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New inter-scale techniques for damage analysis of novel composite architectures

Final Report Summary - INTERCOM (New inter-scale techniques for damage analysis of novel composite architectures)

New inter-scale techniques for damage analysis of novel composite architectures

In a race for efficiency and functionality modern composite become increasingly complicated. At present, the design of fibrous architectures is often tailored to a specific application and optimised for a particular aspect of composite performance. As a result, fibres are organised in complex curved interlaced trajectories and patterns. Every single fibre tow in such architecture may have its own individual path. This level of sophistication becomes typical for the entire suite of composite manufacturing processes ranging from laminated geometries obtained by automatic fibre deposition to intrinsically complex net-shaped textile preforms.
The essential material properties such as stiffness, toughness, strength depend on the interaction of composite constituents at the scale of fibre bundles/yarns/tows. To understand and predict deformation and damage processes, the internal structures of these materials need to be examined with a high resolution. This implies high computational cost of analysis since the fine geometrical features (characteristic sizes of mm and sub-mm) need to be taken into account at the component scale. Thus, the new manufacturing trend creates a need for simple and rational design tools that would allow a fast but comprehensive assessment of the composite performance at the structural and component scales.

Modelling approach
The conventional multi-scale techniques address the challenge of high-resolution – low computational cost by establishing a hierarchy of problems at the component and structural levels. The problem at lower level is typically set for an elementary building block of internal architectures known as representative volume element. This project developed a strategy where the separation of scales is applied to the composites lacking a characteristic representative element through space and scale separation.
It was demonstrated that a complex interaction of unit cells in an arbitrarily nested, and hence, non-periodic textile laminate can be successfully replicated by the analysis of a single unit cell repeat. The key to the efficient deformation analysis is the boundary conditions set on the surfaces of considered structural element and imitating its interaction with the neighbours. The feasibility of generalising this concept was then tested in application to more complicated architectures and loading cases.

Test cases
The applicability and limitations of the proposed modelling concept were examined in a large number of parametric tests problems reproducing the key features of real architectures such as in-plane and out-of plane waviness of fibres, ply crimp, variation in thickness, etc. These test cases were aimed at validating the decomposition approach against the reference solutions obtained at a realistic computational cost. These studies allowed to extend the applicability of classical multi-scale methods beyond the conventional textile architecture and assess the predictive capacity of the approach towards modelling tow-steered, non-crimp, and patterned materials.

Numerical tools developed within the project
A library of new numerical tools was developed within the project. They enabled generating realistic geometrical models of patterned textile reinforcement with imposed features such as thickness variation, distortions, ply curvature and take into account characteristics which are ignored in conventional model but are essential for understanding consolidation mechanisms and fine peculiarities of the load flow in textile composites (such as lateral yarn interaction, side yarn crimp, curvature of textile plies, etc).
The logistics of data flow between the defragmented elements of the architecture demanded novel numerical approaches. This included the transformation of vector field into nodal boundary conditions, the tools for post-processing and superposition of 3D displacement fields, the tools for meshing complex architectures, and post-processing algorithms.

Novel architectures
The particular attention of InterCom was devoted to the characterisation and analysis of real composite architectures obtained using both the conventional and innovative manufacturing methods. In particular three characteristic case studies were examined in details:
(a) Laminated composites obtained by tow steering. Steered architecture are characteristic for high in-plane curvature of tow paths, ply thickness variation, resin rich zones, etc.
(b) Woven architectures patterned through Liquid Resin Print. They feature yarn crimp, superposition of textile and print induced patters, fibre volume fraction variation.
(c) Composites with graded dissimilar matrices and complex distribution of additives applied to redistribute the load around the stress concentration sites.
An experimental testing programme was conducted for these material systems with the focus on the deformation and damage accumulation mechanisms at the tow/yarn scales.

Application of the decomposition approach to model damage
The high resolution assessment of stress distribution allowed modelling damage accumulation and failure. Damage in composites may violate the translational symmetry of even initially periodic structures and hence, the analysis of non-periodic structures becomes particularly relevant in the context of assessment of damaged material. The suggested approach is not constrained by the requirements of a representative volume and hence there is a larger freedom in selecting the volume of considerations at the structural scale. It enables simpler and more efficient modelling of deformation in damaged material. This was demonstrated for layered textile composites prone to failure through the delamination mode.

Further work and synergy with the parallel projects
The InterCom project created a densely cross-linked cluster of studies leading to the experimentally proven, verified multi-scale concept. A number of approaches and techniques developed within InterCom expanded the capabilities of numerical analysis and the design of novel composites which has been developed by the PI and collaborators in a number of parallel projects. These tools allowed simulating and analysing new patterned graded and functionalised composites manufactured by means of Liquid Resin Print method. The methods developed within InterCom will be further exploited and applied to the design materials at the local scale to enhance composite performance at the component level.

The InterCom project facilitated and helped the reintegration of the PI in the new institution. It made a pronounced contribution in establishing a new research niche and allowed to build on the research work initiated previously.