J.M. Molina-Aldareguia

Multiscale modeling of composite materials: a roadmap towards virtual testing.

A bottom-up, multiscale modeling approach is presented to carry out high-fidelity virtual mechanical tests of composite materials and structures. The strategy begins with the in situ measurement of the matrix and interface mechanical properties at the nanometer-micrometer range to build up a ladder of the numerical simulations, which take into account the relevant deformation and failure mechanisms at different length scales relevant to individual plies, laminates and components. The main features of each simulation step and the information transferred between length scales are described in detail as well as the current limitations and the areas for further development. Finally, the roadmap for the extension of the current strategy to include functional properties and processing into the simulation scheme is delineated.

Growth of Ti3SiC2 thin films by elemental target magnetron sputtering

Epitaxial Ti3SiC2(0001) thin films have been deposited by dc magnetron sputtering from three elemental targets of Ti, C, and Si onto MgO(111) and Al2O3(0001) substrates at temperatures of 800–900°C. This process allows composition control to synthesize Mn+1AXn (MAX) phases (M: early transition metal; A: A-group element; X: C and∕or N; n=1–3) including Ti4SiC3. Depositions on MgO(100) substrates yielding the Ti–Si–C MAX phases with (101¯5), as the preferred orientation. Samples grown at different substrate temperatures, studied by means of transmission electron microscopy and x-ray diffraction investigations, revealed the constraints of Ti3SiC2 nucleation due to kinetic limitations at substrate temperatures below 700°C. Instead, there is a competitive TiCx growth with Si segregation to form twin boundaries or Si substitutional incorporation in TiCx. Physical properties of the as-deposited single-crystal Ti3SiC2 films were determined. A low resistivity of 25μΩcm was measured. The Young’s modulus, ascertaine...

Nanostructure formation during deposition of TiN∕SiNx nanomultilayer films by reactive dual magnetron sputtering

Multilayer thin films consisting of titanium nitride (TiN) and silicon nitride (SiNx) layers with compositional modulation periodicities between 3.7 and 101.7 nm have been grown on silicon wafers u ...

Epitaxial stabilization of cubic-SiNx in TiN∕SiNx multilayers

The formation of cubic-phase Si Nx is demonstrated in TiNSi Nx multilayers deposited by reactive dual magnetron sputtering. Transmission electron microscopy examination shows a transition from epit ...

A novel biobased epoxy resin with high mechanical stiffness and low flammability: synthesis, characterization and properties

Exploring renewable biobased epoxy resins possessing intrinsic fire retardancy and high mechanical and thermal properties will greatly advance their potential to satisfy sustainability demands. Herein we develop a biobased route to synthesize a novel eugenol-based difunctional epoxy resin (TPEU-EP) with a full aromatic ester backbone. With 3,3′-diaminodiphenyl sulfone (33DDS) as the curing agent, TPEU-EP is compared with a standard bisphenol A epoxy resin (DGEBA) regarding their cure reactions and ultimate properties. The results show that TPEU-EP/33DDS expresses a higher reaction activation energy and a slower curing rate than DGEBA/33DDS. The isothermal cure reaction of TPEU-EP/33DDS is found to be autocatalytic. We accurately model the curing kinetics and elaborate on the related mechanisms based on the isoconversional analysis. The structure–property study reveals that TPEU-EP/33DDS manifests a 27%, 20% and 17% higher storage modulus (30 °C), Youngs modulus and hardness than DGEBA/33DDS, respectively. TPEU-EP/33DDS displays a high glass temperature (168.4 °C) and thermal stability (up to 300 °C), and shows a much higher damping than DGEBA/33DDS in the glassy state. Moreover, compared with DGEBA/33DDS, TPEU-EP/33DDS shows a 130% and 3.3 increase in char yield (in N2) and limiting oxygen index and a 68% and 40% decrease in the heat release rate and total heat release (microscale combustion test), respectively. Impressively, TPEU-EP/33DDS can self-extinguish in a vertical burning test, and the cone calorimeter test further confirms that TPEU-EP/33DDS has a much improved flame retardancy with a notably lowered smoke production. In brief, TPEU-EP possesses good intrinsic flame retardancy, low smoke production, and excellent mechanical properties, showing high promise for application. Our contribution will open a new avenue to develop sustainable high-performance flame-retardant epoxy resins.

Effect of Glass Fiber Hybridization on the Behavior Under Impact of Woven Carbon Fiber/Epoxy Laminates

The low-velocity impact behavior was studied in hybrid laminates manufactured by RTM with woven carbon and glass (S2) fabrics. Specimens with different thicknesses and glass fiber content (from 0 to 21 vol.%) were tested with impact energies in the range 30-245 J and the resulting deformation and fracture micromechanisms were studied using X-ray microtomography. The results of these analyses, together with those of the impact tests (maximum load and energy absorbed), were used to elucidate the role played by glass fiber hybridization on the fracture micromechanisms and on the overall laminate performance under low-velocity impact.

Optimum high temperature strength of two-dimensional nanocomposites

High-temperature nanoindentation was used to reveal nano-layer size effects on the hardness of two-dimensional metallic nanocomposites. We report the existence of a critical layer thickness at which strength achieves optimal thermal stability. Transmission electron microscopy and theoretical bicrystal calculations show that this optimum arises due to a transition from thermally activated glide within the layers to dislocation transmission across the layers. We demonstrate experimentally that the atomic-scale properties of the interfaces profoundly affect this critical transition. The strong implications are that interfaces can be tuned to achieve an optimum in high temperature strength in layered nanocomposite structures.

Phase stability tuning in the NbxZr1-xN thin-film system for large stacking fault density and enhanced mechanical strength

The phase stability of hexagonal WC-structure and cubic NaCl-structure 4d transition metal nitrides was calculated using first-principles density functional theory. It is predicted that there is a multiphase or polytypic region for the 4d transition metal nitrides with a valence electron concentration around 9.5 to 9.7 per formula unit. For verification, epitaxial NbxZr1−xN (0⩽x⩽1) was grown by reactive magnetron sputter deposition on MgO(001) substrates and analyzed with transmission electron microscopy (TEM) and x-ray diffraction. The defects observed in the films were threading dislocations due to nucleation and growth on the lattice-mismatched substrate and planar defects (stacking faults) parallel to the substrate surface. The highest defect density was found at the x=0.5 composition. The nanoindentation hardness of the films varied between 21GPa for the binary nitrides, and 26GPa for Nb0.5Zr0.5N. Unlike the cubic binary nitrides, no slip on the preferred ⟨11¯0⟩{110} slip system was observed. The inc...

High-temperature nanoindentation behavior of Al/SiC multilayers

Nanoscale Al/SiC composite laminates have unique properties, such as high strength, high toughness, and damage tolerance. In this article, the high-temperature nanoindentation response of Al/SiC nanolaminates is explored from room temperature up to 300°C. Selected nanoindentations were analyzed postmortem using focused ion beam and transmission electron microscopy to ascertain the microstructural changes and the deformation mechanisms operating at high temperature.

An experimental and numerical study of the influence of local effects on the application of the fibre push-in test

Several methods, such as pull-out, microbond and push-in/push-out tests, have been developed to test interfacial adhesion in composite materials. Some of them can only be applied to single-fibre matrix composites, like the microbond test, and others are difficult to perform on brittle fibres due to premature fracture of the fibre. Push-in tests, consisting of pushing the fibre with a micro- or nanoindenter on a bulk specimen, constitute a powerful technique that can be applied directly to composite laminates. However, the interfacial adhesion values obtained from different tests (microbond, push-in) often differ and even the results from one type of test are subjected to a large scatter. This might be due to the fact that the existing analytical solutions that are typically used to interpret the experimental data take into account the constraining effect of the surrounding fibres on a simplified manner. To study interfacial adhesion and the effect of the constraint of the neighbouring fibres, a micromechanical model of the push-in test was developed, coupled with experimental adhesion testing in a glass fibre-reinforced epoxy matrix composite. The model takes into account the interfacial fracture process by means of interface cohesive elements at the fibre–matrix interface and focusses on the study of the constraining effects due to the local configuration of the surrounding fibres.