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Reliability of the interfaces on composite materials

Periodic Report Summary - RICM (Reliability of the interfaces on composite materials)

Composite materials are widely used in aeronautical, civil and mechanical structures that require highly reliable long-term operation. The design and reliability assessment of advanced composite materials involve a better understanding of the fracture initiation process and its propagation to final failure, which is the aim of present research.

O1. Better understanding of the failure mechanisms in composite materials with the focus on the interface reliability. The damage mechanisms were identified in: fibre matrix composites, layered composites, composites with weak interfaces, ceramic matrix composites.
O2. Completely describe the asymptotic stress field around interface cracks and bonded - joints interface as an initial stage of crack initiation and studying the effect of the interface on the fracture parameters (stress intensity factors, energy release rate, T-stress).

New fracture parameters were introduced for better characterising the fracture of interfaces. Development of analytical solutions was based on asymptotic analysis for describing the stress field for a bi-material interface and for cracks approaching the interface. Linear-elastic finite element analysis for a variety of interface cracks refers to determination of the: singular order, asymptotic fields and crack tip parameters. Elasto-plastic analyses include determination of: singular orders, asymptotic fields, characteristic plastic-zone dimension, and interfacial load-phase angle.

O3. Investigating the influence of the material properties on the crack path for the interface cracks and cracks approaching an interface. Numerical predictions of crack propagation path were compared and experimental results. Analytical, numerical and experimental studies were performed on the propagation and interaction of parallel cracks in orthotropic composite material.

For a sandwich structure composed on a rigid polyurethane foam core and with polyester or epoxy faces obtained by impregnation the influence of the faces on mechanical properties has been experimentally evaluated in static and impact loading conditions.

Main results

A new four point Bend bi-material specimen with sub-interface crack was designed. A methodology to determine the elastic asymptotic stress field for bi-material joints, bi-material interface cracks and cracks near interfaces was proposed based on complex variable functions.

For elato-plastic material behaviour the asymptotic stress field was obtained combining Runge-Kutta algorithm for solving non-linear systems in combination with shooting method. A simple engineering formula was proposed to predict the singularity order for Ramberg-Osgood material behaviour.

For modelling the damage process in two-phase ceramic matrix composites a unified multiscale approach was developed, and validated at different scales. New parameters were proposed and calculated for different cracked composites for expressing the reliability of the materials.

The influence of material properties, crack length and crack position for a sub-interface crack propagation path was highlighted by numerical and experimental studies.

Damage mechanisms in notched and un-notched sandwich composite beams under static and impact bending were identified in terms of impact energy and energy absorbed to fracture.

Main deliverables

The appointed researcher Dr Liviu Marsavina become an independent researcher with improved chances of having a successful research career in Europe.

Publication of results in seven peer review journals: Computational Material Science (three papers), Journal of Nonlinear Mechanics, International Journal of Fracture, Journal of Mechanical Science and Technology and Annals of the University Ovidius Constanta.

Participation at four international conferences (12th ICF Conference, Ottawa, Canada, 2009; Xth Fracture and Damage Mechanics, St. Georges, Malta, 2009; 18th ECF Conference, Dresden, Germany, 2010 and 20th IWCMM, Loughborough, UK, 2010).