Search Results for delayed fracture

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.

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.

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.

A generating system in a hydroelectric power plant was shut down to investigate an abnormal sound coming from one of the turbines. A piece of metal that had broken off one of the vanes on the runner was found in the tail race and was subsequently examined along with the runner. Based on the fracture characteristics, as described in the report, the vane failed in fatigue due to a crack that initiated in an area of stress concentration.

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.

This chapter discusses the failure of a compressor blade in an aircraft engine and explains how investigators determined the cause. Based on visual examination and the results of SEM fractography and chemical analysis, it was concluded that blade failed due to fatigue fracture originating from nonmetallic inclusions in the blade root.

During a major servicing of an aircraft, cracks were found in the bottom wing root fitting. Based on dye penetrant inspection and the results of SEM fractography and chemical analysis, investigators concluded that the cracks were due to stress corrosion. They also recommended an inspection of all other aircraft with similar fittings and the consideration of alternate materials that are less prone to stress-corrosion cracking.

This chapter is a brief account of the composition, microstructures, heat treatment, deformation mechanisms, mechanical properties, formability, and special attributes of austenitic stainless steels.

Austenitic stainless steels are iron-base alloys containing more than 50% Fe, 15 to 26% Cr, and less than 45% Ni. This chapter provides a discussion on the types, compositions, microstructures, processing, deformation mechanism, mechanical properties, formability, and special attributes of austenitic stainless steels.

A titanium alloy disc on the fourth stage of an aircraft engine compressor was found cracked in the course of a defect investigation. The disc had not yet reached the halfway point of its expected service life. The chapter explains how the crack was examined and provides relevant details about its location on the disc and various aspects of its appearance. It also explains how failure analysts concluded that the disc had been subjected to a fluctuating load of high magnitude and that the crack was the result of two fatigue cracks, originating from opposite sides of the diaphragm.

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 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 appendix focuses on procedures, techniques, and precautions associated with the investigation and analysis of metallurgical failures that occur in service. It describes the steps of an orderly failure analysis from collecting and examining samples to performing mechanical and nondestructive tests, preparing and examining fractographs and micrographs, determining failure mode, writing the report, and developing follow-up recommendations. It also examines the fundamental mechanisms of failure, why they occur, and how to identify them by their characteristic features.

This chapter presents various case histories that illustrate a variety of failure mechanisms experienced by the high-strength steel components in aerospace applications. The components covered are catapult holdback bar, AISI 420 stainless steel roll pin, main landing gear (MLG) lever, inboard flap hinge bolt, nose landing gear piston axle, multiple-leg aircraft-handling sling, aircraft hoist sling, internal spur gear, and MLG axle. In addition, the chapter provides information on full-scale fatigue testing, nondestructive testing, and failure analysis of fin attach bolts.

Two compressor rotors of similar design and construction were severely damaged during operation. In one rotor, all the blades in the third and fourth stages had been sheared off and some had lifted from the dovetail portion of the drum. The damage in the other rotor was more extensive. Most of the blades in the first four stages had sheared off and many lifted from the dovetail region, particularly in the first two stages where several mounting dovetails had also fractured. Based on visual examination and the results of SEM fractography, metallography, and chemical analysis, investigators concluded that the compressor rotors failed due to stress-corrosion cracking in the dovetail mountings. They also provided recommendations to prevent or mitigate future occurrences.

Failure analysis of steel welds may be divided into three categories. They include failures due to design deficiencies, weld-related defects usually found during inspection, and failures in field service. This chapter emphasizes the failures due to various discontinuities in the steel weldment. These include poor workmanship, a variety of hydrogen-assisted cracking failures, stress-corrosion cracking, fatigue, and solidification cracking in steel welds. Hydrogen-assisted cracking can appear in four common forms, namely underbead or delayed cracking, weld metal fisheyes, ferrite vein cracking, and hydrogen-assisted reduced ductility.

During cyclic spin tests, the turbine disc in an aircraft engine broke apart with a loud noise, followed by a fire. Based on a detailed examination and the results of SEM fractography and hardness measurements, failure analysts concluded that a locking plate became dislodged due to the shearing of the screws that hold it in place. They also provided recommendations to remediate the problem.

This chapter describes the various types of cushion systems used in forming presses and their effect on part quality. It begins with a review of the deep drawing process, explaining that wrinkling, tearing, and fracture are the result of excess or insufficient material flow, which can be prevented by maintaining the correct amount of holding force on the periphery of the blank. It then describes how blank holding force is generated in double-action presses and the extent to which displacement profiles can be adjusted on both the inner and outer slides. The discussion then turns to single-action presses that incorporate some type of cushion system. The chapters describes the many ways that cushion systems are implemented in forming presses and the force and displacement characteristics achievable with each method. It also explains how multipoint cushion systems are designed and how they facilitate uniform metal flow into the die cavity of large deep-drawn parts.

This chapter presents a qualitative and quantitative overview of the stresses, strains, forces, and energy associated with metalworking processes and the tribological behavior of metals. It covers key concepts necessary for understanding metalworking tribology, including plastic deformation, yield criteria, flow strength, and the application of flow rules. It explains how to calculate the work involved in deformation processes, how to assess the propensity for fracture, how to determine temperature rise and strain distribution in the workpiece, and how to classify metalworking processes based on related tribology.