Comparison of polymerization stresses of dental resin composites evaluated by two indentation fracture methods

BACKGROUND: Polymerization stress is a major problem in dental resin composite restorations. Two indentation fracture methods can be applied to evaluate the stress, however, they often calculate different values. OBJECTIVE: To compare polymerization stresses of dental composites determined by the two methods. METHODS: Glass disks with a central hole were used. Two indentation fracture methods (Methods 1 and 2) were employed to determine the polymerization stresses of low-shrinkage and bulk-fill composites. Method 1: Cracks were made in the glass surface at 300 μm from the hole. The hole was filled with the composite. Polymerization stresses at 30 min after filling were calculated from the lengths of crack extension. Method 2: The hole was filled with the composite. Cracks were introduced in the glass at 1,000 μm from the hole at 30 min after the polymerization and the stresses were calculated from the crack lengths. Stresses at composite-glass bonded interface were calculated from the stress values obtained by the two methods. RESULTS: The bulk-fill composite generated the smallest interfacial stress, and Method 1 revealed lower values than Method 2. CONCLUSIONS: The composites yielded relatively small stresses. Method 1 calculated smaller stress values, possibly affected by the lower threshold stress intensity factor.

Effect of Titanium and Boron Microalloying on Sulfide Stress Cracking in C110 Casing Steel

The effect of Ti and B microalloying on the hardenability, prior austenite grain size (PAGS), mechanical properties, and sulfide stress cracking (SSC) of C110 grade steel was studied by Jominy testing, static tensile testing, an optical microscope (OM), scanning electron microscopy (SEM) and double cantilever beam (DCB) testing. The results show that the addition of 0.015% Ti and 0.002% B into a medium-carbon Fe-Cr-Mo-Nb-V steel increased the hardenability and refined the PAGS and quenched martensite packets, and the size of carbides was reduced. It is believed that these behaviors are responsible for the improvement in the threshold stress intensity factor KISSC.

Negligible Crack Growth Thresholds

This paper initiates use of fracture mechanics best practice growth models and tools for pipeline steels with full tri-region da/dN characterization. The utilities associated with establishing negligible crack growth thresholds are demonstrated. Pipeline operators are often presented with decisions that could be supported with scientifically vetted and situationally accurate stress thresholds for negligible crack growth. The threshold stress-intensity factor, ΔKth, is the value for ΔK where the crack growth rate, da/dN, approaches the minimum threshold crack growth rate. Stress-intensity factors at or below this threshold value result in crack growth small enough for operators to practically ignore it in pipeline integrity assessments. Previously, a ΔKth value of 2.0 MPa*m0.5 had been suggested for general use in API 579[1]. The API 579 value appears conservative when compared to industry experience and established ΔKth for similar steel alloys across all stress ratios. By establishing an on-shore pipeline specific ΔKth which considers a pipeline-specific da/dN threshold and stress ratio effects, operators are afforded the opportunity to: • exclude certain pipelines or portions of pipelines from crack growth susceptibility, • identify features with no life limit, • adjust load / boundary conditions to preclude growth, • improve computational efficiency by discarding load cycles below threshold, and, • more accurately simulate crack growth scenarios Pipeline crack growth testing has been researched to derive reasonable and prudent negligible ΔKth values through a closer examination of loading scenarios and environments which affect ΔKth. A da/dN threshold for when diminishingly small crack growth rates can be neglected for typical pipeline assets was determined based on observed pressure fluctuation frequencies. Applications and value derived from deployment of ΔKth are illustrated for North American pipeline assets. Environmental and blunting effects on ΔKth for near-neutral pH stress corrosion cracking previously developed are shown for comparison and utility. Fully established negligible growth thresholds pave the way toward adoption of next-level fracture mechanics best practice models and tools such as AFGROW and NASGRO, and facilitates crack growth simulations and root-cause analysis.