Gruppo Italiano Frattura (IGF)
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A theoretical model for predicting the Peak Cutting Force of conical picks
No full textIn order to predict the PCF (Peak Cutting Force) of conical pick in rock cutting process, atheoretical model is established based on elastic fracture mechanics theory. The vertical fracture model of rockcutting fragment is also established based on the maximum tensile criterion. The relation between verticalfracture angle and associated parameters (cutting parameter n and ratio B of rock compressive strength totensile strength) is obtained by numerical analysis method and polynomial regression method, and thecorrectness of rock vertical fracture model is verified through experiments. Linear regression coefficientbetween the PCF of prediction and experiments is 0.81, and significance level less than 0.05 shows that themodel for predicting the PCF is correct and reliable. A comparative analysis between the PCF obtained fromthis model and Evans model reveals that the result of this prediction model is more reliable and accurate. Theresults of this work could provide some guidance for studying the rock cutting theory of conical pick anddesigning the cutting mechanism
Analisi dell’anisotropia microstrutturale in materiali compositi rinforzati con fibre corte
No full textSynchrotron light microtomography has proved to be particularly efficient in order to analyze
the microstructural characteristics in terms of reinforce fibre distribution and orientation in glass fibre reinforced
composites. The spatial distribution of fibre within the polymeric matrix could be detected even in
case of fibre characterized by a small diameter (10 micrometers average diameter). Differences in orientation
distribution within a sample could be measured using the Mean Intercept Length (MIL) and the fabric
tensor. The results presented herein refer to a sample of a 30% by weight glass fibre reinforced polyamide 6,
extracted form a thin plate
Some practical crack path examples
No full textIt is well known that many engineering structures and components, as well as consumer items,contain cracks or crack-like flaws. It is widely recognised that crack growth must be considered both in designand in the analysis of failures. The complete solution of a crack growth problem includes determinationof the crack path. Macroscopic aspects of crack paths have been of industrial interest for a very long time.At the present state of the art the factors controlling the path taken by a crack are not completely understood.Eight brief case studies are presented. These are taken from the author’s professional and personal experienceof macroscopic crack paths over many years. They have been chosen to illustrate various aspects ofcrack paths. One example is in a component from a major structure, three examples are in laboratory specimens,and four are in nuisance failures. Such nuisance failures cause, in total, a great deal of inconvenienceand expensive, but do not normally receive much publicity
Sismabeton: a new frontier for ductile concrete
No full textThe high ductility of Fiber Reinforced Self-consolidating concrete (called Sismabeton) can be developed not only in tension but also in compression. This aspect is evidenced in the present paper by measuring the mechanical response of normal concrete (NC), plain self-compacting concrete (SC) and Sismabeton cylindrical specimens under uniaxial and triaxial compression. The post-peak behaviour of these specimens is defined by a non-dimensional function that relates the inelastic displacement and the relative stress during softening. Both for NC and SC, the increase of the fracture toughness with the confinement stress is observed. Conversely, Sismabeton shows, even in absence of confinement, practically the same ductility measured in normal and self-compacting concretes with a confining pressure. Thus, the presence of Sismabeton in compressed columns is itself sufficient to create a sort of active distributed confinement
In situ 3D characterization of fatigue cracks displacement fields
No full textThe three dimensional growth of fatigue cracks in samples of nodular graphite cast iron ischaracterized using laboratory X-ray computed tomography. The cracks grow from laser machined artificialdefects, their development is monitored in situ using laboratory X-ray computed tomography (lab. CT) andDigital Volume Correlation (DVC). The combination of both techniques gives access to the 3D displacementfield at the tip of the crack (mainly mode I opening)
High-cycle notch sensitivity of alloy steel ASTM A743 CA6NM used in hydrogenator turbine components
No full textThe presence of notches and other stress concentrations in turbine blades and other notch hydraulic components is a current problem in engineering. It causes a reduction of endurance limit of material. In that sense, specimens of the ASTM A743 CA6NM alloy steel using in several hydrogenator turbine components was tested. The specimens were tested under uniaxial fatigue loading with a load ratio equal to -1, and the considered stress concentration factors, Kt, values, calculated with respect to net area, were 1.55, 2.04 and 2.42. In order to determine the fatigue limit for such notch type, a reduction data method by Dixon and Mood, Staircase method was used. This approach is based on the assumed target distribution of the fatigue limit. For such geometry at least 8 specimens were tested. In addition, the Peterson and Neuber’s notch fatigue factor were compared through fatigue notch reduction factor, Kf, obtained from experimental data. According to results obtained it was possible to conclude that the tested material is less sensitive to notches than the prediction of the Peterson and Neuber’s empirical models
Propagazione di fratture longitudinali in gasdotti caratterizzati da bassa duttilità
No full textIl presente lavoro propone l'analisi di propagazione di una frattura fragile longitudinale di un gasdotto interrato basata sulla valutazione dell'energy release rate. Viene illustrata l'implementazione della suddetta metodologia all'interno di un codice proprietario agli elementi [miti con formulazione esplicita, illustrando gli interventi eseguiti sul preesistente algoritmo di propagazione stazionaria, e proponendo i risultati ottenuti a seguito di simulazioni dinamiche svolte su tubi in acciaio X80 e su provini SENB in pieno spessore sottoposti a prove DW1T
The way the mistery of the Mattei’s case was solved
No full textEnrico Mattei, the President of the Italian oil conglomerate, ENI, was about to land in Milan Linate Airport on October 27, 1962 when his airplane crashed on the ground due to a then unexplained accident. The investigation, reopened more than 30 years later, implied complete re-examining of the theories on macroscopic and lattice deformations under high velocity waves emanating from a small charge explosion.Various macro- and micro-structural changes are induced by an explosion and by the resulting shear stresses in metals exposed to it. At the microstructural level multiple slip bands or mechanical twins, induced from the pressure wave caused by an explosion, can be observed. The occurrence of either ones depend on the type of metal, the pressure and the strain rate. The temperature wave may also cause surface alterations. Different situations regarding stainless steels, aluminium, copper and gold alloys are analysed.Calculations to evaluate which deformation mechanism is eligible for different FCC metals and alloys are reported. Results of field explosion experiments are incorporated into the evaluation of microstructural signs possibly induced on metal targets by an unknown explosive event.Revisited theories were applied to the Mattei forensic case, reaching the conclusion that the aircraft had fallen following an on board small charge explosion
Fatigue crack propagation in turbine disks of EI698 superalloy
No full textIn-service fatigue cracking of turbine disks of EI698 superalloy is discussed based on crack growthanalyses. In the bolt joint for disks to shaft connecting there is high level of stress-state, which directed to earlierin-disks fatigue crack origination in low-cycle-fatigue regime. Fracture surface pattern such as fatigue striationswere used for their spacing measurement and crack growth duration estimating. Developed disk tests on aspecial bench by the equivalent program to in-service cyclic loads have allowed discovering one-to-onecorrelation between fatigue striation spacing and crack increment in one flight. Number of fatigue striations andbeach-marks calculations permitted to estimate crack growth period for the different stages of in-service diskscracking. Equivalent stress level for in-service cracked disks was calculated and compared with stress-level intesteddisks under stress equivalent program to in-service operated cyclic loads. Based on this result nondestructiveinspection intervals were discussed and recommended for in-service disks in dependence on numberof their flights at the moment of developed inspection to exclude in-flight disks fast fracture
Characterization of mechanical damage in granite
No full textThis paper aims to illustrate the use of infrared thermography as a non-destructive and non-contacttechnique to observe the phenomenological manifestation of damage in granite under unconfined compression.It allows records and observations in real time of heat patterns produced by the dissipation of energy generatedby plasticity. The experimental results show that this technique, which couples mechanical and thermal energy,can be used for illustrating the onset of damage mechanism by stress concentration in weakness zones