F. Pratesi

Mechanical strength of dental implants

A series of mechanical strength tests have been carried out in order to characterize some dental implants. Most specimens tested were of the tapered-joint kind, but for comparison further tests have been carried out on screw-joined implants. The loads applied were similar to those existing in the typical working conditions of the component; however, in order to conform to the ISO standard 14801, loads higher than those achieved in working conditions were applied. The static characterization tests determined the strength of the system, with large deformations preceding rupture. The fatigue tests led to the determination of the fatigue limit of the dental implant, defined conventionally in these cases as the maximum value under which the system has a life of at least 2.106 cycles. The fatigue limit is higher than the stress that the teeth endure during the normal process of mastication, and it confirms the suitable structural design of the device. Failure analysis has shown that the fatigue crack leading to rupture commonly occurs in the section in which the hexagonal insert in the tapered joint begins.

Increased liquid droplet erosion resistance of steam turbine blades

Abstract Alloys used for the blades of steam turbines usually do not show a satisfactory resistance to erosion and require surface protection with suitable coatings. Starting from the results obtained with Stellite 6 coatings on AISI 420 stainless steel blades, this research has been developed with the aim to increase erosion resistance by modifying the chemical composition of the coatings. Several samples have been coated by laser cladding and exposed to liquid erosion tests. In particular, concentrations of carbide forming and solid solution strengthening elements have been varied in order to establish their effects against liquid droplet erosion. Results of the tests and EDX analyses on the samples are reported, with a discussion on the various effects on erosion resistance. Two directions have appeared as the most promising, either increasing the concentration of carbide forming elements, or increasing the concentration of nickel in the coatings.

Bithermal Fatigue Testing: Experimental and Prediction Problems

The need of thermomechanical fatigue testing is strongly indicated by the actual working conditions of various machine components used at high temperatures. In many cases, temperature is not uniformly distributed on a component and it also changes over the course of time. In these conditions, isothermal testing has been shown to be unsuitable and life prediction must be based on more complex testing procedures.

Fatigue maps and multistage life prediction methods

Abstract Various methods have been recently developed for life prediction under creep-fatigue conditions. This study is concerned with continuous damage models. In particular, a model developed at ONERA, which evaluates the creep and fatigue contributions separately and which sums the corresponding differential damage, has been used for fitting the high-temperature experimental data for IN 738 test bars. The data were taken both from the literature and from our own tests. In addition, it was found that the model can also be used to derive schematic fatigue maps. Following the introduction of Ashby maps for creep, several other maps have been proposed for fatigue. However, the task for fatigue maps is more difficult to define. This work presents a discussion of the whole problem, a review of existing proposals, and some indicative maps that have been obtained for IN 100. Even though the approach appears to be promising for outlining approximate fatigue maps, metallographic observations are still a necessary complement for mapping single domains where given microstructural mechanisms are predominant.

Fatigue Maps and Mean Load Dependences

The influence of mean stress on fatigue life has been investigated for IN 792 at 800° C The results and their interpretation are reported. For the application of a life prediction model, a suitable input parameter has been determined on the basis of the experimental results obtained. Lastly, a creep-fatigue map is illustrated for IN 792 at 800° C.