Near-Threshold Fatigue Crack Growth under Spectrum Loading: Modeling & Fractographic Observations
Speaker: Dr. R. Sunder, BISS (P) Ltd, Bangalore, India
Date: Monday, Oct. 7, 2019
Time: 10:30 a.m. -11:30 a.m.
Location: NASA/LaRC, Building 1205, Rooms 222
A Tri-Mechanism (TRIM) model is described for cycle-by-cycle estimate of fatigue crack growth under variable-amplitude loading. The three mechanisms are near-tip residual stress, crack closure and crack-tip blunting. The focus is largely on the first of these, which controls material fatigue threshold resistance, a phenomenon that plays a significant role by way of load history effects at crack growth rates below 10-4mm/cycle. The crack tip being a singularity, it will exhibit cyclic stress-strain hysteresis even under threshold conditions.
As a consequence, DKthwill, in fact, exhibit cycle-sequence sensitivity. By implication, DKthcan vary from cycle to cycle, even if the crack does not grow. An experimental and analytical study was performed to validate the TRIM model. Using specially developed test procedures, the relationship between DKth and a certain near-tip residual stress, s* has been established for six different materials, an Al-Cu alloy, an Al-Li alloy and four different steels including 3D printed materials. Given such a relationship, one can explain the effect of load history on near-threshold fatigue crack growth. This is likely to improve the quality of residual fatigue life estimates when low crack growth rates are involved leading to life in the range 106-108cycles.
To render applicability to realistic loading conditions, the study was performed on specimens cut from Al-Cu alloy sheet material 2024-T3 under specially designed long-duration aircraft load spectra, derived from the well-known TWIST load sequence. The Tri-Mechanism model accounting for closure, near-tip residual stress, as well as crack-tip blunting, appears to be able to describe the observed effects.
Dr. R Sunder received his MTech and Ph.D. in Aeronautical Engineering (1978) from Kiev Institute of Civil Aviation. He researched aeronautical fatigue and airframe residual strength at the National Aerospace Laboratories, Bangalore, India (1978-1993). He also worked on Elevated temperature fatigue crack growth in nickel-base superalloys at Air Force Materials Laboratory, Wright-Patterson Air Force Base, Dayton OH (1986-88 and as consulting scientist on Ageing Aircraft Programmes at UDRI, 1990-2001). He founded Bangalore Integrated System Solutions (P) Ltd (BISS) in 1992, a developer and manufacturer of mechanical test systems. BISS is currently an independent business unit of Illinois Tool Works (ITW), USA. From 1992 to date, he has been involved in test technology development for a wide range of applications and researched metal fatigue under service loading.
He is a visiting professor at Kazan Aerospace University, Tomsk Polytechnical University and Volgograd State Technical University. He is a member of ASTM (from 1985) and ASTM Committee E-8 (Fatigue & Fracture) & D30 (Composites), Fellow of Indian Academy of Sciences and President, Indian Structural Integrity Society. He has to his credit 70 peer-reviewed publications.
Dr. R. Sunder
BISS (P) Ltd, Bangalore, India