Strength of Polypropylene Fiber Reinforced Cement Mortar Composites with Variable Water/Cement Ratio

For cement mortars the reinforcement techniques by randomly distributed short fibers, contribute highly to their micro-cracking stabilization and to improvement of their flexibility and tensile strength. Among the wide different types of short fiber reinforcement that have been used in the past for mortars, those made of steel, polymers and natural fibers dominate the area. It is a common sense that the reinforcement of mortars by polypropylene fibers (PP) is considered a very efficient method for reducing their curing shrinkage and enhanced toughness and strength of un-reinforced cemented material. In this study, PP fiber reinforced mortars were prepared with specific composition but with variable water to cement (W/C) ratio and appropriate superplasticizer amounts. For all mixtures their workability, air content and flexural and compressive strengths were measured. In conclusion it can be stated that even though W/C ratio of mortars is varied considerably, and also do their mechanical response, it is possible by appropriate mixture designing, for all studied compositions to produce suitable mortars that can be used successfully in a wide range of demands and applications, achieving high mortar strengths and ideal workability behavior.

The research on the dental bridge model-making process based on the curing shrinkage epoxy and residual stress reduction

Elderly people suffer from more and more teeth problems. The tooth-implant-supported prosthesis provides a reliable solution to missing teeth patients. The proper dental prosthesis design to prevent overstress is essential due to the mechanical characteristics of the dental bridge abutments are different. The finite element method is widely applied, but proper experimental validation is required. The curing shrinkage epoxy is applied for the photoelasticity measurement because its mechanical property is close to the cancellous bone. A series of process developments, including mold design, residual stress releasing and artificial soft film making, is accomplished in this research to simulate the mechanical response of dental bridges in practice. The process is proven and can be accomplished at the dentist's workshop. The transmission photoelasticity technique is applied to measure the residual stress distribution and it nondestructively provides the continuous improvement guideline. The model-making procedure and tools are proven to be available at the dental workshop. Following the model-making procedure, the dental bridge model shows a low residual stress level that the photoelasticity system cannot detect. Excellent reproducibility of the proposed procedure has been validated. These models exhibit stable maximum stress of 2.13 MPa around the natural tooth apex and apical implant region when a 300 N vertical loading is applied upon the dental bridge. Finally, a finite element model of the dental bridge, including the natural tooth and dental implant, is built and validated by the photoelastic measurement.

Resistance Reduction of Conductive Patterns Printed on Textile by Curing Shrinkage of Passivation Layers

Directly printing conductive ink on textiles is simple and compatible with the conventional electronics manufacturing process. However, the conductive patterns thus formed often show high initial resistance and significant resistance increase due to tensile deformation. Achieving conductive patterns with low initial resistance and reduced deformation-induced resistance increase is a significant challenge in the field of electronic textiles (e-textiles). In this study, the passivation layers printed on conductive patterns, which are necessary for practical use, were examined as a possible solution. Specifically, the reduction of the initial resistance and deformation-induced resistance increase, caused by the curing shrinkage of passivation layers, were theoretically and experimentally investigated. In the theoretical analysis, to clarify the mechanism of the reduction of deformation-induced resistance increase, crack propagation in conductive patterns was analyzed. In the experiments, conductive patterns with and without shrinking passivation layers (polydimethylsiloxane) cured at temperatures of 20–120 °C were prepared, and the initial resistances and resistance increases due to cyclic tensile and washing in each case were compared. As a result, the initial resistance was reduced further by the formation of shrinking passivation layers cured at higher temperatures, and reduced to 0.45 times when the curing temperature was 120 °C. The cyclic tensile and washing tests confirmed a 0.48 and a 0.011 times reduction of resistance change rate after the 100th elongation cycle (10% in elongation rate) and the 10th washing cycle, respectively, by comparing the samples with and without shrinking passivation layers cured at 120 °C.

Properties of a New Nanofiber Restorative Composite

SUMMARY A new nanofiber-reinforced hybrid composite (NovaPro Fill, Nanova) was recently introduced with reportedly improved mechanical properties. The purpose of this study was to compare the properties (flexural strength/modulus, degree of conversion [DC], depth of cure, and polymerization shrinkage) of the nanofiber composite to those of traditional hybrid composites (Filtek Z250, 3M ESPE; Esthet-X HD, Dentsply). To determine flexural strength and modulus, composite was placed in a rectangular mold, light-cured, stored for 24 hours, and then fractured in a universal testing machine. For degree of conversion, composite was placed in a cylindrical mold, light-cured, and stored for 24 hours. Measurements were made at the top and bottom surfaces using Fourier Transform Infrared Spectroscopy. To determine depth of cure, composite was placed in a cylindrical mold and light-cured. Uncured composite was scraped until polymerized resin was reached. Remaining composite was measured and divided by two. Polymerization shrinkage was determined by placing the composite material on a pedestal in a video-imaging device while light-curing. Shrinkage was determined after 10 minutes. Data were analyzed with one-way analysis of variance and Tukey post hoc test per property (α=0.05). Compared to Filtek Z250, NovaPro Fill had significantly lower flexural strength and modulus, greater volumetric shrinkage, and similar depth of cure, but greater top and bottom DC. Compared to Esthet-X HD, NovaPro Fill had similar flexural strength, shrinkage, and top and bottom DC, but significantly greater depth of cure and flexural modulus.

Modelling of Residual Stresses in Dental Restorative Polymer Based Materials

A methodology for residual stresses calculations is proposed. Common photo-curing dental restorative materials under different C-factors are considered as case studies. Reaction kinetics, curing shrinkage, and viscoelasticity map were required as input data on a structural FE solver. Post cure effects were considered in order to quantify the residual stresses coming out from natural contraction with respect to those debited to the chemical shrinkage. The analysis showed for a given test case that stresses build-up due to the thermal contraction (after the completion of restoration) are comparable with those emerging due to thermal shrinkage.