Safety-critical Structures Research Articles

An integrated system for bridge management using probabilistic and mechanistic deterioration models: Application to bridge decks

This paper presents a system for the maintenance management of aging highway bridges that integrates two different approaches for deterioration modeling. Probabilistic state-based/time-based models are used to predict the macro-response of bridge components for network level analysis, while reliability-based mechanistic models are used to predict the micro-response of bridge components for project level analysis. Probabilistic state-based/time-based models are developed using qualitative performance indicators (condition ratings) that are determined through visual inspections to identify the overall condition of damaged components in a bridge network. Reliability-based mechanistic models are developed using quantitative performance indicators (physical parameters) that are determined through detailed condition surveys, analytical assessments, and empirical investigations to identify the extent and severity of specific deterioration mechanisms for safety critical structures and/or highly damaged components. The condition rating data obtained from the Ministére des Transports du Québec database and the condition assessment of the Dickson Bridge in Montreal, Canada were used to demonstrate the two approaches for predicting the deterioration of concrete bridge decks at the two levels of management of the integrated system.

Debonding detection using a self-calibration sensor network

Adhesively bonded joints between two plates are gradually receiving increasingacceptance in safety-critical structures, and therefore methods for nondestructiveevaluation (NDE) of adhesive joints have drawn much attention. In this paper,specimens with lap splice joints are prepared for the purpose of guided-wave-baseddebonding damage detection. Fibre optic Doppler (FOD) sensors are used toacquire guided ultrasonic waves propagating in two kinds of bonded specimens,namely aluminium and carbon fibre reinforced plastic (CFRP) assemblies. Toovercome the drawbacks of sensor networks in conventional structural healthmonitoring systems, a self-calibration sensor network is constructed based on elasticanalyses of the bonded structures. By using the proposed sensor network, both thebaseline measurement and the defect-related guided wave signals can be acquired inone excitation experiment. Moreover, complicated signal interpretation is alsoavoided by using the simplest feature, arrival time, of the ultrasonic wave signals.

Optical calibration for both out-of-plane and in-plane displacement sensitivity of acoustic emission sensors

The use of piezoelectric sensors for acoustic emission (AE) monitoring provides an extremely sensitive detection method of AE events. The sensors are used to detect the stress waves, resulting from an AE event, which arrive at the surface of a structure. The sensors provide high sensitivity, and are generally based on a high- Q design where the sensor is used to detect AE events around its resonance. Sensitivity calibration of these sensors is essential for accurate assessment of the AE measurement and particularly important for sensors mounted on safety critical structures. This paper proposes an optical method which enables both the out-of-plane and in-plane displacement sensitivity of an AE sensor to be established independently from each other. In the method, a laser homodyne interferometer is used to measure the out-of-plane and in-plane displacement of the surface of a large test block excited by a repeatable source transducer. The out-of-plane displacement is measured by aligning the laser beam perpendicular to the surface with time gating of the receive waveform used to isolate only the direct arrival of the longitudinal wave produced by the piston source transducer. For the in-plane displacement measurement, the laser beam is aligned parallel to the surface to intersect a small optically reflective step with the time waveform being gated to measure only the direct shear arrival produced using a normal incidence shear wave source transducer. In each case, the interferometer measurement is followed by coupling the sensor under test to the measurement surface, which is then exposed to the same acoustic field and the sensor output signal measured. This substitution method allows the sensor sensitivity to be obtained in terms of volts per unit displacement for both the out-of-plane and the in-plane surface displacement. The method provides a comprehensive description of an AE sensor response to different planes of displacement and offers the potential for a traceable sensor calibration to units of length.

Structural health monitoring – What is the prescription?

Structural health monitoring (SHM) systems can prevent structural failure of safety-critical structures such as aircraft, bridges, nuclear reactors and dams, which cannot be allowed to fail. An SHM system uses the techniques of non-destructive inspection (NDI) to provide continuous (or on-demand) information about the state of a structure, so that an assessment of the structural integrity can be made at any time, and timely remedial actions may be taken as necessary. A large number of sensors forms the front end of an SHM system to provide information on the condition of the structure. The information from the sensors is incorporated into structural analyses and failure models to assess the state of the structure and to predict the remaining lifetime. Thus, the underlying concept is based on detecting and characterizing damage and assessing it in terms of failure mechanics and damage growth laws. Materials engineering and applied mechanics play dominant roles in both the diagnostic and prognostic components of SHM. A probabilistic approach is essential, as will be shown by an example of the growth and detection, or lack thereof, of surface-breaking cracks. The probabilistic approach also plays a key role in the determination of inspection schedules.

Evaluation of Defects in FRP Composites by NDT Techniques

The very fast developments in the technology of composite materials have led to newer and wider applications of such promising materials. Composite materials offer a number of potential advantages in the aerospace field, particularly in safety-critical structures such as primary and secondary aircraft components. The presence of several types of defects such as voids, inclusions, debonds, improper cure, and delamination are almost common during the manufacture and use of composite materials. The proper assessment of such defects is necessary to utilize the full potential of these materials. This present study has been taken up to detect and characterize such defects using a thermal imaging technique. The technique is found to be deterministic in the acceptable level for the assessment of defects in polymer composites. The thermography, ultrasonics (A-scan and C-scan), and also microscopy (photo-microscope and scanning electron microscope) methods were used here to assess the defects. The detection and characterization of the wide range of defects requires a number of specialized non-destructive methods. The proper assessment of defects is essential, particularly in safety-critical structures such as primary and secondary aircraft components, to avoid catastrophic failure.

Fatigue damage monitoring in polymeric composites using multiple fiber bragg gratings

Cyclic loading during service may induce sub-surface defects in polymeric composite materials. These insidious defects may grow and eventually lead to catastrophic failures. The current work aims at exploring the possibility of using embedded fiber Bragg grating to monitor damage evolution. Fatigue damage evolution in Graphite/Epoxy coupons having a [−45/90/+45/0] 2s lay-up sequence with a central circular hole subjected to cyclic loading is evaluated. This study suggested that the embedded fiber bragg gratings have good potentials to detect the appearance of damage. It is hoped that this methodology can be further developed as an online condition monitoring technique for safety-critical structures, thereby improving the safety and reliability of composite structures.

A new structural health monitoring system for composite laminates

The increasing use of composite laminates in safety critical structures has prompted the development of a robust structural health monitoring system for laminates, which uses metastable ferrous alloy inserts embedded within the laminate during component construction to provide an indication of the peak tensile strain encountered by the laminate. The metastable ferrous alloy insert has an austenitic crystal structure at room temperature, but upon application of strain, this transforms to a thermodynamically stable martensite, resulting in a change in magnetic susceptibility, which can be correlated with the peak strain experienced by the material (strain memory effect). This paper presents the test results that show that it is possible to manufacture a smart laminate in this fashion, and that sufficient strain is experienced by the insert to provide a significant change in magnetic susceptibility, thereby warning of a high strain level in the laminate. Various insert geometries and laminate thicknesses are also tested for their effect on the susceptibility measurements.

Measuring Changes of State in Concrete

Currently a combination of varied methods of both intermittent and continuous monitoring have to be employed in the integrity assessment of safety-critical heterogeneous structures such as reinforced-concrete pressure-vessels, buildings or bridges, and those methods vary from simple visual inspection to embedded vibrating-wire strain-gauge techniques. This paper presents ongoing work based on a concept that utilises comparisons of intermittent vibrational response measurements. The non-destructive method employed compares energy-signatures transmitted through a region of material to establish any differentials in response and it then assesses any change-of-state (CoS) from that behaviour observed.

The effects of strain rate and failure modes on the failure energy of fibre reinforced composites

The utilisation of polymer composite materials in safety critical structures necessitates their full characterisation. This will bring about the much needed boost in confidence for their application to industrial situations, especially where high speed impact is a concern. Impact performance can in some way be measured by the energy absorbed or expended to failure of a material. Hence, establishing the rate effect on energy absorption is of paramount importance when designing for impact. Tensile, shear and 3-point bend tests were conducted on a woven glass/epoxy laminate at increasing rates of strain. The results suggest a linear relationship between expended energy and the log of strain rate in the laminate tested. Furthermore, it was found that a relationship exists between the flexural energy obtained at low strain rate and the high speed (2–4 m s −1 ) test values. Magnified views of the failed specimen surfaces, viewed under a scanning electron microscope, indicate a change in failure modes as strain rate is increased, which brought about the increase in energy observed.

Quantitative classification of adhesive bondline dimensions using Lamb waves and artificial neural networks

Adhesive bonding of metal assemblies is gaining acceptance for use with safety critical structures, and there is a need for effective inspection for both quality assurance (QA) and the assessment of condition in service. One aspect of QA is the need for the dimensions of adhesive bondlines to be within tolerance and measurable. This paper describes the application of ultrasonic Lamb waves in the determination of the principal dimensions of two forms of adhered joints (Lap and T-form) between metal plates. Low order Lamb wave modes (s0 and a1) are propagated across adhered bond-lines, and the received signals are transformed to the modulus frequency domain (FD). The FD data are used as input to artificial neural networks (ANNs), which are trained to associate features in the input data with principal bondline dimensions. The performance of different network structures and simplified forms of these is examined, and the technique gives reliable estimates of the required dimensions in bondlines not included in network training. The interconnected weights of simplified networks provide evidence of the features in Lamb wave signals that underlie the successful operation of the method.

The Qualification of Safety Critical Structures by Finite Element Analytical Methods

The Qualification of Safety Critical Structures by Finite Element Analytical Methods

The Qualification of Safety Critical Structures by Finite Element Analytical Methods

The paper introduces the concept of certifying or qualifying structures in a safety critical situation using the finite element method. Error control and error treatment methods for this purpose are discussed together with the associated role of testing. The underlying methodology follows the principles laid down in the SAFESA (SAFE Structural Analysis) method which is described in outline.