3D internal displacement and strain measurements
Linking the microstructure to the mechanical behavior is critical to the design of high-performance components for industry. 3D imaging techniques such as lab/synchrotron X-ray tomography, optical tomography (OCT, OST), and MRI allow researchers to perform in situ experiments with high spatial (down to the micron scale) and temporal resolution (within seconds). These techniques are particularly suitable for capturing complex and very short-lived phenomena such as localization induced by heterogeneities, thermal mismatch between constituents, micro-cracking, fatigue behavior, and phase transitions. Digital Volume Correlation (DVC) exploits the natural texture of materials and has recently emerged as a powerful contactless, bulk strain measurement technique in experimental mechanics, materials science, and biomechanics.
Thermo Scientific Amira and Avizo Software are all-in-one image analysis platforms that allow users to compute 3D full-field displacement and strain maps from volume images acquired during the deformation process of an object. The data can be visualized and analyzed in a sophisticated way to quantify deformation-induced microstructural changes such as strain transfer in multiphase materials, pore growth/coalescence related to fracture, and crack opening displacement (COD).
Bone-cement interface integrity has been identified as the critical element in the success and longevity of both hip and knee cement replacements. The influence of bone type in the overall load transfer and the micromechanics of the bone-cement construct are yet to be fully quantified.
In this example, Digital Volume Correlation was used to compute 3D full-field displacements and strains in one of the possible configurations of an interdigitated bone-implant interface. Using advanced image processing, it was possible to quantify and visualize how the strains are shared between the bone and the cement during mechanical compression. It is observed that most of the deformation is taken by the trabecular bone with a poor strain transfer in the cement region. Other bone-cement composite formulations containing more cortical bone have been tested and proved to be more efficient in diffusing the strains in the cement.
Localization of deformation is a research topic of great importance for the design of new materials. In this example, we illustrate how Digital Volume Correlation can be used to capture the regions of intense strain during tensile loading of a nodular graphite cast iron. A coarse DVC analysis was performed on a regular mesh to track the large displacements, and the data were then used to initialize a more robust multiscale FE-based technique on a finer tetrahedral mesh possessing the exact shape of the tensile specimen. It is observed that high strains seem to develop preferentially at large nodule clusters where the local density is high, which could then be used to improve our understanding of ductile fractures and feed numerical models.
The use of composite materials for gas transportation has recently grown in interest. The damage mechanisms of a polyamide 6 (PA6) matrix reinforced with unidirectional glass fibers have been previously characterized using synchrotron radiation laminography on CT-like specimens. Digital Volume Correlation enables researchers to go one step further to quantify in more detail the 3D interaction of a crack with the microstructure, while extracting valuable information such as Crack Opening Displacement (COD) and matrix volumetric strain.
In this example, the volumetric strain was mapped into the spherical micropores and rice-like macropores, revealing that voids have grown and are likely to coalesce later to form a crack. These results can be used to feed and/or validate numerical models where the damage mechanisms in the matrix are based on void growth.
Zirconia transformation from tetragonal to monoclinic plays an important role in the manufacturing of high-zirconia fused-cast refractories used in glass furnaces. In situ high-temperature experiments were performed using synchrotron radiation tomography at SOLEIL to observe the morphological changes during this transformation. Using Digital Volume Correlation, the strains associated with the volume change were quantified to analyze the microstructural deformation mechanisms.
In this example, inhomogeneous volumetric strains develop on the zirconia grains as a result of swelling. Coupled with numerical simulation, DVC could be of great interest to optimize the fabrication process of next-generation furnaces.
The XVolumeCorrelation extension for Amira-Avizo Software provides dedicated tools for Digital Volume Correlation (DVC):
- 3D full-field continuous displacement and strain maps with high precision/accuracy from volume images acquired during a deformation process (possible precision down to 0.1 pixel and accuracy/bias down to 0.01 pixel, or less, depending on texture and noise)
- Subset based DVC method for large expected displacements (local method)
- Finite element based DVC method for continuous displacements (global method)
- Fusion of local and global methods allowing to tackle a wide range of applications
- The displacements and strains are computed in a 3D mesh which can conform to the exact shape of the object, avoiding the need to trade-off for a smaller region of interest
- Advanced post processing:
- Displacement and strain components visualized at grid elements (hexaedra/tetraedra) or nodes
- Convert grid’s nodal displacement to a regular dataset (3D image)
- Displacement vectors visualized at grid elements, nodes or on regular grid
- Extraction of principal strains, invariants, eigenvectors, equivalent von Mises and Tresca strains
- Iso-displacement and iso-strain mapping
- Deformation animation from the 3D grid
- Link experiment with simulation by creating tetrahedral mesh with the Amira-Avizo XWind extension for FEA/CFD and applying DVC displacements at the mesh boundaries
- Quantify deformation induced microstructural changes such as interface integrity or deformation induced porosity with Amira-Avizo advanced quantification and analysis toolset