Objective
-To investigate the cost and weight saving potential of allowing CFC structures to operate in the post-buckling condition
-To instill greater confidence in the use of CFC in the post-buckled mode, which should stimulate CFC technology in general
-To produce a methodology for the post-buckled design of CFC identifying the type of structures most suitable to post-buckled operations and requiring the achievement of the following sub-objective
-To evolve reliable failure criteria for the through-thickness (z-direction) modes such as "ILSS", interlaminar normal stress and delamination
-To investigate scale (thickness dependent) effects and to develop an appropriate criterion
-To develop methods for the estimation of the through-thickness force levels required to induce panel-stiffener separation
-To develop finite element and other coding as appropriate for the prediction of failure using stress, strain and energy release rate criteria
-To define and perform the testing necessary to validate the methodology being developed.
Carbon fibre composite, stiffened, panels have been designed and manufactured to have significant post-buckling strength. These panels have been tested to failure. The initial failure mechanism in an I-section stiffened panel has been identified by arresting the test before catastrophic collapse. A number of finite element programs have been used to predict post-buckling behaviour.
Recent results from the tests of the I-stiffened panel have precisely identified the initial failure mechanism. The bending moment in the stiffener web was responsible for the delamination propagating from the stiffener base. In the region where this moment has a peak value the only other stress resultant of any significance was the axial compressive load in the panel.
Throughout the tests experimental measurements were used to evaluate the accuracy of the theoretical predictions for the buckled panels. Especially of interest was the margin of load between damage initiation and final collapse. It has been measured to be less than 10% of the collapse load but is likely to be affected by the load history of the panel.
It was anticipated that post-buckling collapse was a consequence of through-thickness stresses. The experimental revelations have confirmed this and have focused attention on some fundamental problems which pervade all attempts to design structures in composite materials.
Final results indicate that:
carbon fibre composites (CFC) can operate successfully in the postbuckled condition;
environmental conditioning and damage and defect simulation to represent an operational environment can be performed satisfactorily;
skin stiffener separation and delamination have been identified as the main factors leading to catastrophic failure;
beaded stiffener designs are unsuitable for compression loading but perform well in static and fatigue shear loading;
finite element models can analyse the postbuckled response and failure criteria, in terms of permissible levels of interlaminar stresses, have been established;
skin stiffener separation failure on stiffened panels can be simulated by a simpler 4-point bending test without the need to apply compression loading.
Savings of up to 10% in weight can result from using postbuckled constructions, and sites on aircraft have been identified where such savings can be achieved. The methodologies developed will lead to cost savings through verified analysis and failure criteria allowing reductions in required testing. Applications in areas other than those considered here are also possible.
Fields of science (EuroSciVoc)
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
- humanities history and archaeology history
- engineering and technology materials engineering composites
- engineering and technology mechanical engineering vehicle engineering aerospace engineering aircraft
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Coordinator
PR4 1AX Preston
United Kingdom
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