In Situ Thermal Inspection of Automated Fiber Placement Operations for Tow and Ply Defect Detection

The advent of Automated Fiber Placement (AFP) systems have aided the rapid manufacturing of composite aerospace structures. One of the challenges that AFP systems pose is the uniformity of the deposited prepreg tape layers, which complicates detection of laps, gaps, overlaps and twists. The current detection method used in industry involves halting fabrication and performing a time consuming, visual inspection of each tape layer. Typical AFP systems use a quartz lamp to heat the base layer to make the surface tacky as it deposits another tape layer. The innovation proposed in this paper is to use the preheated base layer as a through-transmission heat source for inspecting the newly added tape layer in situ using a thermographic camera mounted on to the AFP hardware. Such a system would not only increase manufacturing throughput by reducing inspection times, but it would also aid in process development for new structural designs or material systems by providing data on as-built parts. To this end, a small thermal camera was mounted onto an AFP robotic research platform at NASA, and thermal data was collected during typical and experimental layup operations. The data was post processed to reveal defects such as tow overlap/gap, wrinkling, and peel-up. Defects that would have been impossible to detect visually were also discovered in the data, such as poor/loss of adhesion between plies and the effects of vacuum debulking. This paper will cover the results of our experiments, and the plans for future versions of this inspection system

Emerging technologies for the non-invasive characterization of physical-mechanical properties of tablets

The density, porosity, breaking force, viscoelastic properties, and the presence or absence of any structural defects or irregularities are important physical-mechanical quality attributes of popular solid dosage forms like tablets. The irregularities associated with these attributes may influence the drug product functionality. Thus, an accurate and efficient characterization of these properties is critical for successful development and manufacturing of a robust tablets. These properties are mainly analyzed and monitored with traditional pharmacopeial and non-pharmacopeial methods. Such methods are associated with several challenges such as lack of spatial resolution, efficiency, or sample-sparing attributes. Recent advances in technology, design, instrumentation, and software have led to the emergence of newer techniques for non-invasive characterization of physical-mechanical properties of tablets. These techniques include near infrared spectroscopy, Raman spectroscopy, X-ray microtomography, nuclear magnetic resonance (NMR) imaging, terahertz pulsed imaging, laser-induced breakdown spectroscopy, and various acoustic- and thermal-based techniques. Such state-of-the-art techniques are currently applied at various stages of development and manufacturing of tablets at industrial scale. Each technique has specific advantages or challenges with respect to operational efficiency and cost, compared to traditional analytical methods. Currently, most of these techniques are used as secondary analytical tools to support the traditional methods in characterizing or monitoring tablet quality attributes. Therefore, further development in the instrumentation and software, and studies on the applications are necessary for their adoption in routine analysis and monitoring of tablet physical-mechanical properties

Understanding the Radiation Effects on Fiber Optic Sensors

In this dissertation, the effects of radiation (gamma, neutron or mixed gamma and neutron) on optical fiber sensors are studied and new techniques for real-time measurement of radiation-induced macroscopic changes in optical fibers are presented. It is crucial among the research and development efforts in the nuclear energy field to conduct experiments in Advanced Test Reactor (ATR) to support lifetime extension, novel fuels and materials development, better fuel management, and enhanced safety of existing as well as future nuclear power plants (NPP). Due to their unparalleled and unique advantages over traditional sensors, optical fiber sensors are deemed potential candidates for their use in nuclear environments. However, optical fibers are susceptible to high levels of ionizing radiation emitted by fission reactors which are characterized by the highest levels of gamma dose, high flux of neutrons and potentially high temperatures depending on location in a reactor core. It is essential to accurately determine the information related to physical parameters such as temperature, pressure, and strain in nuclear environments for the safety of the existing and future NPPs. This dissertation starts with inverting a transmission mode long period grating (LPG) to reflection mode using a novel and cost-effective metal coating method since transmission mode LPG limits it applications in tight spaces or in nuclear fields. To understand the metal coating and metal coverage effects on the reflection spectrum of LPG, modeling work was performed, and it was validated by experimental work. We have shown that the sensitivity of LPGs to physical parameters in both transmission and reflection modes are almost the same. Next, we have modeled the radiation effects on different fiber optic sensors, proposed empirical models, and performed numerical analysis to understand the effects of nuclear environments on fiber optic sensors. We analyzed the real-time data from fiber Bragg gratings (FBGs) exposed to high neutron fluence and high temperature environments within the ATR at Idaho National Laboratory (INL). We have found that incoming radiation significantly drifts the characteristic signal of FBGs, leading to a temperature measurement error when FBGs are dedicated to temperature sensing. It is well known that neutron and gamma irradiation compacts silica optical fibers, resulting in a macroscopic change in the refractive index (RI) and geometric structure. The change in RI and linear compaction in a radiation environment is caused by three well-known mechanisms: (1) radiation induced attenuation (RIA), (2) radiation induced compaction (RIC), and (3) radiation induced emission (RIE). While RIA degrades the signal strength by creating different types of color centers in the silica fiber, RIC alters the density, and hence RI by displacing the host material atoms. However, Kramers-Kronig relation states that absorption, and hence the RIA, also modifies the RI of the silica fiber. Apart from RIA and RIC, other phenomena such as temperature, dose rate, stress relaxation, and dopant compositions exchange may change the RI. To overcome these problems, we have proposed an effective technique to measure the change in RI and compaction of optical fiber due to any specific phenomena the fiber is subjected to, including RIC, RIA, dopant diffusion, temperatures, dose, dose rate, etc. By knowing the individual contribution of RI and fiber length to the signal drift, it is possible to reduce the radiation induced signal drift in optical fiber sensors and provide accurate information regarding the temperature inside a radiation environment. Fission gas detection in nuclear environments is another important aspect that needs to be focused on. Pressure induced by fission gases during irradiation may lead to loss of coolant accident (LOCA), which can cause severe damage to the NPPs. We have modeled and fabricated optical fiber-based sensors to enable real-time monitoring of fission gases, which allows understanding the implications of fission gas release during an accident, important for safe and high performance

Efficient Path Delay Test Generation with Boolean Satisfiability

This dissertation focuses on improving the accuracy and efficiency of path delay test generation using a Boolean satisfiability (SAT) solver. As part of this research, one of the most commonly used SAT solvers, MiniSat, was integrated into the path delay test generator CodGen. A mixed structural-functional approach was implemented in CodGen where longest paths were detected using the K Longest Path Per Gate (KLPG) algorithm and path justification and dynamic compaction were handled with the SAT solver. Advanced techniques were implemented in CodGen to further speed up the performance of SAT based path delay test generation using the knowledge of the circuit structure. SAT solvers are inherently circuit structure unaware, and significant speedup can be availed if structure information of the circuit is provided to the SAT solver. The advanced techniques explored include: Dynamic SAT Solving (DSS), Circuit Observability Don’t Care (Cir-ODC), SAT based static learning, dynamic learnt clause management and Approximate Observability Don’t Care (ACODC). Both ISCAS 89 and ITC 99 benchmarks as well as industrial circuits were used to demonstrate that the performance of CodGen was significantly improved with MiniSat and the use of circuit structure

Optimizing Test Pattern Generation Using Top-Off ATPG Methodology for Stuck–AT, Transition and Small Delay Defect Faults

The ever increasing complexity and size of digital circuits complemented by Deep Sub Micron (DSM) technology trends today pose challenges to the efficient Design For Test (DFT) methodologies. Innovation is required not only in designing the digital circuits, but also in automatic test pattern generation (ATPG) to ensure that the pattern set screens all the targeted faults while still complying with the Automatic Test Equipment (ATE) memory constraints. DSM technology trends push the requirements of ATPG to not only include the conventional static defects but also to include test patterns for dynamic defects. The current industry practices consider test pattern generation for transition faults to screen dynamic defects. It has been observed that just screening for transition faults alone is not sufficient in light of the continuing DSM technology trends. Shrinking technology nodes have pushed DFT engineers to include Small Delay Defect (SDD) test patterns in the production flow. The current industry standard ATPG tools are evolving and SDD ATPG is not the most economical option in terms of both test generation CPU time and pattern volume. New techniques must be explored in order to ensure that a quality test pattern set can be generated which includes patterns for stuck-at, transition and SDD faults, all the while ensuring that the pattern volume remains economical. This thesis explores the use of a “Top-Off” ATPG methodology to generate an optimal test pattern set which can effectively screen the required fault models while containing the pattern volume within a reasonable limit

Does OO sync with the way we think?

Given that corrective-maintenance costs already dominate the software life cycle and look set to increase significantly, reliability in the form of reducing such costs should be the most important software improvement goal. Yet the results are not promising when we review recent corrective-maintenance data for big systems in general and for OO in particular-possibly because of mismatches between the OO paradigm and how we think

Study of spin-scan imaging for outer planets missions

The constraints that are imposed on the Outer Planet Missions (OPM) imager design are of critical importance. Imager system modeling analyses define important parameters and systematic means for trade-offs applied to specific Jupiter orbiter missions. Possible image sequence plans for Jupiter missions are discussed in detail. Considered is a series of orbits that allow repeated near encounters with three of the Jovian satellites. The data handling involved in the image processing is discussed, and it is shown that only minimal processing is required for the majority of images for a Jupiter orbiter mission

component of this work in other works. Area-Efficient Synthesis of Fault-Secure NoC Switches

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Tow-Path Characterization for Automated Fiber Placement

Automated Fiber Placement (AFP) is a manufacturing process used to fabricate large composite structures for aerospace applications. During the process, the machine head deposits multiple bands of composite material named tows over a prescribed path. Temperature, speed, and compaction pressure can be varied to obtain a good layup quality. For conventional laminated plate structures manufactured using the AFP process, fibers are laid at constant angles (0°, 90°, ±45°) in straight paths. However, to manufacture complex shell structures or variable stiffness plates, curved paths are necessary in the design leading to a length mismatch between the parallel edges of the towpath. Since finite width fiber tows are originally straight, a form of tow deformation is necessary to absorb the difference in length thus ensuring good adherence to the surface substrate. In this work, several possible tow deformation mechanisms are proposed and classified as follows: (1) elastic strain deformations (tensile, compressive, shear), (2) large in-plane deformations (fiber waviness and bunching), and (3) large out-of-plane deformations (tow wrinkling and folding). Usually, large tow deformations are unfavorable and considered as defects, thus process interruption may be necessary to perform manual repairs. The aim of this proposal is to develop models able to capture tow deformations when placed on a curved path and to validate their occurrence during the AFP process. The proposed work is tackled from three different perspectives: (1) geometrical modeling, (2) physics-based modeling, and (3) experimental investigations. Understanding the geometry of a given layup is necessary to determine critical locations for defect occurrence. A worst-case scenario is considered where all other deformation mechanisms are suppressed in favor of out-of-plane wrinkling. Governing equations for a tow placed on a general surface are derived, and a simple deformation function is applied to the shorter edge of tow showing different wrinkle patterns along the length. A simplified form of the governing equations is provided for the special case of tows steered on a flat surface. Finally, examples are presented visualizing the wrinkles patterns and showing critical locations of wrinkling for a given layup for flat and general surfaces. Relationships between the wrinkles wavelength and the steering radius can be obtained from existing mechanics models and experimental data. This information is used to improve the geometrical model, thus obtaining more accurate results for wrinkles modeling. In the physics-based model, the influence of the material properties and tow geometry on the deformations is studied. The tow is modeled as multiple fiber bundles laying on a stiff foundation. In a first step, only in-plane deformations are allowed, thus capturing the elastic deformation mode (tensile/compressive strains) and the large in-plane deformations (fiber waviness and bunching). In a second step, out-of-plane deformations are allowed, enabling the modeling of additional tow deformations mentioned earlier such as tow wrinkling. In a last step, the interaction between the neighboring fiber bundles is investigated by considering the transverse and shear stiffness properties of the uncured tow. Finally, experimental investigations measuring tow deformations over steered paths are carried using the Digital Image Correlation (DIC) technique. Thermoset pre-impregnated carbon fiber tows are speckled first, then placed using an AFP machine over vi multiple paths with different radii of curvature. Shape and strain measurements of the deformed tows are acquired using a Stereo DIC setup. Quantified measurements of wavelength, width, and amplitude for tow wrinkles are obtained as a function of the steering radius. The effect of the substrate, time, and temperature on the formation of wrinkles is also studied. Other experiments using DIC are carried out to determine the deformation of neighboring tows within a course when steered along a constant curvature path. To understand the effect of AFP process parameters on tow deformations, a benchmark reference path on a flat surface with a linear increase in the curvature is proposed. Based on the location of the first visible defect along the length of the path, a critical steering radius is determined for each set of process parameters used. Finally, steering experiments are performed on a cylindrical tool with varying process parameters. The quality of the manufactured steered paths is assessed through image processing of acquired profilometry scans during manufacturing. Recommendations regarding optimal set of process parameters for future manufacturing activities are provided based on the measured defects along the path