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Über dieses Buch

"Structural and Failure Mechanics of Sandwich Composites" by Leif A. Carlsson and George A. Kardomateas focuses on some important deformation and failure modes of sandwich panels such as global buckling, wrinkling and local instabilities, and face/core debonding. The book also provides the mechanics background necessary for understanding deformation and failure mechanisms in sandwich panels and the response of sandwich structural parts to a variety of loadings. Specifically, first-order and high-order sandwich panel theories, and three-dimensional elasticity solutions for the structural behavior outlined in some detail. Elasticity analysis can serve as a benchmark for judging the accuracy of simplified sandwich plate, shell and beam theories. Furthermore, the book reviews test methods developed for the characterization of the constituent face and core materials, and sandwich beams and plates. The characterization of face/core debonding is a major topic of this text, and analysis methods based on fracture mechanics are described and applied to several contemporary test specimens. Test methods and results documented in the literature are included and discussed. The book will benefit structural and materials engineers and researchers with the desire to learn more about structural behavior, failure mechanisms, fracture mechanics and damage tolerance of sandwich structures.



Chapter 1. Introduction

A structural sandwich typically consists of two thin “face sheets” made from stiff and strong relatively dense material such as metal or fiber composite bonded to a thick lightweight material called “core”. This concept mimics an I-beam, but in two dimensions, where the face sheets support bending loads and the core transfers shear force between the faces in a sandwich panel under load. Figure 1.1 illustrates flat and curved elements from a sandwich structure.
L. A. Carlsson, G. A. Kardomateas

Chapter 2. Characterization of the Mechanical Properties of Face Sheet and Core Materials

Determination of mechanical properties of face sheets and core materials is important for analysis and design of sandwich structures. In many cases, especially for metals, mechanical property data exists in handbooks and textbooks on materials science and strength of materials (e.g., Gere, 2004). For composites, the large variety of fibers, matrix materials, ply lay-ups, and fiber volume fractions makes mechanical testing a necessity. The core may be corrugated, honeycomb, or foam as shown in Figure 1.5. The determination of the mechanical properties may be very challenging, especially for corrugated and honeycomb cores. For balsa wood, the anisotropy of the material causes further complications.
L. A. Carlsson, G. A. Kardomateas

Chapter 3. Classical and First-Order Shear Deformation Analysis of Sandwich Plates

This chapter will present classical laminated plate theory (CLPT) analysis of composite face sheets and sandwich plates. It is recognized that the transverse shear deformation is not incorporated in CLPT. Shear deformation of sandwich plates is important and first-order shear deformation analysis will be outlined. Applications of CLPT and first-order shear deformation analysis to sandwich panels will be presented. Two experimental sandwich plate tests, viz. bending under transverse pressure load and twisting, will be described. Experimental data generated from such tests will be compared to predictions from plate theory analysis and finite elements.
L. A. Carlsson, G. A. Kardomateas

Chapter 4. First-Order Shear Analysis of Sandwich Beams

In this chapter the first-order, two-dimensional shear deformation theory analysis presented in Chapter 3 is specialized to beams. First a general analysis of sandwich beams is developed which is subsequently applied to a threepoint flexure loaded sandwich beam. Simplified beam analysis is developed by reducing the 3 × 3 plate stiffness matrices [A], [B], [C] and [D] to single stiffnesses A, B, C and D, and explicit expressions valid for symmetric beams with thin face sheets are derived. In the final section, three-point flexure testing of sandwich beams and analysis to determine the bending and shear stiffnesses from measured compliance data are outlined.
L. A. Carlsson, G. A. Kardomateas

Chapter 5. Elasticity Solutions for Sandwich Structures

This chapter presents the theory of elasticity solutions for sandwich plates or shells. Elasticity solutions are significant because they provide a benchmark for assessing the performance of the various plate or shell theories or various numerical methods such as the finite element method.Most of these solutions are an extension of the corresponding solutions for monolithic anisotropic bodies which have been developed primarily by Lekhnitskii (1963). This chapter does not cover all problems of the theory of elasticity for sandwich bodies, but presents only some of the most studied ones in an attempt to collect the accumulated recent progress in this field. Section 5.1 on sandwich rectangular plates is adapted from Pagano (1970a), which was extended to the case of positive discriminant materials by Kardomateas (2008a) and Section 5.2 on sandwich shells from Kardomateas (2001).
L. A. Carlsson, G. A. Kardomateas

Chapter 6. High-Order Sandwich Panel Theories

The effects of transverse shear and core compressibility are of high importance in sandwich structures, having an influence on the entire structural behavior including bending, buckling and vibrations. The unusually large transverse shear effects arise due to the very low shear modulus of the core in relation to the extensional modulus of the face sheets. The compressibility effects arise due to the soft nature of the core. This chapter presents two one-dimensional high-order core shear theories for sandwich beams or wide plates, namely the “High-Order Sandwich Panel Theory” (HSAPT), and the more recent “Extended High-Order Sandwich Panel Theory” (EHSAPT). It should be noted that the basic assumptions regarding the face sheets kinematics and face sheet constitutives are the same in all theories and the differences are in dealing with the core kinematics and constitutive relations. In addition, although these theories are presented for the simpler one-dimensional beam configuration, they can be easily extended to the two-dimensional plate or shell geometries. Other high-order theories for sandwich structures available in the literature are briefly outlined.
L. A. Carlsson, G. A. Kardomateas

Chapter 7. Global Buckling of Sandwich Columns and Wide Panels

The most important issue regarding buckling of sandwich structures is the effect of transverse shear which can significantly reduce the Euler critical load. Simply put, the effect of transverse shear absolutely cannot be neglected. Therefore, all formulas for sandwich buckling are essentially ways to include this effect into the Euler formulas. Two basic ways for including transverse shear in column buckling are the Engesser (1891) and the Haringx (1948, 1949) approaches. Both of these approaches are also outlined by Timoshenko (1936).
L. A. Carlsson, G. A. Kardomateas

Chapter 8. Wrinkling and Local Instabilities

Compression loaded faces of sandwich members are sometimes subject to local instability phenomena, the most prominent being the wrinkling or rippling and the intracell buckling or dimpling. This chapter presents the mechanics associated with these phenomena and the classical formulas that predict the conditions for inducing these forms of local instability.
L. A. Carlsson, G. A. Kardomateas

Chapter 9. Fracture Mechanics Analysis of Face/Core Debonds

The superior performance of light-weight sandwich structures requires that the face sheets be successfully bonded to the core. Lack of bonding, or inadequate bonding, will compromise the transfer of shear stress between the face and core, and if debonding occurs over a large area, the debond is likely to grow further. It is also obvious that the face/core adhesion may vary in a large panel with composite face sheets due to inadequate wet-out of the face fabrics resulting in “islands” of poor face/core bonding. Service loads are also known to be a potential source for face/core debonding, in particular wave-slamming loads on the bottom of a ship hull or hard object impact loads transverse to the surface of a sandwich structure.
L. A. Carlsson, G. A. Kardomateas

Chapter 10. Analysis of Debond Fracture Specimens

Several test methods for determining the fracture toughness of the face/core interface in sandwich specimens have been proposed. All debond specimens are beam specimens where a debond typically is implanted in the form of a thin Teflon sheet between face and core during manufacture of the sandwich panel, or in some cases the debond is machined or cut with a thin blade or knife. This and several other experimental issues will be discussed in Chapter 11. In this chapter, we will introduce some of the most popular sandwich debond tests and outline analysis of compliance and energy release rate.
L. A. Carlsson, G. A. Kardomateas

Chapter 11. Debond Fracture Testing

Results from experimental studies using the debond test specimens introduced in Chapter 10, i.e. the double cantilever beam (DCB), tilted sandwich debond (TSD), cracked sandwich beam (CSB), single cantilever beam (SCB), three-point sandwich beam (TPSB), mixed mode bending (MMB), and double cantilever beam-uneven bending moments (DCB-UMB) specimens are discussed. Results of particular interest are the compliance and energy release rate, and the manner in which the crack propagates, i.e., interface propagation or crack kinking and the determination of debond fracture toughness.
L. A. Carlsson, G. A. Kardomateas

Chapter 12. Face/Core Debond Buckling and Growth

The face/core debond is justifiably considered to be a weak link in the use of sandwich structures. This is because such debonds tend to grow and eventually completely delaminate the face sheet. The most common cause of these defects is poor or missing bonding due to careless manufacturing or a mismatch in the geometry. Similar defects may also arise during service due to thermo-mechanical loads, impact events, or structural fatigue. Debonds or delaminations are susceptible to the phenomenon of “delamination buckling” which occurs when local compressive loading is introduced at the debond site.
L. A. Carlsson, G. A. Kardomateas


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