To extend the life of the aging T-37B jet trainer aircraft by
an additional 8,000 flight hours, the U.S. Air Force commissioned a structural life
extension program (SLEP) in which areas of the front wing spar, the fuselage forward
carry-through structure, the horizontal stabilizer, and the empennage structural supports
were redesigned, analyzed, and tested by SwRI under subcontract to Sabreliner Corporation.
Full-scale durability and damage tolerance tests, laboratory coupon spectrum fatigue tests
and durability and damage tolerance analyses (DADTA) were performed to demonstrate by test
and analysis that the modified T-37B SLEP structures met an 8,000 flight hour no
inspection requirement. Since initial inspection intervals are typically defined as
one-half the number of flight hours required to grow a crack from a postulated initial
flaw size to a critical size, the goal of the T-37B SLEP fatigue testing and DADTA was to
demonstrate that initial flaws did not grow to critical crack lengths for at least 16,000
flight hours. This paper describes the validation and verification of the T-37B SLEP
durability and damage tolerance analyses using measured crack growth data obtained from
the full-scale fatigue tests and laboratory coupon spectrum damage tolerance tests.
Durability and damage tolerance analyses were performed for a
total of 12 fatigue critical locations (FCLs) on the T-37B aircraft. The T-37B SLEP DADTA
effort ended the SLEP program by integrating test and analysis results from other phases
of the overall program to demonstrate that the 8,000 hour no inspection goal was achieved.
Fracture mechanics models were developed for each of the 12 FCLs. The predictions of the
crack growth models and the determination of crack growth retardation parameters were
validated against the SLEP coupon spectrum test results and crack growth measurements made
during the full-scale damage tolerance test program.
The paper discusses some of the unique aspects of the T-37B
SLEP DADTA correlations with test results for a selected number of FCLs. These include
cracks growing out of and toward coldworked holes, shorter than expected crack propagation
lives for a crack growing from the edge of an attachment lug towards the lug hole,
correlations (and differences) between full-scale and laboratory coupon test results,
determination of continuing damage sites, and fracture mechanics modeling issues for
unique FCL geometries. The conclusion of the paper presents lessons learned during the
T-37B SLEP program regarding full-scale aircraft testing and the correlation of fracture
mechanics analysis and testing.
Mechanical and Materials Engineering Publications