Devendra Verma

An investigation into environment dependent nanomechanical properties of shallow water shrimp (Pandalus platyceros) exoskeleton.

The present investigation focuses on understanding the influence of change from wet to dry environment on nanomechanical properties of shallow water shrimp exoskeleton. Scanning Electron Microscopy (SEM) based measurements suggest that the shrimp exoskeleton has Bouligand structure, a key characteristic of the crustaceans. As expected, wet samples are found to be softer than dry samples. Reduced modulus values of dry samples are found to be 24.90 ± 1.14 GPa as compared to the corresponding values of 3.79 ± 0.69 GPa in the case of wet samples. Hardness values are found to be 0.86 ± 0.06 GPa in the case of dry samples as compared to the corresponding values of 0.17 ± 0.02 GPa in the case of wet samples. In order to simulate the influence of underwater pressure on the exoskeleton strength, constant load creep experiments as a function of wet and dry environments are performed. The switch in deformation mechanism as a function of environment is explained based on the role played by water molecules in assisting interface slip and increased ductility of matrix material in wet environment in comparison to the dry environment.

Influence of interfacial interactions on deformation mechanism and interface viscosity in α-chitin-calcite interfaces.

The interfaces between organic and inorganic phases in natural materials have a significant effect on their mechanical properties. This work presents a quantification of the interface stress as a function of interface chemical changes (water, organic molecules) in chitin-calcite (CHI-CAL) interfaces using classical non-equilibrium molecular dynamics (NEMD) simulations and steered molecular dynamics (SMD) simulations. NEMD is used to investigate interface stress as a function of applied strain based on the virial stress formulation. SMD is used to understand interface separation mechanism and to calculate interfacial shear stress based on a viscoplastic interfacial sliding model. Analyses indicate that interfacial shear stress combined with shear viscosity can result in variations to the mechanical properties of the examined interfacial material systems. It is further verified with Kelvin-Voigt and Maxwell viscoelastic analytical models representing viscous interfaces and outer matrix. Further analyses show that overall mechanical deformation depends on maximization of interface shear strength in such materials. This work establishes lower and upper bounds of interface strength in the interfaces examined.

Structural-Nanomechanical Property Correlation of Shallow Water Shrimp (Pandalus platyceros) Exoskeleton at Elevated Temperature

This investigation reports the nanomechanical properties of shallow water shrimp exoskeleton at temperatures ranging from 30 °C to 80 °C measured using nanoindentation experiments. Scanning Electron Microscopy (SEM) measurements suggest that the shrimp exoskeleton has the Bouligand structure in its layers, a key characteristic of the crustaceans. The thickness of the layers and packing density are found to be different from that of lobsters and crabs reported earlier in the literature. Mechanical properties at high temperatures are determined using micro materials nanoindentation test set up combined with the hot stage. The properties measured during nanoindentation test are corrected for the creep and thermal drift during the experiments. The reduced modulus values are found to be around 28 GPa at 30 °C that reduces to approximately 24 GPa at 80 °C. The hardness values also decrease from 1.6 GPa at 30 °C to around 1.2 GPa at 80 °C. The indentation size effect is found to be absent at all temperatures. Creep mechanisms of polymers like materials and its temperature dependence are discussed to give more insight into the deformation mechanism.

Strain Rate Dependent Failure of Interfaces Examined via Nanoimpact Experiments

One of the main factors contributing to the failure of composites is the failure initiated at the interfaces. Examples include interface failure at interfaces such as those between HTPB-Ammonium Perchlorate (AP) in an example energetic material. One important characteristic that could be used to develop failure theories under dynamic loading in materials with an account of interface properties is constitutive properties of interfaces under dynamic loading. In this work, interface mechanical strength of a set of HTPB-AP interfaces is characterized using dynamic indentation experiments at strain rates up to 100 s−1. Stress maps were measured in the interface areas using Nano Mechanical Raman Spectroscopy (NRS) to analyze the changes in the stress distribution around interfaces. Measurements of dynamic hardness, strain rates, and plastic-residual depths were correlated to show the relation of interface mechanical strength with the bulk phase mechanical strength. A power law viscoplastic constitutive model was fitted to experimental stress-strain-strain rate data in order to obtain constitutive behavior of interfaces, particle, and matrix. Results show that interfacial properties are affected by the rate of loading and are largely dependent upon the interface structural inhomogeneity. Stress maps are obtained near the interface using In-situ Mechanical Raman Spectroscopy to analyze the changes in the stress distribution around interfaces for different loads. A bilinear cohesive zone model parameters were obtained from the consideration of local stress and the cohesive energy required for delamination.

Spectroscopic Experiments: A Review of Raman Spectroscopy of Biological Systems

Raman spectroscopy is fast emerging as an important characterization tool for biological systems. Raman spectroscopy has proven to be a powerful and versatile characterization tool used for determining chemical composition of material systems such as nanoscale semiconductor devices or biological systems. One major advantage of Raman spectroscopy in the case of biological molecules is that water gives very weak, uncomplicated Raman signal. Biological systems are essentially wet systems; hence, Raman spectrum of a biological system can be easily obtained by filtering the water’s Raman signal. Another advantage of Raman spectroscopy in the case of biological molecules is the ability of Raman spectroscopy to analyze in vivo samples. This aspect gives this technique an edge over other methods such as infrared (IR) spectroscopy which requires elaborate signal preparation for excitation and complex instrumentation for signal processing after the excitation. This chapter focuses on presenting information on advancements made regarding the Raman spectroscopy of algae.

Interface Mechanical Strength and Elastic Constants Calculations via Nano Impact and Nanomechanical Raman Spectroscopy

Interfaces are ubiquitous in important natural and manmade materials. Research evidence has shown that interface chemistry, structure, and thickness together strongly influence material microstructure and mechanical properties. The focus of the present work is on presenting an experiment based theoretic advancement to predict thickness dependent elastic properties of materials interfaces by treating the interfaces and the area around them in a material as an elastic continuum. The experiments are based on the nanomechanical Raman spectroscopy (NMRS) developed by authors earlier with a capability to simultaneously measure stress components in orthogonal directions during an in-situ nanomechanical loading. An analytical model is developed based on boundary conditions of interface to predict thickness dependent interface elastic constants. The interface elastic constants are compared with the relations provided in literature.

Scale Dependence of the Mechanical Properties of Interfaces in Crustaceans Thin Films

Crustacean exoskeletons in the form of thin films have been investigated by several researchers in order to understand the role played by the exoskeletal structure in affecting functioning of species such as shrimps, crabs and lobsters. These species exhibit similar design in their exoskeleton microstructure. Bouligand pattern (twisted plywood structure), layers of different thicknesses across cross section, changes in mineral content through the layers etc. are common feature changes. Different parts of crustacean exoskeletons exhibit a significant variation in mechanical properties based on the variation in the above mentioned features. Mechanical properties have been analyzed by authors using imaging techniques such as SEM (Scanning Electron Microscopy), EDX (Energy Dispersive X-ray) and using mechanical characterization based on nanoindentation. Analyses show that the confinement effect arising from interfaces sandwiched in crustacean microstructure layers along with the strain rates of deformation plays a major role in the deformation of such layered systems. A new constitutive model is proposed that couples the effect of strain-rate and confinement to predict interface deformation behavior. The model predictions are validated based on experiments in glass/epoxy interfaces.

A Nanomechanics Based Investigation into Interface Thermomechanics of Collagen and Chitin Based Biomaterials

From the biological/chemical perspective, interface concepts related to cell surface/synthetic biomaterial interface and extracellular matrix/biomolecule interface have wide applications in medical and biological technology. Interfaces control biological reactions, and provide unique organic microenvironments that can enhance specific affinities, as well as self-assembly in the interface plane that can be used to orient and space molecules with precision. Interfaces also play a significant role in determining structural integrity and mechanical creep and strength properties of biomaterials. Structural arrangement of interfaces combined with interfacial interaction between organic and inorganic phases significantly affects the mechanical properties of biological materials, allowing in particular for a unique combination of seemingly “in-consistent” properties, such as fracture strength and tensile strength being both high—as opposed to traditional engineering materials, which have high fracture strength linked to low tensile strength and vice-versa. This work presents a framework to understand this correlation by presenting a quantified information regarding the effect of interfaces on overall mechanical deformation of two widely simulated materials systems based on Collagen-Hydroxyapatite and Chitin-Calcite interfaces. Analyses point out specific role of interface chemistries in the effect the interfaces have on overall structural mechanical properties.

Multiscaling for Molecular Models: Investigating Interface Thermomechanics

One of the most important aspects of understanding the influence of interfaces on natural material properties is the knowledge of how stress transfer occurs across the organic–inorganic interfaces. The multicomponent hierarchical structure of biomaterials results in organic–inorganic interfaces appearing at different length scales, i.e., between the basic components at the nanoscale, between the mineralized fibrils at the microscale, and between the layers of the multilayered structures at micro- or macroscale. For a given peak tensile strength of a given material, which position of total strength is attributed to interface strength? What is the contribution of interface sliding in time-dependent deformation observed in a simple tension test of a given material sample? This chapter focuses on addressing such questions using molecular simulations.

Multiscaling for Molecular Models to Predict Lab Scale Sample Properties: A Review of Phenomenological Models

One of the defining features of biological materials is that they are highly hierarchical with different structures at different length scales. Often they are complex nanocomposites of soft fibrous polymeric phase and hard mineral phase. For instance, bone has up to seven levels of hierarchy and nacre shows up to six levels of hierarchal structure. In spite of complex hierarchical structures, the smallest building blocks in such biological materials are at the nanometer length scale. The extent of interfacial interaction and the interfacial arrangement are important determinants of the structure–function property relationship of biomaterials and influence the mechanical strength substantially. Challenges lie in identifying nature’s mechanisms behind imparting such properties and its pathways in fabricating and optimizing these composites. The key here is the formation of large amount of precisely and carefully designed organic–inorganic interfaces and synergy of mechanisms acting over multiple scales to distribute loads and damage, dissipate energy, and resist change in properties owing to damages such as cracking. This chapter presents a brief overview of the role of interfacial structural design and interfacial forces in imparting superior mechanical performance to hard biological materials. Focus is on understanding the underlying engineering principles of nature’s materials for use in biomedical engineering and biomaterial development.