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EXTREME Dynamic Loading - Pushing the Boundaries of Aerospace Composite Material Structures

Periodic Reporting for period 3 - EXTREME (EXTREME Dynamic Loading - Pushing the Boundaries of Aerospace Composite Material Structures)

Reporting period: 2018-09-01 to 2019-08-31

Composite materials play a fundamental role for current and future aircraft structures to improve weight, fuel efficiency, reduce CO2 emissions and certification costs. However, the vulnerability of composite structures to localised, dynamic, and unexpected loads such as blade off events or foreign object impact may result in unpredictable, complex, localised damage and a loss of post-impact residual strength.

To overcome these aforementioned challenges, and associated cost implications for the aeronautical industry, this project brings together leading researchers across Europe to develop novel material characterisation methods and in-situ measurement techniques, advanced simulation methods for the design and manufacture of aerospace composite structures under EXTREME dynamic loadings.

The main objectives of the project were to develop:
a. Improved material characterisation techniques allowing for development of new and improved material models, and for damage assessment during and after extreme events i.e blade off events or foreign object damage (hail, runway debris, bird strike etc.)
b. Advanced integrated experimental and numerical procedures and guidelines in support of design and certification of aeronautical structures
c. Smart impact sensing concepts under extreme dynamic loading
i. To reconstruct and warning of occurrence of extreme dynamic events and associated effects
ii. To measure failure parameters as occurs to feed new material models
d. Novel and more accurate multiscale and multilevel simulation tools.
The work carried out involved 1) Manufacturing 2) Material characterisation 3) Development of simulation tools 4) Development of measurement tools.
Several materials were considered for testing and analysis under extreme loadings. Both commercial and new composite material samples were manufactured. A new resin system with a more favourable absorbing energy behaviour compared to a standard aeronautical resin was designed. The use of basalt-fiber composite material to enhance the impact resistance of A/C structures was proposed. Samples and large scale structures were manufactured, successfully tested under extreme dynamic loads.
To characterise these materials, it was necessary to optimize the existing and develop new dynamic material characterisation methods for composite materials. To support the design of future aerospace structures under extreme dynamic loading, significant effort was put in developing material models that can capture how the material breaks, influence of damage on material symmetries, shock wave formation and propagation. For this, rigorous development of a physically based damage model allowing modelling of progressive damage in 3D including damage mode differentiation was performed. Also, meso-scale finite-element based models capable of predicting the failure of composite materials were developed. These models can take into account: non-linear, rate dependent behaviour of the constituents (matrix, fibers/yarns), failure of both the interface between the constituents and the constituents themselves. Composite materials that can be modelled are: unidirectional composites, 2D, 2.5D and 3D woven or braided composites with different fibre architectures. Dedicated algorithms were developed to handle large deformation, complicated failure modes in the following solver Digimat-FE embedded solver, and LS-Dyna. The algorithms for damage evolution within the constituents were fine tuned to optimize the computational time. A number of advances were made in the spatial discretisation development of a novel method for reduction of mesh dependency and treatment of localisation of damage, development of coupled FE-SPH code for modelling impact problems, where a part of the structure is modelled with finite elements and part of the model with the SPH.

To capture extreme dynamic events and understand how the material and structures behave during the impact and associated post-impact damage, several NDT and SHM techniques were developed to detect large extreme loading and post loading caused damages.
The main progress beyond the state of the art and associated impact are summarised below:

1) A new resin system was designed with nanofiller for improved energy absorption. Then a new use of basalt fiber for impact absorptions was suggested with convincing results.
2) Developed new sample geometries, test setups and measurement techniques to accurately characterize the composite materials under high dynamic loading.
3) A new regularisation technique for reducing and removal of mesh dependency was developed to balance effects of the heterogeneous microstructure on local continua and keep the boundary value problem of softening (damaged) continua well-posed. It is not commercially available and it was implemented within the software DYNA3D. Coupling SPH to FE could lead to better modelling hence designing and understanding of composite materials under extreme loading. This will lead to lighter structures and CO2 emissions.
4) New damage and meso-scale models were developed to describe materials behaviour under dynamic loading. The material model, able to describe shock wave formation and propagation in composite with long fibre reinforcement coupled to a physically based damage model, is not currently available in any commercial software and validation demonstrated that it is well above the current state of the art. The new material models would allow better design of aeronautical structures and lead to a new “right at first time” design philosophy and could extend the working boundary of composite materials.
5) Full field optical strain measuring instruments (shearography and high speed 3D DIC instruments) were developed to experimentally measure surface strain components at a very high-temporal (μs) and spatial resolution (sub-millimetre). These experimental results will improve material characterization techniques allowing for development of new and improved material models.
6) Development of smart impact sensing concept based on high-speed fibre-optic sensors and piezosensor networks makes a new level of in situ monitoring of the extreme dynamic loading feasible. The “Supergator” FBG interrogator developed by TFT reaches beyond state of the art performance with 1Mhz sampling rate and resolution below 1.0 pm at a dynamic strain range more than 5%. This is possible due to a novel combination of a spectrometer and an interferometer approaches in a single photonic integrated circuit, which provides significant advantages in size and cost of the interrogator. Integrated sensors and the peripherals as part of SHM concepts capable of sensing impact loadings will provide a valuable contribution for the involved aircraft manufacturers towards more efficient and more intelligent structural components.
7) A new impact detection system was developed capable of localising the extreme event location and measure the force applied during the impact by a foreign body. This was demonstrated at TRL6.
8) Development of algorithm for linear and nonlinear phased array 3D imaging of damage in impacted samples were performed. 3D volume measurement techniques for post event damage characterisation indirectly contribute to safety of the air transport industry. More accurate and reliable assessment of impact damages can be coupled with remaining life predicting tools and measures for repair organisation.
Gator is a spectrometer based commercially available interrogator developed by TFT