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Winfree, W. P.

Publications and source records attributed to Winfree, W. P..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

Thermography Inspection for Detection and Tracking of Composite Cylinder Damage During Load Testing

Two thermography techniques, passive and active, are used to detect damage initiation and progression in a cyclically loaded composite cylinder. The passive thermography tracks damage progression in real time during cyclic loading. Active flash thermography, using a flash tube enclosed within the cylinder, images delaminations in a cylinder under different loads. A differential thermography processing technique eliminates normal material variations and improves sensitivity to and sizing of delaminations. The thermography results were compared to nonimmersion ultrasonic results.

Zalameda, J. N.↗
NASA NTRS2005

Infrared On-Orbit Inspection of Shuttle Orbiter Reinforced Carbon-Carbon Using Solar Heating

Thermographic nondestructive inspection techniques have been shown to provide quantitative, large area damage detection capabilities for the ground inspection of the reinforced carbon-carbon (RCC) used for the wing leading edge of the Shuttle orbiter. The method is non-contacting and able to inspect large areas in a relatively short inspection time. Thermal nondestructive evaluation (NDE) inspections have been shown to be applicable for several applications to the Shuttle in preparation for return to flight, including for inspection of RCC panels during impact testing, and for between-flight orbiter inspections. The focus of this work is to expand the capabilities of the thermal NDE methodology to enable inspection by an astronaut during orbital conditions. The significant limitations of available resources, such as weight and power, and the impact of these limitations on the inspection technique are discussed, as well as the resultant impact on data analysis and processing algorithms. Of particular interest is the impact to the inspection technique resulting from the use of solar energy as a heat source, the effect on the measurements due to working in the vacuum of space, and the effect of changes in boundary conditions, such as radiation losses seen by the material, on the response of the RCC. The resultant effects on detectability limits are discussed. Keywords: Nondestructive Evaluation, Shuttle, on-orbit inspection, thermography, infrared

Howell, P. A.↗
NASA NTRS2005

The Application of Infrared Thermographic Inspection Techniques to the Space Shuttle Thermal Protection System

The Nondestructive Evaluation Sciences Branch at NASA s Langley Research Center has been actively involved in the development of thermographic inspection techniques for more than 15 years. Since the Space Shuttle Columbia accident, NASA has focused on the improvement of advanced NDE techniques for the Reinforced Carbon-Carbon (RCC) panels that comprise the orbiter s wing leading edge. Various nondestructive inspection techniques have been used in the examination of the RCC, but thermography has emerged as an effective inspection alternative to more traditional methods. Thermography is a non-contact inspection method as compared to ultrasonic techniques which typically require the use of a coupling medium between the transducer and material. Like radiographic techniques, thermography can be used to inspect large areas, but has the advantage of minimal safety concerns and the ability for single-sided measurements. Principal Component Analysis (PCA) has been shown effective for reducing thermographic NDE data. A typical implementation of PCA is when the eigenvectors are generated from the data set being analyzed. Although it is a powerful tool for enhancing the visibility of defects in thermal data, PCA can be computationally intense and time consuming when applied to the large data sets typical in thermography. Additionally, PCA can experience problems when very large defects are present (defects that dominate the field-of-view), since the calculation of the eigenvectors is now governed by the presence of the defect, not the "good" material. To increase the processing speed and to minimize the negative effects of large defects, an alternative method of PCA is being pursued where a fixed set of eigenvectors, generated from an analytic model of the thermal response of the material under examination, is used to process the thermal data from the RCC materials. Details of a one-dimensional analytic model and a two-dimensional finite-element model will be presented. An overview of the PCA process as well as a quantitative signal-to-noise comparison of the results of performing both embodiments of PCA on thermographic data from various RCC specimens will be shown. Finally, a number of different applications of this technology to various RCC components will be presented.

Cramer, K. E.↗
NASA NTRS2004

Temporal Treatment of a Thermal Response for Defect Depth Estimation

Transient thermography, which employs pulse surface heating of an inspected component followed by acquisition of the thermal decay stage, is gaining wider acceptance as a result of its remoteness and rapidness. Flaws in the component s material may induce a thermal contrast in surface thermograms. An important issue in transient thermography is estimating the depth of a subsurface flaw from the thermal response. This improves the quantitative ability of the thermal evaluation: from one scan it is possible to locate regions of anomalies in thickness (caused by corrosion) and estimate the implications of the flaw on the integrity of the structure. Our research focuses on thick composite aircraft components. A long square heating pulse and several minutes observation period are required to receive an adequate thermal response from such a component. Application of various time-related informative parameters of the thermal response for depth estimation is discussed. A three-dimensional finite difference model of heat propagation in solids in Cartesian coordinates is used to simulate the thermographic process. Typical physical properties of polymer graphite composites are assumed for the model.

Plotnikov, Y. A.↗
NASA NTRS2000

Thermographic Imaging of Defects in Anisotropic Composites

Composite materials are of increasing interest to the aerospace industry as a result of their weight versus performance characteristics. One of the disadvantages of composites is the high cost of fabrication and post inspection with conventional ultrasonic scanning systems. The high cost of inspection is driven by the need for scanning systems which can follow large curve surfaces. Additionally, either large water tanks or water squirters are required to couple the ultrasonics into the part. Thermographic techniques offer significant advantages over conventional ultrasonics by not requiring physical coupling between the part and sensor. The thermographic system can easily inspect large curved surface without requiring a surface following scanner. However, implementation of Thermal Nondestructive Evaluations (TNDE) for flaw detection in composite materials and structures requires determining its limit. Advanced algorithms have been developed to enable locating and sizing defects in carbon fiber reinforced plastic (CFRP). Thermal Tomography is a very promising method for visualizing the size and location of defects in materials such as CFRP. However, further investigations are required to determine its capabilities for inspection of thick composites. In present work we have studied influence of the anisotropy on the reconstructed image of a defect generated by an inversion technique. The composite material is considered as homogeneous with macro properties: thermal conductivity K, specific heat c, and density rho. The simulation process involves two sequential steps: solving the three dimensional transient heat diffusion equation for a sample with a defect, then estimating the defect location and size from the surface spatial and temporal thermal distributions (inverse problem), calculated from the simulations.

Plotnikov, Y. A.↗
NASA NTRS1995

New nondestructive techniques for the detection and quantification of corrosion in aircraft structures

An overview is presented of several techniques under development at NASA Langley Research Center for detection and quantification of corrosion in aircraft structures. The techniques have been developed as part of the NASA Airframe Structural Integrity Program. The techniques focus on the detection of subsurface corrosion in thin laminated structures. Results are presented on specimens with both manufactured defects, for calibration of the techniques, and on specimens removed from aircraft.

Winfree, W. P.↗
NASA NTRS1992

Assessment of aircraft structural integrity by detecting disbonds through ultrasonic scanning

A study of nondestructive evaluation of aircraft bonded joints using the contact scanning technique is presented. Reflected signals acquired through the contacting transducer characterize the test region as being bonded or disbonded. Ultrasonic signals are attenuated more rapidly in the bonded regions when compared to disbonded regions. A peak amplitude based method and an artificial neural network are used to classify the signals. Results obtained using an artificial neural network exhibited significant insensitivity to signal variation when compared to the peak amplitude. Very good agreement is observed between results obtained using the present technique and those obtained using immersion scanning.

Abedin, M. N.↗
NASA NTRS1992

Automation of disbond detection in aircraft fuselage through thermal image processing

A procedure for interpreting thermal images obtained during the nondestructive evaluation of aircraft bonded joints is presented. The procedure operates on time-derivative thermal images and resulted in a disbond image with disbonds highlighted. The size of the 'black clusters' in the output disbond image is a quantitative measure of disbond size. The procedure is illustrated using simulation data as well as data obtained through experimental testing of fabricated samples and aircraft panels. Good results are obtained, and, except in pathological cases, 'false calls' in the cases studied appeared only as noise in the output disbond image which was easily filtered out. The thermal detection technique coupled with an automated image interpretation capability will be a very fast and effective method for inspecting bonded joints in an aircraft structure.

Prabhu, D. R.↗
NASA NTRS1992

Application of artificial neural networks to thermal detection of disbonds

A novel technique for processing thermal data is presented and applied to simulation as well as experimental data. Using a neural network of thermal data classification, good classification accuracies are obtained, and the resulting images exhibit very good contrast between bonded and disbonded locations. In order to minimize the preprocessing required before using the network of classification, the temperature values were directly employed to train a network using data from an on-site testing run of a commercial aircraft. Training was extremely fast, and the resulting classification also agreed reasonably well with an ultrasonic characterization of the panel. The results obtained using one sample show the partially disbonded vertical doubler. The vertical lines along the doubler correspond to the original extent of the doubler obtained using blueprints of the aircraft.

Prabhu, D. R.↗
NASA NTRS1992

A comparative study of experimental and simulated ultrasonic pulse-echo signals from multilayered structures

The effect on the system acoustic response of variations in the adhesive thickness, coupling thickness, and paint thickness is considered. Both simulations and experimental measurements are used to characterize and classify A-scans from test regions, and to study the effects of various parameters such as paint thickness and epoxy thickness on the variations in the reflected signals. A 1D model of sound propagation in multilayered structures is used to verify the validity of the measured signals, and is also used to computationally generate signals for a class of test locations with gradually varying parameters. This approach exploits the ability of numerical simulations to provide a good understanding of the ultrasonic pulses reflected at disbonds.

Abedin, M. N.↗
NASA NTRS1990

Enhanced ultrasonic attenuation of Tl-Ca-Ba-Cu-O at the superconducting transition

Temperature-dependent ultrasonic attenuation measurements have been performed on polycrystalline samples of Tl-Ca-Ba-Cu-O at various frequencies. An attenuation anomaly was observed close to the superconducting transition temperature T(c) (103 K) whose temperature position was frequency-independent. The attenuation decreased exponentially with temperature in the superconducting state for both transverse and longitudinal waves. It was also found that the temperature-dependent sound velocity showed a slope change around T(c).

Sun, K. J.↗
NASA NTRS1989

Boundary estimation problems arising in thermal tomography

Problems on the identification of two-dimensional spatial domains arising in the detection and characterization of structural flaws in materials are considered. For a thermal diffusion system with external boundary input, observations of the temperature on the surface are used in a output least squares approach. Parameter estimation techniques based on the method of mappings are discussed and approximation schemes are developed based on a finite element Galerkin approach. Theoretical convergence results for computational techniques are given and the results are applied to experimental data for the identification of flaws in the thermal testing of materials.

Banks, H. T.↗
NASA NTRS1989

Relaxation behavior of ultrasonic attenuation in YBa2Cu3O7

It is shown that temperature-dependent ultrasonic attenuation data of YBa2Cu3O7 at various frequencies exhibit anomalies at temperatures close to Tc. These attenuation maxima are found to be the result of a relaxation mechanism added on top of an unusual attenuation background. It is proposed that the grain boundary motions induced by the structural distortion and the propagation of sound waves enhances the energy dissipation around Tc. Whether this structural distortion is the consequence of the onset of a superconducting state remains undetermined. It is also possible that either a tunneling effect or the acoustoelectric effect contributes to sound energy dissipation. The temperature dependence of ultrasonic velocity shows a softening around Tc which may be an intrinsic property of high-temperature superconductors.

Sun, K. J.↗
NASA NTRS1986

Measurement of the degree of cure in epoxies with ultrasonic velocity

The use of ultrasonic longitudinal velocity values to measure the degree of cure (defined for an epoxide system as the concentration of epoxide/amine bonds divided by the initial epoxide concentration) in epoxy resins is investigated. The experimental setup used to measure the changes in longitudinal velocity with time is described, together with the technique used to calculate the degree of cure from the acoustic data, using the principle of additive module. Measurements were done with diglycidyl ether of bisphenol A epoxy resin cured with an amine adduct agent. Good qualitative agreement was shown between the time dependence of the acoustically measured degree of cure and the predicted rate of reaction.

Winfree, W. P.↗
NASA NTRS1986

Acoustic characterization of composite cure

The relationship between the time evolution of ultrasonic longitudinal velocities, measured in the course of the cure of 3501-6 neat and fiber-reinforced resin, and the time evolution of the degree of cure was studied. The resin samples were placed in a mold, one side of which was a graphite plate, while the rest was made of stainless steel. After placing the mold in a press and applying a single-cycle 5-MHz pulse to the transducer bonded to the outer side of the graphite plate, the echo resulting from the resin-graphite interface and the subsequent echoes from within the resin layer were recorded with a 200-MHz, 8-bit digitizer. The degree of cure was calculated from the acoustic response, using the relationships derived by Winfree and Parker (1986). The calculated degree of cure had the expected time dependence, and the agreement between the degree of cure measured for the neat resin and the prepreg was reasonable.

Parker, F. R.↗
NASA NTRS1986

Quantitative thermal diffusivity measurements of composites

A remote radiometric technique for making quantitative thermal diffusivity measurements is described. The technique was designed to make assessments of the structural integrity of large composite parts, such as wings, and can be performed at field sites. In the measurement technique, a CO2 laser beam is scanned using two orthogonal servo-controlled deflecting mirrors. An infrared imager, whose scanning mirrors oscillate in the vertical and the horizontal directions, is used as the detector. The analysis technique used to extract the diffusivity from these images is based on a thin infinite plate assumption, which requires waiting a given period of time for the temperature to equilibrate throughout the thickness of the sample. The technique is shown to be accurate to within two percent for values of the order of those for composite diffusivities, and to be insensitive to convection losses.

Heath, D. M.↗
NASA NTRS1986

Quantitative thermal characterization of thin plates

The principles of quantitative thermal testing of the mechanical integrity of platelike structures are discussed. In this technique, a scanned laser pulse is used to inject a thermal pattern in a sample, and a thermal IR imager is used to document the evolution of the pattern subsequent to injection. The measurements are remote, noncontacting, and one-sided, and the algorithms used to obtain diffusivity are all emissivity-independent. The results yielded by application of the three analytical techniques, used to obtain diffusivity values from the image data on stainless steel, a graphite-epoxy composite, brass, and aluminum 2024 samples, are presented.

Welch, C. S.↗