Effect of Exposure Times of Sodium Hypochlorite before Acid Etching on the Microshear Bond Strength to Fluorotic Enamel

Purpose: To evaluate the effects of different treatment time of 5.25% Sodium hypochlorite (NaOCl) on the microshear bond strength (μSBS), attenuated total reflection Fourier transform infrared (ATR-FTIR) and etching pattern in mild and moderate fluorotic enamel. Study design: Forty-eight fluorotic molars were divided into two groups: mild and moderate fluorotic enamel which were classified by a Thylstrup and Fejerskov index (TFI). Based on the application time (0s, 60s, 120s, 180s) of 5.25% NaOCl, each group was sectioned into four parts. Then the etched enamel was bonded with resin and tested to acquire μSBS. The statistical method was two-way ANOVA and Least Significant Difference (LSD) test at α = 0.05. Besides, fracture modes were observed under a stereo microscope. SEM was used to evaluated the enamel-etching pattern and organic content on the fluorotic enamel surface were investigated by ATR-FTIR. Results: Duration of 5.25% NaOCl at 60s or 120s significantly increased the μSBS of fluorotic enamel compared to 0s (p<0.05). Fracture modes indicated that dominating failures were set in the bonding interface but whose proportion decreased when 5.25% NaOCl was applied. The enamel-etching pattern in 180s was deepest under SEM. Spectra of enamel samples manifested an obvious and gradual removal of its organic phase after duration of NaOCl increased. Conclusion: The maximal μSBS is acquired by using 5.25% NaOCl at 60s for mild fluorotic enamel but 120s for the moderate. The prolonged application time of 5.25% NaOCl prior to phosphoric acid etching improves enamel-etching pattern. Treatment of 5.25% NaOCl decreases proteins on the fluorotic enamel surface.

Thermomechanical Microstructural Predictions of Fracture Nucleation of Zircaloy-4 Alloys with δ and ε Hydride Distributions

A crystalline dislocation-density formulation that was incorporated with a non-linear finite-element (FE) method was utilized to understand and to predict the thermo-mechanical behavior of an hexagonal closest packed (h.c.p.) zircaloy system with hydrides with either face centered cubic (f.c.c.) or body centered cubic (b.c.c.) hydrides. This formulation was then used with a recently developed fracture methodology that is adapted for finite inelastic strains and multiphase crystalline systems to understand how different microstructurally-based fracture modes nucleate and propagate. The interrelated microstructural characteristics of the different crystalline hydride and matrix phases with the necessary orientation relationships (ORs) have been represented, such that a detailed physical understanding of fracture nucleation and propagation can be predicted for the simultaneous thermo-mechanical failure modes of hydride populations and the matrix. The effects of volume fraction, morphology, crystalline structure, and orientation and distribution of the hydrides on simultaneous and multiple fracture modes were investigated for radial, circumferential, and mixed distributions. Another key aspect was accounting for temperatures changes due to the effects of thermal conduction and dissipated plastic work and their collective effects on fracture. For hydrided aggregates subjected to high temperatures, thermal softening resulted in higher ductility due to increased dislocation-density activity, which led to higher shear strain accumulation and inhibited crack nucleation and growth. The predictions provide validated insights of why circumferential hydrides are more fracture resistant than radial hydrides for different volume fractions and thermo-mechanical loading conditions.

Investigation of Ib-Values for Determining Fracture Modes in Fiber-Reinforced Composite Materials by Acoustic Emission

This study analyzed failure behavior using Ib-values obtained from acoustic emission (AE) signals. Carbon fiber/epoxy specimens were fabricated and tested under tensile loads, during which AE signals were collected. The dominant peak frequency exhibited a specific range according to fracture mode, depending on the fiber structures. Cross-ply specimens, with all fracture modes, were used and analyzed using b- and Ib-values. The b-values decreased over the specimens’ entire lifetime. In contrast, the Ib-values decreased to 60% of the lifetime, and then increased because of the different fracture behaviors of matrix cracking and fiber fracture, demonstrating the usefulness of Ib-values over b-values. Finally, it was confirmed that abnormal conditions could be analyzed more quickly using failure modes classified by Ib-values, rather than using full AE data.