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Structural Materials Overview
Transparent Materials
High performance transparent ceramics and composites have wide range of applications as aircraft and automobile windows, IR windows, optical lenses, covers for solar cells and cell phones, visors, watch covers, scanner windows etc. They have high impact and wear resistance and improved optical clarity when compared to conventional glass/polymer designs. Knowledge of the micromechanical deformation and fracture characteristics of these materials is paramount to understand and improve their performance in potential applications. Towards this end, our research focuses on understanding the strain rate sensitivity, impact mechanics, damage interaction mechanisms and characterization of post damage optical performance by employing novel techniques. Transparent materials such as chemically strengthened glasses, sapphire (single crystal Al2O3), and spinels, which have wide range of applications for protection against impact and as high performance optics, are currently being investigated.
Influence of Residual Stresses on Fracture Behavior of Chemically Strengthened Glasses
Chemically surface strengthened lithium aluminosilicate glass with high surface compressive stresses (up to 1 GPa) was investigated to understand the fundamental deformation and fracture characteristics under static and dynamic loads. The depth of the compressive zone layer was determined using photoelasticity. In the current work, the influence of this surface compression on static and dynamic Vickers indentation hardness measurements was investigated and compared with the unstrengthened glass properties. The variation in hardness with depth was also examined by conducting static Vickers indentations on a surface perpendicular to the strengthened face. It was found that the influence of surface strengthening extends much farther beyond the compression zone depth determined using photoelasticity.
The above figures reveal the isochromatic fringe patterns on the unstrengthened face. The magnified image employing green light was used to facilitate the counting of the number of fringes beginning from the zero-order fringe toward the free-surface of the glass (compressions) and toward the interior of the glass (tension). Superimposed on the fringe data is a plot of the fringe order as a function of depth from the free (compressive) surface. This plot illustrates the case-depth of the compressive layer based on the zero-order fringe location. Also superimposed is a plot of hardness variation to exemplify the influence of the residual stress state (compression or tension) on the indentation hardness.
The strengthened glass rods were impacted with steel balls at 330 m/s. The damage propagation was observed using a high-speed camera in normal light, shadow light and photoelasticity. It was observed that the damage front in the unstrengthened glass propagates for a short distance at a maximum velocity of shear wave speed and then drops off quickly to zero. On the contrary, in the strengthened glass the damage propagates at shear wave velocity for the entire length of the rod and forms needle like damage fragments. The damage propagates at a higher rate along the periphery of the rod leaving behind small lobes of undamaged glass in the center of the rod.
3D woven polymer composites
Influence of Weave Architecture on Dynamic Response
High strength, low density and high stiffness of laminated composite materials allow them to be used in the design of strong light-weight structures. However, the propensity for delamination in these composites continues to be a major risk factor when the structures are subjected to impact loads. With continued improvements in delamination strength and damage tolerance of composites, new tests methods are warranted to determine the complex interplay between various structural parameters. In our research, several woven composite specimens with different architectures such as orthogonally woven, angle interlock 3D woven, as well as a 2D woven composites (as shown in the figure) were tested at high rates of flexural loads using a dynamic indentation tester, which is capable of imposing a single controlled impact of known duration and load. The indenter shape was varied to perform both dynamic bend tests as well as low velocity blunt impact on small test specimens. The residual stiffness of each composite specimen was determined by reloading under quasi-static conditions. This procedure allowed for the evaluation of damage tolerance of different architectures. Results from these tests indicate that the addition of z-yarns in 3D woven composites reduce the delamination resistance yet increase the damage tolerance.
References:
1. T.R. Walter, G. Subhash, B. Sankar, and C.F. Yen “Damage modes in 3D glass fiber woven composites under high rate of impact loading” Composites: Part B 40 (2009) 584–589.
2. T.R. Walter, G. Subhash, B. Sankar, and C.F. Yen, “Monotonic and cyclic short beam shear response of 3D woven composites,” Composite Science and Technology 70 (2010) 2190-2197
3. M.P. Rao, B.V. Sankar and G. Subhash, “Effect of Z-Yarns on the Stiffness and Strength of Three-Dimensional Woven Composites” Composites: Part B 40 (2009) 540–551
High-strength structural ceramics and composites
Fundamental investigations to identify the relationship between microstructure and mechanical response of advanced structural ceramics including Al2O3, Si3N4, BN, B4C, ZrO2, SiC and their composites is undertaken. Our intent is to relate microstructural changes and mechanical properties to macroscopic deformation and fracture behavior under high strain rate loading conditions. In particular, our research focuses on understanding the differences between static and dynamic mechanical response of high strength ceramics. We have recently embarked on developing a comprehensive understanding of dislocation based plasticity in ultra-high temperature ceramics such as ZrB2 and HfB2.
Ultra-high Temperature Ceramics (UHTCs)
Development of novel light weight and high strength materials, which can withstand elevated temperatures (above 2000oC), and provide good thermal insulation properties are crucial to meet future demands of many civilian, defense and aerospace applications. Hypersonic vehicles, kinetic energy interceptors and reusable space planes operate in reactive environments with temperatures well above the melting points of traditional materials. For such applications, demand of ultra-high temperature (UHT) materials that can withstand such high temperatures and yet provide adequate mechanical strength is continuously increasing. Towards this end, ultra-high temperature ceramics such as diborides, nitrides and carbides of zirconium and hafnium (melting temperature > 3000oC) have received considerable attention in recent literature to meet the requirements for future ultra-high temperatures materials.
While in service (e.g., during takeoff, landing, or reentry in to atmosphere), the structural components in aerospace vehicles are prone to impact by meteorites and atmospheric debris particles at very high velocities. Under such abrasion-dominated wear scenarios, sharp particles impacting on the exposed surfaces can result in inelastic deformation and material removal. Thus, evolution of fundamental micro-mechanisms of damage and deformation should be fully understood to evaluate the suitability of UHTCs for the aforementioned applications.
In the following figure, the microstructure of zirconium diboride-Silicon carbide ceramics and the slip lines evolved in the vicinity of a scratch as well as the disloation substructure are shown. Due to the metal-metal bond in the ZrB2 crystal, it is theorized that dislocation motion and high thermal conductivity do exist in these UHT ceramics.
References:
1. D. Ghosh, G. Subhash, and N. Orlovskaya, “Slip-line spacing in ZrB2-based ultrahigh temperature ceramics” Scripta Materialia, 62 (2010) 839-842.
2. D. Ghosh, G. Subhash, and G.R. Bourne, “Room-temperature dislocation activity during mechanical deformation of polycrystalline ultra high-temperature ceramics,” Scripta Materialia, 61 (2009) 1075–1078.
3. D. Ghosh, S. Maiti and G. Subhash, “A generalized cohesive element technique for arbitrary crack motion” Finite Elements in Analysis and Design, 45[8-9] (2009) 501-510
4. D. Ghosh, G. Subhash, and G.R. Bourne, “Inelastic Deformation under Indentation and Scratch Loads in a ZrB2-SiC Composite,” Journal European Ceramic Society 29 (2009) 3052-3061.
5. D. Ghosh, G. Subhash, T. Sudarshan, and G. Radhakrishanan “Scratch-Induced Microplasticity and Microcracking in Zirconium Diboride-Silicon Carbide Composite” Acta Materialia, 56/13 (2008) 3011-3022.
6. D. Ghosh, G. Subhash, and N. Orlovskaya, “Measurement of Scratch-Induced Residual Stress within SiC Grains in ZrB2-SiC Composite using Micro-Raman Spectroscopy” Acta Materialia, Vol.56, pp. 5345-5354 (2008)
Plastically Graded Surfaces
Plastically graded surfaces (PGSs) have broad applications in many fields of engineering and technology. For e.g., case hardened steels used in high-performance high-precision bearings and gears have high surface hardness and relatively soft core. The plastic properties at intermediate depths are graded monotonically with depth. Determination of the constitutive response of such plastically graded surfaces is essential for surface engineering design with tailored properties, for many high-performance aerospace applications. In our investigation, we have developed a coordinated experimental and numerical approach utilizing macro and micro indentation and elastic-plastic FEA for determining the monotonic constitutive response of case hardened PGSs. First, micro indentations are conducted on the cross-section of the PGS to obtain the hardness profile (i.e., yield strength values) of the virgin material as a function of depth. Next, a large indent, spanning the graded region of the PGS is conducted on the surface of the material. This causes large plastic deformation in the upper layers, and decreasing plastic strain amplitude as a function of depth. The material is then sectioned and polished close to the center of the indent. Micro indents are performed in the entire plastic zone beneath the macro indent to provide a map of the flow stress reached along the stress-strain curves for each layer of the PGS. Then, this information is utilized in a computational model for the macro indentation process along with the stress-strain response of the homogeneous materials of case and the core materials. The power-law exponents of the two materials, obtained from the power-law fit of the stress-strain responses of the case and core materials, provide the upper and the lower limits of the graded material. In the finite element analysis, we assign gradually varying n values at each depth for the PGS and the computation is repeated until the hardness profile of the deformed region matches that of the computational model. Thus the monotonic constitutive response of the PGS is derived.
References:
1. B. Nathan, A. Nagaraj, G. Subhash, and M. Klecka “Determination of the Constitutive Response of Plastically Graded Materials” International Journal of Plasticity (in press, Sept 2010)
2. B. Nathan, G. Subhash, M. Klecka and A. Nagaraj, “A New Reverse Analysis to Determine the Constitutive Response of Plastically Graded Steels” International Journal of of Solids and Structures (Accepted on Oct 20, 2010)
3. B. Nathan, G. Subhash, M. Klecka and A. Nagaraj, “Material Dependent Representative Plastic Strain for the Prediction of Indentation Hardness,” Acta Materialia 58 (2010) 6487-6494.
Constitutive Response of Biomaterials
Research on biomaterials focuses on determination of constitutive response and material properties under tension, compression and shear. The focus is on agarose gel, ballistic gelatin and brain tissue. Utilizing novel geometries of materials and experimental techniques (e.g., digital image correlation), the constitutive deformation behavior is extracted.
Gelatin is a popular tissue stimulant in biomedical engineering. Due to its biocompatible properties, gelatin is used as a tissue surrogate to assess severity of tissue damage or injury potential in accidents, bullet wounds, knife stabs, etc. Gels, in general, behave as non-Newtonian fluids under high shear rates, i.e., increase in viscosity with rate of shear, called ‘shear-thickening’ behavior. Most of the current modeling studies are focused on quasistatic behavior, which do not capture the rate-dependent response that is germane to these non-Newtonian fluids. Our investigations on gelatin have revealed a 1000-fold increase in dynamic uniaxial compressive strength compared to its quasistatic strength. In addition, we have conducted high rate shear experiments in the range of 1000-8000/s and extracted the power law exponent that characterizes its non-Newtonian (shear thickening) behavior.
Reference:
1. J.W. Kwon and G. Subhash “Compressive Strain Rate Sensitivity of Ballistic Gelatin,” Journal of Biomechanics 43 (2010) 420-425.
2. G. Subhash, Q. Liu, D. Moore, P. G. Ifju, and M. A. Haile, “Concentration Dependence of Tensile Behavior in Agarose Gel using Digital Image Correlation” Experimental Mechanics (in Press, March 2010)
High Rate Deformation Behavior of BrainTissue
Blast-induced traumatic brain injury (bTBI) and post traumatic stress disorder (PTSD) have received increasing attention in recent years because of the ongoing conflicts in Iraq and Afghanistan. During incidences of sudden impact or explosive blasts, sensitive neurological tissues are affected at microstructural levels by pressure waves that travel through the skull and brain. The immediate soft tissue response to these waves is difficult to assess using current in vivo imaging technologies, such as CT or MRI scans, due to limitations in temporal and spatial resolution. However, it is these pressure waves and the resulting local stretching and shearing of tissue within the micro- and milli-second time scale of blast loading that are primary contributors to initial tissue injuries that can lead to bTBI and PTSD.
We have taken a multidisciplinary approach to address the relationship between strains incurred during shock loading and the injury response within living brain tissues. A combination of shock loading, high speed imaging and digital image correlation, optical coherence tomography, microindentation, as well as improved histological methods are utilized in a new ~2D ex vivo model for TBI testing. In this test model, living rat brain tissue slices are subjected to controlled high acceleration and shock loads with simultaneous high speed imaging. Applied strain history maps for tissue slices will be generated for varying levels of incident energy and used to identify damage-sensitive regions where maximum stretching and shearing occur. Regional structural pathology will be analyzed by quantitative assessment of early cell death and supplemental histology. As an additional measure of tissue damage, changes in mechanical properties will be assessed using micro-indentation test methods specifically developed for soft thin tissues so as to map variations in local properties.
Most on-going bTBI studies in the literature focus on the response of the entire brain to impact and blasts. However, our work will include studies on~2D brain tissue slice to obtain detailed time resolved visualization of tissue deformation not yet available. Such data includes measures of relative responses within different brain structures, e.g. cortex and hippocampus, and between heterogeneous tissue regions, e.g. white and gray matter tissues. Fundamentally, these studies will provide new test platforms and experimental data needed to define critical relationships among incident shock waves that impart high strain rate loads, local mechanical responses and the initial injury level within sensitive tissues. Because initial injuries instigate a progressive self-destructive sequence (e.g. excitotoxic responses and apoptosis) that usually leads to even larger zones secondary pathology, it is important to separate out initial injury events. Lack of such data has hindered our ability to fully understand what mechanisms initiate bTBI.
Shear Localization in Amorphous Metals
TBulk metallic glasses or amorphous metals have unique properties compared to metals. While the theoretical yield strength of traditional crystalline materials is expected to be close to 1/100th to 1/1000th of the elastic modulus BMGs are known to possess yield strength close to 1/10th - 1/50th of their theoretical strength. They fail in a quasi-brittle fashion due to rapid development of localized shear bands. It has been experimentally observed that the viscosity of the material drops drastically at the core of these shear bands and therefore, controlling the shear localization behavior at the atomic scale level is essential to improving the mechanical behavior of BMGs.
We have used static and dynamic indentation studies to study evolution of shear bands surrounding and underneath the indentation. Our research has identified three different slip-step patterns beneath the indentation: semi-circular primary shear bands surrounding the indentation at low loads (10-50 g), secondary (radial) shear bands and tertiary shear bands along the periphery of the plastic region at higher loads. These shear bands have been illustrated in the Figure below.
References
1. H. Zhang, S. Maiti, and G. Subhash, "Evolution of Shear Bands in Bulk Metallic Glasses under Dynamic Loading" Journal of the Mechanics and Physics of Solids, 56 (2008) 2171–2187,
2. H. Zhang, G. Subhash, and X. Jing, L. J. Kecskes and R. J. Dowding, "Evaluation of Hardness-Yield Strength Relationships in Bulk Metallic Glasses" Philosophical Magazine Letters, Vol.86, No.5, 333-345 (2006).
3. G. Subhash and H. Zhang, “Shear band patterns in static indentation, dynamic indentation and scratch processes” Metallurgical Transactions, Vol 38[12] 2936-2942 (2007)
Polymeric Foams
We have conducted studies on crushability and dynamic response of polymeric foams using novel polymer split Hopkinson pressure bar specifically developed to test low-impedance materials.
References:
1. G. Subhash and Q. Liu, “Quasistatic and Dynamic Crushability of Polymeric Foams in Rigid Confinement” International Journal of Impact Engineering, 36 (2009) 1303-1311
2. L. Wucherer, J.C. Nino, G. Subhash, “Mechanical Properties of BaTiO3 Open-Porosity Foams for Piezoelectric Composites”, Journal European Ceramic Society, 29 (2009) 1987–1993.
3. T.R. Walter, A. Richards and G. Subhash, “A unified phenomenological model for compression and tension response of polymeric foams,” ASME Journal of Engineering Materials and Technology, 131[1] (2009) 011009 (6 pages)
4. K. Li, X.-L. Gao and G. Subhash, “Effects of Cell Shape and Strut Cross-Sectional Area Variations on the Elastic Properties of Three-Dimensional Open-Cell Foams” J. of Mechanics and Phys¬ics of Solids, Vol.54[4] 783-806 (2006)
5. Q. Liu and G. Subhash, "Characterization of viscoelastic properties of polymer bar using iterative deconvolution in the time domain" Mechanics of Materials,Vol. 38[12], pp. 1105-1117 (2006)
6. G. Subhash, Q. Liu and X.-L. Gao "Quasistatic and high strain rate uniaxial compressive response of polymeric structural foams," Int. J. Impact Engineering, Vol. 32[7], pp.1113-1126, (2006).
7. Q. Liu and G. Subhash, "A phenomenological constitutive model for polymer foams under large deformations" Polymer Engineering and Science Vol.44, pp.463–473, (2004).
8. G. Subhash and Q. Liu, “Crushability maps for structural polymeric foams under uniaxial loading in rigid confinement” Experimental Mechanics, Vol.44 [3], pp. 289-294, (2004).
High strain rate testing of soft materials using a polymer split Hopkinson pressure bar
To test soft materials such as foams and gels, a polymer split Hopkinson pressure bar is used. We have fully analyzed the wave propagation in a polymer bar and have developed novel algorithms to account for attenuation and dispersion in the bars. The following figures illustrate some test results on gelatin specimens, the polymer bar behavior in terms of our ability to predict the wave amplitude and duration at any point on the bar and the algorithm.
Verification of our ability to predict the stress wave profile at any point
along the polymer bar using the algorithm
Quasistatic Stress Strain Response
Dynamic Stress Strain Response