Experimental and numerical investigations were conducted into the damage growth and collapse behaviour of composite blade-stiffened structures. Four panel types were tested, consisting of two secondary bonded skinstiffener designs in both undamaged and pre-damaged configurations. The pre-damaged configurations were manufactured by replacing the skin-stiffener adhesive with a centrally located, full-width Teflon strip. All panels were loaded in compression to collapse, which was characterised by complex postbuckling deformation patterns and ply damage, particularly in the stiffener. For the pre-damaged panels, significant crack growth was seen in the skin-stiffener interface prior to collapse, which caused a reduction in load-carrying capacity. In the numerical analysis of the undamaged panels, collapse was predicted using a ply failure degradation model, and a globallocal approach that monitored a strength-based criterion in the skin-stiffener interface. The pre-damaged models were analysed with ply degradation and a method for capturing interlaminar crack growth based on multi-point constraints controlled using the Virtual Crack Closure Technique. The numerical approach gave close correlation with experimental results, and allowed for an in-depth analysis of the damage growth and failure mechanisms contributing to panel collapse. The successful prediction of collapse under the combination of deep postbuckling deformations and several composite damage mechanisms has application for the next generation of composite aircraft designs.
The European Commission Project COCOMAT (Improved MATerial Exploitation at Safe Design of COmposite Airframe Structures by Accurate Simulation of COllapse) is a currently running four-year project that aims to exploit the large strength reserves of composite structures through a more accurate prediction of collapse. Accordingly, one of the COCOMAT work packages involves the design of test panels with a focus on investigating the progression of composite damage mechanisms. This paper presents the collaborative results of some of the partners for this task. Different design alternatives were investigated for fuselage-representative test panels. Non-linear structural analyses were performed using MSC.Nastran (Nastran) and ABAQUS/Standard (Abaqus). Numerical predictions were also made applying a stress-based adhesive degradation model, previously implemented into a material user subroutine for Abaqus. Following this, a fracture mechanics analysis using Nastran was performed along all interfaces between the skin and stiffeners, to examine the stiffener disbonding behaviour of each design. On the basis of the structural and fracture mechanics analyses, a design was selected as being the most suitable for the experimental investigation within COCOMAT. Though the COCOMAT panels have yet to be manufactured and tested, experimental data on the structural performance and damage mechanisms were available from a separate project for a panel identical to the selected design. This data was compared to the structural, degradation and fracture mechanics predictions made using non-linear finite element solutions, and the application of the design within the COCOMAT project was discussed.
A methodology for analysing the degradation and collapse in postbuckling composite structures is proposed. One aspect of the methodology predicts the initiation of interlaminar damage using a strength criterion applied with a globallocal analysis technique. A separate approach represents the growth of a pre-existing\ud interlaminar damage region with user-defined multi-point constraints that are controlled based on the Virtual Crack Closure Technique. Another aspect of the approach is a degradation model for in-plane ply damage mechanisms of fiber fracture, matrix cracking, and fiber-matrix shear. The complete analysis methodology was compared to experimental results for two fuselage-representative composite panels tested to collapse. For both panels, the behavior and structural collapse were\ud accurately captured, and the analysis methodology provided detailed information on the development and interaction of the various damage mechanisms
Analysing the collapse of skin-stiffened structures requires capturing the critical phenomenon of skin-stiffener separation, which can be considered analogous to interlaminar cracking. This paper presents the development of a numerical approach for simulating the propagation of interlaminar cracks in composite structures. A degradation methodology was introduced in MSC.Marc, which involved the modelling of a structure with shell layers connected by user-defined multiple-point constraints (MPCs). User subroutines were written that employ the virtual crack closure technique (VCCT) to determine the onset of crack growth and modify the properties of the user-defined MPCs to simulate crack propagation. Methodologies for the release of failing
As explicit finite element (FE) codes improve and advanced material models become available, such tools will find more widespread application within the aerospace industry, as ‘what-if ’ simulations become more manageable with increasing computing power and greater modeling realism. This paper describes the investigation of three commercial explicit FE analysis packages, LS-Dyna, MSC.Dytran, and Pam-Shock, to determine their capabilities in predicting barely visible impact damage (BVID) in composite structures. The investigation is conducted by first determining the suitability of the codes in constructing an FE model of a stiffened panel, solving for BVID and retrieving results. The results are in turn compared to experimental data in order to gauge the suitability of the codes for composite design and analysis. Comparisons of the FE simulations to experimental data include damage development and degradation, as well as the time-history responses. The Chang-Chang failure theory with brittle degradation was used for both LS-Dyna and MSC.Dytran, while the biphase model was used for Pam-Shock. Results indicated that the general shape of the force-time curves as well as the peak forces were predicted reasonably well. However, all simulations predicted a trough that was much less significant than the test results, as well as a shorter impact duration.
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