Advanced Thermal Inspection with Pulsed Light Emitting Diodes (PLED) Technology
Sensors
Advanced Thermal Inspection with Pulsed Light Emitting Diodes (PLED) Technology (LAR-TOPS-392)
A high-precision, non-destructive evaluation solution for low-emissivity surfaces
Overview
Thermal nondestructive evaluation (NDE) is a widely used method for detecting defects such as cracks, corrosion, and dis-bond layers in metallic and composite structures. Traditional thermal inspection methods rely on a high-intensity, broadband light heat source (e.g., flash lamp, quartz lamp) that generates heat that is absorbed by the material, and an infrared camera captures the transient thermal response to generate inspection data. However, inspecting low emissivity surfaces (such as unpainted aluminum and titanium alloys) poses challenges including high reflection of the heat source light that can cause inaccurate measurement of the surface temperature response, produce false defect indications, and potential sensor damage due to high-intensity reflections.
In response to this challenge, engineers at NASA Langley Research Center (LaRC) have developed a cutting-edge thermal inspection technology that enhances defect detection on low-emissivity surfaces by eliminating false readings caused by infrared reflections. By using a spectrally narrow, visible-band pulsed light emitting diodes (PLED) heat source and optical filters, this system prevents interference with infrared camera measurements, delivering a higher accuracy, improved defect contrast, and more cost-effective alternative to conventional flash thermography.
The Technology
NASA’s PLED thermal inspection system consists of an array of high- powered LED chips configured to deliver controlled pulses of visible light. The system includes 8 LED chip arrays, mounted on an aluminum heat sink and housed in a hood configuration. The inspection hood is specially designed with filters to prevent internal reflections. The LEDs are powered by regulated power supplies and controlled via a computer interface that synchronizes heat pulses with an infrared camera. An acrylic filter is placed over the LEDs to block residual infrared radiation, ensuring that only visible light reaches the target surface. The system’s infrared camera, operating in the mid-wave infrared (MWIR) range does not detect the visible light and captures the transient thermal response of the material, allowing for precise defect detection. By eliminating the need for high-intensity broadband infrared sources, the PLED system provides a cleaner and more accurate thermal response, particularly for unpainted metals and additively manufactured (AM) components.
Performance validation of the PLED system has demonstrated significant advantages over traditional flash thermography. In tests on aluminum samples with material loss and AM Ti-6Al-4V metal specimens, the PLED system successfully detected defects with superior contrast and no heat source reflections. Principal Component Analysis (PCA) applied to PLED inspection data revealed clearer defect indications compared to flash-based methods, which introduced unwanted artifacts due to transient reflections. Additionally, the PLED system enabled quantitative thermal diffusivity measurements, offering a new approach to single-sided material characterization.
NASA's PLED thermal inspection technology is available for patent licensing. Potential applications include corrosion detection in aerospace components, quality control of AM metal parts, structural health monitoring of industrial materials, and more.
Benefits
- Eliminates reflection artifacts: The visible-band PLED heat source does not produce infrared emissions detectable by the camera, reducing false defect indications.
- Enhanced defect detection: Provides sharper, higher-contrast thermal images, improving accuracy in identifying material loss, cracks, and porosity.
- Improved accuracy for low-emissivity surfaces: Unlike flash lamps, the PLED system is effective on unpainted aluminum and titanium surfaces without requiring emissivity-enhancing coatings.
- Minimized thermal transients: Unlike flash thermography, the PLED system does not introduce unwanted cooling effects that can distort results.
- Energy efficient and cost-effective: NASA’s PLED-based thermal inspection system has the potential to be 50% or less in cost relative to traditional flash lamp-based thermography.
- Customizable heat delivery: The computer-controlled pulse duration allows for fine-tuned heating for different materials and defect depths.
- No burn-in risk for IR cameras: Unlike flash lamps, the PLED system does not produce intense infrared reflections that can damage camera sensors.
Applications
- Aerospace & aviation: Inspection of aircraft components for corrosion, cracks, and composite structural damage.
- Additive manufacturing (AM) quality control: Detection of porosity and fusion defects in 3D-printed metal parts.
- Automotive & transportation: Evaluating bonding integrity, weld defects, and corrosion in vehicle materials.
- Energy & power generation: Inspecting pipelines, turbines, and heat exchangers for material degradation.
- Infrastructure & construction: Non-destructive testing (NDT) of bridges, steel structures, and composite panels.
- Medical device manufacturing: Ensuring structural integrity of metallic implants and surgical tools.
- Defense & military applications: Inspection of aircraft, naval, and armored vehicle components for hidden defects or material degradation.
Technology Details
Sensors
LAR-TOPS-392
LAR-20441-1
“Thermal Inspection of Low Emissivity Surfaces Using a Pulsed Light Emitting Diodes (PLED) Heat Source,” Joe Zalameda, Peter Spaeth, and Samuel Hocker, NASA Langley Research Center, 04/21/2024. https://ntrs.nasa.gov/citations/20240003951
Similar Results
In-situ Characterization and Inspection of Additive Manufacturing Deposits using Transient Infrared Thermography
Additive manufacturing or 3-D printing is a rapidly growing field where solid, objects can be produced layer by layer. This technology will have a significant impact in many areas including industrial manufacturing, medical, architecture, aerospace, and automotive. The advantages of additive manufacturing are reduction in material costs due to near net shape part builds, minimal machining required, computer assisted builds for rapid prototyping, and mass production capability. Traditional thermal nondestructive evaluation (NDE) techniques typically use a stationary heat source such as flash or quartz lamp heating to induce a temperature rise. The defects such as cracks, delamination damage, or voids block the heat flow and therefore cause a change in the transient heat flow response. There are drawbacks to these methods.
Enhanced Software Suite Maximizes Non-Destructive Evaluation (NDE) Methods
This technology provides comprehensive, detailed, and accurate NDE detection and characterization of subsurface defects in composites and some metallic hardware. This complete software suite normalizes and calibrates the data, which provides more stable measurements and reduces the occurrence of errors due to the operator and to camera variability.
When using flash IR thermography to evaluate materials, variations in the thermal diffusivity of the material manifest themselves as anomalies in the IR image of the test surface. Post-processing of this raw IR camera data provides highly detailed analysis of the size and characterization of anomalies. The newly incorporated Transient and Lock-In Thermography methods allow the analysis of thicker material and with better flaw resolution than Flash Thermography alone.
The peak contrast and peak contrast time profiles generated through this analysis provide quantitative interpretation of the images, including detailed information about the size and shape of the anomalies. The persistence energy and persistence time profiles provide highly sensitive data for detected anomalies. Peak contrast, peak time, persistence time, and persistence energy measurements also enable monitoring for flaw growth and signal response to flaw size analysis.
This technology is at a technology readiness level (TRL) of 7 (system prototype demonstration in an operational environment), and the innovation is now available for your company to license. Please note that NASA does not manufacture products itself for commercial sale.
Large Area Structural Damage Nondestructive Evaluation
This technology provides a methodology to measure damage onset and grow at multiple locations in a composite structure during fatigue loading. The thermography inspection is non-contact and can cover very large areas. The acoustic emission sensors require contact at only the sensor attachment points and can cover large areas. The acoustic emission measurement is very sensitive to damage formation events such as matrix cracking, fiber breaks, and delamination, however the event location is approximate. An infrared camera is able to detect damage growth and location at areas of heating (due to fiber breaks, rubbing of disbond areas and matrix cracks) and thus confirm the acoustic emission measurements. If the loading is cyclic the infrared camera is also able to detect the relative depth of the damage. Combining both technologies helps to reduce false indications, confirm damage growth areas and where ultimate failure will occur. This provides a measurement capability to detect growing damage (location and size) for improved structures testing or during in-service applications. Multiple infrared (IR) cameras and multiple acoustic emission sensors can be employed internally or externally for full coverage of the structure. When significant damage growth is detected, the structure can be taken out of service for repair or for further inspections. This technology has been demonstrated for structures testing. In-situ NDE inspections are necessary to provide structural engineers a tool to incrementally control and document damage growth as a function of fatigue cycles before failure. This allows for the comparison of NDE results to develop and validate progressive damage analysis (PDA) models. The ultimate goal is to use the validated PDA models to decrease the time required to certify composite structures and therefore save development costs. Real time NDE can document the progression of damage and provide the documentation of ultimate failure mechanisms.
Method of Non-Destructive Evaluation of Composites
Guided wavefield techniques require excitation of guided waves in a specimen via contact or noncontact methods (such as attached piezoelectric transducers or laser generation). The resulting wavefield is recorded via noncontact methods such as laser Doppler vibrometry or air-coupled ultrasound. If the specimen contains damage, the waves will interact with that damage, resulting in an altered wavefield (compared to the pristine case). When guided wave modes enter into a delaminated region of a composite the energy is split above/below delaminations and travels through the material between delaminations. Some of the energy propagates beyond the delamination and re-emerges as the original guided wave modes. However, a portion
of the wave energy is trapped as standing waves between delaminations. The trapped waves slowly leak from the delaminated region, but energy remains trapped for some time after the incident waves have propagated beyond the damage region.
Simulation results show changes in the trapped energy at the composite surface when
additional delaminations exist through the composite thickness. The results are a
preliminary proof-of-concept for utilizing trapped energy measurements to identify
the presence of hidden delaminations when only single-sided access is available to a
component/vehicle. Currently, no other single-sided field-applicable NDT techniques
exist for identifying hidden delamination damage.
Luminescence-Based Temperature Mapping and Sensing Systems
In order to obtain real-time temperature measurements with this technology, the phosphor can either be applied as a coating onto the surface of the object for temperature mapping, or incorporated onto a sensor and attached to the end of a fiber-optic probe for local temperature measurements. Next, the object is exposed to a pulsed light source, which causes the temperature-sensing phosphor to produce an ultra-bright broadband emission. An algorithm is used to evaluate post-pulse emission decay time, which is then converted to a precise temperature reading.
State-of-the-art temperature sensing phosphors suffer from thermal quenching, or unacceptable loss of signal intensity as temperature increases. Alternative high-temperature measurement systems such as thermocouples and pyrometers, offer only spot measurements. Moreover, thermocouples suffer from attachment issues and electromagnetic interference, while pyrometers frequently suffer from reflected radiation interference and unknown surface emissivity. Cr:GdAlO3, which retains ultra-bright emission intensity to temperatures well above 1000°C, provides an ideal solution to all of these issues with a few added benefits. It has a perovskite structure that is both non-reactive and stable in high-temperature environments. Furthermore, it possesses a favorable electron energy level spacing that enables this sensor to maintain stronger signal intensity than its competitors. Due to the broad absorption and emission bands for Cr:GdAlO3 there is considerable flexibility in the choice of excitation and emission wavelength detection bands. The combination of these factors makes this technology a clear choice for luminescence-based optical high-temperature sensing for a variety of industries from aerospace to manufacturing.
