Matthew Daly

Interfacial Shear Strength of Multilayer Graphene Oxide Films

Graphene oxide (GO) is considered as one of the most promising layered materials with tunable physical properties and applicability in many important engineering applications. In this work, the interfacial behavior of multilayer GO films was directly investigated via GO-to-GO friction force microscopy, and the interfacial shear strength (ISS) was measured to be 5.3 ± 3.2 MPa. Based on high resolution atomic force microscopy images and the available chemical data, targeted molecular dynamics simulations were performed to evaluate the influence of functional structure, topological defects, and interlayer registry on the shear response of the GO films. Theoretical values for shear strength ranging from 17 to 132 MPa were predicted for the different structures studied, providing upper bounds for the ISS. Computational results also revealed the atomic origins of the stochastic nature of friction measurements. Specifically, the wide scatter in experimental measurements was attributed to variations in functional structure and topological defects within the sliding volume. The findings of this study provide important insight for understanding the significant differences in strength between monolayer and bulk graphene oxide materials and can be useful for engineering topological structures with tunable mechanical properties.

Fabrication of a novel laser-processed NiTi shape memory microgripper with enhanced thermomechanical functionality

The thermomechanical properties of nickel-titanium shape memory alloys have sparked significant research efforts seeking to exploit their exotic capabilities. Until recently, the performance capabilities of nickel-titanium devices have been inhibited by the retention of only one thermomechanical response. In this article, the application of a novel laser-processing technique is demonstrated to create a monolithic self-positioning nickel-titanium shape memory microgripper. Device actuation and gripping maneuvers were achieved by thermally activating processed material regions which possessed unique phase transformation onset temperatures and thermomechanical recovery characteristics. The existence of each thermomechanical material domain was confirmed through differential scanning calorimetry analysis. Independent thermomechanical recoveries of each embedded shape memory were captured using tensile testing methods. Deployment of each embedded shape memory was achieved using resistive heating, and in situ resistivity measurements were used to monitor progressive phase transformations.

Strengthening in Graphene Oxide Nanosheets: Bridging the Gap between Interplanar and Intraplanar Fracture

Graphene oxide (GO) is a layered material comprised of hierarchical features which possess vastly differing characteristic dimensions. GO nanosheets represent the critical hierarchical structure which bridges the length-scale of monolayer and bulk material architectures. In this study, the strength and fracture behavior of GO nanosheets were examined. Under uniaxial loading, the tensile strength of the nanosheets was measured to be as high as 12 ± 4 GPa, which approaches the intrinsic strength of monolayer GO and is orders of magnitude higher than that of bulk GO materials. During mechanical failure, brittle fracture was observed in a highly localized region through the cross-section of the nanosheets without interlayer pull-out. This transition in the failure behavior from interplanar fracture, common for bulk GO, to intraplanar fracture, which dominates failure in monolayer GO, is responsible for the high strength measured in the nanosheets. Molecular dynamics simulations indicate that the elastic release from the propagation of intraplanar cracks initiates global fracture due to interlayer load transmission through hydrogen bond networks within the gallery space of the GO nanosheets. Furthermore, the GO nanosheet strength and stiffness were found to be strongly correlated to the effective volume and thickness of the samples, respectively. These findings help to bridge the understanding of the mechanical behavior of hierarchical GO materials and will ultimately guide the application of this intermediate scale material.

Dynamic actuation of a novel laser-processed NiTi linear actuator

A novel laser processing technique, capable of locally modifying the shape memory effect, was applied to enhance the functionality of a NiTi linear actuator. By altering local transformation temperatures, an additional memory was imparted into a monolithic NiTi wire to enable dynamic actuation via controlled resistive heating. Characterizations of the actuator load, displacement and cyclic properties were conducted using a custom-built spring-biased test set-up. Monotonic tensile testing was also implemented to characterize the deformation behaviour of the martensite phase. Observed differences in the deformation behaviour of laser-processed material were found to affect the magnitude of the active strain. Furthermore, residual strain during cyclic actuation testing was found to stabilize after 150 cycles while the recoverable strain remained constant. This laser-processed actuator will allow for the realization of new applications and improved control methods for shape memory alloys.

A kinematic study of energy barriers for crack formation in graphene tilt boundaries

Recent experimental studies have observed a surprisingly wide range of strengths in polycrystalline graphene. Previous computational investigations of graphene tilt boundaries have highlighted the role of interfacial topology in determining mechanical properties. However, a rigorous characterization of deformation energy barriers is lacking, which precludes direct comparison to the available experimental data. In the current study, molecular dynamics tensile simulations are performed to quantify kinematic effects on failure initiation in a wide range of graphene tilt boundaries. Specifically, the process of crack formation is investigated to provide a conservative estimate of strength at experimental loading rates. Contrary to previous studies, significant strain rate sensitivity is observed, resulting in reductions of crack formation stresses on the order of 7% to 33%. Energy barriers for crack formation are calculated in the range of 0.58 to 2.07u2009eV based on an Arrhenius relation that is fit to the coll...

Enhanced thermomechanical functionality of a laser processed hybrid NiTi-NiTiCu shape memory alloy

The exciting thermomechanical behavior of NiTi shape memory alloys (SMAs) has sparked significant research effort seeking to exploit their exotic properties. The performance capabilities of conventional NiTi offerings are limited, however, by current fabrication technologies. In this study, a high power density laser source was implemented to locally alloy Cu into a conventional NiTi material. The effects of laser processing created a localized NiTiCu ternary material domain which possessed a set of unique thermomechanical properties. The combined active responses of the laser processed hybrid NiTi‐NiTiCu SMA represent an enhanced material functionality, which permits a multi-stage thermomechanical recovery and allows for unprecedented novel applications to be realized. (Some figures may appear in colour only in the online journal)

Fabrication of a Novel Monolithic NiTi Based Shape Memory Microgripper via Multiple Memory Material Processing

The exciting thermomechanical behavior of nickel-titanium shape memory alloys have sparked significant research efforts seeking to exploit their exotic shape memory properties. The performance capabilities of conventional nickel-titanium alloys are currently limited, however, by the retention of only one shape memory geometry. In this paper we demonstrate the application of an unprecedented manufacturing process known as Multiple Memory Material technology to create a novel monolithic nickel-titanium shape memory microgripper. In our design, actuation and gripping maneuvers are achieved by thermally activating processed material regions which possess unique shape memory transformation temperatures and shape set geometries. The existence of multiple shape memory regimes is confirmed through differential scanning calorimetry analysis and in situ resistivity measurements.Copyright

Dynamic Actuation of a Multiple Memory Material Processed Nitinol Linear Actuator

Shape memory alloys such as Nitinol, which is a group of NiTi alloys composed of nearly equiatomic nickel and titanium, finds increasing applications in many industries because of its unique properties including the shape memory effect and pseudoelasticity. In past work simple linear actuators have been developed using Nitinol wire which are actuated and controlled using resistive heating. However, traditional Nitinol materials are batch processed and a monolithic component only possesses a single set of transformation temperatures, limiting the functionality of the actuator. In this work a linear actuator processed using the novel multiple memory material processing technology is presented showing multiple transformations and dynamic actuation by resistive heating. This dynamically controlled actuation greatly improves the functionality of the Nitinol actuator allowing for the realization of new applications and improved control methods. The different transformation temperatures embedded in the monolithic wire actuator following processing are identified using thermo-analytical analysis and the dynamic application of load and displacement are presented using a custom test set-up.Copyright

ANALYSIS OF WRINKLED MEMBRANES BOUNDED WITH MACRO-FIBER COMPOSITE (MFC) ACTUATORS

In this paper we focus on a study which involves quantifying the effects of Macro Fiber Composite (MFC) actuators on the pattern and magnitude of wrinkles in a membrane when exposed to various loadings. An ABAQUS finite element code is employed for this research. The membrane in this study has a rectangular shape which is clamped at one edge and is free to move in the horizontal direction at the other edge. MFC actuators are bounded to the membrane to make a bimorph configuration.Copyright

Lamellae spatial distribution modulates fracture behavior and toughness of african pangolin scales

Pangolin scales form a durable armor whose hierarchical structure offers an avenue towards high performance bio-inspired materials design. In this study, the fracture resistance of African pangolin scales is examined using single edge crack three-point bend fracture testing in order to understand toughening mechanisms arising from the structures of natural mammalian armors. In these mechanical tests, the influence of material orientation and hydration level are examined. The fracture experiments reveal an exceptional fracture resistance due to crack deflection induced by the internal spatial orientation of lamellae. An order of magnitude increase in the measured fracture resistance due to scale hydration, reaching up to ~ 25kJ/m2 was measured. Post-mortem analysis of the fracture samples was performed using a combination of optical and electron microscopy, and X-ray computerized tomography. Interestingly, the crack profile morphologies are observed to follow paths outlined by the keratinous lamellae structure of the pangolin scale. Most notably, the inherent structure of pangolin scales offers a pathway for crack deflection and fracture toughening. The results of this study are expected to be useful as design principles for high performance biomimetic applications.