Our group develops techniques to watch the structure of materials as they evolve during dynamic loading.
The mechanical properties of materials—how stiff or strong they are, or how easily they fracture—are of great importance for many engineering applications. Most laboratory tests designed to study these properties are conducted at relatively low loading rates, with the stress on the material gradually increased while the response of the material is monitored. The rate of deformation is usually expressed in terms of a “strain rate,” which for ordinary tests is commonly in the range of 0.00001-0.001 strain per second.
In many applications, however, the loading is very rapid. For example, in an automobile crash the frame can be deformed at rates in excess of 100 strain per second, and in ballistic impacts (a projectile hitting a target) the rates are even higher (1,000,000 strain per second or more). Because the atomic-scale mechanisms by which materials deform and fracture require some time to occur, it should not be surprising that materials can behave quite differently under this dynamic loading than they do under ordinary laboratory conditions. A key question is then how to monitor the response of the material during a dynamic test?
Our group develops techniques to watch the structure of materials as they evolve during dynamic loading. Because a dynamic test is typically over in less than 100 microseconds (sometimes considerably less) we have to develop techniques that can operate on similar time scales. Most of our work involves the use of synchrotrons, which produce extremely intense beams of x-rays. A schematic of a typical experiment, conducted at the Advanced Photon Source, is shown here:
In this experiment the sample is placed between two bars, one of which is struck by a projectile from a gas gun. This causes a rapid strain pulse to propagate down the input bar and into the specimen, producing strain rates of around 1,000 strain per second. This arrangement, commonly used for dynamic testing, is called a compression Kolsky bar or split-Hopkinson pressure bar. The reality is a little more complicated than the simple schematic above would suggest:
Our contribution to this field is to install our Kolsky bar at the Advanced Photon Source to study the structure of the material as it evolves. To do so, we use a fast x-ray shutter to time the arrival of a short (<40 microsecond) pulse of high-energy x-rays coincident with the arrival of the strain pulse at the specimen (or after a suitable delay). The interaction of the x-ray pulse with the specimen produces a diffraction pattern on the the detector, which looks something like this:
That’s for a relatively long exposure (by our standards) of five milliseconds. Because the dynamic test is over in less than 100 microseconds, we need a shorter exposure. Using our fast shutter, a diffraction pattern taken in 42 microseconds looks like this:
The diffraction pattern is much weaker, but from the diameters, shapes, and widths of the diffraction rings we can understand how the structure of the material is responding to the strain pulse. For example, if we “unwrap” the diffraction pattern above to show the radius of the ring as a function of angle, we can see that the ring is distorted such that it is broader in the loading direction (parallel to the bars in the figure above) and narrower in the transverse direction:
This kind of change in the diffraction pattern is characteristic of a material which is being compressed by the strain pulse. Other changes can also appear in the diffraction pattern; for example, some materials undergo a martensitic phase transformation is response to an applied load, which shows up in the diffraction patterns as the appearance of new rings.
You can get a better feel for how the experiment is laid out and how the group members cooperate to make these experiments a success in these two movies:
Look for cameo appearances by Lan Zhou (post-doc) and Emily Huskins (post-doc at the Army Research Laboratory) in the first movie, and by Prof. Hufnagel and Paul Lambert (grad student) in the second. Both were shot and narrated by Caleb Hustedt (grad student).
Going forward, our goal is to understand the dynamic behavior of materials in detail, using a combination of these advanced in situ diffraction techniques and computational modeling of the material response. This work is conducted under the auspices of the Hopkins Extreme Materials Institute, which is supported (in part) by the Army Research Laboratory through the Center for Materials in Extreme Dynamic Environments.