DE102018200514A1 - Method and device for determining a pressure load of a sample caused by a shock wave - Google Patents

Abstract

The invention relates to a method for determining a pressure load of a sample (114) caused by a shock wave. In this case, a sample surface of the sample (114) is irradiated with a laser beam (104) through a transparent cover layer (116) and an absorption layer (118) arranged between the sample surface and the cover layer (116) in order to ablate material of the absorption layer (118). to generate a plasma (204) by which the sample (114) is exposed to the shock wave (202). Subsequently, a plastic deformation of the sample surface caused by the shock wave is detected to obtain at least one deformation parameter (122). Finally, the deformation parameter (122) is evaluated using at least one laser parameter characterizing the laser beam (104) in order to determine the pressure load.

Description

State of the art

The invention is based on a method or a device according to the category of the independent claims. The subject of the present invention is also a computer program.

In the case of injectors, pumps or valves, local shockwave loads, such as cavitation, can limit the service life. Highly dynamic, shock-like loads can lead to a heavy load on the order of gigapascals over a period of a few nanoseconds and result in local material plastification. As the process progresses, material damage, such as cavitation erosion, may occur. The respective behavior and the limits are material-dependent.

For the characterization of materials under shockwave loading and the determination of characteristic characteristics, reliable, standardized test methods and equipment are required that can test the strain rates of the material throughout the entire shockwave load range for individual events.

For example, the following test methods can be used to investigate shockwave events.

In the Split-Hopkinson-Bar experiment, a rod is accelerated to a sample and accelerometers are used to determine the wave propagation of the compression and expansion components of the stress wave in the rod. These allow conclusions to be drawn about the dynamic material properties. This is a single event test.

Another way to introduce shock waves in materials is to investigate cavitation bubble collapse in terms of material and sample proximity. For this purpose, bubbles are generated with a pulsed laser, which generate pressure waves during formation and collapse, which strike the surface and stress the material. It is a quasi-single-event test.

In order to investigate cavitation erosion, the cause of which is shock wave loading, specimens may also be subjected to a stochastic load for a defined period of time. In this case, mass losses resulting from cavitation erosion can be registered. The load is typically generated with cavitation channels (e.g., Franc cavitation tunnels), sonotrodes, or jet cavitation test rigs. This is a stochastic multiple event test. In combination with the pitting method for the evaluation and analysis of local shockwave impressions in the material surface of a ductile metal, also called pits, material laws can be parametrised with these tests. (Material laws have not yet been parameterized, but rather a material law was used to simulatively infer the shockwave pressures.) For example, in the pitting method, a polished metal surface can be subjected to cavitation loading in a multiple test bench for a short period of time. Thus, impressions or pits of individual results in the metal surface can be detected. The surface deformations can be quantified by optical or tactile methods. By a simple, inverse simulation approach, for example, conclusions can be drawn from the pit geometry to the acting shock wave load. (For this, a material model must be adopted for which there are no transferable test procedures yet.)

In order to validate multi-phase simulations, it is necessary to make reliable statements about the magnitude of the impacting shockwaves. It was also to be clarified how the stochastic effect of the strain affects the erosion progress. In order to be able to interpret simulation results from numerical fluid mechanics, also called Computational Fluid Dynamics or CFD for short, in addition to the information about the amplitude of the acting shockwaves, a statement about the number of events per area up to the onset of detectable damage is required.

Disclosure of the invention

Against this background, with the approach presented here, a method for determining a pressure load of a sample caused by a shock wave, a device using this method, and a corresponding computer program according to the main claims are presented. The measures listed in the dependent claims advantageous refinements and improvements of the independent claim device are possible.

The approach presented here is based on the finding that, for example, shockwave pressures occurring in cavitation on a sample can be experimentally determined by selectively applying one or more shock waves to one or more laser pulses and then causing a plastic deformation caused by the shock wave loading Sample surface is evaluated in a suitable manner to the shock wave pressures or pressure loads to determine. On the basis of this experimental determination, CFD results for cavitation erosion, for example, can be validated.

The approach presented here can be regarded as an extension of the surface treatment method Laser Shock Peening, or LSP for short. This can be used to introduce compressive stresses into materials and thereby positively influence the mechanical properties.

For example, it is also possible to examine the material fatigue under repeated shock loading of the sample by means of locally repeated shock waves.

In particular, a test method is presented in the present case with which local shockwave stresses can be generated and investigated under defined conditions. The test method includes, for example, the generation of the load events, the control and adjustment of the set parameters, and the traversal of the material sample to be tested. With a corresponding test facility, defined single and multiple loads with local and temporal variance on the sample can be imprinted. The test setup can be used, for example, to study characteristic characteristics for problems of cavitation erosion or to calibrate pressure sensors for highly dynamic events.

Essential questions in material and shockwave investigations are which pressures or strains actually affect the material and how many events of a pressure or load amplitude are necessary to cause a local material damage.

In the case of the Split-Hopkinson-Bar attempt, the dynamics are problematic in the first place. In the literature, it is assumed that strain rates in the range 10 ⁶ 1 / s (up to 10 ^ 6 1 / s) occur under shock wave loading under cavitation. In the split-Hopkinson bar experiment, only strain rates of up to 10 ⁴ 1 / s (more like 4 × ^10∧ 3 1 / s) are achieved, which for many applications does not fully cover the measurement range.

In the experimental test rig structure of the laser-induced single bubble, in which a cavitation near shock wave load is to be applied to a material surface, two shock waves are always caused per event, namely the optical breakthrough and bladder collapse. Furthermore, the spatial extent of the acting load event in the millimeter range is significantly greater than in the case of real cavitation load events in the micrometer range. Furthermore, the stress can not be reproducibly applied to a site, as nuclei in the liquid lead to a three-dimensional migration of the location of the optical breakthrough. Finally, the generated shock waves are too weak and can not achieve sufficiently large amplitudes to damage the material by the shock wave loading. This is due to the fact that it is hardly possible by the experimental design to induce the bubbles near the wall, without causing thermal damage to the surface. It could be shown that the amplitude of the shock waves drops by two orders of magnitude, for example from 10 GPa to 100 MPa, already 100 μm after the point of initiation by the laser.

If the material is subjected to shock wave loading in multiple-event tests, for example by means of a sonotrode or a dynamometer, an averaged, global image results, which as a rule can not be traced back to a defined number of individual events. It is always superimposed on a variety of individual events of different amplitudes and sizes. Furthermore, there is no information about the load size and the amplitude of the acting shockwave.

The approach presented here proposes a solution to the technical and methodological problems mentioned above. Specifically formulated, the following core advantages result.

On the one hand, the approach can be used for the fundamental investigation of the material behavior of solids at highly dynamic loads. It is possible to apply a very targeted and defined shock wave load on the material, since the accuracy is limited only by the alignment accuracy of the sample holder. Here, the load time (magnitude of the laser pulse duration), the load amplitude (proportional to the introduced intensity I of the laser radiation) and the extent of the load (size of the laser spot) are exactly defined. By modifying the listed parameters, a very precise influence on the characteristics of the generated shockwaves is possible. This makes it possible to investigate and describe the local material behavior under shock rates relevant to shock waves, for example of 10 ⁶ 1 / s.

On the other hand, the approach offers the possibility to generate individual events and to investigate the repeated effects of shock waves of the same amplitude on a single site. Thus, in addition to a statement about the acting pressures, a number-related statement about the shockwave resistance of a material can be generated. For example, in combination with a Truss-controlled sample retention, which allows the sample to move relative to the load, also examines stochastic pitting, which can be used, for example, in life prediction prediction as the basis for a post-processing tool of CFD results.

It is also advantageous that material calibrations and characterizations are possible for a wide variety of metallic materials, including those for the high-strength bearing steel 100Cr6 (annealed at 62HRC).

The variability of the adjustable load events also makes it possible to use a parameter study to calibrate a ductile material as a shockwave sensor. For this example, shock waves of different amplitude and extent can be applied to a polished sample. The amount of plastic deformation acts as an immediate feedback on the amplitude of the stress event. Thus, a nondestructive analysis is possible. By correlating pit depth and pit volume or the ratio of pit depth to pit diameter with the computationally generated pressure, a purposeful statement about the material behavior can be made.

In addition to the direct correlation with individual events from the stochastic test benches, material calibration also offers the possibility of implementing the results as the basis for a shockwave-relevant material law in an FEM calculation. The test stand is a purely mechanical shockwave load without undefined, dissipative friction effects. As a result, the available material laws can be extended by one facet.

The approach presented here can also be used to calibrate pressure sensors, in particular their rise behavior, and to investigate the reaction times. In order to determine the reaction and rise time of pressure sensors, for example, a pressure pulse on the active surface of the pressure sensor can be initiated and the signal can be tapped on an oscilloscope. The beginning and duration of the laser pulse can be determined, for example, by a photodiode. The literature shows that the pressure pulse corresponds approximately to twice the laser pulse duration. By comparing the diode and pressure signal information about the reaction time and the rise time of the pressure sensor can be obtained. As the energy of the laser pulse increases, greater pressures are initiated on the active surface of the pressure sensor. The progression of the voltage signals can for example be used below as input for a calibration curve.

The use of pressure sensors is usually limited by their physical limits. Since the cavitation load events are on the order of gigapascals over the duration of a few nanoseconds, currently available pressure sensors can not resolve these events. On the one hand, the natural frequency of the pressure sensors is usually too low, on the other hand, the pressure amplitudes are too large. Consequently, the pressure sensors usually can not resolve the loads in time to an adequate degree. Another problem with the use of pressure sensors with a suitable rise time is the specific calibration. Furthermore, pressure sensors average the load events across their active area. Thus, one or more cavitation bubble collapses with a loading diameter of about 10 to 100 microns are integrated over the active area.

The approach presented here now creates a method in which defined single and multiple loads with local and temporal variance on a material sample are considered. As a result, a targeted, highly dynamic calibration of the pressure sensor can be carried out. An appropriate test setup can be used to investigate characteristic characteristics for problems of cavitation erosion. The results obtained can be used to apply the material as a kind of calibrated pressure sensor in real cavitation test benches. In summary, the basic idea of the approach presented here is the combination of the pitting method with the laser shock peening method. In this way, conditions can be set experimentally for a physics-based approach, by means of which cavitation erosion can be reliably predicted. The plastic deformation behavior of a material under the action of a shockwave event of specific amplitude and extent can be used as information to correlate the deformations detected by the pitting method with the pressure required for this purpose.

• Bestrahlen einer Probenoberfläche der Probe mit einem Laserstrahl durch eine transparente Deckschicht und eine zwischen der Probenoberfläche und der Deckschicht angeordnete Absorptionsschicht, um unter Ablation von Material der Absorptionsschicht ein Plasma zu erzeugen, durch das die Probe mit der Schockwelle beaufschlagt wird;
• Erfassen einer durch die Schockwelle hervorgerufenen plastischen Verformung der Probenoberfläche, um zumindest einen Verformungsparameter zu erhalten; und
• Auswerten des Verformungsparameters unter Verwendung zumindest eines den Laserstrahl charakterisierenden Laserparameters, um die Druckbelastung zu bestimmen.

A method is provided for determining a pressure load of a sample caused by a shock wave, the method comprising the following steps:

• Irradiating a sample surface of the sample with a laser beam through a transparent cover layer and an absorption layer disposed between the sample surface and the cover layer to produce a plasma by ablation of material of the absorption layer by which the sample is exposed to the shock wave;
• Detecting a caused by the shock wave plastic deformation of the Sample surface to obtain at least one deformation parameter; and
• Evaluating the deformation parameter using at least one laser parameter characterizing the laser beam to determine the pressure loading.

Under a shock wave, also called shock wave, a strong pressure wave can be understood. The sample may in particular be a solid. Under a cover layer can be understood, for example, a glass or plastic layer or a clear liquid such as water. By an absorption layer, for example, a black film or color layer can be understood. Ablation can be understood as the removal of material by heating. A plastic deformation can be understood as meaning at least one permanently remaining deformation point in the form of a small depression in the sample. Such a deformation point can also be referred to as a pit. A deformation parameter may be understood as meaning a parameter characterizing a geometry of the plastic deformation, for example a depth, a diameter, a volume or another parameter derived therefrom. By a laser parameter, for example, an intensity, a pulse duration, a wavelength or a beam diameter of the laser beam can be understood. A plasma can in particular be understood as meaning a high-pressure plasma whose pressure is significantly higher than the pressure of the surrounding atmosphere.

According to one embodiment, in the step of detecting, a depth, a volume, a diameter or a number of deformation points on which the sample is plastically deformed, or a time course of the deformation or a combination of at least two of said quantities can be detected as deformation parameters. By a time course, for example, a pressure curve measured by means of an oscilloscope can be understood. Thereby, the deformation parameter can be easily provided.

According to a further embodiment, a theoretical pressure load of the sample can be determined in the step of the evaluation using the laser parameter. The theoretical pressure load and the deformation parameter can be linked together to determine the pressure load. As a result, the pressure load can be reliably determined.

wobei α ein Maß für die Absorption der Laserenergie ist, Z für den Wellenwiderstand (komplexe akustische Impedanz von transparenter Deckschicht und Covermaterial) und I für die Intensität steht.For example, the theoretical pressure load can be calculated using the following formula: $$p\left[GPa\right]=0.01\sqrt{\frac{\mathrm{<figure-callout\; id="2"\; label="\alpha "\; filenames="DE102018200514A1\_0008.png,DE102018200514A1\_0009.png"\; state="\{\{state\}\}">\alpha </figure-callout>}}{2\alpha +3}}\cdot \sqrt{Z}\left[\frac{\mathrm{<figure-callout\; id="2"\; label="G\; c\; m"\; filenames="DE102018200514A1\_0008.png,DE102018200514A1\_0009.png"\; state="\{\{state\}\}">G\; </figure-callout>}}{c{m}^{}}\right]\cdot \sqrt{I}\left[\frac{GW}{c{m}^2}\right].$$

where α is a measure of the absorption of the laser energy, Z is the characteristic impedance (complex acoustic impedance of transparent cover layer and cover material) and I stands for the intensity.

It can be generated in the step of the evaluation by applying the theoretical pressure load on a quotient of the depth and the diameter of the deformation points, a calibration curve. This can be linked to deformation values representing a cavitation-related plastic deformation of a cavitation sample subjected to shock waves in a cavitation method in order to determine a calibrated pressure load. A quotient can be understood as meaning a similarity number. For example, the cavitation sample may have the same material and / or surface texture as the sample. Alternatively, the sample and the cavitation sample may be the same sample. A cavitation method can be understood as meaning a method in which a sample is subjected to a shock wave, for example by means of ultrasound or fluid jet, in order to bring about cavitation erosion, also called pitting. The cavitation method can therefore also be referred to as a pitting method. By this embodiment, results obtained in the context of a cavitation process can be experimentally verified or corrected. Alternatively, other surface deformation parameters may be used for the calibration. This shows that the energy of the laser shot correlates very well with the volume of a pit, and this correlation is also suitable for the calibration of different materials (deformation resistance to acoustic energy).

According to a further embodiment, in the step of the evaluation, the calibration curve can be offset with a cavitation curve in order to determine the calibrated pressure load. The cavitation curve can represent a relationship between a number of cavitation-related deformation sites where the cavitation sample is plastically deformed due to cavitation and a quotient of a depth and a diameter of the cavitation-related deformation sites as the deformation values. The quotient of the cavitation curve can be understood as meaning a similarity number analogous to the quotient of the calibration curve. This allows a precise experimental determination of the pressure load.

Advantageously, in the step of evaluating a load curve can be generated, the one Represents the relationship between the calibrated pressure load and the number of cavitation deformation points. As a result, the calibrated pressure load can be correlated with the number of cavitation-related deformation points.

According to a further embodiment, in the step of irradiating, the sample surface can be irradiated at at least two different locations. As a result, the accuracy of the method can be increased.

The method may further comprise a step of forming a composite layer of the cover layer and the absorption layer. In this case, in the step of irradiation, the sample surface can be irradiated through the layer composite. For example, the absorption layer may be glued or painted onto the cover layer. As a result, the handling of the two layers can be simplified. In particular, this allows a simple displacement of the sample relative to the two layers, for example to irradiate the sample at several points.

Advantageously, in the step of irradiating, the sample surface may be repeatedly irradiated to subject the sample to multiple shock waves. In this case, the sample can be acted upon by varying the laser beam with different shock waves. This makes it possible to make reliable statements about a shockwave induced material fatigue of the sample.

This method can be implemented, for example, in software or hardware or in a mixed form of software and hardware, for example in a control unit.

The approach presented here also creates a device that is designed to perform the steps of a variant of a method presented here in appropriate facilities to drive or implement. By a device, for example, a corresponding test stand or experimental setup can be understood. Also by this embodiment of the invention in the form of a device, the object underlying the invention can be solved quickly and efficiently.

Also of advantage is a computer program product or computer program with program code which can be stored on a machine-readable carrier or storage medium such as a semiconductor memory, a hard disk memory or an optical memory and for carrying out, implementing and / or controlling the steps of the method according to one of the embodiments described above is used, especially when the program product or program is executed on a computer or a device.

• 1 eine schematische Darstellung einer Vorrichtung gemäß einem Ausführungsbeispiel;
• 2 eine schematische Darstellung einer Probe in Kombination mit einem Schichtverbund gemäß einem Ausführungsbeispiel;
• 3 ein Diagramm zur Darstellung zweier Druckverläufe zur Verwendung in einem Verfahren gemäß einem Ausführungsbeispiel;
• 4 einen realen Laserscan einer Oberfläche einer bestrahlten Probe mit Auswertung gemäß einem Ausführungsbeispiel;
• 5 ein Diagramm zur Darstellung eines beispielhaften Verlaufs einer Materialkalibrierung gemäß einem Ausführungsbeispiel;
• 6 ein Diagramm zur Darstellung von Signalverläufen bei einer Drucksensorkalibrierung gemäß einem Ausführungsbeispiel;
• 7 eine schematische Darstellung einer Oberfläche einer Kupferprobe nach dem Beaufschlagen mit Schockwellen in einem Kavitationsverfahren;
• 8 eine schematische Darstellung einer Oberfläche einer austenitischen Stahlprobe nach dem Beaufschlagen mit Schockwellen in einem Kavitationsverfah ren;
• 9 eine schematische Darstellung einer Oberfläche einer ferritischen Stahlprobe nach dem Beaufschlagen mit Schockwellen in einem Kavitationsverfah ren;
• 10 ein Diagramm zur Darstellung dreier Kavitationskurven zur Verwendung in einem Verfahren gemäß einem Ausführungsbeispiel;
• 11 ein Diagramm zur Darstellung dreier Kalibrierkurven zur Verwendung in einem Verfahren gemäß einem Ausführungsbeispiel;
• 12 ein Diagramm zur Darstellung dreier Lastkurven, generiert in einem Verfahren gemäß einem Ausführungsbeispiel; und
• 13 ein Ablaufdiagramm eines Verfahrens gemäß einem Ausführungsbeispiel.

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description. It shows:

• 1 a schematic representation of a device according to an embodiment;
• 2 a schematic representation of a sample in combination with a layer composite according to an embodiment;
• 3 a diagram showing two pressure curves for use in a method according to an embodiment;
• 4 a real laser scan of a surface of an irradiated sample with evaluation according to an embodiment;
• 5 a diagram illustrating an exemplary course of a material calibration according to an embodiment;
• 6 a diagram illustrating waveforms in a pressure sensor calibration according to an embodiment;
• 7 a schematic representation of a surface of a copper sample after the application of shock waves in a cavitation process;
• 8th a schematic representation of a surface of an austenitic steel sample after exposure to shock waves in a Kavitationsverfah ren;
• 9 a schematic representation of a surface of a ferritic steel sample after exposure to shock waves in a Kavitationsverfah ren;
• 10 a diagram showing three Kavitationskurven for use in a method according to an embodiment;
• 11 a diagram showing three calibration curves for use in a method according to an embodiment;
• 12 a diagram showing three load curves generated in a method according to an embodiment; and
• 13 a flowchart of a method according to an embodiment.

In the following description of favorable embodiments of the present invention, the same or similar reference numerals are used for the elements shown in the various figures and similar acting, with a repeated description of these elements is omitted.

1 shows a schematic representation of a device 100 according to an embodiment. By way of example, a schematic experimental setup for a material test under the action of individual and repeated shock waves is shown. The device 100 has a laser source 102 for emitting a laser beam 104 on, here for example a Nd: YAG laser. The laser beam becomes exemplary 104 via an attenuator unit 106 for energy adjustment, about a micrometer screw, a beam reduction unit 108 , a plurality of mirror elements 110 and a focus lens 112 on a surface of a sample 114 directed. Opposite the surface of the sample 114 there is a composite of a transparent cover layer 116 , here in the form of a glass pane, and a black absorption layer 118 that is between the topcoat 116 and the surface of the sample 114 is arranged. This causes the surface of the sample 114 through the cover layer 116 and the absorption layer 118 through with the laser beam 104 is irradiated, a high pressure plasma is generated, through which the surface of the sample 114 is acted upon by a shock wave. This will be part of the energy of the laser beam 104 through the absorption layer 118 absorbed.

A registration unit 120 , for example in the form of a laser scanner, is designed to provide a plastic deformation of the surface of the sample when it is subjected to the shock wave 114 to capture and at least one deformation parameter characterizing the deformation 122 issue. The deformation can also be referred to as a pit. An evaluation unit 124 is designed to be the deformation parameter 122 using at least one parameter of the laser beam 104 evaluate and so one by the shock wave to the test 114 to determine acting pressure load.

$$\text{D}=\mathrm{8,3}\text{ mm}$$

$$\text{FHBW}=10-12\text{ ns}$$

$$\mathrm{\lambda }=532\text{ nm}$$

The laser beam 104 has, for example, the following parameters: $${\text{<figure-callout id="2" label="M" filenames="DE102018200514A1\_0008.png,DE102018200514A1\_0009.png" state="{{state}}">M</figure-callout>}}^2=1.7-1.9$$

$$\text{D}=8.3\text{mm}$$

$$\text{FHBW}=10-12\text{ns}$$

$$\mathrm{\lambda }=532\text{nm}$$

The laser beam 104 can also be generated with a reference wavelength of 1064 nm instead of 532 nm or any other suitable reference wavelength. The pulse duration (in the nanosecond range), M ² and the output beam diameter can also be varied.

The focus lens 112 , here by way of example with a focal length f = 120 mm, according to this embodiment, in three defined distances to the cover layer 116 can be arranged, wherein each of the three distances a defined beam diameter of the laser beam 104 represents. The corresponding three beam diameters are to the right of the focus lens 112 shown schematically, with a mean beam diameter corresponds to a distance of 10 mm and a maximum beam diameter a distance of 18.5 mm. The dimensions are only dependent on the optics and not relevant. By reducing the distance of the sample to the lens (starting point focus), investigations in the defocused laser beam can be carried out and thus larger spot diameters can be investigated. Alternatively, with a larger focal length also a larger spot diameter can be achieved.

An optional traversing unit 126 is trained to the sample 114 relative to the composite of the two layers 116 . 118 to move. This allows the sample 114 be acted upon at different points with the shock wave.

Hereinafter, embodiments of the approach presented here for better understanding again described in other words.

wobei α ein Maß für die Absorption der Laserenergie ist, Z für den Wellenwiderstand (komplexe akustische Impedanz von transparenter Deckschicht und Covermaterial) und I für die Intensität steht.Similar to laser shock peening pressure curves, for example, a pulsed laser with a nanosecond pulse duration is used to generate shock waves in the material of the sample 114 to create. Shock waves are caused by ablation of material near the surface of the sample 114 generated. This material absorbs the intense laser radiation. In the case of limited ablation, in addition to the ablation material in the form of the absorption layer 118 a transparent cover material in the form of the cover layer 116 applied. The cover layer 116 is formed for example by glass, Plexiglas or water. The cover layer 116 hinders the spread of the plasma, causing very high pressures on the material surface. These pressures can be calculated analytically using, for example, the following formula: $$p\left[GPa\right]=0.01\sqrt{\frac{\mathrm{<figure-callout\; id="2"\; label="\alpha "\; filenames="DE102018200514A1\_0008.png,DE102018200514A1\_0009.png"\; state="\{\{state\}\}">\alpha </figure-callout>}}{2\alpha +3}}\cdot \sqrt{Z}\left[\frac{\mathrm{<figure-callout\; id="2"\; label="G\; c\; m"\; filenames="DE102018200514A1\_0008.png,DE102018200514A1\_0009.png"\; state="\{\{state\}\}">G\; </figure-callout>}}{c{m}^{}}\right]\cdot \sqrt{I}\left[\frac{GW}{c{m}^2}\right].$$

where α is a measure of the absorption of the laser energy, Z is the characteristic impedance (complex acoustic impedance of transparent cover layer and cover material) and I stands for the intensity.

The shock wave pressure is modifiable via the laser intensity I, ie the laser energy per time per area, which can be measured very accurately. Depending on the distance between lens 112 and sample 114 Different spot sizes can be realized. In contrast to the laser shock peening process, it is not primarily the resulting residual compressive stresses that are used as a measure of the applied pressures, but the plastic deformations that are caused by the shockwave. Thus, it can be reliably determined how different materials behave under the influence of a defined shock wave amplitude and number.

As with the pitting method, for example, samples are used which are polished to a very smooth surface, for example with an average roughness depth Rz = 0.02 μm. This ensures that even minimal plastification of the surface is detected. To make a statistical hedge, several laser shots per energy are made. The sample 114 This is achieved by an automated alignment motor system as traversing unit 126 method.

Various parameters are suitable for the evaluation. For example, the displaced volume can be correlated with the incident laser energy. In 5 By way of example, the result of a material calibration for an austenitic and a ferritic steel is shown. The recorded pit depth is correlated with the acting (calculated) shock wave pressures.

Furthermore, it allows in 1 shown experimental setup that the material behavior can be examined after an arbitrary large number of acting shock waves. For this purpose, the black absorption medium is renewed after each shot. This is done by a relative movement between sample 114 and cover material achieved. For example, a black plastic film as the absorption layer 118 adhesive with a glass sheet as cover layer 116 connected. This consequently acts as a transparent carrier material. Thus, it is possible to generate cavitation-relevant Wöhler curves in which shockwave pressure is correlated with the number of events required to cause local damage.

2 shows a schematic representation of a sample 114 in combination with a layer composite 200 according to an embodiment, such as a sample, as described above with reference to 1 is described. Shown is a schematic representation of the generation of laser-generated shock waves 202 , The cover layer is here 116 with the absorption layer 118 to the layer composite 200 connected. The cover layer 116 leads to the amplification of the pulse. The shockwave 202 leads to plastic deformation of the material surface. The layer composite 200 is relative to the sample 114 displaceable. This is achieved, for example, by the absorption layer 118 to one of the sample 114 facing surface of the cover layer 116 is applied. At the absorption layer 118 it is about a color layer or an adhesive film. Also marked is the laser beam 104 generated high pressure plasma 204 ,

3 shows a diagram illustrating two pressure gradients 300 . 302 for use in a method according to an embodiment. Shown are the progressions of pressing the laser shock waves at different intensities and cover materials, wherein the first pressure curve 300 associated with a first cover material and the second pressure profile 302 associated with a second cover material. The pressures were determined by means of the above with reference to 1 calculated formula and can also be referred to as theoretical pressure load. The intensity I is plotted on the abscissa. The theoretical pressure load p is plotted on the ordinate.

4 shows a real laser scan of a surface of an irradiated sample 114 with evaluation according to an embodiment. Shown is the result of a 3-D laser scan of deformation points 400 , also called pits, which were generated on the surface by moving the sample relative to the composite of cover and absorption layer after each laser shot. Height and width of the deformation points 400 are readable in microns. Shown are also a first diagram 402 for representing a respective volume of the deformation points 400 in μm ³ , a second diagram 404 to represent a respective depth of the deformation points 400 in μm and a third diagram 406 to represent a respective quotient of depth and equivalent diameter at half depth.

5 shows a diagram illustrating an exemplary course of a material calibration according to an embodiment. Shown are a first course 500 a pit depth using a sample of austenitic steel and a second course 502 a pit depth when using a sample of ferritic steel. The pit depths in μm are plotted against the calculated pressure in GPa. A print area within which an optical breakthrough takes place in a cover layer of glass is marked with a circle.

6 shows a diagram illustrating waveforms 600 . 602 in a pressure sensor calibration according to an embodiment. In this case, a voltage in V over time in s is plotted. Exemplary are the waveforms 600 one Müller sensor a waveform 602 a highly sensitive photodiode for detecting the plastic deformation of the sensor surface faced. By comparing the signal curves, the reaction and rise time of the pressure sensor can be determined.

7 shows a schematic representation of a surface of a copper sample 700 after exposure to shock waves in a cavitation process.

8th shows a schematic representation of a surface of an austenitic steel sample 800 of austenitic steel 1.4401 after exposure to shock waves in a cavitation process.

9 shows a schematic representation of a surface of a ferritic steel sample 900 of ferritic steel 1.4511 after exposure to shock waves in a cavitation process.

In the in the 7 to 9 dark areas to be recognized are cavitation-related deformation points, so-called pits.

10 shows a diagram showing three cavitation curves 1000 . 1002 . 1004 for use in a method according to an embodiment, wherein the first cavitation curve 1000 the in 7 copper sample shown, the second cavitation curve 1002 the in 8th shown austenitic steel sample and the third cavitation curve 1004 the in 9 associated ferritic steel sample is assigned. The three curves each represent a relationship between a number of cavitation-induced deformation points at which the respective samples were plastically deformed due to cavitation, and a quotient h / d from a depth h and a diameter d of the cavitation-related deformation points. The quotient can also be referred to as a similarity number.

11 shows a diagram showing three calibration curves 1100 . 1102 . 1104 for use in a method according to an embodiment. The calibration curves each represent a relationship between an approximately using the basis of 1 described formula calculated theoretical pressure load and a quotient H / D from a depth H and a diameter D of deformation points, where one by means of the device 1 Shockwave copper sample, austenitic steel sample or ferritic steel sample was plastically deformed. Here is the first calibration curve 1100 the copper sample, the second calibration curve 1102 the austenitic steel sample and the third calibration curve 1104 assigned to the ferritic steel sample. Respective linearizations are indicated by dashed lines. The theoretical or calculated pressure load is plotted in GPa on the ordinate.

12 shows a diagram showing three load curves 1200 . 1202 . 1204 generated in a method according to an embodiment. The first load curve represents this 1200 the result of a combination of the first cavitation curve 1000 out 10 with the first calibration curve 1100 out 11 , the second load curve 1202 the result of a linkage of the second cavitation curve 1002 out 10 with the second calibration curve 1102 out 11 and the third load curve 1204 the result of a combination of the third cavitation curve 1004 out 10 with the third calibration curve 1104 out 11 ,

The three load curves each represent a relationship between a calibrated pressure load, plotted in GPa on the ordinate, with a respective number of cavitation-induced deformation points in the 7 to 9 shown samples ago.

The following is an example of a sequence of a method for determining pressure loads that prevail in a Kavitationserosion, based on 7 to 12 explained. In particular, CFD results can be validated experimentally.

For this purpose, so-called spectra are generated via an automated evaluation of deformation points or pits which were generated on a real cavitation test stand, for example a beam test stand. 10 shows by way of example the spectra recorded for copper, the austenitic stainless steel 1.4401 and the ferritic stainless steel 1.4511. In this case, a so-called similarity number h / d, ie the quotient of pit depth and pit diameter, is plotted over the recorded number of pits. The similarity number h / d is calculated from the measured deformations of a polished material surface, as exemplified in US Pat 7 to 9 shown is calculated. The similarity number h / d serves as a measure of the acting shock wave pressure, which can be demonstrated experimentally by the shock wave test rig. Under a shock wave test stand is a device to understand, as described above with reference to 1 is described by way of example.

In order to get from the similarity number h / d to the real acting shock wave pressure, corresponding calibration curves are now generated by means of the shock wave test stand. The calibration curves for the three materials considered are in 11 shown. The calculated shockwave pressure is plotted here over the similarity number h / d. It is a linear relationship to recognize, the context need not necessarily be linear. The relationship is with the spectra 10 charged. Thus, it is possible to generate real load spectra with a purely experimental approach, as they are used in 12 are shown. The shock wave pressure is directly correlated with the number of pits. As a validation of the method presented here, the fact that the load spectra of the three different materials are very close together can already be considered. This is logically consistent with the assumption that all materials in the test bench have experienced the same load.

13 shows a flowchart of a method 1300 according to an embodiment. The procedure 1300 for determining a pressure load of a sample caused by a shock wave, for example, by means of a previously described with reference to 1 described device are performed. It is in one step 1310 the surface of the sample is irradiated with the laser beam in order to apply a shock wave once or several times to the sample at one or more points. For example, the strength of the shockwave during irradiation can be varied. In a further step 1320 For example, the plastic deformation caused by the shock wave is detected on the surface of the sample to obtain at least one deformation parameter. Finally, in one step 1330 the deformation parameter is evaluated using at least one parameter with respect to the laser beam to determine the compressive stress caused by the shockwave.

Thus, the approach presented here provides essential basic knowledge for all materials that are exposed to shockwave loading in various technical components. This arises, for example, from the collapse of cavitation bubbles near the wall. The laser shock waves can generate important and basic information about the material behavior under high dynamic load. Thus, the gap between CFD simulation results and the resulting load conditions can be correlated with a local material behavior. As already described, the method can also be used to determine the rise time of various pressure sensors.

If an exemplary embodiment comprises a "and / or" link between a first feature and a second feature, then this is to be read so that the embodiment according to one embodiment, both the first feature and the second feature and according to another embodiment either only first feature or only the second feature.

Claims (12)

A method (1300) for determining a compressive load of a sample (114) caused by a shock wave (202), the method (1300) comprising the steps of: Irradiating (1310) a sample surface of the sample (114) with a laser beam (104) through a transparent cover layer (116) and an absorption layer (118) disposed between the sample surface and the cover layer (116) to remove material from the absorption layer (118 ) to generate a plasma (204) by which the sample (114) is exposed to the shock wave (202); Detecting (1320) a plastic deformation of the sample surface caused by the shockwave (202) to obtain at least one deformation parameter (122); and Evaluating (1330) the deformation parameter (122) using at least one laser parameter characterizing the laser beam (104) to determine the pressure loading. Method (1300) according to Claim 1 in which, in the step of detecting (1320), a depth and / or a volume and / or a diameter and / or a number of deformation points (400) on which the sample (114) is plastically deformed, and / or a time course the deformation is detected as a deformation parameter (122). A method (1300) according to any one of the preceding claims, wherein in the step of evaluating (1330) using the laser parameter, a theoretical pressure load of the sample (114) is determined, linking the theoretical pressure load and the deformation parameter (122) to each other To determine pressure load. Method (1300) according to Claim 3 in which, in the step of evaluating (1330), a calibration curve (1100, 1102, 1104) is generated by plotting the theoretical pressure load over a quotient of the depth and the diameter of the deformation points (400), the calibration curve (1100, 1102, 1104 ) is associated with deformation values representing a cavitation-related plastic deformation of a cavitation sample (700, 800, 900) subjected to shock waves in a cavitation process in order to determine a calibrated pressure load. Method (1300) according to Claim 4 in which in the step of evaluating (1330) the calibration curve (1100, 1102, 1104) is compared with a cavitation curve (1000, 1002, 1004) to calibrate the calibration curve To determine pressure load, wherein the cavitation curve (1000, 1002, 1004) has a relationship between a number of cavitation deformation sites where the cavitation sample (700, 800, 900) is plastically deformed due to cavitation, and a quotient of a depth and a diameter of the cavitation deformation points represented as the deformation values. Method (1300) according to Claim 5 in which, in the step of evaluating (1330), a load curve (1200, 1202, 1204) is generated which represents a relationship between the number of cavitation-related deformation points and the calibrated pressure load. A method (1300) according to any one of the preceding claims, wherein in the step of irradiating (1310), the sample surface is irradiated at at least two different locations. The method (1300) according to one of the preceding claims, comprising a step of forming a composite layer (200) of the cover layer (116) and the absorption layer (118), wherein in the step of irradiating (1310) the sample surface passes through the layer composite (200) is irradiated. A method (1300) according to any one of the preceding claims, wherein in the step of irradiating (1310), the sample surface is repeatedly irradiated to provide the sample (114) with a plurality of shock waves (202). Apparatus (100) comprising units (102, 106, 108, 110, 112, 120, 124, 126) adapted to execute and / or drive the method (1300) according to any one of the preceding claims. A computer program configured to execute the method (1300) according to one of Claims 1 to 9 execute and / or control. Machine-readable storage medium on which the computer program is based Claim 11 is stored.

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