Mar 5 2019
The physics of fracturing and characteristics of fracturing materials can be better understood by directly imaging dynamic cracks as they happen. This approach may considerably benefit a number of fields ranging from engineering and construction through to materials science.
Conversely, the fracturing process takes place very quickly, with dynamic cracks traveling via several centimeters of certain soft materials in just one-tenth of a second. Dynamic cracking in certain materials can be directly imaged using high-speed cameras; however, instruments like these are not only costly but also difficult to utilize with certain materials or in specific situations.
John Kolinski of the Ecole Polytechnique Federale de Lausanne in Lausanne, Switzerland, will demonstrate a novel imaging method, called the virtual frame technique, at the 2019 American Physical Society March Meeting in Boston. The technique, developed by Kolinski and his colleagues Shmuel Rubinstein and Samuel Dillavou of Harvard University, allows standard digital cameras to capture a countless number of frames per second for a number of seconds without compromising the high spatial resolution. Kolinski will also take part in a press conference detailing the work.
The new virtual frame technique utilizes the bit depth of a camera sensor to significantly increase the frame rate; bit depth is the amount of data that can be obtained by the sensor. Many physical processes, including cracking, are binary; for instance, either a material is cracked or not cracked. As a result, a crack can be imaged using just two bits. An image sensor having a bit depth of 16 bits has over 65,000 color or grayscale values, implying that thousands of virtual frames can be produced at the time of a single exposure. Frame rates can be increased even further by using a short pulse of intense light and accurate camera timing.
In a recent study using the virtual frame technique, we obtain virtual frame rates exceeding 60 million per second using precise time-gating and a camera sensor with substantial bit-depth.
John Kolinski, Assistant Professor, Ecole Polytechnique Federale de Lausanne
By utilizing the virtual frame method, almost any kind of camera will be able to directly image dynamic cracks as they occur. Moreover, it can even be used for analyzing other rapid physical processes that occur at solid-fluid interfaces like wetting that happens when a drop of liquid strikes the surface of a material. The only condition is that the solid should be opaque, whether it is a soft substance like a polymer, or a construction material.
“Essentially any material could be imaged with the virtual frame technique,” stated Kolinski.
The research team has tested the virtual frame method using different types of cameras having different bit depths and sensitivities spanning from smartphone cameras to advanced high-speed and high-end consumer cameras. By using the virtual frame technique, each kind of camera was able to attain relatively higher frame rates, which according to Kolinski, may lead to its application in forthcoming mobile device apps that can determine the properties of materials.
The novel imaging technique could provide an easier means for studying rapid physical processes, including fracturing, at material interfaces. The use of standard consumer cameras for capturing thousands or more numbers of frames per second makes it viable for analyzing fracture toughness and other characteristics of construction materials, and if applied in mobile apps in the future, the novel imaging technique could complement or even substitute costly testing hardware with a software piece.