Department: Structural Engineering
Faculty Advisor(s): Hyonny Kim

Primary Student
Name: Mimi Ngo
Email: m3ngo@ucsd.edu
Phone: 626-679-7094
Grad Year: 2018

In the aerospace industry, full-scale aircraft are routinely inspected after performing a fixed number of flight hours to mitigate risks from damage that grows during operation, such as delamination (ply separation) in the composite components. This research aims to reduce the resulting cost of maintenance and help improve subsequent aircraft design by analytically and experimentally investigating countersunk fastener-induced delamination. The investigation was done on AS4/3501-6 carbon fiber reinforced polymer at three different 48-ply layups with a countersunk hole, which was drilled, reamed, and countersunk using a CNC milling machine. Specimen quality was ensured through nondestructive evaluation, which included optical microscopy and fluorescent dye penetrant test. A custom designed fastener pin was used during the static and fatigue bearing tests in a double lap shear fixture. These tests were performed on both pristine and pre-delaminated specimens to understand the onset and propagation of delamination. At various load cycles, strength and stiffness were recorded, delamination growth was visualized using ultrasonic C-scans, and hole elongations was measured using a small hole gauge and caliper. Premature failure of the titanium pins, which prevented proper occurrence of delamination in the specimens, promoted a recent switch to hardened stainless steel pins of a larger diameter. Subsequent static bearing tests show damage initiation at 24 kN and the fatigue bearing tests were able to show delamination growth without damaging the pin. In the near future, more fatigue tests will be performed to study the stiffness reductions of pristine versus pre-delaminated specimens and a comparison between double lap shear and single lap shear will be experimentally tested and modeled.

Industry Application Area(s)
Aerospace, Defense, Security | Civil/Structural Engineering

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