D. Michele Heath

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.

Thermal diffusivity imaging of aerospace materials and structures

Thermographic imaging is increasingly being utilized for inspection and characterization of materials and structures. Thermal diffusivity imaging provides a means for quantitative characterization of a materials which is independent of apparatus and measurement configuration. This enables more precise imaging of variations in the material or structure needed to track changes resulting from fatigue or aging processes.

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.

Heat Source Characterization for Quantitative Thermal Nondestructive Evaluation

One aspect of providing more accurate quantitative thermal inspection systems is the characterization of the lamp heat source. By measuring the heat source temporal profile, the lamp response can then be removed from the measurement for more accurate results. In measuring thermal diffusivity, Parker measured the irradiance history of a flash lamp at discrete wavelengths and approximated the energy of the pulse as a triangle [1]. Recently, this has become of more interest since advances in infrared cameras have now allowed for much higher sampling rates allowing for inspection of very fast transient temperature events. These events can potentially be influenced by the thermal heat flux temporal profile.

Thermal Diffusivity Measurements in Carbon-Carbon Composites

In recent years, carbon-carbon composite materials have come into widespread use in aerospace industries. These materials are particularly attractive for high temperature applications due to their thermal and mechanical behavior. Few quantitative measurements, however, have been made to characterize these materials. One problem encountered with carbon-carbon composites is porosity. Materials engineers have determined that degree of porosity is correlated to inter-laminar shear strength in carbon-carbon composites. Since repetition of the carbon-carbon processing cycle reduces porosity, a technique for assessing porosity between processing cycles that is non-contacting and does not contaminate the material would be of value. A material property which is related to density and therefore to porosity, is thermal diffusivity. Thermal diffusivity is easily measured non-contactingly and remotely with infrared techniques and is therefore an attractive candidate measurement for assessing porosity between processing cycles of carbon-carbon composites.

Thermal diffusivity imaging with a moving line source

The thermal line scan technique has been shown to be an effective technique for rapid inspection of aerospace specimens. Past efforts have focused on thermal measurements far behind the line source where the heat flow normal to the surface is negligible. This paper focuses on measurements closer to the line source to enable the measurement of the thermal diffusivity in the surface normal direction. This measurement also enables an independent characterization of the thermal diffusivity in the direction of motion of the thermal line source. An analytical solution is given for a line source moving with constant velocity across an anisotropic plane. A nonlinear least squares fitting routine is used to reduce the temporal response of a specimen to images of the thermal diffusivity in both the directions normal to the surface and parallel to the motion of the line source. Measurements are presented on specimens with known variations in effective diffusivity. Measurements on these specimens allow a comparison of this technique to more conventional techniques for diffusivity measurement.

QUANTITATIVE THERMAL DIFFUSIVITY IMAGING OF DISBONDS IN THERMAL PROTECTIVE COATINGS USING INDUCTIVE HEATING

Problems of adherence of thermal protective coatings to aerospace structures have become important with their use on vehicles such as the space shuttle and for jet engine turbine blades. In these structures a dielectric coating which acts as a thermal barrier, is bonded to a conductive substrate. Failure of these coatings to adhere to the substrate can result in catastrophic failure of the vehicle. Thermal diffusivity is a parameter that can be used to assess the degree of adherence of coatings to a substrate.