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.

Ultrasonic wave phenomena constitute the leading physical mechanism for nondestructive evaluation (NDE) and structural health monitoring (SHM) of solid composite materials such as carbon-fiber-reinforced polymer (CFRP) laminates. Computational models of ultrasonic guided-wave excitation, propagation, scattering, and detection in quasi-isotropic laminates can be extremely valuable in designing practically realizable NDE and SHM hardware and software with desired accuracy, reliability, efficiency, and coverage. This paper presents comparisons of guided-wave simulations for CFRP composites implemented using three different simulation codes: two commercial finite-element analysis packages, COMSOL and ABAQUS, and a custom code implementing the Elastodynamic Finite Integration Technique (EFIT). Comparisons are also made to experimental laser Doppler vibrometry data and theoretical dispersion curves.

Benchmarking of Computational Models for NDE and SHM of Composites

A method of detecting defects in a composite structure includes applying heat to a surface of a composite structure. Thermographic images or frames captured by a moving camera may be utilized to form temporally aligned images that include temperature data (pixels) from a plurality of frames, wherein the (pixels) comprise data captured at a simple (uniform) time delay from the time at which heat was applied. The temporally aligned thermographic data for the surface region may include variations due to differences in thermal transients caused by defects in the composite structure. The variations in the thermographic data may be utilized to detect one or more defects in the composite structure.

Method and means to analyze thermographic data acquired during automated fiber placement

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.

/pdf/in-situ-thermal-inspection-of-automated-fiber-placement-iwbp74x5nj.pdf

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

Automated tow placement has become a widely-used fabrication technique, especially for large aerospace structures. Robotic heads lay down strips (tows) of pre-impregnated fiber along programmed paths. The intention is to lay adjacent tows abutted to one another, but sometimes a gap is left between a tow and the previously-placed tow. If a tow gap exists, it fills with resin during cure, forming a fiber-free volume. In immersion ultrasonic pulse-echo measurements of a cured laminate, the gap can be observed to produce a noticeable echo, without significantly attenuating the back-wall reflection of the laminate. To understand this behavior, we considered a one-dimensional model of the composite laminate, with a thin layer having the ultrasonic sound speed and density of neat resin, sandwiched between two layers of material having the sound speed and density of fiber-reinforced composite and surrounded on both sides by water. Neglecting attenuation, we considered the transmission and reflection coefficients of each interface, as well as that of the thin resin layer. Using the initial water/composite reflection as a reference, we computed the relative magnitude of the back surface/water reflection in the presence and in the absence of a resin-only layer, as well as the relative magnitude of the reflection arising from a thin resin layer in composite. While the one-dimensional model did not fully match the measurements, it did qualitatively explain the observed behavior.

/pdf/one-dimensional-model-for-the-ultrasonic-response-of-resin-2m1s6q3u9w.pdf

One-dimensional model for the ultrasonic response of resin-filled gaps in automated tape layup composites

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. Keywords: Automated Fiber Placement, Manufacturing defects, Thermography

/pdf/advanced-in-in-situ-inspection-of-automated-fiber-placement-6vsxssau1m.pdf

Peter D. Juarez

Papers

Benchmarking of Computational Models for NDE and SHM of Composites

Method and means to analyze thermographic data acquired during automated fiber placement

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

One-dimensional model for the ultrasonic response of resin-filled gaps in automated tape layup composites

Advanced in In Situ Inspection of Automated Fiber Placement Systems