Suction feet are used to attach the mobile scanner to the propeller. Researchers record the ultrasound test data on-site. Image: Fraunhofer ITWM
propellers are as large as a single-family home—and manufacturing them
is quite a challenge. During the casting process, pores and miniscule
cracks can form that in the worst case may cause a blade to break. Now
these massive components can be inspected for defects in a non-invasive
manner, using a new kind of ultrasound process.
can weigh up to 150 tons, and it’s not uncommon for them to measure 9 m
or more in diameter: the ship propellers on huge tankers, container
ships and cruise liners are invisible giants. Damage to these massive
propellers could render a ship unmaneuverable—with unpredictable
consequences for people and the environment. Many defects do not come
from external influences, but instead originate in the production or
repair process. For example, when the molded parts are being cast, any
turbulence could lead to sand inclusions and pores. Left undetected,
critical imperfections could lead to breakage of a blade.
now, propellers have been inspected manually for inner defects when
necessary. To make them visible, the inspector guides an ultrasound test
probe over the component by hand—an error-prone procedure that fails to
capture the entire volume of the component. This method cannot detect
cracks inside the propeller in certain circumstances. To identify
defects in a timely manner, researchers at the Fraunhofer Institute for
Industrial Mathematics (ITWM) developed a mechanized ultrasound process
that can be used for the non-destructive testing of complex components.
The scientists received support from the GL Group (Germanischer Lloyd)
and propeller manufacturer Wärtsilä Propulsion Netherlands.
Mobile scanner can be positioned freely
our mobile ultrasound test system, we can inspect
copper-nickel-aluminum bronzes up to 450 mm thick and detect fissures
down to a few millimeters in length. Because we emit the ultrasound at
defined angles, we also find defects positioned at an angle to the
surface”, says Dr. Martin Spies of ITWM in Kaiserslautern.
system is capable of recording large volumes of digitized ultrasound
test data, taking into account the many and variously intense curvatures
of the propeller surface. The device currently scans test grids of 700
by 400 mm, achieving a rate of up to 100 mm per second. The mobile
scanner can be positioned anywhere on the propeller, and, thanks to its
suction feet, it can be attached in a horizontal as well as vertical
obtained the 3D data about the inside of the component by an imaging
procedure known as SAFT. It provides a detailed display of inclusions
and welding-seam defects. It basically works like computer tomography in
medicine,” explains Spies.
the aid of special computational processes and algorithms, the experts
have succeeded in reducing interference signals and intensifying error
signals—a complicated task, since the various areas of the blade do not
have a homogenously coarse grain. This can weaken the echo
substantially. The specialists also use simulations to calculate in
advance which ultrasound test probe they have to deploy.
researchers use the mobile scan system for their on-site testing at
foundries, at propeller manufacturers, on deck and in dry dock, and are
currently improving scan times and 3D defect imaging. Only recently,
they were able to put the efficiency of their procedure to the test at
the world‘s largest shipbuilder in Korea.
customer wanted to document the quality of its propellers, to gain an
edge over the competition,” says Spies. “With our procedure, we can test
not only propellers but also other complex components made of materials
that are difficult to test, like offshore components made of duplex
steels,” he stressed.
researchers Alexander Dillhöfer, Hans Rieder and Dr. Martin Spies
recently received the Innovation Award from the Deutsches Kupferinstitut
for their outstanding accomplishments with copper and its alloys.