Microstructural analysis of the composite matrix is necessary to understand the performance of the part and its long-term durability. This chapter focuses on the microstructural analysis of engineering thermoplastic-matrix composites and the influence of cooling rate and nucleation on the formation of spherulites in high-temperature thermoplastic-matrix carbon-fiber-reinforced composites. It also describes the microstructural analysis of a bio-based thermosetting-matrix natural fiber composite system.

A compressor blade made of titanium alloy fractured during an engine test. The material and processing conditions of the blade were found to be satisfactory, turning the focus of the investigation to operating anomalies and human error. A photograph of the failed blade shows well-defined chevron marks along the fracture surface that end in a shear lip on the convex side. Further examination using a SEM shows that the failure was due to overload. Based on these observations and the results of tensile testing and microstructural analysis, investigators concluded that a sudden impact load on the concave side of the blade caused it to fracture.

This report describes the failure of a gas turbine in a combined-cycle power plant and the examination and tests that were conducted to determine the cause. Based on microstructural analysis, hardness measurements, and tensile tests, the failure was attributed to inadequate clearances in the seal land region between two stages in the compressor section of the rotor. The report also recommends changes to remediate the problem.

This chapter discusses some of the more advanced methods and procedures used in failure analysis, including in-service material sampling, in situ microstructure analysis, and a form of punch testing that can determine the fracture toughness of any material from a tiny specimen. The chapter also covers quantitative fractography, fracture surface topography analysis, and the use of oxide dating as well as fault tree and failure modes and effects analysis (FMEA) and computational techniques.

A second-stage compressor blade in an aircraft engine fractured after 21 h of service. The remaining portion of the blade was removed and examined as were several adjacent blades. Based on the results of SEM fractography, microstructural analysis, and hardness testing, the blade failed due to stress-corrosion cracking combined with the effects of inadequate tempering.

Two filtration components installed on a developmental aircraft cracked during pressure impulse testing. Both parts were made from an aluminum alloy, solutionized and aged, and cracked due to fatigue. In both cases, the crack initiated at a transition region on an inner surface and progressed circumferentially outward. Based on these observations and the results of SEM fractography and microstructural analysis, the fatigue cracking can be traced to insufficient fillet radius at the transition zone.

This chapter provides perspective on the physical dimensions associated with the microstructure of steel and the instruments that reveal grain size, morphology, phase distributions, crystal defects, and chemical composition, from which properties and behaviors derive. The chapter also reviews the definitions and classifications used to identify and differentiate commercial steels, including the AISI/SAE and UNS designation systems.

A low-pressure turbine rotor blade failed in service, causing extensive engine damage. A section of the blade broke off around 25 mm from the root platform, producing a flat fracture surface that appeared smooth on one end and grainy elsewhere. Based on their examination, investigators concluded that the nickel-base superalloy blade was exposed to high temperatures and stresses, initiating a crack that propagated under cyclic loading. This chapter provides a summary of the investigation and the insights acquired using scanning electron fractography, metallography, and hardness measurements.

A test flight was cut short after a fire warning came on indicating a problem with one of the four engines on an aircraft. A visual examination following the precautionary landing revealed several burned hoses, a melted bolt, and fuel leaking from the base of the main burner. The fuel nozzle was also damaged, and based on its microstructure, came very close to melting. Investigators determined that the burner was mounted backwards, facing the compressor rather than the turbine. They also recommended a redesign to prevent the fuel nozzle from being reversed.

A turbine blade in an aircraft engine failed, fracturing at the root above the fir tree region. Fractography indicated that a fatigue crack initiated at the trailing edge of the blade and the final fracture occurred when the crack reached critical length. Although the exact cause of crack initiation could not be established, material defects, improper root loading, and high operating temperatures were ruled out. This chapter describes how investigators came to their conclusions and what they learned through visual and SEM examination and qualitative elemental analysis. It includes images of the microstructure and fracture surfaces and explains what some of the details reveal about the failure.

A pair of bearings mounted side by side in an aircraft engine failed in service. Photographs show that the inner rings were either broken or deformed, the balls were worn and flattened, and the cages severely damaged. The bearing races were damaged as well, but only on one side indicating a directional thrust. In addition to their examination, investigators also conducted metallographic studies and hardness tests, which indicated that the balls and inner rings reached temperatures above 825 °C (1520 °F). Based on their findings, investigators concluded that the bearings failed due to overheating, possibly as a result of misalignment compounded by insufficient lubrication and high speeds.

An aircraft crashed following the loss of yaw control in full airborne flight. The subsequent discovery of broken shutter bolts in the rear pitch reaction control valve led to an inspection campaign that found bolt failures of a similar nature in valves on several other aircraft. The bolts were removed and analyzed to determine the mode and cause of failure. Based on the results of macroscopy, scanning electron fractography, metallographic examination, and chemical analysis, the failures were caused by stress corrosion cracking, and in one case, overtightening.

This chapter discusses the techniques applicable to the diagnosis of corrosion failures, including visual and microscopic examination of corroded surfaces and microstructure; chemical analysis of the metal, corrosion products, and bulk environment; nondestructive evaluation methods; corrosion testing techniques; and mechanical testing techniques. A guide to investigative techniques used in corrosion failure analysis is provided in a table, describing the advantages and limitations of each technique. The principal stages of the investigation and analysis of corrosion failures discussed in the chapter are: collection of background information and sampling; preliminary laboratory examination; detailed metallographic and fractographic examinations; chemical analysis of corrosion products and bulk materials; corrosion testing for quality control; mechanical testing for quality control; and analysis of results and report writing.

The practical application of metals and alloys is guided largely by information obtained through the study of their microstructure. This chapter examines a wide range of titanium microstructures, identifying characteristic features and explaining what they reveal about processing, properties, and performance. It includes images of elongated and equiaxed structures, primary alpha, transformed beta, and metastable phases as well as spheroidal and intergranular beta, alpha case, and intermetallic compounds. It also defines important terms and provides step-by-step procedures for preparing titanium for metallographic analysis.

This chapter covers the emerging practice of quantitative microscopy and its application in the study of the microstructure of metals. It describes the methods used to quantify structural gradients, volume fraction, grain size and distribution, and other features of interest. It provides examples showing how the various features appear, how they are measured, and how the resulting data are converted into usable form. The chapter also discusses the quantification of fracture morphology and its correlation with material properties and behaviors.

This chapter explains how to safely prepare beryllium alloy samples for metallographic analysis. It describes grinding, polishing, and etching procedures in detail. It also discusses the identification of major and minor constituents and the general appearance of beryllium microstructure.

This chapter elucidates the indispensable role of characterization in the development of cold-sprayed coatings and illustrates some of the common processes used during coatings development. Emphasis is placed on the advanced microstructural characterization techniques that are used in high-pressure cold spray coating characterization, including residual-stress characterization. The chapter includes some preliminary screening of tool hardness and bond adhesion strength, as well as a distinction between surface and bulk characterization techniques and their importance for cold spray coatings. The techniques covered are optical microscopy, X-Ray diffraction, scanning electron microscopy, focused ion beam machining, electron probe microanalysis, transmission electron microscopy, and electron backscattered diffraction. The techniques also include electron channeling contrast imaging, X-Ray photoelectron spectroscopy, X-ray fluorescence, Auger electron spectroscopy, Raman spectroscopy, oxygen analysis, and nanoindentation.

Intermetallic phase precipitates in aluminum alloys can often be identified without resorting to chemical analysis. Very often the determination can be made based on the shape, color, and refractive properties of the particular phase. This chapter explains how these visual attributes can be observed using metallographic techniques. It describes, and in many cases illustrates, the characteristic shapes, colors, and optical properties associated with aluminum alloy intermetallic phases and how they can be enhanced through selective etching. It provides an atlas of microstructures comparing the effects of selective etching procedures on various phase constituents in cast aluminum-silicon alloys. The compilation of images demonstrates the use of two types of reagents: those that reveal discontinuities in crystal orientation and grain boundaries, and those that reveal differences in chemical composition.

The most common methods for preparing polymeric composites for microscopic analysis can be used for most fiber-reinforced composite materials. There are, however, a few composite materials that require special preparation techniques. This chapter discusses the processes involved in the preparation of titanium honeycomb composites, boron fiber composites, titanium/polymeric composite hybrids, and uncured prepreg materials.

This chapter reviews various ways to classify failure categories and summarizes the basic types, causes, and mechanisms of damage, with particular consideration given to whether the likelihood of the types of damage can or cannot be influenced by the heat treating of steel parts. The classical organization for types of damage (failures) is as follows: deformation, fracture, wear, corrosion or other environmental damage, and multiple or complex damage. The chapter also provides some examples of lack of conformance to specification that may at first look like the heat treater did something wrong, but where other contributing factors made it difficult or impossible for the heat treater to meet the specification.