Fatigue Failure Case Studies
Fatigue fracture can occur in many components such as: fasteners, and tubular pole structures. In this paper, fatigue failure mechanisms have been described and the application of the principles for failure analysis for each case will be presented. Cyclic loading at stresses above the fatigue limit of the material can initiate cracks at the surface or at internal defects. Macroscopic and microscopic observations of fatigue crack initiation and approaches for characterization of fatigue failures have been described. Two case studies present application of laboratory analysis techniques to determine primary causes and modes of failures.
A Galvanic Case Study
Material failures occur in products due to changes in the original characteristics that prevent satisfactory performance of a functional system for its intended purpose and could produce losses to human life and economic infrastructure. Metals’ characteristics change due to cyclic stress, strain, and corrosion. This paper presents a case study that deal with the application of corrosion fundamentals in the determination of the origin of failures in a metallic part—galvanic corrosion failure in a water supply plumbing system. Metallographic examination, chemical analysis of the parts, and fractography were conducted, and the cause of failures was determined as a galvanic corrosion.
Oil and Gas Pipelines Aging and Risk Assessment
Pipelines are among the most common means used for transporting hazardous gases and liquids in the United States. However, underground pipelines are aging and are at risk of corrosion failure due to coating degradation/disbondment, pitting corrosion and stress corrosion cracking. Those tasked with maintaining these pipelines require an in-depth understanding of the locations where these aging pipelines are at risk of localized corrosion attack and cracking. Corrosion failures in aging pipelines are either sudden catastrophic ruptures or gradual leaks due to localized corrosion. Many factors associated with these corrosion areas are coating failure, degradation, disbondment, blistering, delamination, mechanical pressure and stress concentration, galvanic action, corrosive ions, the presence of moisture, corrosive soils, AC interference, inadequate cathodic protection and shielding. These areas have a much higher statistical probability of catastrophic failure and rupture. Most of the time initiation of stress corrosion cracking (SCC) and pitting corrosion are detected by coincidence in excavation and digs and is not targeted or predicted by analysis of corrosion performance parameters. Internal or In-line inspection (ILI) tools have limited capability for detecting or identifying stress corrosion cracking and pitting corrosion initiation. Here we would like to elaborate on corrosion risk assessment based on soil corrosively mapping in addition to procedures outlined in NACE SP 0204-2015.
Materials Selection and Design Failure
Safe operation of machinery depends strongly on the design of the system and the employment of proper components and materials. Unique to fluid powered systems are the proper selection of hydraulic lines, fittings, and control valves. This paper describes the cause of fracture in an elbow tube fitting due to deficiencies in the dimensional design and materials selection. The investigation included metallography, fractography, hardness testing and metrology. The results showed that the material was low carbon steel subjected to cold forming and normalized annealing processes during tube production.
Stress Corrosion Cracking
Most materials interact with a diverse environment in service and such interactions cause impairment in physical and mechanical properties. Degradation mechanisms are different in metallic and nonmetallic materials. In recent years, polymers and composites have been introduced in engineering applications, however, metals have remained important in structures because of their strength, stiffness, toughness, and high temperature tolerance. Corrosion in metallic materials due to dissolution and oxidation has been studied extensively. Corrosion can take many forms and one of the most important forms is the interaction of corrosion and mechanical stress to produce a failure by cracking. This type of failure is known as stress corrosion cracking, often abbreviated to SCC.
Anisotropy-in-Al-Li
Studies of the various high strength aluminum alloys highlight the significant interests of aerospace industry to Al-Li alloys. It has been concluded that the various particles that form during natural and artificial aging provide barriers to dislocation motion during the micromechanical process of slip, which is the microstructural mode of deformation for most metallic materials. This impedance provides these materials with improved strength properties over conventional aluminum alloys while their various chemical compositions lead to lower densities than their conventional counterparts. Al-Li alloys are typically anisotropic and they exhibit strong textures. The texture or crystallographic orientation of a material is a measurement of the orientation of crystallographic planes in specific directions, namely the rolling, transverse, and normal directions of a sample.