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Sandwich Structural Composites Sandwich Structural Composites Theory and Practice Wenguang Ma and Russell Elkin First edition published 2022 by CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742 and by CRC Press 2 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN © 2022 Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, LLC Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged, please write and let us know so we may rectify in any future reprint. 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Title: Sandwich structural composites : theories and practices / [edited by] Wenguang Ma and Russell Elkin. Description: First edition. | Boca Raton, FL : CRC Press, 2022. | Includes bibliographical references and index. | Summary: “This book offers comprehensive coverage sandwich structural composites. It describes structure, properties, characterization, and testing of raw materials. In addition, it discusses design and process methods and applications of sandwich structural composites. The book will be of benefit to industrial practitioners, researchers, academic faculty, and advanced students looking to advance their understanding of these increas- ingly important materials”— Provided by publisher. Identifiers: LCCN 2021036440 (print) | LCCN 2021036441 (ebook) | ISBN 9780367441722 (hbk) | ISBN 9781032079073 (pbk) | ISBN 9781003035374 (ebk) Subjects: LCSH: Laminated materials. | Sandwich construction. Classification: LCC TA418.9.L3 S36 2022 (print) | LCC TA418.9.L3 (ebook)| DDC 620.1/18—dc23/eng/20211006 LC record available at https://lccn.loc.gov/2021036440 LC ebook record available at https://lccn.loc.gov/2021036441 ISBN: 978-0-367-44172-2 (hbk) ISBN: 978-1-032-07907-3 (pbk) ISBN: 978-1-003-03537-4 (ebk) DOI: 10.1201/9781003035374 Typeset in Times by codeMantra Contents Preface......................................................................................................................vii Biographies ...............................................................................................................xi Introduction ............................................................................................................xiii Chapter 1 Sandwich Structural Core Materials and Properties ............................1 Wenguang Ma and Russell Elkin Chapter 2 Special Properties and Characterization Methods of Core Materials ....................................................................................73 Wenguang Ma and Russell Elkin Chapter 3 Face Sheet Materials for Sandwich Composites ................................85 Trevor A. Gundberg Chapter 4 Laminating Processes of Thermoset Sandwich Composites ...........125 Wenguang Ma and Russell Elkin Chapter 5 All-Thermoplastic Sandwich Composites ........................................159 Wenguang Ma and Russell Elkin Chapter 6 Characterizations of Sandwich Structures .......................................185 Wenguang Ma and Russell Elkin Chapter 7 Sandwich Structure Design and Mechanical Property Analysis ............................................................................................253 Wenguang Ma and Russell Elkin Chapter 8 Sandwich Composite Structure Modeling by Finite Element Method ...............................................................................295 Guohua Zhou Chapter 9 Application of Sandwich Structural Composites .............................333 Wenguang Ma and Russell Elkin v vi Contents Chapter 10 Sandwich Composite Damage Assessment and Repairing ..............425 Wenguang Ma and Russell Elkin Index ......................................................................................................................449 Preface Wenguang Ma – as a co-author of the book Sandwich Structural Composites – Theory and Practice, earned his Ph.D. degree in plastics engineering from University of Massachusetts Lowell. For more than 20 years, he has been working in the sandwich composites industry area from developing polyethylene terephthalate (PET) foam core materials by extrusion, as well as using balsa wood, plastic foams, and honeycomb core materials for creating sandwich structures for a wide range of applications and developing one of the world’s largest pultrusion line for production of sandwich composite panels. Currently, Dr. Ma is Director of Technical Operation at Vixen Composites USA. Dr. Ma has published about 20 research articles including PET foam core development and application. His other research subjects include balsa and plastic foam cores, and sandwich composite structures. He has given a series of technical semi- nars to international composites conferences on an extensive set of topics including foam core material properties, applications, structural design and characterization, all thermoplastic sandwich construction, and quality control/repairing of sandwich laminates. He contributed one chapter to the book Rigid Structural Foam and Foam-Cored Sandwich Composites in the series on Polymeric Foams published by CRC/Taylor & Francis Group. After that book was published, the editor of CRC, Allision Shatkin, invited him to write a book about the developments in last 30 years on the theories and practices of the sandwich composites. He gladly accepted the invitation, and invited Russell Elkin, his previous colleague, as a co-author to complete the book. He also invited Trevor Gundberg and Guohua Zhou to contribute one chapter each, respectively. Russell Elkin, co-author of the book, is the Product Development Manager with Baltek, Inc. of 3A Composites. He has worked for Baltek and its predecessor com- panies since 1994 after graduating from Carnegie Mellon University with a degree in mechanical engineering. In addition to working peripherally alongside Dr. Ma on the development of PET foam cores, he has designed and assisted on the design of hundreds of sandwich laminates across all market segments. He has also managed dozens of projects converting customers from open molding to vacuum infusion in both structural and process design, training, and implementation. Trevor Gundberg, contributor of Chapter 3, is Vice President of Composite Engineering of Vectorply. He joined Vectorply in 2006 as a Composite Materials Engineer, previously holding technical positions within the composites industry at both Toray Carbon Fibers America and DIAB, Inc. He has been Director of Composites Engineering. Trevor holds a Bachelor of Science in Mechanical Engineering from North Dakota State University (with a minor in Chemistry) and a Master of Science in Mechanical Engineering from the University of Delaware and is a registered Professional Engineer in the state of Alabama. Contributor of Chapter 8, Dr. Guohua Zhou, is a research scientist at Optimal, Inc. working on finite element simulation method development for engineering vii viii Preface application, and he is also an experienced computer-aided engineering (CAE) engineer. Dr. Zhou earned his Ph.D. degree in structural engineering at the University of California, San Diego (UCSD), with a focus on computational mechanics. He has solid background in linear elasticity, composite elasticity, vibration, nonlinear solid mechanics, micromechanics, fracture mechanics, and damage mechanics. “Sandwich” is a special construction of individual materials or structural elements laminated together, where a thick, low-density core material separates thin sheets of stiff, strong, and relatively dense material. The core is typically light-density wood, honeycomb, or polymeric foam. The skins are usually metal, fiberglass, or carbon fiber reinforcements, laminated using thermosetting resins, or embedded in a thermoplastic resin matrix. Faces and core are joined by any variety of lamination and/or bonding methods. The key aspect of sandwich construction is the bond between skin and core, which must ensure an adequate load transfer from one constituent compo- nent to the other. The outcome is an extraordinary product. High bending stiffness, superior strength-to-weight ratios, and good buckling resistance are just a few of the advantages. Sandwich construction also permits function integration. Insulation and fire protection become parts of the structure, for example. Separate materials to provide these traits do not need to be added later. Highly efficient structures may be designed for each specific application by the proper selection and assembly process of integrating the core and facing materials together. The earliest patent for sandwich construction was filed in the late 19th century, but sandwich has been used in modern industry since World War II. Balsa wood was the first core material used for building the de Havilland DH.98 Mosquito bomber’s fuselage. The first foam core material was polyvinyl chloride (PVC) rigid foam, first commercialized in Germany from the 1930s to the 1940s. The first success- ful structural adhesive bonding of honeycomb sandwich structures was achieved in 1938. Today, there is a very wide assortment of materials commercially produced as sandwich cores. Many tailored for some specific performance or application. Metal, plastics, hardwood, especially fiber-reinforced composite and other facing materials have also been developed in the last 50 years for meeting all applications. Sandwich structural composites are being used successfully for a variety of applications such as spacecraft, aircraft, train and car structures, wind turbine blades, boat/ship superstructures, boat/ship hulls, and many other areas. The largest volume of sandwich structure composites has been used in the wind energy industry since the 1980s. The balsa wood, rigid foam, honeycomb, and other core or their combina- tion in cored sandwich constructions are used for building rotor blades and nacelle enclosures of wind energy generators. Since the 1990s, the raw materials, lamination processes, design, and testing methods of the sandwich composites industries have been advanced into a new era. While many still consider high-performance composites and sandwich to be a recent development, the concept and application have been around for more than 75 years. Most engineering students are introduced to composites in an introductory to materials class. Due to the breath of material types (there are material libraries now), FRP and sandwich receive only a brief mention. A similar situation exists in texts about composites. There are a large number of excellent books dedicated to design and analysis, but nothing with a primary focus Preface ix on materials. Furthermore, the sections in many of these texts mainly cover honeycomb and targets the aircraft and aerospace markets. Books on processing are few and those tend to be high on theory, which is important of course, but any discussion of the individual raw materials and how material and process selection affect the resulting laminate is often brief and unsophisticated. Manuals on boat construction and building aircraft often provide more relevant information than many textbooks. Since the turn of the 21st century, the number of published works about composites has grown significantly, but these still cite old and what the authors know to be out- dated and inaccurate references. Current texts continue to contain a great number of assumptions, poorly supported by data, especially about wood-based sandwich cores. We hope this book will fill in a number of those gaps and provide information to the reader which is not available in a product selector app or website. This volume begins with an introduction about the development and signifi- cance of core material. The reader may be introduced to a few new core materials, including polyethylene terephthalate (PET) foam core developed in the last 30 years. Chapters 1 and 2 cover in-depth core types, format, typical properties, and characterization methods. Chapter 3 details various facing materials, reinforcing fibers, and matrix resins. Chapter 4 introduces various laminating processes, adhesive bonding of premade skins to different core materials, and wet laminating processes including hand lay-up, prepreg and autoclave curing, vacuum infusion, and resin transfer molding and pultrusion. All thermoplastic sandwich composites are a new development in the last 20 years. Laminating core and skin sheets using heat and pressure is reviewed in Chapter 5. The structural design, property prediction, computer software simulation, and testing are necessary for developing a new product, optimizing the lamination process, and performing quality control and property confirmation in the sandwich composite industry. Chapters 6–8 present characterizations of sandwich structures, structural design, mechanical property analysis, and finite element analysis (FEA) for mechanical and dynamic predictions. Applications of sandwich composites in different industries are given in Chapter 9, and damage detection, repairing, and nondestructive inspection after repair are presented in Chapter 10. This book is intended to present the latest worldwide developments in sandwich composites over the past 30 years, to benefit the industrial practitioner, researcher, academic faculty, and both undergraduate and graduate school students, and hope- fully direct more interest and dedication into the current industrial activity for an even more prosperous future. A final note: a book like this is never finished. There is still so much to learn.