The goal of this project is to develop and characterise four pilot composites by introducing the following:
1.Improved constituents (new fibre surface treatments, toughened resins).
2.Establishing a reliable selection criteria for fibres and matrices properties to optimise interface bonding.
3.Development of transverse reinforced prepregs.
4.Use of thin thermoplastic resin films as interleaves during laminate production.
Pilot composites with improved damage tolerant behaviour have been developed and characterized by developing improved constituents, innovative reinforcements, and by having a better understanding of the damage growth under load. Up to now, the main conclusions are the following: Fibres Tenax HTA (6K), Tenax J STA (6K) and Tenax J IMS (12K) have been characterized (standard fibres and experimental fibres in their finishes and surface treatments). Fibres Tenax HTA (6K) and Tenax J IMS (12K) have been selected, under standard presentations, to be used as basic fibres for the rest of the Programme.
Matrices: various formulations have been characterized and 2 of them (SXA and SXC) selected to be characterized (on laminates) with reinforcement (tape, fabrics and 2.5 dimensional presentations with or without interleaves). Those matrices present attractive toughness properties (neat resin) compared to the reference (6376) with one of them (SXA) giving a 400% increase of the toughness parameter (810 Joules per square metre). Their dry Tg values are the same as the one of the reference. Tapes, fabrics and innovative 2.5 dimensional reinforcements have been developed. The outputs of feasibility investigations validate the classic autoclave selected process (laminates with or without interleave). Laminates have been tested (impact compression after impact) and a selection of laminates to be tested during the next step of the programme has been done: SXA and SXC for the matrices, Tenax HTA-6K and Tenax J IMS-12K for the fibres under different kinds of presentation (tapes, fabrics, 2.5 dimensional interleaves).
Because of severe design limitations due to poor damage tolerance of currently used composites, development of improved damage tolerant carbon fibre organic matrix composites has been identified as a key factor for widespread use of composites on aircraft primary structures. Research has been carried out in order to develop and characterize pilot composites that will use existing technology.
Fibres have been characterized and 2 of them were selected for further research: Tenax HTA (6K) and Tenax - J IMS (12K). Various formulations have been characterized and 2 of them (SXA and SXC) were selected to be characterized (on laminates) with reinforcement (tape, fabrics and 2,5 D presentations with or without interleaves). They present attractive toughness (delamination test) properties compared to the reference (6376) with one of them (SXA) giving a 400% increase in toughness (810 J m-2).
Tapes, fabrics, innovative 2,5 D reinforcement have been developed. The outputs of feasibility investigations validate the classic autoclave selected process (laminates with or without interleave). Laminates have been tested with respect to impact plus compression after impact (CAI) and a selection of some of them was performed: SXA and SXC for the matrices, Tenax HTA-6K and Tenax J IMS-12K for the fibres.
Damage characterization and modelling activities were performed. Models developed help composite materials choice regarding damage tolerance concept and are very useful for designers during a first analysis of the residual strength of the structure.
After extensive tests performed, it appeared that material HTA/SXA under unidirectional tape presentation showed a 25% increase of CAI property without any degradation of other critical properties such as opened hole tension strength, filled hole compression strength and glass transition temperature compared to the reference first generation HTA/6376 product. For this attractive material (HTA/SXA), an evaluation of its industrial development is under way.
The 2,5 D fabric texture requires some more optimization. Taking into account the weight penalty induced by the use of interleaf, the CAI performance has been increased by 6 to 7% with respect to the reference system (HTA/6376 without interleaf). This was accompanied by a drop of other properties such as opened hole tension.
Because of severe design limitations due to poor damage tolerance of currently used composites, development of improved damage tolerant carbon fibre-organic matrix composites has been identified as a key factor for widespread use of composites on aircraft primary structures. Independent works indicate that an increase of up to 30% of impacted resistance under compressive load is attainable by introducing new manufacturing concepts at each step of the laminate process with no need for major modifications in existing equipment.
During mechanical characterisation on impacted samples of pilot composites, proper instrumentation using acoustic emission technique will be developed in order to perform accurate damage growth characterisation.