Advances in Composites Manufacturing and Process Design

Philippe Boisse

• Publisher: Woodhead Publishing
• Year: 2015
• Print ISBN: 9781782423072
• eBook ISBN: 9781782423201
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

The manufacturing processes of composite materials are numerous and often complex. Continuous research into the subject area has made it hugely relevant with new advances enriching our understanding and helping us overcome design and manufacturing challenges. Advances in Composites Manufacturing and Process Design provides comprehensive coverage of all processing techniques in the field with a strong emphasis on recent advances, modeling and simulation of the design process.Part One reviews the advances in composite manufacturing processes and includes detailed coverage of braiding, knitting, weaving, fibre placement, draping, machining and drilling, and 3D composite processes. There are also highly informative chapters on thermoplastic and ceramic composite manufacturing processes, and repairing composites. The mechanical behaviour of reinforcements and the numerical simulation of composite manufacturing processes are examined in Part Two. Chapters examine the properties and behaviour of textile reinforcements and resins. The final chapters of the book investigate finite element analysis of composite forming, numerical simulation of flow processes, pultrusion processes and modeling of chemical vapour infiltration processes.- Outlines the advances in the different methods of composite manufacturing processes- Provides extensive information on the thermo-mechanical behavior of reinforcements and composite prepregs- Reviews numerical simulations of forming and flow processes, as well as pultrusion processes and modeling chemical vapor infiltration

Part One

Advances in composite manufacturing processes

1

Braiding processes for composites manufacture

B. Arold¹; A. Gessler¹; C. Metzner¹; K. Birkefeld^{2 1} Airbus Group Innovations, Munich, Germany
² University of Stuttgart, Stuttgart, Germany

Abstract

This chapter describes the current status of the production of carbon fiber preforms with braiding technology. It starts with a short introduction to the basics of braiding, typical machines, auxiliary devices and processes, and shows examples for automated preform production. The main part discusses the capabilities and limitations of the technology and the material performance that can be obtained with braiding. The chapter then continues with a detailed description of the advancements in the field of braiding simulation and closes with a short lookout on expected future trends.

Keywords

Braiding

Carbon fiber preforms

Overbraiding of mandrels

Performance of braided materials

Braiding machines

Process automation

Auxiliary processes

Braiding simulation

Future of braiding

1.1 Introduction

Braiding is one of the most versatile and cost-efficient processes for the production of fiber preforms for composites. It is versatile in terms of its huge variability of producible types of textiles with different characteristics, and also because of its very wide range of shapes, being produced directly or by subsequent cutting and draping. And braiding is cost-efficient not only in consequence of its layup speed, but also because the material waste can be reduced to a minimum, which is even more important when expensive carbon yarns are used.

Big automobile companies as well as aircraft manufacturers are using carbon composite parts based on the braided preforms. These applications give evidence of the general mass-market capability of the technology. That usually does not mean production costs are as low as the replaced metal parts, but cost-efficient enough for an economic trade-off between cost and weight (fuel consumption) for commercial products. Yet there are still many open questions, which make the application of braids delicate. Precisely because braiding is extremely versatile, it is rather difficult to find a way toward a common qualification of the material. Lacking a comprehensive calculation model for complex shapes and also a full understanding of the material characteristics of the different textile types, the qualification of products is still made at the part level instead of on the basis of a general material qualification.

Many research projects at universities and industrial sites around the world are dealing with these problems. This chapter will share with you a part of the constantly evolving knowledge about braiding technology, being fully aware that the given picture can be neither comprehensive nor completely up to date.

1.2 Braiding process

1.2.1 Principle

In woven fabrics, the warp runs at 0° along the textile and the weft at 90° perpendicular to it. In braids, the yarns build binding patterns quite similar to woven fabrics, but they run at a desired angle in both directions like a helix around the tube. The yarn angle depends on the relative speed of the bobbins and the take up. A smaller angle is the result of a faster take-up speed (Figure 1.1).

Figure 1.1 Comparison of weave and tubular braid.

Prefabricated hoses are the simplest to use, but to take real advantage of braids, the so-called overbraiding of shape-defining mandrels is the means of choice. Pushed along the middle axis of a braiding machine, the mandrels become covered by a dry fiber preform, one layer each time. The mandrels have to fulfill a series of functions depending on the shape and on the further processing of the preform:

– The material must be pressure-resistant to prevent a deformation of the edges by the yarn forces and to withstand intermediate compaction processes.

– Bending and torsion stiffness of the mandrel are important to ensure a sufficient accuracy of the preform, especially for extremely elongated parts.

– A low thermal expansion coefficient is also essential for long mandrels, if they are to be used for curing high-temperature resins. Sufficient heat resistance is self-evident.

– At the same time, the mandrel should be expandable in thickness to apply pressure from inside during curing in an RTM mold.

– Sometimes, if the shape of the final part does not allow a direct demolding, soluble core materials are needed.

– Finally, the mandrels should be lightweight for easy handling and transport and, of course, low cost because they cause a considerable share of the NRC in serial production.

1.2.2 Variants of braids

Braids may consist of a single-fiber type or of a mixture of fibers. A combination of fibers with different colors, for instance, is sometimes used to achieve a nice characteristic design of the part surface. For special applications, it may be useful to combine reinforcement fibers with high stiffness (carbon) with some of extreme toughness (aramid), to create a material with a certain residual stability after catastrophic impact damage. Apart from this somewhat exotic approach, most of the fiber preforms for technical applications consist only of a single type of reinforcement fiber in all directions, or of a combination of the reinforcement fiber and a so-called support yarn as shown in Figure 1.2.

Figure 1.2 Braided textile styles: unidirectional, biaxial, and triaxial braids.

Biaxial braids—Biaxial braids made of rovings or prestabilized spread tows are the common standard materials produced on circular braiders for composite preforming. The textiles show a more or less wavy surface texture in combination with the typical fiber undulation known from woven fabrics. The total thickness of one biaxial braid layer is twice the yarn height, and the manifestation of the waviness depends mainly on the “a to b” ratio (as shown in Figure 1.2) or, in other words, on the spreadability of the yarn. As a result of the waviness, biaxial braids may have some disadvantages for high-performance applications:

– Higher preform compressibility in the thickness direction;

– Lower stiffness and strength in tension and compression;

– Inhomogeneous fiber content and risk of resin accumulations;

– Difficulty in reaching fiber volume percentages higher than 50%.

Despite this, some applications may benefit from other features deriving from the texture of the textile. First, there is a better interlock between the layers, leading to slightly higher interlaminar shear strength and damage tolerance. Second, the gaps between yarns and layers provide resin channels that allow faster infiltration or the use of resin systems with higher viscosity. In addition, the production of biaxial braids is very cost-effective in te...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of contributors
• Preface
• Woodhead Publishing Series in Composites Science and Engineering
• Part One: Advances in composite manufacturing processes
• Part Two: Mechanical behaviour of reinforcements and numerical simulation of processes in composites manufacturing
• Index