Sandwich Structural Composites

Theory and Practice

Wenguang Ma, Russell Elkin

• Publisher: CRC Press
• Year: 2021
• Print ISBN: 9780367441722
• eBook ISBN: 9781000484632
• Topic: Physical Sciences
• Subtopic: Chemical & Biochemical Engineering

Sandwich Structural Composites: Theory and Practice offers a comprehensive coverage of sandwich structural composites. It describes the structure, properties, characterization, and testing of raw materials. In addition, it discusses design and process methods, applications and damage assessments of sandwich structural composites. The book:

• Offers a review of current sandwich composite lamination processes and manufacturing methods
• Introduces raw materials, including core materials, skin reinforcements, resin substrates and adhesives
• Discusses sandwich structure characterization, finite element analysis of the structures, and product design and optimization
• Describes benefits other than structural, including acoustic, thermal, and fire
• Details applications in various industries, including aerospace, wind energy, marine ships, recreational boats and vehicles, sport equipment, building construction, and extreme temperature applications

The book will be of benefit to industrial practitioners, researchers, academic faculty, and advanced students in materials and mechanical engineering and related disciplines looking to advance their understanding of these increasingly important materials.

1 Sandwich Structural Core Materials and Properties

Wenguang Ma

Russell Elkin

DOI: 10.1201/9781003035374-1

Contents

1. 1.1 Rigid Structural Plastic Foams
   1. 1.1.1 Requirements of Foam Core Materials
   2. 1.1.2 Polyvinyl Chloride (PVC) Foam
   3. 1.1.3 Polyethylene Terephthalate (PET) Foam
   4. 1.1.4 Polyurethane (PUR) Foam
   5. 1.1.5 Poly (Styrene-co-Acrylonitrile) (SAN) Foam
   6. 1.1.6 Polymethacrylimide (PMI) Foam
   7. 1.1.7 Polyetherimide (PEI) and Polyethersulfone (PES) Foam
   8. 1.1.8 Syntactic Foam Core
2. 1.2 Wood-Based Core Materials
   1. 1.2.1 Balsa Wood-Based Core Material
      1. 1.2.1.1 Balsa Tree
      2. 1.2.1.2 Milling, Kiln Dry, and Make Block
      3. 1.2.1.3 Microstructure
      4. 1.2.1.4 Mechanical Properties of Balsa Lumber
      5. 1.2.1.5 Mechanical Properties of Lumber-Based End-Grain Core
      6. 1.2.1.6 Homologation and Density Variation
      7. 1.2.1.7 Humidity, Moisture, and Its Effect on Mechanical Properties
      8. 1.2.1.8 Miscellaneous Properties
      9. 1.2.1.9 Product and Format
      10. 1.2.1.10 Applications
   2. 1.2.2 Cork-Based Sandwich Core
      1. 1.2.2.1 Plantation and Harvest
      2. 1.2.2.2 Mechanical Properties
      3. 1.2.2.2 Other Properties and Applications
3. 1.3 Honeycomb Cores
   1. 1.3.1 Thermoplastics Honeycombs
   2. 1.3.2 Metal Honeycombs
   3. 1.3.3 Honeycombs Made from Composite Materials
   4. 1.3.4 Paperboard Honeycombs
4. 1.4 Special Foam Cores
   1. 1.4.1 Metallic Foams
   2. 1.4.2 Ceramic Foams
   3. 1.4.3 Carbon Foams
5. 1.5 Other Core Materials
   1. 1.5.1 Corrugated and Lattice Truss Cores
      1. 1.5.1.1 Corrugated Core
      2. 1.5.1.2 Lattice Truss Core
   2. 1.5.2 3D Fabric Woven Cores
   3. 1.5.3 Core Mats
6. 1.6 Sheet Formats of Core Materials
7. References

Sandwich construction consists of thin, stiff, and strong sheets of metallic or fiber composite materials separated by a thick layer of low-density material. The thick layer of low-density material, commonly known as the core, may be rigid foam, honeycomb, wood, truss, or lattice. Cellular foams are mostly polymer plastics but can also be metal and ceramics. Honeycombs consist of almost any type of material, as diverse as composite face sheets, but they are usually aluminum, thermoplastic, paper, synthetic fiber paper, and composite sheets. Natural-based core comes from balsa trees, but other light woods, such as paulownia and bamboo have been used. Other core materials, made by fabrics and textiles, have different functions in the composite industries. In this chapter, the processing method of different core materials will be introduced briefly. The special features and typical properties of each core will be discussed. The common formats of the core materials will be presented. Also, the applications of each core in the sandwich composite industries will be revealed.

After reading this chapter, the reader will know how to select the right core type, sheet format and thickness depending on the manufacturing process, and the desired performance of the overall laminate for the end-product application.

1.1 Rigid Structural Plastic Foams

1.1.1 Requirements of Foam Core Materials

Many unique properties are required for structural sandwich cores; thus, prudent selection of the polymer from which the foam is produced is most important. Not every polymer can be foamed commercially, and this is why the polymers available for making core materials are few in number. First of all, the foam should be sufficiently stiff. As the core in a structural sandwich, its shear modulus is a representative indication of sandwich stiffness. PVC foam core with density of 40 kg/m³, a general-purpose core material, has a shear modulus of 12 MPa, while PMI foam core, a high-performance core material, with a lower density of 30 kg/m³ has a shear modulus of 13 MPa. However, a rigid PUR foam with a density of 70 kg/m³ has only a shear modulus of 7.6 MPa, which is why PUR foam is a lower-performance core material.

Equally important are the surface properties of the foam core. It should have a sufficiently high surface energy so that it can be wetted out (compatible) with all the adhesives and liquid thermosetting resins that are used in composite industries, but also it should have good resistance against solvent attack so it cannot be softened by any solvents in the resin or adhesive. The close cell content of the foam core also is a critical property that should be, generally, more than 80%. If the open cell content is too high, the liquid resin will migrant into the core during laminating, increasing the weight of the sandwich laminate. In summary, the foam should have a high-enough surface energy to bond to with the adhesive or resin for laminating the facing but cannot be attacked by them physically or chemically. The bond strength must be high enough to withstand the constant bending, compressive, or tensile forces of dynamic loading, such as the forces on a boat hull or rotating turbine blade. The coefficient of thermal expansion (CTE) of the core, the laminate material, and resin or adhesive must be compatible to ensure that thermal cycling doesn’t cause unequal expansions and contractions, leading to de-bonding.

The hot and cold temperature resistance is another important property of the structural core materials. During lamination, the core will endure a heat history because of the need to cure the resin at elevated temperatures and by the heat released by adhesive or resin curing (exothermic). The bond layer between facing and foam surface will shrink or swell if the foam core does not have a sufficiently high heat resistance, reducing the bond strength and possibly causing a laminate to fail. For the specific laminate application or service, the sandwich structure could be exposed to extremes in temperature environment. The foam core should maintain mechanical properties within an acceptable design range to ensure that the composite sandwich product performs the intended function.

The foam core also needs to be fatigue-resistant. A product made by using the foam cored sandwich construction, such as a boat or a wind turbine blade, will endure cyclical forces. For example, a helicopter or wind blade may be designed for over 10 million cycles in its service life. If the core cannot maintain sufficient mechanical properties for its design li...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Biographies
• Introduction
• Chapter 1 Sandwich Structural Core Materials and Properties
• Chapter 2 Special Properties and Characterization Methods of Core Materials
• Chapter 3 Face Sheet Materials for Sandwich Composites
• Chapter 4 Laminating Processes of Thermoset Sandwich Composites
• Chapter 5 All-Thermoplastic Sandwich Composites
• Chapter 6 Characterizations of Sandwich Structures
• Chapter 7 Sandwich Structure Design and Mechanical Property Analysis
• Chapter 8 Sandwich Composite Structure Modeling by Finite Element Method
• Chapter 9 Application of Sandwich Structural Composites
• Chapter 10 Sandwich Composite Damage Assessment and Repairing
• Index