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Failed blade examination by SwRI engineers revealed crack initiation was caused by environment attack and propagation by high-cycle fatigue. (Inset) SwRI engineers examined rolling element bearings in aeroderivative gas turbines to determine acceptable continuing service.

Gas turbines operate at extreme conditions, often at the design limit of blades, bearings, and combustion components, which means these components are life-limited and more likely to experience failures than other less stressed parts.

• Hot section blades typically fail because of creep, oxidation, low-cycle fatigue (LCF), and high-cycle fatigue (HCF). Contributing factors often include environmental attack, corrosion, cyclic loads, over firing, or inadequate refurbishment.
• Rolling element bearings used in aeroderivative gas turbines often fail because of lack of lubrication, oil contamination, overload, underload (skidding), insufficient cooling, and manufacturing quality control.
• Combustors and combustion components fail because of over firing, inadequate cooling flow control, water injection for NO_x control, defective fuel nozzle spray pattern, and combustion instabilities.

SwRI engineers determine that additional cooling is required in combustion transition pieces.

SwRI® engineers conduct coordinated, multidisciplinary investigations to determine the root cause of failures and to provide corrective action guidance. Often, the primary evidence of a failure is obliterated by consequential damage. Thus, a series of comprehensive examination tests and analyses are required to isolate the primary root cause of the failure before corrective action can be taken. Typically, an investigation to determine the root cause failure of a blade breaks down into a series of steps involving different engineering disciplines.

High magnification (1,000X or more) to measure striation spacing requires an examination of the fracture surface with a scaning electron microscope.

The investigation of other components would follow a similar series of steps.

• Forensic Investigation at the site is essential to develop a scenario of the most likely sequence of events — which component failed first, how did it fail, the trajectories of the failed parts, and the sequence of consequential damage. SwRI engineering teams are available on short notice to collect on-site information before memories dim, environmental damage occurs, or parts disappear.
• Metallurgical Examination includes microscopic examination to determine the failure mechanism and initiation site (LCF, HCF, TMF, creep, corrosion, overheating, oxidation/corrosion) and mechanical and chemical testing to determine if the material’s properties meet specifications. SwRI metallurgical engineering staff specialties include gas turbine materials, coatings, welding, failure analysis, and remaining life assessment.
• Mechanical/Thermal Analyses and testing quantify the underlying forcing functions in the operating environment of the failed component. SwRI staff have experience in engine design, thermal and loading analyses based on Brayton cycle and mean line flow models, finite element analysis and computational fluid dynamics modeling.
• High-Cycle Fatigue Vibration Analyses capabilities for evaluating vibratory stresses of rotating parts are available at SwRI including rotating strain gage telemetry, finite element analysis, and modal impulse testing. Impulse testing can be conducted at SwRI labs or at the customer site using portable equipment.
• Fracture Mechanics Analysis based on crack striation spacing provides a means for estimating fatigue life and endurance under damaging conditions. Combined with knowledge of speed, temperature, and other operating conditions, the actual time to failure can be determined.

Impulse testing is conducted to predict operating oscillating stresses.

A forensic investigation of compressor blade failure Identifies initiating component and subsequent damage.

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