Quantitative Delaminationsbestimmung an geschichteten Verbundwerkstoffen [online]

Abstract

Delamination, i.e. the debonding of adjacent layers, is usually
the dominant and most critical type of damage in layered
composites, e.g. laminates from carbon-fibre reinforced plastic
(CFRP). Especially the size, shape and location of delaminations
are decisive for the remaining strength of a component and the
choice of suitable repair strategies. Therefore, the main goal
of this work is the non-destructive quantification of such
internal damages from easily accessible measurements on the
outer surface.
Specific CFRP samples are investigated under tensile load where
predefined damage domains are incorporated in the prepreg
layering with Teflon foils.
Digital phase-shifting shearography is used for a fast and
accurate experimental determination of the surface deformation
under the given test load.
Additional application of a postprocessing algorithm allows for
the calculation of the displacement field.
The problem of determining internal damages from outer
measurement data is an inverse problem, i.e. in general
ill-posed. As the reconstruction methods in the literature are
mainly concerned with simple crack cases in homogeneous bodies,
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at rst a simplied model problem is dened. In this case, the
reciprocity gap concept of Andrieux and Ben Abda can only be
successfully extended to the case of curved cracks lying on an a
priori known internal surface in a homogeneous anisotropic body.
Therefore, in the following the newly developed reconstruction
algorithm AICRA (Alternating Iterative Crack Reconstruction
Algorithm) is proposed which offers several advantages in
comparison to other algorithms, e.g. the usual optimization
approaches. Favourable convergence and regularizing properties
of this new method are proved theoretically and demonstrated
with several numerical investigations using both the boundary
element method as well as the finite element method.
In the last chapter, AICRA is extended for the case of
delaminations in layered composites. Again fast reconstruction
results are achieved for delaminations in the middle interface
of symmetric laminates using both experimental and numerical
boundary data. Besides, the experimental results are in good
agreement with the FEM simulations of the laminate deformation
behaviour.
As the resolution of AICRA proves to be higher than the
experimental positioning accuracy of the Te foils, only
numerical reconstruction tests are carried out with thicker and
asymmetric laminates. Again, fast and precise quantification
results are achieved, which are promising for further extensions
of AICRA, e.g. to the case of several delaminations in different
interfaces.