Browsing Department of Mechanical Engineering & the Built Environment by Subjects
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Damage in single lap joints of woven fabric reinforced polymeric composites subjected to transverse impact loadingSingle lap joints of woven glass fabric reinforced phenolic composites, having four different overlap widths, were impacted transversely using a hemispherical impactor with different velocities in the low velocity impact range. The resulting damage was observed at various length scales (from micro to macro) using transmission photography, ultrasonic c-scan and x-ray micro tomography (XMT), in support of each other. These experimental observations were used for classification of damage in terms of damage scale, location (i.e. ply, interfaces between plies or bond failure between the two adherends) and mechanisms, with changing overlap width and impact velocity. In addition, finite element analysis was used to simulate delamination and disbond failure. These simulations were used to further explain the observed dependence of damage on overlap width and impact velocity. The results from these experiments and simulations lead to the proposal of a concept of lower and upper characteristic overlap width. These bounds relate the dominant damage pattern (i.e. scale, location and mechanism) with overlap width of the joint for a given impact velocity range.
Micromechanical modeling of 8-harness satin weave glass fiber-reinforced composites.This study introduces a unit cell (UC) based finite element (FE) micromechanical model that accounts for correct post cure fabric geometry, in-situ material properties and void content within the composite to accurately predict the effective elastic orthotropic properties of 8-harness satin weave glass fiber reinforced phenolic (GFRP) composites. The micromechanical model utilizes a correct post cure internal architecture of weave, which was obtained through X-ray microtomography (XMT) tests. Moreover, it utilizes an analytical expression to up-date the input material properties to account for in-situ effects of resin distribution within yarn (the yarn volume fraction) and void content on yarn and matrix properties. This is generally not considered in modeling approaches available in literature and in particular, it has not been demonstrated before for FE micromechanics models of 8-harness satin weave composites. The UC method is used to obtain the effective responses by applying periodic boundary conditions. The outcome of the analysis based on the proposed model is validated through experiments. After validation, the micromechanical model was further utilized to predict the unknown effective properties of the same composite.
Microstructural analysis of TRISO particles using multi-scale X-ray computed tomographyTRISO particles, a composite nuclear fuel built up by ceramic and graphitic layers, have outstanding high temperature resistance. TRISO fuel is the key technology for High Temperature Reactors (HTRs) and the Generation IV Very High Temperature Reactor (VHTR) variant. TRISO offers unparalleled containment of fission products and is extremely robust during accident conditions. An understanding of the thermal performance and mechanical properties of TRISO fuel requires a detailed knowledge of pore sizes, their distribution and interconnectivity. Here 50 nm, nano-, and 1 μm resolution, micro-computed tomography (CT), have been used to quantify non-destructively porosity of a surrogate TRISO particle at the 0.3–10 μm and 3–100 μm scales respectively. This indicates that pore distributions can reliably be measured down to a size approximately 3 times the pixel size which is consistent with the segmentation process. Direct comparison with Scanning Electron Microscopy (SEM) sections indicates that destructive sectioning can introduce significant levels of coarse damage, especially in the pyrolytic carbon layers. Further comparative work is required to identify means of minimizing such damage for SEM studies. Finally since it is non-destructive, multi-scale time-lapse X-ray CT opens the possibility of intermittently tracking the degradation of TRISO structure under thermal cycles or radiation conditions in order to validate models of degradation such as kernel movement. X-ray CT in-situ experimentation of TRISO particles under load and temperature could also be used to understand the internal changes that occur in the particles under accident conditions.
Predicting the effect of voids on mechanical properties of woven composites.An accurate yet easy to use methodology for determining the effective mechanical properties of woven fabric reinforced composites is presented. The approach involves generating a representative unit cell geometry based on randomly selected 2D orthogonal slices from a 3D X-ray micro-tomographic scan. Thereafter, the finite element mesh is generated from this geometry. Analytical and statistical micromechanics equations are then used to calculate effective input material properties for the yarn and resin regions within the FE mesh. These analytical expressions account for the effect of resin volume fraction within the yarn (due to infiltration during curing) as well as the presence of voids within the composite. The unit cell model is then used to evaluate the effective properties of the composite.