This projects aimed at substantially improving the understanding of the role played by environmental factors on the long term hydric fatigue endurance of fibre reinforced composites. This was achieved by an extensive experimental campaign and the development of physically robust finite element based modelling techniques. This allowed the prediction of the onset of fatigue induced damage in laminates subjected to arbitrary temperature and humidity cycles.
Such a modelling capability was the key for establishing equivalence factors between the actual conditions in which composite structural elements are required to operate and the characterisation environment employed in the laboratory.
Fibre-reinforced composites are instrumental for achieving a carbon neutral aviation. State-of-the-art composite airlines, such as the A350, burn approximately 20% less fuel than the previous generation. This is due to the weight savings achieved at airframe level, as well as by the adoption of fuel lean engines. However, the fuel efficiency of current high bypass ratio turbofans is largely due to the adoption of lightweight carbon-fibre composites in the fan system. Hence the adoption of composites has strongly benefitted both the airframe structures and the propulsive system.
The current design philosophy for the design of composites aero-structures is based upon a “no-growth” approach. This implies that composites in service may suffer from damage, for example due to impact, but the resulting flaws are not allowed to propagate into defects that may comprise the overall load-carrying capability. The “no-growth” paradigm is substantiated via extensive material and structural characterisations, following a “pyramid of testing” approach. Mechanical tests are therefore carried out at increasing level of complexity, starting from material coupons and progressing to components, elements and, finally, full airframes. The key objective of these characterisations is to obtain “design allowables”, i.e. values of stresses and strains that must not be exceeded in service in order to meet the “no growth” criterion.
Environmental effects (i.e. temperature and humidity) strongly affect the mechanical performance of materials, hence the structural reliability in service. The definition of robust design allowables requires understanding and quantifying the effect of temperature and humidity on the mechanical performance of composites. Temperature affects the visco-elastic-plastic response of polymer matrices in fibre-reinforced composites. Increasing temperature promotes polymer creep and promotes an increase ductile behaviour of composite matrices. On the other hand, sub-zero temperatures tend to cause an embrittlement of the matrix, with associated micro-cracking. Humidity and the consequent moisture uptake cause swelling of the laminates and cause degradation of the fibre/matrix interface. Hence both temperature and humidity significantly affect the micro-structural behaviour of composites and are detrimental for long-term mechanical performance. In the context of “no growth” design, the role of environmental effects needed to be accounted for when identifying “design allowables”. Historically, “hot & wet” conditions have been considered the most detrimental for the mechanical performance of composites. However, composite structures in service are exposed to variable temperature and humidity. It is therefore crucial to establish whether “environmental cycles” may accelerate the damage accumulation for composites in service. A typical civil airliner cruises at 35,000 ft at temperatures close to -50°C with negligible levels of humidity. On the other hand, landing at a tropical destination may expose the airframe to temperatures in excess of 50°C and 100% relative humidity. The actual severity of the environmental aging experienced by a composite aircraft is therefore a strong function of its operative history. Hence, “hot & wet” conditions do not necessarily constitute the worst-case scenario that has to be mitigated in design. It is necessary to consider the full possible environmental spectrum associated with in-service conditions. This implies the need for reliable methods to predict how damage evolves with varying environmental conditions.
This project had investigated the synergistic role of temperature and humidity on the mechanical strength of fibre-reinforced composites. Mechanical testing at different temperatures and moisture content provides “isolated” data points for finite sets of conditions. Albeit informative, these tests cannot cover the full spectrum of environmental scenarios experienced by an aircraft in service. Hence, the key objective of this project was to develop theoretical approaches and numerical tools that will allow extrapolating “design allowables” for arbitrary operational conditions from a sparse set of experimental data. The method applied to derive the “design allowables” is based on the time-temperature-humidity shift principle. This states that that the effects of temperature and humidity are equivalent to a scaling of the actual service life. A representative curve of the material strength as a function of time, also known as a master curve, can be obtained from tests at reference temperature and humidity conditions. The choice of the reference state is arbitrary, but in this research we considered 20°C temperature and 0% moisture content, i.e. nominally “dry” material. The actual strength of the material after a given time in service at a different temperature and moisture content level is then obtained from the master curve just by stretching or contracting the time axis. The entity of the deformation of the time axis is identified by creep tests performed on small coupons of material at prescribed set of temperature and humidity. The advantage of this approach is that all the matrix dominated properties scale according to the master curve generated by the creep tests. There is no need for performing tests at every possible temperature and humidity on coupons having different configurations depending on the mechanical strength property that is considered. A much smaller set of tests is generally performed with the aim of validating the time-temperature-shift principle, so that a smooth mapping relating different environmental conditions is obtained from a relatively small set of data. The advantage of this approach is that the operative life of a component can be estimated by accumulating the time fractions spent in each service condition in a linear fashion, not dissimilarly from what done when using the Miner’s rule in fatigue analysis. Moreover, the amount of testing that is required for substantiating a “no growth” design in presence of environmental effects is dramatically reduced, and this is highly beneficial in terms of costs.
In this project, the time-temperature-shift principle has been validated for a carbon fire-reinforced epoxy material, namely Cytek Cycom 977-2. This material is extensively employed in primary aeronautical structures, such as fuselages and wings.
The creep response of the material has been characterised at temperatures below the grass transition (180°C) via tests performed in a dynamical mechanical analyser. Master curve of the creep storage modulus have been obtained for both dry and fully saturated specimens. The strength of the material in tension and compression has been characterised via ASTM standard coupons, considering a wide range of service temperatures, for both dry and fully saturated coupons. It has been demonstrated that the master curves obtained for the storage modulus allow predicting the strength of the material in the full range of environmental conditions considered. These results have also been extended to the prediction of the effect of the strain-rate on the strength, which is extremely important for design purposes. The key outcome of this research is that the visco-elastic response of the material has a massive impact on the strength and that environmental effects strongly influence the visco-elastic response. It has also been observed that the shift factor associated with temperature and humidity can be suitably described by characteristic activation energies, which are inherent material properties. The residual strength properties of the fully saturated material have been found to be extremely low, particularly in terms of transverse tension. This poses significant challenges for the design of composite structures in service and highlights the importance of predicting/monitoring the actual moisture content in service, as well as inhibiting the moisture ingress in composite materials via suitable surface protections (i.e. paints).
Future work for assessing the real hazard posed by environmental factors on the durability of composites in service will have to be focussed on the characterisation/prediction of the effects of cyclic temperature and moisture. This project has proven the viability of the time-temperature-humidity shift principles for steady environmental conditions, but it remains to be demonstrated whether the same approach holds for materials that experience cyclic environmental regimes. The data gathered and the analysis methods developed in this project provide a rationale scientific basis for the extension of the time-temperature-humidity shift method to variable temperature/humidity scenarios.