Honeycomb-cored sandwich panels increase part stiffness at a lower weight than monolithic composite materials. This article illustrates an area of a honeycomb-cored sandwich structure composite cross section that is viewed using transmitted polarized light. This area shows the differences in the constituents and resin intermingling. The article discusses the factors that govern the honeycomb core movement and honeycomb core crush, with illustrations. Some common tests performed on honeycomb composites to characterize the skin-to-core bond strength are the climbing drum peel and flatwise tensile tests. The article concludes with a description on the reasons for core failure, which are analyzed after these tests.

Voids in fiber-reinforced composite materials are areas that are absent of the composite components: matrix (resin) and fibers. Voids have many causes but generally can be categorized as voids due to volatiles or as voids that result from entrapped air. This article describes the analysis of various types of voids. It reviews techniques for analysis of voids at ply-drops, voids due to high fiber packing, and voids that occur in honeycomb core composites. The final section of the article discusses void documentation through the use of nondestructive inspection techniques and density/specific gravity measurement methods.

Lightweight structural cores are used on aircrafts to reduce weight and increase payload and fight distance. This article discusses the classification of lightweight structural cores, namely, honeycomb, balsa, and foam. It reviews the four primary manufacturing methods used to produce honeycomb: adhesive bonding and expansion, corrugation and adhesive bonding, corrugation and braze welding, and extrusion. The article describes cell configuration and properties of honeycomb. It discusses the factors influencing specification of structural cores, including materials, size, density, mechanical properties, environmental compatibility, formability, durability, and thermal behavior. The article provides information on the benefits and concepts of a sandwich panel containing lightweight structural cores.

Honeycomb is a product consisting of very thin sheets attached to form connecting cells. This article briefly explains the construction, core characteristics, properties, and testing methods of the honeycomb structures. It discusses the special processes carried out in customizing the shape of core to fit customer's specific needs. The article provides information on the basic concept of creating sandwich structures and its corresponding aspects like material selection, design guidelines, and structural efficiency.

Adhesive bonding is used to assemble composite components into larger structures. Finished components that are damaged during assembly or service are often repaired with adhesive-bonding techniques. This article summarizes criteria for adhesive selection and illustrates typical secondary adhesively bonded joint configurations. It discusses the highly loaded joint considerations of adhesives. The article describes the epoxy adhesives commonly used for the bonding or repair of composite structures. It discusses the surface preparation of composites and metals, and honeycomb processing, including perimeter trimming, mechanical forming, heat forming, core splicing, contouring, and cleaning. The article presents basic steps involved in the adhesive-bonding process and concludes with a discussion on adhesive applications and tooling.

A sandwich structure is comprised of layered composite materials formed by bonding two or more thin facings or facesheets to a relatively thick core material. This article describes the sandwich panel failure modes. It tabulates the nomenclature and definitions for loads, geometry, and material properties. The article illustrates critical strength-check locations for a flat sandwich panel. It discusses the analysis methods formulated for flat rectangular honeycomb panels; curved sandwich panel; and for each of the various sandwich panel failure modes. The article concludes with a discussion on flat panel stability analysis methods.

This article presents general guidelines for machining metal matrix composites (MMC) and honeycomb structures. It provides guidelines for machining of specific MMCs, namely, aluminum-boron, aluminum-SiC, aluminum-Al 2 O 3 , and titanium-SiC MMCs. In addition, the article discusses the various parameters influencing drilling of dissimilar-material laminates.

This article provides information on the applications of fiber-reinforced composites in commercial and military aircrafts. It tabulates the composite components in various types of aircraft. The applications of the composites in the components of Boeing 727, 737, 757, 767, 777, and 777-200 are schematically illustrated.

This article provides non-proprietary and non-competition-sensitive information related to aircraft applications. It presents an overview of reliability and commonly used measurements. Failure modes that cause the negative performance are reviewed based on many types of sources. These include manufacturer service bulletins, reliability and customer service departments, literature reviews, demonstration programs, in-service evaluations, design guides, and surveys of commercial and military aircraft maintenance organizations. The article also describes lessons learned while attempting to avoid overlapping maintainability, reparability, and materials choice.

This article discusses the requirements for designing repairable composite structures such as a honeycomb sandwich panel construction and integrally stiffened co-cured composite structures. It reviews the general and specific design guidelines for bolted or riveted repairs and adhesively bonded repairs of the composite structures. The article presents several examples to illustrate how these repairs can be achieved.

Adhesive-bonded joints are extensively used in aircraft components and assemblies where structural integrity is critical. This article addresses the problem of how to inspect bonded assemblies so that all discrepancies are identified. It describes several inspection techniques and presents drawbacks and limitations of these techniques. Generic flaw types and flaw-producing mechanisms are listed in a table. The article discusses metal-to-metal defects, adherend defects, honeycomb sandwich defects, repair defects, and in-service defects. It reviews the methods applicable to the inspection of bonded structures, including visual inspection, ultrasonic inspection, X-ray radiography, and neutron radiography. The evaluation and correlation of inspection results are also discussed. The article concludes with information on the effects of ultrasonic wave interference in the ultrasonic inspection of adhesive-bonded joints.

This article focuses on the sample preparation methods for titanium honeycomb composites, boron fiber composites, and titanium/polymeric composite hybrids. These include mounting, sectioning, grinding, and polishing. The article also provides information on the sample preparation of unstaged and staged prepreg materials for optical analysis.

This article illustrates the polymer matrices used for composite materials. It describes the use of prepeg materials in manufacturing high-performance composites. The article discusses the various infusion processes for the development of fiber-reinforced composites, namely, resin transfer molding, vacuum-assisted resin transfer molding, and resin film infusion. It explains the composite- and matrix-toughening methods for fiber-reinforced composites, such as dispersed-phase toughening and interlayer toughening. The article concludes with information on optical microscopy, which provides an insight into the micro- and macrostructure of fiber-reinforced composites.

Maintainability is a function of the durability, damage tolerance, and repairability of a structure. This article discusses the configurations of composite structures, such as sandwich, stiffened-skin, and monolithic structures, used in commercial aircraft composites. It describes the considerations for maintainability of the composite structures during the conceptual design phase. Sources of the defects and damage, such as manufacturing defects and in-service defects, are reviewed. The article describes the nondestructive inspection methods that are used in the repair of composite structures to locate damage, characterize the extent of damage, and ensure post-repair quality. It lists suggestions that can be used as design guidelines for adhesive bonding, general composite structure, sandwich structure, material selection, and lightning-strike protection. The article also provides the basic considerations for personnel, facilities, and equipment during maintenance.