The aim of Task 2.1 was to try to improve the ability to accurately and realistically predict the response of impact loaded composite material and structures using computer codes and supplied experimental data. The overall conclusion of this work is that the low-velocity/minor damage region can be adequately modelled, however further work is required to increase reliability. From the experience of the Participants it was thought that the agreement of the codes at high speeds could be much better since one failure mode tends to dominate. It was recommended that work continues in with investigations into mesh dependency, 3-D effects, high strain-rate material, damage tolerance, and modelling to structural features.
The objective of Task 2.2 was to assess various impact test techniques in order to, hopefully, establish an appropriate procedure for producing composite materials property data. This was to be achieved by performing impact tests on supplied specimens and in the process collecting accurate and reliable material property data. The Participants concluded that, in order as assess the best impact test procedure, testing should be carried out under more strictly specified conditions. Also, greater effort should be made to ensure that the parameters to be measured are consistent between test houses. Some reliability in the data gathered was suggested in that, where results overlapped, there were no contradictions to the type of failure occurring. It was concluded that, at a minimum, the amount of test data produced by this Task could be used to validate simulations in future projects. As a rule, most composite components include curved surfaces and designers require data and validated models for such structures. Therefore, it was recommended that any future test programmes in this particular area should include testing on curved surfaces. Also such work should include investigations of the effect of impact on structural features, such as drop-piles.
Task 2.3 was conceived to assess how accurately current analysis codes and associated 'bird' material models simulate the effect of high strain-rate birdstrikes on aerofoil structures. The aim was to develop an improved ''bird' model for use in simulations, thus reducing the amount of testing required. From the work undertaken the Participants concluded that, despite this being one of the few times that birdstrikes against sharp aerofoils had been investigated and modelled, reasonable predictions were produced. The accuracy of the predictive simulations against the test data was sufficient to suggest that, with further parameter refinement, a 'bird' model can be produced to give increased validity and realism to future simulation work. It is noteworthy that the test data is being employed by some industrial Participants to assist in the development of better finite element techniques. To help achieve validated and more realistic models the following work was recommended by the Participants, identifying a fully representative 'bird' model, establish the performance of gelatine relative to a real bird, and, collect more data for asymmetric birdstrikes.
Task 2.4 was set-up to assess the use of smart structures to detect, locate and quantify damage after impact using a variety of sensing techniques. Several sensing technologies, Optical fibres (Per Udsen), Lamb waves (Imperial College), and Electrical Potential/Acoustic Emission (Cranfield), were tested and evaluated using carbon fibre test pieces which were impacted at pre-specified energy levels. It was concluded that each system investigated had advantages and disadvantages which would govern how an end-user would employ them to meet his requirements and it was recommended that the accuracy of the systems should be re-assessed using larger panels with random impact sites. Also that evaluation of the suitability of the systems for use in the monitoring of realistic composite structures and sandwich structures, such as those employed in ships and high-speed trains, should be carried out.
The planned activity under Task 2.5 was to investigate the apparent damage tolerance improvement associated with 3-D braided composites. However due to limited availability of material and, more importantly, time constraints on Participants, this Task was reluctantly not progressed.
Under Task 2.6 of the revised work programme visits were made by a number of Participants to British Airways' inspection and repair facility at Heathrow, and by ISD-Stuttgart to Per Udsen's manufacturing facility. These visits concluded that the sophisticated monitoring and modelling techniques investigated in other Tasks of this Concerted Action would not, at this time, be used in the workshop environment. The modelling systems, particularly, were more likely to be used during the design stage in order to optimise composite structures for damage tolerance and to reduce the amount of testing required.
Composites are being increasingly employed in a wide range of industries ranging from aerospace, marine and automotive industries to the production of hard hats skis etc, in the leisure industry. At present the maximum benefit derived from utilising composites is severely limited by the inability of the technical community to accurately and reliably produce/manufacture, calculate and predict the performance of composites under impact. Potentially, there are enormous economic benefits to be attained by reducing the unnecessarily high factors of safety and over-design.
The Concerted Action aims to encourage interaction and the transfer of technology between industry and universities/research centres through the exchange of ideals, experience and visits. It will identify key areas of significant economic potential and those requiring the formulation of standards/codes of practice.
Further, this Concerted Action will produce a series of newsletters, a database, a dissemination report and arrange several meetings at different levels on various research themes relevant to predictive in the analysis of impact on composites.