Ship 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.
They can weigh up to 150 tons, and it’s not uncommon for them to measure
nine meters 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.
Until 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
"With our mobile ultrasound test system, we can inspect copper-nickel-
aluminum bronzes up to 450 millimeters 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. The 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 millimeters, achieving a rate of up to 100 millimeters 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 test
position. "We 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.
With 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.
The 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. "The 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. ITWM 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.
News Source: http://www.ndt.org/
