Debonding Conditions Research Articles

Effect of interfacial debonding on the failure behavior in a fiber-reinforced composite subjected to transverse tension

Abstract By using computational micromechanics, macroscopic stress–strain curves for a glass/epoxy lamina subjected to transverse tension were determined in this paper. To compute stress for given strain, a finite element model of a three-phased unit cell with the hexagonal symmetry was employed. Mixed mode debonding conditions between reinforcement and matrix were modeled by a bilinear cohesive law. A stress transfer between matrix and fiber was simulated by a inhomogeneous interphase. A detailed analysis of a debonding growth was also presented. Parametrical studies showed that a choice of values for cohesive parameters as well as debonding locations do have an influence on the macroscopic response of material. An ability of the proposed model to simulate the softening behavior of the material under transverse tension and to predict final failure was demonstrated.

Effects of different debonding techniques on the debonding forces and failure modes of ceramic brackets in simulated clinical set-ups

The objectives of this study were to evaluate the effects of different debonding techniques on the in-vitro mean debonding forces and failure modes of ceramic brackets bonded to enamel with clinically simulated setups. Three kinds of ceramic brackets (Clarity; 3M Unitek, Monrovia, Calif; Inspire and Inspire Ice; Ormco, Orange, Calif) were bonded to extracted premolars with the same bonding system. Thirty ceramic brackets, 10 of each type, were removed by hand; 60 ceramic brackets, 20 of each type, were tested on a universal testing machine with the pliers according to the manufacturers' recommendations. To simulate clinical debonding conditions, specially designed setups were used to debond the ceramic brackets. Debonding forces and failure modes were investigated. Fractographic evaluations were performed by using scanning electron microscopy. Most brackets failed at the bracket-adhesive interface. Cohesive bracket fractures were noted in all 3 types of ceramic brackets (debonded by hand: 70% of Inspire, 20% of Inspire Ice, and 10% of Clarity; debonded by machine: 75% of Inspire, 30% of Inspire Ice, and 25% of Clarity). The cohesive ceramic fractures of the Clarity brackets were located at the junction between the wings and the body, and at the slot. However, for the Inspire and the Inspire Ice brackets, the cohesive ceramic fractures were located at the occlusal aspect of the base. The mean debonding forces of Inspire, Inspire Ice, and Clarity brackets were 25.72 +/- 11.98, 17.92 +/- 5.03, and 76.89 +/- 23.47 N, respectively. No enamel damage was found after the brackets were removed. The results of the failure modes showed that the new designs with a ball reduction band in the Inspire Ice bracket and the vertical debonding slot in the Clarity bracket significantly reduced the risk of ceramic bracket fracture during debonding. The force required to debond the Inspire Ice bracket was significantly lower than that of the Inspire bracket.

Macro- to Atomic-Scale Tailoring of Si<sub>3</sub>N<sub>4</sub> Ceramics to Enhance Properties

Silicon nitride ceramics are finding uses in numerous engineering applications because of their tendency to form whisker-like microstructures that can overcome the inherent brittle nature of ceramics. Studies now establish the underlying microscopic and atomic-scale principles for engineering a tough, strong ceramic. The theoretical predictions are confirmed by macroscopic observations and atomic level characterization of preferential segregation at the interfaces between the grains and the continuous nanometer thick amorphous intergranular film (IGF). Two interrelated factors must be controlled for this to occur including the generation of the elongated reinforcing grains during sintering and debonding of the interfaces between the reinforcing grains and the matrix. The reinforcing grains can be controlled by (1) seeding with beta particles and (2) the chemistry of the additives, which also can influence the interfacial debonding conditions. In addition to modifying the morphology of the reinforcing grains, it now appears that the combination of preferential segregation and strong bonding of the additives (e.g., the rare earths, RE) to the prism planes can also result in sufficiently weakens the bond of the interface with the IGF to promote debonding. Thus atomic-scale engineering may allow us to gain further enhancements in fracture properties. This new knowledge will enable true atomic-level engineering to be joined with microscale tailoring to develop the advanced ceramics that will be required for more efficient engines, new electronic device architectures and composites.

エネルギ解放率に基づく熱応力および引張応力下での単繊維複合材料の破壊基準

A crack terminating at an interface in fiber reinforced composites may advance by either penetrating through the interface or deflecting into the interface. To describe such a phenomenon, the energy release rate based on the interfacial mechanics is used in this work as the fracture criterion. The energy release rates for the crack deflecting into the interface and the crack penetrating the interface depend on the branch crack length Δa, which is arbitrarily small compared to the length of the main crack. When loading condition is a tensile load, the ratio of those energy release rates is independent of Δa. However, the ratio depends on Δa in case of thermal loading condition. A relation between the ratio and the stiffness ratio of the components (fiber and matrix) is investigated in cases of two loading conditions, and the debonding conditions are shown. The results can be used for determination of the range of interface toughness relative to fiber one, to ensure the cracks to be deflected into the interface for prevention of catastrophic fracture.

Debonding behavior between β-Si 3N 4 whiskers and oxynitride glasses with or without an epitaxial β-SiAlON interfacial layer

In order to gain insight on the influence of intergranular glass on the fracture toughness of silicon nitride, the debonding behavior of the interface between the prismatic faces of β-Si 3N 4 whiskers and oxynitride glasses was investigated in model systems based on various Si–(Al)–Y(Ln)–O–N (Ln: rare-earth) oxynitride glasses. It was found that while the interfacial debonding strength increased when an epitaxial β′-SiAlON layer grew on the β-Si 3N 4 whiskers, the critical angle for debonding was lowered with increasing Al and O concentrations in the SiAlON layer. Only in the absence of a SiAlON epitaxial layer, were debonding conditions altered by residual stresses imposed on the interface due to thermal–mechanical mismatch. A possible explanation for the effect of SiAlON formation and its composition on the debonding behavior is suggested by first-principles atomic cluster calculations. It is concluded that by tailoring the densification additives and hence the chemistry of the intergranular glass, it is possible to improve the fracture resistance of silicon nitride.

Non-linear numerical study of the single-fiber fragmentation test. Part II: A parametric study

A three phase, non-linear finite element analysis is applied to investigate the stress transfer phenomenon in the single-fiber fragmentation test of a model composite. The effect of fiber properties, interphase properties and thickness on the stress distribution in the vicinity of a fiber break is evaluated. The stress fields for various fiber/matrix interface debonding conditions and the effect of frictional stress transfer is investigated. The experimental determination of the interphase shear stress is discussed.

Fiber Debonding in Residually Stressed Brittle Matrix Composites

The competition between initial fiber debonding versus fiber failure marks a crucial event of the microstructural failure process in fiber‐reinforced brittle matrix composites. In this study, the role of a thermal residual stress field on the debonding conditions is examined theoretically and analytically. The analysis is based on two critical observations, the first being that the mechanics at the tip of a kink crack are driven only by the singularity at the main crack tip. Following from the first is the second observation that any thermal stress effects on the debonding criteria should enter only through the phase angle ψT of the total stress intensity factor at the main crack tip. In general, this stress intensity factor has a thermal as well as a mechanical load contribution. It is shown that when the thermal and mechanical stress intensities, KR and Kt, respectively, are in phase, i.e., ψR=ψt, the existing debonding conditions are universal and can be used even in the presence of thermal loads. On the contrary, when KR and Kt are out of phase, i.e., ψR≠ψt, events such as the delamination of thick films or debonding of inclined aligned fibers in brittle matrix composites become sensitive to the presence of the thermal stresses.