Buckling and Postbuckling of Cocured Composite Stiffened Panel Under Axial Compression Load Computation and Testing

Abstract

A cocured carbon fibre composite stiffened panel was designed to have an early onset of skin
buckling and larger postbuckled strength. IMA/M21 prepreg was used for fabricating the panel
using autoclave moulding process. Special end fixtures were developed using a metallic casing
and the panel was encased in it using a mixture of epoxy resin and aluminium powder. The
panel was tested under axial compression to its full load bearing capacity in a universal testing
machine. Full field displacement measurement was carried out using advanced techniques like
Digital Image Correlation (DIC) apart from acoustic emission technique and strain gauges
for monitoring the structural response. After the test, the panel was subjected to ultrasonic
scan to check for any delamination or disbonding that might have occurred due to post buckled
response. Concurrently, a finite element model was developed to predict the buckling and
postbuckling response of panel. DIC captured the onset of skin buckling and deformations/
mode shapes in postbuckling regime. These experimental observations were correlated
with numerical simulations. In the postbuckled regime, severe bending and twisting of skin
and stringers was observed, resulting in complete loss of global axial stiffness of the panel. It
is suspected that such a state of stress in the panel could lead to delamination, debonding or
fiber failures. However, acoustic emission sensors bonded to the panel did not record any
significant events during tests, even in the deep postbuckled regime. This was further corroborated
through ultrasonic inspection conducted after unloading the panel.