Kinetics of martensitic interface motion

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1983.MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE.Vita.Includes bibliographical references.by Mica Grujicic.Ph.D

Meso-scale computational investigation of polyurea microstructure and its role in shockwave attenuation/dispersion

In a number of recently published studies, it was demonstrated that polyurea possesses a high shockwave-mitigation capacity, i.e. an ability to attenuate and disperse shocks. Polyurea is a segmented thermoplastic elastomer which possesses a meso-scale segregated microstructure consisting of (high glass-transition temperature, Tg) hydrogen-bonded discrete hard domains and a (low Tg) contiguous soft matrix. Details of the polyurea microstructure (such as the extent of meso-segregation, morphology and the degree of short-range order and crystallinity within the hard domains) are all sensitive functions of the polyurea chemistry and its synthesis route. It has been widely accepted that the shockwave-mitigation capacity of polyurea is closely related to its meso-phase microstructure. However, it is not presently clear what microstructure-dependent phenomena and processes are responsible for the superior shockwave-mitigation capacity of this material. To help identify these phenomena and processes, meso-scale coarse-grained simulations of the formation of meso-segregated microstructure and its interaction with the shockwave is analyzed in the present work. It is found that shockwave-induced hard-domain densification makes an important contribution to the superior shockwave-mitigation capacity of polyurea, and that the extent of densification is a sensitive function of the polyurea soft-segment molecular weight. Specifically, the ability of release waves to capture and neutralize shockwaves has been found to depend strongly on the extent of shockwave-induced hard-domain densification

Nacre-like ceramic/polymer laminated composite for use in body-armor applications

Nacre is a biological material constituting the innermost layer of the shells of gastropods and bivalves. It consists of polygonal tablets of aragonite, tessellated to form individual layers and having the adjacent layers as well as the tablets within a layer bonded by a biopolymer. Due to its highly complex hierarchical microstructure, nacre possesses an outstanding combination of mechanical properties, the properties which are far superior to the ones that are predicted using the techniques such as the rule of mixture. In the present work, an attempt is made to model a nacre-like composite armor consisting of boron carbide (B4C) tablets and polyurea tablet/tablet interfaces. The armor is next investigated with respect to impact by a solid right-circular-cylindrical rigid projectile, using a transient non-linear dynamics finite element analysis. The ballistic-impact response and the penetration resistance of the armor is then compared with that of the B4C monolithic armor having an identical areal density. Furthermore, the effect of various nacre microstructural features (e.g. surface profiling, micron-scale asperities, mineral bridges between the overlapping tablets lying in adjacent layers, and B4C nano-crystallinity) on the ballistic-penetration resistance of the composite-armor is investigated in order to identify an optimal nacre-like composite-armor architecture having the largest penetration resistance. The results obtained clearly show that a nacre-like armor possesses a superior penetration resistance relative to its monolithic counterpart, and that the nacre microstructural features considered play a critical role in the armor penetration resistance

Polyurea/Fused-silica interfacial decohesion induced by impinging tensile stress-waves

All-atom non-equilibrium molecular-dynamics simulations are used to investigate the problems of polyurea-borne tensile-stress waves interacting with a polyurea/fused-silica interface and fused-silica tensile-stress waves interacting with a fused-silica/polyurea interface, and the potential for the accompanying interfacial decohesion. To predict the outcome of the interactions of stress-waves with the material-interfaces in question, at the continuum level, previously determined material constitutive relations for polyurea and fused-silica are used within an acoustic-impedance-matching procedure. These continuum-level predictions pertain solely to the stress-wave/interface interaction aspects resulting in the formation of transmitted and reflected stress- or release-waves, but do not contain any information regarding potential interfacial decohesion. Present direct molecular-level simulations confirmed some of these continuum-level predictions, but also provided direct evidence of the nature and the extent of interfacial decohesion. In the molecular-level simulations, reactive force-field potentials are utilized to properly model the initial state of interfacial cohesion and its degradation during stress-wave-loading. Examination of the molecular-level interfacial structure before the stress-wave has interacted with the given interface, revealed local changes in the bonding structure, suggesting the formation of an “interphase.” This interphase was subsequently found to greatly affect the polyurea/fused-silica decohesion strength and the likelihood for interfacial decohesion during the interaction of the stress-wave with the interface

Use of the Materials Genome Initiative (MGI) approach in the design of improved-performance fiber-reinforced SiC/SiC ceramic-matrix composites (CMCs)

New materials are traditionally developed using costly and time-consuming trial-and-error experimental efforts. This is followed by an even lengthier material-certification process. Consequently, it takes 10 to 20 years before a newly-discovered material is commercially employed. An alternative approach to the development of new materials is the so-called materials-by-design approach within which a material is treated as a complex hierarchical system, and its design and optimization is carried out by employing computer-aided engineering analyses, predictive tools and available material databases. In the present work, the materials-by-design approach is utilized to design a grade of fiber-reinforced (FR) SiC/SiC ceramic matrix composites (CMCs), the type of materials which are currently being used in stationary components, and are considered for use in rotating components, of the hot sections of gas-turbine engines. Towards that end, a number of mathematical functions and numerical models are developed which relate CMC constituents’ (fibers, fiber coating and matrix) microstructure and their properties to the properties and performance of the CMC as a whole. To validate the newly-developed materials-by-design approach, comparisons are made between experimentally measured and computationally predicted selected CMC mechanical properties. Then an optimization procedure is employed to determine the chemical makeup and processing routes for the CMC constituents so that the selected mechanical properties of the CMCs are increased to a preset target level

Penetration resistance and ballistic-impact behavior of Ti/TiAl₃ metal/intermetallic laminated composites (MILCs): A computational investigation

A comprehensive computational engineering analysis is carried out in order to assess suitability of the Ti/TiAl3 metal/intermetallic laminated composites (MILCs) for use in both structural and add-on armor applications. This class of composite materials consists of alternating sub-millimeter thick layers of Ti (the ductile and tough constituent) and TiAl3 (the stiff and hard constituent). In recent years, this class of materials has been investigated for potential use in light-armor applications as a replacement for the traditional metallic or polymer-matrix composite materials. Within the computational analysis, an account is given to differing functional requirements for candidate materials when used in structural and add-on ballistic armor. The analysis employed is of a transient, nonlinear-dynamics, finite-element character, and the problem investigated involves normal impact (i.e. under zero obliquity angle) of a Ti/TiAl3 MILC target plate, over a range of incident velocities, by a fragment simulating projectile (FSP). This type of analysis can provide more direct information regarding the ballistic limit of the subject armor material, as well as help with the identification of the nature and the efficacy of various FSP material-deformation/erosion and kinetic-energy absorption/dissipation phenomena and processes. The results obtained clearly revealed that Ti/TiAl3 MILCs are more suitable for use in add-on ballistic, than in structural armor applications