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.

Analyzing the structure of composite materials is essential for understanding how the part will perform in service. Assessing fiber volume variations, void content, ply orientation variability, and foreign object inclusions helps in preventing degradation of composite performance. This article describes the optical microscopy and bright-field illumination techniques involved in analyzing ply terminations, prepreg plies, splices, and fiber orientation to provide the insight necessary for optimizing composite structure and performance.

The analysis of composite materials using optical microscopy is a process that can be made easy and efficient with only a few contrast methods and preparation techniques. This article is intended to provide information that will help an investigator select the appropriate microscopy technique for the specific analysis objectives with a given composite material. The article opens with a discussion of macrophotography and microscope alignment, and then goes on to describe various illumination techniques that are useful for specific analysis requirements. These techniques include bright-field illumination, dark-field illumination, polarized-light microscopy, interference and contrast microscopy, and fluorescence microscopy. The article also provides a discussion of sample preparation materials such as dyes, etchants, and stains for the analysis of composite materials using optical microscopy.

This article focuses on characterizing the fracture-surface appearance at the microscale and contains some discussion on both crack nucleation and propagation mechanisms that cause the fracture appearance. It begins with a discussion on microscale models and mechanisms for deformation and fracture. Next, the mechanisms of void nucleation and void coalescence are briefly described. Macroscale and microscale appearances of ductile and brittle fracture are then discussed for various specimen geometries (smooth cylindrical and prismatic) and loading conditions (e.g., tension compression, bending, torsion). Finally, the factors influencing the appearance of a fracture surface and various imperfections or stress raisers are described, followed by a root-cause failure analysis case history to illustrate some of these fractography concepts.

This article describes the failure analysis procedures for composites and the techniques to be used in these analyses. These procedures include a review of the available in-service records, materials and processing methods, print requirements, and manufacturing records; visual analysis and nondestructive part evaluation; and verification of materials and processing methods. The article discusses the determination of fiber, matrix, and void volume fractions and verification of ply lay-up and orientation. A review of composites processing parameters; fractography and surface analysis; and mechanical testing and stress analysis is also presented.

Any model that describes the early stage of cavitation must therefore address experimental observations of continuous nucleation, cracklike interface cavities, cavity growth from nanometer-scale sizes, and debonding at particle interfaces and formation of large-faceted cavities. This article summarizes the microstructural details of the early stages of cavitation in metals for understanding the interface-constrained plasticity cavitation model. It discusses formulation, predictions and implications, involved in analysis of cavitation under constrained conditions.

This article reviews how process variations influence the characteristics of thermal spray coatings. It describes various specimen preparation techniques, which allow accurate microstructural analysis. These techniques include sectioning, cleaning, mounting, planar grinding, fine grinding, rough polishing, and etching. The article provides information on the problems associated with specimen preparation. It concludes with a discussion on the various methods of analysis for thermal spray coatings.

This article discusses a set of experimental and computational studies aimed at understanding the effect of various processing parameters on the extent of burr and other defect formation during sheet edge-shearing and slitting processes. It describes the development of experimentally validated finite-element models for analyzing the classes of shearing processes. The article also discusses the use of microstructural characterization with stereology to render three-dimensional volumetric parameters. It concludes with information on the numerical simulation of an edge-shearing process, along with sensitivity studies with respect to process and tool parameters.

This article describes how composite specimens are sectioned, documented, and labeled during sample preparation. The mounting procedures for the specimen are summarized. The article explains sample clamping, which involves not mounting the specimens using an adhesive or casting resin and corresponds to clamped samples used in automated polishing heads. It details that cavity molds involve mounting the composite specimens using a casting resin in a preset mold. The article also discusses the mounting of composite materials for hand polishing.

The goals of modeling diffusion bonding can be regarded as twofold: to optimize the selection of the process variables for a given material and to provide an understanding of the mechanisms by which bonding is achieved. This article describes the existing models of diffusion bonding with an assumption that the surfaces to be joined are free of contaminants and oxide, that bonding occurs between similar materials, and that the materials are single-phase metals. It discusses the mechanisms considered for diffusion bonding and limitations of existing models.

Microscopy is a valuable tool in materials investigations related to problem solving, failure analysis, advanced materials development, and quality control. This article describes the sample preparation techniques of composite materials. These techniques include mounting, rough grinding, and polishing. The preparation techniques of ultrathin sections are also summarized. The article explains the illumination methods used by reflected light microscopy to view a specimen. These consist of epi-bright-field illumination, epi-dark-field illumination, epi-polarized light, and epi-fluorescence. The article also provides information on transmitted light microscopy.