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Development of a methodology (test, measurement, analysis) to characterize the BEhaviour of composite structures under DYNamic loading

Periodic Reporting for period 1 - BEDYN (Development of a methodology (test, measurement, analysis) to characterize the BEhaviour of composite structures under DYNamic loading)

Reporting period: 2020-07-01 to 2021-12-31

When advanced polymer-based composite materials are to be used in aeronautical structural components, a design development program is generally initiated during which the performance of the structure is assessed prior to its use. Typically, the process of design starts with the analysis of a large set of simple small specimens and, when sufficient knowledge is acquired at this level, it is changed over to a more complex structure but carrying out fewer tests. This methodology is quite mature and well stablished for static and fatigue loads. However, for intermediate and high dynamic loading conditions, the methods are still under development and often limited to academic research levels, without any type of standardization.

During their service life, aerospace structures can be subjected to a variety of dynamic loading cases. Crash/impact is one of the most concerning cases due to its possible disastrous consequences. Impacts on aerospace structures can be produced by the accidental or deliberate hit of an object into aircraft. Hailstones, bird strikes, runaway debris, tyre fragments or even other fragments from the aircraft structure that could be ejected in case of an accident (i.e. uncontained rotor engine failure) are the main examples produced in the aerospace sector. Therefore, it is crucial to understand how the materials used in the aerospace sector behaves under dynamic loadings.
Composite materials may exhibit strain rate effects, therefore robust and industrial dedicated dynamic coupon and element level tests, analysis and modelling methods are then necessary to design and certify composite airframe structures. The analysis tools based on static formulations could be far away from the actual material and structural response, and hence a dedicated methodology is needed for dynamic loading states. This is what the proposed BEDYN project will deal with.

The aim of BEDYN project is to address a methodology to properly characterize the dynamic behaviour up to rupture of thermoset polymer-based composite structures submitted to dynamic loading. Different main objectives can be defined:
Ob. 1) Define a modelling approach for dynamic loading events, suited to industrial needs for emergency situations applications.
Ob. 2) Define dynamic tests, which include the definition of: specimens, test setups, and data reduction methods. Three different specimen levels are set: “coupon”, for characterizing basic properties of the composite material (ply), interlaminar (delamination) and adhesive interfaces; “element”, they include what can be understood as small size demonstrator (under this category the response of the flexural, notch effects and bearing will be analysed); “structure”, they are devoted for characterizing the behaviour at a subcomponent level under out-of-plane dynamic loads. In order to describe properly the possible dynamic effect in some material/structure behaviours, a quasi-static test campaign is also considered for any of the specimen levels accounted for.
Ob. 3) Define a calibration and validation process of the models.
Ob. 4) Demonstrate and evaluate the proposed methodology based on tests performed.

The BEDYN project will contribute towards the consolidation of the use of numerical simulation in the design phase of polymer-based composite structures under dynamic loading. The BEDYN project will address innovative technologies, allowing better product development thanks to an increased knowledge of the behaviour of composite materials under dynamic loading. Maturing and validation of technologies is a key aspect of integrating research in the development process of industrial activities and next generation aircrafts.
BEDYN project can be split into two work sets: experimental and numerical. For each set, different activities have been carried out until the current reporting period (M1-M18).

For experimental activities, the main work done is related to the definition of the tests to be performed. It includes the definition of the specimens, the test setups and appropriate data reduction methods, for both quasi-static and dynamic loading conditions. In some cases, the definition is based on works published in the literature and adapted, based on numerical simulations, to the consortium facilities. Other tests requested more research and new test methods are developed. In addition, other experimental activities performed are: manufacturing of specimens and test rigs, and execution of characterization tests mostly under quasi-static conditions and for basic material characterization (coupon level), including data reductions, reporting and analysis.

Regarding the numerical activities, the main activity performed is the literature review related to constitutive models that take into account the strain rate effect on the linked material properties (including stiffness, strengths and fracture toughness). Based on that review, the strategy for updating former constitutive models thought for quasi-static simulations, has been defined.
The current state of the art has been identified, but none of the activities has been concluded to quantify the progress beyond the state of the art. It is expected that once the dynamic test campaign is finished and the updated constitutive models are implemented, impacts will be clearly identified.