Two thermography techniques, passive and active, are used to detect damage initiation and progression in a cyclically loaded composite cylinder. The passive thermography tracks damage progression in real time during cyclic loading. Active flash thermography, using a flash tube enclosed within the cylinder, images delaminations in a cylinder under different loads. A differential thermography processing technique eliminates normal material variations and improves sensitivity to and sizing of delaminations. The thermography results were compared to nonimmersion ultrasonic results

Johnston, P. H.

Seebo, J. P.

Winfree, W. P.

Zalameda, J. N.

NASA Technical Reports Server

 
 
 
 
 
 
 
 
 
 
 
THERMOGRAPHY INSPECTION FOR DETECTION AND 
TRACKING OF COMPOSITE CYLINDER DAMAGE DURING 
LOAD TESTING  
 
 
 
J. N. Zalameda1, W. P. Winfree1, J. P. Seebo2 and P. H. Johnston1 
 
1 NASA Langley Research Center, MS231 Hampton, VA 23681 
2 Lockheed Martin Corporation NASA Langley Research Center, MS231    
  Hampton, VA 23681 
 
 
ABSTRACT. Two thermography techniques, passive and active, are used to detect 
damage initiation and progression in a cyclically loaded composite cylinder. The 
passive thermography tracks damage progression in real time during cyclic loading. 
Active flash thermography, using a flash tube enclosed within the cylinder, images 
delaminations in a cylinder under different loads. A differential thermography 
processing technique eliminates normal material variations and improves sensitivity 
to and sizing of delaminations. The thermography results were compared to non-
immersion ultrasonic results. 
 
Keywords: Thermal NDE, Thermal Diffusivity, Composite Matrix Cracks, Delamination 
 
PACS: 81.70.Fy, 81.70.Bt, 07.57.-c, 44.35.+c 
 
INTRODUCTION 
      
 Composite structures are commonly tested under controlled loading to understand 
the initiation and progression of damage [1]. These tests are critical for validating design 
concepts and structural analysis tools [2].  One of the most significant damage modes for 
composite structures is delamination.  For a given composite design, understanding how 
delaminations initiate and grow improves confidence in predicting fatigue life. 
Nondestructive evaluation (NDE) is used to detect the onset and growth of delamination 
damage.  It is advantageous that the inspection is performed while the structure is being 
tested as opposed to periodic removal of the structure from a test fixture.  This is more 
time efficient and removes the potential for handling induced damage.  To perform the in-
situ inspection requires the sensor be mounted on or in close proximity to the load frame.  
Thermal nondestructive evaluation (NDE) has been shown to be an effective technique for 
https://ntrs.nasa.gov/search.jsp?R=20100042259 2019-08-30T13:51:13+00:00Z
 
 
detecting damage in composites during load testing because the inspection is non-
instrusive [3]. 
In this work the combination of two noncontact thermography techniques, passive 
and active, are used to detect the initiation and progression of damage in a composite 
cylinder during cyclic loading. The types of damage detected were matrix cracking and 
delaminations.  An active flash thermography technique detected and tracked delamination 
damage progression.  The flash thermography technique utilizes a flash tube enclosed 
within the cylinder.   This setup allows for the measurement of the transmission of the heat 
through-the-thickness.  If the thickness is known then a 1-Dimensional model can be fitted 
to the data to estimate the effective thermal diffusivity [4]. This setup also allows for 
thermography of the tube during the load application.  A differential thermography 
processing technique is applied to remove thermal variations associated with normal 
material variations.  It has been found that application of a static torsion load during the 
flash inspection improves the sensitivity to and sizing of defects.  
In addition, the passive thermography technique detects and tracks damage 
progression in real time as dynamic torsion loading is applied.   The torsion loading will 
cause damaged areas to rub or clap together thus providing frictional heating that can be 
detected with an infrared camera [3].  The frictional heating from the damage is typically 
very small and subsurface.  A principal component analysis data reduction algorithm is 
applied to improve defect contrast and thus enhance visualization of the damage [5].  
Lastly, the thermography results were compared to non-immersion ultrasonic inspection 
results. 
 
SAMPLE DESCRIPTION  
 
 A picture of the composite cylinder is shown in Fig. 1.  The composite cylinder’s 
overall length is approximately 51 cm with an inner diameter of 8.1 cm.  The cylinder’s 
ply layup consists of +/- 45 degree layers with ply drops and overlaps (4-5 plies total).  
Aluminum mounting fixtures are bolted on the top and bottom of the cylinder to allow for 
attachment to the load frame.  The composite cylinder’s thickness is tapered from the 
fixtures down to a uniform thickness of 0.1524 cm.  The uniform thickness represents the 
area of interest and is 25 cm in length along the tube axis as shown in Fig. 1.  In this area 
three Teflon inserts A, B, and C, are placed with respective sizes of 1.27 x 1.27 cm, 0.63 x 
0.63 cm, and 1.27 x 1.27 cm.  The Teflon inserts are used to simulate delamination damage 
and possible damage initiation sites.  The top of the tube is held stationary by the tension 
torsion machine and the torsion load is applied at the bottom with a force of +10,000 to -
9,000 inch-lbs at 4 Hz.  The axial load was held to a minimal value (target of 0 lbs) by the 
tension torsion machine. 
 
 
 
 
 
 
 
 
                     
 
 
Figure 1.  Composite cylinder sample with Teflon insert locations. 
 
 
THERMOGRAPHY SETUP  
 
The thermal measurement setup is shown in Fig. 2. The system consists of an 
infrared camera operating in the 3–5 micrometer infrared band, an image data acquisition 
computer, and a linear flash tube mounted within the composite cylinder.  The flash tube is 
approximately 19 cm long. The thermography system utilizes a pair of infrared mirrors 
(polished aluminum plates) to allow for a full circumference inspection in one view.  The 
infrared camera is mounted on a XY scanning system for precise positioning.  The 
computer triggers the flash lamp that is synchronized to the image acquisition card.  The 
temperature rise from the flash lamp was measured within the tube to be less than 10 
degrees Celsius above ambient.  The acquired data are composed of a series of 12 bit 
digital images with a resolution of 256 x 320 pixels captured at 60 Hz.  For a typical 
measurement, 180 images are acquired.  Thermal diffusivity images are obtained by fitting 
a 1-Dimensional model to the temporal thermal data pixel by pixel [4].  The passive 
thermography inspection did not use an externally applied heat source.  The passive 
inspection captured the thermal variations generated in the specimen during the torsion 
loading.  The images were captured at 1 Hz and approximately 3800 frames where 
acquired for a total inspection time of approximately an hour.  A principal component 
analysis reduction of the data improved defect contrast and enhanced damage visualization 
[5].       
 
CHARACTERIZATION OF FATIGUE INDUCED DAMAGE  
 
Differential Thermography with Static Torsion Loading 
 
 After a number of load cycles, typically around 50,000, a flash inspection was 
performed.  Three flash inspections were performed: 0 inch-lbs static torsion load, + 3,000 
inch-lbs static torsion load, and -3,000 inch-lbs static torsion load.  For each static load, an 
effective thermal diffusivity inspection image was obtained.  The effective thermal 
diffusivity image for the negative static load is shown in Fig. 3a and the top and bottom 
inserts A and B are detected as compared to the effective thermal diffusivity image for the 
positive static load shown in Fig. 3b where the top and bottom inserts, A and B, are not as 
apparent. For these inserts, the thermal contact resistance decreases (lower contrast) when 
a positive load was applied and increases (higher contrast) with a negative load. The 
difference in these measurements provides an image that shows all the Teflon inserts, Fig. 
3c.  The contact resistance change at insert A is seen in the measured temporal responses 
shown in Fig. 4a.  When a negative static load is applied, the thermal diffusivity values 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2.  Thermography inspection setup on load frame. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3.  Differential thermal diffusivity imaging. 
  
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4.  Change in temporal response at insert A and comparison of insert thermal 
diffusivity values as a function of static load. 
 
at inserts A and C decrease and the thermal diffusivity value at insert B increases, Fig. 4b. 
The increases and decreases in the contact resistance, at the inserts, can be attributed to 
increases and decreases in the air gap.  This data indicates the response in the cylinder 
depends on the composite layup in the region of the inserts.  
 
Application of Thermal Model for Air Gap Thickness Estimation 
 
 The changes in contact resistance can be modeled as an air gap change due to the 
applied load.  For inserts A and C the application of the negative torsion load increases the 
air gap, which resulted in an increase in the contact resistance.  The application of a 
positive load decreases the air gap and thus the contact resistance, which resulted in a 
higher effective thermal diffusivity value.  A one dimensional model for this configuration 
is shown in Fig. 5.  The multilayered model includes the inner and outer composite layers, 
the insert, and air gap.  The model can be solved analytically in the Laplace domain using 
Equation (1) where V2 is the temperature response, f1 is the instantaneous heat flux input, 
l1 is the inside composite layer thickness, l2 is the outer composite layer thickness, K is the 
composite thermal conductivity, 
 
 comp  is the composite thermal diffusivity, KTeflon is the 
Teflon thermal conductivity, lTeflon is the Teflon thickness, Kair gap is the air gap thermal 
 
 
conductivity, lair gap is the air gap thickness and s is the Laplace complex argument. A 
matrix equation which represents the model is given as:  
 
 
(1) 
 
 
 
                               where 
 
The Fixed Talbot numerical inversion algorithm is used to solve for the temporal response 
of equation (1) [6]. The material property values used are shown in Table 1 for a nominal 
composite fiber volume fraction [7-9].  An example fit to the normalized data at insert A is 
shown in Fig. 6.  The air gap thickness is measured with a negative static load applied.  
The estimated air gap thickness change at inserts A and C, due to cyclic loading is shown 
in Fig. 7.  As shown, the air gap thickness increases as the number of fatigue cycles 
increases.  This is due to the repeated opening and closing or exercising of the insert 
delamination during cyclic loading.  The air gap over insert C appeared to grow faster as 
compared to insert A.  This was perhaps related to a matrix crack that was detected using 
flash thermography at insert C.  This is shown in the top right image in Fig. 7. 
 
 
 
 
 
  
 
 
 
 
 
 
 
Figure 5.  Multi-layered model of composite at insert with air gap. 
 
 
Table 1.  Values used for curve fitting multi-layered model to the thermal data. 
 
Parameter Symbol Value 
Composite Thermal Conductivity K 0.00211 cal/sec/cm/K 
Composite Thermal Diffusivity comp 0.00373 cm2/sec 
Composite Layer 1 Thickness l1 0.101 cm 
Composite Layer 2 Thickness l2 0.051 cm 
Teflon Thermal Conductivity KTeflon 0.000586 cal/sec/cm/K 
Teflon Thickness lTeflon 0.0013 cm 
Air Thermal Conductivity  KAir Gap 0.0000614 cal/sec/cm/K 
Air Gap Thickness lAir Gap Estimated from data fit 
 
 
q 2   q 1 s  comp .
2v
 0




cosh [ q2 l2]        -
1
K q2
sinh [ q2 l2]
-K q2 sinh [ q2 l2]         cosh [ q2 l2]  








 
1        - 1Teflon
K Teflon
0         1  








 
1        -
1air gap
K air gap
0         1  










 
cosh [ q1 l1]        -
1
K q1
sinh [ q1 l1]
-K q1 sinh [ q1 l1]         cosh [ q1 l1]  








  1v
1f





  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6.  Multi-layered model fit to data at insert A with air gap. 
 
 
 
Figure 7. Estimated air gap change due to cyclic loading with flash inspection images.  
 
Early Detection and Tracking of Delaminations using Differential Thermography  
 
 The flash thermography differential inspection results are shown in Fig. 8.  
Unexpectedly, the damage initiated on the right backside of the cylinder and not at the 
Teflon inserts.  There was good agreement between the non-immersion ultrasonic image 
and the thermography results, Fig. 8. The non-immersion inspection used an ultrasonic 
transducer assembly comprised of an entrapped water column with a flexible membrane.  
The transducer assembly was mounted on the XY scanner to produce the inspection image.  
The ultrasonic inspection required about 2.5 hours to perform, and therefore, the 
thermography inspection (inspection time of several minutes) was used periodically to 
determine when an ultrasonic inspection should be performed.  The ultrasonic inspection 
provides greater resolution of damage and at multiple interfaces. 
 
PASSIVE THERMOGRAPHY MEASUREMENTS 
 
 Thermal data was also taken in real time during cyclic loading. The passive 
inspection was performed without application of external heat.  Any detectable heating 
was a result of matrix cracks rubbing or delamination or disbond interfaces clapping or 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 8.  In-situ flash thermography inspection of delamination damage growth. 
 
rubbing.  Time domain processing using a principal component analysis data reduction 
algorithm was used to improve defect contrast and thus enhance visualization of the 
damage [5].  Examples of matrix cracks and delamination growth are shown in Fig. 9 and 
Fig. 10 respectively. 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 9.  Example passive thermography results showing matrix crack growth. 
 
 
 
 
 
  
  
 
 
 
 
 
 
 
 
 
Figure 10. Passive thermography results showing delamination growth. 
 
 
 
CONCLUSIONS 
 
 Flash and passive thermography were used to track composite cylinder damage 
during cyclic fatigue testing.  Thermography results agreed with and complemented the 
non-immersion ultrasonic inspection.  It has been shown that application of static torsion 
loading and difference thermography increases sensitivity to delamination damage and 
helps quantify the size.  It is desired to compare the thermography estimates of air gap 
thickness with another inspection technique such as computed tomography X-ray. 
 
ACKNOWLEDGEMENTS 
 
 This work was performed under the NASA Aviation Safety Program in cooperation 
with the Center for Rotorcraft Innovation.  The authors would also like to acknowledge 
Chris Wright of NASA Langley and James Bly of Lockheed Martin for data acquisition. 
 
REFERENCES 
 
1. T. K. O'Brien, “Development of a Delamination Fatigue Methodology for Composite 
Rotorcraft Structure”, NASA Aviation Safety Technical Conference Denver, CO, 
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2. R. Krueger, “An Approach to Assess Delamination Propagation Simulation 
Capabilities in Commercial Finite Element Codes”, NASA/TM-2008-215123, 2008. 
3. E. G. Henneke, K. L. Reifsnider, and W. W. Stinchcomb, Journal of Metals 31(9) ,11-
15, (1979). 
4. W. P. Winfree and D. M. Heath, “Thermal diffusivity imaging of aerospace materials 
and structures”, Proceedings of  SPIE, Thermosense XXIII, Vol. 3361,1998, pp. 282-
290. 
5. N. Rajic, Composite Structures 58, 521--528, 2002. 
6. A. Talbot, IMA J Appl Math 23 (1): 97-120, (1979) 
7. J. N. Zalameda, J. Compos. Technol. Res. 21, 98–102, 1999. 
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d_156.html. 
9. Teflon® Specifications, Boedeker Plastics, Inc., www.boedeker.com/teflon_p.htm. 


Thermography Inspection for Detection and Tracking of Composite Cylinder Damage During Load Testing

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