Colin P. Ratcliffe

Vibration technique for locating delamination in a composite beam

An experimental nondestructive vibration-based technique for locating a delamination in a composite beam is presented. The method operates on the fundamental displacement eigenvector, which is converted to a curvature mode shape. The application of a unique, gapped smoothing damage detection method to the curvature yields a damage index that locates the delamination, irrespective of its position along the beam or depth within the beam. The procedure can operate solely on data obtained from the damaged structure. Models or data from the undamaged structure are specifically not required or used during the analysis. The procedure reported here is highly satisfactory for experimental data, where small variations in the measurement cause false features in other published curvature-based damage detection methods. The damage location method is demonstrated with a finite element model of a composite beam, where a delamination is modeled by relaxing connectivity between elements at the desired location of the delamination. The results of an experimental investigation of a composite beam with a manufactured delamination are also presented. When the gapped smoothing method was used on the experimental modal data, the delamination was successfully located.

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.

Type A and Type B Elemental Uncertainties

Elemental uncertainties are from two sources: uncertainty in transducers, and uncertainty in the measurand. This chapter provides a non-mathematical introduction to the sources of many of these elemental uncertainties. The chapter also introduces the GUM (Guide to Uncertainty of Measurement), and Type A and B uncertainties.

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.

Sensitivity by Perturbation

When detailed mathematical equations for a process are not available or are intractable, we can often use a perturbation method to help determine sensitivities—critical aspects required for uncertainty propagation. This chapter introduces and demonstrates the concept of perturbation which can be used for an existing process by a real experiment, or by numerical modelling if there is a computer simulation of the process.

Expanded Uncertainty of a Measurement and an Uncertainty Budget for a Single Measurement

This chapter takes the standard uncertainty calculated in the previous chapter and shows how to apply a statistical confidence in order to determine the expanded uncertainty, which is commonly referred to as “the uncertainty of a measurement.” The chapter demonstrates how uncertainties quoted at one level of confidence (e.g., on a calibration certificate) can quickly be converted to a different level of confidence. The concept of uncertainty budgets is introduced, and used to show how they can be used to identify where effort (money!) needs to be put to improve the accuracy of a measurement, and where extra effort is unwarranted.

Standard Uncertainty of a Measurement

Standard uncertainty can be thought of as the building block for uncertainty analysis—the lowest common denominator. Analysts have to combine uncertainties that are quoted in different ways, and this chapter demonstrates how to convert these uncertainties into standard uncertainties. Uncertainties due to scale size, resolution, rounding and truncation are also introduced. The chapter then introduces the combination of several standard uncertainties in order to determine the total standard uncertainty for a measurement.

Propagation of Uncertainty An Uncertainty Budget Example

Finding the uncertainty of a single measurement is just part of the story. In this chapter we show how to combine the uncertainties from several measurements to determine the total uncertainty for a calculated quantity. For example, while generating a cost estimate for repaving a rectangular parking lot, you may need to combine the uncertainties in measured distances (length and width of the lot) to find the uncertainty in area. The chapter develops two extensive examples: determination of the elastic modulus from an experiment measuring several different quantities, and a radiation heat transfer problem. In both examples uncertainty budgets are developed and demonstrate how they can be used, for example, to minimize the cost of equipment.

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.