Induction Welded Repairs in Carbon Fiber Thermoplastic Aircraft Structures

Abstract

This master thesis research investigates the possibility to repair carbon fiber thermoplastic aircraft
structures using induction welding. Carbon fiber is conductive and heats up when placed inside an
alternating magnetic field. A generator, coil, and pressure frame are needed to perform induction
welding. The support plates needed to pressurize the carbon fiber parts need to be non-conductive,
non-magnetic, temperature resistant, stiff at high temperatures, and thermally insulating.
The material used is five harness satin weave carbon fiber PPS supplied in pre-consolidated plates
and unconsolidated semi-preg. Three different joint geometries are used in this investigation: a conventional
scarf, a continuous scarf, and a stepped lap joint. The stepped lap joint and the conventional
scarf are milled using a CNC machine. The continuous scarf is produced in a press using specialized
tooling and a press program prescribed by TenCate.
Of each type two specimens are produced: an induction welded specimen and a press joined specimen.
The specimens are tested in tension to determine the tensile strength and stiffness. This gives
the performance of the induction welded joints with respect to the press joined specimens. Additionally,
both of the welded specimens are compared to the pristine specimens.
By measuring the temperature along the weld-line of multiple test specimens, an induction welding
program is obtained for each joint type. Achieving a consistent temperature along the weld-line is
challenging due to the thermal conductivity of carbon fibers and other effects inside the laminate. Similar
to the continuous scarfed specimens, the press joined specimens are created using specialized tooling.
Before testing the specimens in tension, they are scanned using a C-scan. The press bonded
specimens show no flaws, whereas the induction welded specimens do not return the signal to the
transducer. Micrography of the specimens shows similar results as the C-scan. The press joined
specimens show little to no flaws and the induction welded specimens show a significant amount of
voids and small unjoined sections near the tips for the continuous scarf and stepped lap specimen.
The tensile tests show that all induction welded joints perform less than the press joined specimens
with a percentage of about 73-78%. The conventional scarf and the continuous scarf result in a similar
failure strength recovery of about 44% of the pristine failure strength for the press joined specimens.
Both the conventional scarf and the continuous scarf show similar behavior in welding and the tensile
testing, but the continuous scarf is more difficult to produce. Therefore the conventional scarf joint is
preferred over the continuous scarf joint. The stepped lap joint has the best performance, recovering
about 59% of the pristine stiffness. The stiffness of all tested specimens is between 92%-99% of the
pristine stiffness.
Inspection using digital image correlation and finite element modeling indicates the failure initiates
inside the PPS matrix in between the parts at the location of the first or last step. Finite element modeling
also suggests that a smaller angle for the scarf joint or a longer overlap for the stepped lap joint would
increase failure strength.