Giuseppe Pasquero

Fatigue Sensitivity to Small Defects of a Gamma–Titanium–Aluminide Alloy

The fatigue properties of a Ti-48Al-2Cr-2Nb alloy obtained by electron-beam melting (EBM) with a patented process has been examined by conducting high cycle fatigue tests performed at different R ratios at room temperature. Fatigue-crack propagation tests have been performed for the purpose of characterizing the fatigue-crack growth rate and threshold of the material. Additionally, specimens with artificially introduced defects have been fatigue tested with the objective of studying the growth behavior of small cracks. Artificial defects with different sizes have been generated in the gauge section of the specimens by electron-discharge machining (EDM). After EDM defects are produced, the specimens are pre-cracked in cyclic compression, so that small cracks can be generated at the root of the EDM starter defects. Fatigue tests are conducted by applying the staircase technique with the number of cycles of censored test (runout) fixed at 107 cycles. By employing the Murakami model for the calculation of the range of stress intensity factor, the threshold stress intensity factor range dependence on the loading ratio R and on the defect size is evaluated, highlighting the relevant parameters that govern the specific mechanisms of failure of the novel γ–TiAl alloy studied in the present work.

Fatigue Properties and Design Criteria of a Gamma Titanium Aluminide Alloy

The fatigue properties of a Ti-48Al-2Cr-2Nb alloy obtained by electron beam melting (EBM) with a patented process has been examined by conducting high cycle fatigue tests performed at different loading ratios both at room temperature and at high temperatures, comparable to those experienced by the components during service. Some tests have been conducted in the superlong life regime well exceeding 10 million cycles, highlighting individual fatigue characteristics of the studied TiAl alloy.

Assessment of Fatigue Reliability of Power Transmission Gearing for Aerospace Propulsion Applications

The fatigue strength in presence of inclusions is one of the main topics for the fatigue of high strength steels, especially the case hardened ones adopted for power transmission. In this paper we analyze the fatigue strength of carburized gears in presence of inclusions in order to develop the reliability assessment of a power transmission for aerospace propulsion. The high cycle fatigue properties of a high strength carburizing steel have been experimentally determined by conducting bending fatigue tests with specimens with artificial defects and a coherent model for the assessment of fatigue crack growth thresholds by taking into account of the residual stresses due to carburizing has been developed. In parallel, the distribution of non-metallic inclusions has been estimated by carrying out high cycle fatigue tests onto cylindrical smooth specimens and by inspection of mirror polished sections of the material. Eventually, by employing a simplified finite element model of a highly loaded gear wheel, the bending fatigue strength of gear teeth and the reliability of a turbo-propeller power transmission was determined by incorporating the fatigue crack growth thresholds, the measured residual stress profiles and the statistics of extreme non-metallic inclusions into a competing risks model applied to FE discretization.Copyright

Coating Pre-Cracking Effect in Combined Cycle Fatigue Tests of Superalloys for Gas Turbine Blades

In gas turbine engines for aerospace propulsion, the application of coatings on HP and LP stage blading where the highest temperatures are experienced is a common practice to prevent environmental degradation. However, since the strength of the coating is lower than that of the substrate material, upon loading the static strength of the coating may be exceeded and coating cracking may occur. In order to assess the effect of cracking in the coating on polycrystalline nickel superalloy MAR-M002, a number of combined cycle fatigue (CCF) and low cycle fatigue (LCF) tests with and without dwell have been carried out, at temperatures up to 870 °C. In order to experimentally assess the potential detrimental effect of coating cracking, controlled cracking in the coating prior to fatigue testing has been generated by using a special procedure. CCF tests have carried out by superimposing to strain controlled zero to maximum LCF cycles with dwell time stress controlled smaller HCF cycles, simulating the high loading ratio vibrations occurring in the blades. The loading mode applied in the CCF tests, even if much simpler than effective service conditions, is sufficiently representative of the loading experienced by the materials in correspondence of critical geometrical features of the turbine blades, where HCF amplitudes due to blade vibrations are superimposed to major (ground-air-ground) LCF cycles occurring during the regular service of the gas turbine engines. Comparison of the CCF and of the LCF tests with dwell with conventional LCF tests is presented herein, with special consideration of the effect of coating cracking.Copyright

Thermo-Structural Analysis of Ceramic Vanes for Gas Turbines

Advanced structural ceramics such as Hot Pressed Silicon Nitride (HPSN) and Reaction Bonded Silicon Carbide (RBSC), thanks to their low density (3.1 ÷ 3.4 gr/cm3) and to their thermostructural properties, are interesting candidates for aerospace applications. This research investigates the feasibility of employing such monolithic advanced ceramics for the production of turbine vanes for aerospace applications, by means of a finite element analysis. A parametric study is performed to analyse the influence of the coefficient of thermal expansion, the specific heat, the thermal conductivity, and the Weibull modulus on structural stability, heat transfer properties and thermomechanical stresses under take-off and flying conditions. A nodal point that is evidenced is the high intensity of thermal stresses on the vane, both on steady state and in transient conditions. In order to reduce such stresses various simulations have been carried out varying geometrical parameters such as the wall thickness. Several open questions are evidenced and guidelines are drawn for the design and production of ceramic vanes for gas turbines.