Engineering topics
Publications and source records attributed to Welch, Christopher S..
Analysis of temperature data from a probe in a vertical Bridgman furnace growing germanium crystals revealed a displacement of the temperature profile due to conduction error. A theoretical analysis shows that the displacement compensation is independent of local temperature gradient. A displacement compensation value should become a standard characteristic of temperature probes used for temperature profile measurements.
A light source directs ultraviolet light onto a test surface and a detector detects a current of photoelectrons generated by the light. The detector includes a collector which is positively biased with respect to the test surface. Quality is indicated based on the photoelectron current. The collector is then negatively biased to replace charges removed by the measurement of a nonconducting substrate to permit subsequent measurements. Also, the intensity of the ultraviolet light at a particular wavelength is monitored and the voltage of the light source varied to maintain the light a constant desired intensity. The light source is also cooled via a gas circulation system. If the test surface is an insulator, the surface is bombarded with ultraviolet light in the presence of an electron field to remove the majority of negative charges from the surface. The test surface is then exposed to an ion field until it possesses no net charge. The technique described above is then performed to assess quality.
Optically stimulated electron emission (OSEE) used in inspection for contamination of critical bonding surfaces in solid rocket motors of Space Shuttle prior to formation of adhesive bonds on surfaces during manufacture and refurbishment. Fundamental OSEE inspection technique described in "Surface-Contamination Inspection Tool for Field Use" (MFS-25581) and "Detecting Contamination With Photoelectron Emission" (MFS-25619). OSEE measurement head easily portable, and measurement operation convenient and rapid, making it useful inspection technique in industrial environment. Reveals contamination in many situations in which other techniques do not work.
The melt-solid interface position was measured during the Bridgman growth of germanium by both temperature profiling and by X-ray imaging. The temperature field of the ampoule, growth material and measurement thermocouples was analyzed by means of numerical analysis. It is shown that both the temperature shift and the discontinuity in temperature gradient is a function of the imposed thermal field and the thermal properties of the thermocouples.
A full-field-thermographic technique for imaging stress patterns in dynamically loaded structures using general purpose IR imaging and image processing hardware is described. The inspection technique is based on the thermoelastic effect. A simple geometry is examined, and the experimentally determined values for the stress invariant are shown to be consistent with theoretical and numerical calculations. The application of full-field-thermographic measurement would ensure that the observed stress field has a common sampling period, thus allowing the observation of rapidly occurring stress anomalies such as the propagation of a fatigue crack. Fatigue crack propagation in two consecutive thermoelastic stress images from an aluminum sample is shown.
A highly reproducible optically stimulated electron emission (OSEE) response steel was achieved in laboratory conditions through suppression of surface photochemistry by replacing air with argon. This permitted the characterization of the OSEE process on D6AC steel as a photodiode. With a low-pressure mercury light source, most of the OSEE current is attributed to photoelectrons released by the 185-nm mercury line with only about 5 percent of the photocurrent associated with the 254-nm line. The forward current-voltage characteristic of the OSEE diode is linear for small fields changing to a square root response at larger fields. These results are all consistent with established theories.
The Solid Rocket Motor Witness Test was undertaken to examine the potential for using thermal infrared imagery as a tool for monitoring static tests of solid rocket motors. The project consisted of several parts: data acquisition, data analysis, and interpretation. For data acquisition, thermal infrared data were obtained of the DM-9 test of the Space Shuttle Solid Rocket Motor on December 23, 1987, at Thiokol, Inc. test facility near Brigham City, Utah. The data analysis portion consisted of processing the video tapes of the test to produce values of temperature at representative test points on the rocket motor surface as the motor cooled down following the test. Interpretation included formulation of a numerical model and evaluation of some of the conditions of the motor which could be extracted from the data. These parameters included estimates of the insulation remaining following the tests and the thickness of the charred layer of insulation at the end of the test. Also visible was a temperature signature of the star grain pattern in the forward motor segment.
A theoretical and experimental analysis of the inspection of reiveted plates using the thermoelastic effect is presented. Theoretically, a model is employed to estimate the thermoelastic response expected from a cracked and uncracked rivet hole. The stress invariant, which causes the thermoelastic response, is calculated for the cracked case using an FEM model and for the uncracked case using a theoretical approximation. The differences in the two thermal responses are reduced when the effects of optical spatial averaging, thermal conduction, and a concentric blind area representing the rivet heat are considered. It is suggested that discrimination between the cracked and uncracked cases for short cracks will improve with increasing frequency of stress oscillation. Measurements are shown for aerospace aluminum samples with and without fatigue cracks propagating from holes in a plate. The effect of mean stress on the thermoelastic response is illustrated, and the results are qualitatively compared with the finite element analysis.
Some of the applications to materials evaluation and property determination of thermographic NDE using digital postprocessing of sequences of thermograms are demonstrated. A generic description is given of the steps used in postprocessing for obtaining material property values.
Research at the NASA/Langley Research Center concerning quantitative NDE of composites and metals is reviewed. The relationship between ultrasonics and polymer cure is outlined. NDE models are presented, which can be used to develop measurement technologies for characterizing the curing of a polymer system for composite materials. The models can be used to determine the glass transition temperature, the degree of cure, and the cure rate. The application of the model to control autoclave processing of composite materials is noted. Consideration is given to the use of thermal diffusion models combined with controlled thermal input measurements to determine the thermal diffusivity of materials. Also, a two-dimensional physical model is described that permits delaminations in samples of Space Shuttle Solid Rocket Motors to be detected in thermograms in the presence of cooling effects and uneven heating.
Thermal Remote Anemometer Device developed for remote, noncontacting, passive measurement of thermal properties of sample. Model heated locally by scanning laser beam and cooled by wind in tunnel. Thermal image of model analyzed to deduce pattern of airflow around model. For materials applications, system used for evaluation of thin films and determination of thermal diffusivity and adhesive-layer contact. For medical applications, measures perfusion through skin to characterize blood flow and used to determine viabilities of grafts and to characterize tissues.
A method of determining thermal diffusivity in thin plates is presented. The method, using infrared images of evolving thermal patterns previously injected with a laser, is noncontacting, one-sided, and remote. It does not require independent estimates of either the emissivity of the sample or the sample thickness. With a line-segment pattern for thermal input, it yields the inplane components of the diffusivity tensor in anisotropic materials and also the rate of heat loss to the environment of the plate. Two methods of data analysis are presented, one corresponding to a heating line of general cross section and the other considering a Gaussian cross section, thereby saving considerable computer time. Both methods produce a statistical evaluation of measurement quality as well as estimates of diffusivity and loss rate. Results are shown for plates of metals and graphite-epoxy composite materials. Principal components and orientation for the diffusivity tensor are obtained in the anisotropic graphite-epoxy sample.