Adrian C. Orifici

A Finite Element Methodology for Analysing Degradation and Collapse in Postbuckling Composite Aerospace Structures

A methodology for analysing the degradation and collapse in postbuckling composite structures is proposed. One aspect of the methodology predicts the initiation of interlaminar damage using a strength criterion applied with a global-local analysis technique. A separate approach represents the growth of a pre-existing interlaminar damage region with user-defined multi-point constraints that are controlled based on the Virtual Crack Closure Technique. Another aspect of the approach is a degradation model for in-plane ply damage mechanisms of fiber fracture, matrix cracking, and fiber-matrix shear. The complete analysis methodology was compared to experimental results for two fuselage-representative composite panels tested to collapse. For both panels, the behavior and structural collapse were accurately captured, and the analysis methodology provided detailed information on the development and interaction of the various damage mechanisms.

AN ANALYSIS TOOL FOR DESIGN AND CERTIFICATION OF POSTBUCKLING COMPOSITE AEROSPACE STRUCTURES

In aerospace, carbon-fiber-reinforced polymer (CFRP) composites and postbuckling skin-stiffened structures are key technologies that have been used to improve structural efficiency. However, the application of composite postbuckling structures in aircraft has been limited due to concerns related to both the durability of composite structures and the accuracy of design tools. In this work, a finite element analysis tool for design and certification of aerospace structures is presented, which predicts collapse by taking the critical damage mechanisms into account. The tool incorporates a global–local analysis technique for predicting interlaminar damage initiation, and degradation models to capture the growth of a pre-existing interlaminar damage region, such as a delamination or skin–stiffener debond, and in-plane ply damage mechanisms such as fiber fracture and matrix cracking. The analysis tool has been applied to single- and multistiffener fuselage-representative composite panels, in both intact and predamaged configurations. This has been performed in a design context, in which panel configurations are selected based on their suitability for experimental testing, and in an analysis context for comparison with experimental results as being representative of aircraft certification studies. For all cases, the tool was capable of accurately capturing the key damage mechanisms contributing to final structural collapse, and suitable for the design of next-generation composite aerospace structures.

Progressive damage in single lap countersunk composite joints

This paper presents an experimental and computational investigation of the influences of countersink and bolt torque on the progressive failure of single-lap composite joint. Using the Abaqus® software, delamination damage and ply fracture are modelled using cohesive element approach and continuum damage mechanics method, respectively. The model is first validated against a filled-hole tension test to calibrate the composite damage model. Comparison with the experimental results indicates that the computational model is capable of accurately predicting the joint strength and the damage progression process.

Experimental Validation of the 2D Meshless Random Grid Method

This paper presents the first systematic effort for the experimental validation of the 2D Meshless Random Grid Method (MRGM) for the full field measurement of displacement and strain fields. Although the MRGM has been demonstrating very promising characteristics of accuracy, performance and ease of application based on previously conducted sensitivity analysis supported by virtual data, extensive experimental validation was not available until now. This work comes to fill this gap and presents preliminary validation results against strain gauge data collected from open hole tension experiments of composite specimens. In addition, strain and displacement field verification is performed by comparison studies with finite element analysis results.Copyright

Preliminary Validation of Composite Material Constitutive Characterization

This paper is describing the preliminary results of an effort to validate a methodology developed for composite material constitutive characterization. This methodology involves using massive amounts of data produced from multiaxially tested coupons via a 6-DoF robotic system called NRL66.3 developed at the Naval Research Laboratory.

Effect of disbonds on the fatigue endurance of composite scarf joints

The certification of scarf repairs requires that the repair is capable of handling flight loads in the presence of disbonds. This paper presents a study of the fatigue disbond growth behaviour of scarf joints. By determining the strain energy release rates of a disbond in a scarf joint subjected to a unit load, a predictive model based on linear elastic fracture mechanics is presented, which is shown to correlate well with experimental results. This method offers a promising technique of predicting the fatigue life of composite scarf joints with disbonds.

Automated modal parameter-based anomaly detection under varying wind excitation

Wind-induced operational variability is one of the major challenges for structural health monitoring of slender engineering structures like aircraft wings or wind turbine blades. Damage sensitive features often show an even bigger sensitivity to operational variability. In this study a composite cantilever was subjected to multiple mass configurations, velocities and angles of attack in a controlled wind tunnel environment. A small-scale impact damage was introduced to the specimen and the structural response measurements were repeated. The proposed damage detection methodology is based on automated operational modal analysis. A novel baseline preparation procedure is described that reduces the amount of user interaction to the provision of a single consistency threshold. The procedure starts with an indeterminate number of operational modal analysis identifications from a large number of datasets and returns a complete baseline matrix of natural frequencies and damping ratios that is suitable for subsequent anomaly detection. Mahalanobis distance-based anomaly detection is then applied to successfully detect the damage under varying severities of operational variability and with various degrees of knowledge about the present operational conditions. The damage detection capabilities of the proposed methodology were found to be excellent under varying velocities and angles of attack. Damage detection was less successful under joint mass and wind variability but could be significantly improved through the provision of the currently encountered operational conditions.