Zahra Sharif Khodaei

A Bayesian Approach for Sensor Optimisation in Impact Identification

This paper presents a Bayesian approach for optimizing the position of sensors aimed at impact identification in composite structures under operational conditions. The uncertainty in the sensor data has been represented by statistical distributions of the recorded signals. An optimisation strategy based on the genetic algorithm is proposed to find the best sensor combination aimed at locating impacts on composite structures. A Bayesian-based objective function is adopted in the optimisation procedure as an indicator of the performance of meta-models developed for different sensor combinations to locate various impact events. To represent a real structure under operational load and to increase the reliability of the Structural Health Monitoring (SHM) system, the probability of malfunctioning sensors is included in the optimisation. The reliability and the robustness of the procedure is tested with experimental and numerical examples. Finally, the proposed optimisation algorithm is applied to a composite stiffened panel for both the uniform and non-uniform probability of impact occurrence.

Instantaneous Baseline Damage Localization Using Sensor Mapping

In this paper, an instantaneously recorded baseline method is proposed using piezoelectric transducers for damage localization under varying temperature. This method eliminates need for baselines required when operating at different temperatures by mapping a baseline area onto the interrogation area. Instantaneously recorded baselines and current interrogation signals are calibrated based on the sensor mapping. This allows the extraction of damage scatter signal which is used to localize damage. The proposed method is used to localize actual impact damage on a composite plate under varying temperatures. The method is also applied to a stiffened fuselage panel to accurately localize impact damage.

Airborne Transducer Integrity under Operational Environment for Structural Health Monitoring

This paper investigates the robustness of permanently mounted transducers used in airborne structural health monitoring systems, when exposed to the operational environment. Typical airliners operate in a range of conditions, hence, structural health monitoring (SHM) transducer robustness and integrity must be demonstrated for these environments. A set of extreme temperature, altitude and vibration environment test profiles are developed using the existing Radio Technical Commission for Aeronautics (RTCA)/DO-160 test methods. Commercially available transducers and manufactured versions bonded to carbon fibre reinforced polymer (CFRP) composite materials are tested. It was found that the DuraAct transducer is robust to environmental conditions tested, while the other transducer types degrade under the same conditions.

Impact Damage Localisation with Piezoelectric Sensors under Operational and Environmental Conditions

Guided-wave structural health monitoring (SHM) systems with piezoelectric sensors are investigated for localisation of barely visible impact damage in CFRP plates under vibration and different thermal conditions. A single baseline set is used in a delay-and-sum algorithm with temperature correction for damage localisation in a large temperature range. Damage localisation is also demonstrated under transient thermal conditions, with signals recorded while the temperature is rapidly decreased. Damage severity due to successive impact events is studied under constant temperature. Damage is also localised when the plate is subjected to random vibration.

Determination of the Impact Location and Damage Characterization Based on Guided Waves

The aim of this paper is to understand the effects of the damage criteria modelling on the training phase (performed by means of Finite Element simulations) of an artificial neural network (ANN) enabled to locate impacts onto a CFRP laminate. The developed FE models have been also used to investigate the intra-laminar damage mode, which, among different ones, has the most effects on the residual strength of the panel.

Damage severity assessment in composite structures using ultrasonic guided waves with chirp excitation

This paper explores the feasibility of using ultrasonic guided Lamb waves to characterise the type and through thickness severity of damage present in composite plate-like structures. Two cases were considered, the first compared isolated subsurface delaminations between plies whilst the second case looked at more complicated barely visible impact damages caused by a low velocity impactor. In this study, the ultrasonic guided Lamb waves were generated by a surface mounted piezoelectric transducer and were sensed by a Laser Doppler Vibrometer. This allowed full wavefield imaging of the Lamb wave interaction with damage without the need for a previously acquired damage free baseline signal. In order to save time and improve the signal to noise ratio, the narrowband toneburst signals are reconstructed from a singular chirp response and a post-processing algorithm. Both cases showed similar results in that the first symmetric mode, S0, which is dominant at higher frequencies, caused mode conversions when interacting with the defects whilst the first anti-symmetric mode, A0, dominant at lower frequencies, mainly caused a change in phase and amplitude across the defects. Both cases also showed that as the damaged area got more severe, the effects of the damage on both modes became more pronounced.

Damage Detectability Model of Pitch-Catch Configuration in Composite Plates

Detectability of damage using Lamb waves depends on many factors such as size and severity of damage, attenuation of the wave and distance to the transducers. This paper presents a detectability model for pitch-catch sensors configuration for structural health monitoring (SHM) applications. The proposed model considers the physical properties of lamb wave propagation and is independent of damage detection algorithm, which provides a generic solution for probability of detection. The applicability of the model in different environmental and operational conditions is also discussed.

Damage Detection in Composite Skin Stiffener with Hybrid PZT-FO SHM System

A hybrid piezoelectric (PZT)/fibre optic diagnostic system has been developed for damage detection in built up composite structures. The hybrid system uses PZT transducers to actuate the structure and fibre optic (FO) sensors to capture the propagating wave. The diagnostic system will then have the advantages of both PZT and FO sensors. The applicability of the system is then tested for detecting an artificial damage at a skin/stiffener interface of a thick composite structure. The response of the FO sensors is then compared to PZT sensors and presented.

Spectral BEM for the Analysis of Wave Propagation and Fracture Mechanics

This paper presents a spectral boundary element formulation for analysis of structures subjected to dynamic loading. Two types of spectral elements based on Lobatto polynomials and Legendre polynomials are used. Two-dimensional analyses of elastic wave propagation in solids with and without cracks are carried out in the Laplace frequency domain with both conventional BEM and the spectral BEM. By imposing the requirement of the same level of accuracy, it was found that the use of spectral elements, compared with conventional quadratic elements, reduced the total number of nodes required for modeling high-frequency wave propagation. Benchmark examples included a simple one-dimensional bar for which analytical solution is available and a more complex crack problem where stress intensity factors were evaluated. Special crack tip elements are developed for the first time for the spectral elements to accurately model the crack tip fields. Although more integration points were used for the integrals associated w...

Reliability Assessment of SHM Methodologies for Damage Detection

Probability-based imaging which illustrates a distribution map of probability of damage presence in structures is a diagnostic method well established for damage detection in sensorized structures. Since the quality of the recorded signal is directly linked to the reliability of the diagnostic outcome, the assessment of robustness of the damage detection methodology is of high significance. In this paper, robustness and reliability of the current probability based imaging algorithms have been assessed for detecting BVID in a composite panel. Consequently, a proposed outlier analysis and DI probability distribution damage detection algorithm was shown to improve the reliability of the detection method.