Variable-stiffness composite panels – Defect tolerance under in-plane tensile loading
O. Falcó a, J.A. Mayugo a, C.S. Lopes b, N. Gascons a, J. Costa
Automated Fiber Placement is being extensively used in the production of major composite components
for the aircraft industry. This technology enables the production of tow-steered panels, which have been
proven to greatly improve the structural efficiency of composites by means of in-plane stiffness variation
and load redistribution. However, traditional straight-fiber architectures are still preferred. One of the
reasons behind this is related to the uncertainties, as a result of process-induced defects, in the mechanical performance of the laminates. This experimental work investigates the effect of the fiber angle discontinuities between different tow courses in a ply on the un-notched and open-hole tensile strength of
the laminate. The influence of several manufacturing parameters are studied in detail. The results reveal
that ‘ply staggering’ and ‘0% gap coverage’ is an effective combination in reducing the influence of defects
in these laminates.
In the present experimental work, the effect of the fiber angle
discontinuities between different tow courses in a ply on the unnotched and open-hole laminates tensile strength was investigated. The influence of the percentage of coverage and the staggering technique were studied in detail.
Test specimens were designed and manufactured to represent a
sub-domain of a VSP containing a worst-case tow-angle discontinuity, and different practical manufacturing strategies to tackle
such discontinuity: (i) tow-dropping with 0% gap coverage (gaps);
(ii) tow-dropping with 100% gap coverage (overlaps); and (iii) towdropping with 0% gap coverage and ply staggering.
After a geometrical observation of overlapping and gap defects
produced by the tow-drop manufacturing method, test specimens
were loaded until failure. In comparison with the baseline specimens representing straight-fiber panels without defects, the configuration with 0% gap coverage and no staggering presents the
most critical strength reduction; as a results of the clustering of
gaps. When the ply staggering technique is applied, these effects
are highly mitigated. Therefore, ‘ply staggering’ and ‘0% gap coverage’ would be expected to be an effective combination to lessen the
influence of defects in VSP.
The experimental results show that the stress concentration,
induced by the presence of the hole in OHT specimens, has a higher
influence on failure than that induced by the presence of manufacturing defects in un-notched specimens. Additionally, the analysis
Fig. 12. (a) Comparison of load-crosshead displacement for different open-hole laminate configurations. (b) Delamination and damage on tested specimen. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
30 O. Falcó et al. / Composites: Part A 63 (2014) 21–31of failure mechanisms in un-notched specimens reveals that large
delaminations initiate in the vicinity of the tow-drop defects,
which are then followed by extensive matrix cracking and finally
fiber failure. This is probably caused by the amplification of the
interlaminar stresses around the defects.
The authors acknowledge the financial support of the Ministerio de Economía y Competitividad of Spain under Project
MAT2009-07918. The first author would also like to thank Generalitat de Catalunya for the FI predoctorate Grant 2010FI_B00658
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