Showing papers by "John W. Hutchinson published in 2016"

Buckling of spherical shells revisited

Abstract: A study is presented of the post-buckling behaviour and imperfection sensitivity of complete spherical shells subject to uniform external pressure. The study builds on and extends the major contrib...

Analysis of necking based on a one-dimensional model

Abstract: Dimensional reduction is applied to derive a one-dimensional energy functional governing tensile necking localization in a family of initially uniform prismatic solids, including as particular cases rectilinear blocks in plane strain and cylindrical bars undergoing axisymmetric deformations. The energy functional depends on both the axial stretch and its gradient. The coefficient of the gradient term is derived in an exact and general form. The one-dimensional model is used to analyze necking localization for nonlinear elastic materials that experience a maximum load under tensile loading, and for a class of nonlinear materials that mimic elastic-plastic materials by displaying a linear incremental response when stretch switches from increasing to decreasing. Bifurcation predictions for the onset of necking from the simplified theory compared with exact results suggest the approach is highly accurate at least when the departures from uniformity are not too large. Post-bifurcation behavior is analyzed to the point where the neck is fully developed and localized to a region on the order of the thickness of the block or bar. Applications to the nonlinear elastic and elastic-plastic materials reveal the highly unstable nature of necking for the former and the stable behavior for the latter, except for geometries where the length of the block or bar is very large compared to its thickness. A formula for the effective stress reduction at the center of a neck is established based on the one-dimensional model, which is similar to that suggested by Bridgman (1952).

Dependence of confined plastic flow of polycrystalline Cu thin films on microstructure

Abstract: Axial compression was conducted on micro-pillars, in which polycrystalline Cu thin films were sandwiched between CrN and Si. Plastic flow of Cu was achieved, when the Cu films are inclined either at 90° or 45° with respect to the pillar axis. The texture of Cu films was altered by changing the template on which film growth occurred. The Cu microstructure was further altered by post-deposition annealing. The flow stress shows little dependence on the film texture in the as-deposited state. However, annealing influences the flow stress of confined Cu films significantly. The implications on strain gradient plasticity models are discussed.

Preliminary Experimental Investigation of Initial Onset of Sand Deposition in the Turbine Section of Gas Turbines

Abstract: Particle ingestion and deposition is an issue of concern for gas turbine engines operating in harsh environments. The ingested particles accelerate the deterioration of engine components and thus reduce its service life. This effect is observed to a greater extent in aircrafts/helicop ters operating in particle laden environment. Understanding the effects of particle ingestion at engine representative condition leads to improved designs for turbomachinery. Experiments have been in an Aerothermal Rig facility at Virginia Tech to study particle deposition at engine representative temperatures. The Aerothermal Rig was upgraded to achieve air temperatures of up to 1100°C at the test section. The experiments are performed using Arizona Road Dust (ARD) of 20-40 μm size range. The temperature of air and particles are around 1100°C at a constant velocity of 70 m/s. The target coupon is made of Hastelloy X, a nickel-based alloy and the angle at which the particles impact the coupon varies from 30° to 80°. The experiments were performed with different amounts of total particle injected, concentration, and coupon angle to understand their effects on deposition. Similar research was carried out in the past at the same facility to study particle deposition at temperatures up to 1050°C and 70 m/s flow velocity. However, this previous research only studied how the coupon angle affects particle deposition; other parameters such as total particle input and particle concentration were not studied. It was found that particle deposition increases significantly at higher temperatures beyond 1050°C for higher coupon angle and amount of sand injected. Results from current study also show that deposition increases with increase in total sand injected, concentration, and coupon angle for a given temperature and velocity.

Rolling at Small Scales

Abstract: The rolling process is widely used in the metal forming industry and has been so for many years. However, the process has attracted renewed interest as it recently has been adapted to very small scales where conventional plasticity theory cannot accurately predict the material response. It is well-established that gradient effects play a role at the micron scale, and the objective of this study is to demonstrate how strain gradient hardening affects the rolling process. Specifically, the paper addresses how the applied roll torque, roll forces, and the contact conditions are modified by strain gradient plasticity. Metals are known to be stronger when large strain gradients appear over a few microns; hence, the forces involved in the rolling process are expected to increase relatively at these smaller scales. In the present numerical analysis, a steady-state modeling technique that enables convergence without dealing with the transient response period is employed. This allows for a comprehensive parameter study. Coulomb friction, including a stick–slip condition, is used as a first approximation. It is found that length scale effects increase both the forces applied to the roll, the roll torque, and thus the power input to the process. The contact traction is also affected, particularly for sheet thicknesses on the order of 10 μm and below. The influences of the length parameter and the friction coefficient are emphasized, and the results are presented for multiple sheet reductions and roll sizes.