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high sensitivity. With this method even smallest defects could be found. However, after a
short period Hi was discarded for NDT purposes (not for quantitative deformation measurement),
mainly due to two disadvantages. On the one hand the complicated chemical development
process of holographic media was not acceptable for measurements in production
environment (even the thermoplastic film with semi-automatic development could not solve
this problem). Secondly, the price for the high measurement sensitivity was the sensitivity of
the measurement apparatus to vibrations.
The next step in interferometric measurement was Electronic Speckle Pattern Interferometry
(ESPI), which replaced the holographic film by an electronic target, but led the vibration problem
still unsolved. So ESPI was also an intermediate step.
Finally Shearography also solved the vibration problem, because both interfering beams are
guided via the possibly vibrating component. Meanwhile complete shearography systems
with laser, optical head and computer controlled data recording and evaluation are commercially
available.
Airbus applied shearography to several CFRP inspection problems. One example is shown
in figure 8. Flaws due to incorrect repair of the spoiler are clearly visible in the unwrapped
phase map. Shearography is mature for aircraft applications, but it competes with existing
solutions, mainly with ultrasonic testing. A big advantage of shearography is that it is contactless
and delivers directly two-dimensional information. Debondings and delaminations are
easily detectable. On the other hand porosity is difficult to detect, because there is no direct
correlation between the measured surface deformation and the pores inside. Shearography
is therefore a candidate for in-service application, where porosity detection is not required.
Possible applications are detection of stringer debondings or other flaws caused by damage,
e. g. impact damages.
8
Figure 8: Shearography Measurement
8. Conclusions and Outlook
Ultrasonic testing is a standard method for CFRP inspection. Manual testing with single element
transducers is applied for small parts, big parts are tested with multi-channel facilities.
The phased array technology (with linear arrays) is entering more and more in production
and in-service. Laser ultrasonic testing still in the laboratory phase, but has potential for inspection
of parts with small and medium size and complex geometry. Shearography is
meanwhile an established method, but until now only a few real applications in production or
in-service exist.
Future challenges in NDT for CFRP are driven by a potentially higher percentage of composite
in aircraft structure. This requires more in-line inspection tools, i. e. an integration of NDT
in the manufacturing process chain. Today “parts meets the NDT system”, in future this
should be vice versa - “the NDT system should meet the part”. In some cases manufacturing
processes could be supervised by pure on-line system, replacing classical NDT.
Furthermore production needs NDT methods, which are sensitive only to defects. It is not
economic for an inspector to spend 99% of the evaluation time to look on faultless areas. It is
much better to have a system, which guides him directly to suspicious areas. A further step is
then to develop intelligent defect identification tools, which can decide between genuine defects
and spurious indications.
For preparation of NDT procedures it is often necessary to perform extensive laboratory investigations,
in which all potential combinations of part geometry, flaw size and position,
probe configurations etc. are tested. In future NDT modeling tools may help to reduce such
investigations by replacing them partially by numerical simulations.
76
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Part No.
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Part No.
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Part No.
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Description
Handle
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