Correlation of T-37B SLEP Damage Tolerance Analysis with Full-Scale and Coupon Fatigue Test Results

Joseph W. Cardinal or David H. Wieland

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.

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