Dynamic deformation occurs when bodies are subjected to rapidly changing loads and can differ significantly from deformation that occurs under static or quasi-static situations. It is of great significance to understand the deformation and failure mechanisms of advanced materials, and there are potential applications in which dynamic deformation and failure can occur. Two classes of advanced materials, ultrafine-grained (UFG) (~500 nm and ~100 nm) titanium and high-entropy alloys (HEAs) (Al0.3CoCrFeNi and CoCrFeMnNi) are the focus of this doctoral investigation.

The deformation and adiabatic shear localization at cryogenic temperatures (173 K and 77 K) in ultrafine-grained (100 and 500 nm) titanium are investigated. In comparison with conventionally-grained titanium, the strength of ultrafine-grained titanium is higher due to the classic Hall-Petch effect while the strain-hardening rate approaches zero. Our results show that shear localization in dynamic deformation is also altered. The width of the shear band of coarse-grained titanium decreases from 30 µm at 293 K to 18 µm at 77 K (a 40% decrease). In contrast, for 100 nm titanium, the width of shear band decreases more significantly from 4 µm at room temperature to 1 µm (a 75% decrease) at cryogenic temperature (77 K). This difference is attributed to the combined effects of a decrease in the thermal conductivity and the specific heat capacity, and an increase in the thermal softening, which can lead to a band with thickness of 1 µm. These changes agree with the predictions of the Grady and Bai-Dodd theories. The dislocation evolution and the subgrain rotation mechanisms responsible for forming ultrafine- and nano- recrystallized grains are modeled. In addition, the Zener-Hollomon parameter is incorporated in the analysis to predict the critical dislocation density for shear localization and the recrystallized grain size in titanium .

The mechanical behavior of three single-phase face-centered-cubic (fcc) Al0.3CoCrFeNi, annealed CoCrFeMnNi and as-processed CoCrFeMnNi high-entropy alloys (HEAs) was studied in both quasi-static and high strain-rate regimes. Based on Hall-Petch strengthening, solid-solution strengthening, order hardening, cutting forest dislocations, and twinning hardening mechanisms, a constitutive equation was proposed to describe the flow of the annealed CoCrFeMnNi high-entropy alloy under dynamic impact. The resistance to shear localization is being established by dynamically-loading hat-shaped specimens that induce forced shear localization. Adiabatic shear band formation required an imposed shear strain of ~7 for the annealed CoCrFeMnNi HEA and cannot be observed at a strain of 1.1 for the Al0.3CoCrFeNi HEA. The structural and mechanical response that give rise to a remarkable resistance to shear localization are characterized by a combination of (1) a high strain-hardening ability, enabled by solid solution hardening, forest dislocation hardening, order hardening, and twinning hardening, (2) a high strain-rate sensitivity and (3) modest thermal softening; these combination effects give rise to the remarkable resistance to shear localization. First, the low stacking-fault energies in as-received high-entropy alloys lead to the formation of twinned segments inside the coarse grains. Then, when the thermal softening overcomes strain hardening, the shear bands would form, and dynamic recrystallization occurs inside the segments for the further break-up of the grains. Classical Straker equation is applied to predict the critical shear strain for shear localization, which was quite comparable to the experimental values in the high-entropy alloys. It was revealed that the as-processed CoCrFeMnNi HEA was prone to shear localization due to the initially high dislocation density which results in a relatively low work-hardening effect.

The dynamic deformation of these two metallic materials leads to adiabatic shear band formation at extreme shear strains. The resultant of the ultrafine grain structure observed in these two materials with diverse structures (HCP for Ti and FCC for HEAs) is remarkably similar and reinforces the concept of rotational dynamic recrystallization as the mechanism responsible for localization.

The fragmentation and constitutive response of tailored aluminum-based compacts is examined under dynamic conditions. Mesostructured Al compacts having interfaces between the powders (with sizes of 40, 100, and 400 μm) with controlled strengths were produced by swaging. Nickel-aluminum compacts were also produced and studied because the Ni-Al system is an optimum model system to investigate exothermic reactions initiated by dynamic events, such as ballistic impact.

In order to obtain a fundamental quantitative understanding of the process the fragmentation of pre-compacted powders is essential for the modeling of the energetics and kinetics of exothermic reactions. Thus, in this work the Mott theory was extended by incorporating fracture toughness. The fracture toughness and yield stress are used to calculate fragment sizes in a variety of particle sizes and geometries which are compared with experimental results.

The fragmentation generated in the explosively-driven rings expanded at a velocity of approximately 100 m/s was captured by high-speed photography. The fragment size distributions obtained varied widely and correlated with the interfacial strength of the compacts as well as with powder size. Finite element simulations were also conducted to help better understand the influence of material properties on fragmentation. Experimental results and finite element simulations both confirm that the modified Mott theory, which incorporates fracture toughness, can successfully calculate fragment size.

Skin is the outermost layer of the body and acts as a primary protective barrier against external agents such as heat, light, infection, and injury. Additionally, skin regulates a broad range of physiological parameters and hosts several vital components. In order to fulfill these functions throughout life, skin must be able to withstand and recover from significant deformation as well as mitigate tear propagation that can occur during growth, movement, and injuries affecting its integrity. Hence, characterizing the mechanical behavior of skin and understanding the underlying mechanisms of deformation at different spatial scales is essential in a large spectrum of applications such as surgery, cosmetics, forensics, biomimetics and engineering of protective gear or artificial grafts.

In this dissertation, a comprehensive list of experimental techniques that have been developed over the years to test skin’s nonlinear elastic, viscoelastic, and dissipative properties are reviewed. To identify parameters affecting its behavior, a significant number of models have been developed, some of which are detailed here. The principal structural elements within the dermis, and especially the arrangement and orientation of the collagen fibrils and fibers, are presented; their incorporation into the constitutive models is discussed.

We conduct a detailed investigation of the evolution of the collagen architecture of the dermis as a function of deformation, which reveals new aspects that extend our understanding of the mechanical response of porcine skin. The dermis is found to have a tridimensional woven structure of collagen fibers, which evolves with deformation. After failure, we observe that the fibers have straightened and aligned in the direction of tension. Time-dependent and dissipative effects are quantitatively established. Digital image correlation techniques were implemented to quantify skin’s anisotropy; measurements of the Poisson ratio and their evolution are reported for the first time. Based on new observations, we propose that fiber braiding is at least partially responsible for the monotonic increase of the tangent modulus of skin with strain, as well as its dissipative response to cyclical loads.

We incorporate these findings in a constitutive framework incorporating fibril stiffness, interfibrillar frictional sliding, and the effect of lateral fibers on the extension of a primary fiber, using analytical and coarse-grained modeling approaches. The representation of these important physical processes that occur during deformation of the dermis represents an advance in our understanding of these phenomena.

Finally, we estimate the toughness of porcine skin by conduct two types of experiment on pre-notched specimens, placing the tissue under shear Mode III and opening Mode I. We obtain two distinct toughness values of J_IIIc≈20.4 kJ/m^2 and J_Ic=30.4 kJ/m^2, indicating notable differences between these two modes of crack propagation. Digital image correlation is used to plot strain profiles around the tip of the crack, from which a strain-based criterion for crack propagation is established. The evaluation of the structure at the crack tip and regions undergoing more uniform states of deformation is conducted by ex situ transmission electron microscopy and in situ environmental scanning electron microscopy.

In bird flight, the majority of the wing surface consists of highly refined and hierarchically organized β-keratinous feathers. Thus, flight feathers contain ingenious combinations of components that optimize lift, stiffness, aerodynamics, and damage resistance. Their design involves two main parts: a central shaft which prescribes stiffness and lateral vanes that allow for the capture of air. Within the feather vane, barbs branch from the shaft and barbules branch from barbs, forming a flat surface and ensuring lift. Microhooks at the end of barbules hold barbs tightly together, providing a close-knit, unified structure and enabling repair of the vane through the reattachment of un-hooked junctions.

In this dissertation, unique aspects of feather architecture are explored to uncover principles translatable to the design of modern aerospace materials and structures. Specifically, understudied aspects of the feather’s lightweight yet resilient properties are investigated. This research has revealed several novel characteristics of the feather. Allometric scaling relationships are developed linking the geometry of a bird’s wing components to its flight characteristics and total mass. Barbule spacing within the feather vane is found to be 8-16 µm for birds ranging from 0.02-11 kg. Additionally, it is discovered that strength is recovered with the shape recovery property of feathers, and a mechanism for this phenomenon is proposed. Barbule adhesion within the vane is found to prevent barbs from twisting in flexure, maintaining the vane’s stiffness, and the extent to which unzipping these connections affects the feather’s ability to capture air is related to barb shape. Directional permeability of the feather vane is experimentally confirmed and related to the intricate microstructure of barbules. Lastly, the exceptional architecture of the feather motivated the design of novel bioinspired structures with tailored and unique properties. The avian feather serves as an excellent springboard for designs that can be adapted to enhance synthetic materials and structures.

High entropy alloys are newly discovered class of metal alloys. It gains many researchers’ attention due to their good and exceptional mechanical properties. This work aims to examine CoCrFeMnNi mechanical response at different strain rates and temperatures. In addition, to studying the adiabatic shear band evolution and the mechanism of grains refinement inside the shear band. Microhardness were also measured inside and outside the shear band. The microstructure was examined by optical microscopy, SEM, EBSD and TEM.

CoCrFeMnNi improve its strength with lowering temperature and increasing the strain rate. In addition, adiabatic shear bands were clearly overserved in CoCrFeMnNi hat shape specimens at 153K and 300K. The grains inside the shear band were extremely refined to equiaxed nanograins at 153K while grains were refined to ultrafine equiaxed grains at the test conducted at 300K.

There were dual refinement mechanisms; the deformation twinning and rotational dynamic recrystallization. The initial average grain size was 10 µm while the refined equiaxed nanograins were between 0.05-0.2 µm for 153K test while between 0.25-0.35 µm for 300K test. TEM images shows evidence that the difference in the refinement factor for these tests is due to the increase of twinning density at lower temperature for specimen tested at 153K. CoCrFeMnNi show excellent microharndess inside the shear band and superior property compared with 316L austenitic stainless steel.

Adiabatic shear band was also observed in finite element model. The numerical model shows good approximation with the actual results and also used to validate the developed Johnson Cook plasticity model.

Mother nature is an ingenious master on developing efficient structural and functional materials, which manifest various fascinating properties that are superior to synthetic materials. Here, we systematically investigate two topics: the materials design for piscine defense and hydro-actuated reversible deformation of plant organs. In each topic, we selected several types of biological materials to study and unravel the connection between their intelligent hierarchical structure and superb mechanical properties.

We selected three fish and fully investigated the structure and mechanical properties of their scales. The first is the legendary lobe-finned fish coelacanth, which is extant for 400 million years. This defense is provided by primitive elasmoid scales having a double-twisted Bouligand structure of collagenous lamellae. The collagen fibrils in coelacanth scale form bundles which are embedded in a matrix composed of fibers arranged perpendicular to the layered structure, providing added rigidity and resistance to deformation. The second one we studied is a more evolved scale from common carp. Carp has typical modern elasmoid fish scales which are commonly found on most current teleosts. Like coelacanth scale, the outer surface of carp scale is composed of mineralized layers and the inner core is comprised of collagenous lamellae. The lamella orientations in carp scale follows a single twisted Bouligand pattern, with a rotation angle of 36°. Moreover, perpendicular to the lamellae, thinner collagen fibrils, which are called threading fibrils here, form a “sheet-like” structure oriented from the basal part to the external layer. Using in situ synchrotron small-angle x-ray scattering during uniaxial tensile testing, the deformation mechanisms of the collagen in these two scales are identified in terms of fibril stretching, reorientation, sliding, bending and delamination. The third scale we studied is from one of the largest freshwater fish, Arapaima, which has successfully survived in piranha-infested seasonal lakes of the Amazon. To quantitatively investigate the scale’s fracture toughness, we developed a new fixture and measured the J-integral based fracture toughness of the scale and find that the crack-growth toughness as high as ~200 kJm-2, which is one of the toughest flexible biological materials. This toughness is primarily the result of multiple mechanisms which are identified by in situ SEM observation. Our results may bring some critical thinking for developing novel armor materials.

Instead of evolving body armor to protect themselves passively, some fish develop powerful weapons to defend actively. We fully investigated the thorny catfish (Doradidae; order Siluriforme), which has barbed pectoral fin spines and mid-lateral scutes that work in concert to provide an active mechanical defense capability. The structural design of these two weapons is very impressive, including a hollow structure, porous components, and gradient transitions, leading to an outstanding performance by maintaining strength, toughness and light weight synergistically. These designs can provide inspiration for developing new structural materials.

The second topic we studied is the hydro-actuated reversible deformation in plant organs. Plants have developed many intelligent strategies to respond to external stimuli, significantly benefiting their survival. Here we studied a classic: pine cone, which opens to release the pods upon drying and can reclose upon rehydrating. We unraveled a novel mechanism of reversible deformation actuated by hydration. Our findings provide an interdisciplinary perspective combining materials science, structural engineering and biology, as well as offering some design models for development of novel smart responsive materials which have improved mechanical properties and biocompatibility yet simpler strategy.

Pulsed lasers with a power of the order of terawatts, once deposited on a target surface, will launch a stress pulse that propagates into material. Owing to the ultrashort duration of the laser pulses, unprecedented experimental conditions which combine high pressures (and/or shear stresses), strain rates and temperatures can be generated in materials, yielding a yet unexplored regime of study: materials science at extremes.

High-power, short-duration, laser-driven, shock compression and recovery experiments were carried out on four covalently bonded materials, namely, silicon (Si), germanium (Ge), boron carbide (B4C) and silicon carbide (SiC). These materials were chosen because of their high Peierls-Nabarro stress and negative Clapeyron slope. The profile of the shock waves was measured by a velocity interferometer system for any reflectors (VISAR). The shock deformation microstructure has been revealed by high resolution transmission electron microscopy and all the materials exhibit shock-induced amorphization. For Si and Ge with [001] orientation, two distinct amorphous regions were identified: (i) a bulk amorphous layer close to the surface and (ii) amorphous bands initially aligned with {111} slip planes. The VISAR measurements show that the estimated thresholds for such a crystalline-to-amorphous transition is estimated to be ~10 GPa (for silicon) and ~4 GPa (for germanium). Further increase of the shock stress leads to the crystallization of amorphous domain into nanocrystals with high density of nano-twins. For polycrystalline boron carbide, only amorphous bands inclined to the direction of shock wave propagation have been observed at a shock stress above ~45 GPa. At lower shock stress, planar faults have been seen below the shocked surface. For [0001] oriented monocrystalline silicon carbide, in addition to the amorphous bands inclined to the shock direction, some amorphous bands perpendicular to the direction of shock wave propagation were observed.

We propose that the amorphization is produced by the combined effect of high magnitude hydrostatic and shear stresses under dynamic shock compression. This study reveals that amorphization is a general inelastic deformation mechanisms in covalently bonded elements and compounds subjected to shock compression. Their formation yields a decrease in the overall hydrostatic and deviatoric elastic energy. Shock-induced defects play a very important role in the onset of amorphization. Calculations of the free energy changes with pressure and shear, using the Patel-Cohen methodology, agree with the experimental results. Molecular dynamics simulation corroborates the amorphization, showing that it is initiated by the nucleation and propagation of partial dislocations. The nucleation of amorphization is analyzed by classical nucleation theory.

Owing to superior mechanical properties such as high fatigue life and high yield strength of many non-equiatomic high entropy alloys (HEA), a novel division of non-equiatomic FeNiCoAlTaB (NCATB) HEAs has been introduced. The novel NCTAB HEAs exhibit a very high yield stress which is around 1.1~1.2 GPa with appreciable ductility under quasi-static compression. The strengthening mechanisms in NCATB-HEA system are strongly dependent on the grain precipitates, solid solution and martensite transformation under dynamic loading. Martensitic transformation can commonly be attributed to athermal induction and deformation induction. In respect of athermal induced martensitic transformation, temperature of bulk materials needs to be lowered below Ms temperature rapidly. Whereas, martensitic transformation can commence above Ms temperature. The chemical driving force for phase transition can be balanced through drastic cooling or external loading. Different shape of martensite (lath, butterfly, lenticular, thin plate) can be formed depending on the temperature while the lath-like martensite tends to appear at highest forming temperature and thin plate tends to appear at lowest forming temperature. Due to its superior mechanical properties including higher strengthen and strain hardening rate and high endurance limit, this novel HEA system is considered to be conducted into wider application including high damping materials and aviation application. The strain rate effect on martensitic transformation is also studied in current research. Microband is a very common phenomenon in high stacking fault energy material, whereas its formation mechanism remains unclear. Microband formation and its effect on martensitic transformation are also dedicated to study in this research.

Keratinous materials are omnipresent, encompassing terrestrial, aerial and aquatic territories; they form diverse epidermal appendages and serve various interesting functions, triggering the curiosity of humans and inspiring the inventions of novel structures. Among these, pangolin scales and the feather shaft are systematically and analytically studied, answering the questions how the scales function as an armor and the shaft fulfills flight, which contribute to advancing the current knowledge of natural keratinous materials and providing valuable insights in developing new bioinspired designs.

The pangolin is the only known mammal that has keratinous scales covering the main body. Arboreal and ground pangolin scales show an overlapping pattern in a hexagonal arrangement to provide multi-layered coverage. Scales show three regions in the solid interior that features flattened cells forming crossed lamellae, rarely seen in most keratinous materials. Two dimensional x-ray diffraction reveals the presence of α-helices and possible β-sheet, and the microfibrils are crossed in a range of directions. Scales show an interlocking interface between lamellae, which results from the suture-like cell membrane complex. Scales are transverse isotropic and show a strain-rate dependence, which favor the function in resisting external forces from multi-directions.

The feather shaft, a naturally refined lightweight and stiff flight material, is distinguished in having a changing shape factor: circular at the calamus but square towards the distal rachis; this produces a tailored flexural stiffness along the shaft length fulfilling the local stress requirements. The cortex has a complex fibrous hierarchical structure, both making the shaft longitudinally strong, dorsal-ventrally stiff and torsionally rigid, yet capable of desirable deflection and twisting. Filling the cortex with foamy medulla introduces a synergistic strengthening and toughening mechanisms. Flexure of the shaft reveals decreasing flexural properties towards the distal end; nevertheless, the density-normalized flexural stiffness almost remains the same, and the specific flexural strength increases by 48% at the distal shaft. Knowledge from the structure design of the feather shaft have potentials in developing aircraft materials and biomedical scaffolds.

The role of microstructure on material properties is a foundational principle in materials science. The processing of a material will change its microstructure which, in turn, will change its properties. Understanding the strength of condensed matter at ultrahigh pressures and strain rates is challenging. The material’s behavior changes unexpectedly and, in some conditions, may be surprisingly insensitive to the initial microstructure, which is contrary to the results of conventional static and quasi-static experiments. Therefore, experiments are needed in these extreme regimes to better understand and model material properties. Iron is a largely used material in many applications but it is also relevant to geophysical phenomena. Accurately quantifying iron strength would aid in geophysical modeling of planet impact and formation, the understanding of more complex and relevant structural iron alloys, and fundamental material modeling at high strain rates. As a material, it is especially interesting in its pure form because it goes through phase transitions that change its mechanical properties. So, by understanding a relatively simple material like iron, one can begin to gain insight into more complex phase-changing materials that are more universally used.Quasi-static and dynamic compression of iron was performed to determine microstructural effects on iron strength at various, increasing strain rates. Quasi-static compression strength experiments obey the well-known Hall-Petch relationship in which strength increases with decreasing grain size. Contrarily, the spall strength under dynamic compression increases with increasing grain size. This is due to spallation being a failure process that is dominated by defects. It has also been found that the timescale of the reverse phase transformation is only 8 ns and completes before spall. Preliminary results of iron strength under the highest pressures achievable suggest that it is higher than models are predicting and insensitive to initial microstructure. The high-pressure phase transformation in iron is thought to be a crucial factor in its high-pressure behavior. A measured reduction in grain size is thought to contribute to this microstructural insensitivity during compression. Hydrodynamic simulations are also used to design and predict results for these various high-pressure laser-driven experiments.