By N. Jayaraman, Ph.D., Surface Enhancement Technologies, LLC, Part of Lambda Technologies Group
Gas turbine engine components are prone to high- and low-cycle fatigue (HCF and LCF), cracking in critical areas of high stress. Cracks often initiate from local stress concentrations created by surface damage including foreign object damage (FOD), fretting, corrosion pitting, and erosion. Fatigue in critical components during service can lead to catastrophic failure. Understanding failure mechanisms and applying damage mitigation methods to critical parts can balance the cost of scrapping parts prematurely and the risk of failure in service.
For over a decade, the damage tolerance (DT), performance, and service life of gas turbine engine components have been improved by Lambda Technologies introducing beneficial residual compression. Remarkable improvement is achieved by stable compressive residual stress (RS) fields through patented methods like low-plasticity burnishing (LPB®) in fatigue critical components. Design credits for improvement can be taken through proven analytical methods, and the process can be readily implemented in initial production or during maintenance, repair, and operations (MRO). LPB technology has been applied to military and commercial aircraft rotating parts with FAA approval, and is equally applicable to ground-based turbines.
Design Credit for Surface Compression
Surface compression is well known to improve resistance to cracking. Many parts are routinely shot peened for this reason. Shot peening introduces a shallow layer of surface compression, and a rough surface that is highly cold worked. The cold work causes relaxation of the beneficial compression in service. Therefore, design credit for beneficial residual compression is not generally taken because: magnitude and depth of compression are not reliably measured, compression may not be stable (due to cold work), depths and magnitudes of compression are not well controlled, compression may not be reproducible, and a reliable RS design method has not been available. Many surface enhancement technologies suffer from one or more such limitations.
Compression by Design
Conventional fatigue design methods use the Goodman (or Haigh) diagram with classic Stress-Life and/or Strain-Life equations. These methods do not incorporate compressive residual stresses in predicting performance. Using the Smith Watson Topper (SWT) method to account for mean stresses extends the Haigh diagram to account for beneficial compression. Including a Neuber type of damage parameter to account for damage leads to a powerful fatigue design diagram (FDD). The FDD combined with finite element analyses (FEA), predicts the compressive RS field required to improve life or damage tolerance. When combined with linear elastic fracture mechanics (LEFM) and damage tolerance analyses (DTA), the FDD is a powerful tool for designing RS fields, allowing design credits for surface enhancement technologies.
Incorporating the technology for surface enhancement into a repair shop using existing machine tools could be cost effective and logistically convenient. Lambda’s LPB® is currently in use for improving component performance and damage tolerance of gas turbine engine components. The LPB® tools and process codes have been incorporated into existing machine tools like CNC lathes, CNC mills, and robots.
In legacy gas turbine systems, LPB® has been used for life extension and improved damage tolerance on serviced parts. After inspection for severe damage, accepted parts are LPB® treated in an MRO shop setting and reintroduced into service. Since LPB® treatment provides DT of three to 10 times the damage detectable by the NDE technique, this approach provides serviced components that are safer than even the original parts. LPB® has also been introduced into newly designed components to mitigate unanticipated design flaws including stress concentrations, and to provide extra design credit. In both cases, the component performance has been significantly improved without the need to change either the design or material.
Taking Design Credits and Cost Benefits
Cost benefits of LPB® treatment include reduced inspection requirements, as well as improved DT, service life, and safety. Reductions in inspection frequency and machine downtime significantly improve overall operational savings.