Effect of material in a RSDM system - A275

Effect of material in a RSDM system
Systems knowledge article
Document Type	Article
Document Identifier	275
Themes	Residual stress and dimensional control management
Relevant Classes	Material
Tags	Material parameters System control Thermal properties Thermal transformation Material Thermal management interaction

Introduction

“Material” under the context of residual stress and deformation management refers to material structure and the associated material properties. As a subset of parameters that can affect RSDM, these material properties affect the stress built up within the composite parts during processing, leading to residual stresses and deformations.

Scope

This article discusses the impact of material on the residual stress and deformation of composite parts. The page links to foundational knowledge content. Important material properties such cure shrinkage and thermal expansion and contraction of different material forms are discussed.

Significance

Residual stresses, process induced deformation and dimension control of the final part should be considered from the designing and composite material selection phase. If not controlled, these factors can lead to matrix failure and unexpected dimensional changes which can significantly increase manufacturing time and cost. When a material system is selected, the material structure and associated material properties have direct impact on the part deformation during processing. An understanding of the deformation mechanisms and the roles of those material properties is highly important.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• General material properties
• Polymer properties
• Reinforcement properties
• Composite properties
• Heat transfer
• Thermal management
• System interactions
• Material (system class)
• Residual stress and dimensional control management

Analysis

A detailed breakdown of the material structures and material properties are as following:

• Material structure
  • Fibre
  • Matrix
  • Core
  • Insert
• Material properties
  • General material properties
    • Coefficient of cure shrinkage
    • Coefficient of thermal expansion
  • Polymer properties
    • Heat of reaction
    • Degree of cure
    • Viscosity
    • Gel time
    • Tg
  • Composite properties
    • Elastic moduli
    • Mechanical properties
    • Fiber volume fraction

Effect of material anisotropy on RSDM

The main driver for composite process induced deformation is material anisotropy. After being processed at an elevated temperatures, the strain caused by resin cure shrinkage and thermal contraction in resin property dominated directions are higher than the strain in directions where fibre exists. This strain mismatch can happen on the microscale between individual fibres and matrix or on the macroscale between different directions in a laminate (for example, in the in-plane and through thickness directions). Material anisotropy is one of the major driving mechanisms of stresses and deformation.

Thermal expansion and contraction

As thermoset composites are processed with high temperature cure cycles, CTEs of the fibre and resin must be carefully evaluated to understand internal stress development. On the microscale, fibre and resin have different CTEs. Carbon fibre has a constant and very low (to slightly negative) CTE in the longitudinal direction. Whereas resin typically has higher CTE that evolves as the cure progresses.

In the context of orthotropic fibre reinforced composites, minimal internal stresses are formed during the early curing stages because resin is a viscous liquid before the onset of gelation. However, once gelled, the resin-fibre interface is established; thermal strains in resin caused by cure cycle heat ups and cool downs can result in stresses within the laminates. Generally, CTEs in the fibre directions are lower than the CTEs in directions where there are no fibre constraints and are resin dominant. Hence, when temperature changes, different thermal strains are produced.

Cure shrinkage

As cross-linking progresses, volume in the molecular arrangement and molecular chain mobility decrease. Physically, resin shrinks in volume and becomes more viscous. Like thermal strain, the cure shrinkage strain in a composite also depends heavily on fibre orientation.

Effect of core and inserts on RSDM

Core and inserts can affect the part flexural rigidity which plays an important role in deformation. In general, the thicker the core, the stiffer the parts, the more resistant it is to process induced deformation. Run-out geometry around the core may also affect process induced deformation.

Effect of polymer properties on RSDM

As mechanisms of RSDM are heavily dependent on the evolution of resin properties during cure, characterizing the polymer is crucial for understanding and mitigating residual stresses and deformations. State variables of the polymer such as degree of cure, viscosity, Tg as well as various moduli determine how and how much stress is built up in a composite part. Resin heat of reaction can also significantly affects the composite thermal history which can lead to changes in properties of resin during cure.

Effect of composite properties on RSDM

Aside from the anisotropic properties mentioned above, the stiffness of the cured laminate also plays an important role in the final deformation. The stiffer the parts, the more resistant it is to process induced deformation.

Effect of fiber volume fraction gradients on RSDM

Fiber volume fraction gradient through the thickness can also significantly influence the part residual stress and deformation. These stresses are mainly driven by resin chemical shrinkage and thermal shrinkage. The higher the resin content (lower fiber volume fraction) the more shrinkage and higher the stresses. This through thickness fiber volume fraction gradient is common in thick pre-preg systems that require bleeding. Bleeding is typically done on the bag side to remove excess resin. This results in the bag side being "dryer" comapred to the tool side. Fiber volume fractions of 52% on the bag side and 59% on the tool side have been observed. As the resin cures, the tool side experiences more chemical and thermal shrinkage due to being resin rich, generate higher stress.

Fiber volume fraction gradients can also occur in parts that are made with wet layup or vacuum infusion. It is common in wet layup parts that the resin or fiber are not deposited evenly, whereas in infusion, resin content is higher closer to the outlets compared to at the inlet. However, if the resin used shrinks very little and those parts are not cured at elevated temperatures, the fiber volume fraction gradients can be less of an issue.

References

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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 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

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.

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.

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