Damage in brittle composites: high fidelity simulation of microstructure effects and its experimental validation
Professor James Marrow, University of Oxford, UK
Novel analysis methods, such as digital image correlation, provide experimentalists with the tools to detect and quantify damage that can be below the threshold of visible resolution; three dimensional observations within composites may be achieved in suitable microstructures using in situ X-ray computed tomography. These data may inform and also test high-fidelity simulations that model the development of damage in composites. The challenge lies in appropriate treatment of the high-resolution characterisation of large representative volumes of microstructure, which can easily lead to inoperable models unless homogenisation is done to reduce the model's computational cost. Homogenisation, however, results in a loss of detail.
We will present examples of how this challenge may be addressed, using high fidelity modelling and experimental observation of damage in ceramic matrix composites to demonstrate how X-ray tomography and image correlation can be applied to validate computed simulations. A new approach, 'Cellular-Automa Finite Element - Microstructural Adaptive Meshfree framework' is also presented; this offers significant improvements in computional efficiency, whilst retaining high fidelity to the microstructure. Its application to a self-healing laminate composite is described.
The aim of this research is to be able to extract critical material parameters from small specimen tests of composite structures. Applied to monitor the degradation of materials, this approach has the potential to simulate the influence of an aging microstructure on the performance of larger scale components, with applications from aerospace to advanced nuclear energy.
A-FEM for high fidelity multi-scale failure analysis of structural composites
Professor Qingda Yang, University of Miami, USA
Advanced composites have been increasingly used as primary load-bearing structures or structural members. While they offer many excellent properties, significant concerns regarding their long term durability and safety remain. One particular unsolved challenge is to quantify the uncertainty associated with the service life. Typical composites exhibit complex, multiple damage events including multiple crack initiation events within and between plies/tows from pristine materials, through their coupled growth, involving multiple bifurcation, coalescence, and ply-jumping, to eventual failure. Numerical and/or analytical methods capable of predicting the progressive damage evolution in composites under general thermal-mechanical loadings and quantifying their effects on structural integrity are in high demand.
High-fidelity thermal-mechanical analyses to composite structures with explicit consideration of arbitrary crack-like damage events are challenging because the heterogeneous nature of composites makes it impossible to know the cracking locations a priori, yet efficient accounting of the nucleation, coalescence, and bifurcation of multiple damage processes is critical to simulation fidelity. There are several methods that can deal with arbitrary cracking in solids such as the extended FEM (X-FEM) and the phantom-node-method (PNM). These methods typically employ crack tip enrichment functions through portioning-of-unity, which are inherently nonlocal and are difficult in handling merging or bifurcating cracks. The need for extra DoFs or nodes for new or propagating cracks also makes these methods inflexible in treating crack interaction.
In this study, we present a multiscale simulation platform based on our newly developed augmented finite element method (A-FEM). We shall demonstrate that the A-FEM is able to achieve high fidelity simulations of multiple, arbitrary damage evolution processes in composite structures with an improved numerical efficiency of several orders of magnitude. The capability of the new A-FEM will be demonstrated through several case studies on realistic composite systems at various important scales. Important information regarding how the microscopic damage processes and their evolution can impact the macroscopic structural performance/integrity will be discussed.
Multi-scale characterization and representation of composite materials during processing
Professor Anoush Poursartip, University of British Columbia, Canada
The worlds of structural simulation and process simulation are rapidly converging as it becomes increasingly clear that failure predictions can be significantly enhanced if process-induced conditions and defects are captured in the structural simulation. Processing outcomes that affect structural response range from thermal outcomes (e.g. degree of cure or crystallinity, physical morphology), to quality outcomes (e.g. ply thickness, volume fraction, wrinkling and waviness, porosity), or mechanical outcomes (e.g. dimensional change and residual stress).
The state of the art in modelling these outcomes as a function of processing conditions is quite varied in terms of rigour, completeness, breadth, integration, and application. Broadly speaking, there are three ranges of matrix response where different simulations strategies can be applied: (a) high temperature, low cure/crystallinity where the system is essentially fluid like and viscous; (b) mid-temperature, gelled or crystallized where the system is viscoelastic; and (c) lower temperature, below the glass transition temperature, where the system is essentially elastic. The behaviour of the system must also be considered at all scales, which in processing includes not only the classical micro-scale, meso-scale, layer/lamina scale, and structural scale, but also the tooling, equipment and other associated manufacturing initial and boundary conditions. Here we present some of our most recent work in creating a coherent and consistent framework for process modelling, and provide experimental and simulation results to highlight both successes and needs. In particular, we focus on residual stress development under different thermal histories, and show how a multi-scale modelling approach can be exercised to evaluate and predict different aspects of this problem.
Computational micromechanics of composites: a mature discipline?
Professor Carlos González, Polytechnic University of Madrid and IMDEA Materials Institute, Spain
Computational micromechanics is emerging as an accurate tool to study the mechanical
behaviour of composites at the micro scale due to the sophistication of the modeling tools and to the ever-increasing power of digital computers . Within this framework, the macroscopic properties of a composite or heterogeneous material can be obtained by means of the numerical simulation of the deformation and failure of a statistically representative volume element of the microstructure. As compared with classic homogenization techniques, computational micromechanics presents two important advantages. Firstly, the influence of the geometry and spatial distribution of the phases (i.e. size, shape, clustering, connectivity, etc.) can be accurately taken into account. Secondly, the details of the stress and strain microfields throughout the microstructure are resolved, leading to precise predictions of the onset and propagation of damage.
Recent advances in this area include the analysis of the effect of different fiber (carbon and glass ), interfaces , spatial distribution and ply thickness or the influence of damage on the final mechanical performance of fiber-reinforced composites [4,5], as well as the physical determination by detailed experimental techniques based on instrumented nanoindentation [6,7] of the parameters governing the mechanical behavior of the composite at this length scale.
 Llorca, J; Gonzalez, C; Molina-Aldareguía, J M; Segurado, J; Seltzer, R; Sket, F; Rodriguez, M; Sadaba, S; Munoz, R; Canal, L P, Multiscale modeling of composite materials: a roadmap towards virtual testing, Advanced materials, 23, 5130-47, 2011, DOI: 10.1002/adma.201101683.
 E. Totry, J. M. Molina-Aldareguia, C. González, J. Llorca. Effect of fiber, matrix and interface properties on the inplane shear deformation of carbon-fiber reinforced composites. Composites Science and Technology, 70, 970-980, 2010.
 C. González and J. Llorca. Mechanical behaviour of unidirectional fiber-reinforced polymers under transverse compression: Microscopic mechanisms and modelling. Composites Science and Technology, 67, 2795-2806, 2007.
 LP. Canal, C. González, J. Segurado, J. LLorca, Intraply fracture of fiber-reinforced composites: Microscopic mechanisms and modeling, Composites Science and Technology, 72, 1223-1232,
 C. González and J. Llorca. Multiscale modelling of fracture in fiber-reinforced composites. Acta Materialia, 54, 4171-4181, 2006.
 M. Rodriguez, JM. Molina Aldareguia, C. González, J. LLorca, A methodology to measure the interface shear strength by means of the fiber push-in test, Composite Science and Technology, 72, 1924-1932,
 M. Rodriguez, JM. Molina-Aldareguia, C. González, J. LLorca, Determination of the mechanical properties of amorphous materials through instrumented nanoindentation, Acta Materialia, 60, 3953-3964,
DOI: 10.1016/j.actamat.2012.03.027, 2012.