This aperture field is then prescribed to move at a constant speed from one side of the sample to the other. In this loading configuration, stable crack growth is actually maintained through the controlled failure of the specimen. The change in thickness contrast has several effects on the relative properties of the photopolymer material.
Note that the presence of the grating hardens the front of the sample so that the crack on the front (red arrow) has not propagated as far as the crack on the back (blue arrow), which is visible through the transparent polymer.
Powder Grid Method
These new meshes are formed by making a textured mesh pattern on the specimen using photopolymer and then filling the spaces in this textured pattern with dark powder pigment to achieve the contrast needed for the mesh method. Therefore, the bias of the epoxy layer is eliminated and the sample processing time is greatly reduced. The result of this is a silicon mass which has a uniform pattern of SU-8 pillars that are 50-70 µm high and have spacing corresponding to the lattice pitch.
In this study, the grid pitch was chosen to be 120 µm based on the specifications of the camera used for imaging, but this pitch can be adjusted to be smaller or larger depending on the needs of the imaging setup. The sample is then placed under UV light for 14-30 minutes (depending on sample thickness) to fully cure the liquid PR48 polymer into the shape of the silicone template. Since the polymer is physically constrained by the shape of the template, overexposure resulting in distortion of the grid pattern is not of particular concern.
Once polymerized, the photopolymer adheres preferentially to the sample rather than to the silicone template, so that the template can be easily removed for repeated use. At this point, the surface of the sample is covered with a uniform array of pillars that match the grid template, but this grid pattern has no optical contrast. Because the screening method depends only on the pitch and frequency of the waveform created by the applied screen, the color contrast of the screen can be varied based on the color of the sample itself.
Verification of Powder Grid Functionality
Once the powder is spread, this same flat edge is used to remove any excess powder, leaving a mesh pattern of white alumina on the sample. The samples investigated in this study were transparent or dark, and white powder was used to make the grid, but if the sample is light colored or white, a dark pigmented powder can be used to create a grid of black lines on the sample. with no change in network efficiency. Mathematical details of the construction of digital networks as well as mathematical analyzes of network operation are available in the reference.[1] All three networks are shown in Figure 0.7.
The fluctuating behavior seen in translation error is a product of error arising from subpixel interpolation, and the variation in digital gratings is due to aliasing effects due to imperfect subpixel interpolations of perfect sine waves. However, this increase in error is still relatively small and is considered a reasonable trade-off to allow the mesh method to be used in fracture analysis of low-toughness materials. To verify the functionality of this new grid method in fracture studies, homogeneous samples of PR48 were fractured using surfing loading conditions and characterized using the powder grid method.
The powder grids developed were used to measure the displacement and strain fields, and the J-integral was then calculated from the strain field to determine the critical stress intensity factor. The measured critical stress intensity factor of the 3D printed polymer sample was determined as MPa√. This is very similar to the previously measured value of 2 MPA√. m, which both verifies the functionality of the powder grids as well as provides a suitable baseline of homogeneous photopolymer toughness for later experiments. a) Powder grids (b) Powder grids (enlarged).
Results and Discussion .1 Numerical Simulations
Experimental Results
As the elastic contrast was introduced by the thickness variation, the crack was forced to bend outward around the thickness change as it exited the inclusion, resulting in additional hardening. As the crack sticks into more compliant inclusions, the change in thickness also results in a decrease in the crack front length. After the crack reaches the end of the inclusion and propagates back into the matrix, the attached crack front is forced to bend due to the change in geometry to continue propagating into the thicker matrix, as shown in Figure 0.14. .
In simulation, it is easy to perfectly center the crack between rows of inclusions in the composite structure so that it deflects evenly as it propagates. In the case of simulations this is not a problem as it is relatively easy to make the load move evenly with the tip of the crack. However, the experimental setup relies on steady propagation of the crack to prevent tensile load build-up as the sample moves along the rail.
Still, when the crack paths of the different anisotropic samples are analyzed, differences are noticeable between different inclusion orientations. In the case of leftward inclusions, the crack enters and exits the circular arc region of the containment, as predicted by the simulation, which would indicate a hardening effect similar to that of isotropic circles. Alternatively, the crack path of the samples with rightward heterogeneities follows successive stress concentrators, which would indicate lower toughness behavior, as predicted by simulations. a) Left facing (higher toughness) (b) Right facing (lower toughness).
Outlook
Experimental Limitations
This behavior of pinning followed by rapid propagation was also observed in almost all anisotropic specimens, as the presence of stress concentrators further increased the tendency for unstable crack propagation. Consequently, no reliable toughness measurements could be made and the anisotropic samples could only be characterized based on post-fracture crack path analysis. Photopolymer PR48 was a very suitable model material due to its brittle nature, good shape retention, and compatibility with the Ember DLP printer used to make the samples.
However, with PR48, as with all other acrylate photopolymers, there are some material stability issues. Many acrylate photopolymer systems used in stereolithography and digital light processing are based on photochemistry originally developed for mask lithography of silicon wafers. These systems have extremely good definition and shape retention, but were not designed for long-term use or stability, especially in environments containing UV light and oxygen.
Each polymer was subjected to both UV and oxygen irradiation, and the degree of degradation was characterized by the rate of evaporation of low molecular weight groups as well as oxidative cross-linking of side chains. The general trends in decomposition behavior were that methacrylic polymers were more stable than acrylics and polymers with smaller side groups tended to be more stable, e.g., poly(butyl methacrylate) exhibited the fastest and most wide, as well as the greatest interconnection. of side chains.[50] Although to date, no quantitative analysis of the degradation of 3D printed polymers has been published, the trends investigated by Chiantore et al. These polymers tend to be acrylates with very large side chain clusters to minimize the amount of network formation necessary to achieve a solid free standing.[7] This implies that these polymers are likely to be highly susceptible to degradation in environments containing oxygen and UV light, which includes the environment in which the surf load tests were performed.
The Potential of Anisotropic Heterogenities
Extension to Ceramics
For the PR48 photopolymer, the rail used was made of 6061 aluminum (McMaster Carr, Elhurst, IL) and the divergence used was 1-2 mm. Therefore, a ceramic testing rail using this same design would need to have a tolerance of tens of microns with very tight tolerances, and almost all slippage in the pins and bearings used to move the sample along the rail should be eliminated.
Summary
Because of this, surf loading proved impractical for use on ceramics, and other characterization techniques were explored in Chapters 4 and 5. In the case of anisotropy, the presence of stress concentrators on one side of the circular arc of the sample reduced the toughness improvement due to elastic clamping of the contrast in one direction , but the opposite direction showed similar hardening to the isotropic case, albeit with a smaller proportion of inclusion phase. Experimental analysis of large inclusions and anisotropic inclusions has been limited by unstable growth arising from a combination of heterogeneous structure and the limitations of experimental loading conditions in surfing.
Because the hardening is governed by the elastic contrast and the local structure of the interface, it is possible to make anisotropic inclusion structures that attenuate crack propagation similarly to isotropic ones as long as the loading is biased in a certain direction. The benefit of anisotropy in this case is that anisotropic heterogeneities use a significantly smaller volume fraction of the inclusion phase, making them much more favorable for maintaining bulk matrix properties, which is desirable in structural ceramics or systems ceramics designed for motor environments.
An fft-based numerical method for calculating the mechanical properties of composites from images of their microstructures. Digital image correlation using the Newton-Raphson method of partial differential correction. Experimental Mechanics, September 1989. 38] Huimin Xie, Satoshi Kishimoto, Anand Asundi, Chai Gin Boay, Norio Shinya, Jin Yu, and Bryan K A Ngoi.
PRODUCTION AND TRANSFER OF LOW SPATIAL FREQUENCY NETWORKS FOR MEASUREMENT OF DISPLACEMENT FIELDS WITH MOIRE AND GRID METHODS. Characterization of a cracked sample by full-field measurements: direct determination of the crack tip and calculation of the energy release rate.
