Measurement and analysis of residual stresses in zirconia dental composites using micro X-ray diffraction

Due to their aesthetic value and high compressive strength, dentists are utilizing ceramics as a material of choice for dental restorations. Among ceramics, zirconia provides high toughness and crack resistance. Residual stresses develop in processing due to factors including coefficient of thermal expansion mismatch, geometry, and grain anisotropy. In the present study, advanced methods were adapted including polychromatic X-ray (Laue) micro-diffraction, which provided grain orientation and residual stresses, and monochromatic diffraction in the sin-squared-psi mode. Analysis tools for both methods were developed to provide grain averaged and grain specific material properties on clinically relevant specimens.Large type II residual stress variations ranging from - 1 to + 1 GPa were observed between grains. Most grains in monolithic zirconia have a mean compressive deviatoric stress of 70 MPa, depending on direction. Another important part of the study was the interface residual stresses which occur at the interface of a base/core region of a dental restoration and the veneer. Type I residual stresses as high as 800 MPa were observed at the interface. A detailed analysis of the effects of simulated mouth motion impact fatigue cycling on the residual stress states revealed significant relaxation of the residual stress from 800 MPa to around 150 MPa within 45,000 cycles. Results from the sin-squared-psi; residual stress measurement technique revealed significantly higher tensile stresses in the zirconia core. Equivalent to approximately one year's service, 45,000 fatigue cycles at a load of 150N led to fracture of the porcelain veneer cusp. Residual stress at the site of fracture indicates relaxation of residual stress. At the interface, stress induced phase transformation of zirconia forming monoclinic phase was observed. Residual stresses can promote crack growth and thereby catastrophic failure. These results have implications for all materials particularly structural ceramics

Combined in situ mechanical testing and scale-bridging 3D analysis of nanoporous gold

In this work we present results on in situ small scale testing of nanoporous gold (npg) in scanning electron microscopy (SEM) and transmission electron microscopy (TEM). By combining nano- and micromechanical testing of pillar structures with advanced tomographic imaging, a 3D characterization of the plastic deformation process in different states of deformation is achieved. For small strut sizes 360° electron tomography (ET) is applied enabling high quality reconstructions of the 3D morphology of npg without missing-wedge artefacts. Combining the geometric information with mechanical data from in situ testing in SEM and TEM the yield strength is precisely determined. Furthermore, the experimentally derived 3D data are used as input for large-scale molecular dynamics (MD) simulations in order to understand the role of strain localization and identify predominant defect processes. Please click Additional Files below to see the full abstract

4-D XRD for strain in many grains using triangulation

Determination of the strains in a polycrystalline material using 4-D XRD reveals sub-grain and grain-to-grain behavior as a function of stress. Here 4-D XRD involves an experimental procedure using polychromatic micro-beam X-radiation (micro-Laue) to characterize polycrystalline materials in spatial location as well as with increasing stress. The in-situ tensile loading experiment measured strain in a model aluminum-sapphire metal matrix composite using the Advanced Light Source, Beam-line 7.3.3. Micro-Laue resolves individual grains in the polycrystalline matrix. Results obtained from a list of grains sorted by crystallographic orientation depict the strain states within and among individual grains. Locating the grain positions in the plane perpendicular to the incident beam is trivial. However, determining the exact location of grains within a 3-D space is challenging. Determining the depth of the grains within the matrix (along the beam direction) involved a triangulation method tracing individual rays that produce spots on the CCD back to the point of origin. Triangulation was experimentally implemented by simulating a 3-D detector capturing multiple diffraction images while increasing the camera to sample distance. Hence by observing the intersection of rays from multiple spots belonging to the corresponding grain, depth is calculated. Depth resolution is a function of the number of images collected, grain to beam size ratio, and the pixel resolution of the CCD. The 4DXRD method provides grain morphologies, strain behavior of each grain, and interactions of the matrix grains with each other and the centrally located single crystal fiber

Quantitative and qualitative bone imaging: A review of synchrotron radiation microtomography analysis in bone research.

All levels of the unique hierarchical structure of bone, consisting of collagen and hydroxyapatite crystals at the nanoscale to osteon/lamellae structures at the microscale, contribute to its characteristic toughness and material properties. Elements of bone's density and size contribute to bone quantity (or bone mass), whereas elements of bone's material composition, material properties, internal structure, and organization describe bone quality. Bone quantity and quality can be degraded by factors such as aging, disease, treatments, and irradiation, compromising its ability to resist fracture and sustain loading. Accessing the morphology and architecture of bone at the microscale to quantify microstructural features and assess the degree of mineralization and path of crack propagation in bone provides crucial information on how these factors are influencing bone quantity and quality. Synchrotron radiation micro-computed tomography (SRμCT) was first used to assess bone structure at the end of the 1990's. One of the main advantages of the technique is that it enables accurate three-dimensional (3D), non-destructive quantification of structure while traditional histomorphometry on histological sections is inherantly destructive to the sample and two-dimensional (2D). Additionally, SRμCT uses monochromatic, high-flux X-ray beams to provide high-resolution and high-contrast imaging of bone samples. This allows the quantification of small microstructural features (e.g. osteocyte lacunae, canals, trabeculae, microcracks) and direct gray value compositional mapping (e.g. mineral quantification, cement lines) with greater speed and fidelity than lab-based micro-computed tomography. In this article, we review how SRμCT has been applied to bone research to elucidate the mechanisms by which bone aging, disease, and other factors affect bone fragility and resistance to fracture