<p> A research programme is presented to more fully understand the behaviour of Round Hollow Structural Sections subjected to inelastic strain reversals. An attempt is made to establish a preliminary guideline for choosing a reasonable value for the slenderness ratio (D/t) that qualifies the Round HSS for cyclic loading. </p> <p> An experimental programme on nine sections was performed to evaluate the loss in load capacity due to inelastic cyclic loading and to construct the load-deflection hysteresis loops. </p> <p> Three different limiting cyclic deflections were imposed to the smallest D/t section and a comparison was made between the performance of the tube in each case. </p> <p> Prasad's computer program was used to select the strain value at which local buckling will appear and to calculate the half buckle wave-length. The results were compared to the experimental results obtained by the author. </p> <p> A comparison is made between two Round HSS having different diameters and thicknesses but the same D/t ratio. </p> <p> A comparison is made between the behaviour of Round HSS and Square HSS subjected to the same cyclic loading programme. </p> <p> An attempt is made to suggest a reasonable curve relating the number of cycles to failure and the D/t ratio. </p> / Thesis / Master of Engineering (ME)

Continuous Fiber Reinforced Ceramic Composites (CFCC's) are being currently investigated as potential materials for high temperature applications such as combustor liners in stationary gas turbines. The creep behavior of woven Enhanced SiC/SiC composites was studied at temperatures from 600 to 1200 °C and at 140 to 220 MPa stress levels. Most researchers studying the creep behavior of ceramic matrix composites (CMCs) use the time hardening model and rate equations for expressing the dependence of creep strain on time, temperature and stress. Such laws, although simple and easy to use, are inadequate to represent the creep behavior over a range of stress levels and temperatures and cannot be used to quantify the pest phenomenon commonly observed in CMCs. Hence, these laws were modified to include the pest phenomenon and an empirical equation was developed that can be used to represent the creep behavior at various stresses and temperatures. The modified equation was used in the finite element analysis and the results were compared with the time and strain hardening models. Microscopic observations on the fractured surfaces revealed the pseudo-ductile behavior of the material at high temperatures. A quasi-static test was conducted at 1200 °C to determine the unloading response of the material. The stress-strain response of the composite demonstrates a hysterisis loop and a small amount of permanent strain, which are characteristic of the CMC's [3]. Finally, a test was conducted at 1200 oC to investigate the recovery behavior of the material. The material exhibits a tendency to recover the accumulated creep strain as well as the small permanent strain upon unloading, if sufficient time is allowed for recovery. The creep data were also modeled using the representations such as Monkmon-Grant and Larsen-Miller equations. A modified Monkman-Grant equation was used to model the stratification of the creep strain rate data with temperature. A finite element model based on the plasticity theory was developed to simulate the quasi-static cyclic behavior of the material. Though the loading behavior of CMCs can be modeled using the bilinear or multilinear kinematic hardening plasticity models, the unloading behavior as predicted by the models is entirely different from the experimentally observed behavior. Hence, these models were modified to correctly predict the stress-strain behavior. The model, which was input via a user defined subroutine into the ANSYS finite element program uses the concept of state or internal variables to define the unloading portion of the stress-strain curve. The results were compared with the test data and they show very good agreement. The model was then used to predict the stress-strain response of a plate with a notch. The results from the analysis were compared with the experimental data and they show good agreement if average values of strains are considered. / Master of Science