La opinión de los ex-residentes entrevistados

zone models. To determine the interplay between cylindrical aircraft structures and the response of individual fatigue cracked panels (e.g. pre-existing MSD in aging aircraft), well-controlled and minimal experiments for dynamic fracture of blast loaded barrel tests have been performed on three popular aerospace materials; Aluminium 2024-T3, Glare and CFRP [26]. The dynamic event induced crack growth speeds in order of magnitude of several hundred meters per second, metrics obtained via image processing of high speed images. Glare exhibited the lowest crack growth speeds and displayed a combi-nation dynamic ductile behaviour and fibre bridging. The results also highlighted the poor blast attenuating qualities of CFRP, displaying crack speeds nearly ten times that of Glare with evidence of crack bifurcation-branching.

Finally, in an effort to model the dynamic ductile crack growth of Aluminium 2024-T3 from the previous barrel tests, a numerical cohesive zone approach is followed; a layer of interface elements which behave according to a traction-separation law is inserted along the fracture path. Static cohesive properties were extracted from standard frac-ture toughness tests and extrapolated to the aforementioned barrel tests. This method proved inaccurate to predict the rate of fracture as a considerable difference was found between the experiments and predictive results. This discrepancy was attributed to the rate-independence of the cohesive formulation which failed to take into account the influ-ence of triaxiality and the opening rate on the local cohesive traction within the fracture process zone. To circumvent this problem, a Perzyna visco-plastic rate-dependent cohe-sive formulation is discussed and implemented which gave better representative results in terms of crack-growth rates. However the visco-plastic parameters were derived from one set of experimental data.

7.2 Recommendations for future work

In order to define some interesting areas of further research, it is useful to present first a perspective of this research in relation to the main aims and objectives.

Blast mitigation of structural materials to withstand acts of sabotage require a funda-mental understanding of its use in real-life applications, whether military, civil or in urban areas. This thesis focusses on thin-walled structures which are predominantly used in aircraft applications where stiffness and weight are driving performance factors.

It has been shown that if a cylindrical thin-walled structure, such as the fuselage, is explosively detonated then the combination of internal pressurisation and the explo-sively driven blast impulse can cause severe structural damage and generate cracks that propagate along the length of the aircraft at supersonic speeds. According to the au-thor’s opinion based on the numerical work performed in Chapter 4, a small amount of

explosive (IED) can lead to catastrophic failure (both in wide-body and narrow bodied aircraft) posing great threats to passengers and ground civilians. It is very difficult to quantify the amount of explosive charge (impulse) which is needed to breach the fuselage as there are a multitude of scenarios which could take place on-board an aircraft. As shown in the pressurised barrel tests, the blast pressure reduces rapidly with distance (1/2 metre) and so the implications for aircraft security is that the explosive charge has to be in close proximity to the skin of the fuselage or be of large size/quantity to cause breaching of the structure. It is assumed in this research that current baggage screening capabilities in airports can detect large amounts of explosive and therefore only small amounts, such as IEDs, should be considered. However, the smaller the explosive the more difficult it is to detect. Further work needs to be done to quantify these issues, such as how much explosive could bring down an aircraft, for a multitude of vulnerable locations such as the passenger and luggage cabin sections. Ideally, this should be done for large-scale aircraft (wide and narrow bodied) which encompass all of the elements which were neglected in this thesis, such as flooring, window openings etc.

The present work has focussed on Glare, a fuselage skin material for the Airbus A380, which has shown to outperform monolithic aluminium in blast events. Nevertheless, it would useful to the wider community if the tearing threshold of this material in relation to other conventional structural materials was determined.

Moving away from Glare, there are also many other promising material variations (such as Titanium-CFRP laminates) which should be explored. Although it is recognized that such materials will seldom reach technological maturity for fuselage applications, efforts could focus on blast mitigated luggage containers. Such containers can help reduce the blast pressures at vulnerable locations on the airframe. This is by no means a trivial feat, since economics (manufacturing, materials) and weight are crucial driving factors.

Furthermore, if successful, such designs must be retro-fitted to existing aircraft to reduce costs and meet the demand of passenger security in this ever changing climate.

It is acknowledged by the author that further small-scale fracture tests for Glare should be performed to (a) validate the derived cohesive parameters and, if possible, (b) extract experimentally measured deformation fields to obtain accurate Mode I cohesive zone laws. The traction-separation law should encompass the fibre bridging mechanism and the process of stress transfer between the aluminium layers and glass-fibre in the wave of the crack. Furthermore, a revision of the numerical methodology should be performed to generate models (mesh design generation, boundary and loading input parameters, cohesive zone element formulation) which are more manageable for the numerical ana-lyst, particularly in industry. One such area of further research is the development and implementation of shell cohesive zone elements which can be integrated seamlessly to

7.2 Recommendations for future work 143

the global shell structure. This would preclude the use of shell-solid connections and significantly reduce the computational effort. To help alleviate some of the shortcomings of the finite element method in relation to crack growth (where the direction of the path must be known to the user), the XFEM (partition of unity) method should be consid-ered to model the propagation of discontinuities in large-scale shell structures. These enriched elements can reproduce the challenging features of fracture without the user knowing or tracking the crack path. Moreover, treating problems with discontinuities with the XFEM method suppresses the need to define time-consuming mesh refinement in the vicinity of the crack tip and avoid re-meshing the discontinuity surfaces. These factors can alleviate the computational cost and errors associated with conventional finite element methods.

Appendix A