Final Qualification & Implementation of Laser Peening on F-35B & C Fatigue and Fracture Critical Components

Carlson, Scott S.

Since 2014 the F-35 Service Life Analysis Group has developed a team to investigate, certify and implement one of the largest aerospace structural Engineered Residual Stress (ERS) projects on record. This project has included over 300 building block fatigue tests in both notched and un-notched configurations, in 7085Al forging and Ti-6Al-4V BA ELI forging material. These fatigue tests reduced the risks associated with qualifying Metal Improvement Company’s (MIC) Laser Peening (LP) process to be applied on the F-35 airframe. Each test had specific marker band sequences applied at the end of each test spectrum block. Through the use of post fracture quantitative fractography it was possible to quantify the effect of laser peening on crack formation, short crack growth, and stable crack growth.

This presentation will provide final life results for work performed for the F-35B&C variants, covering both material classes. Lessons learned from coupon design, spectrum development, fatigue testing, quantitative fractography, and the design of the engineered residual stress field from the LP process will all be discussed. Specific focus will be on methods used to address the formation of subsurface cracking due to a specific LP process setting and how new LP processing parameters were developed, analyzed, applied and tested to address the formation of subsurface cracking, while still required to meet a specific fatigue life requirement. This work was performed within a significantly restricted timeframe due to the closure of the F-35’s Demonstration phase and the induction of the first F-35C for LP.

Through the use of advanced digital tools it was possible to develop quality control methods and checks that allow the laser operator, quality inspector, and structural integrity engineer the type and quality of engineering data needed to ensure that the laser peening process applied the correct process to each defined spot on the aircraft.