This paper presents an undergraduate research project developed to enhance mechanical engineering education at the University of Oklahoma. Selective Laser Sintering (SLS) is a promising additive manufacturing method for high-temperature materials with high spatial resolution and surface quality. As one of the most capable engineering-grade thermoplastics, polyether ketone (PEEK) can be used in additive manufacturing due to its elevated working temperature. This printer will use multiple heat zones, adjustable layer height, and a controlled hopper system to allow the user to fine-tune every print. In this paper, students are required to analyze the technical challenges of SLS based 3D printing technology. Using three separate controlled heat zones, the user will be able to hold the part above its glass transition temperature until the entire part finishes, therefore, annealing it in the process. This will additionally allow for testing and documentation of the effect of heat during preheating, pre-sintering, and post sintering. These features in a small-scale machine will allow thorough documentation of how controlled heated environments can alter the physical properties of a 3D printed part. Using a full steel platform with CNC machined parts and an off the shelf laser, the cost will be reduced to under ten thousand dollars. This undergraduate project to design an SLS based 3D printer provide a unique opportunity for students to fully understand the challenges of SLS manufacturing and gain experience in developing a complex 3D printing system.

This paper presents the fabrication and characterization of porous polydimethylsiloxane (PDMS) plates. The framework for obtaining porous PDMS is based on the solvent evaporation induced phase separation technique. A mixture of PDMS, water, and tetrahydrofuran (THF) with different concentrations is prepared. The three phases are stirred to reach a highly stable and viscous solution. The THF and water phases are evaporated during a curing cycle by applying a stepping heat treatment. The porous PDMS sheets with a wide range of pore sizes are fabricated by controlling the ratio of water to THF in the mixture. The confocal microscopy images are used to characterize the average pore size and the pore size distribution in the structures. Dogbone samples following the ASTM standard D412 are cut from the porous plates by utilizing a designed cutting die and mechanical press. The specimens are tested under tensile loading to evaluate the effect of the pore size on the mechanical properties of the porous structure. The results demonstrate the ability of the proposed solvent evaporation method to control the stiffness of the porous structure by changing the non-solvent to the solvent ratio in the mixture.

In this study, a three-dimensional model of single carbon-fiber composites enhanced by radially grown zinc oxide (ZnO) nanowires is investigated numerically. Due to the different length scales of the composites and the theories used in the system, a multi-scale analysis is employed to simulate the behavior of the fiber-reinforced composites. The effective mechanical properties of the enhancement layer are extracted at the micro-scale by the homogenization analysis of an appropriate representative volume element. The fiber interface is modeled at the meso-scale utilizing the cohesive zone method. A thin layer of interface with the cohesive element is modeled around the fiber. The material properties of the interface are evaluated based on the properties of fiber and the enhancement layer. The macro-scale damage behavior of fiber is defined by user-defined mechanical material behavior. Single fiber fragmentation test is simulated in ABAQUS by applying the tensile loads on the structure. The load transfer mechanism is evaluated by capturing the number of fiber fragmentation and calculating the interfacial shear strength. The effect of different ZnO diameters and volume fractions are also investigated. The results show stronger interface and higher load transfer capacity in the enhanced composite compared to the bare composite.

In recent years, highly flexible nanocomposite sensors have been developed for the detection of a variety of human body movements. To precisely detect the bending motions of human joints, the sensors must be able to conform well with the human skin and produce signals that effectively describe the amount of deformation applied to the material during bending. In this paper, a carbon nanotube-based piezoresistive strain sensor is developed via the direct ink writing based embedded 3D printing method. The optimum weight concentration range of carbon nanotubes in the nanocomposite inks, appropriate for embedded 3D printing, is identified. Samples with complex 2D and 3D geometries are printed to demonstrate the manufacturing capabilities of the embedded printing process. The sensitivity of the piezoresistive strain sensor is optimized by determining the ideal nanofiller concentration, curing temperature, and nozzle size to produce the highest gauge factor in a wide strain range. The piezoresistive and mechanical properties of the optimized sensors are fully characterized to verify the suitability for skin-attachable strain sensing applications. The developed sensors have a wide sensing range, high sensitivity, and minimal strain rate dependence. In addition, their low elasticity and high biocompatibility allow them to be comfortably bonded on the human skin.

Carbon nanotubes (CNTs) have the unique ability to absorb microwave radiation and efficiently transfer the energy into substantial heat. When adequately dispersed in a thermoset polymer, such as polydimethylsiloxane (PDMS), the nanocomposite can be fully cured in seconds in a microwave oven rather than in hours in a convection oven. In this paper, cylindrical PDMS nanocomposites containing well-dispersed CNTs are fabricated by either microwave-curing or conventional thermal-curing. The mechanical, electrical, and piezoresistive properties of the fabricated samples are compared to understand the effects of different curing methods. Microwave-cured nanocomposites exhibit a significantly reduced compressive modulus for different CNT loadings. In addition, the electrical conductivity of microwave-cured nanocomposites is significantly enhanced over the thermally-cured counterparts. Experimental results demonstrate that the one-step microwave-curing procedure can improve the electrical conductivity of 1 wt% nanocomposites by almost 150 % over thermal-curing. However, their piezoresistive sensitivity remains remarkably similar, showing the potential for microwave-curing to replace thermal-curing for the manufacturing of highly flexible CNT-based nanocomposites.

In this study, the damage analysis of hybrid carbon fiber reinforced polymer (CFRP) composited with vertically aligned zinc oxide (ZnO) nanowires is investigated numerically. The effect of growing nanowires on improving the interlaminar shear strength (ILSS) of the hybrid structures is explored. The multi-scale model developed to make a bridge between the materials with different length scales available in the hybrid structures, including micro-scale, meso-scale, and macros-scale. The vertically aligned ZnO nanowires on the lamina and embedded in the epoxy matrix creates an enhancement layer. The effective material properties of this layer are evaluated at micro-scale by homogenization analysis. The cohesive zone method is employed in the meso-scale to explore the interfacial behavior and delamination (interlaminar damage) between the homogenized stacking layer and the CFRP lamina. Besides, the strain-based failure criterion is implemented at the macro-scale to investigate the progressive damage of fiber and matrix in CFRP plies. This analysis is programmed in user-defined subroutine linked to ABAQUS finite element software. The three-dimensional hybrid composite short beam in the three-point bending load is simulated in ABAQUS Explicit packager, and the ILSS is obtained. The damage behavior of hybrid composite is compared to the bare CFRP beam. The results indicate that aligning nanowires on the plies improves the performance of CFRP composites.

Intracranial aneurysms have the potential to be fatal; when detected, they must be treated promptly by surgical clipping or by endovascular methods. The latter, while having better long-term overall survival than the former, fail to provide complete occlusion of the aneurysm lumen, creating risks for therapy-related adverse events, such as embolic device migration or recanalization. Polyurethane shape memory polymers (SMPs) have the potential to provide patient-specific treatment to reduce rates of incomplete occlusion and mass effect. In this study, SMP matrices are infiltrated with carbon nanotubes (CNTs) to induce electrical conductivity and provide a precise triggering method for deployment of the embolic device. Through thermomechanical characterization of the composite, it was determined that CNTs play a significant role in resistivity of the SMP foam and its ultimate shape recovery properties. Cyclic mechanical testing allowed to determine that CNTs might induce polymeric matrix damage, creating the need for new approaches to CNT infiltration. The studied composite foams were able to occlude an in vitro idealized aneurysm phantom model, which allowed to conclude that the proposed CNT-infiltrated SMP foams exhibit potential as biomedical devices for endovascular therapy of intracranial aneurysms.

The alignment of high aspect ratio reinforcing nanoparticles within a polymer matrix can have significant effects on the mechanical, electrical, and thermal properties of the nanocomposite. Therefore, in order to tailor the properties of the composite, it is imperative to develop novel methods to control the alignment of these filler particles in various polymeric matrices. This paper reports a unique approach to alter the alignment of carbon nanotubes (CNT) within polydimethylsiloxane (PDMS) nanocomposites using 3D printing technology. A line of the reinforced PDMS resin is printed on a PDMS substrate using direct ink writing technology, which can produce alignment in the print direction depending on printing parameters, the loading of the reinforcing particle, and the rheology of the ink. Then, the substrate is stretched and placed in an oven to cure the printed nanocomposites line with increased alignment in the stretch direction. These two techniques have the advantage of simplicity over other techniques and can efficiently manufacture nanocomposites with the alignment of nanoparticles. Optical microscopy will be used to quantify the alignment within the printed line. Electrical and mechanical properties will be tested to determine the effects of the different alignments within the elastomer. The ability to control the alignment of elastomeric CNT composites is advantageous for the growing field of polymer-based electronics.

Flexible and sensitive strain sensors can be utilized as wearable sensors and electronic devices in a wide range of applications, such as personal health monitoring, sports performance, and electronic skin. This paper presents the fabrication of a highly flexible and sensitive strain sensor by 3D printing an electrically conductive polydimethylsiloxane (PDMS)/multi-wall carbon nanotube (MWNT) nanocomposite on a PDMS substrate. To maximize the sensor’s gauge factor, the effects of MWNT concentration on the strain sensing function in nanocomposites are evaluated. Critical 3D printing and curing parameters, such as 3D printing nozzle diameter and nanocomposites curing temperature, are explored to achieve the highest piezoresistive response, showing that utilizing a smaller deposition nozzle size and higher curing temperature can result in a higher gauge factor. The optimized 3D printed nanocomposite sensor’s sensitivity is characterized under cyclic tensile loads at different maximum strains and loading rates. A linear piezoresistive response is observed up to 70% strain with an average gauge factor of 12, pointing to the sensor’s potential as a flexible strain sensor. In addition, the sensing function is almost independent of the applied load rate. The fabricated sensors are attached to a glove and used as a wearable sensor by detecting human finger and wrist motion. The results indicate that this 3D printed functional nanocomposite shows promise in a broad range of applications, including wearable and skin mounted sensors.

For decades, aircraft disasters have always been a concern for airline companies and especially for consumers. Scientists all over the world have been constantly trying to study, discover and invent new methods for testing and prevention to reduce future aircraft accidents. One of those methods is non-destructive testing, which is a widely adaptive process for analyzing structural integrity over wide arrays of object. X-rays, ultrasound and computed tomography (CT) are non-destructive testing applications commonly used for the commercial aircraft maintenance. These non-destructive testing methods for aircraft structures give us high-quality images of structural damage but, there are some disadvantages related to resolution and the contrast mechanism of the image. The goal of this study is to demonstrate the concept of X-Ray Induced Acoustic Computed Tomography (XACT) imaging method for defect detection and localization through simulations using k-wave MATLAB toolbox. XACT is a technique based on the X-ray induced acoustic effect. In XACT, a short pulsed of X-rays are required to achieve thermal response and generate acoustic waves. X-ray travels to an object, the photons are absorbed causing the temperature in the object to raise, which generates acoustic waves due to thermoelastic expansion. These acoustic waves are then detected by ultrasonic transducers. Within the fuselage of the aircraft, the aircraft’s stiffener is designed using SolidWorks. along with two different types of defects through voids due to manufacturing imperfection process. As well as, cracks in the surface of the model due to mechanical failures are created in MATLAB. Two properties of Aluminum 6065 and Inconel 625 materials were selected for our simulation study since it is often used for the fuselage and/or aircraft engines. XACT images are generated under the combination of high X-ray absorption and ultrasonic transducers that will be able to overcome the disadvantages of the X-ray imaging technique and ultrasound imaging technique in image resolution and contrast mechanisms. The results from this simulation study demonstrate that the XACT method not only gives us high-resolution images but moreover, higher contrast of images that also allows us to detect position accuracy of the cons created.

This paper presents the novel development of highly porous carbon nanotube (CNT)/shape memory polymer (SMP) nanocomposites for potential endovascular treatment of intracranial aneurysms (ICAs). Intracranial aneurysm is a cerebrovascular disorder that can significantly weaken the wall of a brain artery, resulting in a localized dilation of the blood vessel with risk of rupture and subarachnoid bleeding. Current therapeutic options include surgical clipping and endovascular coil embolization. Clipping of intracranial aneurysms is invasive, and, therefore, it has gradually been replaced by non-invasive endovascular embolization. Recent studies have shown that aneurysmal recanalization and incomplete occlusion are still emerging clinical challenges in endovascular coil embolization. Therefore, there is an urgent need to develop new medical devices and surgical procedure to treat intracranial aneurysms with improved long-term outcomes. CNT/SMP nanocomposites are fabricated by directly coating CNTs on sugar particles before fabricating the sugar template for porous nanocomposites. Pristine SMP prepolymer is infiltrated into the pores of sugar template. All the sugar is dissolved in water after the fully curing of PDMS, resulting in SMP based nanocomposites with well dispersed CNTs. The porous nanocomposites are characterized to identify key parameters, such as electrical resistivity and shape memory capability. A resistive-heating mechanism is developed to trigger shape recovery of the nanocomposites. The results of this work will lay a solid foundation for our subsequent development of new personalized biomedical devices to treat ICAs using a catheter-based endovascular embolization procedure.

This paper presents the implementation and preliminary analysis of a multi-scale material and mechanics education module for the improvement of undergraduate solid mechanics education. 3D printed and conventional wrought aluminum samples were experimentally characterized at both the micro- and macro-scales. At the micro-scale, we focus on the visualization of material’s grain structure. At the macro-scale, standard material characterization following ASTM standards is conducted to obtain the macroscopic behavior. Digital image correlation technology is employed to obtain the two-dimensional strain field during the macro-scale testing. An evaluation of students understanding of solid mechanics and materials behavior concepts is carried out in this study to obtain the student data and use it as baseline for further evaluation of study outcomes. We plan to use the established multi-scale mechanics and materials testing dataset in a broad range of undergraduate courses, such as Solid Mechanics, Design of Mechanical Components, and Manufacturing Processes. Our current effort is expected to demonstrate the real materials’ multi-scale nature and their mechanical performance to undergraduate engineering students. The successful implementation of this multi-scale approach for education enhances students’ understanding of abstract solid mechanics theories and establishing the concepts between mechanics and materials. In addition, this approach will assist advanced solid mechanics education, such as the concept of fracture, in undergraduate level education throughout the country.

Damage precursor in composites can lead to large structural damages, such as delamination, in carbon fiber reinforced plastic (CFRP) composites due to complex load conditions and environmental effects. In addition, multiple types of damage precursors including micro-scale matrix cracks, fiber pull-out from matrix, and fiber breakages, are extremely difficult to detect due to the limitation of resolution of current non-destructive evaluation (NDE) technologies. This paper presents a photo-acoustic based non-contact NDE system for the detection of damage precursors with extremely high resolution up to one hundred micrometers. This system consists of three major components: picoseconds pulsed laser based ultrasonic actuator, ultrasound receiver, and data processing and computing subsystem. Picoseconds pulsed laser is used to generate ultrasonic propagations in composites during the NDE process, and the ultrasound signals are recorded by the ultrasound receiver. Three-dimensional microstructure of the individual composites grid within the composite is able to be reconstructed for further analysis. The size and position of the damage precursors are evaluated with high accuracy up to 100 μm. The experimental results demonstrate that this imaging system is able to provide a novel non-contact approach with extremely high resolution for damage detection of CFRP composites. In addition, the developed NDE system has a wide industrial application in aerospace, automobile, civil, mechanical, and other key industries.

This paper presents the fabrication, optimization, and characterization of in-situ pressure sensor arrays using polydimethylsiloxane (PDMS) and carbon nanofibers (CNFs) nanocomposites. We first synthesize and characterize the nanocomposites to identify the optimal material formulation and fabrication procedure. Structural optimization algorithms and finite element method are employed to optimize the geometries of sensors. Pressure sensing units in cylinder and conical shapes are fabricated using the optimized material formulation and geometries. Two prototypes of sensors arrays are assembled and tested under different pressure load conditions. The long term sensor performance is validated using cyclic compression tests.

This paper presents a novel medical device developed using shape memory polymer (SMP) foams for the endovascular treatment of intracranial aneurysms (ICAs). The SMP foam is fabricated, characterized, and experimentally investigated to better understand their potential for endovascular embolization of ICAs. Polyurethane-based SMP is successfully synthesized and characterized. The SMP foam is manufactured using cast molding, and characterized using an electro-thermal triggering mechanism to fully understand their shape recovery capability. The successful completion of this work will serve as a solid foundation for the development of new biomedical devices to treat intracranial aneurysms and develop an optimal releasing procedure for future animal study.

This paper presents the additive manufacturing of electrically conductive polydimethylsiloxane (PDMS) nanocomposites for in-situ strain sensing applications. A straight line of pristine PDMS was first 3D printed on a thin PDMS substrate using an in-house modified 3D printer. Carbon nanotubes (CNTs) were uniformly sprayed on top of uncured PDMS lines. An additional layer of PDMS was then applied on top of CNTs to form a thin protective coating. The 3D printed PDMS/CNT nanocomposites were characterized using a scanning electron microscope (SEM) to validate the thickness, CNT distribution, and microstructural features of the sensor cross-section. The strain sensing capability of the nanocomposites was investigated under tensile cyclic loading at different strain rates and maximum strains. Sensing experiments indicate that under cyclic loading, the changes in piezo resistivity mimic, both, the changes in the applied load and the measured material strain with high fidelity. Due to the high flexibility of PDMS, the 3D printed sensors have potential applications in real-time load sensing and structural health monitoring of complex flexible structures.

The objective of this paper is to develop in-situ structural health monitoring in polymer matrix composites using embedded bucky paper. Bucky paper based sandwich composites has been used for damage and load sensing in aerospace and defense applications due to high electrical conductivity, low density, and outstanding load sensitivity. Recent research focuses on improving mechanical, electrical, thermal properties of certain composites with improved gauge factor for sensing applications. To better understand certainly quantity strain change effects, it is essential to design composite materials and sensors for in-situ and embedded strain monitoring in composites using piezoresistance feedback. In this paper nanocomposite bucky papers are manufactured to monitor the load and damage condition in fiber reinforced polymer matrix composites. We first investigated the fabrication of bucky papers using different nanomaterials. Then the micro-scale morphology and structures are characterized using a scanning electron microscopy. The sensing function is achieved by correlating the piezoresistance variations to the stress or strain applied on the sensing area. Due to the conductive network formed and the tunneling resistance change in neighboring nanoparticles, the electrical resistance is able to show a good correlation with the load conditions. The prepared bucky papers are embedded in composites and the sensing capability is experimentally characterized under three-point bending experiments. The characterized membrane structures have the potential to be further applied to in-situ structural health monitoring and structural state awareness during their entire service lives.

Shape memory polymers (SMPs) have been developed as an emerging technology platform for biomedical applications in the past decades. In particular, SMPs are clinically essential for the development of novel medical devices to significantly improve long-term surgical outcomes. In this paper, we synthesized and characterized thermally-activated aliphatic urethane SMPs fabricated with nanocomposites for the design and development of biomedical devices. The thermal activation of shape memory function was investigated by direct thermal activation. Critical polymer properties, such as the glass transition temperature and shape memory function, have been tailored to desired applications, by adjusting the polymer composition. Carbon nanotubes were uniformly dispersed within the polymer during nanocomposite fabrication to significantly enhance the thermal and electrical properties. The synthesized SMPs and nanocomposites were characterized to understand their thermal and mechanical properties using dynamic mechanical analysis (DMA). Scanning electron microscopy was employed to evaluate the dispersion of carbon nanotubes in polymer matrix. The mechanical properties of SMPs and nanocomposites at temperature above their glass transition temperature were evaluated using dog-bone samples in a dual-column mechanical testing system and an environmental chamber. SMPs and nanocomposites will then be fabricated in the form of foam for the development of novel devices applicable to endovascular embolization of cerebral aneurysms.

In this paper, a novel shape memory epoxy based composites is employed to design and develop new flexible composites for aerospace applications. The epoxy and carbon fiber reinforced epoxy were first synthesized and characterized to understand their shape memory property. The characterized epoxy is used as the polymer matrix in carbon fiber reinforced composites. The shape memory function of the fabricated composites is experimental characterized using the Joule-heating based method. The Joule-heating based activation method using direct electrical current is systematically characterized to understand the shape recovery speed at different temperature. It was observed that the developed composites have good flexibility and response to Joule heat quickly during deployment. Such composite materials can be further used to design and develop flexible composites and aerospace structures, such as satellite panel trusses and solar sail boom structures.

In this paper a nanocomposite system is proposed to accurately measure pressure generated by normal force. The strain sensing function is achieved by correlating the piezoresistance variations to the normal force applied on the sensing area. Due to the conductive network formed by carbon nanofibers (CNFs) and the tunneling resistance change between neighboring CNFs the electrical resistance measured using the four-probe method shows a clear correlation with the load conditions. In order to improve the piezoresistivity, CNF are uniformly dispersed in PDMS using a solvent assisted ultra-sonication method. The proposed nanocomposite based strain sensor is experimentally characterized under both quasi-static and cyclic load conditions.