Roger M. Crane

Fiber Optics for a Damage Assessment System for Fiber Reinforced Plastic Composite Structures

There are a limited number of nondestructive evaluation techniques available for field inspection of large composite structures and practically no viable techniques for in-service inspection. With this in mind, an innovative Damage Assessment System is proposed which is based on a concept of using an optical fiber mesh, implanted into the body of a fiber reinforced composite structure. Such a mesh would become an integral part of the structure during the course of its fabrication. The selection of the mesh fibers would be predicated on their strain to failure characteristics and strain compatibility with the base, composite reinforcing fibers. This optical system will be capable of locating damage, assessing severity and monitoring damage growth. A successful implementation of the total Damage Assessment System would involve the interaction of the optical fiber mesh with an adequately designed interrogative electronic package. This paper focuses on the former aspect of the total system. It will address some recent experimental work showing the practicality of the concept in assessing various modes of failure due to impact of composite plates, optical fiber selection, location and spacing of fibers, as well as the utility of the system for damage assessment in large, complex composite structures.

Fabrication and mechanical properties of multidimensionally (X-D) braided composite materials

This paper describes research concerning the resin impregnation and characterization of multidimensionally braided fiber-reinforced composite materials. These materials are an alternative to traditional laminated structures, having the potential for being more damage tolerant. Three graphite fiber systems were used in this investigation, and three processes were investigated for resin impregnation of the multidimensionally braided material using vacuum or pressure. Two were resin transfer techniques and the third was a resin film lamination technique. While all three methods are presented, the latter technique was chosen for impregnating the test specimens due to the consistently low void content and superior surface quality achieved by this method. Three variables having an important bearing on the performance of braided materials were investigated. These included the effect of braid pattern, tow size, and edge condition on the tensile, compressive, flexural, and interlaminar shear properties. The properties were obtained in the braid direction only. The cutting of the specimen edges substantially reduced both tensile and flexural strengths and moduli. Of the three braid patterns investigated (1 x 1, 1 x 1 × ½F and 3 x I), the 3 x I braid showed superior tensile performance and the 1 x I x ½F pattern exhibited superior flexural properties. Variation in fiber tow size caused variations in tensile, flexural, and short-beam shear properties. The 12K tow size specimens exhibited the best performance. All braided composite materials in the uncut edge condition showed significant improvements in their short-beam shear strengths, being equal to or greater than unidirectional laminated composites. This latter characteristic may be one of several indicators that multidimensionally braided composites are inherently damage tolerant.

High damping composite joint for mechanical vibration and acoustic energy dissipation

A design for a high damping composite joint which dissipates vibrations through the use of air gaps and viscoelastic material minimizing the transfer of vibrations to the metallic coupling. Viscoelastic material is used with adhesive to provide for increased energy dissipation by the joint. As the load increases on the joint, the load transfers from adhesive to the viscoelastic. The viscoelastic begins to take the load of the joint at the point where the adhesive becomes plastic. Acoustic vibrations are then dissipated in the viscoelastic and are prevented from being transferred to the metallic coupling by air gaps provided in the joint. The amount of viscoelastic and adhesive used depends on the anticipated load. Finite element analysis is used to calculate optimal amounts of viscoelastic and adhesive.

Broad area damage detection in composites using fibre Bragg grating arrays

This article reports on the development of a technique for broad area detection of structural irregularities in composites using an integrated fibre optic sensing network. The technique is founded on a broadband vibration-based methodology known as the structural irregularity and damage evaluation routine that uses features in complex curvature operating shapes to locate damage and other areas with structural stiffness variations. The original structural irregularity and damage evaluation routine methodology relied on impact excitation at a series of grid points on the structure with the response recorded using a small number of reference accelerometers to determine the operating curvature shapes. This methodology has been modified to allow for single-point or environmental excitation with measurement of the curvature shapes provided by a spatially dense network of fibre Bragg grating strain sensors’ surface mounted on the structure. This modified approach is known as the inverse structural irregularity and damage evaluation routine and has been experimentally validated on an impact-damaged composite panel. The technique was then applied to a full size composite structure (ship’s rudder) containing structural features, where it was shown to successfully locate impact damage.

Broad-area detection of structural irregularities in composites using fibre Bragg gratings

The Structural Irregularity and Damage Evaluation Routine (SIDER) is a broadband vibration-based technique that uses features in complex curvature operating shapes to locate damage and other areas with structural stiffness variations. It is designed for the inspection of large-scale composite structures not amenable to more conventional inspection methods. The current SIDER methodology relies on impact excitation at a series of grid points on the structure and records the response using a small number of accelerometers to determine the operational curvature shapes. This paper reports on a modification to the SIDER technique whereby the acceleration measurements are replaced with in-plane strain measurements using Fibre Bragg Gratings (FBGs). One of the major challenges associated with using Bragg gratings for this type of response measurement is that the strains induced by structural vibrations tend to be low, particularly at higher frequencies. This paper also reports on the development of an intensity-based, swept wavelength interrogation system to facilitate these measurements. The modified SIDER system was evaluated on an E-glass/vinyl ester composite test beam containing a machined notch. The measurements accurately detected the presence and location of the notch. The distributive capacity of FBGs means that these sensors have the potential to replace the excitation grid with a measurement grid, allowing for single point or environmental excitation. The spatially separated measurements of strain can be used to provide the curvature shapes directly. This change in approach could potentially transition SIDER from an interval-based, broad-area inspection tool to an in-service structural health monitoring system.

Experimental investigation into the use of vibration data for long-term monitoring of an all-composite bridge

This paper presents the results of an ongoing investigation into using broadband vibration data to monitor the structural integrity and health of an all-composite bridge. Bridge 1 - 351 on Business Route 896 in Glasgow, Delaware, was replaced with one of the first state-owned all-composite bridges in the nation in the fall of 1998. The bridge consists of two E- Glass/vinyl ester sandwich core sections (13-ft X 32 ft) joined by a longitudinal joint in the traffic direction. Each sandwich core section consists of a 28-inch deep core and 0.4 - 0.7-inch thick facesheets. Vibration data were obtained from a mesh of 1050 test points covering the upper and lower surfaces of the bridge. From the modal information and the visualization of the data, several aspects of the structural behavior of the bridge were obtained. These characteristics include the interactions between the bridge and abutments; the effectiveness of the longitudinal joint to couple the deck sections; the effectiveness of the core to couple the face sheets; and the structural integrity and dynamic consistency of the entire structure. In addition, mode shapes and natural frequencies were determined and are correlated with theoretical calculations and vibration analyses conducted for this bridge. A novel algorithm using the vibration data is being developed that enables local perturbations sensitive to the state of the material (e.g. manufacturing defects, material degradation or service damage) to be detected and spatially located in the bridge. This technique has been successfully validated for locating damage in 1-D beam structures and is being extended to the 3-D sandwich structure. Applications for quality assurance/quality control and health monitoring of large composite bridge structures using this technique will be discussed.

Development of an integrated fibre optic sensing network for a composite rudder

This paper outlines the various steps considered in the design, development and application of a network of 294 optical fibre based strain sensors on a glass fibre reinforced rudder for a mine counter measures vessel. The sensing array is designed for use together with a vibration-based analysis tool to be implemented as an in-service structural health assessment system.

An Experimental Comparison of the In-Air and Underwater Damping Properties of Composite Cylinders

Acoustic radiation and self noise are potentially problematic in many underwater applications since both can interfere with the performance of onboard sensors. This is especially prevalent when the structural material forthe pressure hull is metallic. In an effort both to reduce radiated noise and minimize structural vibrations, experiments were undertaken to investigate the use of composite materials for cylinders which would be subjected to hydrostatic pressure loading. The damping characteristics of cylinders with various designs and material combinations were obtained initially from modal analysis testing in air. This technique very quickly provides very consistent results. The results desired for this venture, however, were the damping characteristics of these cylinders underwater, which significantly is a more complex experimental configuration. This paper presents the test procedure and results of modal analysis testing conducted in air and underwater on some composite cylinders. The experimental results, which are presented and compared herein, include the resonant frequencies, damping factors, and mode shapes at resonance. In order to ensure a valid comparison, the natural frequencies and damping factors are compared on a mode-by-mode basis, that is, the result for two cylinders are compared when the resonance has the same mode shape. Despite the increased complexity of the experiments required to obtain these underwater measurements, it is shown that the in-air and underwater levels of damping observed at relatively low frequencies are comparable. The results show that an appropriate choice of composite materials can increase the levels of damping by at least an order of magnitude over conventional metals. The damping can be increased additionally by using a double hollow core or triple-shell configuration with reinforced polyurethane ribs. The paper discusses this new reinforced polyurethane as a structural component of the cylinder, and proposes various mechanisms of damping that this material form provides.

Experimental modal analysis comparison of the vibration damping properties of composite cylinders measured in‐air and underwater

Self‐noise is problematic in many underwater applications, since it interferes with the performance of onboard sensors. To reduce self‐noise, experiments investigated using composite materials for cylinders subjected to hydrostatic pressure loading. This paper presents the test procedure and results of modal analysis testing conducted in‐air and underwater on some composite cylinders. The in‐air tests used impact force excitation, whereas the underwater tests used sound excitation. The results include resonant frequencies, damping factors, and mode shapes at resonance. In order to ensure a valid comparison, the natural frequencies and damping factors are compared on a mode‐by‐mode basis. Despite the increased complexity of the underwater experiments, it is shown that the in‐air and underwater levels of damping observed at relatively low frequencies are comparable. Further, an appropriate choice of composite materials can increase damping by at least an order of magnitude over conventional metals. The damp...

Composite cylinder cross section tailoring for radiated noise reduction

This paper presents the results of an ongoing investigation into the reduction of transmitted noise for underwater cylindrical structures. A reduction in the radiated self‐noise of cylindrical structures, compared with the conventionally utilized aluminum structures, was undertaken, investigating the use of composite materials with varying cross sections to meet the specified requirements. The cross sections investigated included several variations of a state‐of‐the‐art triple skin construction, along with monolithic, constrained layer and a semi‐debonded cross section. Cylinders have been fabricated and tested, comparing the dynamic vibration properties of the forced vibrations in air of the composite scaled cylinders to the baseline aluminum. The modal analysis determined mode shapes, natural frequencies, and damping loss factors. These were then tested in reverberant conditions under water to assess their acoustical performance. The in air and underwater tests were compared to identify similarities and differences between force excitation and acoustic excitation. The comparison showed where in air tests can be used as an aid to estimating underwater performance. The results also highlighted where in air tests are inadequate for predicting underwater behavior. The effect of the cross‐sectional design for the tailoring of the acoustic signature will be discussed.