Professor Eckart Schnack from the Institute of Solid Mechanics, University of Karlsruhe in Germany, visited the Institute and gave a lecture on “A rigorous model formulation for characterisation of composites”.
Eckart is well known in this field of research and he has worked on the quantitative determination of damage in layered components using integral methods. The development of the work was the topic of his lecture, expanding on numerical methodologies and the application of a Richardson-type iterative approach for the identification of delamination.
He first formulated the problem in boundary elements terms as a boundary value problem using a variational formulation involving the standard formulation and its complimentary expression. Considering the overdetermined problem, he applied an iterative procedure to find the solution by switching from the original to the complementary problem all the time.
The formulation was used to identify delamination cracks inside a composite consisting of different layers using outer surface data. The importance of the work is that in this way delamination can be determined from comparatively simple data.
The delamination problem is rather complex due to the presence of singularities and the anisotropy of the media.
The technique has been used to study the main damage type for CFRP laminates used by Boeing. They found that delamination accounts for 60% of all damages in CFRP airplane parts detected during inspection. The experimental measurement technique was shearography which allows the determining of surface displacements more accurately than other techniques.
An isotropic specimen consisting of two layers and a delamination zone on the internal boundary was studied using the technique. This was based on having the complete field on the external boundary, which in practice is not possible, hence it is necessary to deal with incomplete data sets.
Several carbon fibre reinforced test specimens were experimentally tested. Numerical results were shown to converge to the experimental measurements with a low number of iterations.
Eckart described the practical applications of the method and how that could be combined with shearographic techniques for cases as complex as helicopter blades. The results from shearographic experiments are mainly qualitative and the numerical technique developed can significantly add to our understanding of delamination problems in composites.
Wessex Institute