Reference - Aspects of Flow and Compaction of Laminated Composites Shapes During Cure

Draft

Create review

Type	Thesis
Title	Aspects of Flow and Compaction of Laminated Composites Shapes During Cure
Abstract	[[Abstract::Cost-effective manufacturing has become the primary objective for many industries using composite materials in primary structures. To realize this objective, it is often desired to use process modelling, as it helps understand the interaction between the parameters affecting the product quality. Among the multitude of phenomena occurring during composites processing, resin flow is a critical issue. It affects the fibre volume fraction distribution, the mechanical properties of the laminate and the final dimensions of the part. For thermoset matrix composites, the percolation flow approach is typically used to model flow and compaction. In this approach, the resin flows relative to the fibres and the flow has to be coupled with the fibre bed compaction behaviour to obtain the final shape of the laminate. In the present work, a percolation flow-compaction model is implemented in a two-dimensional finite element processing model for complex shapes such as angles or hat sections. Material properties required for the flow-compaction model, such as resin viscosity and the fibre bed compaction curve are measured for two carbon-epoxy composites: AS4/3501-6 and AS4/8552. An experimental technique to measure the fibre bed compaction curve directly from the prepreg is presented. The fibre bed compaction curves are validated with results from uniaxial compaction tests. The flow-compaction model is used to study the effect of a variation of the material properties on the compaction of angle laminates. The results from this sensitivity analysis show that the compaction curve significantly affects the final laminate thickness while the fibre bed shear modulus controls the ability of the fibres to conform to a curvilinear shape. A series of angle laminates were cured to study the effect of fibre orientation, bagging conditions, material and tool type on compaction behaviour. Simulations of the angle laminates in bleed conditions are in good agreement with the experiments. However, the model does not predict the experimentally observed magnitude of the deformation at the corner of the [90°] layup. This behaviour is attributed to shear flow, where the fibres and the resin move together as a very viscous anisotropic fluid. Direct observations of the compaction behaviour during transverse flow are presented. They confirm the kinematic relations used in shear flow theory and show that friction is present at the plate-laminate interface. Next, the conditions required to produce percolation or shear flow are investigated in a simple uniaxial compaction experiment. Fibre orientation and resin viscosity are the primary variables determining the dominant flow mechanism. Shear flow is primarily affected by the fibre orientation and occurs essentially in the direction perpendicular to the fibres. Percolation flow depends on the resin viscosity and occurs mainly in the direction where shear flow is not possible.]]
Authors	Hubert, Pascal
City	Vancouver
Department	Metals and Materials Engineering
Date	1996
University	The University of British Columbia, Vancouver
Publication	UBC Open Collections
Websites	https://circle.ubc.ca/handle/2429/6139
DOI	10.14288/1.0078499
Country	Canada

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

The use of multiphysics models to predict the outcome of real-world scenarios. May be analytical (closed form, "hand calculations") or computational (implementation on computers is required due to the large number of calculations involved. e.g. finite element method, finite difference method)

Most often in composite materials engineering, modelling refers to either:

• Process modelling (predicting manufacturing outcomes, or the inverse problem of predicting manufacturing parameters to achieve a desired outcome), or
• Performance modelling (predicting the stiffness/strength/suitability of a structure, or the inverse problem of the material properties and dimensional requirements to achieve a desired stiffness/strength/suitability of a structure)

(same as "Simulation")

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Volume fraction of either matrix or fibres with respect to total composite volume (matrix + fibre).

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

In an unconstrained state, fibre has loft (springiness), when it goes into a mould it must be compacted to obtain the desired V_f.

As the compaction increases, the permeability decreases.

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.