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Results Project 4

Evaluation of hypervelocity impact damage of rocks using elastic waves

The project:

The main task of project 4 is to deal with the laboratory perspective of the cratering process using elastic wave techniques that are obtained from non-destructive testing (NDT) of materials and that will be related to data from geophysical field methods. On one hand we will be responsible for data e.g. to characterize the geophysical (petro-physical) properties of the targets and the extent of the damage zone in three-dimensions underneath the crater. On the other hand the data will be compared to the simulation results as well as the microstructural damage observations and field data. The laboratory perspective can help to understand the cratering in a way field data cannot, since all environmental conditions and the target itself can optimally be controlled in the lab. The cratering experiments include variations of the target material and the projectile parameters. These variations and in particular the effect on the crater structure cannot be studied at terrestrial craters alone using geophysical prospecting methods. The simulation needs real data to calibrate the models and therefore one can claim that non-destructive measurements are the missing link between the (destructive) laboratory observation, the numerical models, and the field data.

Acoustic emission, similar to seismic measurements, is used for a localization of the source (the impact or single cracks inside the target) and an evaluation of the wave velocity of the first propagating wave field.

Ultrasound tomography is an imaging method, which illustrates the alteration of the p-wave velocity map before and after an impact experiment. The result is a 3D velocity map.

During the impact experiments, important parameters like the target material, projectile material, and impact velocity and energy, respectively, were altered. Applying different parameters led to differences in the wave propagation of the impact signal and in the damaging zone underneath the impact craters, which are detectable by ultrasound tomography.

Experimental setup for measurements between the impacts:

It is not easy to deliver data of the crater formation during an impact, because the targets are located in a chamber with low pressure. Acoustic emission (AE) techniques are useful in this case since elastic waves, radiated during crater formation, can be recorded with a very high resolution at the accessible surfaces of the target. Therefore the measuring sensors are placed at the surface of the target with a good coverage. While the recording of the impact itself is a low risk the identification of cracks induced by the impact underneath the crater has a much higher risk. The size and therewith the very small differences in the travel times for each sensor, as well as the high amplitude of the Impact complicates the evaluation of these data. In addition to the localization of impact and cracks the evaluation of the propagating wave induced by the impact is of interest. Important for the AE is the preparation of the target with the sensor system. Depending on the target size the sensors are positioned at the surfaces of the target with a good covering and for the bigger targets some sensors are positioned in borehole.

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After preparation of the target the measurement starts to record the acoustic sound between the impact.

 

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The analysis of the AE-signal gives information about the real impact point. This can be calculated by the differences of the travel times.

 

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Additionally the propagation of the first wave front can be recorded and enables a reconstruction of the wave (p-wave) velocity.

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Experimental setup for measurements before and after the impacts:

The characterization of the target before is important to analyze the measured results after the impact. If we have the knowledge of existing cracks we can assess the information about the impact induced cracks.

Ultrasound-tomography measurements give us a three dimensional velocity map of the whole target with a 1 cm grid. Therefore through transmission measurements with 1 cm spacing were done at the surface of the target. With the measured travel times and the knowing travel length the p-wave velocity can be calculated. As we conduct these measurements in each spatial direction we can calculate a velocity map for the whole target. The velocities were calculated with the assumption of straight ray paths and a constant velocity for all these ray paths. The following figures (a) and (b) show two slices of an undamaged Quartzite target. In figures (c) and (d) the same slices after the impact experiment is shown.

 

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Ultrasound tomography videos:

Quarztite target before impact

Quartzite target after impact


Further we undertook radiographic micro-computer tomography measurements for a few targets after the impact experiments. These measurements were done in cooperation with the WIWeB (Wehrwissenschaftliches Institut der Bundeswehr).

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The damage zone underneath the crater structure is an important parameter for the impact survey. We compare the target material, impact energy, crater dimension and the damage zone of each experiment to give a statement for an expected damage zone underneath terrestrial impact craters and to have an approach for geophysical measurements. Until now we can see, that the damage underneath the crater increases with lower densities of the target and a higher impact energies.

A second method to characterize the target before and after an impact experiment is the measurement of the elastic modulus. Here we can distinguish different elastic parameters for a material with and without damage. Therefore we use modal analysis to record this changings. The damage induces a decreasing of the frequency of the torsional, flexural and longitudinal eigenmodes. The eigenmodes are used to calculate the elastic moduli, which show the same perceptual decrease like the frequency.

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Cooperation with other MEMIN projects:

Finally the non-destructive measured results are compared and combined with the results of other MEMIN project. The AE measurements were analyzed to reconstruct the real impact point (Project 6, crater morphology) and shall use to find some individual cracks inside the target after the impact. The velocity of the propagated wave is used to refine the numerical calculation of the wave (p-wave) propagation and damage expansion in project 5.

The results of the damage zone with tomographic methods can compare with the results of project 2, which is looking for the crashed zone near the crater surface. Both results, of project 2 and project 4, give also information for project 5 to calculate the damage expansion in the targets.

As well the calculation of the elastic moduli helps to refine the numerical models.

Dorothee Moser

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