PhD in Technology, Innovation and Management – TIM XXXIII cycle
Report about the original contributions of the PhD thesis
Emanuele Vincenzo Arcieri
Tutor: Prof. Sergio Baragetti
In these three years Emanuele Vincenzo Arcieri has deepened the characterization of Ti-6Al-4V titanium and 7075 aluminum alloys. Their favorable strength-to-weight ratio helps to reduce the total mass of the components, making them highly performing and eco-compatible. The lower the weight, the lower the inertia and therefore the lower the fuel consumption. For this reason, they are commonly adopted in the aeronautical, naval and racing fields. Since the sectors in which these alloys are applied are very competitive, Emanuele Vincenzo Arcieri’s thesis contributes to describe the behavior of these materials under different loading and environmental conditions. The study takes into account the presence of damage, which are unfortunately always present in machine components. The thesis also helps to identify the effects induced by the deposition of innovative coatings on fatigue life. The application of these coatings can in fact improve resistance to corrosion and wear. However, even though these coatings can induce compression on the surface of components, their contribution to fatigue strength is questionable. The entire analysis was conducted by means of experimental tests and numerical simulations in order to assess the failure mechanism.
The research activity is divided into three sections: (i) quasi-static tests on Ti-6Al-4V; (ii) fatigue tests on uncoated and coated 7075-T6 samples; (iii) numerical studies on damaged 7075-T6 samples.
Quasi-static tests were performed on Ti-6Al-4V specimens. The alloy is not subject to STOA (Solution Treatment and Over-Aging). Without this treatment, components could be cheaper. Geometries with notches of different severity were tested in inert (air, paraffin oil) and corrosive (methanol) environments. Methanol and sharp notches are detrimental to the alloy, causing a significant reduction in its strength. However, moisture in the air can contaminate methanol, inducing a reduction in its aggressiveness. Methanol and steep stress gradients made cracks initiate at the notches. The analysis highlighted the biphasic nature of the material, which showed an intermediate brittle/ductile behavior. The alloy’s strength was influenced by the immersion time in methanol and the applied strain rate. The second part of the thesis presents the experimental campaign and the numerical analyses carried out to assess the fatigue strength of 7075-T6 specimens coated by physical vapor deposition. The performance was compared to that of the uncoated samples. High scatter was observed for the results of the tests on coated specimens. The reasons for this scatter were investigated, assuming the presence of defects in the coating, partial adhesion to the bulk material or spallation. The superposition of the bending stresses generated by a rotating bending moment with the residual stresses induced by the deposition process helped identify the areas subjected the highest stresses, tensile in nature, where crack nucleation is expected in the presence of a uniform defect distributions. The favorable compressive state induced by the deposition leads to crack nucleation below the sample surface. Observation of the fracture surfaces confirms this result.
Modifications in the fatigue strength of 7075-T6 components induced by damage were investigated with numerical simulations. In detail, a normal and an oblique impact of a steel sphere on 7075-T6 hourglass samples were simulated. The results revealed that the oblique collision generates higher axial tensile stresses than the normal impact. The samples were then considered tested with a rotating bending test. The areas where crack nucleation is expected were again determined by summing the residual axial stresses with the bending stresses. The areas where cracks can nucleate are located near the crater and on the crater rim. The maximum total stresses were reached in the vicinity of the crater, while on the crater the high stresses plus the maximum relative tangential displacements induce crack nucleation. For the oblique collision, the most stressed area is nearer the crater in the part of the section towards which the velocity of the sphere is directed.
Globally, the maximum total stresses found in normal and oblique collisions are very high, tensile in nature, inducing a low fatigue life.
The results obtained by the PhD candidate are excellent. The work was presented at various national and international conferences and published in scientific international journals.
Emanuele Vincenzo Arcieri is willing to listen and learn, and proactive too. In these three years, the candidate has shown great predisposition to scientific research, curiosity, effort, and ability to work both alone and in a team. He is strongly oriented to build his skills through practical work. He has improved his already appreciable skills in experimental activities and numerical modelling. He has combined the two approaches to verify the results by acquiring a valid research method.
For the reasons indicated above, I think that the research activity conducted by the candidate is excellent.
Bergamo, 29/09/2020
