Tensile test

The tensile test provides the basic material characterization to identify if the material is either Ductile or Brittle, and allows the accurate determination of the Young’s modulus, the Yield and Ultimate strengths and the percentage elongation at fracture. Three hydraulic testing machines are available at the DICI Mechanical department at maximum loads 100, 150 and 250 kN for various specimen sizes and materials. Specific extensometers are also available, along with dedicated software, to perform Fracture Mechanics tests both for plane-strain fracture toughness and for the J-R curve determination.

Hole-drilling method

Residual stresses are self equilibrated loads, embedded in any mechanical component, primarily dependent on the technological process history. The external loads applied to the component are superimposed to the residual stresses and their combination can either increase the mechanical strength or exacerbate the structural loads. The Hole-Drilling Method is an experimental technique to measure the surface residual stresses of metal components. A small hole is drilled concentrically with a strain gauge rosette, the stressed material is removed and the so called relaxed strains are measured by the rosette grids and then elaborated to find the residual stresses. RESTAN – MTS 3000 is an automatic system produced by SINT Technology that implements the Hole-Drilling Method providing a dedicated software for strain elaboration and residual stress determination. The collaboration with the DICI department was aimed at the elaboration software development and also experimental validation of the measuring procedure.

Fatigue

Drill strings are tubular long structures to drive the perforating drill bit for producing the oil borehole. The curved path followed by a drill string generates rotating and then cyclic load that can induce fatigue fracture especially at the drill pipe connections. The fatigue strength of these connections, either steel to steel or aluminum alloy to steel, is modeled and experimentally investigated at the DICI department in collaboration with ENI SpA Exploration and Production division. Large tubular sections are tested by means of a mechanical test rig designed on the basis of the resonance principle. A portion of the pipe, with the connection under testing in the middle, is dynamically excited near the first natural mode with rotating eccentric masses. The resonance implies both a lighter test rig structure, and a quite high working frequency allowing fatigue testing in the high cycle regime.

Deep Rolling for fretting application

Deep Rolling introduces a plastic strain field distribution at the surface and initial subsurface which in turn generates a compressive residual stress to improve the crack closure and then the fatigue strength. Different rollers can be used, such as the Ball roller for the Low Plasticity Burnishing, or alternatively others dedicated to specific notch details, e.g. a conical tungsten carbide roller with a rounded fillet and contacting edge. The Deep Rolling can also be applied to the fretting fatigue application where the alternating stress is exacerbated by the micro-wear at the contact edge of two mating parts, such as the shrink-fitted shaft-hub connection where, besides the residual stress, the material hardening produced a beneficial synergistic effect.