Damage Tolerance Analysis of Grinding Burn Cracks in High Strength Steels

TECHNOLOGY AREA(S): Materials
OBJECTIVE: Develop special methods, data, or applications for the modeling and crack growth analysis of thermally induced cracks located in grinding burns of high strength steel landing gear parts.
DESCRIPTION: Landing gear are specialized structures designed to sustain the high stresses and loads of landing aircraft. They are often made of high strength steels (300M, 4340, steels with Ftu>180ksi) which are sensitive to elevated temperatures due to material microstructure and low tempering temperatures. On occasion, during manufacture, rework, or chrome grinding, landing gear are overheated resulting in an under/over tempered martensitic condition (burn). Generally, these conditions are associated with the formation of microstructurally and physically small cracks in the 0.001 - 0.010 in. range. It is desirable to better understand the fracture mechanics of small cracks in burned high strength steel parts and methods/models that can be used to manage such cracks in landing gear, and more generally, aerospace specific high strength steel parts.
PHASE I: Investigate the types of machining conditions that encourage the formation of microstructurally and physical small cracks in under/over tempered steels. Define a test plan that will result in valuable data that can be used within the assumptions and limitations of LEFM methodologies to predict crack growth in burned high strength steel materials.
PHASE II: Initiate and complete the test plan developed in Phase I. Phase II testing results will be documented in a technical report and submitted to the government. All lessons learned and additional testing needed for a Phase III effort will be included in this report. The test plan shall include at a minimum test data development, stress intensity validation for specific specimen types selected, and fracture surface analysis to determine stress intensity solutions for failed parts/specimens.
PHASE III: Finalize the results of all testing in a technical report, and create methodology and models that allow for the management of burn induced cracks. Develop specialized tools and techniques that will enable the quick evaluation of grinding burn cracks in support of landing gear sustainment.
REFERENCES: 1. JSSG-2009, DEPARTMENT OF DEFENSE JOINT SERVICES SPECIFICATION GUIDE: AIR VEHICLE SUBSYSTEMS.2. JSSG-2006, DEPARTMENT OF DEFENSE JOINT SERVICE SPECIFICATION GUIDE: AIRCRAFT STRUCTURES.ANODIC ETCHING - A METHOD OF DETECTING GRINDING BURNS ON CHROMIUM PLATED STEEL PARTShttp://www.dtic.mil/dtic/tr/fulltext/u2/a017689.pdf.3. Crack Extension in Several High-Strength Steels Loaded in 3.5% Sodium Chloride Solutionhttp://www.dtic.mil/dtic/tr/fulltext/u2/685377.pdf.
KEYWORDS: Grinding, Martensitic, High Strength Steel, Fatigue, Fracture, Damage Tolerance Analysis, Manufacturing, Modeling, Material, Transition, Stress Intensity Factor, Mode, Airworthiness, Integrity