Research Project

Dynamic Fracture of Rocks and Engineering Ceramics

The ways in which materials break has long fascinated scientists and engineers and is also of tremendous practical importance. If you want to make a tough, lightweight ceramic armor you need to know how the ceramic will fracture when a projectile hits it. Similarly, if you want to understand how rock fractures due to an explosion or an earthquake, you need to understand the mechanisms of fracture.

The ways in which materials break has long fascinated scientists and engineers and is also of tremendous practical importance. If you want to make a tough, lightweight ceramic armor you need to know how the ceramic will fracture when a projectile hits it. Similarly, if you want to understand how rock fractures due to an explosion or an earthquake, you need to understand the mechanisms of fracture.

 

Fracture is a tricky thing to study, however. When rocks or engineering ceramics are subjected to loading at high rates, cracks can form in a fraction of a microsecond and propagate at rates of more than 1000 meters per second. How can we hope to observe such rapid processes?

 

To address this challenge, we are developing high-speed x-ray phase-contrast imaging techniques for studying dynamic fracture. These techniques involve using the extremely intense but extremely short pulses of radiation from a synchrotron source (such as the Advanced Photon Source) to take images of a material as it fractures. The experimental setup looks like this:

 

 

The experiment begins when a projectile fired from a gas gun strikes the input bar, creating a strain pulse that propagates down the input bar and into the specimen. The specimen has a special “notched three-point bend” geometry that creates a stress concentration at the notch, causing a crack to initiate there:

 

 

By placing the sample in the synchrotron x-ray beam, we can form an image of the fracture on a scintillator crystal placed downstream. The scintillator is in turn viewed by a high-speed optical camera, which records the images. Here’s an example of fracture in fused silica, a kind of glass:

 

 

From an experiment of this kind we can determine the precise conditions under which the crack initiates, its velocity as it propagates, and whether or not it propagates stably as a single crack or bifurcates into two (or more) independent crack fronts.

 

Another kind of experiment is dynamic compression, in which a cube-shaped sample is compressed between two platens at a high rate. Because there is no notch, multiple fractures can initiate and propagate:

 

 

One detail about these experiments is that the images above are not simple radiographs, like the ones a doctor would take if you had a broken bone. The contrast in a radiograph comes from absorption of x-rays by the specimen, which is all you get if your source of x-rays is incoherent (as most x-ray sources are). But if your x-ray source has some spatial coherence you can get another kind of contrast, called phase contrast, which comes from gradients in the electron density of the specimen. In the figure at left below, with an incoherent x-ray source, contrast comes only from absorption of the x-rays so the object appears as a dark patch of low intensity on the detector:

 

 

The right-hand figure shows what happens when we have a coherent source; constructive (and destructive) interference among x-rays refracted by gradients in electron density enhance the contrast at edges, making it easier to see kinds of cracks and other defects. This quick discussion glosses over many of the details, but you can easily see the effects of phase contrast in a simple experiment. It turns out that the amount of phase contrast we see depends on the distance between the scintillator and the sample. The movie below shows phase-constrast images of a quartz sample with internal cracks, imaged several times with the scintillator at different distances from the specimen:

 

 

As the distance increases the amount of phase contrast (relative to the absorption contrast) also increases, making it easier to see the internal cracks.

 

This work is conducted under the auspices of the Hopkins Extreme Materials Institute and was funded by the Army Research Laboratory through the Center for Materials in Extreme Dynamic Environments and by the Defense Threat Reduction Agency. If these experiments look like fun, and you’re interested in joining the group as a graduate student or post-doc, please get in touch with Prof. Hufnagel.

 

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