Nondestructive Evaluation Technique Utilizing Embedded Thermal Fiber Optic Sensors

Abstract

We investigated a thermal nondestructive evaluation (NDE) technique based on thermography that uses optical fiber thermal sensors to detect damage within a laminated graphite epoxy composite specimen. Two sets of composite samples were used for testing, one set had fiber optic sensors embedded between the layers while the other relied on thermocouple sensors attached to the exterior. A simulated impact system was used to induce damage of varying degrees into the samples. The damage was confirmed either with X-ray or C-scan imaging. The test apparatus in addition to the test data obtained from both sets of samples are presented. The data confirms that the fiber optic thermal sensors can not only detect the presence of damage, but can measure the severity of damage as well. This is the first known attempt to use in situ fiber optic point sensors with thermography to detect damage.

Health Monitoring Technique for Composite Materials Utilizing Embedded Thermal Fiber Optic Sensors

“…This is expected because Teflon has a smaller conductivity (approximately 0.25 W/m K) than the composite in the direction perpendicular to the fibers (approximately 0.87 W/m K), and thus a larger insert should impede the heat flow to a greater extent, creating a larger temperature difference. This data is consistent with the data taken from the samples containing actual damage [5].…”

“…The fiber optic sensors were manufactured at UCLA (Figure 2(a)). The sensors are based on the EFPI principle (see Figure 2(b)) and the fabrication procedure and mode of operation was detailed in [5]. However, a brief description follows.…”

“…To overcome these limitations, a fundamentally different approach to thermography is proposed which replaces the external IR cameras with internally embedded fiber optic thermal sensors [5]. Damage to the composite is identified by measuring the local temperature response with the embedded fiber optic sensors.…”

“…A proof-of-concept study of the proposed technique utilizing embedded thermal sensors and applied impact damage has been published by the authors [5]. The continuing work presented here used simulated impact damage in composite specimens to facilitate the development of a numerical model.…”

Thermal-based damage detection in porous materials

“…Additional advantages of thermal NDE are that it is a single sided, noncontacting and a large area technique making inservice evaluation feasible. It is also possible to embed temperature sensors in the material [18] for continuous structural heath monitoring. We treat here the problem of thermal NDE in porous materials.…”