By Dr. Wesley De Boever, Product Marketing Manager, TESCAN

New developments in time-resolved micro-CT allow researchers to study material behaviors in situ as the materials respond dynamically to environmental influences. Dynamic micro-CT, which acquires data continuously, is a significant evolution in micro-CT technology, enabling scientists to capture data from unpredictable events that may be missed with conventional time-lapse techniques.

Time-resolved micro-CT
The ability to watch the effects of environmental influences on materials and structures is not a new technique, but recent developments really enhance its value. It is worth taking a closer look at micro-CT in the context of time-resolved recordings of dynamic processes and the shift it enables from looking at material characteristics to material behaviors.

All CT techniques combine a set of 2D projections acquired from multiple perspectives around the sample to create a 3D model. Either the sample or the source and detector rotate through 360° to acquire the image set. Ultimately, the time resolution is limited by the time it takes to finish a complete rotation. That time can vary greatly — depending on analysis requirements and instrument capabilities — but is usually on the order of minutes or even hours. Processes that produce significant changes over a shorter period will not be well resolved because they are simply faster than the time resolution of the experiment.

Time resolved micro-CT has been referred to as 4D CT, a reference to time as the fourth dimension in addition to the three physical dimension: length, width and depth. Most 4D data acquisition can be compared to a time-lapse recording, where sets of 2D images, each set acquired over a single rotation, are separated in time during which data are not acquired. The structural models reconstructed from each set are then displayed in sequence, yielding an accelerated recording of the changes.

4D data acquisition can be considered as a form of stroboscopic imaging, whereby a dynamic process is captured by a discrete number of snapshots — ideally at regular time intervals during the duration of the experiment. In the case of micro-CT, all that is required for this kind of 4D data acquisition is to capture a full 360° scan at a sufficiently fast scanning rate to obtain the state of the material at regular time intervals, Figure 1. If the rate of change of the sample is sufficiently slow, 4D experiments could even tolerate samples taken in and out of the micro-CT if the system does not cater to highly complex in-situ setups (for example experiments conducted inside the micro-CT system). A time-lapse video produced from a series of micro-CT snapshots will then reveal the changing properties of the sample, with a temporal resolution defined as the duration of the experiment, divided by the number of snapshots incorporated in the time-lapse video.

4D data acquisition can be improved with dynamic data acquisition, in which data are acquired continuously from one revolution to the next, over the entire duration of the experiment without time intervals between the data snapshots. The most important benefit of true dynamic micro-CT over conventional 4D or time-lapse micro-CT, is its ability to capture an event that occurs at a time that cannot be predicted with reasonable accuracy. This is valuable for events that depend on achieving some threshold before the event occurs. This is typical for stress–strain experiments, which often involve a rather abrupt transition from elastic to inelastic strain as gradually increasing stress is applied to the sample.

Other examples of such thresholds are the wetting of surfaces when nucleation sites connect, and phase transitions when critical pressure and temperature conditions are reached. While 4D time-lapse micro-CT is adequate for dynamic experiments where events take place gradually and consistently over time, true dynamic-CT is better suited to dynamic experiments where thresholds are expected to accelerate events at an unpredictable time during the experiment. Often, it is the timing of the abrupt transition that is the experimental parameter of interest. The higher the temporal resolution of the system, the more detailed the data will be from the accelerated event and the more valuable for subsequent analysis. Dynamic micro-CT is significant step in the evolution of time-resolved X-ray microscopy, from time-lapse 4D microscopy to uninterrupted dynamic micro-CT.

Along with optimizing standard parts of the system (x-ray source, sample rotation and detector) for high temporal resolution, a number of other key elements are in place for dynamic CT. Specifically, “no cable wrap” continuous rotation for in situ apparatuses, in situ interfaces for simple swapping of test rigs and unique temporal reconstruction and analysis techniques that allow users to leverage the full benefit of continuous data. The hardware and software tools allow individual scans down to a few seconds and temporal discretization near the rate of individual radiographs, which may be on the order of 20 msec or better.

Researchers can now have the best of both worlds. TESCAN recently launched the first dynamic micro-CT system to offer both high temporal resolution for uninterrupted 4D dynamic CT experiments and sub-micron resolution 3D non-destructive imaging for static studies.

Emerging applications of dynamic micro-CT
The ability to image material behavior at the micrometer scale is an exciting prospect. This dynamic, transparent microscopy of even the most opaque materials, will help materials scientists develop new materials for a wide variety of applications, including light-weight, high-strength metals and alloys; new energy storage materials; micro and nano-sensors; materials optimized for challenging environments, such as batteries and sensors to be used in fast-moving cars at high temperature and high vibrational loads; composite materials in light-weight aircrafts; and organic particles to carry drugs in the human body. A few examples are provided below.

Compression testing of aluminum foam

In one example, we looked at the behavior of an aluminum foam as it underwent continuous compression in a uni-axial compression stage. For this test we collected scans at about 15 seconds temporal resolution with a constant displacement rate. Some key takeaways for this example are shown in the graphic below where there is a clear change in loading behavior at certain points during the test. If this test had been performed in an interrupted manner, the most common testing method prior to lab-based dynamic CT, these particular events would almost certainly be missed, and the load curve would show sample relaxation during the experiment interruptions. With the associated CT data, we analyze the tomograms around this load change and identify specific details of change; individual struts in the sample necking and failing over the course of a short time, less than 24 seconds in this case as shown in Figure 2.

Quality assurance in additive manufacturing

As with many manufactured parts, additive manufacturing (AM) products can be prone to both external and internal defects. These defects, such as voids, cracks, delamination and contaminants, may influence the mechanical performance of a product. However, the complex geometries possible with AM creates unique challenges for inspection. Micro-CT provides non-destructive 3D information on a component and has become essential for detection and analysis of internal imperfections in these intricate parts. However, aside from basic quality analysis, it is also essential to understand how these imperfections influence the behavior of the part when they are actually used.

Dynamic CT can provide detailed information on a part’s actual performance. As mentioned previously, most lab-based in situ micro-CT involving loading a part in compression or tension is done in an interrupted form, where the applied force must be held constant during the tomography collection. This can create issues involving sample relaxation and missing information during the actual loading procedure. Dynamic CT, with continuous acquisition and uninterrupted loading, helps to alleviate these issues.

As an example, several test specimens were created in plastic using 3D printing. A total of six cylindrical samples, with different internal supports, were formed. Each sample was then compressed continuously using a Deben CT5000 load stage, while tomography data were collected on a TESCAN CoreTOM system at a temporal resolution of 5.8 seconds with a voxel resolution of 59 µm. This resulted in 210 full sample scans for each sample. Figure 3 provides an overview of three of these samples including their internal structure, 3D rendered snapshots of the sample throughout the process, and their associated load curves. Through this experimental evaluation of deformation of different geometries, one can develop more precise simulations to best understand optimization for the specific needs of an application. As can be seen on the graph, no relaxation took place during the experiment due to a constant displacement.

Dynamic micro-CT is an extremely useful and powerful technique. For academic and industrial users, the ability to supplement their micro-CT needs for non-destructive 3D imaging and ROI location, with the new and exciting possibilities of dynamic micro-CT, will further studies that correlate new material properties with the desired material behavior.