Anton Khomenko

Integrated slanted microneedle-LED array for optogenetics

This paper presents a three-dimensional (3-D) flexible micro light emitting diode (μ-LED) array for selective optical stimulation of cortical neurons. The array integrated individually addressable μ-LED chips with slanted polymer-based microneedle waveguides to allow precise light delivery to multiple cortical layers simultaneously. A droplet backside exposure method was developed to monolithically fabricate slanted microneedles on a single polymer platform. A wafer-level assembly technique was demonstrated, which permits large-scale, high-density system integration. The electrical, optical, thermal, and mechanical properties of the 3-D slanted microneedle-LED array were characterized experimentally.

A polycrystalline diamond-based, hybrid neural interfacing probe for optogenetics

This paper reports a hybrid optoelectronic neural interfacing probe, combining micro-scale light emitting diode (μLED) and microelectrodes on a polycrystalline diamond (PCD) substrate for optogenetic stimulation and electrical recording of neural activity. PCD has superior thermal conductivity (up to 1800 Wm-1K-1) [1], which allows rapid dissipation of localized LED heat to a larger area to improve heat exchange with surrounding perfused tissues, and thus significantly reduce the risk of thermal damage to nerve tissue. During repetitive stimulation with 100ms and 1Hz pulses, the maximum rise in surface temperature of the PCD probe is less than 1 °C, which is ~90% lower than that of a polymer-based probe. A PCD based probe with two stimulating sites and four recording sites was fabricated. The capacity of the probe for neural stimulation and recording has also been demonstrated in vivo by successfully observing light evoked action potentials.

Fiber Bragg-Grating Sensor Array for Health Monitoring of Bonded Composite Lap-Joints

The detection and characterization of defects in adhesively bonded composite joints is of special interest for determining the load carrying capacity and structural integrity of resulting components for automobile, marine and aerospace applications. Embedded fiber Bragg-grating (FBG) sensors are being increasingly used to monitor the bonded region in adhesive joints as they do not affect the intrinsic bonding properties. This paper presents a highly reliable system that uses embedded FBG sensors for health monitoring in glass-fiber composite joints. Particularly, an array of strategically placed FBG sensors characterizes the extent and location of defects in the joints studied. Experimental data from the embedded FBGs can be further used to develop experimentally validated simulations (EVS) which can be used as a design tool and also to evaluate residual capacity of damaged joints. Preliminary results demonstrate potential of the developed technique for a wide variety of bonded joints with similar and dissimilar adherends.

Bolt tension monitoring with reusable fiber Bragg-grating sensors

Bolted/mechanical fastening is one of the oldest and most widely used joining techniques. While it has many advantages such as ease of assembly and repair, it also has some important limitations. One such concern is bolt clamping load control and monitoring during and after joint assembly. Conventionally used torque wrenches can provide only an approximation of the clamping load and cannot be used for load monitoring. Clamping force transducers are bulky, expensive, and cannot usually be incorporated into the bolted joint for continuous load monitoring in the field. In this work, a novel implementation of a transducer device, called here for convenience the “bolt tension monitor,” is described and tested. It utilizes removable and reusable fiber Bragg-grating sensor(s) embedded in a bolt shaft for preload and retained clamping force measurements. While the inherent small size of the fiber Bragg-grating provides precise monitoring without significant effect on the intrinsic properties of the bolt, the embedding of an fiber Bragg-grating sensor with temporary adhesives allows quick assembly, disassembly, and reassembly of the sensor. Furthermore, these instrumented bolts can be used for structural health monitoring and defect detection in joints and structures. The technique shows great potential in simple adaptation to conventional manufacturing practices, precise clamping load measurement, and structural health monitoring of bolts and resulting joints.

Optical transmission scanning for damage quantification in impacted GFRP composites

Glass fiber reinforced polymer (GFRP) composites constitute nearly 90% of the global composites market and are extensively used in aerospace, marine, automotive and construction industries. While their advantages of lightweight and superior mechanical properties are well explored, non-destructive evaluation (NDE) techniques that allow for damage/defect detection and assessment of its extent and severity are not fully developed. Some of the conventional NDE techniques for GFRPs include ultrasonics, X-ray, IR thermography, and a variety of optical techniques. Optical methods, specifically measuring the transmission properties (e.g. ballistic optical imaging) of specimens, provide noninvasive, safe, inexpensive, and compact solutions and are commonly used in biomedical applications. In this work, this technique is adapted for rapid NDE of GFRP composites. In its basic form, the system for optical transmission scanning (OTS) consists of a light source (laser diode), a photo detector and a 2D translation stage. The proposed technique provides high-resolution, rapid and non-contact OT (optical transmittance)-scans, and does not require any coupling. The OTS system was used for inspection of pristine and low-velocity impacted (damaged) GFRP samples. The OT-scans were compared with conventional ultrasonic C-scans and showed excellent agreement but with better resolution. Overall, the work presented lays the groundwork for cost-effective, non-contact, and rapid NDE of GFRP composite structures.

High resolution imaging of impacted CFRP composites with a fiber-optic laser-ultrasound scanner

Damage induced in polymer composites by various impacts must be evaluated to predict a component’s post-impact strength and residual lifetime, especially when impacts occur in structures related to human safety (in aircraft, for example). X-ray tomography is the conventional standard to study an internal structure with high resolution. However, it is of little use when the impacted area cannot be extracted from a structure. In addition, X-ray tomography is expensive and time-consuming. Recently, we have demonstrated that a kHz-rate laser-ultrasound (LU) scanner is very efficient both for locating large defects and evaluating the material structure. Here, we show that high-quality images of damage produced by the LU scanner in impacted carbon-fiber reinforced polymer (CFRP) composites are similar to those produced by X-ray tomograms; but they can be obtained with only single-sided access to the object under study. Potentially, the LU method can be applied to large components in-situ.

Monitoring of fatigue damage in composite lap-joints using guided waves and FBG sensors

Adhesive bonding is being increasingly employed in many applications as it offers possibility of light-weighting and efficient multi-material joining along with reduction in time and cost of manufacturing. However, failure initiation and progression in critical components like joints, specifically in fatigue loading is not well understood, which necessitates reliable NDE and SHM techniques to ensure structural integrity. In this work, concurrent guided wave (GW) and fiber Bragg grating (FBG) sensor measurements were used to monitor fatigue damage in adhesively bonded composite lap-joints. In the present set-up, one FBG sensor was strategically embedded in the adhesive bond-line of a lap-joint, while two other FBGs were bonded on the surface of the adherends. Full spectral responses of FBG sensors were collected and compared at specific intervals of fatigue loading. In parallel, guided waves were actuated and sensed using PZT wafers mounted on the composite adherends. Experimental results demonstrated that...

Monitoring the effect of micro-/nanofillers on curing-induced shrinkage in epoxy resins

Abstract The curing process of polymers/resins in composites introduces volumetric shrinkage, which, based on the component type and boundary conditions, can detriment the strength of the resulting materials and components. In this work, the effect of resin reinforcement (micro- and nanofillers) and curing cycle on curing-induced shrinkage is presented. Dynamic measurements of the pristine and reinforced resin volume changes during the curing process were performed using a pycnometer. A special technique was developed to correct the existing pycnometer measurement methodology to make it independent of the testing chamber temperature, thereby allowing accurate measurements of volumetric changes during exothermic curing of resins. Overall, the study lays a foundation for measuring shrinkage-induced effects on the strengths of resins and resulting composites.

Novel Hybrid Fastening System with Nano-additive Reinforced Adhesive Inserts

Structural joining of materials and components involves complex phenomena and interactions between several elements of either similar or dissimilar materials. This complex behavior, coupled with the need for lightweight structures and safety (human occupants in aerospace, automotive and ground vehicles), propels the need for better understanding and efficient design. A novel joining technique that incorporates the advantages of both bonded (lightweight) and bolted (easy disassembly) techniques was invented (Provisional Patent 61/658,163) by Dr. Gary Cloud at Michigan State University. The most basic configuration of this invention consists of a bolt that has a channel machined through the bolt-shaft that allows injection of an insert compound that fills the hole-clearance of the work-pieces and acts a structural component. The hole may contain additional sleeves or inserts. Several combinations of the proposed technique are possible, and in particular, the effect of the adhesive inserts, with and without nano-modification was studied in this work. Glass Fiber Reinforced Plastic (GFRP) composite plates were used as adherends with 12.5 mm holes, grade 8 bolts and preloaded to a torque of 35 N m. Pristine and Cloisite® 30B nanoclay reinforced SC-15 epoxy were used as adhesive inserts in the hybrid bolts. Tension lap-shear tests were performed on conventional (no-inserts) and hybrid bolted joints (inserts: adhesives + nanoclay), and their performance was compared. Results reveal that hybrid bolted joints can eliminate joint slip and considerably delay the onset of delamination. The addition of nanoclay increases the strengths but most importantly can prevent moisture from reaching the bolts shaft due to its excellent barrier properties. The proposed joining technique holds great promise for multi-material joining and a wide range of applications.

Evaluation of progressive damage of nano-modified composite laminates under repeated impacts

However, studies on the effect of nano-reinforcements in repeated impact scenarios are relatively limited. This work investigates the effect of resin nanoclay modification on the impact resistance of glass-fiber reinforced polymer (GFRP) composites subjected to repeated impacts. Three impact energy levels were used in experiments with a minimum of four specimens per case for statistical significance. Each sample was subjected to 40 repeated impacts or was tested up to perforation, whichever happened first. The impact response was evaluated in terms of evolution of the peak force, bending stiffness, visual damage inspection and optical transmission scanning (OTS) at critical stages as a function of number of impacts. Also, the damage degree (DD) was calculated to monitor the evolution of damage in the laminates. As expected, the impact response of the GFRP composites varied based on the presence of nano-clay and the applied impact energy. The modification of the resin with nano-clay introduced novel phenomena that changed the damage progression mechanism under repetitive impacts, which was verified by visual observation and optical transmission scanning. A better understanding of these phenomena (e.g. crack-bridging, tortuosity) and their contributions to enhancements in the impact behavior and modifications of the types of damage propagation can lead to better design of novel structural composites.