Brett A. Williams

Mechanical properties and structure of the biological multilayered material system, Atractosteus spatula scales.

During recent decades, research on biological systems such as abalone shell and fish armor has revealed that these biological systems employ carefully arranged hierarchical multilayered structures to achieve properties of high strength, high ductility and light weight. Knowledge of such structures may enable pathways to design bio-inspired materials for various applications. This study was conducted to investigate the spatial distribution of structure, chemical composition and mechanical properties in mineralized fish scales of the species Atractosteus spatula. Microindentation tests were conducted, and cracking patterns and damage sites in the scales were examined to investigate the underlying protective mechanisms of fish scales under impact and penetration loads. A difference in nanomechanical properties was observed, with a thinner, stiffer and harder outer layer (indentation modulus ∼69 GPa and hardness ∼3.3 GPa) on a more compliant and thicker inner layer (indentation modulus ∼14.3 GPa and hardness ∼0.5 GPa). High-resolution scanning electron microscopy imaging of a fracture surface revealed that the outer layer contained oriented nanorods embedded in a matrix, and that the nanostructure of the inner layer contained fiber-like structures organized in a complex layered pattern. Damage patterns formed during microindentation show complex deformation mechanisms. Images of cracks identify growth through the outer layer, then deflection along the interface before growing and arresting in the inner layer. High-magnification images of the crack tip in the inner layer show void-linking and fiber-bridging exhibiting inelastic behavior. The observed difference in mechanical properties and unique nanostructures of different layers may have contributed to the resistance of fish scales to failure by impact and penetration loading.

Characterization of multi-layered fish scales (Atractosteus spatula) using nanoindentation, X-ray CT, FTIR, and SEM.

The hierarchical architecture of protective biological materials such as mineralized fish scales, gastropod shells, ram’s horn, antlers, and turtle shells provides unique design principles with potentials for guiding the design of protective materials and systems in the future. Understanding the structure-property relationships for these material systems at the microscale and nanoscale where failure initiates is essential. Currently, experimental techniques such as nanoindentation, X-ray CT, and SEM provide researchers with a way to correlate the mechanical behavior with hierarchical microstructures of these material systems1-6. However, a well-defined standard procedure for specimen preparation of mineralized biomaterials is not currently available. In this study, the methods for probing spatially correlated chemical, structural, and mechanical properties of the multilayered scale of A. spatula using nanoindentation, FTIR, SEM, with energy-dispersive X-ray (EDX) microanalysis, and X-ray CT are presented.

Multi-Scale Mechanical Performance of High Strength-High Ductility Concrete

A new fiber-reinforced cement-based composite, called High Strength-High Ductility Concrete (HSHDC) with unparalleled combination of compressive strength (>150 MPa) and tensile ductility (>3 %), has been recently developed. Due to such unique combination of properties, the specific energies of HSHDC under tension and compression at both pseudo-static and high strain rates are extremely high. The design of this engineered material is based on the fundamental principles of micromechanics which focus on the synchronous functioning of the fiber, the cementitious matrix, and their interface to achieve the desired material properties for a given structural application. For such micromechanics-based design to succeed, the material has been researched at several length scales ranging from micro-scale fiber/matrix interactions to structural-scale impact resistance of HSHDC slabs. This paper summarizes the mechanical properties of HSHDC at various length scales to facilitate further development of this material and explore its potential for use in enhancing structural impact and blast resistance.

Mechanical Response and Damage Evolution of High-Strength Concrete Under Triaxial Loading

Current weapons effects modeling efforts rely heavily on quasi-static triaxial data sets. However, there are fundamental knowledge gaps in the current continuum modeling approach due to limited experimental data in the areas of dynamic effects and damage evolution. Arbitrary scalar values used for damage parameters have experimentally unverified mathematical forms that often do not scale to different geometries, stress states, or strain rates. Although some preliminary tests have been performed through dynamic triaxial compression experiments, the results are difficult to interpret due to changes in specimen diameter and length-to-diameter ratio. In this study, a high-strength concrete (f’c ∼130 MPa) was investigated under triaxial loading conditions at confining pressures up to 300 MPa. Three cylindrical specimen sizes were used to determine size effects, including 50 × 114 mm, 25 × 50 mm, and 25 × 13 mm. For a limited number of specimens, X-Ray Computed Microtomography (XCMT) scans were conducted. It was noted that size and length-to-diameter ratio have substantial effects on the experimental results that must be understood to determine dynamic effects based on specimen geometries used in dynamic triaxial compression experiments. Additionally, by quantifying pore crushing and crack development under a variety of triaxial loading conditions, future multi-scale modeling efforts will be able to incorporate systematically defined damage parameters that are founded on experimental results.

Influence of Distribution Modulus of Particle Size Distribution on Rheological and Mechanical Properties of Ultra-High-Strength SHCC Matrix

Particle packing models, such as the modified Andreasen and Andersen (A&A) method, have been adopted by researchers for determining the target particle size distribution (PSD) with given ingredients in an Ultra-High Performance Concrete (UHPC). The curvature of the target PSD is governed by a parameter known as the distribution modulus (q). It determines the ratio of aggregate/cementitious paste content needed for achieving the densest possible particle packing which likely achieves the greatest compressive strength. In addition to the hardened properties, q influences the rheological properties of a UHPC. While achieving the densest particle packing may be the primary objective in a UHPC design, controlling the plastic viscosity of the fresh matrix for homogenous fiber dispersion and reducing the matrix fracture toughness for improved tensile ductility are important objectives for a Strain-Hardening Cementitious Composite (SHCC) design. The design of an Ultra-High-Strength SHCC (UHS-SHCC) therefore requires achieving both the objectives, simultaneously. There is lack of knowledge about the correlations between the distribution modulus and rheological and mechanical properties of SHCC matrices. This paper attempts to address this knowledge gap. In this experimental research, central composite experimental design for reducing the number of trials, along with modified A&A method for mixture proportioning, are employed to investigate the aforementioned correlations for a UHS-SHCC matrix. Results show that there exists an optimum value of q for UHS-SHCC design with given set of ingredients.