Yards Moving Forward - GL Group

MODERN DAMAGE ANALYSIS
To Be or not to Be
“In cases like these, we always begin by checking the
material properties,” is how Manfred Feyer describes the
usual procedure. “After all, a mistake in material selection or
application cannot be ruled out until the opposite has been
proven. First of all, we have to answer what is perhaps the
most trivial question of all: is the material in question really
what it claims to be, i.e. according to specifications?”
To determine this, the component fragment must be subjected
to a certain amount of violence. It is pulled, bent and
etched literally beyond the limits of endurance. The first
ordeal is called the test of tensile strength. Here the laboratory
technicians extract a tensile sample from the shaft of the bolt
and apply a precisely defined tensile test to ascertain the
strength of the material. This value is then compared with the
requirements set by the corresponding standard of the supposed
material. Then the so-called “notched bar impact test”,
which measures the tenacity of the material, is performed. For
this, a specimen is notched on one side and attacked with a
pendulum hammer. Depending on how much energy the
sample absorbs – how tough it is – the pendulum hammer
swings back up to a varying degree. However, the tenacity is
not only a function of the material, but also depends on the
environmental conditions, particularly the temperature. The
environmental parameters must therefore be monitored
closely to obtain reliable data.
If the values determined in the tests correspond to the
standard values for the material, then it is most probable that
the correct material has indeed been used. Final certainty is
offered by the chemical analysis. With the aid of a spectrometer,
the technicians draw an electric arc – like that used in
welding – from the material sample. The spectrometer analyses
the spectrum of the electric arc, which is characteristic for
a certain material composition. This method, which can be
used for all metallic materials, e.g. those with an iron, nickel,
aluminium and copper basis, now provided an exact analysis
of the alloying constituents used in our mystery material.
High-resolution Analysis
In the case of the cylinder-head bolt, the conclusion was
crystal-clear: it had indeed been made of the specified
chrome-molybdenum steel – there had been no error in the
choice of material. The actual reasons for failure were still
unknown, but Feyer was now able to rule out a number of
likely causes. The breakthrough then came with the microscopic
examination. In addition to the light-microscopy with
a magnification of some 1000 times, the test technicians at
GLP also use the 20,000-fold magnification of a scanning electron
microscope. The sharp vision of this high-resolution
analysis device proved to be irresistible: In the thread root –
the starting point for crack formation – minute traces of corrosion
could be seen. “The question as to why the crack had
started in the first place could therefore be answered conclusively,”
Feyer was pleased to report. The corrosion disturbed
the overall balance of admissible stresses in the bolt, resulting
in a micro-crack, which then grew to a size that compromised
the integrity of the component. The rest of the investigation
was routine: “In the servicing instructions for the engine, we
found a clear description of how the cylinder-head bolts had
to be mounted,” explained Feyer, “namely with the use of a
sealant to keep moisture out of the thread. This had evidently
62 nonstop 3/2006
Ruptured surface with the lines of rest that are typical of fatigue fractures
been neglected during the last maintenance.” The servicing
error was thus proven beyond doubt. A silver lining in the
cloud for the shipowner, but bad luck for the servicing company
which had been entrusted with the maintenance. “Once
this test report was received, they had to call up their insurance
company right away.”
Look before You Leap
The financial benefit which the customer can obtain from
the advanced apparatus and high-level expertise of GLP is not
always as clearly visible and rapidly applied as in this example.
In particular, failure prevention calls for a long-term perspective.
“The significance of consultative material examination
– another source of business for us – is increasing steadily,”
Feyer explains. The realization is growing that not all
structural design features can be optimized by computer –
residual uncertainties will always remain. Sometimes, it is
merely that the operating data with which a process or component
could be optimized is missing. Take spot-welding in
the processing of sheet metal, for instance: to be able to meet
the quality objectives it has set itself, a certain company is
having the preproduction series of a safety-relevant tank
examined by GLP. With success, as Manfred Feyer emphasizes:
“We were able to give specific advice on optimizing the
methods being applied.”
A valuable contribution towards preventing damage is
undoubtedly the close cooperation of GLP with the central
damage management of GL. The constant feedback of information
helps to detect and avert unfavourable developments
at an early stage. In this regard, the broad line-up of the GL
Group is proving its worth, as it not only certifies but also tests
and monitors the application of technology. “Whenever we
identify a pattern in cases of damage, this is incorporated into
our rules and guidelines,” Feyer notes.
Not all of the assignments that land on Feyer’s desk, or on
those of his colleagues in Mülheim, Herne and Stuttgart,
demand such a “good nose”, as in the case of the corroded
cylinder-head bolt. There are also the many everyday jobs –
routine examinations that are performed within the scope of
material certifications, product modifications, or changes of
vendor or materials. But they too can save the customer a lot
of money – when a company switches its supplier of cast parts,
for example. The dimensional accuracy of the components
can be checked by any suitably equipped incoming inspection
department, but the material properties are a matter for a fullblown
laboratory. Manfred Feyer is convinced: “Calling in GLP
could help prevent quite a few product recalls!” ■ JI
For further information: Dr. Manfred Feyer, Managing Director of Germanischer Lloyd
Prüflabor, Phone: +49 40 20003-970, manfred.feyer@gl-group.com