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Design of Pneumatically Actuated Torsional Loading for High Strain Rate Testing

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In real world applications, materials undergo a simultaneous combination of tension, compression, and torsion as a result of high velocity impact. The split Hopkinson pressure bar (SHPB) is an effective tool for analyzing stress-strain response of materials at high strain

In real world applications, materials undergo a simultaneous combination of tension, compression, and torsion as a result of high velocity impact. The split Hopkinson pressure bar (SHPB) is an effective tool for analyzing stress-strain response of materials at high strain rates but currently little can be done to produce a synchronized combination of these varying impacts. This research focuses on fabricating a flange which will be mounted on the incident bar of a SHPB and struck perpendicularly by a pneumatically driven striker thus allowing for torsion without interfering with the simultaneous compression or tension. Analytical calculations are done to determine size specifications of the flange to protect against yielding or failure. Based on these results and other design considerations, the flange and a complementary incident bar are created. Timing can then be established such that the waves impact the specimen at the same time causing simultaneous loading of a specimen. This thesis allows research at Arizona State University to individually incorporate all uniaxial deformation modes (tension, compression, and torsion) at high strain rates as well as combining either of the first two modes with torsion. Introduction of torsion will expand the testing capabilities of the SHPB at ASU and allow for more in depth analysis of the mechanical behavior of materials under impact loading. Combining torsion with tension or compression will promote analysis of a material's adherence to the Von Mises failure criterion. This greater understanding of material behavior can be implemented into models and simulations thereby improving the accuracy with which engineers can design new structures.

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2016-05

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Investigating the Mechanical Behavior and Deformation Mechanisms of Ultrafine-grained Metal Films Using Ex-situ and In-situ TEM Techniques

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Nanocrystalline (NC) and Ultrafine-grained (UFG) metal films exhibit a wide range of enhanced mechanical properties compared to their coarse-grained counterparts. These properties, such as very high strength, primarily arise from the change in the underlying deformation mechanisms. Experimental and simulation

Nanocrystalline (NC) and Ultrafine-grained (UFG) metal films exhibit a wide range of enhanced mechanical properties compared to their coarse-grained counterparts. These properties, such as very high strength, primarily arise from the change in the underlying deformation mechanisms. Experimental and simulation studies have shown that because of the small grain size, conventional dislocation plasticity is curtailed in these materials and grain boundary mediated mechanisms become more important. Although the deformation behavior and the underlying mechanisms in these materials have been investigated in depth, relatively little attention has been focused on the inhomogeneous nature of their microstructure (particularly originating from the texture of the film) and its influence on their macroscopic response. Furthermore, the rate dependency of mechanical response in NC/UFG metal films with different textures has not been systematically investigated. The objectives of this dissertation are two-fold.

The first objective is to carry out a systematic investigation of the mechanical behavior of NC/UFG thin films with different textures under different loading rates. This includes a novel approach to study the effect of texture-induced plastic anisotropy on mechanical behavior of the films. Efforts are made to correlate the behavior of UFG metal films and the underlying deformation mechanisms. The second objective is to understand the deformation mechanisms of UFG aluminum films using in-situ transmission electron microscopy (TEM) experiments with Automated Crystal Orientation Mapping. This technique enables us to investigate grain rotations in UFG Al films and to monitor the microstructural changes in these films during deformation, thereby revealing detailed information about the deformation mechanisms prevalent in UFG metal films.

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2017

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Structure-property relationships in aluminum-copper alloys using Transmission X-ray Microscopy (TXM) and micromechanical testing

Description

Aluminum alloys are ubiquitously used in almost all structural applications due to their high strength-to-weight ratio. Their superior mechanical performance can be attributed to complex dispersions of nanoscale intermetallic particles that precipitate out from the alloy’s solid solution and offer

Aluminum alloys are ubiquitously used in almost all structural applications due to their high strength-to-weight ratio. Their superior mechanical performance can be attributed to complex dispersions of nanoscale intermetallic particles that precipitate out from the alloy’s solid solution and offer resistance to deformation. Although they have been extensively investigated in the last century, the traditional approaches employed in the past haven’t rendered an authoritative microstructural understanding in such materials. The effect of the precipitates’ inherent complex morphology and their three-dimensional (3D) spatial distribution on evolution and deformation behavior have often been precluded. In this study, for the first time, synchrotron-based hard X-ray nano-tomography has been implemented in Al-Cu alloys to measure growth kinetics of different nanoscale phases in 3D and reveal mechanistic insights behind some of the observed novel phase transformation reactions occurring at high temperatures. The experimental results were reconciled with coarsening models from the LSW theory to an unprecedented extent, thereby establishing a new paradigm for thermodynamic analysis of precipitate assemblies. By using a unique correlative approach, a non-destructive means of estimating precipitation-strengthening in such alloys has been introduced. Limitations of using existing mechanical strengthening models in such alloys have been discussed and a means to quantify individual contributions from different strengthening mechanisms has been established.

The current rapid pace of technological progress necessitates the demand for more resilient and high-performance alloys. To achieve this, a thorough understanding of the relationships between material properties and its structure is indispensable. To establish this correlation and achieve desired properties from structural alloys, microstructural response to mechanical stimuli needs to be understood in three-dimensions (3D). To that effect, in situ tests were conducted at the synchrotron (Advanced Photon Source) using Transmission X-Ray Microscopy as well as in a scanning electron microscope (SEM) to study real-time damage evolution in such alloys. Findings of precipitate size-dependent transition in deformation behavior from these tests have inspired a novel resilient aluminum alloy design.

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2017