Magnetostrictive composites are of considerable interest for real-time remote force sensing and structural health monitoring. In this study, we produced wireless thin-layer force sensors based on washers made of magnetostrictive composite materials featuring terfenol-D particles embedded in an epoxy matrix. The magnetostrictive composite materials were characterized by a combination of characterization techniques, including x-ray diffraction, magnetic hysteresis measurements, and dedicated magneto-mechanical tests. We demonstrated an operation range over which the magneto-mechanical response is linear, repeatable, has a minor amount of hysteresis and demonstrates no relaxation or rate effects. In addition, we demonstrated ways for producing force sensors with a higher sensitivity and signal-to-noise ratio by increasing the poling magnetic field and the temperature during the curing of the epoxy. Moreover, we identified the sequence of processes that occur during the curing and poling stage and the critical (hardest) process that eventually determines sensor sensitivity.
Epoxy resins are the most commonly used adhesives in industry due to their versatility, low cost, low toxicity, low shrinkage, high strength, resistance to moisture, and effective electrical resistance. These diverse properties can be tailored based on the chemical structure of the curing agent and the conditions of the curing process. Molecular simulations of epoxy resins have gained attention in recent years as a means to navigate the vast choice of chemical agents and conditions that will give the required properties of the resin. This work examines the statistical uncertainty in predicting thermodynamic and mechanical properties of an industrial epoxy resin using united atom molecular dynamics simulation. The results are compared with experimental measurements of the elastic modulus, Poisson’s ratio, and the glass transition temperature obtained at different temperatures and degrees of curing. The decreasing trend of the elastic modulus with increasing temperature is accurately captured by the simulated model, though the uncertainty in the calculated average is large. The glass transition temperature is expectedly overpredicted due to the high rates accessible to molecular simulations. We find that Poisson’s ratio is particularly sensitive to sample anisotropy as well as the method of evaluation, which explains the lack of consistent trends previously observed with molecular simulation at different degrees of crosslinking and temperatures.
No abstract
One major application for smart materials is measuring stresses or strains in load bearing structures. The ability to monitor structural health, and observe real time stress levels in load bearing platforms is a field of great interest. In this work, we develop and characterize a method for stress monitoring adhesively bonded joints by incorporating a magnetostrictive filler into the polymeric matrix. Magnetostrictive materials create a change in their surrounding magnetic field when subjected to strain, and thus serve as natural strain sensors, that require neither power supply nor any kind of wiring. A clear correlation between the stress and the magnetic field, which is measured at a distance of 20–60 mm from the specimen, is observed under both shear and compression loads. Moreover, there is a significant stress region in which the relationship between the stress and the magnetic field is approximately linear. This behavior demonstrates the possibility of monitoring the average stress in a specimen by a single magnetic sensor mounted at a distance from the specimen. Additionally, complete three dimensional mapping of the magnetic field around loaded specimens reveals that the specimen magnetization is not uniform and implies the existence of a correlation between the specimen magnetization and the stress field which was numerically computed. This behavior indicates the potential of mapping the local stress profile within a specimen by using an array of several magnetic sensors. The effects of magnetostrictive particle size and of applying a magnetic field during specimen polymerization are also discussed.
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