Sergio Baragetti

Effects of over-torque on stress relief in conical threaded connections

The analysis of the stress state induced by the make-up torque of tapered threaded connections is very important for the improvement of both the static and the fatigue resistance of such connections. Make-up torque induces a nonuniform stress state; in the same way, the axial load induced by the make-up torque is distributed in a nonuniform way along the sections of the coupled elements of the connections. The most important consequence is the overloading at the first threads engaged, which has to be summed to the stress concentration in the core section due to the notch effect. The aim of this paper is to analyze the stress and strain fields induced by make-up torque in tapered threaded connections; in particular, the purpose is to investigate the effects of the application of over-torque before putting the connections into service (stress relief due to over-torque before nominal make-up torque application). The evaluation of the local stress state and of stress relief, with a good level of precision, was performed by using finite element models in order to assess the percentage of load carried by each thread after the application of the make-up torque and further axial tensile or compressive loads. Numerical results confirmed the favorable thread load distribution induced by overloading. Experimental tests carried out on full-scale connections, used in the oil and gas industry for the joining of pipes, confirmed the numerical results.

Notch Corrosion Fatigue Behavior of Ti-6Al-4V

The aim of this paper is to map the corrosion fatigue characteristics of Ti-6Al-4V alloy through the evaluation of the corrosion fatigue initiation and failure mechanisms. The study included the effect of the stress concentration factor at very high Kt values and the role of different inert or corrosive environments. This alloy is widely used in naval-structures and aero-engine communities and the outcomes of the work will have direct relevance to industrial service operations. Axial fatigue tests (R = 0.1; 2 × 105 cycles; f = 10 Hz) were carried out on smooth and high notched (Ktmax = 18.65) flat specimens in laboratory air, paraffin oil, laboratory air + beeswax coating, recirculated 3.5% NaCl solution. The step loading procedure was used to perform the fatigue tests and the surface replica method and crack propagation gages were used to check crack nucleation and propagation until failure. Log-Log plots of σmax vs. Kt showed a bilinear behavior and enabled the demonstration of the presence of a threshold stress intensity factor (Kt = 8–9), after which the environment has no effect on the fatigue damage for all the tested environments.

Bending behaviour of double-acting hydraulic actuators

Abstract The buckling behaviour of hydraulic actuators is often established by using simple analytical models. However, existing research does not take into account rectilinear imperfections and friction effects at the restrained ends and in the area of the rod-cylinder connection (the port through which the rod enters the cylinder). Recent research has tried to establish the effects of actual geometrical imperfections and friction on plunger-rod actuators, thus enabling better use of these very common mechanical components. This paper analyses the bending behaviour of hydraulic piston-type actuators using analytical models, able to take into account friction, rectilinear imperfections and the actual geometric configuration of piston-type actuators. The reliability of these analytical models was confirmed by means of experimental tests carried out on full-scale equipment.

A numerical procedure for shot peening optimisation by means of non-dimensional factors

Shot peening is widely used to improve the fatigue behaviour of mechanical components. The improved performances of shot peened components are generally put into relation with the residual stress field induced. The aim of the present paper is to define a calculation approach, based on FE analyses, able to predict, once the shot peening parameters are known, the residual stress profile. In this way, it is possible to optimise the results of shot peening by taking into account the applied loads. The possible application of a non-dimensional number to summarise the shot peening conditions was evaluated. It was also possible to express non-dimensional residual stress profiles (residual stresses/yield stress of the material) as a function of this non-dimensional number. The measurements carried out on shot peened specimens by means of a X-ray diffractometer allowed for critical analyses of the results.

Fatigue behaviour of DLC coated Al 7075-T6 alloy in an aggressive mixture

7075-T6 aluminium alloy is commonly adopted in high performance structures and components. Its fatigue behaviour is however dramatically worsened by exposure to aggressive environments. The deposition of PVD coatings, which are commonly adopted to increase the surface properties of structural elements in terms of hardness, contact fatigue and wear resistance, could be beneficial also for the fatigue behaviour of a 7075-T6 substrate in an aggressive environment. In the present work, Diamond Like Carbon (DLC) PVD coated 7075-T6 specimens immersed in methanol have been analysed, by means of step-loading rotating bending fatigue tests (R = -1) at 2·105 cycles. Coated specimens were tested in laboratory air for comparison, and uncoated polished samples were studied in both the environments to obtain reference values. SEM micrographs of the fracture surfaces were taken to investigate the effects of the corrosive environment on the failure mechanism.

Fatigue behaviour of thin coated Al 7075 alloy with low temperature PVD coatings

7075-T6 is one of the most performing aluminium alloys, considering its mechanical properties and good fatigue behaviour. In this work the influence of WC/C and DLC PVD coatings on the fatigue behaviour was investigated by rotating bending tests at 2·105 cycles. The fatigue behaviour of polished and untreated specimens was considered as reference. In order to decouple the effect of the deposition temperature from the coating one, some uncoated specimens were submitted to the thermal cycles of the WC/C and DLC processes and then tested under fatigue loading. Fatigue life was determined using a step-loading technique. SEM micrographs of the fracture surfaces were taken in order to characterize the fatigue mechanisms of coated and uncoated specimens.

Experimental and Numerical Study of Shot Peened Thin Hard-Coated Components

A test bench was designed and assembled to carry out impact tests on samples and components. The system allows simple and rapid adjustment of the test parameters, such as the shot size and air pressure, with good repeatability of the results. Tests on steel and light alloys were carried out under both as-produced condition and on thin hard-coated samples. Significant reductions in dimple dimensions were seen after coating. FE models simulating the experiments overestimated the dimple depths, although the parameter trend was satisfactorily captured. The residual stresses from coating and shot peening determined numerically are believed to have been proven effective against fatigue.

Friction and tightening force of conical threaded

In this study the friction behaviour of conical threaded connections was investigated in various conditions by performing full scale make up tests. A new procedure for friction condition evaluation in threaded joints is proposed. Several make up/break out (M/B) tests were carried out and the behaviour of the threaded connections at different make up speeds was investigated; the results were summarised and organised through the use of probability charts. All the results of the tests provide the user with a procedure, based on a large data spectrum, that enables the evaluation of whether or not to reject a connection before inserting it in the perforation drillstring.

A numerical study on the fatigue and rolling contact fatigue behaviour of PVD-coated steel and titanium spur gears

Thin hard coatings deposited with physical vapor deposition (PVD) can enhance both the fatigue and the rolling contact fatigue resistance of mechanical components. In this work a cheap and fast way to evaluate the best parameter levels of coating and bulk material is proposed. Design of Experiments (DoE) was applied to the numerical results obtained from the simulation of meshing PVD-coated spur gears. Preliminary analyses were performed to assess the fatigue behaviour of PVD-coated standard specimens for rotating bending tests. The coating elastic modulus and thickness, and the trend of the residual stresses induced by the deposition process were considered among the variables affecting the fatigue and the rolling contact fatigue behaviour. Different bulk materials, including steel and titanium alloys, were analyzed. The proposed method may help to define the optimal coating design, especially when the replacement of traditional steels with light alloys constitutes a goal that is strongly recommended.

A Numerical and Experimental Study of the RCF Behaviour of PVD-coated Spur Gears

In this work the rolling contact fatigue (RCF) behaviour of WC/C PVD-coated and uncoated case hardened transmission gears for competition motorcycles was studied both numerically and experimentally. Finite element models of the gears were processed and a theoretical-numerical procedure was applied to the numerical results to predict their RCF life. The presence of the coating and the residual stress fields induced by the surface treatments were simulated. Besides 16NiCr11, which is the actual material of the gears analyzed, the Ti-6Al-4V alloy was also considered with the numerical models. Preliminary RCF tests were carried in both dry and lubricated condition to observe the damage on the tooth flanks with and without the presence of the coating.