Troubleshooting part quality during cure when changing the reinforcement - P131

 

Troubleshooting part quality during cure when changing the reinforcement
Practice document
Document Type	Practice
Document Identifier	131
Themes	Thermal management Material deposition management Flow and consolidation management Residual stress and dimensional control management
Tags	Continuous fibre reinforcement Reinforcement architecture Reinforcement material Thermoset cure kinetics
Objective functions	Cost Reduce Rate Maintain Quality Maintain
MSTE workflow	Troubleshooting
Prerequisites	Cure of thermosetting polymers Effect of material in a thermal management system Reinforcement properties

Q: "I have used a given prepreg resin system to cure a wide variety of good quality parts, but having switched reinforcement types to lower cost, I can no longer make good quality parts with the same cure cycle."

A: It is likely that the change in reinforcement resulted in a significant change in the composite's material properties. Common properties to check would be the thermal conductivity, specific heat capacity, density, volume, thickness, and the coefficient of thermal expansion (CTE). If the specific heat capacity or mass of the reinforcement changed dramatically then so too would have the thermal mass of the composite. This will affect the thermal lag experienced by the composite during heat up and cooldown. A change in thermal conductivity, specific heat capacity, or density will have altered the thermal diffusivity of the reinforcement, and therefore the composite. This will affect the manner in which heat is distributed across the composite, potentially leading to hot and cold spots if the diffusivity is poor. In extreme cases, a change in thermal mass or thermal diffusivity may result in uneven cure. A change in the fibre volume fraction or ply thickness may have altered the exothermic response of the part which could have led to thermal degradation. This may manifest as reduced mechanical properties. Finally, if the CTE changed significantly, then the stress imparted onto the part by the tool during cure may be notable. If the part is deforming upon demoulding this may be the cause. Another potential cause for deformation following demoulding could be an uneven degree of cure (DOC).

Overview

Altering any aspect of the material will influence the composite's material properties. If the change in reinforcement is small, there may be no noticeable affect on quality. However, if the change is significant, it is possible that quality will be notably affected. If the thermal properties of the composite change significantly, then the consequences may influence not only the cure behaviour of the composite, but its flow and stress behaviour as well. As with all troubleshooting workflows, the best course of action is to list the defects, relate them to the changes made, and determine what may be the cause of those defects.

Thermal management considerations

Link to thermal management

From a thermal management perspective, the following should be considered at a minimum:

1. Is the fibre different? Changing from carbon fibre to glass fibre is a significant change, but even different grades of the same fibre type can lead to changes in specific heat capacity, thermal conductivity, and thermal diffusivity of the reinforcement form. This can affect the thermal response of the system.
2. Is the fibre volume fraction (Vf) different? This can affect the composite properties, and again affect the thermal response of the system. To learn how to measure Vf, visit the following page: How to measure reinforcement content.
3. Is the ply thickness different? If so, the total laminate thickness will change, and this can again affect the thermal response of the system.

To learn more about the former two points, visit the following page:

• Effect of material in a thermal management system

To learn more about how laminate thickness may affect the thermal response of the system, visit the following page:

• Effect of shape in a thermal management system

Material deposition management considerations

Link to material deposition management

Content coming soon.

Flow and consolidation management considerations

Link to flow and consolidation management

Content coming soon.

Residual stress and dimensional control management considerations

Link to residual stress and dimensional control management

If the coefficient of thermal expansion (CTE) of the new reinforcement has changed significantly, then the residual stressed induced by the tool-part interaction will have also changed. This may act to increase or decrease the chance of part warpage upon demoulding depending on the disparity between the composite CTE and the tool CTE. A higher disparity will tend to increase CTE-induced deformation. Below is a table of common CTE values. Generally speaking, it is best to have tools with a low CTE for carbon fibre-epoxy composites.

CTE values for common tooling and part materials

Material	CTE (x10-6/°C)
Aluminum	23
Steel	11
Invar	0.6 to 1.5
Epoxy	45 to 62
Polyester	60 to 200
Vinylester	100 to 150
Carbon fibre (longitudinal)	-0.2 to -0.5
Carbon fibre (transverse)	10 to 15
E-glass fibre (longitudinal)	5
E-glass fibre (transverse)	5

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents	Maintaining equivalency during cure for different fibre architectures - P118 Troubleshooting part quality during cure when changing the reinforcement - P131
Case Studies
Perspectives Articles	Composite materials engineering webinar session 1 - Introduction - A120 Composite materials engineering webinar session 2 - Constituent Materials - Fiber - A121 Fabric Forming: how it affects design and processing, and how simulation can address this - A310 Fibre Architecture: Availability, pros and cons, and selection for my application - A309

References

About	Help

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.

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.

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.

A quantitative measure of how a material will respond to transient thermal conditions. It is defined as the ratio of thermal conductivity to the volumetric heat capacity of the material (density times the specific heat capacity).

Materials with large thermal diffusivity have a high thermal conductivity relative to their capacity to store energy (heat capacity) and will rapidly distribute the thermal energy throughout its volume and rapidly change temperature to reach the new equilibrium temperature.

Materials with small thermal diffusivity have a low thermal conductivity relative to their capacity to store energy (heat capacity) and will have a sluggish temperature response to a change in thermal conditions (large temperature lag). This is because it takes more energy per unit volume to change the temperature, and the low thermal conductivity means it takes longer to distribute the thermal energy throughout its volume.

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

Degree of cure (DOC) is an indication of how far the chemical curing reaction (crosslinking process) has advanced in a thermoset resin.

DOC is defined with a number between 0 and 1 (or 0% and 100%) where 100% is a fully cured resin. It does not have to fully reach 100% for the resin to become solid or the part to be used. In some aerospace applications, resins are only cured to about 90%. Higher the degree of cure, higher the mechanical properties.

A set of steps/procedures that are intended to provide guidance in manufacturing and/or decision making activities.

There are two types of workflows:

• Standard workflows are intended to formalize practices where the manufacturing science base exists. The focus is to provide guidance using manufacturing simulation as an enabling tool (e.g. design activities/decisions relating to thermal management).
• Complex workflows are intended to reduce the level of effort in practices where the existing manufacturing science base is not sufficiently mature to support production scale problems. The focus is to provide guidance using simulation based thinking and/or checklists (e.g. design activities/decisions relating to porosity management).

A central processing theme in the manufacturing cycle. This theme is concerned with managing the thermal response of materials during storage and handling or parts/tools when they are subsequently heated.

Carbon fibres are composed of large aromatic sheets similar to those in graphite. These graphitic layers form the basic structural units in the shape of ribbons. The structure of carbon fibre ribbon is believed to be a columnar arrangement of disoriented graphite crystallites parallel to the ribbon length. The idealized tetragonal crystallites are stacked above one another, with slight disorientation between the crystals in the direction of fibre axis, trapping sharp needle like voids, where the boundaries between the stacks represent the disordered regions.