Jamie J. Kruzic

Crack blunting, crack bridging and resistance-curve fracture mechanics in dentin: effect of hydration.

Few studies have focused on a description of the fracture toughness properties of dentin in terms of resistance-curve (R-curve) behavior, i.e., fracture resistance increasing with crack extension, particularly in light of the relevant toughening mechanisms involved. Accordingly, in the present study, fracture mechanics based experiments were conducted on elephant dentin in order to determine such R-curves, to identify the salient toughening mechanisms and to discern how hydration may affect their potency. Crack bridging by uncracked ligaments, observed directly by microscopy and X-ray tomography, was identified as a major toughening mechanism, with further experimental evidence provided by compliance-based experiments. In addition, with hydration, dentin was observed to display significant crack blunting leading to a higher overall fracture resistance than in the dehydrated material. The results of this work are deemed to be of importance from the perspective of modeling the fracture behavior of dentin and in predicting its failure in vivo.

Indentation techniques for evaluating the fracture toughness of biomaterials and hard tissues

Indentation techniques for assessing fracture toughness are attractive due to the simplicity and expediency of experiments, and because they potentially allow the characterization of both local and bulk fracture properties. Unfortunately, rarely have such techniques been proven to give accurate fracture toughness values. This is a concern, as such techniques are seeing increasing usage in the study of biomaterials and biological hard tissues. Four available indentation techniques are considered in the present article: the Vickers indentation fracture (VIF) test, the cube corner indentation fracture (CCIF) test, the Vickers crack opening displacement (VCOD) test and the interface indentation fracture (IIF) test. Each technique is discussed in terms of its suitability for assessing the absolute and relative toughness of materials or material interfaces based on the published literature on the topic. In general, the VIF and CCIF techniques are found to be poor for quantitatively evaluating toughness of any brittle material, and the large errors involved (approximately +/-50%) make their applicability as comparative techniques limited. Indeed, indentation toughness values must differ by at least by a factor of three to conclude a significant difference in actual toughness. Additionally, new experimental results are presented on using the CCIF test to evaluate the fracture resistance of human cortical bone. Those new results indicate that inducing cracking is difficult, and that the cracks that do form are embedded in the plastic zone of the indent, invalidating the use of linear elastic fracture mechanics based techniques for evaluating the toughness associated with those cracks. The VCOD test appears to be a good quantitative method for some glasses, but initial results suggest there may be problems associated with applying this technique to other brittle materials. Finally, the IIF technique should only be considered a comparative or semi-quantitative technique for comparing material interfaces and/or the neighboring materials.

Fatigue of mineralized tissues: cortical bone and dentin.

Gaining a mechanistic understanding of the mechanical properties of mineralized tissues, such as dentin and cortical bone, is important from the perspective of developing a framework for predicting and preventing failure of teeth and whole bones, particularly with regard to understanding the effects of microstructural modifications from factors such as aging, disease, or medical treatments. Accordingly, considerable research efforts have been made to determine the specific mechanisms involved in the fatigue and fracture of mineralized tissues, and to discover how these mechanisms relate to features within the respective microstructures. This article seeks to review the progress that has been made specifically in the area of fatigue, focusing on the research that moves our understanding beyond simple fatigue life (S/N) concepts and instead addresses the separate mechanisms for microdamage initiation, crack propagation, and in the case of bone, repair and remodeling.

A highly efficient degradation mechanism of methyl orange using Fe-based metallic glass powders

A new Fe-based metallic glass with composition Fe76B12Si9Y3 (at. %) is found to have extraordinary degradation efficiency towards methyl orange (MO, C14H14N3SO3) in strong acidic and near neutral environments compared to crystalline zero-valent iron (ZVI) powders and other Fe-based metallic glasses. The influence of temperature (294–328 K) on the degradation reaction rate was measured using ball-milled metallic glass powders revealing a low thermal activation energy barrier of 22.6 kJ/mol. The excellent properties are mainly attributed to the heterogeneous structure consisting of local Fe-rich and Fe-poor atomic clusters, rather than the large specific surface and strong residual stress in the powders. The metallic glass powders can sustain almost unchanged degradation efficiency after 13 cycles at room temperature, while a drop in degradation efficiency with further cycles is attributed to visible surface oxidation. Triple quadrupole mass spectrometry analysis conducted during the reaction was used to elucidate the underlying degradation mechanism. The present findings may provide a new, highly efficient and low cost commercial method for azo dye wastewater treatment.

Quantification of free volume differences in a Zr44Ti11Ni10Cu10Be25 bulk amorphous alloy

Deformation of metallic glasses requires the existence of free volume to allow atomic movement under mechanical loading. Accordingly, quantification of the free volume state of the alloy is crucial to understand its mechanical behavior. By annealing below the glass transition temperature, the free volume of a Zr44Ti11Ni10Cu10Be25 bulk metallic glass was varied via structural relaxation. Differential scanning calorimetry was used to quantify enthalpy differences between the relaxed and as-cast materials, which were then quantitatively related to average free volume differences. These results can be used to characterize the average free volume in this alloy for future mechanical property studies.

R-curve behavior and toughening mechanisms of resin-based dental composites: Effects of hydration and post-cure heat treatment

OBJECTIVES To test the hypothesis that the fracture resistance of two different particulate resin composites degrade after water hydration and improve after post-cure heat treatment, and to correlate those changes with salient failure micromechanisms. METHODS Two composites with different filler morphology were selected, denoted microhybrid (Filtek Z250) and nanofill (Filtek Supreme plus). Following initial light curing, hydrated samples were aged in water for 60 days at room temperature while post-cured samples were heat treated at 120 degrees C for 90 min. Fracture resistance was assessed using fracture resistance curves (R-curves) utilizing pre-cracked compact tension, C(T), specimens. The flexural strength of the hydrated composites also was evaluated in four-point bending using unnotched beams. Scanning electron microscopy (SEM) of crack paths and fracture surfaces was performed to determine the micromechanisms of fracture and toughening. The results were compared by two-way ANOVA and Tukeys multiple comparison test (p< or =0.05). RESULTS SEM observations revealed a predominantly interparticle matrix crack path for all cases except the hydrated nanofill composite, which showed evidence of particle matrix debonding. Hydration lowered the strength for both composites and the peak toughness for the nanofill composite. The strength decrease was attributed to resin matrix plasticization and hydrolytic degradation in both cases, with additional interfacial degradation causing a larger strength decline and concomitant peak toughness decrease in the nanofill composite. The post-cure heat treatment noticeably changed the R-curve shape causing the peak toughness to be reached after shorter amounts of crack extension. Such changes help explain the increases in strength reported in other studies and is attributed to improved resin matrix properties. SIGNIFICANCE Results from this study provide new insight into the micromechanisms of fracture in resin-based dental composites which should aid the future development and improvement of these materials.

R-curve behavior and micromechanisms of fracture in resin based dental restorative composites

The fracture properties and micromechanisms of fracture for two commercial dental composites, one microhybrid (FiltekZ250) and one nanofill (FiltekSupreme Plus), were studied by measuring fracture resistance curves (R-curves) using pre-cracked compact-tension specimens and by conducting both unnotched and double notched four point beam bending experiments. Four point bending experiments showed about 20% higher mean flexural strength of the microhybrid composite compared to the nanofill. Rising fracture resistance was observed over approximately 1 mm of crack extension for both composites, and higher overall fracture resistance was observed for the microhybrid composite. Such fracture behavior was attributed to crack deflection and crack bridging toughening mechanisms that developed with crack extension, causing the toughness to increase. Despite the lower strength and toughness of the present nanofill composite, based on micromechanics observations, large nanoparticle clusters appear to be as effective at deflecting cracks and imparting toughening as solid particles. Thus, with further microstructural refinement, it should be possible to achieve a superior combination of aesthetic and mechanical performance using the nanocluster approach for dental composites.

Effects of plastic constraint on the cyclic and static fatigue behavior of metal/ceramic layered structures

Abstract The role of metal layer thickness and resultant plastic constraint in the metal layer during the failure of metal/ceramic layered structures is examined under cyclic and static loading conditions. Crack-growth experiments were conducted on sandwich specimens consisting of 99.999% pure aluminum layers bonded between 99.5% pure polycrystalline alumina with the metal layer thickness varying from 5 to 100 μm. Under cyclic loading, crack growth occurred primarily at the interface separating the two materials; additionally, stable fatigue cracks deviated into the alumina for thin-layered samples at high driving forces. Under monotonically increasing loads, the fracture toughness increased with Al layer thickness, whereas under cyclic loads the threshold driving force for crack growth conversely decreased with increasing layer thickness. Under static loading in a moist environment, interfacial crack growth was never observed at measurable rates (⩾10−9 m/s) for driving forces up to 200 J/m2; however, for thin-layered samples, subcritical cracks did deviate off the interface and grow, sometimes stably, into the alumina. Trends in crack-growth rates and crack trajectories are examined in terms of the level of constraint, loading conditions and environmental influences.

Bioactive glass fillers reduce bacterial penetration into marginal gaps for composite restorations

OBJECTIVE Bioactive glass (BAG) is known to possess antimicrobial and remineralizing properties; however, the use of BAG as a filler for resin based composite restorations to slow recurrent caries has not been studied. Accordingly, the objective of this study was to investigate the effect of adding 15wt% BAG to a resin composite on bacterial biofilms penetrating into marginal gaps of simulated tooth fillings in vitro during cyclic mechanical loading. METHODS Human molars were machined into approximately 3mm thick disks of dentin and 1.5-2mm deep composite restorations were placed. A narrow 15-20 micrometer wide dentin-composite gap was allowed to form along half of the margin by not applying dental adhesive to that region. Two different 72wt% filled composites were used, one with 15wt% BAG filler (15BAG) and the balance silanated strontium glass and one filled with aerosol silica and silanated strontium glass without BAG (0BAG-control). Samples of both groups had Streptococcus mutans biofilms grown on the surface and were tested inside a bioreactor for two weeks while subjected to periods of cyclic mechanical loading. After post-test biofilm viability was confirmed, each specimen was fixed in glutaraldehyde, gram positive stained, mounted in resin and cross-sectioned to reveal the gap profile. Depth of biofilm penetration for 0BAG and 15BAG was quantified as the fraction of gap depth. The data were compared using a Students t-test. RESULTS The average depth of bacterial penetration into the marginal gap for the 15BAG samples was significantly smaller (∼61%) in comparison to 0BAG, where 100% penetration was observed for all samples with the biofilm penetrating underneath of the restoration in some cases. SIGNIFICANCE BAG containing resin dental composites reduce biofilm penetration into marginal gaps of simulated tooth restorations. This suggests BAG containing composites may have the potential to slow the development and propagation of secondary tooth decay at restoration margins.

On the growth of small fatigue cracks in γ-based titanium aluminides

Gamma-based TiAl intermetallic alloys have received considerable attention recently as candidate materials for high-temperature aerospace applications. Two classes of microstructure have been prominent in the two-phase ({gamma} + {alpha}{sub 2}) alloys: a lamellar structure consisting of lamellar colonies containing alternating {gamma} and {alpha}{sub 2} grains. In general, duplex structures display better elongation and strength, whereas lamellar structures show better toughness and fatigue crack-growth resistance. However, a problem with both microstructures, as with most intermetallics, is that fatigue-crack growth rates, da/dN, show a very strong dependence upon the applied stress-intensity range, {Delta}K, i.e., da/dN {proportional_to} {Delta}K{sup m}, where m is greater than {approximately}10. Small cracks (typically < {approximately}500 {micro}m in length) are known to grow at applied {Delta}K below the long crack threshold, and to exhibit growth rates in excess of those corresponding to long cracks (typically larger than 2--3 mm) at the same applied {Delta}K levels. Accordingly, the objective of the present study is to examine the small-crack effect in a commercial {gamma}-based TiAl alloy by comparing the growth-rate behavior of long (through-thickness) cracks with that of small surface cracks for both duplex and fully lamellar microstructures.