Tonguy Liu

In-situ process and condition monitoring of advanced fibre-reinforced composite materials using optical fibre sensors

This paper presents a general overview of a number of optical fibre sensor systems which have been developed and used in advanced fibre-reinforced composites for in-situ process and condition monitoring. The in-situ process monitoring techniques were optical-fibre-based evanescent wave spectroscopy, transmission near-infrared spectroscopy and refractive index monitoring. The optical fibre sensors were successful in tracking the cure reaction. The condition monitoring of advanced fibre-reinforced composites was carried out using two intensity-based optical fibre sensor systems: an extrinsic multi-mode Fabry-Perot sensor and Bragg gratings. In addition to this, the feasibility of using the reinforcing fibre as a light guide was demonstrated. These sensor systems were evaluated under quasi-static, impact and fatigue loading. The test specimens consisted of prepreg-based carbon-fibre-reinforced epoxy and glass-fibre-reinforced epoxy filament-wound tubes. Excellent correlation was obtained between surface-mounted strain gauges and the embedded optical fibre sensors. The feasibility of using these sensor systems for the detection of impact damage and stiffness reduction in the composite due to fatigue damage was successfully demonstrated.

A multiplexed optical fibre-based extrinsic Fabry-Perot sensor system for in-situ strain monitoring in composites

The detection of impact damage in fibre reinforced composites is of significant concern because such damage can reduce the load-bearing ability of the composite. A number of factors can influence the nature and extent of impact damage development in composites including: (a) the type of reinforcing fibre and resin system; (b) the magnitude of the residual (fabrication) stresses; (c) the lay-up sequence; and (d) other factors such as the nature of the impactor, impact velocity, impact energy, temperature, moisture content in the composites, etc. From a structural health monitoring point of view, it is necessary to investigate the distribution of damage through the thickness of the composite. This paper reports on a simple, partially multiplexed optical fibre strain sensor system for in-situ strain and residual strain measurements in a carbon fibre reinforced epoxy composite. An extrinsic Fabry-Perot interferometric (EFPI) sensor design was used along with single-mode fibres. The multiplexing scheme was based on wavelength division via the use of two super luminescent diodes (SLDs) at different wavelengths. A low-cost fibre optic CCD spectrometer was used as the detector. The multiplexing scheme was demonstrated using two EFPI sensors. In principle, a number of EFPI sensors can be multiplexed using the proposed scheme provided that each sensor is illuminated at a specified and different wavelength. The feasibility of detecting the residual strain in the composite was demonstrated successfully at two specified positions within a 16-ply carbon fibre reinforced composite panel. Preliminary results indicated that the sensor system was also capable of detecting the effects of a 3.2 J impact. Excellent correlation was obtained between the EFPI sensor output and that obtained using surface mounted strain gauges.

A multi-mode extrinsic Fabry - Pérot interferometric strain sensor

This paper reports on the fabrication and evaluation of a multi-mode extrinsic Fabry - Perot interferometric (EFPI) sensor which is capable of measuring both tensile and compressive strains. A scanning monochromator was used to measure the absolute cavity length of the EFPI sensor. Sensors of this type were embedded within a 16-ply carbon-fibre-reinforced epoxy composite and tested under quasi-static tensile and compressive loading conditions. Excellent correlation was observed between the EFPI sensor and a surface-mounted extensometer. The sensor system can operate in the strain range from -1 to 1% with an accuracy of better than 30 micro-strain. Preliminary results indicated that the sensor design was relatively insensitive to temperature in the range 38 - 180 . An analysis of the relationship between the insensitivity and the sensor geometry is also presented.

Simultaneous strain and temperature measurements in composites using a multiplexed fiber Bragg grating sensor and an extrinsic Fabry-Perot sensor

Optical fiber Bragg grating (FBG) sensors have significant potential for use as embedded devices to monitor the structural integrity of engineering materials. The main drawback of the FBG strain sensor is its cross-sensitivity to temperature. This paper reports a simple scheme for multiplexing a FBG and an extrinsic Fabry-Perot interferometric (EFPI) sensor to enable the decoupling of strain from temperature. The EFPI sensor was constructed using a precision bore quartz capillary tube which housed two cleaved optical fibers. The gap between the fiber end- faces served as the Fabry-Perot cavity. Since the coefficients of thermal expansion between the optical fiber and the capillary tube were similar, the EFPI sensor has a very low sensitivity towards temperature. Therefore, when both sensors are placed close together, the EFPI sensor can act as the strain sensor, and temperature can be determined from the FBG wavelength shift after taking out the strain effect. The signal processing for the EFPI sensor was based on a channelled spectrum method using a CCD spectrometer. The same CCD spectrometer was also used to determine the wavelength shift of the FBG. The cross-talk between the EFPI and FBG sensors was evaluated. The feasibility of conducting simultaneous strain and temperature measurements was demonstrated.

A multi-purpose optical fibre sensor design for fibre reinforced composite materials

This paper reports on the evaluation of a multi-functional extrinsic Fabry - Perot optical fibre-based sensor design. The sensor was constructed using multimode and single mode optical fibres and a precision bore capillary tube. Fusion joints were used to secure the optical fibres into the capillary tube. The separation between the cleaved end-faces of the optical fibres defined the cavity length for the Fabry - Perot sensor and the distance between the fusion joints defined the gauge length for this strain and temperature sensor. The sensor design was modified to: (i) monitor the progress of cure in an epoxy/amine resin system; (ii) detect the ingress of moisture in a cured epoxy/amine resin system; (iii) monitor the vibration characteristics of a pre- and post-impact damaged carbon fibre reinforced epoxy panel; and (iv) discriminate between strain and temperature measurements. The feasibility of using this type of sensor for cure monitoring, strain, temperature, residual stress measurements and damage detection in advanced fibre reinforced composites is demonstrated.

Design, fabrication, and evaluation of an optical fiber sensor for tensile and compressive strain measurements via the use of white light interferometry

An embedded, intensity-based fiber optic sensor was previously designed and evaluated for strain monitoring in advanced fiber reinforced composites under dynamic loading conditions. The original sensor design involved the use of two multimode fibers, each with a cleaved end. These fibers were fitted into a glass capillary and were secured in position via a fusion splice at each end of the capillary. However, the effective operational strain range of this sensor design was limited primarily to tensile loading. In order to use this sensor under compressive loading regimes, it was necessary to develop a technique to construct the sensor with a known separation of the fiber end-faces. In effect, the sensor is an extrinsic Fabry-Perot interferometric sensor. The signal processing was based on a scanning monochromator. The feasibility of using the optical fiber sensor for tensile and compressive strain measurements was demonstrated. The sensor was also used to obtain in-situ stiffness reduction data during the fatigue testing of a cross-ply carbon fiber reinforced composite. An analysis of the relationship between detection sensitivity and sensor geometry is also presented.

In-situ strain monitoring in composites using an embedded extrinsic Fabry-Perot interferometric sensor and a CCD detection system

This paper reports on the use of a multimode extrinsic fiber Fabry-Perot interferometric sensor for quasi-static and dynamic fatigue loading experiments. A surface mounted extensometer was also used to measure the strain in the composite as a function of applied load. Excellent correlation was obtained between the strain data from the extensometer and the embedded EFPI sensor. With reference to dynamic loading, the sensor was found function reliable up to 1,600,000 cycles when the fatigue test was terminated. The fatigue tests were carried out using a peak stress of 260 MPa with a stress ratio of negative 0.40, and a frequency of 5 Hz. The signal processing technique was based on a channelled spectrum CCD spectrometer. The sensitivity of quasi-static strain measurements was approximately 30 micro-strain with a strain range of approximately negative 1% to 1%. The feasibility of using the EFPI sensor for stiffness-decay measurements during fatigue testing of composites was demonstrated. Preliminary results form the use of a single-mode EFPI sensor design for strain measurements in composites is also presented.

Simultaneous strain and temperature measurements in composites using extrinsic Fabry-Perot interferometric and intrinsic rare-earth-doped fiber sensors

This paper reports on a novel optical fiber-based sensing scheme for conducting simultaneous strain and temperature measurements. The sensor design involved the use of an extrinsic Fabry-Perot interferometric strain sensor and a rare-earth doped fiber fluorescence decay-time based temperature sensor. The combined sensors were embedded in a carbon fiber reinforced composite system and evaluated. The feasibility of using this embedded sensor configuration for simultaneous strain and temperature measurements was demonstrated.

Quartz and E-glass fiber self-sensing composites

This paper reports on developments in the field of self- sensing fiber reinforced composites. The reinforcing fibers have been surface treated to enable them to act as light guides for short distances. The reinforcing fiber light guides were embedded in carbon fiber reinforced epoxy prepregs and processed into composites. The resultant composite was termed the self-sensing composite as any damage to these fibers or its interface would result in the attenuation of the transmitted light. The self-sensing fibers were capable of detecting a 2 J impact.

Simultaneous strain and temperature measurement using an integrated fiber Bragg grating/extrinsic Fabry-Perot sensor

This paper reports on a novel optical fiber sensor configuration for conducting simultaneous strain and temperature measurements. The sensor consisted of an optical fiber-based extrinsic Fabry-Perot interferometer (EFPI) with an integrated fiber Bragg grating (FBG). The FBG was located within a glass capillary which housed the EFPI sensor and was thus in a strain-free condition. The FBG is primarily sensitive to temperature, while the EFPI was sensitive to both strain and temperature. The integrated FBG/EFPI sensor was embedded in a carbon fiber reinforced composite and evaluated. The standard deviation of strain measurement was 36 (mu) e in the range 0 to 1200 (mu) e, and the temperature measurement had a standard deviation of 3.5 degrees C in the range 30 degrees to 70 degrees C. The thermal expansion of the cross-ply composite was investigated and was found approximately 4.05 X 10-6 degrees C.