Prediction of delamination in glass fibre reinforced composite materials using elasto-plastic modelling

Glass Fibre reinforced composite (GFRC) has been used for numerous structural applications in Aerospace, Chemical, Automotive and Civil infrastructure fields over a hundred of years. Due to this reason, understanding the intricate fracture behaviour of GFRC materials is crucial and essential for designing critical structural components. Voids and micro-cracks are considered as imperfections in Glass Fibre Reinforced composites. Much research has been undertaken on approaches to calculate and evaluate the effects of the imperfections on mechanical properties. However, it is an established fact that the micro-mechanical approach alone is not sufficient to understand a complete damage accumulation process during delamination. The damage mechanism which largely affects the performance of GFRC structures is commonly known as 'delamination'. Since the delamination is invisible, and hard to detect with ordinary non-destructive evaluation methods, therefore it is considered as a hidden killer which can cause catastrophic failure without any prior warnings. Due to this reason, research work on delamination modelling, damage detection and self-healing materials have been the highly placed research topics for more than five decades. Unfortunately there are a number of unresolved problems in delamination damage modelling and prediction, and few grey areas regarding application of Structural Health Monitoring systems to monitor delamination damages. This thesis has proposed to study the insight into the cause of delamination damage and its propagation mechanisms, by analytical modelling and experimental verifications. Within this research project, extension of the work by Tsukrov and Kachanov (2000) – “An innovative Elasto-plastic model” has been undertaken to evaluate, investigate and model the onset and propagation of delamination damages. Mode I, Mode II as well as Mixed Mode I/II delamination damage analysis has been utilised to study the proposed model predictions for GFRC structures for both in-plane and out-of-plane load applications. The proposed model has been validated using the Double Cantilever Beam (DCB), End Notch Flexure configurations (ENF) and Cracked Lap Shear (CLS) experiments on 0/90-glass woven cloth specimens. For the validation process, the procedures stipulated by ASTM standards were employed. It was observed that there were significant discrepancies between calculated fracture energies using standard procedures and the proposed model. Interestingly these observations have revealed some inconsistencies associated with the standard method for strain measurements that majorly controls the fracture energy calculations. This research project has demonstrated and evidently proven the accuracy of the proposed model predictions using the strain measured with embedded Fibre Bragg Grating (FBG) sensors, located inside the sample in proximity of the crack tip. The extended use of FBG strain measurement has created a breakthrough in Structural Health Monitoring (SHM) of composite structures. Non-availability of a suitable damage prediction model is an issue for accurate damage monitoring process. The proposed model has also demonstrated the potential for its integration with Structural Health Monitoring (SHM) systems. Additionally, Thermoplastic Stress Analysis (TSA) has been employed to monitor delamination. The potential for integration of FBG sensors and TSA techniques has been experimentally demonstrated during this project and, it is another breakthrough in SHM field as a result of this research. In addition to analytical model, a detailed Finite Element model was also created on ABAQUS commercial software. The cohesive elements with state variables (SDV) and UMAT codes were used for FEA simulations. Interestingly, the FEA results have shown an excellent correlation with the experimental results. Finally, this thesis has evidently proved the validity of the proposed model and integration of model with SHM system based on FBG sensors and TSA techniques. The outcomes of the thesis have provided a novel and innovative damage prediction model and a breakthrough technology for SHM systems

Detection and characterisation of delamination damage propagation in woven glass fibre reinforced polymer composite using thermoelastic response mapping

This paper details a study on the application of Thermoelastic Stress Analysis (TSA) for the investigation of delamination damage propagation in glass fibre reinforced composite materials. A woven Glass (0/90)/ Epoxy composite sample containing a purposely created delamination was subjected to a step-cyclic loading (varying mean level) whilst monitoring the thermoelastic response of the sample with an infrared camera. A finite element analysis (FEA) was performed using cohesive elements to simulate the propagation of the delamination under a monotonically increasing axial load. It is shown that the delamination crack length inferred from the TSA results is consistent with microscopic analysis of the sample, and that the measured crack growth rate is in reasonable agreement with simulation results

Numerical assessment of heat sink for pressure sensor connections

Pressure sensors, converting pressure force to electrical outputs such as 4-20 mA or 0-10 V, are used in a vast variety of areas while being facing numerous challenging thermal conditions. A common way is to design a heat sink for establishing natural convection cooling to protect the sensor. This work assesses a heat sink design and conveys its performance as a heat sink for an application interval. Special orientation as well as design geometry is introduced. Computational fluid dynamics were utilized for evaluation and assessment. A core region of heat transfer was identified. Natural convection wake boundaries were detected. It is concluded that the design can successfully protect the pressure sensor at the pressure tap. Future projections and aspects are also described in the paper

Use of fiber Bragg grating sensors for monitoring delamination damage propagation in glass-fiber reinforced composite structures

Embedded fiber Bragg grating (FBG) sensors have been widely used for damage monitoring of fiber composite structures for a few decades. However, many remaining engineering challenges have delayed FBG based in situ structural health monitoring (SHM) systems. One of the major problem associated with FBG based SHM system is the unavailability of reliable data processing algorithms. The present work details a study which has been undertaken for identification of delamination crack propagation in fiber reinforced polymer (FRP) composite plate under uniaxial loading. The strain measured by embedded FBG sensors closer to the crack tip was used to qualitatively and quantitatively analyse delamination damage propagation using recently proposed elasto-plastic model. Strain energy release rate was calculated and compared with the model prediction. The study has concluded that the delamination crack propagation in a FRP composite can be monitored successfully using an integral approach of FBG sensors measurements and the predictions of proposed elasto-plastic model

Evaluation of delamination crack tip in woven fibre glass reinforced polymer composite using FBG sensor spectra and thermo-elastic response

This paper details a study which was carried out on the application of FBG sensors for investigation of delamination crack status in glass fibre reinforced composite materials. A woven glass (0/90) epoxy composite sample containing a purposely created delamination and an embedded FBG sensor was investigated to study the behaviour of delamination crack under applied axial quasi-static tensile load. The reflected spectra from FBG sensor and the thermo-elastic response using an infrared camera were recorded to detect the propagation of delamination crack tip. In addition a finite element analysis (FEA) was performed using cohesive elements to simulate delamination crack tip. It has been seen that the propagation of delamination crack tip monitored from thermal stress analysis (TSA) was consistent with the prediction of the FBG sensor response. Further it has been observed that the experimentally observed delamination damage propagation was in a good agreement with FEA simulation results

Use of an elasto-plastic model and strain measurements of embedded fibre Bragg grating sensors to detect Mode I delamination crack propagation in woven cloth (0/90) composite materials

Mode I fracture analysis being employed to study delamination damage in fibre-reinforced composite structures under in-plane and out-of-plane load applications. However, due to the significantly low yield strength of the matrix material and the infinitesimal thickness of the interface matrix layer, the actual delamination process can be assumed as a partially plastic process (elasto-plastic). A simple elasto-plastic model based on the strain field in the vicinity of the crack front was developed for Mode I crack propagation. In this study, a double cantilever beam experiment has been performed to study the proposed process using a 0/90-glass woven cloth sample. A fibre Bragg grating sensor has embedded closer to the delamination to measure the strain at the vicinity of the crack front. Strain energy release rate was calculated according to ASTM D5528. The model predictions were comparable with the calculated values according to ASTM D5528. Subsequently, a finite element analysis on Abaqus was performed using ‘Cohesive Elements’ to study the proposed elasto-plastic behaviour. The finite element analysis results have shown a very good correlation with double cantilever beam experimental results, and therefore, it can be concluded that Mode I delamination process of an fibre-reinforced polymer composite can be monitored successfully using an integral approach of fibre Bragg grating sensors measurements and the prediction of a newly proposed elasto-plastic model for Mode I delamination process

Development of fracture and damage modeling concepts for composite materials

An overview of proposed micro-crack based damage models for fibre reinforced composite plates is presented. A critical analysis has been performed on the potential application of those models for damage accumulation analysis of wide range fibre reinforced composite materials. The flaws and drawbacks of those models were critically analysed and presented in the text. Interestingly, it has been found that some proposed models can be extended or modified to address unresolved issues in crack propagation and damage accumulation in fibre reinforced composites. It can be concluded that the micromechanical approach alone is not sufficient to evaluate complete damage accumulation of composite and a significant theoretical modifications are required for existing brittle damage models before applying them to fibre reinforced composite materials. The goal of the current paper is an overview of the numerical approaches and approaches’ limitation of plate with multiply cracks problem. The used approaches is discussed and summarized. Equations for two or three dimensions of plate is given for studying effect of crack density on effective moduli. The insensitive effective moduli to the sizes, orientation and location of individual microcracks is also discussed. In addition, the problem associated with limitation of the exciting approaches with increasing cracks density conditions is summarized to approve that necessary modifications the numerical approach and corrections are required. Due to an increase interest in using a fracture mechanics based on microcracks numerical approaches to assess the damage of composite structures, the laminated composites selected as example. The micromechanics approach is not enough to evaluate damage and also most of current theoretical need modification for simulate real conditions

Integrated FBG sensor responses and full field thermo-electric stress approach to monitor damage accumulation in glass fibre reinforced composite plate

Monitoring internal damage status of advanced composite components with distributed sensor network has shown significant success in recent research works. However, application of such a system in a full scale structure is a critically challenging task and maintaining such a system during life time operations is an extremely difficult. An additional non-contactable full field strain measurement system being used to measure outer surface strain field of a composite sample while an embedded FBG sensor closer to an internal void being used to monitor localized strain variation. Recent developments in miniature low-cost microbolometer technology have paved the way to use full field thermo-elastic stress mapping using relatively inexpensive Infra-Red cameras. This paper details a comparison of strain measurements observed from FBG sensors embedded in a composite plate sample at a closer location to a void and full field thermo-elastic stress map. The test coupons were fabricated with a purposely created delamination and sample was loaded by quasi-static and low cycle fatigue uni-axial loads. The FBG responses and IR images were recorded in frequent intervals in order to track the delamination growth. Further the strain variations were studied using a detailed FEA and compared with experimental strain and full field Thermo-elastic stress map. Copyright © 2015 by DEStech Publica tions, Inc

Adaptive Robust Controller Design-Based RBF Neural Network for Aerial Robot Arm Model

Aerial Robot Arms (ARAs) enable aerial drones to interact and influence objects in various environments. Traditional ARA controllers need the availability of a high-precision model to avoid high control chattering. Furthermore, in practical applications of aerial object manipulation, the payloads that ARAs can handle vary, depending on the nature of the task. The high uncertainties due to modeling errors and an unknown payload are inversely proportional to the stability of ARAs. To address the issue of stability, a new adaptive robust controller, based on the Radial Basis Function (RBF) neural network, is proposed. A three-tier approach is also followed. Firstly, a detailed new model for the ARA is derived using the Lagrange–d’Alembert principle. Secondly, an adaptive robust controller, based on a sliding mode, is designed to manipulate the problem of uncertainties, including modeling errors. Last, a higher stability controller, based on the RBF neural network, is implemented with the adaptive robust controller to stabilize the ARAs, avoiding modeling errors and unknown payload issues. The novelty of the proposed design is that it takes into account high nonlinearities, coupling control loops, high modeling errors, and disturbances due to payloads and environmental conditions. The model was evaluated by the simulation of a case study that includes the two proposed controllers and ARA trajectory tracking. The simulation results show the validation and notability of the presented control algorithm