391 results found
Yoo K, Bacarreza Nogales O, Aliabadi MHF, 2021, Multi-fidelity probabilistic optimisation of composite structures under thermomechanical loading using gaussian processes, Computers and Structures, Vol: 257, Pages: 1-14, ISSN: 0045-7949
A multi-fidelity probabilistic optimisation method for the design of composite structures subjected tothermomechanical loading isintroduced in this work for the first time. The proposed multi-fidelity approach offersconsiderable computation efficiency as well as sufficient accuracy, enabling probabilistic optimisation to includemore design variables in the early design phase. This approach incorporates both nonlinear information fusionalgorithms and multi-level optimisation to achieve increased accuracy and computation time savings. In thisoptimisation process, a High-Fidelity Model (HFM) covers only a part of the entire design space with informationcollected uniformly while providing high-fidelity information of other design spaces sparsely without causingextra computational cost. Simultaneously, a Low-Fidelity Model (LFM) explores the whole design space tocompensate lack of high-fidelity information. In this manner, the number of high-fidelity information to constructa multi-fidelity model is dramatically reduced. The Reliability-Based Design Optimisation (RBDO) demonstratedthe proposed multi-fidelity method of a mono-stringer stiffened composite panel under thermomechanical loadingusing Gaussian Processes (GPs)
Seno AH, Aliabadi MHF, 2021, Uncertainty quantification for impact location and force estimation in composite structures, STRUCTURAL HEALTH MONITORING-AN INTERNATIONAL JOURNAL, ISSN: 1475-9217
Yue N, Khodaei ZS, Aliabadi MH, 2021, Damage detection in large composite stiffened panels based on a novel SHM building block philosophy, SMART MATERIALS AND STRUCTURES, Vol: 30, ISSN: 0964-1726
Yoo K, Bacarreza Nogales O, Aliabadi MHF, 2021, Multi-fidelity robust design optimisation for composite structures based on low-fidelity models using successive high-fidelity corrections, Composite Structures, Vol: 259, Pages: 1-12, ISSN: 0263-8223
In this paper, a novel multi-fidelity modelling-based optimisation framework is developed for the robust designof composite structures. The proposed framework provides significant savings on computation time compared toboth conventional multi-fidelity and high-fidelity modelling methods while maintaining an acceptable level ofaccuracy. Artificial neural networks (ANNs) and multi-level optimisation approach are both incorporated into thismulti-fidelity modelling formulation. The framework utilises varied High-Fidelity Model (HFM) and LowFidelity Model (LFM) covering different design spaces. This means that the HFM has only a few design variables,whereas the LFM explores the entire design spaces during the optimisation process. The proposed multi-fidelityformulation is demonstrated by the Robust Design Optimisation (RDO) of a mono-stringer stiffened compositepanel considering design uncertainty under non-linear post-buckling regime.
Morse L, Khodaei ZS, Aliabadi MH, 2020, Reliability-based fracture analysis for plate bending problems with the dual boundary element method, FRACTURE AND DAMAGE MECHANICS: Theory, Simulation and Experiment, Publisher: AIP Publishing, Pages: 1-7
A novel methodology is presented for the efficient reliability analysis of plate structures containing cracks with the Dual Boundary Element Method (DBEM). The derivatives of the DBEM plate formulations for the Crack Surface Displacement Extrapolation (CSDE) method have been derived for the first time and are used as part of an Implicit Differentiation Method (IDM) for the efficient calculation of Stress Intensity Factor (SIF) sensitivities. A numerical example is investigated in which results from the presented CSDE methodology are compared to those from the J-integral. The SIF sensitivities were used with the First-Order Reliability Method (FORM) to determine the reliability of a plate structure containing a crack. Results indicate that the proposed CSDE methodology is capable of providing estimates for reliability that are very similar to those provided by the J-integral. Given that the proposed CSDE methodology can be easier to implement, it could prove to be an effective alternative to the J-integral for the efficient reliability analysis of plate structures containing cracks.
Giannakeas IN, Sharif-Khodaei Z, Aliabadi MH, 2020, Assessment of the performance of different element types for guided wave simulations in abaqus, FRACTURE AND DAMAGE MECHANICS: Theory, Simulation and Experiment, Publisher: American Institute of Physics, Pages: 1-10, ISSN: 1551-7616
Accurate numerical tools for the simulation of wave propagation are essential for the development and optimization of guided wave based Structural Health Monitoring systems. This article aims in delivering a systematic comparison of the Finite Element simulation of Lamb wave propagation using different types of elements. The numerical simulations are all realized within the environment of the commercial software Abaqus. In total three different element types are considered: conventional shell, continuum shell and 3D solid elements. To evaluate the performance of each element type, the numerically simulated signals are compared with experimental measurements from two panels. The first panel is made of Aluminium while the second is a layered composite panel. When continuum or 3D solid elements are used, the numerical predictions are closely correlated to the experimental observations. Accurate predictions were also made using conventional shell elements to model wave propagation in the first panel however, the group velocity for the first symmetric wave mode is over-estimated for the second panel when the excitation frequency is fc = 250kHz.
Giannakeas IN, Sharif-Khodaei Z, Aliabadi MH, 2020, On the estimation of material properties using guided wave measurements for the calibration of finite element models, ISSN: 0094-243X
The design and development of robust and reliable guided wave Structural Health Monitoring systems require accurate information of the structure's material properties. This is needed by various analysis tools for the determination of wave propagation characteristics in order to evaluate and optimize the system's performance. Estimation of the mechanical properties of composite materials using Lamb wave measurements is not straightforward as it requires the solution of an inverse problem. In this study, a numerical procedure is presented for the material properties estimation. The aim is to obtain estimates of the unknown material properties using circular surface mounted piezoelectric transducers that can be used for material characterization or calibration of numerical models. The procedure utilizes the Semi-analytical finite element method for the efficient computation of the dispersion curves and a genetic algorithm for the extraction of the material properties that fit best the experimental observations.
Li J, Khodaei ZS, Aliabadi MH, 2020, Boundary element analysis of lamb wave scattering by a through-thickness crack in a plate, ISSN: 0094-243X
This paper aims to analyse Lamb-wave scattering by a through-thickness crack in a plate using the boundary element plate formulations. These formulations allow to discretize the plate boundaries including plate edges and crack surfaces using simple one-dimensional line elements. Since infinite nature of the plate can be rigorously treated using the boundary element method, truncation of the plate domain is avoided to model the infinite domain. The crack is modelled using an efficient dual boundary element method, which allows to treat the crack surfaces as extra plate boundaries. The detailed scattering process is illustrated using snapshots at different time points.
Seno AH, Aliabadi MHF, 2020, Correction: Seno, A.H.; Ferri Aliabadi, M.H. Impact Localisation in Composite Plates of Different Stiffness Impactors under Simulated Environmental and Operational Conditions. Sensors 2019, 19, 3659, Sensors (Basel, Switzerland), Vol: 20, ISSN: 1424-8220
The author wishes to make the following correction to this paper [...].
Seno AH, Aliabadi MHF, 2020, A novel method for impact force estimation in composite plates under simulated environmental and operational conditions, Smart Materials and Structures, Vol: 29, Pages: 1-16, ISSN: 0964-1726
During its lifetime, an aircraft structure is subjected to various impacts from various sources such as tool drops, hail, ground service equipment, etc. In modern composite structures, these impacts have a significant chance of generating barely visible damage (BVID) which may lead to catastrophic failure of a structure if left undetected to grow. However, BVID is difficult to detect during routine visual inspection without specialised non-destructive inspection and thus there is large interest in developing monitoring systems for estimating the location and severity of impact events. Currently, most systems and methods have been developed for controlled lab conditions and do not consider the wide range of impact parameters in real life operation (environmental conditions, vibration, impactor stiffness, angle, etc) which may severely compromise the accuracy of these methods. In this study we have explored two methods for maximum impact force estimation, deconvolution and a novel gradient method, for the purpose of reliable severity assessment in composite aircraft structures under simulated environmental and operational conditions. It is shown that both methods allow accurate and robust estimation of the maximum impact force from various cases of impacts (variation of impact energy, mass, stiffness, angle, temperature, source) using minimum initial data from a single impact case. From further testing it is demonstrated that the gradient method is robust towards the effects of impact localisation errors and noise. The gradient method also has much less computational and storage requirements and is thus more feasible to integrate with current data acquisition systems being developed for structural health monitoring. Thus, we conclude that the proposed gradient method is suitable for impact force monitoring and severity assessment in composite aircraft structures in the simulated environmental and operational conditions.
Salehzadeh Nobari AE, Aliabadi MHF, 2020, A multilevel isolation forrest and convolutional neural network algorithm for impact characterization on composite structures, Sensors, Vol: 20, Pages: 5896-5896, ISSN: 1424-8220
In this paper, a Deep Learning approach is proposed to classify impact data based on the type of impact (Hard or Soft Impacts), via obtaining voltage signals from Piezo-Electric sensors, mounted on a composite panel. The data is processed further to be classified based on their energy, location and material. Minimalistic and Automated feature extraction and selection is achieved via a deep learning algorithm. Convolutional Neural Networks (CNN) are employed to extract and select important features from the voltage data. Once features are selected the impacts, are classified based on either, Hard Impacts (simulated from steel impactors in a lab setting), Soft Impacts (simulated from silicon impactors in a lab setting) and their corresponding location and energy levels. Furthermore, in order to use the right data for training they are obtained from the signals as anomalies via Isolation Forests (IF) to speed up the process. Using this approach Hard and Soft Impacts, their corresponding locations and respective energies are identified with high accuracy.
Morse L, Sharif Khodaei Z, Aliabadi MH, 2020, Statistical inference of the equivalent initial flaw size for assembled plate structures with the dual boundary element method, Engineering Fracture Mechanics, Vol: 238, ISSN: 0013-7944
The statistical inference of the Equivalent Initial Flaw Size Distribution (EIFSD) is developed for the first time using the Dual Boundary Element Method (DBEM) for assembled shear deformable plate structures. As part of this inference procedure, Bayesian updating is employed to enable the continuous refinement of the EIFSD via data obtained by many simulated routine inspections of a stiffened panel from a fleet of aircraft. Fatigue crack growth is modelled using an incremental crack growth procedure that only requires modelling of the boundary of the 2.5D structure with line elements and requires no remeshing during crack growth simulations. Stochastic Kriging is employed to account for the stochastic nature of fatigue crack growth and to offset the high computational cost associated with modelling complex built-up structures. To demonstrate the efficiency of the proposed inference methodology, a numerical example featuring a stiffened panel subjected to complex loading in the form of combined tension and bending is presented. Once the EIFSD has been inferred, it can be used to optimise the intervals between routine aircraft inspections via the use of reliability analysis techniques as part of a combined reliability-EIFS approach. It is demonstrated that the proposed methodology offers the capability to reduce the costs associated with inspections.
Chen YH, Aliabadi MH, 2020, Meshfree-based micromechanical modelling of twill woven composites, Composites Part B: Engineering, Vol: 197, Pages: 1-13, ISSN: 0961-9526
This paper presents a novel, meshfree-based micromechanical model for homogenising the elastic properties and analysing the deformation and microscopic strains/stresses of twill woven composites. The proposed model was based on a minimum unit cell (mUC) whose internal features such as the cross-sectional shape and waviness of yarns were described by using sophisticated functions. The boundary conditions imposed on the mUC were derived by applying an equivalence approach, which converts the standard form of periodic boundary conditions into a generic set of fixed and relative displacement constraints. Theoretical formulations were developed to implement the micromechanical model within the framework of the moving kriging (MK)-based element-free Galerkin (EFG) method. An in-house computer program implementing the proposed model was developed for analysing a typical twill woven composite. Good agreements were found between the meshfree-based predictions and the reference results, highlighting the proposed model capable of homogenising twill woven composites and meanwhile avoiding the commonly required pre-processing tasks such as building an explicit geometry model and generating identical meshes on the mapping surfaces to enforce boundary conditions. Three case studies were also performed to identify the sensitivities of the predicted results to three numerical parameters, i.e. the total number of field nodes, the total number of background cells, and the support domain scaling factor. The results of these studies suggest that the numbers of field nodes and background cells used must be sufficiently large, while the support domain scaling factor in an appropriate range (e.g. 2.0−3.25) to achieve convergent results.
Sharif Khodaei Z, Li J, Aliabadi MH, 2020, Boundary element modelling of ultrasonic Lamb waves for structural health monitoring, Smart Materials and Structures, Vol: 29, Pages: 1-19, ISSN: 0964-1726
In this paper, a novel boundary element plate formulation is proposed to model ultrasonic Lamb waves in both pristine and cracked plates for structural health monitoring (SHM) applications. Lamb waves are generated and sensed by piezoelectric discs. An equivalent pin-force model is newly proposed to represent the actuation effect of piezoelectric discs, which is more accurate than the classical pin-force model. The boundary element formulation is presented in the Laplace-transform domain based on plate theories, which allows three-dimensional analysis of Lamb wave behaviours, such as propagation and interaction with cracks, in thin-walled structures. A damage detection algorithm is used for crack localization alongside the BEM-simulated data. The BEM solutions show excellent agreement with both 3D finite element simulation and experimental results.
Yue N, Aliabadi MH, 2020, A scalable data-driven approach to temperature baseline reconstruction for guided wave structural health monitoring of anisotropic carbon-fibre-reinforced polymer structures, Structural Health Monitoring: an international journal, Vol: 19, Pages: 1487-1506, ISSN: 1475-9217
To account for the temperature effect on guided wave signals in complex structures, a significant amount of baseline measurements typically need to be collected over a large temperature range to serve as a library of signals at all possible temperatures, which, if not impossible, is highly impractical. This article presents a data-driven temperature baseline reconstruction approach that is applicable for various structures made from the same material. The influence of temperature on the amplitude and phase of guided wave measurements are experimentally quantified as dimensionless compensation factors. The derived compensation factors are used to reconstruct baselines at various temperatures for guided wave measurements in a simple flat plate and a stiffened panel. With a single baseline measurement at 20°C and the reconstructed baseline using the predetermined temperature compensation factors, impact damage was successfully detected and located when current measurements were up to 25°C and 20°C higher than the baseline temperature, respectively.
Yue N, Aliabadi MH, 2020, Hierarchical approach for uncertainty quantification and reliability assessment of guided wave-based structural health monitoring, Structural Health Monitoring: an international journal, ISSN: 1475-9217
In this article, a hierarchical approach is proposed for the design and assessment of a guided wave-based structural health monitoring system for the detection and localisation of barely visible impact damage in composite airframe structures. The hierarchical approach provides a systemic and practical way to establish guided wave-based structural health monitoring systems for different structures in the presence of uncertainties and to quantify system performance. The proposed approach is carried out in four steps: (1) determine optimal sensor placement for the target structure and its plausible impact scenarios, (2) set detection threshold for global damage index based on the noise level present in the required environmental and operations conditions, (3) detect damage in critical locations and quantify detection performance by calculating the probability of detection, probability of false alarm and detection accuracy and (4) locate the detected damage while also quantifying the accuracy of location estimation and the probability of correctly indicating if the damage is in an area critical to the integrity of the structure. The proposed approach is demonstrated in aircraft carbon fibre-reinforced polymer structures from coupon level (simple flat panels) to sub-component level (large flat panel with multiple carbon fibre-reinforced polymer stringers and aluminium frames) for the detection and localisation of barely visible impact damage.
Feng T, Dimitrios B, Aliabadi MHF, 2020, Active health monitoring of thick composite structures by embedded and surface-mounted piezo diagnostic layer, Sensors, Vol: 20, ISSN: 1424-8220
An effective approach for an embedded piezo diagnostic layer into thick composite material is presented. The effectiveness of the approach is assessed in comparison to the surface-mounted layer. The proposed manufacturing alleviates difficulties associated with trimming edges of composites when embedding wires. The Electro-Mechanical Impedance technique is used to access the integrity of the piezoelectric sensors bonding process. Comparisons of ultrasonic guided waves are made between embedded and surface-mounted diagnostic layers and their penetration through and across the thickness of the composites. Temperature influences with the range from −40 °C up to 80 °C on embedded and surface-mounted guided waves are investigated. An investigation is carried out into the relationship between amplitude and time-of-flight with temperature at different excitation frequencies. The temperature has significant but different effects on amplitude and phase-shift of guided waves for the embedded layer compared to the surface-mounted layer. A Laser Doppler Vibrometer is used to identify the blue tack and impact damage. Both embedded and surface-mounted layers are shown to be an effective means of generating detectable wave scatter from damage.
Yoo K, Bacarreza O, Aliabadi MHF, 2020, A novel multi-fidelity modelling-based framework for reliability-based design optimisation of composite structures, Engineering with Computers: an international journal for simulation-based engineering, Pages: 1-14, ISSN: 0177-0667
A new multi-fidelity modelling-based probabilistic optimisation framework for composite structures is presented in this paper. The multi-fidelity formulation developed herein significantly reduces the required computational time, allowing for more design variables to be considered early in the design stage. Multi-fidelity models are created by the use of finite element models, surrogate models and response correction surfaces. The accuracy and computational efficiency of the proposed optimisation methodology are demonstrated in two engineering examples of composite structures: a reliability analysis, and a reliability-based design optimisation. In these two benchmark examples, each random design variable is assigned an expected level of uncertainty. Monte Carlo Simulation (MCS), the First-Order Reliability Method (FORM) and the Second-Order Reliability Method (SORM) are used within the multi-fidelity framework to calculate the probability of failure. The reliability optimisation is a multi-objective problem that finds the optimal front, which provides both the maximum linear buckling load and minimum mass. The results show that multi-fidelity models provide high levels of accuracy while reducing computation time drastically.
Farokhi H, Bacarreza O, Aliabadi MHF, 2020, Probabilistic optimisation of mono-stringer composite stiffened panels in post-buckling regime, Structural and Multidisciplinary Optimization: computer-aided optimal design of stressed solids and multidisciplinary systems, Vol: 62, Pages: 1395-1417, ISSN: 1615-147X
In this paper, a multi-objective probabilistic design optimisation approach is presented for reliability and robustness analysis of composite structures and demonstrated on a mono-omega-stringer stiffened panel. The proposed approach utilises a global surrogate model of the composite structure while accounting for uncertainties in material properties as well as geometry. Unlike the multi-level optimisation approach which freezes some parameters at each level, the proposed approach allows for all parameters to change at the same time and hence ensures global optimum solutions in the given parameter design space (for both probabilistic and deterministic optimisations) within a certain degree of accuracy. The proposed approach is used in this study to conduct extensive multi-objective probabilistic and deterministic optimisations (without considering safety factors) on a mono-stringer stiffened panel. In particular, a global surrogate model is developed utilising the computational power of a high-performance computing facility. The inputs of the surrogate model are the omega-stringer geometry and the mechanical properties of the composite material, while the outputs are the fundamental linear buckling load (LBL) and the nonlinear post-buckling strength (NPS). LBL and NPS are obtained via detailed parametric finite element models of the mono-stringer stiffened panel; in the nonlinear model, the interface between the skin and the omega-stringer is modelled via cohesive elements to allow for debonding in the post-buckled regime. Extensive multi-objective optimisations are conducted on the surrogate model using deterministic and probabilistic approaches to examine the omega-stringer geometric parameters mostly affecting the system robustness and reliability. The differences between deterministic and probabilistic designs are highlighted as well.
Li J, Sharif Khodaei Z, Aliabadi FMH, 2019, Dynamic fracture analysis of plates loaded in tension and bending using the dual boundary element method, Advances in Fracture and Damage Mechanics XVIII, Publisher: Trans Tech Publications, Ltd., Pages: 440-445
The purpose of this paper is to solve dynamic fracture problems of plates under both tension and bending using the boundary element method (BEM). The dynamic problems were solved in the Laplace-transform domain, which avoided the calculation of the domain integrals resulting from the inertial terms. The dual boundary element method, in which both displacement and traction boundary integral equations are utilized, was applied to the modelling of cracks. The dynamic fracture analysis of a plate under combined tension and bending loads was conducted using the BEM formulations for the generalized plane stress theory and Mindlin plate bending theory. Dynamic stress intensity factors were estimated based on the crack opening displacements.
Li L, Aliabadi MH, 2019, Elastic property prediction and damage mechanics analysis of 3D braided composite, THEORETICAL AND APPLIED FRACTURE MECHANICS, Vol: 104, ISSN: 0167-8442
Chen YH, Aliabadi MH, 2019, Micromechanical modelling of the overall response of plain woven polymer matrix composites, International Journal of Engineering Science, Vol: 145, Pages: 1-18, ISSN: 0020-7225
This paper presents a novel approach to micromechanical modelling of plain woven polymer matrix composites and predicting the overall response including the nonlinear and rate-dependent behaviour. The nonlinearity and rate-dependence of plain woven composites is evaluated by describing the behaviour of the polyer matrix using a viscoplastic model. The damage evolution of the yarn material and deformation of the woven fabric are investigated by considering Weibull distribution based formulations and a shear-modulus discount approach, respectively. The explicit meshfree method with time-dependent periodic boundary conditions for unit cell (UC) models that describe the internal architecture of plain woven composites is presented for the first time. For validation, numerical examples are performed to simulate the EP121-C15-53 plain woven composite subjected to in-plane normal/off-axis tensile loading conditions and at three different strain rates, i.e. 10−1 s−1, 10−3 s−1 and 10−5 s−1. Good agrements are found between the numerical and experimental results, with both the quasi-linear, rate-insensitive behaviour in the normal direction and the nonlinear, rate-dependent response in the off-axis direction successfully predicted.
Bekas DG, Saenz-Castillo D, Sharif Khodaei Z, et al., 2019, Smart bondline monitoring of an efficient industrial thermoplastic aircraft window frame, Advances in Fracture and Damage Mechanics XVIII, Publisher: Trans Tech Publications, Ltd., Pages: 470-475
In this work, a smart thermoplastic window frame for a regional aircraft has been designed and manufactured. The aim of the work was to design a smart sensing system for monitoring of a bonded thermoplastic aircraft window frame in operation. The conductive tracks were designed and inkjet-printed onto the window frame and their disruption was used as an indication of a damage event created within the bondline. Based on the electrical resistance measurements, the method was able to detect a damage that was created in the bondline due to an impact event. To verify the proposed methodology, ultrasonic C-scan inspection was also performed.
Bekas DG, Mendias MM, Sharif Khodaei Z, et al., 2019, SHM of composite mono-stringer elements based on guided waves, Advances in Fracture and Damage Mechanics XVIII, Publisher: Trans Tech Publications, Ltd., Pages: 464-469
In this work, the applicability of structural health monitoring (SHM) technique for damage detection in two composite mono-stringers representative of composite fuselage are investigated. The two different manufacturing technologies are co-curing and co-bonding of composite mono-stringers to the skin. The panels were then impacted at the foot of the stringer to cause Barely Visible Impact Damage (BVID). Piezoelectric transducers were surface mounted on the mono-stringers, guided wave measurements before and after impact were taken and used for detecting damage based on Weighted Energy Arrival Method (WEAM).
Morse L, Sharif Khodaei Z, Aliabadi MH, 2019, A dual boundary element based implicit diﬀerentiation method for determining stress intensity factor sensitivities for plate bending problems, Engineering Analysis with Boundary Elements, Vol: 106, Pages: 412-426, ISSN: 0955-7997
A novel methodology for determining Stress Intensity Factor (SIF) sensitivities for plate bending problemsusing the Dual Boundary Element Method (DBEM) is presented. The direct derivatives of the DBEM integralequations for plate bending have been derived for the first time and are used as part of a DBEM-based ImplicitDifferentiation Method (IDM or DBEM-IDM) for calculating the sensitivities of SIFs to changes in differentgeometric parameters such as crack length and crack rotation angle. The SIFs and their sensitivities arecalculated using the J-integral and the derivative of the J-integral respectively. A numerical example featuringa thick plate subjected to membrane, bending, and pressure loads is presented. In the first half of the numericalexample, the SIF sensitivities from the IDM are compared with those obtained from the more common, butrelatively crude, Finite Difference Method (FDM or DBEM-FDM). Results show that the IDM is a significantlymore efficient and robust alternative to the FDM. The accuracy of the FDM showed significant dependence onthe step size used, necessitating a time-consuming optimization procedure to determine the optimal step size.Once this optimal step size was found, both methods provided very similar results. As part of the second halfof the numerical example, a demonstration of one possible application of the SIF sensitivities from the IDMis presented. This involved carrying out reliability analyses using the First-Order Reliability Method (FORM)with a large number of design variables.
Li J, Khodaei ZS, Aliabadi MH, 2019, Dynamic fracture analysis of Kane–Mindlin plates using the dual boundary element method, Engineering Analysis with Boundary Elements, Vol: 106, Pages: 217-227, ISSN: 0955-7997
In this paper, a new dual boundary element formulation is presented for dynamic crack problems in finite-thickness plates under in-plane loadings. The proposed formulation is based on a first-order plate theory (Kane–Mindlin theory) taking into account a coupled out-of-plane fracture mode due to in-plane shear loading and the effect of plate thickness on stress intensity factors, which cannot be considered within the framework of the two-dimensional elastic theories. The dynamic stress intensity factors are evaluated based on the crack opening displacements. Three benchmark examples are presented including the mixed-mode fracture of a finite plate. The effect of plate thickness on the dynamic stress intensity factors is investigated. The dynamic stress intensity factors obtained using the proposed formulation are shown to be in good agreement with the results from 3D finite element simulations.
Seno AH, Aliabadi MHF, 2019, Impact localisation in composite plates of different stiffness impactors under simulated environmental and operational conditions, Sensors (Basel, Switzerland), Vol: 19, ISSN: 1424-8220
A parametric investigation of the effect of impactor stiffness as well as environmental and operational conditions on impact contact behaviour and the subsequently generated lamb waves in composite structures is presented. It is shown that differing impactor stiffness generates the most significant changes in contact area and lamb wave characteristics (waveform, frequency, and amplitude). A novel impact localisation method was developed based on the above observations that allows for variations due to differences in impactor stiffness based on modifications of the reference database method and the Akaike Information Criterion (AIC) time of arrival (ToA) picker. The proposed method was compared against a benchmark method based on artificial neural networks (ANNS) and the normalised smoothed envelope threshold (NSET) ToA extraction method. The results indicate that the proposed method had comparable accuracy to the benchmark method for hard impacts under various environmental and operational conditions when trained only using a single hard impact case. However, when tested with soft impacts, the benchmark method had very low accuracy, whilst the proposed method was able to maintain its accuracy at an acceptable level. Thus, the proposed method is capable of detecting the location of impacts of varying stiffness under various environmental and operational conditions using data from only a single impact case, which brings it closer to the application of data driven impact detection systems in real life structures.
Seno AH, Sharif Khodaei Z, Aliabadi MHF, 2019, Passive sensing method for impact localisation in composite plates under simulated environmental and operational conditions, Mechanical Systems and Signal Processing, Vol: 129, Pages: 20-36, ISSN: 0888-3270
A novel feature extraction method is developed for impact localisation based on Artificial Neural Networks (ANNs) in sensorized composite structures subjected to environmental and operational conditions. Impact induced lamb waves are investigated for the first time for different impact scenarios (angle, mass and energy) on flat and curved plates under environmental (temperature range) and operational (vibration) conditions. The Time of Arrival (TOA) is significantly influenced by these conditions hence complicating the impact localisation. To overcome this complication, a novel and robust TOA extraction method is proposed. It is based on Normalised Smoothed Envelope Threshold (NSET) coupled with a high pass filter to remove vibration noise prior to TOA extraction. Localisation ANNs were trained with data from a single baseline impact condition and were tested under impacts with varying conditions. It was shown that by using the proposed method for TOA extraction, the trained ANN is able to better predict the location of impacts compared to an ANN trained with data from common TOA extraction methods (detection area 0.99–56.08% of sensing region versus 0.28–1.55% for NSET). The developed method gives consistent accuracy and significantly reduces the required training data, making ANN based impact localisation more feasible for real life application.
Fu H, Sharif Khodaei Z, Aliabadi M, 2019, An energy-efficient cyber-physical system for wireless on-board aircraft structural health monitoring, Mechanical Systems and Signal Processing, Vol: 128, Pages: 352-368, ISSN: 0888-3270
In this paper, an energy-efficient cyber-physical system using piezoelectric transducers (PZTs) and wireless sensor networks (WSN) is proposed, designed and experimentally validated for on-board aircraft structural health monitoring (SHM). A WSN is exploited to coordinate damage detection using PZTs distributed on the whole aircraft. An active sensing methodology is adopted for PZTs to evaluate the structural integrity in a pitch-catch manner. The system configuration and operation principle are discussed in the first place. Then, the detailed hardware design was introduced. The proposed system is not only characterized as low-power, high-compactness and wireless, but also capable of processing actuating-sensing signals at megahertz, generating actuating signals with great flexibility, handling multiple actuating-sensing channels with marginal crosstalk. The design was implemented on a 4-layer printed circuit board (8 × 6.5 cm) and evaluated on a large-scale composite fuselage. A 5 MHz sampling rate for actuating and 1.8 MHz for sensing (8 channels) were realized, and the accuracy was validated by comparing the results with those from an oscilloscope. The crosstalk issue caused by actuation on sensing channels is properly addressed using a 2-stage attenuation method. An ultra-low current (81.7 μA) was measured when no detection was required; the average current was 0.45 mA with a detection rate of twice per hour, which means the system can continuously work for up to 12.6 months for 2 AA batteries. Eventually, an example of damage detection is provided, showing the capability of such a system in SHM.
Bekas DG, Sharif Khodaei Z, Aliabadi MH, 2019, A smart multi-functional printed sensor for monitoring curing and damage of composite repair patch, Smart Materials and Structures, Vol: 28, ISSN: 0964-1726
A novel multifunctional diagnostic sensor is developed as a cost-effective, in-service structural health monitoring (SHM) system for determining the initial quality of curing of a bonded composite repair patch and assessing its long-term durability on composite structure. The proposed multi-functional sensor technology involves the creation of a "tailor-to-order" 2D conductive patterns onto step-sanded repair surface of composite repair patch using inkjet printing. In employing this methodology, bondline quality during curing and in service was successfully assessed via Impedance spectroscopy and resistance change measurements, respectively. The ability of this technology to effectively monitor the integrity of the bondline and the extent of damage in real-time was investigated by subjecting the scarf-repaired CFRP panels to 3-point bending fatigue and low-velocity impact tests. The obtained results were compared with those of transient infrared thermography (IrT) and ultrasound inspection techniques, thus validating the proposed method.
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