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The outstanding challenge in solid mechanics: engineering structures subjected to extreme loading conditions

Periodic Reporting for period 2 - OUTCOME (The outstanding challenge in solid mechanics: engineering structures subjected to extreme loading conditions)

Reporting period: 2018-01-01 to 2019-12-31

We formed an international network composed of 3 academic beneficiaries and 2 industrial beneficiaries to train 8 early-stage researchers in what is referred to as an outstanding challenge in Solid Mechanics: developing novel solutions for the analysis and design of aerospace and civilian-security structures subjected to extreme loading conditions. Structural elements used in these sectors are frequently subjected to a large variety of unusually severe thermo-mechanical solicitations. New specific structural solutions are constantly developed to fulfill such requirements, which place these industrial sectors in the forefront of the technological innovation. Hereby, aerospace and security industries constitute the natural meeting point between academia and entrepreneurial fabric. A deep understanding of the response of structures under the aforementioned sharp solicitations is mandatory for design purposes. Unfortunately, not even today is easy to find researchers in the labor market with such specific understanding. Aerospace and security industries require highly-qualified technical staff capable of developing research and innovation within the framework of structural mechanics. The training and research program devised in OUTCOME has provided to 8 early-stage researchers with the specific technical background and soft skills required to tackle the challenging problems that aerospace and civilian-security industries face within the framework of structures engineering.
The early-stage researchers enrolled in OUTCOME developed ready-to-use analytical and numerical models, backed up with own experiments, to predict damage and failure in different types of engineering materials and structures subjected to extreme loading. Their research has yielded to several papers published in the most renowned journals of Solid Mechanics, and different tools, which include demonstrators and manufacturing techniques, which have been integrated into the services portfolio of the industrial beneficiaries. The 8 early-stage researchers obtained a PhD degree from their corresponding host institutions or academic partner, and they found jobs both in academia and industry. The success of the network was based on the design of an ambitious research program; in which all the early-stage researchers faced specific scientific problems with different experimental, numerical and analytical techniques; and a unique training program in which all the fellows were exposed to academic and industrial environments trough secondments and other networking activities. Namely, OUTCOME has organized two international symposia and several thematic sessions in international conferences, 2 industrial workshops, 1 technical course in the International Center of Mechanical Sciences and 2 non-technical courses devised to help the fellows to design the roadmap of their careers. All the research and training activities have been communicated to the general public trough different public engagement initiatives, and the project was included in the Common Dissemination Booster initiative to increase the impact of the network activities.
The 8 PhD projects of the early-stage researchers led to the following salient results:

ESR 1. A crystal plasticity model to analyze the effect of Triaxiality, Lode angle and crystal orientation in the mechanical response of metallic materials was developed. A key point was the generation of prescribed boundary conditions such that the external Triaxiality and Lode angle could be controlled considering periodicity and using a multipoint constraint subroutine. Numerical calculations were developed with different boundary conditions to study the mechanical response of single crystals with voids, bicrystals with voids at the grain boundaries and polycrystals with voids.

ESR 2. A constitutive model to describe the dynamic response of porous metallic materials under plane strain condition was developed. The approach consists of considering a cylindrical void of circular section embedded in a cylindrical shell made of viscoplastic matrix. The key point was to show that the length of the cylinder plays an important role in the macroscopic response of the material. Micro-inertia effects were investigated, considering various cases of loading conditions including spherical loading, plane stress loading and uniaxial loading paths. The analytical model was validated against numerical simulations.

ESR 3. A constitutive model to study material’s rate sensitivity on interfacial crack growth in brittle materials was developed and validated with experiments. The tests showed that a weak interface under static loading conditions can be transformed into a strong interface under dynamic loads, thus diverting the crack trajectory. The interaction of a dynamically loaded crack with different pre-existing flaws was also studied. While similar problems were previously investigated in the literature, it was shown that much of the experimental results concerning the effect of a pre-existing hole’s size are in fact a result of stress wave scattering from the hole, prior to any crack growth.

ESR 4. A two-scale damage model with distributed heat sources resulting from energy dissipation during rapid evolutions of microcracks in brittle materials has been developed. The model has been obtained by asymptotic homogenization, extending a procedure available in the literature to thermo-elastic media with dynamically propagating microcracks having heat sources at their tips. The model is implemented in a finite element code and numerical simulations of the Compact Compression Test showed good agreement with the experimental temperature evolutions.

ESR 5. Development of a perturbation model to assess the joint effect that inertia, strain rate, loading path, porosity, tension-compression asymmetry and anisotropy have on the onset and development of necking instabilities in ductile metals subjected to dynamic loading. The distinctive feature of this work is to consider material anisotropy, tension-compression asymmetry and porosity. The results of the analytical model were compared with finite element calculations and specific experiments that we performed for this task.

ESR 6. Formulation of a multi-scale dislocation-based constitutive model to define the thermomechanical response of metallic materials under dynamic loading. Based on the evolution of the microstructure during deformation, the model can predict the material response at the macroscopic level. The thermal evolution is estimated by considering the heat dissipation related with internal features of the material structure. This model will establish the base for future models for material instabilities, such as those considering shear localization.

ESR 7. Development of an operating system to measure surface imperfections and damage on aircraft structures. This procedure further developed to investigate the characteristics of fracture surfaces at a lower scale. To do so, an algorithm was created based on Deep Learning and Artificial Intelligence methods that can automatically perform topographic characterization of the fracture surface for ceramic materials.

ESR 8. Development of a methodology to extract the delamination energy of composite structures from peel tests. The methodology, which combines experimental data and modeling, has been applied to obtain new guidelines for the mechanical design of Printed Circuit Boards.