The development and application of a delamination prediction method to composite structures.
A method of predicting delamination in fibre-reinforced composite materials including
several previously disregarded strength issues is presented. Thermal residual stresses,
volume of stressed material, in-plane stresses and the hydrostatic stress in the polymer
matrix are introduced and their influence on composite material strength discussed. These
factors are then applied in a stress based method for predicting delamination which can
deal with both unidirectional and general laminates. The results from a series of scaled
unidirectional specimens designed to produce interlaminar tensile strength data are used
to determine the strength parameters for the method.
The method is shown to be effective in predicting failure in the fill-in region of two 'T'-
piece specimen designs to within 14%. The failures were dominated by tension acting
between fibres in large blocks of unidirectional material which had high thermal residual
stresses and tensile hydrostatic stress due to constraint from the surrounding material.
The method is also applied to a series of test pieces which used general laminates. The
designs are based on sandwich panel sections and a tapered I-beam specimen. In the sandwich
panel specimens, the edge closure sections were constructed using 0,90 and ±45°
plies. Delamination occurred in a region of dropped plies and curvature making all the
stress components important in producing accurate predictions, which are within 16% of
the failure loads in testing.
The tapered I-beam specimens were designed to delaminate in a doubly-curved laminate
region of 90 and ±45° plies. The delamination predictions were within 13% of the
first delamination loads found in testing.
The method produced failure predictions which were all within 16% of the failure loads
of the tested specimens. It is found that the local geometry of the delamination region
is critical in determining the stress levels in the specimens and therefore their strength.
Variations in the manufacture of such specimens and components is therefore clearly important
in establishing the delamination loads of composite structures.