Development of a Degradation Model for the Collapse Analysis of Composite Aerospace Structures

For stiffened structures in compression the most critical damage mechanism leading to structural collapse is delamination or adhesive disbonding between the skin and stiffener. This paper presents the development of a numerical approach capable of simulating interlaminar crack growth in composite structures as a representation of this damage mecha-nism. A degradation methodology was proposed using shell layers connected at the nodes by user-defined multiple point constraints (MPCs), and then controlling the properties of these MPCs to simulate the initiation and propagation of delamination and disbonding. A fracture mechanics approach based on the Virtual Crack Closure Technique (VCCT) is used to detect growth at the delamination front. Numerical predictions using the degradation methodology were compared to experimental results for double cantilever beam (DCB) specimens to dem-onstrate the effectiveness of the current approach. Future development will focus on address-ing the apparent conservatism of the VCCT approach, and extending the application of the method to other specimen types and stiffened structures representative of composite fuselage designs. This work is part of the European Commission Project COCOMAT (Improved MA-Terial Exploitation at Safe Design of COmposite Airframe Structures by Accurate Simulation of COllapse), an ongoing four-year project that aims to exploit the large strength reserves of composite aerospace structures through more accurate prediction of collapse

Effects of bondline flaws on the damage tolerance of composite scarf joints

Scarf repairs to aircraft structures need to sustain design ultimate load in the presence of flaws due to manufacturing and impact by foreign objects, in order to demonstrate compliance with airworthiness regulations. This paper presents an investigation into the effect of disbonds on the load-carrying capacity of adhesively bonded scarf joints. Experiments were conducted on scarf joints containing disbonds of varying lengths. The results showed that the load-carrying capacity of scarf joints decreases with the size of the bondline flaw at a faster rate than the reduction in the effective bond area. Fractographic analysis showed that the fracture occurred in the composite matrix adjacent to the adhesive-composite interface, at a distance equal to a small fraction of ply thickness. Computational analyses using the virtual crack closure technique (VCCT) and the cohesive zone model (CZM) confirmed these experimental observations: model predictions using composite material properties were in better correlation with experimental results than those using adhesive properties. Furthermore, CZM is capable of predicting the effects of flaws of all sized being considered, while the VCCT model is only applicable to joints containing flaws greater than a certain size

Effect of disbonds on the fatigue endurance of composite scarf joints

The certification of scarf repairs requires that the repair is capable of handling flight loads in the presence of disbonds. This paper presents a study of the fatigue disbond growth behaviour of scarf joints. By determining the strain energy release rates of a disbond in a scarf joint subjected to a unit load, a predictive model based on linear elastic fracture mechanics is presented, which is shown to correlate well with experimental results. This method offers a promising technique for predicting the fatigue life of composite scarf joints with disbonds

Investigation into optimised composite scarf repairs with practical constraints

This paper focuses on using a genetic algorithm to optimise composite scarf repairs subject to tensile loading by minimisng the normalised shear stress in the adhesive. Several parameters are modified to investigate their role in the effectiveness of the optimisation, including constraints to the optimised stacking sequence to increase the practicality of the design. These include enforcing a symmetric and stiffness-matched optimised layup to the parent structure. The studies show that additional constraints do reduce the effectiveness of the optimisation; but still provide adequate reduction in the maximum normalised adhesive shear stress

Efecto del método de secado del mucílago de chía (Salvia hispanica L.) sobre sus propiedades fisicoquímicas y funcionales

Se analizó comparativamente el efecto del método de secado del mucílago de chía sobre sus propiedades fisicoquímicas y funcionales. El mucílago se extrajo mediante exudación con agitación mecánica, secado mediante liofilización (MI) y estufa por circulación de aire caliente (MII) y separación por tamizado. El MI permitió obtener un mayor rendimiento de extracción que el MII (12,4 y 3,4% base seca b.s.-, respectivamente), alcanzando en una segunda etapa de extracción un 6,8 (MI) y un 3,5 (MII) % (b.s.). El mucílago obtenido según MI presentó una textura más suave, esponjosa y liviana y un color beige más claro y opaco que el obtenido por el MII. Los diferentes métodos de secado no afectaron significativamente la humedad de los mucílagos (p>0,05). Ambos mucílagos evidenciaron altos porcentajes de proteínas (16,4 y 14,7% b.s.) y bajos en fibra cruda (7,9 y 9,3% b.s., para MI y MII, respectivamente). El MI permitió generar un mucílago con mejores propiedades relacionadas con componentes lipídicos, mientras que el MII un mucílago con mejor capacidad de retención de agua. En general, el método de secado empleado afectó las propiedades fisicoquímicas y funcionales de los mucílagos, observándose algunas características particulares que los diferenciaron entre sí.Centro de Investigación y Desarrollo en Criotecnología de Alimento

Efecto del método de secado del mucílago de chía (Salvia hispanica L.) sobre sus propiedades fisicoquímicas y funcionales

Se analizó comparativamente el efecto del método de secado del mucílago de chía sobre sus propiedades fisicoquímicas y funcionales. El mucílago se extrajo mediante exudación con agitación mecánica, secado mediante liofilización (MI) y estufa por circulación de aire caliente (MII) y separación por tamizado. El MI permitió obtener un mayor rendimiento de extracción que el MII (12,4 y 3,4% base seca b.s.-, respectivamente), alcanzando en una segunda etapa de extracción un 6,8 (MI) y un 3,5 (MII) % (b.s.). El mucílago obtenido según MI presentó una textura más suave, esponjosa y liviana y un color beige más claro y opaco que el obtenido por el MII. Los diferentes métodos de secado no afectaron significativamente la humedad de los mucílagos (p>0,05). Ambos mucílagos evidenciaron altos porcentajes de proteínas (16,4 y 14,7% b.s.) y bajos en fibra cruda (7,9 y 9,3% b.s., para MI y MII, respectivamente). El MI permitió generar un mucílago con mejores propiedades relacionadas con componentes lipídicos, mientras que el MII un mucílago con mejor capacidad de retención de agua. En general, el método de secado empleado afectó las propiedades fisicoquímicas y funcionales de los mucílagos, observándose algunas características particulares que los diferenciaron entre sí.Centro de Investigación y Desarrollo en Criotecnología de Alimento

Loading, support and geometry effects for pin-reinforced hybrid metal-composite joints

Reliable joining technologies are increasingly required for multi-material lightweight structures. For metal-composite joints, there is significant potential to integrate through-thickness pins onto the metal surface to improve bond strength. We investigated Selective Laser Melting manufactured Ti-64 adherends with integrated pins bonded to carbon fibre-reinforced polymer composite, and characterised joint performance from pure tension to shear-dominated pin loading. Both single pin and multi-pin double cantilever beam specimens were examined and correlated using experimental and finite element methods. Adherend support conditions affected the single pin pull-out process and related energy absorption up to 35% for all pin offset angles. The pin alignment with respect to the crack direction and fibre angle had little effect on joint performance. Pins with grooved surface features further increased energy absorption by 60% compared to smooth cylindrical pins. This work adds significant insight into pin-reinforced hybrid metal-composite joints and their performance and optimisation in realistic structural scenarios

Improved analysis of the scalar and vector form factors of kaon semileptonic decays with Nf = 2 twisted-mass fermions

We investigate the vector and scalar form factors relevant for Kl3 semileptonic decays using maximally twisted-mass fermions with two flavors of dynamical quarks (Nf = 2). The simulations cover pion masses as light as 260 MeV and four values of the lattice spacing, ranging from ~0.05 up to ~0.1 fm, which allow to compute directly, for the first time, the continuum limit for the vector form factor at zero-momentum transfer, f_+(0). The preliminary result is f_+(0) = 0.9544(68), where the error is statistical only. We also extrapolate both form factors to the physical point and study their momentum dependence. Our results are in good agreement with those obtained from a dispersion analyses of the experimental data. Together with the form factors, we analyze the ratio of the leptonic decay constants f_K / f_pi, by imposing the constraint coming from the Callan-Treiman theorem, obtaining at the physical point f_K / f_pi = 1.190(8). Combining our results for f_+(0) and f_K / f_pi with the experimental measurements of the leptonic and semilpetonic decay rates, and using the determination of |V_ud| from nuclear beta decays, we determine the values of the Cabibbo angle |V_us| from both Kl3 and Kl2 decays, obtaining |V_us|^{Kl3} = 0.2266(17) and $|V_us|^{Kl2} = 0.2258(16)