This chapter provides an overview of the tools and techniques used to examine failure specimens and the wealth of information that can be obtained from fracture surfaces, cracks, wear patterns, and other such features. It discusses the use of metallography, fractography, and optical and electron microscopy. It presents a number of images recorded using these methods and explains what they reveal about the mode of fracture and the state of the component prior to failure.

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.

After several failed attempts to lower their starboard wheels for landing, pilots engaged the help of gravity through g-force maneuvers and managed to coax the wheels into place. An inspection following the incident revealed a broken universal joint in one of the linkages that opens and closes the doors to the undercarriage compartment. The failed component was removed from the aircraft and examined using optical and electron microscopes. Under low magnification, the fracture surface appeared jagged except for one corner that was relatively smooth. SEM fractography revealed the presence of fatigue striations in the smooth region and dimpling elsewhere. Based on their findings, investigators concluded that fatigue loading initiated a crack in the universal joint that progressed with time and that the final fracture occurred due to bending tensile overload.

The quill shaft in an aircraft engine was found in two pieces following a flameout. One piece was short, straight, and otherwise undamaged; the other piece was bent in several places as was the sleeve that covered it. The facture surface, as viewed under optical and scanning electron microscopes, was flat and shiny with deformation marks and dimples, typical of torsional overload, and signs of severe rubbing on the periphery. Based on their observations, investigators concluded that the quill shaft failed by torsional overload, the source of which could not be determined.

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.

This chapter discusses the failure of an aircraft control cable and the investigation that followed. It explains how visual examination showed that the failure occurred in a heavily worn section that runs over a pulley. Further examination under a stereoscope and in a SEM found that 40 out of 49 wires had thinned and broke apart, and that the remaining nine wires failed in tension evidenced by cup and cone fracture and the presence of dimples on the fracture surface. Based on their findings, investigators concluded that cable snapped because of excessive thinning due to contact wear.

An aircraft was heavily damaged when it was forced to land due to a throttle malfunction. Investigators determined that one of the studs linking the throttle to the engine fractured from fatigue, initiated by cracks formed during a riveting procedure. This chapter provides a summary of the investigation, which included the use of scanning electron fractography, along with recommendations on acceptable hole patterns for rivets.

A pair of bolts on a connecting rod failed during a test run for a prototype engine. They were replaced by bolts made from a stronger material that also failed, one due to fatigue, the other by tensile overload. The fracture surfaces on all four bolts were examined using optical and electron microscopes, indicating that the operating loads on the bolts far exceeded the design loads. Based on their observations, which are summarized in the report, failure analysts concluded that the design of the connecting rod system needs to be reassessed.

A second-stage turbine blade in an aircraft engine failed in service, fracturing along a path through the shroud hole. Cracks were also found in the shroud holes of the two adjacent blades. Based on the results of visual examination and SEM fractography, investigators concluded that the fracture and cracks were due to the fretting action of the pins inside the shroud holes.

A tie rod on a 70-ton aircraft towing tractor failed during a test run, fracturing near a welded bracket that connects to a hydraulic jack. This chapter discusses the failure and the investigation that followed. It presents a close-up view of the fracture surface showing what appears to be a brittle fracture that initiated from a zone of poor-quality weld. It also provides photographic evidence of a weld crack in the heat-affected zone and includes a drawing of a modified weld design that passed subsequent testing.

A low-pressure turbine rotor blade failed during a test run, causing extensive damage to an aircraft engine. Visual examination showed that the nickel-base superalloy blade broke above the root platform in the airfoil section, leaving a fracture surface with two distinct regions, one characteristic of fatigue, the other, overload. Two dents were also visible on the leading edge, near the origin of the fracture. Based on these observations and the results of SEM fractography, investigators concluded that the blade failed due to fatigue aided by cracks in the surface coating caused by mechanical damage.

Ductile fracture results from the application of an excessive stress to a metal that has the ability to deform permanently, or plastically, prior to fracture. Careful examination and knowledge of the metal, its thermal history, and its hardness are important in determining the correct nature of the fracture features. This chapter is a detailed account of the general characteristics and microstructural aspects of ductile fracture with suitable illustrations. It describes some of the complicating factors extraneous to the fracture itself.

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 describes an investigation following an aircraft accident in which the main undercarriage struts had failed. Visual examination revealed that the starboard strut fractured about 13 cm from the end nearest the underside of the wing. A close-up view of the fracture surface indicated that cracking initiated at the outer periphery of the strut and propagated inward until overload fracture occurred. SEM imaging revealed fatigue striations along the outer periphery and dimples elsewhere, indicative of tensile overload. Based on these observations, investigators concluded that the starboard strut failed by fatigue, which overloaded the port side strut as evidenced by its slant type fracture pattern.

This article depicts typical fractographic features for a number of different composite materials, covering interlaminar and translaminar fracture characteristics in composite materials.