Christopher S. Welch

Remote measurement of in-plane diffusivity components in plates

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 in‐plane 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.

SHORT TERM CHANGES IN THE VERTICAL SALINITY DISTRIBUTION OF THE YORK RIVER ESTUARY ASSOCIATED WITH THE NEAP-SPRING TIDAL CYCLE

A multidisciplinary investigation of hydrographic-nutrient-phytoplankton interactions was undertaken in the lower York River estuary of Virginia during August, 1978. The study centered on a spring tide-associated water column destratification event predicted to occur on or soon after August 19, the date of the maximum monthly spring tide. A station in the lower York River (depth 19 m) was occupied during four different periods, August 7–10, 16–17, 21–24 and 28–30, and temperature and salinity were measured periodically at 1 m depth intervals. During August 16–20 salinities were measured through the water column at seven stations in the York River extending from the mouth to 35 km upriver. During the first two sampling periods the water column was moderately to strongly stratified. Destratification was first observed 15–20 km upriver on August 18 and the lower river was destratified by August 21. Destratification persisted in the lower river for four days at which time increasing bottom salinities indicated the beginning of the restratification process. By August 28 restratification, resulting primarily from an increase in bottom salinities, was complete. The results illustrate the highly dynamic hydrographic nature of this estuarine system and the predictability of the stratification-destratification sequence. The effects of this hydrographic cycle on nutrient distributions, phytoplankton dynamics and benthic nutrient fluxes in this estuary are discussed in accompanying papers.

Thermal Coating Characterization Using Thermoelasticity

Synchronous demodulation techniques have been used for some time to measure thermal radiation produced by the small temperature changes associated with elastic stress in materials, the thermoelastic effect[l]. As reviewed in [2], these measurements have been employed primarily for inferring stress distributions in materials from the associated temperature distributions. Images of stress distribution have been made most often by assembling a large number of individual synchronous component measurements using specialized equipment [3]. Other investigators [4,5] have used subtraction of stroboscopic samples to build up similar images. Recently, techniques using full-field synchronous demodulation [6,7] have been reported using equipment [8] which may be configured to perform a large number of thermal measurements. In this paper, the elements of the measurement system are used to provide information useful in characterizing the thermal properties of a coating on a substrate.

Thermographic Detection of Delaminations in Laminated Structures

The detection of disbonds in a laminated structure is the focus of many nondestructive techniques.One of the promising techniques is thermography, where heat is applied to a structure, and the subsequent temperature profiles are detected with an infrared (IR) imager.If there is an even application of heat, an elevated temperature profile will appear, as a result of the reduction in heat flow from the surface layer to subsurface layers.Two advantages of the thermographic technique over more conventional ultrasonic techniques are that it can be easily made noncontacting and that large areas can be inspected in a short period of time.

Quantitative NDE Applied to Composites and Metals

This paper reviews recent advances at LaRC in quantitative measurement science applied to characterizing materials in a nondestructive environment. Recent demands on NDE have resulted in new thrusts to achieve measurements that represent material properties rather than indications or anomalies in a background measurement. Good physical models must be developed of the geometry, material properties, and the interaction of the probing energy with the material to interpret the results quantitatively. In this paper are presented NDE models that were used to develop measurement technologies for characterizing the curing of a polymer system for composite materials. The procedure uses the changes in ultrasonic properties of the material to determine the glass transition temperature, the degree of cure, and the cure rate. A practical application of this technology is a closed feedback system for controlling autoclave processing of composite materials. An additional example is in the area of thermal NDE. Thermal diffusion models combined with controlled thermal input/measurement have been used to determine the thermal diffusivity of materials. These measurements are remote, require no contact with the material under test and thus have interesting promise for NDE applications.

Material property measurements with post-processed thermal image data

Post-processing of infrared thermal image thta is a technique which finds many uses in a laboratory devoted to non-destructhe evaluation (NDE) of materials. Among these are determination ofmaterial pmperty values and detection/location of delaminations. Exanples are shown in which thermal diffusivity is measured for technique verification, as a verification of the tensor nature of diffusivity measurements and as a proxy for porosity in a test sample of a material under developmenL Another example is given in which the coefficient of thennal expansion is determined through the phenomenon of thermoelasticity. A final example is given in which post-processing extrts the thermal signature of a delamination from an image dominated by an unwanted feature. Following these examples of materials evaluation using post-processing, a set of procedures common to the data analysis in the examples is extracted. Generic requirements are given so that each procedure can operate consistently within the entire process to produce appropriate values of the material characteristics sought.

A Numerical Grid Generation Scheme for Thermal Simulations in Laminated Structures

Significant efforts are being made to improve the safety of the solid rocket motor (SRM) for the shuttle. The SRM is a laminated structure consisting of four layers of materials: a steel casing, bonded to NBR insulation, the liner, and the propellant. One of the candidate inspection techniques is a thermal technique which analyzes the response of the SRM to an external heat source for detection of disbonds at the interfaces between the steel, NBR and fuel. Computational simulations of experimental measurements can provide limits of the effectiveness of the technique and easily assume a variety of different defect geometries to determine their detectability without the expense of making many different samples. Simulations can also provide useful information for the experimenter including the heating protocol that will provide the greatest contrast and the typical flaw size that can be detected.

Thermographic measurement of in-plane diffusivity in very thin plates using diffusion of thermal patterns

We report a measurement of in-plane thermal diffusivity in very thin titanium samples. The measurement uses least squares analysis of radiatively imaged thermal patterns produced by heating samples through a patterned mask from the back side. Multiple flashes from a flash lamp through the mask are used to build up data, during which time the lamps, mask, sample and imager are stationary relative to one another. Coatings to enhance emissivity are avoided because thermal thermal mass can be substantial in comparison with that of the underlying titanium, with resulting alteration of the measured diffusivity value. The length scale is obtained from the analysis, which evaluates, besides diffusivity, the position of the mask pattern in the thermal image, the image scale factor and the sharpness of focus in the particular set of data. By inferring parameters from the data rather than determining them separately, the measurement becomes more robust and easier to apply outside the controlled laboratory environment.

Combined Thermoelastic and Photoelastic Full-Field Stress Measurement

Accurate determination of stress distributions is essential in assessing the structural integrity of a component. Photoelasticity and thermoelasticity are full field nondestructive methods used to measure the stress state of an object.

Numerically Enhanced Thermal Inspection of Shuttle Solid Rocket Motor Inhibitor/Liner/Fuel Bondline

The Solid Rocket Motors (SRM’s) for the Space Shuttle are fabricated by laying up and molding materials, such as insulation, which are joined using adhesives, the adhesives being set along with the materials in a final curing process for an entire motor segment at a time. The resulting structure (Fig. 1) has two important properties from the point of view of inspection: integrity of the adhesive joints (bondlines) is of great importance, and the structure cannot be disassembled for inspection prior to use. Of particular interest are disbonds producing small voids (delaminations) at the bondlines. Specific delaminations can be located in service hardware only by using NDE methods. This paper reports on the application of thermal methods for performing such inspections.