Patricia A. Howell

Thermographic imaging of cracks in thin metal sheets

The presence of cracks significantly decreases the structural integrity of thin metal sheets used in aerospace applications. Thermographic detection of surface temperature variations due to these cracks is possible after external heating. An approximate line source of heat is used to produce an inplane flow of heat in the sheet. A crack in the sheet perturbs the inplane flow of heat and can be seen in an image of the surface temperature of the sheet. An effective technique for locating these perturbations is presented which reduces the surface temperature image to an image of variations in the inplane heat flow. This technique is shown to greatly increase the detectability of the cracks. This thermographic method has advantages over other techniques in that it is able to remotely inspect a large area in a short period of time. The effectiveness of this technique depends on the shape, position and orientation of the heat source with respect to the cracks as well as the extent to which the crack perturbs the surface heat flow. The relationship between these parameters and the variation in the heat flow is determined both by experimental and computational techniques. Experimental data is presented for through-the-thickness, subsurface and surface EDM notches. Data for through-the-thickness fatigue cracks are also presented.

Quantitative thermal imaging of aircraft structures

Aircraft structural integrity is a major concern for airlines and airframe manufacturers. To remain economically competitive, airlines are looking at ways to retire older aircraft, not when some fixed number of flight hours or cycles has been reached, but when true structural need dictates. This philosophy is known as `retirement for cause. The need to extend the life of commercial aircraft has increased the desire to develop nondestructive evaluation (NDE) techniques capable of detecting critical flaws such as disbonding and corrosion. These subsurface flaws are of major concern in bonded lap joints. Disbonding in such a joint can provide an avenue for moisture to enter the structure leading to corrosion. Significant material loss due to corrosion can substantially reduce the structural strength, load bearing capacity and ultimately reduce the life of the structure. The National Aeronautics and Space Administrations Langley Research Center has developed a thermal NDE system designed for application to disbonding and corrosion detection in aircraft skins. By injecting a small amount of heat into the front surface of an aircraft skin, and recording the time history of the resulting surface temperature variations using an infrared camera, quantitative images of both bond integrity and material loss due to corrosion can be produced. This paper presents a discussion of the development of the thermal imaging system as well as the techniques used to analyze the resulting thermal images. The analysis techniques presented represent a significant improvement in the information available over conventional thermal imaging due to the inclusion of data from both the heating and cooling portion of the thermal cycle. Results of laboratory experiments on fabricated disbond and material loss samples are presented to determine the limitations of the system. Additionally, the results of actual aircraft inspections are shown, which help to establish the field applicability for this technique. A recent application of this technology to aircraft repairs using boron/epoxy patches is shown illustrating the flexibility of the technology.

Thermographic Detection of Corrosion in Aircraft Skin

The aging of aircraft and the integrity of lapjoints and doublers is a growing concern. With the increasing average age of the commercial aircraft fleet, there exists an increased need for the development of new NDE techniques for the detection of critical flaws in aircraft airframes. The current techniques being either too time consuming or unreliable are a primary reason for a requirement of major mandatory modification to the existing fleet. Improved NDE techniques offer the possibility for increased safety and reliability at reduced costs.

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.

Numerical simulations of thermographic responses in composites

Numerical simulations of thermographic responses in composite materials have been useful for evaluating and optimizing thermographic analysis techniques. Numerical solutions are particularly beneficial for thermographic techniques, since the fabrication of specimens with realistic flaws is difficult. A quadrupole method for performing the simulations in two dimensions is presented. The results are compared to a finite element simulation of the same geometry. The technique is shown to be in good agreement with a finite element simulation of the same geometry, however, it requires about one hundredth of the computational time.

Determination of Flaw Size from Thermographic Data

Conventional methods for reducing the pulsed thermographic responses of delaminations tend to overestimate the size of the flaw. Since the heat diffuses in the plane parallel to the surface, the resulting temperature profile over the flaw is larger than the flaw. A variational method is presented for reducing the thermographic data to produce an estimated size for the flaw that is much closer to the true size of the flaw. The size is determined from the spatial thermal response of the exterior surface above the flaw and a constraint on the length of the contour surrounding the flaw. The technique is applied to experimental data acquired on a flat bottom hole composite specimen.

Quantitative thermal depth imaging of subsurface damage in insulating materials

A thermal technique is presented for imaging subsurface damage and computing the depth of damaged areas for low diffusivity materials. The measurement technique presented uses uniform heating with quartz lamps over a large area. The surface temperature of the sample is collected using a scanning IR radiometer and a real time image processor during the cooling of the sample after heating. Flaw depths are computed by performing a numeric approximation to the surface Laplacian on each temperature image in the time series. The depth of the damage is then calculated from the time required for the amplitude of the surface Laplacian to reach a minimum in the region over the damage. Experimental results from the application of the technique to low diffusivity materials with surface and subsurface defects at various depths are presented showing the techniques ability to give quantitative depth of damage information. Additionally, the effects of variations in defect size on the time for flux minimum, and thus on the calculated depth, is also investigated. Finally, finite element simulations are compared with experimental results.

Parametric Studies of Thermographic Detection of Disbonds in Laminated Structures Using Computational Simulations

A quantitative assessment of a structure’s material characteristics contributes to the safety, reliability, and useful lifetime of a structure. Thermographic nondestructive evaluation has advantages over other methods in that it is a noncontacting, quantitative measurement of the material integrity which can inspect large areas in a short period of time. A disbond between layers of a laminated structure will prevent heat from penetrating from the surface layer to the subsurface layers and will result in an increase in temperature over the disbond. The limits of this technique for detection of disbonds in solid rocket motors was investigated by computational simulation of the thermographic technique. This has an advantage over an experimental investigation, since many sample configurations and flaw sizes can be investigated at a fraction of the cost and time required for sample fabrication, data acquisition and analysis. This paper presents a series of simulations varying parameters that affect the thermal contrast such as heating time, disbond size, and thickness of the surface layer. Experimental results are presented for comparison.