Cohesive Strength Of Interface Research Articles

Explicit finite element analysis of failure behaviors of thermoplastic composites under transverse tension and shear

This paper studies failure behaviors of glass fiber/PC resin thermoplastic composites under transverse tension and shear loads using explicit finite element method. Microscopic long fiber/matrix composite representative volume elements with randomly distributed fibers and periodic boundary conditions are established by ABAQUS-PYTHON script language. The finite-deformation viscoplastic Gurson model is used for predicting the plastic deformation and void growth of thermoplastic glassy polymer matrix, which is implemented using ABAQUS-VUMAT explicit subroutine. The bilinear cohesive model with finite-thickness interface element is used to simulate the fiber/matrix debonding. Three important issues are explored: first, effects of the cohesive interface strength on the plastic deformation and void growth of composites are studied using three mesh models and three fiber distributions. Second, failure surface under different tensile and shear displacement ratios is obtained, compared with the predictions using the Hashin and Puck failure criteria developed mainly for thermoset composites. Finally, the influence of the plastic softening effect of matrix on the strain concentration or localization behaviors is discussed. Results show the tension/shear load ratio and the cohesive interface strength produce large effects on the void growth and failure surface of composites, but fiber distribution shows a small effect on transverse load-bearing ability of composites.

Multi-scale model of effects of roughness on the cohesive strength of self-assembled monolayers

Self-assembled monolayers (SAMs) are aggregates of small molecular chains that form highly ordered assemblies at the nanoscale. They are excellent contenders of molecular-level tailoring of interfaces because of the wide choice of terminal groups. Molecular dynamics (MD) simulations and experimental observations of spallation of two SAM-enhanced gold-film/fused silica-substrate interfaces have shown that the cohesive strength of SAM-enriched transfer-printed interfaces is strongly dependent on the choice of terminal groups. Though the MD results of perfectly ordered atomistic surfaces show the same qualitative trend as the experiments, they over-predict the interfacial cohesive strengths by a factor of about 50. Previous studies have revealed that the roughness of these interfaces may significantly impact their cohesive strength. In this manuscript, we perform a multiscale study to investigate the influence of surface roughness on cohesive strength of an interface between a Si/SAM substrate and a transfer-printed gold film. We approximate the film as a 2D deformable medium while the rough SAM-enhanced substrate is modeled using 2D harmonic functions with the cohesive interaction between the SAM and the film described by a simple exponential relation. Spallation is simulated on this system to evaluate the effective traction-separation response for the rough SAM-gold interface. Beyond the idealized harmonic interface, we extend our studies to real surface profiles obtained by AFM. We demonstrate how interfacial roughness can reduce the cohesive strength of the SAM-enhanced interface by more than an order of magnitude.

Role of microstructure on fracture of dentin

Dentin possesses unique hierarchical structure, which has a significant influence on the mechanical properties. Understanding the relationship between structure and mechanical properties of dentin is essential for preventing and curing oral diseases, as well as, potentially for developing man-made engineering materials with superior mechanical performance. In this study, the effect of the two-layered structure, where hard peritubular dentin (PTD) containing dentin tubules are embedded in soft intertubular dentin (ITD), on the fracture behavior of dentin is investigated. A numerical model is developed, in which PTD cracking, ITD cracking and the debonding of the interface between PTD and ITD are all taken into account. Numerical simulations reveal that PTD fracture and interface debonding are the major failure mechanisms, which are consistent with experimental observation. It is identified that the cohesive strength and critical separation of interface are the key parameters controlling which of the mechanisms is active. The low cohesive strength of interface and small critical separation of interface can lead to interface debonding, while the large cohesive strength and critical separation give rise to PTD fracture. In addition, it is found that large volume fraction of dentin tubules and small volume fraction of PTD can enhance the toughness of dentin, which provides a new insight into the degraded mechanical properties of old dentin.

Atomistically derived metal–ceramic interfaces cohesive law based on the van der Waals force

The cohesive interface strength of metal–ceramic system is the key factor for these composites. In this paper, a cohesive law for nanograined metal–ceramic interfaces based on the van der Waals force was established, while the metal and ceramic grains may not be well bonded. The theoretical equations for calculating tensile and shear cohesive stresses are derived in terms of the volume density of metal and ceramic atoms, grain size, as well as the parameters in the van der Waals force. This law is very helpful for promoting the understanding on the interface strength of both layer and particles reinforced metal–ceramic composite materials.

Polymer Adhesion vs Substrate Receptor Group Density

The effect of substrate receptor group density on polymer adhesion was investigated. Model substrates with varying −NH2 density on Al2O3 were prepared by a self-assembly of mixed amine-terminated silanes and methyl-terminated silanes. The model polymer was synthesized through a high-pressure carboxylation of polybutadiene. The polymer adhesion was determined by the fracture energy of the polymer−solid interfaces, GIC, from a T-peel test. The strong acid−base interactions between the polymer sticker groups (−COOH) and the substrate receptor groups (−NH2) at the interface caused an interesting fracture energy variation in the range of annealing times examined. At short time, GIC increased as the density of substrate receptor groups increased. However, at long time, GIC increased up to 30% receptor group coverage and then decreased with increasing coverage. A maximum in GIC occurred because the adhesive interface strength increased between the solid surface and the adsorbed chains, and simultaneously the cohesive interface strength decreased between the adsorbed chains and the neighboring free chains with increasing density. Failure occurred at the weaker of the two interface strengths. In this paper, it was shown that the polymer adhesion was not a monotonic function of the surface energetics.

Role of Interface Structure on Mechanical Properties of Fluorine-Doped Si<sub>3</sub>N<sub>4</sub>-SiC Ceramics

Changes in interface chemistry and, accordingly, in interface structure of a Si3N4-SiC ceramic, due to the presence of fluorine, was found to markedly affect the mechanical response of the material. Owing to a lowered cohesive interface strength, as a consequence of fluorine segregation at grain and phase boundaries, an increased fraction of intergranular fracture was monitored upon crack propagation. Moreover, in comparison to undoped specimens, fluorine-doped samples revealed markedly higher creep rates, which are related to a decrease in the apparent grain-boundary viscosity. The experimental results allow a correlation between the macroscopic mechanical properties and the atomic interfacial glass structure. A glass structure model is presented, which emphasizes the influence of broken (non-bridging) bonds on the bulk mechanical performance of Si3N4-SiC-based composites.

Structure and Chemistry of Interfaces in Si<sub>3</sub>N<sub>4</sub> Ceramics Studied by Transmission Electron Microscopy

Interfaces in liquid-phase sintered Si3N4 materials were investigated employing both analytical and high-resolution transmission electron microscopy techniques (AEM, HREM). Owing to the sintering process that involves the addition of metal oxides to the Si3N4-starting powder, densified materials are composed of Si3N4-grains and amorphous as well as partly crystalline secondary phases. Most Si3N4 ceramics contain continuous, thin interlayers of residual glass along grain boundaries. HREM studies in conjunction with electron energy-loss spectroscopy (EELS) focused on the characterization of these grain-boundary films. A comparison between the diffuse dark field, the Fresnel fringe and the high-resolution lattice imaging technique revealed the latter technique to be most accurate in order to determine intergranular film width quantitatively. Depending on the interface composition, HREM imaging revealed conspicuous differences in film thickness. However, with a given batch composition each as sintered material is characterized by a constant film width within ±0.1nm. Crystallization of secondary phases can alter the interface chemistry and thus change intergranular film thickness. The latter result is in line with the variation of interfacial film widths before and after oxidation. Owing to cation outward diffusion, a thinning of the interlayer was observed. In contrast, with increasing impurity-cation concentration at the interface, first a thinning and furtheron a widening of the intergranular film was observed. Si3N4 materials with only SiO2 present at interfaces showed a film thickness of 1.0nm, independent of glass volume fraction. Apart from cations, the influence of anion segregation at Si3N4-grain boundaries was studied. Fluorine was found to markedly affect the mechanical response of the material. Owing to a lowered cohesive interface strength, as a consequence of F-segregation at grain boundaries, intergranular fracture was predominantly monitored. Moreover, in comparison to undoped specimens, anion-doped samples revealed markedly higher creep rates, which are related to a decrease in the apparent grain-boundary viscosity, as elaborated from internal friction measurements.In addition to the characterization of internal interfaces at room temperature, microstructures at high service temperatures were studied by rapid cooling Si3N4 materials from high temperatures. Quenching a MgO-doped Si3N4 from 1350°C produced no observable variation in the grain-boundary film thickness. A substantial increase and a non-uniform width of the amorphous films was, however, observed when rapidly cooling from temperatures of about 1420°C. Specimens quenched from higher temperatures or longer residence times above the eutectic temperature again revealed an equilibrium thickness that was slightly wider compared to the film widths observed in the material slowly cooled down to room temperature.By studying a MgO-doped Si3N4 that was exposed for about 14, 000h to 1100°C, the question was addressed as to whether internal grain-boundary films are actually stable and can sustain a long-term exposure to high testing temperatures. It is shown that, apart from other microstructural changes induced during high-temperature exposure, the grain-boundary structure was fundamentally altered and depleted grain boundaries were formed. Moreover, the characteristics of interfaces, observed in Si3N4/SiC microstructures obtained by the pyrolysis and subsequent crystallization of organometallic precursors, are briefly discussed. Here, similar to the long-term exposure experiment, complex interface structures without the presence of continuous grain-boundary films were observed. In brief, the content of this