Initiation and growth of microcracks in high strength steel butt welds.

378 Pages·1993·9.8 MB·en_US

by  Olsen, Edward.

INITIATION AND GROWTH OF MICROCRACKS IN HIGH
STRENGTH STEEL BUTT WELDS
bv
EDWARD QLSEN
B.S., Mechanical Engineering
University of South Carolina, 1985
Submitted to the Department ofOcean Engineering
in Partial Fulfillment ofthe Requirements for the Degrees of
Naval Engineer
and
Master of Science in Mechanical Engineering
at the
Massachusetts Institute ofTechnology
May 1993
Edward Olsen, 1993 All rights reserved.
The author hereby grants to MIT permission to reproduce and to
distribute publiclv copies ofthis thesis document in whole or in part
Protessor A Douglas Carmichael, Department ofOcean Engineering
Department Graduate Committee
T260800

INITIATION AND GROWTH OF MICROCRACKS IN HIGH
STRENGTH STEEL BUTT WELDS
by
EDWARD OLSEN
Submitted to the Department ofOcean Engineering
on 7 May 1993 in partial fulfillment ofthe
requirements for the Naval Engineer Degree and
Master ofScience in Mechanical Engineering.
ABSTRACT
Early tests such as the explosion bulge test created a preference for overmatched
welds (welds which are stronger than the base metal) which eventually became codified for
many structural applications. While an overmatched system offers advantages such as the
shedding ofstrain to the base plate, it requires the use ofexpensive fabrication procedures
to avoid cracking. Undermatched welding ofsome high strength steels may offer
reductions in welding costs with little sacrifice in weld performance or low cycle fatigue
integrity.
An experimental study was carried out to observe microcrack initiation and growth
ofovermatched and undermatched butt welded high strength steel samples using globally
elastic low cycle fatigue testing. First, 1 inch thick HY-80 and HY-100 base plates were
multipass, spray gas metal arc welded (GMAW) with overmatching and undermatching
filler metal using a semiautomatic welding machine. Second, 1/4 inch thick MIL-A-46100
GMAW
high hardness armor plates (HHA) were manually, two pass spray welded with
two grades ofundermatching consumables. Weld reinforcements were removed from all
HY HHA
specimens and six specimens. All specimens had a crack initiator slit machined
in the test section. The specimens were fatigue tested by transverse tensile loading with a
12 to 13 Hz tension-tension profile. The loading range was from 10% to 85% ofthe
HY HHA
tensile strength ofthe steel base plate and weld metal respectively. Crack
initiation and propagation was observed in situ using a confocal scanning laser microscope.
HY
In the 80 tests, the undermatched specimens failed between 14,500 and 16,500
cycles, overmatched specimens failed between 21,000 and 26,500 cycles, and the base
HY
plate failed at 20,000 cycles. In the 100 tests, undermatched specimens failed between
16,500 and 21,000 cycles, overmatched specimens failed between 17,800 and 21,600
HHA
cycles, and the base plate failed at 32,000 cycles. All "as welded" specimens failed
HHA
between 8,000 and 17,500 cycles. ground 100S-1 welds failed at 10,000 cycles and
the 70S-6 welds failed at 17,500 cycles
Thesis Supervisor: Professor Koichi Masubuchi
Title: Kawasaki Professor ofEngineering

Acknowledgments
I would like to extend special thanks and gratitude to the people who have helped me in
this endeavor.
Professor Koichi Masubuchi for his expertise, guidance and support.
Professor Frank McClintock for his expertise in fracture mechanics and helping me to
understand it better.
Captain Osie Combs, USN, for providing the HY plate specimens and the requisite testing.
Tom Melvin ofthe Army Research Laboratory for providing the HHA material specimens.
Warren Mayott and Bob Pierce ofElectric Boat for preparing the HY specimen plates,
their NDT, and documentation.
Gene Franke ofDavid Taylor Research Center, Annapolis for allowing me to study their
research material and work with the materials group during "Professional Employment".
The Navy for making this MIT graduate program possible and my colleague 13A's for
keeping it fun.
My brothers Andy and Greg who provided me with ongoing inspiration and friendly phone
calls.
Dr. Sally Boyd who through her love and patience taught me the value offriendship and
helped me grow immeasurably.
Finally, to my wife, Kathy R, and daughter, Brooke, thanks for being there throughout
these past three years. Your love and devotion have given me strength throughout these
trying times.