From WillowRun's summary of
Aviation Week dated April 22-May 5, 2024(?)
(I don't know a link, probably behind paywall: Bold again is my markup)
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(Remark from WillowRun: The person quoted is the vp and chief engineer etc ....) :
In-service reviews of the oldest and most used airframes underpin results from tests done early in the program to validate the 787’s design—tests that unknowingly trialed production flaws discovered years later.
“The most impactful data is the data that we got from our full-scale fatigue test,” Chisholm said. “The build condition [was] the same as what we saw in the later build.”
Put another way, the same nonconformances that crept into the first 980 aircraft were present in the fatigue test article. While the revelation does not reflect well on Boeing’s original quality assurance process, it bolsters the argument that its design—including material selection and tolerances—is robust.
“In over 165,000 cycles, there were zero fatigue issues in the composite structure,” Chisholm said.
The tests, which began in September 2010 on ZY998, the third 787 airframe built, ran through 2015 and simulated entire flights, from taxi through ascent, cruise, descent and back to taxi (AW&ST Dec. 21, 2015-Jan. 3, 2016, p. 51). Targeted at creating a dataset for the airframe’s durability, the tests subjected the structure to loads that simulated more than 3.6 times the design life of 44,000 flight cycles.
Testing was conducted in a steel rig weighing more than 1 million lb. at the Boeing manufacturing plant in Everett, Washington. The rig included more than 100 mechanical connections to push, pull and twist the 182-ft.-long fuselage, wing forward leading edge and vertical stabilizer. The 787 structure incorporated over 3,000 sensors that evaluated more than 40 million discrete load conditions as the airframe was subjected to shear forces, bending moments and torsion loads typically experienced during five flight conditions ranging from benign to extremely turbulent.
Although some parts failed over the course of testing, they were all metallic. Components and parts that cracked or failed prematurely included a metal bearing pad in the main landing-gear support structure and tie-rod end lug and support fittings. There were no catastrophic failures during the test, Boeing adds.
The fatigue-test airframe incorporated additional sensors along the side-of-body wingbox joint, which was reinforced after earlier static tests revealed weakness in the area. The redesign, which pushed back first flight of the 787 to December 2009, was validated in static tests the previous month and later incorporated into ZY998. Compared with fatigue tests on earlier metallic airframes, the 787 test unit included more sensors on various stiffener terminations and more extensive periodic inspections than required by the baseline maintenance program.
Average 787 utilization is about 600 cycles per year, Boeing said. The busiest aircraft operate 900-1,400 cycles annually, Aviation Week Network Fleet Discovery data show. The highest-time 787, an All Nippon Airways 787-8, has flown 16,500 cycles in its 11-year service life.
The 44,000-cycle life expectancy is reflected in the extent of the 787 tests compared with previous fatigue tests on conventional aluminum-built airframes, such as the 777, which completed the then-record number of 120,000 simulated cycles in 1997. This represented the equivalent of 60 years in operation or twice the 777’s design service objective (DSO) of 60,000 flights. The DSOs of the 787 and 777 have both been significantly extended beyond previous generations, such as the 757 and 767, which underwent fatigue tests simulating 100,000 flights, or twice a 20-year DSO of 50,000 flights.