Real-time observation of compressive fracture of porous material by 10-millisecond 4D X-ray microtomography

We report on an application of 4D synchrotron radiation microtomography with a temporal resolution of 10 ms. Our device compresses a sample while rotating it at high speed, making it possible for the first time to capture the moment of a local fracture inside a porous composite material. It was visualized that the fracture was caused by local tensile shear stress, not local compressive stress.

As products become more sophisticated, the need for measurement to understand the function of materials is increasing.It is expected that capturing the moment when a function occurs will provide feedback to material design and contribute to speeding up the product development process.[3][4][5][6][7][8][9][10][11] Using such a method, 4D visualization of non-equilibrium voids generated by tensile fracture of tire rubber was successfully realized. 5,8)This paper reports on the application of this method to the visualization of the moment of a local fracture inside a porous composite material.
Porous composite materials [12][13][14][15][16][17][18][19] have attracted attention as materials that have both the flexibility of porous materials and the functions of fillers. Rgarding the flexibility of porous materials, they must have excellent shape-following performance on uneven surfaces and compression recovery properties and must be able to be used repeatedly.The functions of fillers include electrical and thermal conductivities.There is a problem with the function of the filler: its function degraded when it was compressed under high stress.In this paper, we demonstrate that 4D synchrotron radiation microtomography, which directly visualizes the morphological change when uniaxial pressure is applied, should be effective in clarifying the cause of this functional degradation.
The experiment was performed at BL28B2 at SPring-8, which is a bending-magnet beamline, where highly brilliant white synchrotron radiation is available.The experimental setup is shown in the upper panel of Fig. 1.The sample of porous composite material was fixed on the high-speed rotation device that was reported in a previous paper, 5) which consists of two synchronized coaxial motors movable along the direction of the axis and can stretch or compress a rotating sample.X-ray transmission images were acquired by an X-ray image detector, consisting of a 10 μm thick singlecrystalline scintillator [Ce: Gd 3 Al 2 Ga 3 O 12 (GAGG) 20) ] plate, a lens-coupling system for scintillation light, and a high-speed CMOS camera (Photron FASTCAM NOVA S12) with a pixel size of 20 μm, while the sample is rotated and compressed.The lens coupling system had a magnification factor of 4, resulting in an effective pixel size of 5 μm.
The sample was located at a distance of 44.7 m from the bending-magnet source and the X-ray image detector was located at 4.8 m downstream from the sample, resulting in an effective pixel size of 4.5 μm at the sample.The rotation speed of the sample was 3,000 rpm, corresponding to a temporal resolution of 10 millisecond (a half turn) for the 4D microtomography.The X-ray transmission images during the compression of the sample were captured at 1024 × 1024 with a frame rate of 12,800 fps.The compression speed was 4.4 μm/10 ms, which was determined by the X-ray transmission images.First, the sample is rotated, and immediately after the projection image acquisition begins, the sample is compressed.
From X-ray transmission images with propagation-based contrasts, [21][22][23] X-ray phase images are obtained by ANKAPhase, 24) a single-distance non-iterative phase-retrieval algorithm proposed by Paganin et al. 25) CT reconstruction was performed using a conventional filtered-back projection (FBP) algorithm.The projection images required for CT reconstruction were cropped every 180 degrees.
The sample used was a porous composite material consisting of resin (silicone) and filler (boron nitride and graphite).A section image of the material captured by an optical microscope is shown in the lower panel of Fig. 1 with a schematic illustration.The material has a porous structure which has excellent shape-following performance on uneven surfaces and compression recovery properties, and can be used repeatedly.Its function (prevention of water, sound, vibration, transmission of conductivity, heat transfer, and so on) can be controlled by the change in its three-dimensional structure.108002-2 © 2023 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd However, the correlation between the function and the structure is not clear especially when the structure is changed due to external stresses such as compression.
Experimental results are shown in Figs. 2 and 3.The upper panel of Fig. 2 shows examples of X-ray transmittance images obtained at different times (0 s, 1 s, and 2 s after the start of compression; see also Supplementary movie 1 for transmittance images from one direction).The lower panel of Fig. 2 shows images of a coronal section obtained by the CT reconstruction (see also Supplementary movie 2).In the middle figure of the lower panel of Fig. 2, a composite layer of the filler layer and the resin layer is seen to be fractured by compression.Figure 3 shows magnified images around the fractured composite in the lower panel of Fig. 2 for every 40 msec (arranged from left to right, top row to bottom row, in time order).The process of the local fracture of the composite layer due to compression is clearly captured.Importantly, it is considered that the thin composite layer is fractured by localized tension rather than compression since the two neighboring voids were deformed and fractured in such a way that they were displaced vertically in opposite directions.Such local tensile processes and fractures are considered to be responsible for the deterioration of the function of the filler when compressed.Note that the total exposure time was relatively short, and no obvious visible irradiation damage was observed, such as color change or deformation of the sample.
In summary, an application of 4D synchrotron radiation microtomography with a temporal resolution of 10 ms was reported.Our device, which can compress a sample while rotating it at high speed, made it possible for the first time to visualize that a local fracture inside a porous composite material was caused by local tensile shear stress, not local compressive stress.In the future, for example, 4D synchrotron radiation microtomography can be performed while measuring changes in electrical conductivity.One of the advantages of high-speed 4D microtomography is that it can measure a large number of samples at high throughput, and together with big data analysis, it is expected to be highly effective in clarifying correlations between structural changes and functional degradation.

Fig. 1 .
Fig. 1. (Upper) Experimental setup of 4D synchrotron radiation microtomography (top view).(Lower) Optical microscope image of a section of porous composite material used (right) and its schematic illustration (left).

Fig. 3 .
Fig. 3. Magnified images around fractured filler in the lower panel of Fig. 2 for every 40 msec (arranged from left to right, top row to bottom row, in time order).

Fig. 2 .
Fig. 2. (Upper) Examples of X-ray transmittance images at 0 s (left), 1 s (middle), and 2 s (right) after start of compression (see also Supplementary Movie 1 for X-ray transmittance images from one direction).(Lower) Images of a coronal section obtained by CT reconstruction for 0 s (left), 1 s (middle), and 2 s (right) after the start of compression (see also Supplementary Movie 2 for coronal section).