The Use of Fibre Optic Sensors for Damage Detection and Location in Structural Materials

Abstract

Abstract. The measurement of changes in the properties of ultrasonic Lamb waves propagating through structural material has frequently been proposed as a method for the detection of damage. In this paper we describe work that uses optical fibre sensors to detect the Lamb waves and show that the directional properties of these sensors allow us to not only detect damage, but also to locate it.We look at two types of optical fibre sensor, a polarimetric sensor and the fibre Bragg grating. The polarimetric sensor measures the change in birefringence of a fibre caused by the pressure wave of the ultrasound acting upon it. This is an integrated sensor since the fibre length bonded to the sample needs to be greater than the ultrasonic wavelength in order to obtain the required sensitivity. The maximum sensitivity of this sensor is when the fibre is positioned normal to the direction of wave propagation. Fibre Bragg gratings are essentially point sensors since the grating length needs to be a fraction of the ultrasound wavelength to obtain maximum sensitivity. Ultrasound is detected mainly through the in-plane strain it produces and maximum sensitivity is therefore produced when the grating is aligned parallel to the direction of wave propagation.Holes drilled into sample plates can be detected using both type of sensor by examining the changes in either the transmitted Lamb wave or through detection of the reflections produced by the hole. The sensitivity of the technique is shown to be determined by the relative positions of the acoustic source, the hole and the sensor. If we use fibre Bragg gratings in a rosette configuration (i.e. 3 gratings forming an equilateral triangle) then the direction of the Lamb wave can be determined using the directional sensitivities of the gratings. Using two such rosettes allows us to calculate the source of the wave from the intersection of two of these directions. If the source of the wave is the hole (which acts as a passive source), then the location of that hole can be determined.

Damage detection under a composite patch using an embedded PZT-FBG ultrasonic sensor array

“…But the best results were obtained from those PZT-FBG combinations, where the direction of the FBG sensor is perpendicular to the line connecting it to the PZT exciter. Due to the directional sensitivity of the FBG sensor [4], the direct signal picked from the PZT exciter, being perpendicular to the FBG axis, produces a relatively weak response, while the signal reflected from the hole is received at the most sensitive angle of 0 deg. Having an array of sensors, it is a relatively simple matter to locate the damage.…”

“…Moreover, for composite structures, such optic fibers sensors can be easily embedded into the structure, eliminating the need for sensor protection, and can be also used to monitor strain and temperature during curing or service [2,3]. Unlike the omnidirectional sensitivity of the commonly used piezoelectric discs, FBGs have directional sensitivity [4], which can be advantageously used for damage detection and location. Quite a few investigators have described successful attempts to measure Lamb waves using FBGs [4][5][6][7][8][9], either in surface-attached or embedded forms.…”

“…Unlike the omnidirectional sensitivity of the commonly used piezoelectric discs, FBGs have directional sensitivity [4], which can be advantageously used for damage detection and location. Quite a few investigators have described successful attempts to measure Lamb waves using FBGs [4][5][6][7][8][9], either in surface-attached or embedded forms. Moreover, measurement results have clearly indicated the potential of embedded FBGs to detect debonding in a composite laminate and to estimate its size and evolution [8].…”

Acousto-Ultrasonic Optical Fiber Sensors: Overview and State-of-the-Art

“…The change in length due to an applied strain is given simply by (10) The change in the propagation constant depends on a change in the refractive index , and a change in the fiber's diameter . This is given as [13] (11)…”

“…Fiber Optic Sensors, edited by Udd [8], also includes a number of acoustic sensors and hydrophones. A more recent review for optical fiber ultrasonic sensing is given by Atique et al [9], and to a lesser extent by Thursby et al [10]. What follows is a broader review of the literature for optical fiber acoustic and ultrasonic sensing, summarizing the vast majority of sensing methods and techniques currently in the literature.…”