Jane W. Maclachlan Spicer

Analysis methods for full-field time-resolved infrared radiometry

A data analysis procedure is described to enable quantitative information about defect depth and surface and subsurface thermal properties to be obtained from the time series of images acquired in a time-resolved infrared radiometry (TRIR) measurement. While the approaches described have been previously considered for single point measurements, in this work the algorithms are applied to full field images. As a result, images presenting defect depth and amount of thermal mismatch at subsurface interfaces can be constructed. Results are presented for composite test panels with flat-bottomed holes milled to different depths, two-layer specimens with differing thermal properties between the top layer and the substrate and a graphite/epoxy-honeycomb composite panel with simulated delaminations.

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.

Detection of microwave emission from solid targets ablated with an ultrashort pulsed laser

In addition to visible and near-IR emission, recent investigations have shown that electromagnetic pulses (EMP) in the microwave and RF regions of the spectrum are generated during femtosecond laser-matter interactions if the laser source is sufficiently intense to ablate and ionize an illuminated solid target material. Although the mechanisms for the laserinduced EMP pulse are not fully characterized, it is reported that this phenomenon arises from two mechanisms associated with terawatt to petawatt level laser interactions with matter: (1) ionization via propagation in air, and (2) plasma generation associated with the laser-excited solid material. Over the past year, our group has examined the microwave emission profiles from a variety of femtosecond laser ablated materials, including metals, semiconductors, and dielectrics. We have directed our measurements towards the characterization of microwave emission from ablated surfaces in air using laser peak powers in excess of 1012 Watts (energy/pulse ~50 mJ, pulse width ~30 fs, laser diameter at target ~200 microns). We have characterized the temporal profile of the microwave emission and determined the emission from all samples is omni-directional. We have also observed a difference in the minimum fluence required to generate emission from conducting and insulating materials although the peak amplitudes from these materials were quite similar at the upper laser energy levels of our system (~50 mJ).

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.

Enhancement of electromagnetic pulse emission from ultrashort laser pulse irradiated solid targets

Ultrashort laser pulses (~100 fs duration) are known to generate charge separation in solid, liquid and gas targets through a variety of nonlinear mechanisms. This process results in the emission of a broadband electromagnetic pulse (EMP) in the microwave and terahertz (THz) regions of the electromagnetic spectrum. Possible applications of this phenomenon include remote RF and THz generation for material detection and diagnostics. We investigate the energy and spectrum of the EMP emitted from copper and glass targets irradiated by single 800 nm, 38 fs duration pulses with varying energy. The detector is two feet from the target and the detection bandwidth is 2-18 GHz. We also demonstrate our ability to enhance the emitted EMP energy from a copper target by more than an order of magnitude by irradiating the target with a 1064 nm, 14 ns duration pulse at a specific time delay relative to the ultrashort pulse. We attribute the increased optical to RF energy conversion to enhanced absorption of the ultrashort pulse by the nanosecond pulse-generated plasma at the surface of the target.

Thermal inspection of SiC/SiC ceramic matrix composites

Measurements of both through-thickness and lateral thermal diffusivities are described for a SiC/SiC ceramic matrix composite material. The measurement approach is one-sided and is based on the time-resolved IR radiometry method in which the specimen is illuminated with a step heating pulse and the resulting temperature rise is monitored with an IR focal plane array. An area heating source is used for the through-thickness measurement of thermal diffusivity and a line source is used for the lateral thermal diffusivity measurement. The result indicate that the thermal diffusivity values obtained using a model assuming surface absorption and emission have to be corrected since the composites show some IR transparency accompanied by strong scattering. Additional measurements are presented on CMC components structures with possible porosity at bonds.

Microwave-source time-resolved infrared radiometry for monitoring of curing and deposition processes

Thermal wave methods employing optical heating sources have been used successfully in the past to determine layer thicknesses or thermal diffusivities. However, monitoring of curing or deposition processes can be difficult with such techniques due to changes in the sample surface properties such as optical absorption during processing. The method introduced in this paper, microwave-source time-resolved infrared radiometry (TRIR) in conjunction with intentionally embedded carbon or metal fibers allows the determination of surface layer thickness and thermal diffusivity almost independently of the surface properties of the layer. This suggests the use of fibers as embedded sensors in applications where layer thickness or thermal properties need to be controlled during processing.

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.

Comparison of ultrasonic, microwave, and photothermal imaging of defective graphite-epoxy composite panels

We compare results of ultrasonic, microwave, and thermal imaging of graphite-epoxy composite panels with artificial defect structures.