M.H. Ferri Aliabadi

Structural Health Monitoring of Bonded Patch Repaired Composite

Bonded repair of composite structures still remains a crucial concern for the airworthiness authorities because of the uncertainty about the repair quality. This works, investigates the applicability of Structural Health Monitoring (SHM) techniques for monitoring of bonded repair. Active sensing method has been applied to two case studies: a sensorised panel impacted to cause barely visible impact damage (BVID) and repaired afterwards, the tensile and fatigue testing of a composite strap repair. In the first case, the previous sensors have been used to detect an artificially introduced damage. In the second case the failure of the adhesive during the tensile testing is used as basis of the load levels in the tensile-tensile fatigue test. In both cases PZT transducers have been used to monitor the bonded patch. An electromechanical impedance (EMI) and Lamb wave analysis have been carried out to check the overall integrity of the repair patch between. In both cases the state of the repaired composite was monitored successfully and reported.

A MeshFree Micromechanical Analysis for Twill Composite

This paper presents homogenization of twill woven composites. It includes presenting formulated geometrical unit cell models, establishing meshfree micromechanical and CDM models for twill woven composites. The results cover meshfree nodal distributions, meshfree yarn geometries, predicted mechanical properties, effectiveness of yarn waviness, non-linear stress-strain relation of unit cell, stress distributions inside unit cell volume, and necessary comparisons with existing literatures.

Reliability-Based Optimization of the Stacking Sequence Layup in Post-Buckled Composite Stiffened Panels

This paper presents a methodology to carry out Reliability-Based Design Optimization (RBDO) in composite stiffened panels. The target is to maximize the reaction force that the panel can withstand before collapse, setting the shortening of failure as the probabilistic constraint. The design variables are the stacking sequence orientations of the composite plies while the random parameters are the elastic properties of the material. In order to predict the collapse load properly, post-buckling and progressive failure analyses are considered within the FE solver employed.

Passive Sensing of Sensorized Composite Panels: Support Vector Machine

Strain readings recorded by surface mounted piezoelectric sensors due to impact events on composite panel are used to detect and characterize the impact. Sensor signals on a composite stiffened panels have been simulated by a valid numerical model. Applicability of least square support vector machines (LSSVM) on creating a meta-model to detect and characterize impact event has been investigated. In particular, the main advantage of LSSVM over other meta-modeling technique was found to be the smaller number of training data that is required. Experimental results on a composite panel has been used to validate the findings.

Optimal Sensor Positioning for Damage Detection in Composite Sensorised Panels

In this work the optimal configuration of transducers for damage detection and localization has been investigated. A particular interest is given to three optimization methods: mini-max, average Probability of Non Detection (POND) and ray tracing approach, coupled with genetic algorithm. After optimal configurations have been computed for each technique, they are experimentally tested and compared on a composite panel with one or two damages by generating and receiving Lamb waves signals. Damage detection is carried out with the Probability Based Damage Index Method (PBDIM). It was found that, in most cases, the ray tracing method and the average POND technique give better results, with a good detection of damages in comparison to the minimax POND technique, even if the latter seems numerically better.

Micromechanical Boundary Element Modelling of Transgranular and Intergranular Cohesive Cracking in Polycrystalline Materials

In this paper a cohesive formulation is proposed for modelling intergranular and transgranular damage and microcracking evolution in brittle polycrystalline materials. The model uses a multi region boundary element approach combined with a dual boundary element formulation. Polycrystalline microstructures are created through a Voronoi tessellation algorithm. Each crystal has an elastic orthotropic behaviour and specific material orientation. Transgranular surfaces are inserted as the simulation evolves and only in those grains that experience stress levels high enough for the nucleation of a new potential crack. Damage evolution along (inter-or trans-granular) interfaces is then modelled using cohesive traction separation laws and, upon failure, frictional contact analysis is introduced to model separation, stick or slip. Moreover some physical consideration based on cohesive energies were made, in order to guarantee the cohesive model in consideration was appropriate for the purpose of this work. Finally numerical simulations have been performed to demonstrate the validity of the proposed formulation in comparison with experimental observations and literature results.

Boundary Element Analysis of Stiffened Panels with Repair Patches

In this paper BEM for analysis of fractured stiffened panels repaired with riveted patches is presented. Several boundary element formulations involving the membrane and bending of displacement, and, stress resultants are coupled together to analyse the model. The Crack Opening Displacements (COD) method and the J-integral are implemented to evaluate the required fracture parameters.