinder is shown, a _p and v _p are greater than zero. The piston is still at rest.

In the sub-figure c, the piston hits the cone 19 and the piston is accelerated, a _k and v _k become greater zero.

In the partial figure d the beginning of the experiment is shown. The piston and the test cylinder move at a constant speed v _p and v _k , the accelerations a _p and a _k are zero again.

Thus, the experimental condition is reached, a constant test speed is applied to the piston 18 and thus on the holder 5 applied.

In 5 are plots over the test time t shown in the solid line show the force applied to a sample F and in the broken line the deformation rate v of the sample. Furthermore, the characteristic values Fu for the damping element and Fo for the elastic range of the sample are entered. The force value Fu marks the upper limit of the flatter force increase due to the damping element. The force value Fo defines the upper limit of the elastic range of the sample. In the example shown here, Fu is about 3.5 kN and Fo about 14 kN. These traces were taken on a sample of the material X5CrNi18-10, in which the joint was made with a high-strength blind rivet.

Of the The course of the curve illustrates the effect of the damping element. It distributes the acceleration phase over a longer period Period. The damping element only works in the lower force range and the further increase in power remains unaffected.

there is due to the exponential spring characteristic of the damping element Achieved that a sudden increase in acceleration in the beginning the acceleration phase of the joining zone is avoided. Thereby become high vibration-inducing inertial effects suppressed. For comparison with the increase in power is also the deformation rate depending on the Test time shown. It becomes clear that it was not until the End of the flat force increase to an acceleration of the sample and thus to an increase in the deformation rate at the Joint is coming. At maximum force, the load corresponds at the joint of the target speed. The deformation speed is largely constant over the test period and rises only after the sample break by the sudden discharge of the chuck strand jumped.

1

Tester

2

sample

3

High speed cylinder

4

accelerator

5

holder

6

joint

7

clamping block

8th

High-speed camera

9

Servohydraulic quick tearing

10

measuring system

11

High-pressure oil storage

12

Valve

13

wells

14

joining surface

14a

Retaining compound

15

piston of the high-speed cylinder

16

piston rod

17

pole of the test piston

18

test tube

19

cone

20

test cylinder

21

damping element

22

markers

A, A '

Vibration Superimposed readings

B

vibration-free force signals

F

test load

Fo

upper force value

Fu

lower power value

s

deformation

t

Test time

v

strain rate

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 102007010426 [0010]

Claims (2)

Test device for determining velocity-dependent connection nuclei values of a sample under impact load, in which the sample is held in a clamping section and in which an accelerator device ( 4 ) via a flow path accelerated piston ( 18 ) the load in the form of a substantially elastic impact on the sample ( 2 ), whereby by a specific integration of a plastic impact component by means of a in the accelerating device ( 4 ) mounted damping element ( 21 ), whose damping behavior increases exponentially during the impact load, and by a defined ratio of the energy required to accelerate the piston mass to the fracture energy of the sample, the determination of force-displacement or force-time curves at approximately constant deformation rate during the sample failure and without appreciable Overlapping of vibrations from bounce processes is achieved, characterized in that the damping element only at load start up to a defined force (Fu) affects the force-time curve at the sample. Test device according to claim 1, characterized characterized in that the defined force (foot) is not higher as a quarter of the maximum force (Fo) is to which the sample is elastic.