Jonathan P.-H. Belnoue

An experimental investigation of the consolidation behaviour of uncured prepregs under processing conditions

This paper presents a methodology and research study that characterises toughened materials, as is needed for optimisation of composite manufacturing processes. The specific challenge is to cover all of the stages of advanced composite manufacturing: fibre deposition by automatic fibre placement machines, hot or room temperature debulking, and consolidation in an autoclave. In these processes the material experiences a wide range of processing parameters: pressure, load rate, temperatures, and boundary constraints. In these conditions, toughened prepregs exhibit complex rheological behaviour, with diverse flow and deformation mechanisms at various structural scales. Here a series of experimental results are presented in order to describe the temperature, viscosity, flow mechanisms, and scale-effects of simple uncured prepreg stacks. The driver for this study is to obtain a further understanding of flow mechanisms throughout the consolidation phase of composites manufacture since fibre path defects are most likely to occur during compaction, prior to vitrification.

Multi-scale modelling of non-uniform consolidation of uncured toughened unidirectional prepregs

Consolidation is a crucial step in manufacturing of composite parts with prepregs because its role is to eliminate inter- and intra-ply gaps and porosity. Some thermoset prepreg systems are toughened with thermoplastic particles. Depending on their size, thermoplastic particles can be either located in between plies or distributed within the inter-fibre regions. When subjected to transverse compaction, resin will bleed out of low-viscosity unidirectional prepregs along the fibre direction, whereas one would expect transverse squeeze flow to dominate for higher viscosity prepregs. Recent experimental work showed that the consolidation of uncured toughened prepregs involves complex flow and deformation mechanisms where both bleeding and squeeze flow patterns are observed [1]. Micrographs of compacted and cured samples confirm these features as shown in Fig.1. A phenomenological model was proposed [2] where bleeding flow and squeeze flow are combined. A criterion for the transition from shear flow to resin bleeding was also proposed. However, the micrographs also reveal a resin rich layer between plies which may be contributing to the complex flow mechanisms during the consolidation process. In an effort to provide additional insight into these complex mechanisms, this work focuses on the 3D numerical modelling of the compaction of uncured toughened prepregs in the cross-ply configuration described in [1]. A transversely isotropic fluid model is used to describe the flow behaviour of the plies coupled with interplay resin flow of an isotropic fluid. The multi-scale flow model used is based on [3, 4]. A numerical parametric study is carried out where the resin viscosity, permeability and inter-ply thickness are varied to identify the role of important variables. The squeezing flow and the bleeding flow are compared for a range of process parameters to investigate the coupling and competition between the two flow mechanisms. Figure 4 shows the predicted displacement of the sample edge with the multi-scale compaction model after one time step [3]. The ply distortion and resin flow observed in Fig.1 is qualitatively retrieved by the computational model.Consolidation is a crucial step in manufacturing of composite parts with prepregs because its role is to eliminate inter- and intra-ply gaps and porosity. Some thermoset prepreg systems are toughened with thermoplastic particles. Depending on their size, thermoplastic particles can be either located in between plies or distributed within the inter-fibre regions. When subjected to transverse compaction, resin will bleed out of low-viscosity unidirectional prepregs along the fibre direction, whereas one would expect transverse squeeze flow to dominate for higher viscosity prepregs. Recent experimental work showed that the consolidation of uncured toughened prepregs involves complex flow and deformation mechanisms where both bleeding and squeeze flow patterns are observed [1]. Micrographs of compacted and cured samples confirm these features as shown in Fig.1. A phenomenological model was proposed [2] where bleeding flow and squeeze flow are combined. A criterion for the transition from shear flow to resin b...

Coupled Damage-Plasticity Modelling of Ductile Failure in an Aluminium Alloy

The ductile failure of metallic alloys is characterized by the long plateau of the stress-strain response during plastic deformation. In aluminium alloys this complex process is principally mediated by crystal slip associated with dislocation nucleation, motion, interaction, and locking. This results in hardening, i.e. the increase in the flow stress and progressive exhaustion of ductility, eventually leading to damage. Therefore, in the advanced stages of deformation the strength increase at the material level competes with overall stiffness and strength decrease due to effective cross-section reduction by decohesion and voiding. Capturing the complex hierarchical failure of these materials requires developing sophisticated concurrent constitutive descriptions of both plastic deformation and damage at different stages of failure. In the present study the modelling of aluminium alloy failure is accomplished using a plasticity-based model with nonlinear hardening coupled with isotropic damage in a thermodynamically consistent framework. The model developed in this way is enhanced with nonlocal regularization to deal with material instabilities issues due to softening. Emphasis is placed on the correspondence between experimental measurements of the essential work of fracture and the non-essential work of fracture, and both local and spatial sets of model parameters. This approach is the key to assuring a constitutive response consistent with experimental observations, an issue usually overlooked in nonlocal constitutive modelling. Numerical examples are used to demonstrate the features of the new approach.

Modeling Crack Initiation and Propagation in Nickel Base Superalloys

Nickel base superalloys are the primary class of materials used in the manufacture of high temperature components for gas turbine aeroengines, including combustion casings and liners, guide vane and turbine blades and discs, etc. These components are subjected to complex cyclic loading induced by the combination of mechanical loading, changing temperatures and thermal gradients, inducing plastic deformation and creep, that ultimately may lead to crack initiation and propagation. The purpose of the present paper is to provide a necessarily brief overview of recent modeling activities in this field, including polycrystalline crystal plasticity modeling for the study of crack initiation, coupled non-local damage-plasticity modeling for crack initiation and propagation studies, and the incorporation of time and environment dependent processes (creep and oxidation) in the predictive modeling of fatigue crack growth rates in nickel base superalloys.