Quadruple Simulations of Thermographic Inspections of Impacted Composites

Thermography has been shown to be a viable technique for inspection of composites. Impact damage in composites typically contains multiple overlapping delaminations at different depths. Understanding the limitations of the thermographic inspection is enhanced by performing simulations of the technique. Most simulations of composite thermographic inspections have focused on simulations of a single delamination at a fixed depth. The quadrupole method has been shown as a viable technique for rapid three-dimensional thermographic simulations of a delamination. This method is expanded to enable rapid simulation of multiple overlapping delaminations at different depths. Quadrupole simulations are compared to finite element simulations of multiple delaminations at different depths. The simulations are also compared to the thermographic measurements on impacted composites where shape and depth of the delaminations are known from x-ray computed tomography data

Numerical Solutions for Heat Flow in Adhesive Lap Joints

The detection of disbonds in riveted lap joints is of increasing interest to the aerospace community. Adhesively bonded and riveted lap joints are used to bond the thin overlapped sheets of aluminum which comprise the outer skin of an aircraft. Through time, the integrity of the bond can become compromised by disbanding, leading to corrosion and stress concentrations at the rivets and subsequent cracking leading to joint failure. A thermal technique for determining bond integrity in these structures has been investigated by Winfree, et al [1]. This technique involves active heating of the aircraft fuselage with a measurement of the temperature on the outer surface of the structure with an infrared imager. By even application of heat to the outer surface of the lap joint, details of the inner structure become thermographically detectable. A disbond will prevent heat from penetrating from the surface layer to the subsurface layers, resulting in an increase in surface temperature over the disbond. Thermographic detection of disbonds excels over other methods by being a noncontacting, quantitative method for inspecting large areas in a short period of time

Method of remotely characterizing thermal properties of a sample

A sample in a wind tunnel is radiated from a thermal energy source outside of the wind tunnel. A thermal imager system, also located outside of the wind tunnel, reads surface radiations from the sample as a function of time. The produced thermal images are characteristic of the heat transferred from the sample to the flow across the sample. In turn, the measured rates of heat loss of the sample are characteristic of the flow and the sample

Fiber Optic Thermal Detection of Composite Delaminations

A recently developed technique is presented for thermographic detection of delaminations in composites by performing temperature measurements with fiber optic Bragg gratings. A single optical fiber with multiple Bragg gratings employed as surface temperature sensors was bonded to the surface of a composite with subsurface defects. The investigated structure was a 10-ply composite specimen with prefabricated delaminations of various sizes and depths. Both during and following the application of a thermal heat flux to the surface, the individual Bragg grating sensors measured the temporal and spatial temperature variations. The data obtained from grating sensors were analyzed with thermal modeling techniques of conventional thermography to reveal particular characteristics of the interested areas. Results were compared and found to be consistent with the calculations using numerical simulation techniques. Also discussed are methods including various heating sources and patterns, and their limitations for performing in-situ structural health monitoring

Auantitative Thermal Diffusivity Measurements on Composite Fiber Volume Fraction (FVF) Samples

A composite’s strength is determined by the interaction between the fiber and matrix. Since the matrix distributes the load onto and between the fibers it is important to know the respective volume amounts to insure proper load distribution. Studies [1] have been done relating axial tensile and axial compressive strengths to fiber volume fraction (FVF). Current methods to determine FVF are destructive and time consuming. They involve removal of the matrix by heat or chemical digestion. In this work a thermal diffusivity measurement technique is investigated for the characterization of FVF in graphite composite plates. A thermal technique is advantageous since it is noncontacting, fast, and nondestructive

Fiber Optic Thermographic Detection of Flaws in Composites

Optical fibers with multiple Bragg gratings bonded to surfaces of structures were used for thermographic detection of subsurface defects in structures. The investigated structures included a 10-ply composite specimen with subsurface delaminations of various sizes and depths. Both during and following the application of a thermal heat flux to the surface, the individual Bragg grating sensors measured the temporal and spatial temperature variations. The obtained data were analyzed with thermal modeling to reveal particular characteristics of the interested areas. These results were found to be consistent with the simulation results

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 Measurements on Composite Porosity Samples

Porosity is a defect which can arise from moisture or gases being introduced to the resin system before cure and also during the curing process when poor bagging techniques are used. The effect of porosity results in a degradation in compressive, transverse tensile, and interlaminar shear strengths. For example, for a 1% porosity level there is approximately a 7% decrease in the interlaminar shear strength [1]. Ultrasonics is the current state of the art NDE method for the characterization of porosity in composites using the back scatter and frequency dependent attenuation measurements. In this work a thermal diffusivity technique is investigated for the characterization of porosity in graphite composite parts. The advantages of using thermal techniques is the noncontacting nature of the measurements and the ability to capture large areas using a thermal imager

Air-coupled acoustic thermography for in-situ evaluation

Acoustic thermography uses a housing configured for thermal, acoustic and infrared radiation shielding. For in-situ applications, the housing has an open side adapted to be sealingly coupled to a surface region of a structure such that an enclosed chamber filled with air is defined. One or more acoustic sources are positioned to direct acoustic waves through the air in the enclosed chamber and towards the surface region. To activate and control each acoustic source, a pulsed signal is applied thereto. An infrared imager focused on the surface region detects a thermal image of the surface region. A data capture device records the thermal image in synchronicity with each pulse of the pulsed signal such that a time series of thermal images is generated. For enhanced sensitivity and/or repeatability, sound and/or vibrations at the surface region can be used in feedback control of the pulsed signal applied to the acoustic sources