L. C. Aamodt

Measurement of coating physical properties and detection of coating disbonds by time-resolved infrared radiometry

This paper describes the principles and applications of time-resolved infrared radiometric (TRIR) imaging to characterization of coating systems. Examples are given of its application to the measurement of coating properties such as thickness and thermal diffusivity and to the detection of regions of coating disbond. Results are shown for coatings of different thicknesses, for test specimens containing artificial disbonds, and for thermal barrier coating specimens exhibiting real disbonds. A theoretical model describing the time development of the surface temperature of a coating during step heating is presented and the experimental results show good agreement with this model. Methods for applying the technique for inspection of large areas of coating as would be required in a process control or in service inspection environment are discussed and examples of parallel data acquisition using line heating sources are presented.

Image Contrast Processes in Thermal and Thermoacoustic Imaging

Recent developments in thermal wave or more generally modulated source imaging using laser-, electron-, and ion-beam sources are discussed. Thermal wave imaging is further classified on the basis of the type of detection process used, and recent work relating to contrast mechanisms in the several classes of thermal imaging is discussed.

Time-Resolved Infrared Radiometry (Trir) for Characterization of Impact Damage in Composite Materials

A quantitative thermographie NDE technique for the characterization of impact damage in composite materials is under development along with supporting theoretical analysis. We have previously shown that the technique of time-resolved infrared radiometry (TRIR) is an effective method for quantitatively detecting coating thickness variations and for characterizing the degree of coating disbonding in terms of equivalent air gaps [1,2]. Here we extend the TRIR technique to the study of composite systems by applying the results of a multilayer analytical model [3]. Experimental results in both simple and hybrid composite systems are discussed. The depth and lateral extent of interlaminar separation in composites subjected to impact loading is presented and the use of lateral heat flow techniques to image defect structures is examined.

Time-resolved infrared radiometry of multilayer organic coatings using surface and subsurface heating

The thermal inspection technique of time-resolved infrared radiometry (TRIR) has been developed as a nondestructive characterization method for examination of multilayer materials systems. Applications of the technique include detecting defects such as disbonds in protective coatings which could lead to coating failure. This technique is an extension of earlier thermal wave imaging techniques developed for examination of material microstructure and defects. The TRIR technique is totally noncontacting and allows quantitative measurements to be made of both the coating thickness and the integrity of the bond between the coating and the substrate within the same measurements. In the present work we review the basics to the TRIR technique and examine the use of the TRIR technique for inspection of organic coatings.

Microwave thermoreflectometry for detection of rebar corrosion

A microwave-based approach under development for detecting corrosion of rebar is described. The rebar inside the concrete is heated with an induction heater and then the surface temperature of the rebar inside the concrete is probed using a microwave reflectance method. This is in contrast to infrared thermographic approaches which monitor the surface temperature of the concrete and are dependent on waiting for considerable lengths of time for heat flow from the rebar to the concrete surface. Results will be presented for a series of test specimens produced by deliberately corroding rebar inside concrete in the laboratory. Microwave thermoreflectance measurements made in a 5 second measurement time are compared with conventional thermographic measurements of the temperature distribution at the concrete surface which require a 10 minute measurement time. Theoretical results are also presented of the predicted temperature versus time curves expected for rebar inside concrete with and without air defects at the rebar-concrete interface. These results predict that a rebar-concrete interface could be distinguished from a rebar-air interface with only 1 second of heating. The theoretical results further show that the presence of an air layer of finite thickness between rebar and concrete after about 2 seconds could be detected with a 2 second heating time.

Time-resolved infrared radiometry (TRIR) using a focal-plane array for characterization of hidden corrosion

A time-resolved infrared radiometry technique is developed which is applicable to detection of hidden corrosion in airframe structures and which implements an InSb focalplane array. The TRIR method can measure the loss of skin thickness due to corrosion and can also detect second layer corrosion when a sealant material is present at a lap joint. It has also been shown that the presence of corrosion product can be detected on the back surface of an aluminum plate free standing in air. Other experimental studies suggest that the presence of corrosion product, corrosion thinning of the plate and morphology changes at the aluminum-corrosion layer interface all affect the TRIR temperature-time signature. This raises the possibility of separating these contributions experimentally.

Source patterning in time-resolved infrared radiometry of composite structures

A quantitative thermographic NDE technique for the characterization of composite materials is under development along with supporting theoretical analysis. The TRIR technique differs from other pulsed thermography techniques in that the surface temperature of the specimen is monitored as a function of time during the application of a step heating pulse to the sample. Full-field images with temporal resolution faster than video frame rates are acquired with an infrared scanner by disabling the vertical galvanometer in the infrared camera and scanning the heating source across the sample. Alternatively, an InSb focal plane array is used to acquire the time-resolved images. The geometry of the heating source is selected to optimize the acquisition of information about the structure of composite materials. Experimental results in both simple and hybrid composite systems are discussed. The depth and lateral extent of interlaminar separation in composites subjected to impact loading are presented and the use of lateral heat flow techniques to image vertical defect structures is examined.

Determination of Degree of Thermal Barrier Coating Disbonding by Time-Resolved Infrared Radiometry (TRIR)

We have previously described the time-resolved infrared radiometry (TRIR) technique and demonstrated how both measurement of coating thickness and detection of coating disbonding can be made within the same measurement [1]. We have also reported indications of the sensitivity of the TRIR technique to differing degrees of coating disbonding [2]. In the present work we examine this question in detail and compare TRIR experimental results with both an analytical multilayer theory and a destructive analysis of the coating-substrate interface. While the material system studied in this work is a zirconia thermal barrier coating on a superalloy substrate, the methodology and analytical basis of the technique are applicable to a wide variety of materials and components including printed circuit boards and composite materials. The capability of the TRIR technique for characterization of these specific systems will depend on details of the thermal properties and layer thickness in these specimens.

Characterization of Hidden Airframe Corrosion by Time-Resolved Infrared Radiometry (TRIR)

Since hidden corrosion is expected to be the primary factor limiting the service life of military and civilian aircraft, the problem of detection of hidden corrosion needs to be solved to allow for life extension. A quantitative thermographic NDE technique for the characterization of hidden corrosion is under development along with supporting theoretical analysis. In earlier work [1,2] we have shown that the technique of time-resolved infrared radiometry (TRIR) is an effective method for quantitatively detecting coating thickness variations and for characterizing the degree of coating disbonding in terms of equivalent air gaps. In this paper we examine the applicability of TRIR techniques to the characterization of corrosion damage in airframes by investigating plate specimens of 2024-T3 aluminum with both milled defects and corroded regions produced by an accelerated corrosion test.

Time-Resolved Infrared Radiometry for Characterization of Structured Materials

Time resolved radiometry is demonstrated both theoretically and experimentally as a parallel, noncontacting method for quantitative imaging of laterally and vertically heterogeneous systems such as composites and coatings systems.