Resource Library

We have developed this library of resources to provide access to the tremendous amount of content that we have developed over the years. It includes product-specific literature, application-based technical resources, and media presentations of our unique technologies.

If you know which specific product or application you’re looking for, simply click on that link below and you’ll find in-depth content. Alternatively, you can use the search function to connect you to literature and media related to your specific interest.

Published Papers

Title
Product Area
Additively Manufactured Components with Embedded Instrumentation
  • ODiSI/HD-FOS
Abstract

Additively manufactured components enable complex structures to be rapidly fabricated and tested for use in the automotive and aerospace industries. Additive manufacturing capabilities have expanded to include a variety of plastics, metal alloys, and fiber-reinforced polymers. Luna Innovations has developed and demonstrated methods to embed high definition fiber optic sensing (HD-FOS) technology into components that have been additively manufactured using ABS plastic as well as a cobalt chrome alloy.

Citation
• Davis, Matthew, Middendorf, John, Garg, Naman, Ohanian, O. John. "Additively Manufactured Components with Embedded Instrumentation." ASME International Mechanical Engineering Congress and Exposition, November 11-17, 2016. Phoenix, AZ. IMECE2016-66697
Defect Detection during Manufacture of composite Wind Turbine Blade with embedded fiber optic distributed strain sensor
  • ODiSI/HD-FOS
Short Summary
We present results from using optical frequency domain reflectometry for high-density distributed fiber optic measurement of strain in a composite wind blade during dynamic fatigue testing. This work illustrates the potential of distributed fiber optic strain measurement for early defect detection in large-scale composite structural health monitoring.
Abstract

High resolution fiber optic strain sensing is used to monitor the distributed strain throughout the manufacturing process of a 9-meter wind turbine blade with intentionally introduced defects.  Standard telecommunications-grade optical fiber was embedded in several layers of the carbon fiber spar cap and used to sense distributed strain during the VARTM process.  The amplitude and phase of the light reflected from the fibers are measured using a commercial optical frequency domain reflectometer (OFDR). Changes in the amplitude and phase of the backscattered light were measured to determine the strain along the entire length of the spar cap with 5 millimeter resolution.  Distributed strain measurements throughout the depth of the spar cap provide valuable information at intermediate points in the manufacturing process which elucidate defects both prior to and during infusion.  The embedded sensors will subsequently be used to measure strain during fatigue testing of the blade to provide a cradle-to-grave method for non-destructive testing of composite structures.

Citation
S. M. Klute et al., “Defect Detection During Manufacture of Composite Wind Turbine Blade with Embedded Fiber Optic Distributed Strain Sensor,” 43rd Proc. International SAMPE Tech. Conf, Ft. Worth, TX, 2011.
Embedded and Surface Mounted Fiber Optic Sensors Detect Manufacturing Defects and Accumulated Damage as A Wind Turbine Blade is Cycled to Failure
  • ODiSI/HD-FOS
Short Summary
High resolution fiber optic strain sensing is used to monitor the distributed strain during fatigue testing of a 9-meter CX-100 wind turbine blade with intentionally introduced defects.
Abstract

High resolution fiber optic strain sensing is used to monitor the distributed strain during fatigue testing of a 9-meter CX-100 wind turbine blade with intentionally introduced defects.  Commercially available telecommunications-grade optical fiber was embedded in several layers of the carbon fiber spar cap and surface mounted along the spar cap and leading edges of the finished blade.  The amplitude and phase of the light reflected from the fibers are measured using a commercial Optical Frequency Domain Reflectometer (OFDR). Changes in the amplitude and phase of the backscattered light are used to determine the strain along the entire length of the fiber with 2.5 millimeter spatial resolution.  Distributed strain measurements throughout the depth of the spar cap provide an unprecedented view into the strain field within a composite wind turbine blade with defects during fatigue testing to failure.  

Citation
J. R. Pedrazzani et al., “Embedded and Surface Mounted Fiber Optics Sensors Detect Manufacturing Defects and Accumulated Damage as a Wind Turbine Blade is Cycled to Failure” SAMPE Tech. Conf. Proc.: Emerging Opportunities: Materials and Process Solutions, Baltimore, MD, 2012.
In-situ Monitoring and Control of Induction Welding in Thermoplastic Composites Using High Definition Fiber Optic Sensors
  • ODiSI/HD-FOS
Abstract

This paper presents a novel method for providing temperature feedback to the control system of an induction welder during the joining of thermoplastic composite components. Thermoplastic composites are attractive due to their ability to be re-heated and melted repeatedly without degrading the strength of the materials. This enables joining components via fusion bonding or welding, bypassing mechanical fasteners or adhesive bonding completely. In order to ensure a successful joint, the relevant process parameters need to be dialed in and controlled, for specific levels and durations. Induction welding has the advantage of applying a very localized heat, minimizing geometrical distortion of the parts being joined. For the induction welding processes, current and pressure are controlled in an effort to achieve the appropriate temperature at the weld surface with sufficient force to join the two components. Thermocouples are the typical sensors used for temperature measurements, but their size prevents them from being accepted as an inclusion in the final part. As a viable alternative, high definition fiber optic sensing (HD-FOS) is explored as a method for providing a temperature measurement every 1.3 mm along the joint. The small form factor of the sensor lends itself to permanent embedding within the final part. In this work, a high-definition fiber optic sensor is used to provide spatially dense temperature measurements within an induction weld. A control scheme is set up to use the sensor’s measurements as feedback to the controller and to adjust the settings accordingly. This functionality is demonstrated in a dynamic thermoplastic weld setup where the sensor is sandwiched in a lap shear joint configuration. The strength of this weld is evaluated after manufacture and correlated to in-situ temperature measurements. It is shown that HD-FOS could significantly benefit the quality of the final composite part by providing spatially resolved in-situ feedback to the control system to insure uniform temperature profiles at the weld zone and proper processing conditions for each production part.

In-Situ Structural Health Monitoring of Composite-Overwrapped Pressure Vessels
  • ODiSI/HD-FOS
Short Summary
Demonstrates integration of fiber sensor into composite overwrapped pressure vessel as a built-in structural integrity assessment system, potentially eliminating the need for hydrotesting recertification.
Abstract

Currently, most composite pressure vessels must be recertified every 2-5 years via hydrostatic testing to confirm the structural integrity of the pressure vessel. The test requires pressurization in a fluid filled chamber with the global volumetric expansion compared to acceptance criteria. This requirement poses significant cost and time out-of-service issues across many industries. In this work, Luna has teamed with Worthington Industries (WI) - a leading commercial and military supplier of composite air flasks and contracted designer of 30 year flasks, with the objective of providing composite flasks with a built-in structural integrity assessment system that will eliminate the need for hydrotesting recertification. High-definition (HD) distributed strain sensing is used to monitor strain along the axis of circumferentially-wrapped embedded fiber optic sensors in composite-overwrapped pressure vessels (COPVs) during qualification testing and following blunt and highly localized damage events. Luna demonstrates that the use of strain sensors embedded in the composite flask during manufacture will allow rapid assessment of the composite flask structural integrity on-site, while the flask is still mounted in the rack. In addition to the potential for replacing hydrotesting, risk associated with the use of 30 year extended service life flasks will be mitigated by utilizing this efficient health monitoring capability to identify damage and weakened flask structure. The core technology behind Luna’s HD strain measurement systems is Optical Frequency Domain Reflectometry (OFDR) technology, which allows continuous strain measurements at hundreds of gage locations per meter of fiber. Application of the sensor is directly integrated into current flask fabrication methods and the technology utilizes standard telecommunication optical fiber. Therefore, the added cost associated with the embedded sensor and interrogation equipment will be minimal when compared to the recertification costs currently required.

Integrated Fiber Optic Structural Health Sensors for Inflatable Space Habitats
  • ODiSI/HD-FOS
Abstract

Inflatable space habitats offer many advantages for future space missions; however, the long term integrity of these flexible structures is a major concern in harsh space environments. Structural Health Monitoring (SHM) of these structures is essential to ensure safe operation, provide early warnings of damage, and measure structural changes over long periods of time. To address this problem, the authors have integrated distributed fiber optic strain sensors to measure loading and to identify the occurrence and location of damage in the straps and webbing used in the structural restraint layer. The fiber optic sensors employed use Rayleigh backscatter combined with optical frequency domain reflectometry to enable measurement of strain every 0.65 mm (0.026 inches) along the sensor. The Kevlar woven straps that were tested exhibited large permanent deformation during initial cycling and continued to exhibit hysteresis
thereafter, but there was a consistent linear relationship between the sensor’s measurement and the actual strain applied.  Damage was intentionally applied to a tensioned strap, and the distributed strain measurement clearly identified a change in the strain profile centered on the location of the damage. This change in structural health was identified at a loading that was less than half of the ultimate loading that caused a structural failure. This sensing technique will be used to enable integrated SHM sensors to detect loading and damage in future inflatable space habitat structures.

Citation
Osgar John Ohanian, Naman Garg, and Matthew A. Castellucci, “Integrated fiber optic structural health sensors for inflatable space habitats”, Proc. SPIE 10172, March 25, 2017. doi:10.1117/12.2260106
Residual Strains using Integrated Continuous Fiber Optic Sensing in Thermoplastic Composites and Structural Health Monitoring
  • ODiSI/HD-FOS
Abstract

The evolution of spatially resolved internal strain/stress during the manufacturing of thermoplastic composites and subsequent relaxation from water intake are evaluated using an in-situ fiber optic sensor corresponding to a coated optical glass fiber with a nominal diameter of 160 μm. Unidirectional carbon fiber-polyamide 6 composites are produced using compression molding with an embedded fiber optic for strain measurement. The distributed fiber optic based strain sensor is placed in an arrangement to capture 0, 45, and 90° strains in the composite to resolve in-plane strain tensor. Strains are monitored in the direction of fiber optic sensor along its length at high resolution during the various stages of compression molding process. Results indicate considerable internal strains leading to residual stress at the end of processing step along the off-axis (45°) and transverse (90°) directions, and small strains in the carbon fiber pre-preg (0°) direction. At the end of compression molding process, an average of 7000 and 10,000 compressive micro-strains are obtained for residual state of strain in the off-axis and transverse direction. Since water/moisture infusion affects the mechanical properties of polyamide-6 matrix resin, these composite panels with embedded sensors targeted for marine applications are monitored in a water bath at 40 °C simulating accelerated testing conditions. Using the same fiber optic sensor based technique, the strain relaxation was observed during water uptake demonstrating in-situ strain monitoring during both manufacturing and subsequent composite implementation/application environment. The technique presented in this paper shows the potential of optimizing time-temperature-pressure protocols typically utilized in thermoplastic manufacturing, and continuous life-cycle monitoring of composite materials using a small diameter and inexpensive distributed fiber optic sensing.

Citation
Arhant, M., Meek, N., Penumadu, D. et al. Exp Mech (2017). https://doi.org/10.1007/s11340-017-0339-2
Three-Axis Distributed Fiber Optic Strain Measurement in 3D Woven Composite Structures
  • ODiSI/HD-FOS
Short Summary
In the present application optical fiber for distributed sensing was embedded on a commercial weaving loom and used for three-axis strain measurement and subsequent spatial frequency analysis.
Abstract

Recent advancements in composite materials technologies have broken further from traditional designs and require advanced instrumentation and analysis capabilities. Success or failure is highly dependent on design analysis and manufacturing processes. By monitoring smart structures throughout manufacturing and service life, residual and operational stresses can be assessed and structural integrity maintained. Composite smart structures can be manufactured by integrating fiber optic sensors into existing composite materials processes such as ply layup, filament winding and three-dimensional weaving. In this work optical fiber was integrated into 3D woven composite parts at a commercial woven products manufacturing facility. The fiber was then used to monitor the structures during a VARTM manufacturing process, and subsequent static and dynamic testing. Low cost telecommunications-grade optical fiber acts as the sensor using a high resolution commercial Optical Frequency Domain Reflectometer (OFDR) system providing distributed strain measurement at spatial resolutions as low as 2mm. Strain measurements using the optical fiber sensors are correlated to resistive strain gage measurements during static structural loading.

Citation
M.A. Castellucci, S. Klute, E.M. Lally, M.E. Froggatt, D. Lowry. Three-Axis Distributed Fiber Optic Strain Measurement in 3D Woven Composite Structures, SPIE Smart Structures 2013