Non-contact technique for characterizing full-field surface deformation of shape memory polymers at elevated and room temperatures

Abstract Thermally activated shape memory polymers (SMPs) can display modulus changes of approximately three orders of magnitude in transitioning from the high modulus, "glassy" state below the glass transition temperature (Tg) to the low modulus, "rubbery" state above the Tg. In the high temperature region, SMPs can achieve strain levels well above 100%. Their complex behavior includes large modulus changes to as low as ∼1 MPa, extremely high strain levels, and path dependent properties, thus precluding the use of traditional strain gages and low-contact force extensometers. The present study presents a comparison of thermomechanical testing techniques developed to characterize the material behavior of SMPs. Specifically, the performance of strain measurements using contact methods (clip-on extensometers and adhesive strain gages) are compared to non-contact methods (laser extensometer and digital image correlation). An MTS environmental chamber with an observation window allows for non-contact optical measurements during testing. A series of tensile tests are performed on a commercial SMP (with a Tg of ∼105 °C) at 25 °C and at 130 °C. It is observed that the clip-on extensometer significantly affects the SMP behavior even in the low temperature, high modulus state. Overall, the laser extensometer provides a robust method for controlling the axial strain in the gage section of the samples at moderate strain rates. The digital image correlation allows for full field measurement of both axial and transverse strains of SMPs over a range of temperatures and strain rates

Scaling the Stiffness, Strength, and Toughness of Ceramic‐Coated Nanotube Foams into the Structural Regime

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108652/1/adfm201400851-sup-0001-S1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/108652/2/adfm201400851.pd

Cohort profile : The resilience for eating and activity despite inequality (READI) study

The Resilience for Eating and Activity Despite Inequality (READI) cohort was established to address the following two key aims: to investigate the pathways (personal, social and structural) by which socio-economic disadvantage influences lifestyle choices associated with obesity risk (physical inactivity, poor dietary choices) and to explore mechanisms underlying ‘resilience’ to obesity risk in socio-economically disadvantaged women and children. A total of 4349 women aged 18–46 years and 685 children aged 5–12 years were recruited from 80 socio-economically disadvantaged urban and rural neighbourhoods of Victoria, Australia, and provided baseline (T1: 2007–08) measures of adiposity, physical activity, sedentary and dietary behaviours; socio-economic and demographic factors; and psychological, social and perceived environmental factors that might impact on obesity risk. Audits of the 80 neighbourhoods were undertaken at baseline to provide objective neighbourhood environmental data. Three-year follow-up data (2010–11) have recently been collected from 1912 women and 382 children. Investigators welcome enquiries regarding data access and collaboration

Fabrication and structural studies of sequentially adsorbed polyelectrolyte multilayers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1997.Includes bibliographical references (leaves 165-174).by Jeffery W. Baur.Ph.D

HEALING OF MICROSPHERE CORES IN SANDWICH COMPOSITES

ABSTRACT The primary purposes of a core in a sandwich composite are to keep the face sheets separated by a fixed distance and to transmit shear stresses. Syntactic foam cores consisting of hollow glass microspheres and resin can form strong, lightweight cores. By underfilling the interstitial space in a packed microsphere bed with a binder, a three-phase syntactic foam is created that has a percolated void network. In a sealed sandwich composite, a void network allows for the entire core of the sandwich composite to be evacuated and mechanically compressed by the exterior pressure. By combining this compression with a heating cycle, it is possible to repair core cracking and core/face sheet interface debonding when a reversible binder is used. Upon cooling, the healed sandwich restores its properties. We examine the relation between the mechanical properties of these sandwich composites and the healing methodologies

Effects of Loading Rate on the Relaxation and Recovery Ability of an Epoxy-Based Shape Memory Polymer

The majority of aerospace structural composites use thermoset resins for their processing flexibility, temperature capability, and environmental durability. In this study, the recovery behavior of Veriflex-E, an epoxy-based, thermosetting, thermally-triggered shape memory polymer (SMP) resin, is investigated in glassy (25 °C) and rubbery (130 °C) states, as a function of shape memory cycles, and as a means to evaluate its potential as a resin for a shape memory composite. At 25 °C, Veriflex-E exhibits a promising high elastic modulus and a positive, nonlinear strain rate sensitivity in monotonic loading. At 130 °C, the strain rate sensitivity in monotonic loading decreases. Stress relaxation after storage in the deformed temporary state and subsequent shape recovery is of particular interest, a challenge to measure, and has not been widely reported for SMPs. The current experimental program measures the influence of strain rate changes in the 10−4–10−2 s−1 range on the stress relaxation response of the material, as well as on the strain recovery behavior at both 25 °C and 130 °C. As expected, the post-relaxation strain is larger with faster loading. Unexpectedly, the total strain recovered after shape memory cycling is more similar to the low temperature deformation. Overall, the results suggest that, while being influenced by both the loading rate and the test temperature, Veriflex-E is a promising candidate for a shape memory composite which could enable adaptive structures

High-Bandwidth and Sensitive Air Flow Sensing Based on Resonance Properties of CNT-on-Fiber Hairs

Artificial hair flow sensors were fabricated using piezoresistive, radially grown carbon nanotube arrays on glass fibers and investigated for their dynamic aerodynamic response as measured within an instrumented plane-wave tube. The sensors were experimentally observed to provide both a large bandwidth of operation below first resonance and a strong resonance response at selected frequencies above first resonance. The frequency of first resonance was easily tunable by adjusting the length of the exposed hair and could be made to vary from a few hundred hertz to over 13 kHz. Higher frequency bands were accessible for a given hair length using higher-order resonance modes, up to five of which were observed. All of the responses were understood and modeled using a vibrating Euler-Bernoulli beam analysis

Printable epoxy carbon fiber composites and their mechanical properties

Additive manufacturing (AM) of fiber reinforced thermosetting resins has not been well explored due to material and processing restrictions including high viscosity, poor reinforcement dispersion, dimensional tolerance, and porosity. Yet, such a process would enable tool-less and agile manufacturing of small and complex composite parts with temperature and environmental stability superior to traditional thermoplastics. Printed thermoset composites could also be combined with traditional epoxy prepregs to make complex co-cured structures. In this work, a direct write AM process for epoxy/chopped carbon fiber composites is used which has been shown to provide cured composites tensile modulus and strength values 90% and 66% of the fully aligned composite, respectively. The fibers are largely oriented in the print direction, but have sufficient localized material flow to achieve acceptable transverse properties and low porosity. Sandwich structures created with additively printed cores and co-cured prepreg face sheets were fabricated and evaluated. Rather than creating a sandwich structure by adhesively bonding standard sizes of honeycomb core to cured composite face sheets, this work examined co-curing customized additively printed core structures with prepreg face sheets. This process can be used to design and optimize properties for complex geometries while reducing the number of manufacturing steps

Modeling the Role of Bulk and Surface Characteristics of Carbon Fiber on Thermal Conductance across the Carbon-Fiber/Matrix Interface

The rapid heating of carbon-fiber-reinforced polymer matrix composites leads to complex thermophysical interactions which not only are dependent on the thermal properties of the constituents and microstructure but are also dependent on the thermal transport between the fiber and resin interfaces. Using atomistic molecular dynamics simulations, the thermal conductance across the interface between a carbon-fiber near-surface region and bismaleimide monomer matrix is calculated as a function of the interface and bulk features of the carbon fiber. The surface of the carbon fiber is modeled as sheets of graphitic carbon with (a) varying degrees of surface functionality, (b) varying defect concentrations in the surface-carbon model (pure graphitic vs partially graphitic), (c) varying orientation of graphitic carbon at the interface, (d) varying interface saturation (dangling vs saturated bonds), (e) varying degrees of surface roughness, and (f) incorporating high conductive fillers (carbon nanotubes) at the interface. After combining separately equilibrated matrix system and different surface-carbon models, thermal energy exchange is investigated in terms of interface thermal conductance across the carbon fiber and the matrix. It is observed that modifications in the studied parameters (a–f) often lead to significant modulation of thermal conductance across the interface and, thus, showcases the role of interface tailoring and surface-carbon morphology toward thermal energy exchange. More importantly, the results provide key bounds and a realistic degree of variation to the interface thermal conductance values at fiber/matrix interfaces as a function of different surface-carbon features