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Lithium disilicate glass-ceramic (LS2) can be biomimetically textured by crystal orientation, mimicking the microstructure of dental enamel and inducing anisotropic mechanical responses to crack growth. The deliberate texturization of LS2 plays to the clinical advantage of reinforcing weak sites of typical fracture initiation in dental prosthetic constructs. Similar to enamel, adhesion of biomimetically textured LS2 could also show anisotropic behavior. Therefore, tensile bond strength (TBS) before and after thermocycling (TC) and interfacial characterization of biomimetically textured LS2 (parallel (PAR) or perpendicular (PER) crystal orientation) after different pre-treatment modes (hydrofluoric acid etching (HFE), grit blasting (GBL) and self-etching glass-ceramic primer (SGP)) were investigated. TC reduced significantly TBS for all specimens except for GBL. Biomimetic textured LS2 after HFE showed anisotropic behavior regarding adhesion. Crystal orientation reduced TBS for the PER HFE specimens after TC significantly. In general, the TBS of HFE specimens was still higher or not significantly lower compared to other pre-treatment modes like GBL or SGP. For the GBL and SGP specimens, crystal orientation had no influence on TBS and interfacial characteristics. In contrast HFE specimens showed different interfacial characteristics depending if they were PAR or PER texturized. Based on these findings, HFE of biomimetic textured LS2 can be recommended. For bonding, the PAR orientation is recommended as its adhesion potential is less prone to degrade.

We intend by this short critical review to highlight important aspects regarding the mechanical testing of fracture toughness. The final aim is to increase the awareness to the test sensitivity, ultimately increasing the quality and reliability of reported testing results. In a well-intended attempt to facilitate testing procedures or provide alternatives for testing material interfaces, authors are resorting to adaptation of testing methodologies without proper theoretical and experimental validation. The assumption of validity in such cases endangers the perpetration of testing strategies that are not safeguarded by sound theoretical bases. The use of improper statistical treatments based on extreme-value distributions further aggravates this scenario. We supply here some directions for authors concerning method selection, interpretation of data scatter, statistical treatment, and possibilities for test validation.

We subjected a commercial dental composite formed by interconnecting polymer-ceramic scaffolds to extensive quasi-static, static and cyclic experiments under biaxial flexure. By this we meant to obtain static and cyclic subcritical crack growth exponents that, based on established relationships describing the degradation of frictional bridging mechanisms, challenging the notion of a suggested R-curve behavior. By exploring the fracture statistics of specimens with increasing effective volumes and effective areas, we demonstrated the presence of a bi-modal defect size distribution in disaccord with the Weibull behavior across length scales. Lifetime distributions seemed to follow the strength distributions, and were used to derive crack growth velocity diagrams for combined levels of applied stress. Ultimately, the claim of an R-curve behavior could not be supported based on the absence of any significant cyclic fatigue effect, i.e. bridging degradation.

The objective of the study was to investigate the relationship between pH-neutralization, hydrophilicity, and free hygroscopic expansion stress of self-adhesive resin cements (SARCs) after storage in artificial saliva. The SARCs RelyX Unicem Automix 2 (RX2, 3 M ESPE), iCEM (iCEM, Heraeus) and Maxcem Elite (MCE, Kerr) were under investigation in this study. Cylinders (height × diameter, 6 × 4mm) were prepared from each material and stored in artificial saliva (7d at 37 °C). Cylinder height was measured at baseline and after 7 days. After storage, the compression modulus was measured to calculate the free hygroscopic expansion stress. For pH-neutralization and hydrophilicity assessment, disks (height × diameter, 1 × 1.5 mm) were prepared, covered with electrolyte, and monitored over 24 h at 37 °C. Hydrophilicity was assessed using the static sessile drop technique at baseline and at different time intervals up to 24 h. Data were analyzed using one-way ANOVA and post hoc Student-Newman-Keuls test (S-N-K, α = 0.05). After 24 h, RX2 (pH24h 4.68) had a significantly higher (p

Objectives This study was intended to separate residual stresses arising from the mismatch in coefficients of thermal expansion between glass and zirconia (ZrO2) from those stresses arising solely from the cooling process. Slow crack growth experimentes were undertaken to demonstrate how cracks grow in different residual stress fields. Methods Aluminosilicate glass discs were sintered onto ZrO2 to form glass–ZrO2 bilayers. Glass discs were allowed to bond to the ZrO2 substrate during sintering or prevented from bonding by means of coating the ZrO2 with a thin boron nitrade coating. Residual stress gradients on “bonded” and “unbonded” bilayers were assessed using birefringence measurements. Unbonded glass discs were further tested under biaxial flexure in dynamic fatigue conditions in order to evaluate the effect of residual stress on the slow crack growth behavior. Results When fast-ccoling was induced, residual tensile stresses on the glass increased significantly on the side toward the ZrO2 substrate. By allowing the bond between glass and ZrO2, those tensile stresses observed in unbonded specimens are overwhelmed by the contraction mismatch stresses between the ZrO2 substrate and the glassy overlayer. Specimens containing residual tensile stresses on the bending surface showed a time-dependent strength increase in relation to stress-free annealed samples in the dynamic biaxial bending test, with this effect being dependent on the magnitude of the residual tensile stress. The phenomenon observed is explained here on the basis of the water toughening effect, in which water diffuses into the glass promoting local swelling. An additional residual tensile stress at the crack tip adds an applied-stress-independent (Kres) term to the total tip stress intensity factor (Ktip), increasing the stress-enhanced diffusion and the shielding of the crack tip through swelling of the crack faces. Significance Residual stresses in the glass influence the crack growth behavior of veneered-ZrO2 bilayered dental prostheses. The role of water in crack growth might be of higher complexity when residual stresses are present in the glass layer.