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.

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.

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.

The nose landing gear on an aircraft malfunctioned during landing roll. After the incident, two fractured studs were found in the retraction jack support beam. Based on visual examination and the results of SEM fractography, investigators concluded that the studs failed by fatigue, a vulnerability because of the way they were mounted. A sketch showing the correct mounting configuration is included in the report.

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.

Several components from the tail boom of a helicopter were found fractured at a crash site, including gusset plates, the hat section near the lower yoke, and a cable that controls the pitch of the tail rotor. The components were recovered from the wreckage and taken to a lab for closer examination. Based on their observations and the results of SEM fractography, failure analysts concluded that the gusset plates failed due to a downward bending overload in tension and that the tail rotor control cable snapped due to tensile overload. There were no indications of delayed failure in any of the areas examined.

From a fundamental standpoint, there are only two modes, or ways, in which metals can fracture under single, or monotonic, loads: shear and cleavage. There are fracture modes other than shear and cleavage. These include intergranular and quasi-cleavage fracture modes for single-load applications, and fatigue for multiple-load applications. Each of these fracture modes are discussed in this chapter. The factors affecting the ductile brittle relationship are also covered.

A design modification intended to reduce dowel bolt failures in an aircraft engine proved ineffective, prompting an investigation to determine what was causing the bolts to break. As the chapter explains, failure specimens were examined under various levels of magnification and subjected to chemical analysis and low-cycle fatigue tests. Based on their findings, investigators concluded that the bolts failed due to fatigue compounded by excessive clearances and poor surface finishes. The chapter provides a number of recommendations addressing these issues and related concerns.

This chapter discusses the causes and effects of ductile and brittle fracture and their key differences. It describes the characteristics of ductile fracture, explaining how microvoids develop and coalesce into larger cavities that are rapidly pulled apart, leaving bowl-shaped voids or dimples on each side of the fracture surface. It includes SEM images showing how the cavities form, how they progress to final failure, and how dimples vary in shape based on loading conditions. The chapter, likewise, describes the characteristics of brittle fracture, explaining why it occurs and how it appears under various levels of magnification. It also discusses the ductile-to-brittle transition observed in steel, the characteristics of intergranular fracture, and the causes of embrittlement.