Kristin Schaaf

Dynamic Mechanical Analysis of Fly Ash Filled Polyurea Elastomer

In this work, the material properties of a series of fly ash/polyurea composites were studied. Dynamic mechanical analysis was conducted to study the effect of the fly ash volume fraction on the composites mechanical properties, i.e., on the materials frequency- and temperature-dependent storage and loss moduli. It was found that the storage and loss moduli of the composite both increase as the fly ash volume fraction is increased. The storage and loss moduli of the composites relative to those of pure polyurea initially increase significantly with temperature and then slightly decrease or stay flat, attaining peak values around the glass transition region. The glass transition temperature (measured as the temperature at the maximum value of the loss modulus) shifted toward higher temperatures as the fly ash volume fraction increased. Additionally, we present the storage and loss moduli master curves for these materials obtained through application of the time-temperature superposition on measurements taken at a series of temperatures.

Mechanical properties of composite materials with integrated embedded sensor networks

We present efforts to develop structural composite materials which include networks of embedded sensors with decision-making capabilities that extend the functionality of the composite materials to be information-aware. The next generation of structural systems will include the capability to acquire, process, and if necessary respond to structural or other types of information. We present work related to the development of embedded arrays of miniature electronic-based microsensors within a structural composite materials, such as GFRP. Although the scale and power consumption of such devices continues to decrease while increasing the functionality, the size of these devices remain large relative the typical scale of the reinforcing fibers and the interlayer spacing. Therefore, the question of the impact of those devices on the various mechanical properties is relevant and important. We present work on characterizing some of those effects in specific systems where sensors, or suitable dummy sensors, are arrayed with ~1 cm spacing between elements. The typical size of the microelectronic sensing element is ~1 mm, and here is orthorhombic. Of particular importance are the effects of inclusion of such devices on strength or fatigue properties of the base composite. Our work seeks to characterize these effects for 1 and 2 dimensional arrays lying in planes normal to the thickness direction in laminated composites. We also seek to isolate the effects due to the sensing elements and the required interconnections that represent the power-carrying and data communications capabilities of the embedded network.

Properties of Elastomer-based Particulate Composites

In this work, an attempt has been made to develop fly ash filled polyurea matrix composites with low density and good dynamic mechanical behavior. Fly ash (105μm –149μm in diameter) was introduced into polyurea, and its volume fraction was varied to study its effects on the overall properties of the composites. Scanning electron microscopy was used to observe the morphology of the composites. The storage and loss moduli of the composites were determined using dynamic mechanical analysis (DMA) from -80 to 70°C at low frequencies and using ultrasonic measurements at high frequencies under ambient conditions. Results showed that fly ash particles were distributed homogeneously in the polyurea matrix, and the density of the composites decreased as the volume fraction of fly ash increased. Compared to neat polyurea, increases in storage and loss moduli at high temperature were achieved by increasing fly ash content. The peak in the ratio of the moduli of the composites system over that of neat polyurea occurred near glass transition temperature Tg. The speed of sound in the composites increased with increasing fly ash content. Longitudinal modulus and acoustic impedance had similar trends.

Optimization of the mechanical properties of composite materials with integrated embedded sensor networks

The increasing demand for in-service structural health monitoring has stimulated efforts to integrate self and environmental sensing capabilities into materials and structures. To sense damage within composite materials, we are developing a compact network microsensor array to be integrated into the material. These structurally-integrated embedded microsensors render the composite information-based, so that it can monitor and report on the local structural environment, on request or in real-time as necessary. Here we present efforts to characterize the structural effects of embedding these sensors. Quasi-static three-point bending (short beam shear) and fatigue three-point bending (short beam shear) tests are conducted in order to characterize the effects of introducing sensors, or suitable dummy sensors in the form of chip resistors, and commonly used circuit board material, namely G-10/FR4 Garolite on the various mechanical properties of the host structural composite material. Furthermore, various methods and geometries of embedding the microsensors are examined in order to determine the technique that optimizes the mechanical properties of the host composite material. The work described here is part of an ongoing effort to understand the structural effects of integrating microsensor networks into a host composite material.

Effect of particle size and volume fraction on tensile properties of fly ash/polyurea composites

Fly ash, which consists of hollow particles with porous shells, was introduced into polyurea elastomer. A one-step method was chosen to fabricate pure polyurea and the polyurea matrix for the composites based on Isonate® 2143L (diisocyanate) and Versalink® P-1000 (diamine). Scanning electron microscopy was used to observe the fracture surfaces of the composites. Particle size and volume fraction were varied to study their effects on the tensile properties of the composites. The tensile properties of the pure polyurea and fly ash/polyurea (FA/PU) composites were tested using an Instron load frame with a 1 kN Interface model 1500ASK-200 load cell. Results showed that fly ash particles were distributed homogeneously in the polyurea matrix, and all of the composites displayed rubber-like tensile behavior similar to that of pure polyurea. The tensile strength of the composites was influenced by both the fly ash size and the volume fraction. Compared to the largest particle size or the highest volume fraction, an increase in tensile strength was achieved by reducing particle size and/or volume fraction. The strain at break of the composites also increased by using fine particles. In addition, the composites filled with 20% fly ash became softer. These samples showed lower plateau strength and larger strain at break than the other composites.

Optimization studies of self-sensing composites

The demand for real-time or in situ structural health monitoring has stimulated efforts to integrate self and environmental sensing capabilities into structural composite materials. Essential to the application of smart composites is the issue of the mechanical coupling of the sensor to the host material. In this study various methods of embedding sensors within the host composite material are examined. Quasi-static three-point bending (short beam) and fatigue three-point bending (short beam) tests are conducted in order to characterize the effects of introducing the sensors or suitable simulated sensors. The sensors that are examined include simulated sensors in the form of chip resistors with the original packaging geometry and thin film sensors (PVDF). The sensors are integrated into the composite either by placement between the layers of prepreg or by placement within precision punched cut-outs of the prepreg material. Thus, through these tests we determine the technique that optimizes the mechanical properties of the host composite material.

Blast Resistant Elastomeric Polymer-by-Design

The essence of this research is to mitigate shock through material design. Here we seek to develop a thorough understanding of the material through experimental characterization methods that lend themselves to creating verifiable constitutive relations, all while working towards the development of a new blast resistant elastomeric composite material. The host elastomer, polyurea, is created by reacting Versalink P-1000 with Isonate 143L. This study evaluates the impact of both chemistry modifications and the integration of micro-scale additives on the polyurea material system properties and performance. The properties of the resultant elastomers and elastomeric composite materials are mechanically and thermally characterized using durometer testing, dynamic mechanical analysis (DMA) testing, and differential scanning calorimetry (DSC) testing in order to determine the hardness, storage and loss moduli, and glass transition temperature of the composites, respectively. Preliminary results indicate that the durometer and dynamic mechanical properties of the material can be significantly altered through such modifications. The work described here is part of an ongoing effort to develop and verify rules and tools for creating elastomer-based composite materials with optimally designed compositions and characteristics.Copyright

Experimental Investigation of Dynamic Mechanical Properties of Polyurea-Fly Ash Composites

Polyurea has been the material of choice in many applications due to its thermomechanical properties. These applications span a wide spectrum from abrasion-resistant coating to reinforcement against blast damage in structures, ships, and vehicles. The improved observed performance is linked to its microstructure, a lightly crosslinked (elastomeric) block copolymer. The constitutive modeling of such materials is generally based on a wide variety of experimental measurements, including dynamic mechanical analysis. Furthermore, the response under stress-wave propagation may be measured through ultrasonic tests. In this work we present our research towards improvement and modification of the dynamic mechanical properties of polyurea through inclusion of various size and distributions of fly ash hollow spherical particles. The extensive experimental results are reported and a micromechanical homogenization model is presented. The extent of application of such model will be established and alternative modeling techniques which include the possible inertia effects at high rates of deformation will be surveyed.

Characterization of Elastomeric Composite Materials for Blast Mitigation

In this work, we seek to develop elastomeric composite materials capable of shock mitigation through material design by small-scale heterogeneity. The host elastomeric material is a polyurea system that is a lightly cross-linked two-phase polymer, which consists of the diamine component Versalink P-1000 as the soft segment and the diisocyanate component Isonate 143L as the hard segment. This study evaluates the impact of additives in the form of untreated and surface treated milled glass fibers. The properties of the resultant elastomeric composite materials are mechanically and thermally characterized using durometer testing, dynamic mechanical analysis (DMA) testing, and differential scanning calorimetry (DSC) testing in order to determine the hardness, storage and loss moduli, and glass transition temperature of the composites, respectively. Preliminary results indicate that the dynamic mechanical properties of the material can be significantly altered through such modifications. The work described here is part of an ongoing effort to understand the impact of additives on the ultimate properties and performance of the host elastomeric material.

Composite materials with self-contained wireless sensing networks

The increasing demand for in-service structural health monitoring, particularly in the aircraft industry, has stimulated efforts to integrate self sensing capabilities into materials and structures. This work presents efforts to develop structural composite materials which include networks of sensors with decision-making capabilities that extend the functionality of the composite materials to be information-aware. Composite panels are outfitted with networks of self-contained wireless sensor modules which can detect damage in composite materials via active nondestructive testing techniques. The wireless sensor modules will communicate with one another and with a central processing unit to convey the sensor data while also maintaining robustness and the ability to self-reconfigure in the event that a module fails. Ultimately, this research seeks to create an idealized network that is compact in size, cost efficient, and optimized for low power consumption while providing a sufficient data transfer rate to a local host.