Richard E. Martin

Cleveland State University, Cleveland, Ohio

Evaluation of Ultrasonic and Thermal Nondestructive Evaluation for the Characterization of Aging 

Degradation in Braided Composite Materials

NASA/CR—2010-216793

October 2010
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Richard E. Martin

Cleveland State University, Cleveland, Ohio

Evaluation of Ultrasonic and Thermal Nondestructive Evaluation for the Characterization of Aging 

Degradation in Braided Composite Materials

NASA/CR—2010-216793

October 2010

National Aeronautics and

Space Administration

Glenn Research Center

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NASA/CR—2010-216793 1 

Evaluation of Ultrasonic and Thermal Nondestructive Evaluation 

for the Characterization of Aging Degradation in Braided 

Composite Materials

Richard E. Martin 

Cleveland State University 

Cleveland, Ohio 44115 

Abstract 

This paper examines the ability of traditional nondestructive evaluation (NDE) techniques to measure 

the degradation of braided polymer composite materials subjected to thermal-humidity cycling to simulate 

aging. A series of braided composite coupons were examined using immersion ultrasonic and pulsed 

thermography techniques in the as received condition. These same specimens were then examined 

following extended thermal-humidity cycling. Results of this examination did not show a significant 

change in the resulting (NDE) signals. 

Introduction 

Polymer composites have seen an increasing role in aerospace structures due to their light weight and 

high strength properties. Even though these materials have beneficial properties, concerns exist regarding 

long term performance in hostile environments. In particular, long term exposure to temperature and 

humidity can lead to material changes such as weight loss, and growth of microcracks and corresponding 

changes in mechanical properties such as loss in stiffness and strength (Refs. 1 to 4). 

Although much work has been done to understand and characterize degradation mechanisms in 

composite materials, little work has been published regarding methods to nondestructively measure the 

condition of a material subjected to environmental loadings. Methods employing ultrasound, 

thermography, eddy currents, and x-rays are regularly employed to locate and measure discrete damage 

such as cracks and delaminations, but not for the measurement of aging related material degradation. 

When degradation occurs in a distributed fashion throughout the entire material volume and without any 

visible changes of physical and geometrical properties or when surface changes do not correspond to the 

degree of volume degradation, a simple visual inspection of the structure will not reveal any dangerous 

conditions. Therefore, there is a compelling need for nondestructive techniques to measure the degree of 

degradation. Limited success in this area has been achieved using methods such as ultrasonic 

spectroscopy and specialized x-ray techniques for measurement of microcrack growth in composite 

materials (Refs. 5 and 6). These methods are typically limited to small area inspections, simple composite 

architectures, or require highly specialized equipment. The purpose of this research is to examine the 

ability of ultrasonics and thermography to detect and characterize the aging related degradation of a 

braided carbon/epoxy polymer composite material subjected to thermal-humidity cycling. 

Ultrasonics 

Ultrasonic techniques are widely used for the inspection of composite materials (Ref. 7). Typically 

methods employ either a handheld contact method where a part is inspected point by point or more 

elaborate scanning techniques where ultrasonic probes are coupled to the part through immersion in water 

or through the use of a squirter or bubbler system. In either case, material discontinuities are identified by 

examining the amplitude and time response of an ultrasonic pulse after interacting with the material of 

interest. This is typically accomplished in either a single sided pulse-echo mode or in a through 

transmission mode where a separate transducer is used to send and receive the ultrasonic signal. In this 
NASA/CR—2010-216793 2 

study, both pulse-echo and through transmission immersion ultrasonic scanning techniques are used. 

Measurement of degradation mechanisms is conducted by comparing the amplitude of the received 

signals in both the baseline and aged condition. 

Thermography 

One of the more widely used methods for the thermographic inspection of materials and components 

is pulsed thermography. As the name implies, the technique imparts a pulse of thermal energy, usually 

provided by a photographic flash lamp, on the surface of a specimen. The thermal energy on the surface 

will conduct into the cooler interior of the sample. In turn, there is a reduction of the surface temperature 

over time (Ref. 8). This surface cooling will occur in a uniform manner as long as the material properties 

are consistent throughout the specimen. Subsurface defects that possess different material properties 

(thermal conductivity, density, or heat capacity) will affect the flow of heat in that particular region. This 

resistance in the conductive path causes a different cooling rate at the surface directly above the defect, 

when compared to the surrounding, defect free, material. This results in a non-uniform surface 

temperature profile. Unprocessed, as-received data is typically displayed sequentially on a computer 

monitor and contrast enhanced. Images are then visually inspected for signs of a subsurface defect by 

locating areas with anomalous surface temperatures. This method depends on the skill of the operator and 

a material with a relatively uniform surface condition. 

Another technique for the processing of the thermal image data has been developed to improve the 

detection capabilities (Refs. 9 and 10). This method involves the creation of a set of mathematical 

equations that represent the time response of each pixel in the raw data set. This “reconstruction” process 

exploits certain general features of the thermal response of materials to pulsed heating, and has been 

shown to allow significant improvement in detection of subsurface features without the use of a reference 

(Ref. 11). 

The mathematical expressions used in the thermal image reconstruction process are based on the 

response of a uniform material subjected to an instantaneous heat pulse. Through the application of these 

relationships, the time dependent logarithmic behavior of each pixel can be approximated by a 

polynomial function of order, n: 

           

n

n

2

10 2   tlna...tlnatlnaatTln (1) 

Using the coefficients calculated in Equation (1), it is possible to create a reconstruction of the 

original thermal signal based on the relationship: 

            n}

n

2

1o 2   tlna...tlnatlnaaexptT (2) 

By applying this approximation to the collected data, the time dependent temperature response of 

each pixel can be represented by the coefficients of the polynomial, greatly reducing the storage 

requirements for large data sets. Depending on the order of the polynomial, high frequency noise 

components of the thermal data can be eliminated, in turn, sharpening the thermal image. 

Application of the reconstruction technique has benefits beyond those mentioned above. Since the 

image data has been reduced to coefficients of a polynomial, instantaneous derivatives of the image 

sequence can be calculated and displayed in the same manner as the original data set. Through the 

examination of the rate of change in the thermal data, more defect data can be extracted than is possible 

with time based images alone. 
NASA/CR—2010-216793 3 

Experimental Setup 

Specimen 

The specimens used in this study consisted of a triaxially braided T700S/E862w composite in the 

form of 23 flat coupons 50.8 mm (2 in.) wide by 305 mm (12 in.) long by 3.2 mm (0.125 in.) thick. 

Specimens oriented both in the direction of and transverse to the axial tows were cut from larger panels. 

Eight of the samples were removed following 4 months of aging and the remaining samples removed after 

13 months. 

Aging 

Aging was accomplished by subjecting the material to a cyclic temperature-humidity profile. A single 

cycle consists of a ramp up to 121 oC (250 oF) , hold for 2 hr, ramp down to 29 oC (85 oF) and hold for 

5 hr at 85 percent humidity and then ramp down to –54o (–65 oF) and hold for 1hr. The profile is shown 

graphically in Figure 1. This profile was continuously repeated until specimen removal. 

Ultrasonics 

Specimens were examined using both through transmission and pulse echo ultrasonic techniques 

using a commercially available ultrasonic scan system. In the through transmission mode, a 5MHz 

focused transducer (focal length = 69.9 mm (2.75 in.), and focused on front surface) was used to send the 

ultrasonic signal which was received by a 5MHz flat focus transducer. Transducer excitation was 

provided using a UT340 square wave pulser/receiver manufactured by UTEX Scientific. The received 

signal was digitized and stored for later analysis. Scans were performed over the specimen area with data 

collected in 0.5 mm (0.020 in.) increments in both scan and step directions. For pulse echo scanning, the 

same 5MHz focused transducer was used as both the sending and receiving transducer. Scanning was 

performed using the same increments as the through transmission mode. 
NASA/CR—2010-216793 4 

Pulsed Thermography 

Pulsed thermography data was collected using a commercial system manufactured by Thermal Wave 

Imaging Inc. A schematic representation of the setup is shown in Figure 2. The system uses a cooled 

640 by 512 InSb focal plane array camera operating in the 3 to 5 m range for imaging. A series of 

thermal images were collected at a 30 Hz frequency and for a 30 sec duration. Thermal excitation is 

provided using 2 linear xenon flash lamps with a pulse duration of approximately 5 ms. Thermal response 

at the specimen surface is stored for later analysis and processing. 

Thermography data was processed within the acquisition software by applying a polynomial fit to the 

front surface temperature time response following thermal excitation as mentioned previously. Analysis 

of the image sequence data in the baseline and aged condition as well as analysis of the rate based cooling 

response were used as measures for identifying distributed damage that may be present due to aging. 

Results 

Ultrasonics 

Figures 3 and 4 show representative ultrasonic C-scans of samples in the baseline and aged condition. 

Visual inspection of the scan results did not reveal any obvious discrete damage. Further examination was 

conducted by comparing the attenuation (received peak amplitude response) of the signal in both the 

baseline and aged conditions over the entire sample. Results indicated a large range of values resulting 

from the scatter of the ultrasonic signal by the braided architecture of the material. This scatter prevented 

the measurement of any significant change in the material response. 

Pulsed Thermography 

Figure 5 shows representative images for a single specimen in the baseline condition and following 

13 months of aging. Visual examination of thermography images for all of the specimens did not reveal 

any obvious discrete damage within the specimens. Further examination of the cooling curves was 

conducted in order to identify any possible distributed damage within the specimen volume. Figure 6(a) 

shows a typical log-log display of the temperature-time curve following thermal excitation. The two 

curves represent a similar point on the same specimen in the baseline and aged condition. The point of 

interest is identified in Figure 5. Figures 6(b) and (c) show the first and second derivative curves 

respectively for the same location. 

Using the information contained in the derivative images further analysis was conducted in an attempt 

to quantify any changes in the response of the material. This was achieved by calculating the time at 

which the second derivative curve reaches a peak value. This point was chosen to provide a consistent 

measurement location within the time series of data. In addition, this point in time is associated with the 

thermal front reaching the back wall of the sample, thus allowing interrogation of the entire specimen 

volume. It is expected that distributed flaws within the material will act as barriers to the flow of heat thus 

affecting the resulting time to reach a peak value. Second derivative peak times were calculated for a 

region of interest of each sample in both the baseline condition and following aging and an average was 

calculated for all samples in the baseline and aged condition. Results of this analysis are shown in 

Figure 7. Based on these values and the examination of individual samples, no significant trend was 

identified. 
NASA/CR—2010-216793 5 

 
NASA/CR—2010-216793 6 
NASA/CR—2010-216793 7 

Summary 

A series of braided composite samples were nondestructively investigated to determine quantitative 

and qualitative measurements of degradation due to thermal-humidity cycling. Results from examination 

using pulsed thermography and ultrasonics did not reveal a measureable change following aging and 

showed a consistent visual appearance following aging. There are several possible reasons for this result. 

In the case of ultrasonic measurements, the size of the expected damage is most likely hidden by the 

complex composite architecture. This complexity limits the frequency that can be used for inspection and 

thus limits the sensitivity of the technique. The large amount of scatter in the measured attenuation values 

within the specimens highlights this masking effect. In the case of thermography, the situation may be 

similar in the sense that the overall scatter in the data due to composite architecture may overwhelm the 

sensitivity of the technique. 

Further research in this area may yield methods for assessing aging related degradation. For a 

technique to be successful, there must be a clear separation of the environmental degradation effects from 

the composite architecture response or by utilizing techniques that measure indirect effects such as 

strength or stiffness changes. Further data regarding the effect of aging on mechanical properties and 

microstructure will be examined and correlated with NDE data as it becomes available. 

References 

1. A. Nakai, S. Ikegaki, H. Hamada and N. Takeda, “Degradation of braided composites in hot water,” 

Composites Science and Technology 60 (2000): 325–331. 

2. Y.Z. Wan, Y.L. Wang, Y. Huang, et. al., “Moisture sorption and mechanical degradation of 

VARTMed three-dimensional braided carbon-epoxy composites,” Composites Science and 

Technology 65 (2005): 1237–1243. 

3. J. Wolfrum, S. Eibl, L. Lietch, “Rapid evaluation of long-term thermal degradation of carbon fiber 

epoxy composites,” Composites Science and Technology 69 (2009): 523–530. 

4. X. Fang, X. Chen, Z. Wang, S. Gifford, W. Lin, D. Hanks and J. Woods, “Fiber preform effect on 

composite microcrack,” Presented at SAMPE 39th ISTC—Cincinnati, OH, Oct. 29–Nov. 1, 2007. 

5. J. Schors, K. Harbich, M. Hentschel and A. Lange, “Non-destructive Microcrack Detection in Modern 

Materials,” Proceedings of the 2006 ECNDT, Berlin, Germany. 
NASA/CR—2010-216793 8 

6. J. Walker, S. Russell M. Suits and G.Workman, “Fatigue Crack and Porosity Measurement in 

Composite Materials by Thermographic and Ultrasonic Methods,” Presented at the 5th Conference on 

Aerospace Materials, Processes, and Environmental Technology, Huntsville, Al, 2002. 

7. Nondestructive Testing Handbook Volume 7—Ultrasonic Testing, Patrick O. Moore Editor, ASNT, 

2007 

8. P.V. Xavier Maldague, Theory and Practice of Infrared Technology for Nondestructive Testing, John 

Wiley, Interscience, New York, 2001. 

9. Nondestructive Testing Handbook Volume 3—Infrared and Thermal Testing, X. Maldague and 

P. Moore Eds., ASNT, 2001 

10. S.M. Shepard, “Advances in Pulsed Thermography,” Proc. SPIE Thermosense XXIII,Vol. 4360, 

pp. 511–515, April 2001. 

11. R.E. Martin, A.L. Gyekenyesi and S.M. Shepard, “Interpreting the Results of Pulsed Thermography 

Data,” Materials Evaluation, Vol. 61 No. 5, May 2003, pp. 611–616. 
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14. ABSTRACT

This paper examines the ability of traditional nondestructive evaluation (NDE) techniques to measure the degradation of braided polymer 

composite materials subjected to thermal-humidity cycling to simulate aging. A series of braided composite coupons were examined using 

immersion ultrasonic and pulsed thermography techniques in the as received condition. These same specimens were then examined 

following extended thermal-humidity cycling. Results of this examination did not show a significant change in the resulting (NDE) signals.

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