Michael W. Keller

Strong, Low-Density Nanocomposites by Chemical Vapor Deposition and Polymerization of Cyanoacrylates on Aminated Silica Aerogels

Strong polymer-silica aerogel composites were prepared by chemical vapor deposition of cyanoacrylate monomers onto amine-modified aerogels. Amine-modified silica aerogels were prepared by copolymerizing small amounts of (aminopropyl)triethoxysilane with tetraethoxysilane. After silation of the aminated gels with hexamethyldisilazane, they were dried as aerogels using supercritical carbon dioxide processing. The resulting aerogels had only the amine groups as initiators for the cyanoacrylate polymerizations, resulting in cyanoacrylate macromolecules that were higher in molecular weight than those observed with unmodified silica and that were covalently attached to the silica surface. Starting with aminated silica aerogels that were 0.075 g/cm(3) density, composite aerogels were made with densities up to 0.220 g/cm(3) and up to 31 times stronger (flexural strength) than the precursor aerogel and about 2.3 times stronger than an unmodified silica aerogel of the same density.

Self-healing flexible laminates for resealing of puncture damage

A flexible self-healing system capable of healing puncture damage has been manufactured. Our material consists of three layers: a poly(dimethyl siloxane) (PDMS) composite, embedded with a self-healing microcapsule system, sandwiched between two layers of poly(urethane) coated nylon. The total structure thickness ranges between 0.84 and 1.5 mm. A protocol is established in which samples are damaged using a hypodermic needle or a razor blade, and a successful heal is defined as the ability to reseal the damage to withstand a pressure differential across the laminate of 103 kPa (∼1 atm). Trends in healing success are analyzed as a function of microcapsule size, self-healing layer thickness, and puncture diameter. Healing varied significantly with microcapsule size, with the maximum healing success rate (100% successfully healed) occurring in samples with 220 µm microcapsules and a puncture diameter of 0.49 mm. For this puncture size, an increase in microcapsule diameter corresponds to a decrease in healing efficiency. However, samples with larger microcapsules (up to 500 µm avg.) demonstrate more effective healing for larger puncture diameters, up to 1.61 mm. Additionally, healing increased with composite layer thickness, and decreased with increasing puncture hole size. (Some figures in this article are in colour only in the electronic version)

Targeted Self-Healing by Magnetically Guiding Microcapsules

Magnetically guided microcapsules are used to achieve self-healing with 1/10th of the healing components required using traditional self-healing approaches. Microcapsules are rendered responsive to magnetic fields by suspending magnetic nanoparticles in the core material. The nanoparticles are surface-modified to enable urea-formaldehyde encapsulation within a phenyl acetate core. Magnetic fields are used to guide the microcapsules to the expected fracture location in tapered double-cantilever beam (TDCB) epoxy specimens. This guiding method achieves an order of magnitude increase in local microcapsule concentration over controls, resulting in successful self-healing at microcapsule concentrations as low as 0.025 wt %. Additionally, the observed healing is both more consistent and significantly higher than that of control specimens, remaining relatively constant across all weight percentages tested.

Experimental Fatigue Specimen and Finite Element Analysis for Characterization of Dental Composites

Dental composites are becoming more popular due to their semi white color and appearance. Mechanical damage such as cracks are causing the majority of short-term failures of dental composites. Using self-healing materials most of these failures can be prevented. Fatigue loads are a proper method to characterize the crack initiation and propagation. As healing makes uncertainty about the location of the crack tip, samples of tapered double cantilever beam (TDCB) are frequently used for their crack length independent in the measurements of healing efficiency and fracture toughness of self-healing. Due to the high cost of dental composite materials, tiny, inexpensive TDCB samples, about 30 % of the standard size, were developed and optimized with Rapid Prototyping (Objet 3D printer). FEA is also performed in order to visualize the stress field of the crack tip.

Coupon specimen–based approach for the simulation of crossbore stress and strain state

Direct investigation of the stress and strain state of structures with crossbore geometries is typically complex to achieve. Crossbores are often located at the center of reciprocating machinery with few options for access. Thus, the number of studies that have directly characterized these structures is limited and continues to be an experimental challenge. In order to provide an alternative approach for the investigation of material performance in these structures, a coupon specimen is presented as an alternative to direct measurement. A notched tensile specimen geometry is adopted in this study and is designed to match the stress gradient in a representative crossbore intersection. An approach for specimen design is presented and the design is validated using digital image correlation to measure the full-field stress gradient. The proposed design is shown to match the stress gradient to within 10% for the first 50% of the specimen. In addition to the validation of the gradient, an initial set of fatigue experiments is also conducted in order to compare to the modified Smith–Watson–Topper fatigue life approach. Finally, a simulated autofrettage cycle is completed and the resulting residual stress is analyzed by digital image correlation.

Compact Fracture Specimen for Characterization of Dental Composites

Dental composites are becoming increasingly popular due to their tooth-like color and appearance. Most short-term failures of dental composites are due to mechanical damage, such as cracking. Therefore, many of these failures might be prevented through the use of a self-healing dental composite. High-quality characterization is critical in the development of self-healing materials. Since healing creates uncertainty about the location of the crack-tip, tapered double cantilever beam (TDCB) specimens are often used for their crack length independence when measuring the fracture toughness and healing efficiency of self-healing materials. Because of the high cost of dental composite materials, small cost-effective TDCB samples, about one third of the standard size, were designed and optimized using rapid prototyping (Objet 3D Printer).

Notch Strain Analysis of Crossbore Geometry

Many high-pressure components have intersecting bore geometries, such as fluid end module of fracture pumps. Imposition of compressive residual stresses at crossbore intersections can extend the fatigue life, thus approaches such as autofrettage are typically used. Understanding the stress–strain response during autofrettage in the crossbore is critical for fatigue life estimation and design. Crossbore geometry is frequently complex and no closed-form analytical solution is available for prediction of residual stresses. As such, numerical methods like FEA are frequently used. Applying FEA to complicated geometries requires extensive parametric studies which are computationally expensive and time consuming, thus notch strain analysis methods are promising. Elastoplastic stress–strain responses due to varying internal pressures in a crossbore geometry were evaluated using FEA and notch strain analysis formulas including both Neuber and Glinka approaches. To define the theoretical elastic stress concentration factor based on ratios between maximum Mises or hoop stress and pressure or nominal stress four different values were calculated and imported into the notch strain analysis formulas. It was observed that the results of Glinka approach match better to the FEA results particularly by applying the ratio between maximum Mises stress and nominal stress as elastic stress concentration factor.

Fatigue Behavior of Fluid End Crossbore Using a Coupon-Based Approach

Fracture or mud pumps are known as the heart of the drilling and hydraulic fracturing. Crossbore geometries are central to the design of fluid end module in these positive displacement reciprocating pumps. Intersection between bores emerges as a stress concentrator and because the fluctuating pressure history is extreme, fatigue limits the useful life of the pump. Approaches such as autofrettage are typically used to extend fatigue lives through the imposition of compressive residual stresses at crossbore intersections. Direct investigation of the impact of residual stresses in working pumps is not typically possible. In order to improve understanding of the impact of residual stresses on fatigue life and to optimize the fatigue-strength improvement provided to fluid ends, unique sample geometry was designed to simulate the stresses in the crossbore. These samples are tested on laboratory-based servohydraulic fatigue frames and eliminate the need for complicated in-situ stress analysis on the fluid ends. Using notch strain analysis and modified Smith–Watson–Topper approach a life prediction algorithm was also developed to calculate the fatigue life of the coupon. To optimize the autofrettage load and cyclic loading simulation, elastoplastic FEA was accomplished utilizing a combined nonlinear isotropic/kinematic hardening material model for 4300-series alloy steel.

6.15 Self-Healing Composite Materials

This chapter discusses the current status of self-healing composite materials. Self-healing materials are capable of responding to mechanical damage by initiating a self-repair mechanism, similar to a biological entity. These materials are classified into two broad categories, both of which are reviewed. Extrinsic self-healing materials are synthesized through the addition of a second or third material phase that provides the self-healing capability. Intrinsic self-healing materials have the ability to self-repair built into the material at the molecular level. Both material types can also be further classified into those that require no intervention to complete healing and those that require additional energetic input to complete healing. The mechanisms and performance of these materials are critically discussed in the context of long term success.

Fatigue Characterization of In-Situ Self-Healing Dental Composites

The short-term failures of dental composites are a common limitation of these materials. Oral conditions apply fatigue load cycles in the form of chewing and thermal loads, and the damage due to fatigue loads plays a major role in restorative failures in the form of cracks. A lab-based fatigue test is a suitable technique to characterize the crack propagation in dental composites. In this paper, we investigate the ability of a self-healing material to repair or arrest propagating fatigue cracks. In-situ self-healing resin composites were prepared using 15 wt% of activator resin microcapsules and 5 wt% of initiator microcapsules, and equal amounts of 40 wt% each of dental filler and dental resin. Compact Tapered Double Cantilever Beam (cTDCB) specimens of self-healing dental resin composites were prepared by integrating two sets of microcapsules of diameter 45 ± 10 μm containing an acrylate monomer and a polymerization initiator (BPO). Samples were tested in a servo-hydraulic load frame in air at room temperature. Three specimen types are investigated, dental composite without microcapsules, dental composite with non-healing microcapsules, and dental composite with in-situ self-healing microcapsules. The results show that the in-situ self-healing dental composite successfully extended the life of the composite compared to the control samples.