Mr. Julio Hernandez Profile

Julio Hernandez

at Purdue University

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Publications (4)

Proceedings Article | 18 April 2022 Presentation + Paper

Electrode spacing-induced signal filtering for transient piezoresistivity

J. Hernandez, H. Zhu, F. Semperlotti, T. Tallman

Proceedings Volume 12046, 120460B (2022) https://doi.org/10.1117/12.2612903

KEYWORDS: Nanocomposites, Multifunctional materials, Electrodes, Structural health monitoring, Resistance, Composites, Sensors, Electronic filtering, Epoxies, Wave propagation

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Nanofiller-modified composites have shown much potential for structural health monitoring (SHM) and nondestructive evaluation (NDE) because they exhibit self-sensing behavior via the piezoresistive-effect. To date, the piezoresistive effect has been used predominantly in isolation for cases of (quasi-)static loading. There is a comparative lack of work that investigates the piezoresistive-effect under mechanically dynamic conditions. This is important because combined piezoresistive-elastodynamic approaches could leverage the relationship between electrode geometry/topology and piezoresistive information carried in elastic waves. In other words, the question of how certain factors of electrode design, such as spacing between electrode pairs, impacts the measured piezoresistive signal has been little explored within the context of elastodynamics. Addressing this gap in the state of the art may yield insights into multiphysics approaches to SHM and NDE which seamlessly marry conductivity and vibration-based techniques. To this end, a simple prismatic carbon nanofiber (CNF)-modified epoxy rod was manufactured. Piezoresistive measurements were taken as a function of time by way of normalized resistance measurements between surface mounted electrodes. An electromagnetic shaker was employed to inject highly controlled one-dimensional stress waves into the CNF-modified epoxy rod. It was found that surface mounted piezoresistive measurements were able to accurately reconstruct the profile of propagating subsurface wave packets across the length of the rod. Transmission and propagation of the wave packets were extrinsically validated with the shaker force sensor and an external laser vibrometer (LV) system, respectively. Furthermore, artificial signal filtering was achieved by changing the distance between the electrode pairs. Lastly, the dispersion curves were constructed from piezoresistive measurements and extrinsically validated. Results from this preliminary investigation seeks to lay the foundational work for a new multiphysics SHM tool, piezoresistivity-coupled elastodynamics, that aims to provide an unparalleled level of understanding into material dynamic properties from direct electrical interrogation.

Proceedings Article | 22 March 2021 Presentation + Paper

On the performance of additively manufactured CNF/PLA piezoresistive strain sensors

J. Hernandez, C. Maynard, D. Gonzalez, M. Viz, J. Garcia, B. Newell, T. Tallman

Proceedings Volume 11591, 115910B (2021) https://doi.org/10.1117/12.2582165

KEYWORDS: Fused deposition modeling, Structural sensing, Resistance, Printing, Nanofibers, Sensors, Additive manufacturing, Structural health monitoring, Sensor technology, Nondestructive evaluation

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Robust and adaptable sensor technology is essential for achieving meaningful structural health monitoring (SHM) and integrated nondestructive evaluation (NDE). Unfortunately, prevailing sensor technologies are most often pre-packaged and therefore lack much adaptability. In other words, sensors are rarely structure-specific or application-specific. Rather, an existing pre-packaged sensor must be retrofit to the component or structure under inspection. Multifunctional additive manufacturing (AM) has immense potential to overcome this limitation by permitting stimulus-responsive materials to be printed onto or directly embedded within structures for application-specific sensing. Herein, we explore this concept for strain sensors fabricated via multifunctional AM. Specifically, pelletized polylactic acid (PLA) is modified by the addition of carbon nanofibers (CNFs) at 7.5% by weight. This modification is done through a dry-mix process which is followed by multiple reclaiming and re-extrusion cycles through a single-screw filament extruder. Through this process, the CNFs form an electrically conductive network within the PLA structure. Because the electrical conductivity of the CNFmodified PLA is deformation-dependent (i.e. the material is piezoresistive), the sensors printed from CNF/PLA filament can be leveraged for strain sensing. In this work, we utilize a commercially available fused deposition modeling (FDM) printer to print the CNF-modified PLA into small and thin dog-bone shapes. These sensors then are adhered to a comparatively stiff substrate such that resistance changes across the sensor can be monitored as a function of strain as the substrate is deformed within a load frame. Our preliminary results show that AM-produced CNF-modified PLA strain gauges can indeed be used to track strains consistently. These successful preliminary results show that multifunctional AM has considerable potential for the development of highly adaptive, application-specific, and on-demand sensing technology.

Proceedings Article | 23 April 2020 Presentation + Paper

The effect of extrusion temperature and cycles on electrical resistivity in carbon nanofiber-modified PLA filament for multi-functional additive manufacturing

Cole Maynard, J. Hernandez, D. Gonzalez, T. Tallman, J. Garcia, B. Newell

Proceedings Volume 11379, 113791O (2020) https://doi.org/10.1117/12.2557298

KEYWORDS: Multifunctional materials, Fused deposition modeling, Additive manufacturing, Manufacturing, Printing, Sensors, Carbon, Polymers, Structural health monitoring

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Proceedings Article | 27 March 2019 Presentation + Paper

On the transient piezoresistive response of impacted nanofiber-modified epoxy

J. Hernandez, T. Tallman

Proceedings Volume 10970, 1097013 (2019) https://doi.org/10.1117/12.2514259

KEYWORDS: Structural health monitoring, Manufacturing, Resistance, Composites, Mechanics

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