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# Effect of shape in a RSDM system - A276

Effect of shape in a RSDM system
Systems knowledge article
Document Type Article
Document Identifier 276
Themes
Relevant Classes
• Shape
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## Introduction

The shape of a composite part can influence its residual stresses on a component scale (See Residual stress and dimensional control management (RSDM) for different scales of residual stresses and mechanisms). In other words, the shape of a part can influence its shape change during processing. The general geometry of the part (for example: L-shape or C-shape), curvature, use/presence of core, and part size are some of the major influencers.

## Scope

This article discusses the impact of part shape on the residual stress and deformation of composite parts. The interaction between part shape and material properties such as cure shrinkage and thermal expansion/contraction leading to residual stress build up and deformation are discussed.

## Significance

Deformations intrinsically associated with the shape, layup and size of a part should be considered during the design stage. For example, the angle change of an L-shaped bracket. If not controlled, the stresses and dimensional changes can cause matrix failure and increase manufacturing time and cost significantly. Since polymer composites shrink during cure and some are being cured at elevated temperature, residual stresses are impossible to eliminate completely. Thus, it is important to understand the potential deformation associated with the nominal designed shape.

## Prerequisites

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

## Analysis

The shape of a part implies the following parameters:

• General geometry: flat, L-shape, C/U-shape, S-shape, Z-shape, cylindrical or a combination of the geometries
• Layup: fiber orientation and laminate thickness
• The use of core
• Part size: length and width

### Effect of general geometry on RSDM

Given the general geometry, a number of possible deformation outcomes are shown in the table below. The deformation behavior in each case is driven by multiple factors including: tool-part interaction, layup, material anisotropy and environmental conditions such as physical aging or moisture absorption.

Warpage for flat plates/sheets and large parts with very mild curvature (such as a wing skin) are mainly driven by tool-part interaction and layup. Tool-part interaction is discussed in more detail in Effect of tooling in a RSDM system and layup is discussed in Effect of layup on RSDM section below. Commonly, warpage can be observed as the part deforms to become either concave or convex with respect to the tool it was made on.

Spring-in can occur in any corner section of a part or curved areas. L-shape, C-shape, S-shape or Z-shape are prone to this type of deformation. Material anisotropy is the main driver for spring-in; it refers to the mismatch of through-thickness and in-plane cure shrinkage strain and thermal contraction strain. (See more details on material anisotropy on Residual stress and dimensional control management (RSDM)). As the composite part is being cured and subsequently cooled down from the processing temperature, the through-thickness strain is higher than the in-plane strain because fiber is not oriented in the through-thickness direction and the resin matrix will contract due to cure shrinkage and CTE. This strain mismatch causes the radius of curved areas to decrease. Another factor is that laminates at male corners can thin out due to pressure intensification. Whereas if bridging happens at females corners, there can be a lack of pressure which lead to increased thickness. This kind of deformation is commonly observed as the angle deviates from the tool after the part is made.

C/U-shapes and convex (male) portion of some S-shapes are prone to geometrical locking where the part is forced to expand/contract with the tool[1][2]. This can causing additional stresses to build up in the web and eventually more deformation. It was shown that with materials, tooling and equipment and all other processing conditions being the same, C-shapes have higher deformation than L-shapes by roughly 30% [3][4]. When the C/U-shape is locked to the tool, spring-in makes the two flanges clamp onto the tool, potentially making de-moulding difficult.

### Effect of corner radius on RSDM

This disagreement within the literature could potentially be due to the narrow bandwidth of processing conditions and geometries such as corner radius, flange length and laminate thickness considered in the individual studies. A recent meta-analysis pooled experimental data of three representative materials systems: HEXCEL AS4/8552, TORAY T800/3900-2 (and its variants) and CYCOM IM7/5320-1 in literature. The study showed no dependence of spring-in angle on the corner radius [15].

### Effect of layup and laminate thickness on RSDM

Thickness and layup are two of the most thoroughly studied processing parameters that can significantly affect residual stress and part deformation. Composite materials are intrinsically anisotropic, meaning the residual stresses and deformation are different in different ply directions. For example, a unidirectional ply will expand very little in the 0° direction due to its low A272, while it will expand more in the 90° direction because it is dominated by the expansion of the matrix. Similar residual stresses occur at all ply interfaces with different orientations. If the laminate is not balanced and symmetric, warpage will certainly occur. A balanced laminate has the same number of plies in the -θ as in the +θ direction, while a symmetric laminate forms a mirror image on both sides of the middle of the laminate. An example of a balanced and symmetric laminate is [0/45/-45/90/90/-45/45/0], which can also be written as [0/45/-45/90]s. Note that in reality, very slight deviation of the ply orientations (1°-2°) can cause the layup to be un-balanced and produce warpage in thin laminates. These deviation are un-avoidable in hand layup.

Most studies in the literature reported the deformation (spring-in and warpage) decreases with increasing thickness [16][10][17][4][18][19][20][2][21][8][22][23][3]. This is expected because for a given geometry and layup, the laminate bending stiffness increases as thickness increases, making it harder to deform. Thicker laminates also better facilitate interlaminar shear, alleviating some residual stress and reducing deformation [24]. However, a few studies reported that laminate thickness has no influence on part deformation [11][25][7][14][9]. This could also be attributed to the narrow bandwidth of processing conditions in those individual studies. It is worth noting that a thick laminate exhibiting less deformation than a thin laminate does not necessary mean that the residual stress level is lower. Residual stress in the thicker laminates can lead to matrix microcracking.

The same recent meta-analysis mentioned in the effect of corner radius on RSDM above also corroborate that the bending stiffness coefficient (D22), which captures the effect of both layup sequence and laminated thickness, can be used as a proxy for calculating spring-in [26][15]. Given a laminate thickness, the layup sequence that creates a higher bending stiffness will lead to less deformation. For example, for the same material and number of plies (laminate thickness), when varying the layup sequence for an L-shape, the deformation should follow: ± 45 > Quasi > cross ply 0/90 > UD 0 > UD 90.

The spring-in of UD [90]n parts is worth discussing. As mentioned, the fundamental driver for spring-in is the in-plane and through thickness strain difference. Since the properties in the in-plane (along the flange length) and through thickness directions of a [90]n laminate are both resin dominated and similar, the common understanding is that spring-in would be minimum for [90]n L-shape and C-shape specimens. However, non-negligible spring-in has been observed in [90]n specimens in literature. The discrepancies are attributed to tool-part interaction, through thickness cure, Vf gradient, or fiber misalignment. Thus, processing conditions and defects can also impact part deformation, especially in the case on UD [90]n parts, where the bending stiffness along the flange length direction is very low.

### Effect of core and inserts on RSDM

Core material can significantly affect the through thickness strain as the composite part is being cooled from curing temperature. Thus the residual stress and deformation of a curved Sandwich Panels structure depends on the coefficient of thermal expansion of the constituents as well as the constraints between the core and the skin. Among which, the coefficient of thermal expansion of the core in through-thickness direction is very important[27]. Core and inserts can affect the flexural rigidity of the part, 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[28]. However, the effect of core thickness increase on reduced deformation is non-linear. Run-out geometry around the core may also affect process induced deformation.

### Effect of part size on RSDM

Part size can also affect the part deformation, particularly due to warpage via tool-part interaction. Large acreage of flat parts or parts with mild curvature (such as a wing skin) are prone to this type of deformation. Part aspect ratio (length/thickness), processing pressure and surface condition, which affects the friction between the part and the tool are a few of the determining processing parameters. Although larger parts have been observed to warp more compared to smaller parts, regardless of processing conditions and part thickness[29]. Tool-part interaction is discussed in more details on Effect of tooling in a RSDM system.

## Related pages

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
Case Studies
Perspectives Articles

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## Welcome

Welcome to the CKN Knowledge in Practice Centre (KPC). The KPC is a resource for learning and applying scientific knowledge to the practice of composites manufacturing. As you navigate around the KPC, refer back to the information on this right-hand pane as a resource for understanding the intricacies of composites processing and why the KPC is laid out in the way that it is. The following video explains the KPC approach:

## Understanding Composites Processing

The Knowledge in Practice Centre (KPC) is centered around a structured method of thinking about composite material manufacturing. From the top down, the heirarchy consists of:

The way that the material, shape, tooling & consumables and equipment (abbreviated as MSTE) interact with each other during a process step is critical to the outcome of the manufacturing step, and ultimately critical to the quality of the finished part. The interactions between MSTE during a process step can be numerous and complex, but the Knowledge in Practice Centre aims to make you aware of these interactions, understand how one parameter affects another, and understand how to analyze the problem using a systems based approach. Using this approach, the factory can then be developed with a complete understanding and control of all interactions.

## Interrelationship of Function, Shape, Material & Process

Design for manufacturing is critical to ensuring the producibility of a part. Trouble arises when it is considered too late or not at all in the design process. Conversely, process design (controlling the interactions between shape, material, tooling & consumables and equipment to achieve a desired outcome) must always consider the shape and material of the part. Ashby has developed and popularized the approach linking design (function) to the choice of material and shape, which influence the process selected and vice versa, as shown below:

Within the Knowledge in Practice Centre the same methodology is applied but the process is more fully defined by also explicitly calling out the equipment and tooling & consumables. Note that in common usage, a process which consists of many steps can be arbitrarily defined by just one step, e.g. "spray-up". Though convenient, this can be misleading.

## Workflows

The KPC's Practice and Case Study volumes consist of three types of workflows:

• Development - Analyzing the interactions between MSTE in the process steps to make decisions on processing parameters and understanding how the process steps and factory cells fit within the factory.
• Troubleshooting - Guiding you to possible causes of processing issues affecting either cost, rate or quality and directing you to the most appropriate development workflow to improve the process
• Optimization - An expansion on the development workflows where a larger number of options are considered to achieve the best mixture of cost, rate & quality for your application.