This chapter describes nondestructive evaluation (NDE) test methods and their relative effectiveness for diagnosing the cause of stress-corrosion cracking (SCC) service failures. It discusses procedures for analyzing various types of damage in carbon and low-alloy steels, high-strength low-alloy steels, hardenable stainless steels, austenitic stainless steels, copper-base alloys, titanium and titanium alloys, aluminum and aluminum alloys, and nickel and nickel alloys. It identifies material-environment combinations where SCC is known to occur, provides guidelines on how to characterize cracking and fracture damage, and explains what to look for during macroscopic and microscopic examinations as well as chemical and metallographic analyses. It also includes nearly a dozen case studies investigating SCC failures in various materials.

This chapters discusses the basic steps in the failure analysis process. It covers examination procedures, selection and preservation of fracture surfaces, macro and microfractography, metallographic analysis, mechanical testing, chemical analysis, and simulated service testing.

This atlas contains images showing how sintering conditions (time, temperature, and atmosphere) and compaction pressure affect the microstructure of different types of stainless steel. It also includes images of stainless steel powders, fracture surfaces, and test specimens characterized by the presence of compounds, such as oxides, carbides, and nitrides, and various forms of corrosion.

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.

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.

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 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.

Structural members in a radar antenna system are held together by cadmium-plated high-strength steel bolts, several of which had fractured along the fillet near the head. Investigators determined that the bolts did not seat properly, making contact only at the periphery, which subjected them to high stress concentrations in the fillet region. They also concluded that the intergranular nature of the fracture, as revealed by scanning electron fractography, pointed to hydrogen embrittlement as a contributing factor. This chapter provides a summary of the investigation along with a recommendation to consider adding spring washers to the assembly.

This chapter documents the key findings of an investigation into the failure of an aircraft’s main wheel bearing housing. Using annotated images and a detailed SEM fractograph, it shows what investigators observed that led them to conclude that the flange on one of the hubs broke off due to a combination of fatigue, bending stresses, and wear. It also includes a recommendation to assess the structural integrity of the bearing housing after every 100 h of service using nondestructive techniques.

An adaptor and a bolt were overloaded during a flight causing them to fracture. This chapter recounts the circumstances that led to the failure and the investigation that followed. It includes images of the fracture surfaces which show that both components failed quickly due to overload conditions. It also recommends the use of twin suspension hooks to make attachment points more stable under difficult flight conditions.

This chapter explains how investigators determined that a stabilizer link rod fractured due to overload, possibly by a combination of tension and bending forces that occurred during an accident. It includes images comparing the fractured rod with its undamaged counterpart recovered from the starboard side of the aircraft. A close-up view of the threads near the fracture surface provides evidence of bending, while the presence of dimples in an SEM fractograph supports the theory that the link rod failed as a result of overload.

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.

An aircraft tire burst while inflating, causing one of the flanges on the wheel hub to fracture. This chapter provides a summary of the investigation along with key findings. It includes images of the damaged hub and describes how various parts failed as the pressure in the tire increased. It explains that the hub material was of good quality under uniform load and that it fractured quickly by cleavage due to the force exerted by the overinflated tire.

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.

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.

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.