Fatigue behavior of lubricated Ni-Ti endodontic rotary instruments

The use of Ni-Ti alloys in the practice of endodontic comes from their important properties such as shape memory and superelasticity phenomena, good corrosion resistance and high compatibility with biological tissues. In the last twenty years a great variety of nickel-titanium rotary instruments, with various sections and taper, have been developed and marketed. Although they have many advantages and despite their increasing popularity, a major concern with the use of Ni-Ti rotary instruments is the possibility of unexpected failure in use due to several reasons: novice operator handling, presence manufacturing defects, fatigue etc. Recently, the use of an aqueous gel during experimental tests showed a longer duration of the instruments. The aim of the present work is to contribute to the study of the fracture behavior of these endodontic rotary instruments particularly assessing whether the use of the aqueous lubricant gel can extend their operative life stating its reasons. A finite element model (FEM) has been developed to support the experimental results. The results were rather contradictory, also because the Perspex (Poly-methyl methacrylate, PMMA) cannot simulate completely the dentin mechanical behavior; however the results highlight some interesting points which are discussed in the paper

Experimental measurement and model validation of COD in pipe under bending with off-centered circumferential crack

The leak area of circumferential through-thickness crack in pipe under bending depends on the position of the crack with respect to the bending plane. In leak-before-break (LBB) analysis, the assumption that the crack is symmetrically placed with respect to the bending plane is not necessarily conservative. In this work, the crack opening of circumferential cracks, off-centered with respect to the bending plane, was investigated experimentally. Here, three pipe geometries and two crack lengths were investigated. For each crack, the centred and two off-centered configuration were examined. The crack opening displacement (COD) distribution along the crack length was measured for two selected bending load levels using digital image correlation (DIC) technique. These measurements have been used for verifying the solution provided by the hodograph cone method (HCM) as proposed by Bonora [1]

Quantitative estimating size of deep defects in multi-layered structures from eddy current NDT signals using improved ant colony algorithm

Detection and quantitative estimation of deep defects in multi-layered structures is an essential task in a range of technological applications, such as maintaining the integrity of structures, enhancing the safety of aging aircraft, and assuring the quality of products. A novel approach to accurately quantify the two-dimensional axisymmetric deep defect size from eddy current nondestructive testing (NDT) signals is presented here. The method uses a finite element forward model to simulate the underlying physical process and an improved ant colony algorithm (IACA) to solve the inverse problem. Experiments are carried out. The performance comparison between the IACA method and the least square method is shown. The comparison results demonstrate the feasibility and validity of the IACA method. Between them, the IACA method gives a better estimation performance than the least square method at present

Crack propagation in micro-chevron-test samples of direct bonded silicon-silicon wafers

Wafer bonding describes all technologies for joining two or more substrates directly or using certain intermediate layers. Current investigations are focused on so-called low temperature bonding as a special direct bonding technology. It is carried out without intermediate layers and at temperatures below 400 °C. In addition to the wafer materials, the toughness of the bonded interface also depends on the bonding process itself. It can vary for different pre-treatments. Furthermore, an increase of the annealing temperature leads to a higher toughness of the bonded interface. The fracture toughness is a suitable value to describe the damage behaviour of the bonded interface. Based on a micro-chevron-specimen, the fracture toughness can be determined either numerically or by combining numerical analysis with experimental measurement of the maximum force. The maximum force is measured during a micro-chevron-test using a Mode I loading. The minimum of the stress intensity coefficient can be determined by a FE-simulation only. One possibility to estimate the stress intensity coefficient is the compliance method. The compliance of the whole specimen increases with a growing crack. The stress intensity coefficient can be directly derived from the simulated compliance and the crack length itself. The paper is focused on the micro-chevron-test for direct bonded silicon-silicon wafers. Additional to the estimation of dimensionless stress intensity coefficient as a function of geometry, the influence of different pre-treatments and annealing temperatures on the measured maximum force are analysed and discussed

Numerical modelling of intergranular fracture in polycrystalline materials and grain size effects

In this paper, the phenomenon of intergranular fracture in polycrystalline materials is investigated using a nonlinear fracture mechanics approach. The nonlocal cohesive zone model (CZM) for finite thickness interfaces recently proposed by the present authors is used to describe the phenomenon of grain boundary separation. From the modelling point of view, considering the dependency of the grain boundary thickness on the grain size observed in polycrystals, a distribution of interface thicknesses is obtained. Since the shape and the parameters of the nonlocal CZM depend on the interface thickness, a distribution of interface fracture energies is obtained as a consequence of the randomness of the material microstructure. Using these data, fracture mechanics simulations are performed and the homogenized stress-strain curves of 2D representative volume elements (RVEs) are computed. Failure is the result of a diffuse microcrack pattern leading to a main macroscopic crack after coalescence, in good agreement with the experimental observation. Finally, testing microstructures characterized by different average grain sizes, the computed peak stresses are found to be dependent on the grain size, in agreement with the trend expected according to the Hall-Petch law

A four-parameters model for fatigue crack growth data analysis

A four-parameters model for interpolation of fatigue crack growth data is presented. It has beenvalidated by means of both data produced by the Authors and data collected from Literature. The proposedmodel is an enhanced version of a three-parameters model already discussed in a previous work that has beensuitably modified in order to overcome some drawbacks raised when applied to a quite wider experimental dataset. Results of validation study have also revealed that the new model, besides interpolating accurately crackgrowth data, allows to identify the presence of anomalies in the data sets. For this reason, by a suitable filter tobe chosen depending on the size and number of anomalies, it can be used to remove them and obtain sigmoidalcrack propagation curves smoother than those obtained when the current analysis techniques are used. In theend, possible model parameters correlations are analysed

The use of thermally expandable microcapsules for increasing the toughness and heal structural adhesives

In this research, the effect of thermally expandable microcapsules (TEMs) on mode I fracture toughness of structural adhesives were investigated. The single-edge-notch bending (SENB) test was used. Firstly, a standard toughness test was performed on adhesives with microcapsules. Secondly, since TEMs start their expansion at approximately 60ºC, the next specimens were fatigue tested expecting a local heating in the notch leading to the desired expansion before being statically loaded for fracture toughness determination. Thirdly, a manual local heating at 90ºC was applied in the notch before the fracture static test. The experimental results were successfully cross-checked through a numerical analysis using the virtual crack closure technique (VCCT) based on linear elastic fracture mechanics (LEFM). The major conclusion is that fracture toughness of the modified adhesives increased as the mass fraction of the TEMs increased

Relay-race deformation mechanism during uniaxial tension of cylindrical samples of carbon steel: using digital image correlation technique

The work deals with experimental study of macro localization of plastic yielding occurrences of structural carbon steel, research of singularity of deformation wave processes by complex use of contemporary test equipment and high effective digital image correlation method. Evolution of nonuniform axial strain fields on surface of cylindrical samples during uniaxial tension was registered, time dependences were drawn, and a ‘relay-race’ mechanism of material deformation was found out at the stage of yield plateau forming. Strain concentration ratio was estimated for several material deformation stages

Failure of steam line causes determined by NDT testing in power and heating plants

This paper examines leakage and damages of steam and provides an overview of NDT testing inorder to determine the cause of steam lines failure in power plants and heating plants. This approach may beapplied to similar structures and its application in preventive maintenance would help extend the life of steampipes

Coupled FEM-DBEM method to assess crack growth in magnet system of Wendelstein 7-X

The fivefold symmetric modular stellarator Wendelstein 7-X (W7-X) is currently underconstruction in Greifswald, Germany. The superconducting coils of the magnet system are bolted onto a centralsupport ring and interconnected with five so-called lateral support elements (LSEs) per half module. Afterwelding of the LSE hollow boxes to the coil cases, cracks were found in the vicinity of the welds that couldpotentially limit the allowed number N of electromagnetic (EM) load cycles of the machine. In response to theappearance of first cracks during assembly, the Stress Intensity Factors (SIFs) were calculated andcorresponding crack growth rates of theoretical semi-circular cracks of measured sizes in potentially criticalposition and orientation were predicted using Paris’ law, whose parameters were calibrated in fatigue tests atcryogenic temperature. In this paper the Dual Boundary Element Method (DBEM) is applied in a coupledFEM-DBEM approach to analyze the propagation of multiple cracks with different shapes. For this purpose,the crack path is assessed with the Minimum Strain Energy density criterion and SIFs are calculated by the Jintegralapproach. The Finite Element Method (FEM) is adopted to model, using the commercial codes Ansysor Abaqus;, the overall component whereas the submodel analysis, in the volume surrounding the cracked area,is performed by FEM (“FEM-FEM approach”) or alternatively by DBEM (“FEM-DBEM approach”). The“FEM-FEM approach” considers a FEM submodel, that is extracted from the FEM global model; the latterprovide the boundary conditions for the submodel. Such approach is affected by some restrictions in the crackpropagation phase, whereas, with the “FEM-DBEM approach”, the crack propagation simulation isstraightforward. In this case the submodel is created in a DBEM environment with boundary conditionsprovided by the global FEM analysis; then the crack is introduced and a crack propagation analysis has beenperformed to evaluate the effects of the crack shape and of the presence of nearby cracks on the allowednumber of EM load cycles