DE102007010426B4 - Test device for samples under impact load - Google Patents

Abstract

Tester for determining speed-dependent connection characteristics a sample under impact load, wherein the sample is taken in a clamping section in which a in an accelerating device (4) accelerated over a delay line Piston (18) the load in the form of a substantially elastic shock to the sample (2), thereby characterized in that the accelerating device (4) is a test cylinder (20) has, whose end (19) is cone-shaped, and a correspondingly adapted conical piston (18), between which a damping element (21) works, and thus by a specific integration of a plastic shock share by means of the damping element (21) and by a defined ratio of the acceleration needed the piston mass Energy to the fracture energy of the sample the determination of force-displacement or force-time courses approximately constant deformation rate during sample failure and without significant overlay of Vibrations from bouncing processes is reached.

Description

The The invention relates to a testing device for determining speed-dependent connection characteristics a sample under impact load, wherein the sample is taken in a clamping section, in which one in one Acceleration device over a Advance run accelerated piston the load in the form of a essentially elastic shock transfers to the sample.

The Knowledge about the resistance, the deformability and the capacity to absorb materials and material connections under 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 highest 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 be able to perform high prediction accuracy is a knowledge of exact strength and deformation behavior of materials and joined Connections under high load speeds necessary. For the experimental determination of this strength and deformation behavior become testers used with a corresponding velocity profile. To one such testing device for Determination of characteristic values under impact load are very high requirements posed. Dynamic effects during the high speed test can the superimpose recorded 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 above those for quasistatic experiments, and the use of commercial testing machines is only in rare cases and with restrictions possible.

in the Over time, different concepts for testing of materials, joining compounds and components developed at high speeds. For the determination the strength and deformation behavior of the relevant for the automotive industry Strain rate of max. 500 / sec are here in particular the servohydraulic quick tearing, to call the rotary impact mechanism and the pendulum impactor.

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

The Servohydraulic quick tearing machine is with a pressure oil reservoir equipped, which emits the hydraulic fluid via a control valve and thus a piston in an accelerator on the Test speed brings. At the end of the delay line, the cylinder couples to the accelerator to the end of the inboard 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 over an inductive Langwegaufnehmer (LVDT = Longitudinal Velocity Differential transformer). When rotary impact, is accelerates the flywheel by an electric motor. As soon as the desired Circumferential speed is reached, the previously in the flywheel The hidden hammer is unfolded and strikes the yoke on which the sample is attached. On the other side of the sample is that Force measuring attached, the strain is measured by a strain gauge. The measuring signal is amplified and recorded by a storage oscilloscope.

The Pendulum works with a split fin and can be added Impact tensile tests are used. At the beginning of the 20th century the first pendulum impact testers for the implementation developed by impact test. These first became later too for the Determination of material and connection parameters further developed. At that time was at higher Speeds due to the lack of electronic possibilities only an estimate of the strength behavior by energy measurement with the pendulum or defined deformations possible. With the beginning of the 40s of the last century emerged with the development of piezo-quartz and strain gauges new options to record the loads and stresses during the Experimental procedure, so that more accurate conclusions on the deformation behavior of the materials became possible.

A Determination of sample deformation via the measurement of the displacement of the test cylinder provides in particular in experiments under sudden load often not sufficient information about the actual Deformation behavior of the samples.

In contrast to the tests under quasi-static and oscillatory loading, dynamic influences in the test procedure make it even more difficult at higher test speeds the interpretation and evaluation of the force signals. Thus, during the acquisition of the measured values during the experiment, superpositions of the force signals can occur due to vibrations which do not correspond to the load of the sample. Depending on the strain rate, these interferences arise from the superimposition of the actual measurement results with vibration effects from natural oscillations, inertial effects and wave phenomena, and can falsify the measurement signals in extreme cases so that an evaluation is no longer possible.

In front In this background, the force measurement is generally not done only over the testing device, z. B. on the crosshead in conventional servohydraulic quick-release machines, but beyond by the evaluation of the measurement signals applied to the sample DMS. Such a local data acquisition on the sample is in the frame of material testing State of the art. Attached Samples for determining characteristic values as a function of the test speed and the load introduction angle is a data acquisition directly at the joining point but not possible. Here is a complex test arrangement with many interfaces and comparatively large Masses necessary. These properties are in contradiction to the for abrupt Investigations required low masses and high stiffnesses in the clamping strand. In addition to the described problem area is that it generally fails to achieve a constant strain rate or local To achieve deformation speed.

In the German patent DE 38 02 500 C1 is a hydraulic drive for impact tensile tests described. Here is the piston to be accelerated within an oil volume. By the necessary displacement of the oil volume a desired sudden acceleration of the working piston is prevented. As a result, it is not possible to load directly at the required test speed at the start of the test. Although there is also a slight damping, dampens the bouncing effects, but a defined damping element is not provided.

It It is therefore an object of the invention to provide a test apparatus which works from the beginning with the intended test speed and while suppressing vibrations in the sample and in the measuring system, and to disclose a method, characteristic values of joints under impact load reliably determine.

The solution the task for the tester With the features of claim 1, the method with the features dissolved in claim 15. embodiments The invention are specified in the subclaims.

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).

in the Meaning the for abrupt investigations required low masses at high Stiffnesses of the clamping are the joints and holders in the clamping strand Reduced weight. The holder is made of titanium.

The necessary rigidity of the clamping section is caused by a bias voltage or achieved by friction.

In the sense of an optimized impact mechanism, a conical design of the receptacle and a correspondingly shaped piston head are also used. A key approach to reducing dynamic spurious effects during high-speed tensile tests is the minor ge increase the plastic shock component in the sudden engagement of the piston. Without this central optimization, the engagement of the piston in the receptacle - in this case steel und steel on steel - can be regarded in a first approximation as an elastic shock. In the case of this elastic impact, however, significant bounce effects occur, which are reflected in the significant vibrations in the force measurement, so that the evaluation of the measurement signals does not allow any information about the actual stress of the sample. This succeeds only through the targeted integration of a small amount of plastic impact into the operating principle of the test device. In this case, a thin damping element is mounted directly in the accelerator device.

Farther has been found by a defined ratio of energy needed to accelerate the piston mass to the fracture energy the sample the vibrations in measured to determine the characteristics Force signal during the impact load largely suppressed become.

On this way, the determination of force-distance / force-time progressions is added The examination of samples at near constant strain rate during sample failure and without significant overlay of vibrations in the force signal from bounce operations possible.

Of the test cylinder in the accelerator device is conical at its end. The piston is adapted to this cone accordingly, the Angle of the cone between 5 ° and 45 °.

On the cone-shaped Piston is the damping element non-slip applied.

By the damping element, a targeted and defined plastic component is integrated in the first approximation as elastic to be designated impact mechanism of the test bed. As a result, the vibrations in the force signal are reduced to a minimum or completely eliminated without the impact of the cylinder on the cone of the piston resulting in a significant reduction in the piston speed V _K in comparison to the purely elastic impact process. The local deformation rate during sample failure is comparatively constant during sample breakage.

The on the piston applied wear-resistant damping element, made of rubber or is made of elastic plastic, is in its thickness of the moving Mass in the clamping section dependent. 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 rather takes with increasing pressure load through the test cylinder exponentially too. Furthermore, such a damping element is characterized by very good wear properties out.

The Tester based on a servohydraulic quick-tearing machine, a rotary impact mechanism or a drop tower. In the servohydraulic quick tearing becomes the test load is generated by means of a Hochgeschwindigskeitsprüfzylinders, the from a high pressure oil storage is fed.

The Sample, with the characteristic values in dependence the load rate and the load application angle determined are typically a so-called KS-II sample. Such a KS-II Sample consists of two U- or L-shaped profile parts whose Floor parts over a joining technique connected to each other. The joint connection can be a welded joint, Adhesive bond, a mechanical joint or the like be.

The Legs of the sample are held in a two-piece holder over which the sample is held in the clamping section.

there is the holder of the sample is designed so that the initiation the impact load Centric in the sample and at different, defined angles to the joint connection he follows. usual related to the joining surface Load introduction angles are 0 °, 30 °, 60 ° or 90 °, so that Characteristic values for the Kopfzug, the Scherzug and for Intermediate angle and the peel train can be determined.

to torque-free force transmission are in the clamping strand on both sides The probe integrates joints that make the impact load centric and torque-free transferred to the test.

By a defined ratio the energy needed to accelerate the piston mass to the fracture energy the sample becomes the vibrations in order to determine the characteristic values measured force signal during the impact load largely suppressed. For example, at a fracture energy of the sample of about 40 J at a set test speed of 2 m / s and a mass of the piston of 7 kg can be almost a constant deformation rate at failure of the samples achieved become.

With the test device described above, characteristic values of compounds can be determined precisely. These characteristics, which include Breaking force, fracture energy, stiffness and fracture path are automatically evaluated on a measuring computer.

To are the locally measured on the sample deformation of the sample and determines the force measured locally on or on the sample. For force value recording The application of strain gauges (strain gauges) on the Sample or in the chuck between the sample and the joints proven.

to Detection of sample deformation under impact load has increased the gray value correlation method based optical deformation measurement established. In this measuring system, with the area coordinates, strains and other derived quantities can, the local deformation directly on the the sample be determined. The measured value was recorded during the Load tests on the testing machine through a high-speed camera system directly on the sample.

embodiments The invention is illustrated by way of example in the figures.

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 a KS-II sample.

6 shows a holder with sample.

7 shows a force-deflection diagram based on use of the optimized accelerator.

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 measurement is carried out on the sample or in the immediate vicinity of the sample "quasilocal" via applied strain gauges 10 fed and evaluated. For this purpose, a special software tool is available that enables the evaluation of the connection characteristics (breaking load, energy to break, 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 is a perspective view ei a sample 2 shown. It is designed as a typical KS-II sample. The KS-II sample consists of two U-profiles produced and joined together by forming technology. Depending on the joining process, close tolerances must be observed in the production of semifinished products so that the samples do not undergo any damage due to their being clamped in the testing device. About the holes 13 Screws are inserted, with which jaws for non-slip force transmission are attached to the sample. The bottoms of the two profiles are over a joining surface 14 connected, in which the joint connection 14a located. From this joint connection, the characteristic values are determined. This sample can be tested under different load introduction angles to the joint surface ( 14 ) being checked. In the KS-II peel test, the semi-finished products are provided with only one leg forming technology.

In 6 is shown as the sample 2 in the holder 5 is clamped, consisting of the two parts 5a . 5b consists. Here is a holder 5 for a shear pull (load introduction angle to the joining surface 0 °). The force is thus centered by the holder over an angle of 0 ° to the joint surface 14 introduced into the sample.

At the rehearsal 2 are optical marks 22x . 22y whose displacements are recorded by means of the camera system during the test. The evaluation then takes place using the gray value correlation method.

The local deformation describes the difference of the displacements of a point on the upper compared to a point 22y or more on the lower half of the sample. The local strain rate is the local strain over time.

In 7 a comparison between measured values A of the force F with oscillations and measured values B measured by means of the optimized testing device is shown. The deformation s corresponds to the local deformation determined directly on the sample by means of optical deformation measurement.

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

s

deformation

Claims (21)

Test device for determining speed-dependent connection characteristics of a sample under impact load, wherein the sample is gripped in a clamping section in which a test 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 ), characterized in that the accelerating device ( 4 ) a test cylinder ( 20 ) whose end ( 19 ) is cone-shaped, and a correspondingly adapted cone-shaped piston ( 18 ), between which a damping element ( 21 ), and thus by a specific integration of a plastic impact component by means of the damping element ( 21 ) 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 strain rate during sample failure and without significant superposition of vibrations from bounce processes is achieved. Tester according to claim 1, characterized in that the damping element non-slip on the cone of the piston or the test cylinder is attached. Test device according to claim 1, characterized in that the damping element ( 21 ) has a non-constant damping behavior. Test device according to claim 3, characterized in that the damping behavior of the damping element ( 21 ) increases exponentially during impact loading. Test device according to claim 1, characterized in that the damping element ( 21 ) a layer of wear-resistant rubber or elas is plastic. Test device according to claim 1, characterized in that the angle of the cone ( 19 ) is between 5 ° and 45 °. Tester according to claim 1, characterized in that the impact load in the clamping section over a servohydraulic quick tearing machine, a rotary impact mechanism or a drop tower is generated. Test device according to claim 1, characterized in that the impact load by means of a Hochgeschwindigskeitsprüfzylinders ( 3 ) produced from a high pressure oil reservoir ( 11 ) is fed. Test device according to claim 1, characterized in that the sample ( 2 ) consists of two U- or L-shaped profile parts, the bottom parts of a joint surface ( 14 ) connected by a joining technique and the legs by a two-part holder ( 5 ). Test device according to claim 9, characterized in that the holder ( 5 ) of the sample is designed so that the initiation of the impact load in the sample centric and at different, defined angles to the joint surface ( 14 ) he follows. Tester according to claim 10, characterized in that the load introduction angle based on the joint surface of 0 °, 30 °, 60 ° and 90 ° provided are. Test device according to claim 10, characterized in that in the clamping strand on both sides of the sample joints ( 6 ), the impact load is centric and torque-free on the sample ( 2 ) transfer. Tester according to claim 1, characterized in that the clamping distance by bias or friction has a high rigidity. Tester according to claim 10, characterized in that the holder made of titanium is made. Test device according to claim 1, characterized in that at a foreseeable fracture energy of the sample ( 2 ) of about 40 J at a set test speed of 2 m / s and a mass of the piston of 7 kg a nearly constant deformation rate is achieved in the failure of the samples. Method for determining speed-dependent connection characteristics of a sample ( 2 ) under impact load, with a test device ( 1 ) according to any one of the preceding claims. A method according to claim 16, characterized in that to the determined characteristics locally on the sample ( 2 ) measured deformation of the sample and the force (F) measured locally on or on the sample. A method according to claim 17, characterized in that the force measurement by means of strain gauges on the sample ( 2 ) and / or one or both sides of the sample between the sample ( 2 ) and the joints ( 6 ) he follows. A method according to claim 18, characterized in that the local deformation of the sample directly on the sample ( 2 ) is determined by means of optical deformation measurement. A method according to claim 19, characterized in that the local deformation measurement by means of a high-speed camera ( 8th ) and evaluation of the camera images according to the grayscale correlation method. Method according to claim 16, characterized in that that an automatic, computer-based evaluation of the connection characteristics such as breaking strength, fracture energy, rigidity or fracture path.