November 2014 

NASA/TM–2014-218545 

Nondestructive Evaluation (NDE) for Inspection 

of Composite Sandwich Structures 

Joseph N. Zalameda and F. Raymond Parker 

Langley Research Center, Hampton, Virginia 
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November 2014 

NASA/TM–2014-218545 

Nondestructive Evaluation (NDE) for Inspection 

of Composite Sandwich Structures 

Joseph N. Zalameda and F. Raymond Parker 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 
Abstract 

Composite honeycomb structures are widely used in aerospace applications due to 

their low weight and high strength advantages. Developing nondestructive evaluation 

(NDE) inspection methods are essential for their safe performance. Flash thermography 

is a commonly used technique for composite honeycomb structure inspections due to its 

large area and rapid inspection capability. Flash thermography is shown to be sensitive 

for detection of face sheet impact damage and face sheet to core disbond. Data 

processing techniques, using principal component analysis to improve the defect 

contrast, are discussed. Limitations to the thermal detection of the core are investigated. 

In addition to flash thermography, X-ray computed tomography is used. The aluminum 

honeycomb core provides excellent X-ray contrast compared to the composite face 

sheet. The X-ray CT technique was used to detect impact damage, core crushing, and 

skin to core disbonds. Additionally, the X-ray CT technique is used to validate the 

thermography results. 

Nomenclature

V = temperature 

l = thickness 

K = thermal conductivity 

f = heat flux 

α 	




 



1
Introduction 

Composite honeycomb structures continue to be used widely in aerospace 

applications due to low weight and high strength requirements. There is a growing 

interest in the application of thermal methods for nondestructive evaluation (NDE) of 

composite sandwich structures [1-3]. Some of the advantages of thermal NDE are 

noncontact, rapid, capable of imaging large areas, applicable to complex geometries, 

and quantitative. The technique is safe where only a small amount of heat is applied to 

the surface of the structure. Thermography has shown good potential for detection of 

various defects in composite structures. Defects such as delaminations, disbonds, gross 

porosity, and fiber volume fraction variations are detectable using thermography. X-ray 

computed tomography (CT) is also used to detect defects in sandwich structures [4]. 

The X-ray CT inspection results provide a true volumetric measurement of the damage, 

however the inspection must be performed within a radiation enclosure. The aluminum 

honeycomb core provides excellent X-ray contrast compared to the composite face 

sheets and therefore, is an ideal inspection technique. The X-ray system is used to 

detect impact damage, core crushing, and skin to core disbonds. 

Specific to composite honeycomb structures, face sheet delaminations, face sheet to 

core disbonds, and core crushing are defects of interest. Typically the composite face 

sheets are relatively thin compared to the overall thickness and therefore, thermography 

is effective for detection of face sheet delaminations. Detection of the face sheet to core 

disbonds is also critical. Under load, face sheet to core disbonds can grow leading to 

disbond buckling failure which can lead to catastrophic structural failure [5]. Both 

thermography and X-ray CT have the potential to detect face sheet to core disbonds. 

The thermal detection limitations of face sheet to core disbonds are investigated using 

analytical and finite element thermal modeling. 

Flash thermography results are presented on composite honeycomb samples with 

face sheet delaminations, face sheet to core disbonds, and core crushing. The thermal 

results are compared to the X-ray CT measurements. Thermal modeling is used to 

optimize the inspection. The thermal modeling consisted of a 1-dimensional (1-D) 

analytic model and 2-dimensional (2-D) finite element model. The 1-D analytical model 

is used to provide an understanding of the effect of the adhesive between the face sheet 

and core. The 2-D finite element model is used for two purposes: first to determine the 

optimal thermal processing parameters for detection of face sheet delamination damage 

and face sheet to core disbonds and second to determine the thermal detection 

limitations caused by the face sheet to core thickness ratio. This model is also used for 

differentiating between impact damage and face sheet disbonds using early time and 

late time analysis. X-ray CT is used to image the skin to core disbond and impact 

damage areas by viewing averaged image data through-the-thickness. The core 

crushed areas are detected by measuring the deviation of the core away from the nondamaged side. 

Sample Configuration 

The configuration of the composite sandwich samples studied is shown in Figure 1. 

The sandwich structure is comprised of an aluminum hexagonal core with outer graphite 

plain weave composite face sheets. The core thickness is 2.54 cm and the composite 

face sheet thickness is approximately 0.1 cm. The core cell wall thickness is 

2
approximately 0.02 cm with a cell size of 0.32 cm. The ratio of the face sheet to core wall 

thickness is 5 to 1. 

Figure 1. Typical configuration of composite face sheet with aluminum core. 

Flash Thermography 

Thermographic single side flash inspection data were acquired using a commercially 

available thermal inspection system. The thermal inspection system uses two flash 

lamps that are mounted within a hood to contain the flash, as shown in Figure 2. The 

flash power is provided from two power supplies delivering 4.8 kJ of energy to the flash 

tubes. The flash duration is typically less than 10 milliseconds and is instantaneous 

compared to the frame rate of the camera and the thermal response time of the 

composite [6]. An infrared camera was used to record the surface temperature 

response. The camera operates in the mid infrared band with a pixel resolution of 

640x512. The camera sensitivity or noise equivalent temperature difference, quoted by 

the manufacturer, is less than 0.02 Kelvin. The camera is configured with a 25 mm 

germanium optical lens. The data were acquired at 60 Hz for 720 frames. The total 

acquisition time was 12 seconds. 

Figure 2. Thermal inspection system with flash lamps contained in hood. 

Thermal Modeling 

The thermal modeling efforts included a 1-D approach to investigate the effects of 

the adhesive between the face sheet and core. A multi-layer 1-D thermal model was 

used by comparing the difference in the thermal response between the face sheet 

bonded directly to the core and a thin layer of adhesive between the face sheet and 

core. From these results, a 2-D finite element model was configured similar to the right 

3
image of Figure 3 using a “T” configuration. This was used to study the effect of the 

lateral heat flow in reducing thermal contrast for imaging a core disbond. Simulated 

camera noise, determined from actual thermal data, was added to the model results. An 

X-ray CT image showing the aluminum core bonded to a composite face sheet is shown 

in the left image of Figure 3. For both of these models, the fillet bonds between the core 

and adhesive along the length of the core wall were not considered. 

Figure 3. X-ray CT image (left) showing actual thermal configuration of attached core 

to the face sheet and thermal model configuration (right). 

1-D Modeling 

The 1-D analytical model provided an understanding of the effect of the adhesive 

between the face sheet and core and is given in equation (1) [7]. The multilayered 

model includes the face sheet composite layer, the adhesive layer and the core layer. A 

contrast function is defined by the temperature difference response to impulse flux 

heating at the surface between points A, B, and C as shown in Figure 3. For point A, the 

1-D analytical model contains 3 layers: the face sheet, the adhesive, and the core, 

equation (1). For points B and C the analytical model contains 1 layer (face sheet layer 

only) and 2 layers (face sheet and the core), equations (2) and (3) respectively. The 

equations are given below as: 

 

vl

 0

⎡

⎣

⎢ ⎤

⎦

⎥= cosh [ q2 l2 ] - 1

Kcore q2

sinh [ q2 l2 ]

-Kcore q2 sinh [ q2 l2 ] cosh [ q2 l2 ] 

⎡

⎣

⎢

⎢

⎢

⎤

⎦

⎥

⎥

⎥

1 - 1adhesive

K adhesive

0 1 

⎡

⎣

⎢

⎢

⎢

⎤

⎦

⎥

⎥

⎥

cosh [ q1 l1] - 1

Kcomp q1

sinh [ q1 l1]

-Kcomp q1 sinh [ q1 l1] cosh [ q1 l1] 

⎡

⎣

⎢

⎢

⎢

⎤

⎦

⎥

⎥

⎥

 vA

f 1

⎡

⎣

⎢

⎢

⎤

⎦

⎥

⎥

 (1)

 v2

 0

⎡

⎣

⎢ ⎤

⎦

⎥= cosh [ q1 l1] - 1

Kcomp q1

sinh [ q1 l1]

-Kcomp q1 sinh [ q1 l1] cosh [ q1 l1] 

⎡

⎣

⎢

⎢

⎢

⎤

⎦

⎥

⎥

⎥

 vB

f 1

⎡

⎣

⎢

⎢

⎤

⎦

⎥

⎥

 (2) 

 v3

 0

⎡

⎣

⎢ ⎤

⎦

⎥= cosh [ q2 l2 ] - 1

Kcore q2

sinh [ q2 l2 ]

-Kcore q2 sinh [ q2 l2 ] cosh [ q2 l2 ] 

⎡

⎣

⎢

⎢

⎢

⎤

⎦

⎥

⎥

⎥

 cosh [ q1 l1] - 1

Kcomp q1

sinh [ q1 l1]

-Kcomp q1 sinh [ q1 l1] cosh [ q1 l1] 

⎡

⎣

⎢

⎢

⎢

⎤

⎦

⎥

⎥

⎥

 vC

f 1

⎡

⎣

⎢

⎢

⎤

⎦

⎥

⎥

 (3) 

 where q 1 = s

α comp

 and q 2 = s

α core

 

.

4
The model can be solved analytically in the Laplace domain [8] where VA, VB and VC 

are the 3 layer, 1 layer, and 2 layer surface temperature responses in the Laplace 

domain respectively, f1 is the instantaneous heat flux input estimated to be 

approximately 1 cal/cm2

 [9], l1 is the face sheet composite layer thickness = 0.1cm, l2 is 

the core layer thickness = 2.54 cm, Kcomp is the composite thermal conductivity = 0.0021 

cal/cm sec o

C, Kaluminum is the aluminum thermal conductivity = 0.33 cal/cm sec oC, acore is 

the composite thermal diffusivity = 0.004 cm2

/sec, aaluminum is the aluminum thermal 

diffusivity = 0.53 cm2

/sec, Kadhesive is the adhesive thermal conductivity = 0.0006 cal/cm 

sec o

C, ladhesive is the estimated adhesive thickness = 0.001 cm [10], and s is the Laplace 

complex argument. The temperature results are normalized before taking the difference. 

The results are shown in Figure 4 where the contrast between the model with adhesive 

and no adhesive (VA – VC) is plotted. In addition, the difference between the face sheet 

only and face sheet and core (VB – VC) is plotted for the core contrast. Compared to the 

core contrast the adhesive difference response is minimal. 

Figure 4. 1-D model temperature difference of adhesive and no adhesive compared 

to face sheet and face sheet and core. 

2-D Finite Element Modeling 

The 2-D finite element method (FEM), via a commercially available software package 

[11], was used to determine the thermal detection limitations caused by the face sheet to 

core thickness ratio. The face sheet thickness to core thickness ratios studied were 4:1, 

5:1, 6:1, 7:1 and 8:1. These models were simplified using the face sheet bonded directly 

to the core (no adhesive) based on the 1-D results. An example finite element analysis 

model of a composite face sheet with aluminum core is shown in Figure 5. The 2-D 

model contains 1038 elements and 620 nodes. The face sheet thickness is 0.1 cm and 

the aluminum core cell wall thickness is 0.02 cm. The ratio of the face sheet to core 

thickness is 5:1. The width of the face sheet is 0.5 cm and the length of the core is 0.4 

cm, as shown in Figure 5. The surface temperature (room temperature subtracted) as a 

function of time is also shown in Figure 5 (right plot). For early times around 0.3 

seconds there is no contrast caused by the core. This reveals that defects within the 

face sheet can be detected around this time and therefore can be differentiated from 

core defects. Later in time, around 0.97 seconds, the aluminum core lowers the 

temperature at the face sheet surface over the core. The temperature contrast fades 

away around 8.3 seconds. Figure 6 shows a comparison between the 2-D FEM and the 

1-D analytical model results. For early times up to around 0.75 seconds, there is fairly 

good agreement between the 2 models. Past 0.75 seconds the models diverge and as 

expected the 2-D model would predict lower temperature contrast since the lateral heat 

flow is taken into account. Comparisons to 2-D model results, with camera noise 

added, of the contrast evolution vs. time for various face sheet to core cell wall thickness 

ratios are shown in the right plot of Figure 6. The temperature difference was calculated 

5
by subtracting a point over the core from a point away from the core. The core wall 

thickness is 0.020 cm and the thickness of the face sheet values are 0.08, 0.10, 0.12, 

0.14, and 0.16 cm, resulting in face sheet thickness to core thickness ratios of 4:1, 5:1, 

6:1, 7:1 and 8:1 respectively. The temperature difference is barely above the noise for 

the 8:1 ratio configuration indicating a face sheet to core disbond would not be 

detectable. 

Figure 5. Finite element model of a composite face sheet with aluminum core (left 

image) and corresponding surface temperature evolution (right plot). 

Figure 6. Contrast evolution comparison of 2-D finite element model comparison to 

1-D analytical model (left plot) and temperature difference comparison of face sheet 

to core cell wall thickness ratios (right plot). 

Data Processing 

Principal component analysis (PCA) is common for processing of thermal data 

[12,13]. This algorithm is based on decomposition of the thermal data into its principal 

components or eigenvectors. Singular value decomposition is a routine used to find the 

singular values and corresponding eigenvectors of a matrix. Since thermal NDE signals 

are slowly decaying waveforms, the predominant variations of the entire data set are 

usually contained in the first or second eigenvectors, and thus account for most of the 

data variance of interest. The PCA is computed by defining a data matrix A, where the 

time variations are along the columns and the spatial image pixel points are row-wise. 

The matrix A is adjusted by subtracting the mean along the time dimension. The matrix 

A can then be decomposed using singular value decomposition as: 

6
 

 AT

A = U* Γ 2 * U T

 (4) 

where Γ is a diagonal matrix containing the squares of the singular values and U is an 

orthogonal matrix, which contains the basis functions or eigenvectors describing the time 

variations. The eigenvectors can be obtained from the columns of U. The PCA image is 

calculated by dot product multiplication of the selected eigenvector times the 

temperature response (data matrix A), pixel by pixel. Typically the first or second 

eigenvector PCA image provides good contrast for defect detection. The first 

eigenvector is dominated by the transient cool down of the face sheet layer and is more 

suited for early time detection of face sheet delamination. The second eigenvector is 

less dominant and defines the temporal contrast evolution of the face sheet bonded to 

the core. This is shown in Figure 7 where the 2nd eigenvectors, determined from the 2-D 

model data for each of the thickness to core ratios, are plotted for comparison (no 

camera noise added). The maximum contrast point moves later in time due to the 

increases in the face sheet thickness, as expected. This indicates the 2nd eigenvector is 

the best candidate for imaging the face sheet to core disbond regions. Also the 

maximum contrast time determined from the 2nd eigenvectors are different compared to 

the maximum time determined from the difference calculation with no camera noise 

added (see Table 1 in Figure 7). While both agree that with increasing face sheet 

thickness the maximum point moves slightly later in time; the maximum contrast time 

determined from the 2nd eigenvector is more accurate because all the surface points are 

used from the model instead of only the 2 surface points used in the difference 

calculation. Also shown in Figure 7, the 8:1 ratio eigenvector curve does not have a 

clear maximum point, thus indicating the core response is not as dominant in the data 

and cannot be detected as confirmed by the 8:1 ratio temperature difference plot in 

Figure 6 (especially with camera noise added). 

Figure 7. 2nd Eigenvectors determined from the 2-D FEM results (left plot) and table 

of maximum contrast times determined from temperature difference calculations 

(from Figure 6) and the local maximum contrast time from 2nd eigenvector 

calculations. 

X-ray Computed Tomography (CT) System 

The advanced micro-focus X-ray system used in this study is shown in Figure 8. This 

system is capable of resolving details down to 5 microns, and with magnifications up to 

7
Figure 8. X-ray CT system used for sandwich structure inspections. 

160 times. The X-ray source voltage can be varied from 25 to 225 kilovolts. The 

detector array size is 2,000 x 2,000 with a 200μ pitch. The sample can be manipulated 

with 5 axes of freedom, while continuously viewing the image on a monitor. Defects can 

be rapidly located, zooming in for detailed analysis. The system is contained in a 

radiation enclosure, with X-ray, manipulator and imaging controls housed in a separate 

control console. Sample sizes can range up to 25.4 x 25.4 cm. 

Comparison of Thermography to Computed 

Tomography X-ray 

Based on the 2-D modeling results from sections 3.2 and 4.1, an early time window 

can be used for detection of defects within the composite face sheet. A late time window 

can be used to detect a face sheet to core disbond. Shown in Figure 9 is an early time 

(0.33 – 1.5 seconds) PCA image of a composite honeycomb sample with impact 

damage delamination defects (7 total impacts). The impact damage shows up as lighter 

Figure 9. Photograph of impacted sandwich structure (left image) and early time 

thermography inspection results (middle image) along with X-ray CT (right images). 

8
regions. The X-ray CT images show the core still attached to the face sheet with some 

core crushing. The PCA image was calculated from the first eigenvector. A comparison 

of the early time and late time (1.5 – 8.3 seconds) PCA images to X-ray CT inspection 

images are shown in Figure 10. There is good qualitative agreement between the 

thermography inspection image (early time analysis) compared to the X-ray CT image 

for detection of impact damage. Figure 11 shows the areas of crushed core using X-ray 

CT. The crushed core areas are determined by locating the core edges just below the 

back face sheet and following the edges toward the front surface until they deviated by 

three pixels from a linear fit and then traced back to a one pixel deviation; there are 

minor core deformations deeper into the core that are ignored. The crushed core areas, 

as shown in Figures 9 and 11, are not detectable with thermography. 

Figure 10. Thermography inspection results of early time (left image) and late time 

(middle image) of an impacted sandwich structure along with X-ray CT (right image). 

Figure 11. Images of the core crushed areas using X-ray CT. 

Another sample, containing a face sheet crack, impact damage and core damage 

(cut core) was inspected. Shown in Figure 12 is an early time (0.33 – 1.5 seconds) PCA 

image of the honeycomb sample with a face sheet crack and impact damage defects. 

The bottom left image of Figure 12 is the late time (5.0 – 8.3 seconds) PCA image of the 

honeycomb sample showing the face sheet to core disbonds along the edge. A later 

9
start time (5.0 seconds) was used in the PCA processing to image the cut core areas 

and this was found to provide optimal results. The later time requirement is most likely 

due to some portion of the aluminum core still attached to the face sheet. This can be 

clearly seen in the left X-ray CT images of Figure 13 where some of the cut core is still 

attached to the face sheet and therefore later times were required to produce the surface 

temperature contrast. The attached core along with the resin fillet bonding changes the 

thermal response of the face sheet (more thickness) thus requiring a later start time to 

reveal contrast. Shown in Figure 13 is a close up area of the boxed region of Figure 12 

where the impact damage is over an area of cut core. The early time image and late 

time image (shown in Figure 13) clearly reveals the face sheet impact damage and the 

face sheet to core disbond respectively. The face sheet to core disbond region is larger 

than the impact damage. The cut core cannot be detected under the impact damage 

area, as expected due to the delamination blocking the heat flow. 

Figure 12. Thermography inspection results (early and late time) of the damaged 

sandwich panel with X-ray CT showing skin to core disbond. 

Summary 

Flash thermography and X-ray CT was used to detect defects in composite sandwich 

structures with an aluminum core. A thermal method was presented to detect damage in 

10
aluminum core composite face sheet sandwich structures using PCA and time window 

analysis. The early time window PCA image using the first eigenvector was applied to 

detect face sheet impact damage. The late time window PCA image using the second 

eigenvector was applied to detect the face sheet to core disbond areas. The FEM 

analysis was helpful in selecting the eigenvector number and determining the optimal 

time windows. The flash thermography technique was not able to detect the core 

crushed areas. The X-ray CT inspection was able to detect impact damage, core 

crushed areas, and skin to core disbonds. The flash thermography results compared 

well with X-ray CT for impact damage detection and skin to core disbonds. 

 

 

Figure 13. Thermography inspection close up of impact damage over cut core with 

X-ray CT. 

References 

1. Sfarra, S. et al, “A comparative investigation for the nondestructive testing of 

honeycomb structures by holographic interferometry and infrared 

thermography“,J. Phys.: Conf. Ser. 214 012071, (2010). 

2. Clemente Ibarra-Castrando, et al, “Comparative Study of Active Thermography 

Techniques for the Nondestructive Evaluation of Honeycomb Structures”, 

Research in Nondestructive Evaluation Volume 20, Issue 1, (2009). 

3. Genest, M., Brothers, M., Beltempo, C. A., Rutledge, R. S., “Nondestructive 

Evaluation of Aluminum Honeycomb Sandwich Structures”, NATO Unclassified, 

STO-MP-AVT-224, (2013). 

4. Liu, Tong, Malcolm, Andrew A., and Xu, Jian, “High Resolution X-ray CT 

Inspection of Honeycomb Composites Using Planar Computed Tomography 

Technology”, 2nd International Symposium on NDT in Aerospace (2010). 

11
5. Southward, T., Horrigan, D.P. W., Mallinson, G. D., and Jayaraman, K., “Failure 

of Sandwich Composite Structure Containing Face-sheet/Core Disbonds – An 

Experimental Study”, 5th Australasian Congress on Applied Mechanics, ACAM 

2007 10-12, Brisbane, Australia December (2007). 

6. Zalameda, J. N., "Heat Source Finite Pulse Time Effects in Thermal 

Nondestructive Evaluation," Materials Evaluation Journal, American Society for 

Nondestructive Testing, Vol. 60, Nos.3, pp.425-429 March (2002). 

7. Carslaw H. S. and Yeager J. C., [Conduction of Heat in Solids], 2nd. Ed. Oxford 

Univ. Press, (1959). 

8. Talbot, A., IMA J Appl Math 23 (1): 97-120, (1979). 

9. Shepherd, S. M., Lhota, J. M., and Ahmed, T., “Measurement Limits of Flash 

Thermography”, Proceedings of SPIE, Thermosense XXXI, edited by Douglas D. 

Burleigh and Ralph B. Dinwiddie, Vol. 7299, (2009). 

10. Devadas, N. P., Sunilkumar, G., and Sajeeb, R.,”Finite Element Analysis of Flat 

Joints in Metallic Honeycomb Sandwich Beams”, Journal of Mechanical and Civil 

Engineering, ISSN: 2278-1684, Vol. 3, Issue 2, Sept-Oct. (2011). 

11. LISA Finite Element Analysis, software version 8.0.0, (2013). 

12. Rajic, N., "Principal Component Thermography for Flaw Contrast Enhancement 

and Flaw Depth Characterisation in Composite Structures", Composite 

Structures, Vol. 58, pp. 521--528, (2002). 

13. Cramer, K. E. and Winfree, W. P., “Fixed Eigenvector Analysis of Thermographic 

NDE Data”, Proceedings of SPIE, Thermosense XXXIII, edited by Morteza Safai 

and Jeff Brown, Vol. 8013, (2011). 

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Nondestructive Evaluation (NDE) for Inspection of Composite Sandwich 

Structures

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 6. AUTHOR(S)

Zalameda, Joseph N.; Parker, F. Raymond

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NASA Langley Research Center

Hampton, VA 23681-2199

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National Aeronautics and Space Administration

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14. ABSTRACT

Composite honeycomb structures are widely used in aerospace applications due to their low weight and high strength 

advantages. Developing nondestructive evaluation (NDE) inspection methods are essential for their safe performance. Flash 

thermography is a commonly used technique for composite honeycomb structure inspections due to its large area and rapid 

inspection capability. Flash thermography is shown to be sensitive for detection of face sheet impact damage and face sheet to 

core disbond. Data processing techniques, using principal component analysis to improve the defect contrast, are discussed. 

Limitations to the thermal detection of the core are investigated. In addition to flash thermography, X-ray computed 

tomography is used. The aluminum honeycomb core provides excellent X-ray contrast compared to the composite face sheet. 

The X-ray CT technique was used to detect impact damage, core crushing, and skin to core disbonds. Additionally, the X-ray 

CT technique is used to validate the thermography results. 

15. SUBJECT TERMS

Aluminum; Composite; Inspection; Nondestructive test; Structures

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gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and 

Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person 

shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 11 - 2014