MECHANICS OF MATERIALS
High temperature fatigue and creep
Studies in this area have focused on the characterization of the creep-fatigue interaction phenomenon and on the qualification of the related analysis techniques and prediction models. As far as the second aspect is concerned, the influence of complex load histories and/or multiaxial stresses has been studied in particular.
The research activities in this sector have been carried out in collaboration with Italian and foreign research institutions and scientific committees (ENEA, EC Working Groups on “Codes and Standard” AG 2 and AGT 9B)
Fatigue in corrosive environment
Prof. Bertini has conducted a broad theoretical and experimental activity concerning the behavior of structural steels under fatigue-corrosion conditions. The research concerned in particular the propagation speed of fatigue cracks in a marine environment (steels for “offshore” applications) and in operating environments typical of geothermal applications (steels for drilling strings).
Fatigue in the presence of high mean stress
The behavior of steels under cyclic stress conditions characterized by high levels of average stress (R>0.7) is a complex problem, still poorly characterized to date, which is of great importance for the prediction of the fatigue behavior of components characterized by high preloads combined with cyclic external loads (e.g. large threaded joints). A large experimental campaign was conducted on HSLA steels for use in compressors, which highlighted the possibility of complex phenomena of “ratcheting” and/or creep under stress, interacting with the external stress cycle
Fracture mechanics
In this field the research work has mainly been carried out in the sector of “Weight Functions”, for the determination of which a new approach has been proposed based on the use of a particular property of the WFs themselves, which requires significantly reduced computational costs compared to the traditional techniques.
A result of significant scientific and applicative interest was then the demonstration that the WF allows an accurate evaluation of the complete state of stress (also including the non-singular terms) in front of the apex of the defect.
Finally, the WFs were developed for some cases of significant applicative interest, including the surface semi-elliptical crack subject to completely general stress states and the inclined surface crack, demonstrating their usability also for the solution of non-linear problems (contact between the fracture surfaces).
Hydrogen embrittlement
Studies have been conducted aimed at characterizing the influence of some relevant parameters (chemical composition, heat treatment, multiaxiality of the state of stress, presence of notches) on the susceptibility to hydrogen embrittlement of some classes of low activation steels proposed for the production of the first wall of future fusion reactors. Innovative test techniques were also developed, which made it possible to conduct “tests” in the presence of very high hydrogen concentrations and high temperatures.
Ceramic coatings
In this sector, an articulated experimental activity was carried out, aimed at the development of reliable techniques for measuring the main mechanical properties (Young’s modulus, Poisson’s ratio, coefficient of thermal expansion) of ceramic coatings. The measurement techniques developed were then employed for the characterization of two-layer coatings (bond coat + top coat) on a Nickel superalloy substrate intended to be used in the field of gas turbines.
Additive Manufacturing
An extensive activity was conducted to develop an integrated multi-level analytical-FEM simulation of Additive Manufacturing process, aimed at predicting mechanical properties, shape distortions and residual stress distribution. Additional studies were devoted to mechanical properties characterisation of metallic components obtained by AM, in particular as regards fatigue behaviour.
FATIGUE STRENGTH OF WELDED JOINTS
Relevant theoretical and experimental studies on the fatigue behavior of welded joints under different conditions (light alloys or steels, low or high temperatures) have been conducted in collaboration with the industries of the sector. The main aim of these studies was the development/qualification of techniques for predicting the fatigue life of welded joints present in mechanical components (e.g. automotive suspension arm) taking into account the possibilities offered by modern Finite Element analysis tools.
Studies have also recently been conducted concerning the behavior of “full-scale” welded joints of the “pipe-to-plate” type in the presence of simultaneous bending and torsion stresses acting with different phase shifts between them, in order to determine the combined evolution of fatigue damage.
EXPERIMENTAL METHODS
Residual stresses
As far as residual stresses are concerned, research activities have been oriented towards the development of new measurement techniques and the analysis of the effects on the propagation of cracks due to fatigue.
A hybrid destructive technique has been developed, based on strain gauge measurements and influence coefficients obtained through Finite Element (EF) models, which allows a complete estimate of the self-tension fields present inside mechanical components, even of large size.
Two important extensions of the semi-destructive technique known as “(blind or through) hole” have also been developed, which allow the measurement of residual stresses having a very high value (up to 90% of the yield strength) or variable in thickness.
Finally, a methodology for analysing the effect of residual stresses on the advancement of fatigue fractures was developed, based on the “Weight Functions” (WF) technique.
Modal analysis
A robotic station for the fully automatic modal analysis of structures and components has been set up.
The CAD solid model of the component to be analyzed is inserted into the Control Computer and the grid of points in which to carry out the measurement is defined.
Subsequently, while the objective component of the test is excited via a shaker, a robotic arm positions a Doppler effect laser velocimeter, in succession, on the grid of measurement points previously defined.
As a result, we have the automatic determination of the proper modes of the structure and of the related mode shapes.
BEHAVIOR OF OIL DRILLING BATTERIES
Prof. Bertini has been responsible for a series of research activities, in collaboration with ENI, aimed at determining and improving the fatigue resistance of full-scale components of oil drilling strings (heavy rods or “Drill Collars” and light rods or “Drill Pipes”).
In this context, resonance machines have been developed capable of subjecting full-scale battery elements to rotating or alternating bending fatigue tests, and test campaigns have been conducted on both steel and aluminium components.
Theoretical-design studies have also been conducted aimed at improving the performance of the drilling strings, both through the improvement of the distribution of tension in the threaded joints, and through the introduction of new construction concepts aimed mainly at increasing the distance that can be reached with horizontal drilling (“extended reach drilling”).
ANALYSIS OF STRUCTURES AND COMPONENTS
Prof. Bertini has carried out some research activities on the application of reliability techniques to the evaluation of the integrity of structures, dealing in particular with the development of a methodology for predicting the probability of rupture of drilling strings for geothermal wells.
Prof. Bertini conducted an extensive study aimed at developing a mathematical model for predicting the operating efficiency of V-belt expandable pulleys transmission (“Continuously Variable Transmissions or CVT) for vehicle use.