While polymer additive manufacturing (AM) has advanced significantly over the past few decades, the limitations in material properties, speed of manufacture, and part size have relegated this technology to the space of rapid prototyping rather than the legitimate manufacture of end-use parts. Carbon fiber offers a low density, a low coefficient of thermal expansion, and high thermal conductivity and is an ideal material for bringing polymer-based AM from the realm of form and fit to that of form, fit, and function. Use of carbon fiber in AM can improve material properties, reduce the time required to manufacture functional parts compared with traditional subtractive technologies, and reduce warping, thereby enabling a larger possible build envelope. Therefore, the addition of carbon fiber to various AM technologies is of increasing interest in academic and industrial communities. This paper examines the work performed in this fast-growing area to date. Specifically, the effects of fiber reinforcement on the structure and mechanical properties of 3D printed parts are investigated within the body of literature. Upper bounds for tensile properties of carbon fiber composites are theoretically evaluated and compared with experimentally measured values. Moreover, current and potential applications of additively manufactured carbon fiber composites in the context of desktop 3D printing and big area AM are discussed. Recent innovations and industry breakthroughs in this field are also examined. This review is intended to organize and synthesize the present body of work surrounding AM of carbon fiber reinforced plastics, identify the most promising technologies, and prescribe viable research and development paths forward to advance AM from the application space of rapid prototyping to that of functional, load-bearing, end-use parts.

Additively manufactured carbon fiber-reinforced composites: State of the art and perspective

Inductor–capacitor ( $LC$ ) passive wireless sensors use a transformer with loose coupling between an external readout coil and an inductor that receives power through this inductive coupling. Changes in the sensor are wirelessly and remotely detected by the readout coil, which makes them highly useful in applications that require the sensor to be powered remotely and to occupy a small volume, such as harsh and sealed environments, where physical access to the sensor is difficult. Although the sensor to accomplish this function dates from the 1960’s, its rapid extension over the past decades has benefited from microelectromechanical systems. This paper provides an overview of the status and challenges in the $LC$ passive wireless sensor toward a wireless sensing platform. The basic sensing principles are first categorized into detecting changes of the sensor in response to the capacitance, resistance, inductance, or coupling distance due to the parameter of interest through monitoring the impedance magnitude and phase spectrum. The present state of the art in sensor applications for pressure, strain, temperature, humidity, biochemical, gas, and so on is then reviewed and compared. For emerging applications from many Internet of Things scenarios, geometrical constraints, such as small and non-invasive coils, reduce the magnetic coupling between the sensor and the readout coil, resulting in a limited interrogation distance. Furthermore, an increasing number of applications also require the simultaneous measurement of multiple parameters. Recent efforts to increase the interrogation distance and to extend the measurement of single parameter to multiple parameters are finally outlined. [2016-0093]

LC Passive Wireless Sensors Toward a Wireless Sensing Platform: Status, Prospects, and Challenges

1. Bulk Acoustic and Surface Acoustic Waves.- 2. Grating.- 3. Interdigital Transducers.- 4. Transversal Filters.- 5. Resonators.- 6. Selection of Substrate Material.- 7. Coupling-of-Modes Theory.- 8. Simulation of SH-type SAW Devices.- A. Physics of Acoustic Waves.- A.1 Elasticity of Solids.- A.2 Piezoelectricity.- A.3 Surface Acoustic Waves.- A.4 Effective Acoustic Admittance Matrix and Permittivity.- A.5 Acoustic Wave Properties in 6mm Materials.- A.5.1 Rayleigh-Type SAWs.- A.5.2 Effective Permittivity for BGS Waves.- A.5.3 Effective Acoustic Admittance Matrix.- A.6 Wave Excitation.- A.6.1 Integration Path.- A.6.2 Electrostatic Coupling.- A.6.3 BGS Wave Excitation.- A.6.4 SSBW Excitation.- References.- B. Analysis of Wave Propagation on Grating Structures.- B.1 Summary.- B.2 Metallic Gratings.- B.2.1 Blotekjr's Theory for Single-Electrode Gratings.- B.2.2 Wagner's Theory for Oblique Propagation.- B.2.3 Aoki's Theory for Double-Electrode Gratings.- B.2.4 Extension to Triple-Electrode Gratings.- B.3 Analysis of Metallic Gratings with Finite Thickness.- B.3.1 Combination with Finite Element Method.- B.3.2 Application to Extended Blotekjaer Theories.- B.4 Wave Excitation and Propagation in Grating Structures.- B.4.1 Effective Permittivity for Grating Structures.- B.4.2 Evaluation of Discrete Green Function.- B.4.3 Delta-Function Model.- B.4.4 Infinite IDTs.- References.

Surface acoustic wave devices in telecommunications : modelling and simulation

Corrosion has been a great concern in the oil and natural gas industry costing billions of dollars annually in the U.S. The ability to monitor corrosion online before structural integrity is compromised can have a significant impact on preventing catastrophic events resulting from corrosion. This article critically reviews conventional corrosion sensors and emerging sensor technologies in terms of sensing principles, sensor designs, advantages, and limitations. Conventional corrosion sensors encompass corrosion coupons, electrical resistance probes, electrochemical sensors, ultrasonic testing sensors, magnetic flux leakage sensors, electromagnetic sensors, and in-line inspection tools. Emerging sensor technologies highlight optical fiber sensors (point, quasi-distributed, distributed) and passive wireless sensors such as passive radio-frequency identification sensors and surface acoustic wave sensors. Emerging sensors show great potential in continuous real-time in-situ monitoring of oil and natural gas infrastructure. Distributed chemical sensing is emphasized based on recent studies as a promising method to detect early corrosion onset and monitor corrosive environments for corrosion mitigation management. Additionally, challenges are discussed including durability and stability in extreme and harsh conditions such as high temperature high pressure in subsurface wellbores.

https://www.mdpi.com/1424-8220/19/18/3964/pdf

Corrosion Sensors for Structural Health Monitoring of Oil and Natural Gas Infrastructure: A Review

https://ntrs.nasa.gov/api/citations/20190025948/downloads/20190025948.pdf

Simulation of guided-wave ultrasound propagation in composite laminates: Benchmark comparisons of numerical codes and experiment.

Multi-frequency local wavenumber analysis and ply correlation of delamination damage.

https://ntrs.nasa.gov/api/citations/20150000579/downloads/20150000579.pdf

Emerging Needs for Pervasive Passive Wireless Sensor Networks on Aerospace Vehicles

Automated fiber placement (AFP) provides precision and repeatable manufacturing of both simple and complex geometry composite parts. However, AFP also introduces the possibility for unique flaws such as overlapping tows, gaps between tows, tow twists, lack of layer adhesion and foreign object debris. These types of flaws can all result in a significant loss of performance in the final part. The current inspection method for these flaws is a costly and time intensive visual inspection of each ply layer. This work describes some initial efforts to incorporate thermal inspection on the AFP head and analysis of the data to identify the previously mentioned flaws. Previous bench-top laboratory experiments demonstrated that laps, gaps, and twists were identified from a thermal image. The AFP head uses an on- board lamp to preheat the surface of the part during layup to increase ply consolidation. The preheated surface is used as a thermal source to observe the state of the new material after compaction. We will present data collected with the Integrated Structural Assembly of Advanced Composites (ISAAC) AFP machine at Langley Research Center showing that changes to the temperature profile is sufficient for identifying all types of flaws.

/pdf/in-situ-thermography-of-automated-fiber-placement-parts-329sjaez1k.pdf

In-situ thermography of automated fiber placement parts

Automated Fiber Placement (AFP) systems have been developed to help take advantage of the tailorability of composite structures in aerospace applications. AFP systems allow the repeatable placement of uncured, spool fed, preimpregnated carbon fiber tape (tows) onto substrates in desired thicknesses and orientations. This automated process can incur defects, such as overlapping tow lines, which can severely undermine the structural integrity of the part. Current defect detection and abatement methods are very labor intensive, and still mostly rely on human manual inspection. Proposed is a thermographic in situ inspection technique which monitors tow placement with an on board thermal camera using the preheated substrate as a through transmission heat source. An investigation of the concept is conducted, and preliminary laboratory results are presented. Also included will be a brief overview of other emerging technologies that tackle the same issue.

Advances in in situ inspection of automated fiber placement systems

The advent of Automated Fiber Placement (AFP) systems have aided the rapid manufacturing of composite aerospace structures. One of the challenges that AFP systems bring is the variability of the deposited prepreg tape layers, which are prone to 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 discussed in this paper is to use the preheated base layer as a through transmission heat source and inspect the newly added tape layer in situ using a thermographic camera mounted onto the AFP structure. Such a system would not only increase manufacturing throughput by reducing inspection times, but would also aid in process development for new structural designs or material systems. 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 revealed 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 recent progress on the data reduction algorithms in preparation for machine learning development.

Peter D. Juarez

Papers

Multi-frequency local wavenumber analysis and ply correlation of delamination damage.

Emerging Needs for Pervasive Passive Wireless Sensor Networks on Aerospace Vehicles

In-situ thermography of automated fiber placement parts

Advances in in situ inspection of automated fiber placement systems

In situ thermal inspection of automated fiber placement manufacturing