Giorgio Olmi

Introduction to Inverse Problems

Experimental Stress Analysis has been traditionally applied—through a direct or forward approach—for solving structural mechanical problems as an alternative and complementary methodology to the theoretical one. The great development of numerical methods has largely overruled this task. In addition, the increased accuracy of numerical tools has confined the forward approach to the role of experimental verification restricted to cases of complex and non-conventional numerical modeling, such as stress states resulting from singularities, material anisotropy, etc. If, however, causes (such as forces, impressed temperatures, imposed deformations) or system parameters such as geometry, materials and boundary conditions are unknown, the case is totally different, and the experimental inverse approach has no alternatives. Through measurements of the effects like displacements, strains and stresses, it is possible to find solutions to these inverse problems by identifying the unknown causes, integrating a series of experimental data into a theoretical model. The accuracy of data together with a proper selection of the quantities that must be measured are a necessary premise for limiting the experimental errors that can influence the accuracy of the inversely estimated results.

Low Cycle Fatigue Experiments on Turbogenerator Steels and a New Method for Defining Confidence Bands

The 26 NiCrMoV 14 5 and 18Mn18Cr steels have wide applications in the manufacturing of rotors and coil retaining rings. Current studies in the literature mainly regard their metallurgical properties, while little information is available about their cyclic and fatigue behavior. There is also little information regarding possible anisotropic behavior along the tangential and radial forming directions of the aforementioned components. The cited items were tackled by running an experimental campaign with specimens machined from real components in the tangential and radial directions. The experimental procedure was very careful from the points of view of methodology, of strain control, and of fracture evaluation. Static, cyclic, and low cycle fatigue curves were determined. The data were processed for the computation of curve parameters. While an original statistical analysis inspired by ASTM E739-10 was conducted to determine hyperbolic confidence bands “wrapped around” the entire fatigue curves, relating total strain amplitudes to lives. To tackle the question of anisotropy, a novel formulation of the analysis of variance was applied to compare the fatigue curves of both materials along the two forming directions.

The influence of lubrication on the frictional characteristics of threaded joints for planetary gearboxes

Planetary gearboxes generally consist of a ring gear, or gear body, connected with the input and output flanges by means of several screws, equally spaced along the diameter. The ring gear is manufactured with steel, whereas the flanges are usually made of cast iron. These screws must provide axial preload between the parts, allowing the assembly withstanding the breakaway torque given by the difference between the output and input torque applied to the gearbox. For a given screw geometry, the axial preload can be calculated, provided that the friction coefficients in the thread and in the underhead are known. Most often, the tightening torque is the only parameter being controlled during assembly and service operations. Hence, it is mandatory to know the friction coefficients of the joint. These depend, among others, on the hardness, roughness and texture of the mating surfaces, as well as on the lubrication state of the joint. In fact, the addition of a lubricant modifies the tribological behavior of the joint, thus the wearing evolution of the surfaces across repeated tightening operations. The present work tackles the following two aspects: (i) the characterization of the preloading force–tightening torque relationship on the actual component by means of a dedicated specimen, (ii) the evaluation of the influence of lubrication on the evolution of the frictional characteristics of the joint across several re-tightening operations. The present work has been carried out by means of both numerical finite element analyses and experimental stress analysis techniques.

Analysis of the Stress State in Brake Caliper Mounts of Front Motorbike Suspensions

The aim of this work is to analyze the stress and the strain fields in the brake caliper mounts of front motorbike suspensions produced by the braking action. First of all, some formulae useful to evaluate the maximum braking force in function of the vehicle features (e.g., total mass, centre of gravity position, tyre dimension, and brake disk diameter) have been developed. A mathematical model useful to calculate the axial, the bending, and the torque stresses on the braking caliper mounts has been then defined. The model has been developed by comparing the theoretical results with those obtained by some numerical analyses, based on the finite element method. An ad hoc test equipment has been, finally, designed and manufactured in order to define and analyze experimentally the strength of different types of brake caliper mounts and, at the same time, to verify the proposed model.

Fatigue Life Improvement of Holed Plates Made of an Innovative Medium C Micro-Alloyed Steel by Local Plastic Deformation

This paper deals with the influence of local plastic deformation on the fatigue strength of holed plates manufactured with an innovative medium-carbon micro-alloyed steel with high silicon content (hi-Si MCM). Local deformation around the hole is achieved by means of an interference fitted pin. The effect was investigated both experimentally and numerically. Microstructural characterization, hardness, and tensile tests were carried out first. Tension–tension fatigue tests were performed under two different conditions: open-hole (OH) specimens and specimens with a press fitted pin with 0.6% nominal specific interference. A 2D elastic–plastic finite element analyses (FEAs) investigation was done as well, in order to analyze the stress field in the vicinity of the hole. The stress history and distribution in the neighborhood of the hole indicate a significant reduction of the stress amplitude produced by the external loading (remote stress) when a residual stress field is generated by the pin insertion. In fact, experimental stress-life (SN) curves pointed out increased fatigue strength of the interference fit specimens, compared with the OH ones. Finally, scanning electron microscope (SEM) analyses of the fractured fatigue specimens were carried out, in order to investigate the mechanisms of failure and to relate them to the peculiar microstructural features that characterize this innovative steel.

Analysis of the Influence of Fretting on the Fatigue Life of Interference Fitted Joints

The present paper deals with the fatigue behaviour of shaft hub interference fitted joints. The investigation is carried out by means of a down-scaled ad-hoc specimen, which has been developed by the authors in order to perform accelerated fatigue tests. The specimen is similar to those suggested by standard ISO 1143, but it consists of two parts, joined together by shrink fitting. In a previous work, the authors compared the performances of a plain specimen manufactured according to ISO 1143 and those of the shrink fitted specimen. Both the plain and the shrink fitted specimens were made of C40 EN 10083 low carbon steel. The outcome of that research was that shrink fitting determines a degradation of the fatigue response of the assembly, with respect to those of the plain specimen. Moreover, such decrease cannot be predicted by means of FEA alone, since it is partly due to fretting phenomena. In fact, fretting takes place on the mating surface between the shaft and the hub, and especially in the vicinity of the end of the contact. The present paper deals with the observation of failed and survived specimens by means of optical and SEM microscopes, in order to determine the actual tribological characteristics of the contact surface. For instance, the amplitude of the sliding zone observed experimentally is compared with that given by FEA for different choices of the contact formulation. Since fretting is often associated with the presence of secondary fatigue cracks, which do not propagate, the authors set up an experimental method for locating the secondary cracks prior to sectioning the specimen for microscope observation.Copyright

Fatigue Life Characterisation of Interference Fitted Joints

Reversed bending fatigue tests to be performed on full scale assemblies, joined by interference fitting, are very complicated to set up, as well as very expensive to carry out, due to material costs and to low achievable testing frequencies. Moreover, along with the interference level and the shape of the hub edge, the method by which interference is created is deemed to influence the fatigue behaviour of the assembly. A quicker and cheaper way to obtain information about the fatigue behaviour of such assemblies could be performing fatigue tests on down scaled specimens, similar to those suggested by standard ISO 1143. Since no standardized specimen exists for characterising the fatigue behaviour of interference fitted shaft hub joints, the authors designed an ad-hoc specimen (“notched” specimen) and set up a convenient assembly procedure for performing rotating bending fatigue tests. The present investigation focuses on an interference fitted joint, made of EN 10083-2 C40 low carbon steel, whose hub bore edge is rounded. The rationale of the experimentation is retrieving the fatigue stress concentration factor by comparing the endurance limit results of the “notched” specimen to those of a plain specimen manufactured according to ISO 1143. Results were analysed according to standard ISO 12107. Furthermore, a three dimensional numerical model was developed, suitable for evaluating the local stress state of an axisymmetric interference fitted joint. The numerical model accounts for the effect of an external bending moment applied to the shaft. Experimental results are then compared to those given by finite elements analyses, in order to find out the relationship between the fatigue life and the local stress state of interference fitted components.Copyright

Strain gauge analysis of implant-supported, screw-retained metal frameworks: Comparison between different manufacturing technologies

Over the past decades, the technological development in the medical field, coupled with the ongoing scientific research, has led to the development and improvement of dental prostheses supported by screw-retained metal frameworks. A key point in the manufacture of the framework is the achievement of a passive fit, intended as the capability of an implant-supported reconstruction to transmit minimum strain to implant components as well as to the surrounding bone, when subject to any load. The fitting of four different kinds of screw-retained metal frameworks was tested in this article. They differ both in materials and manufacturing process: two frameworks are made by casting, one framework is made by computer-aided design and computer-aided manufacturing and one framework is made by electric resistance spot welding (WeldONE, DENTSPLY Implants Manufacturing GmbH, Mannheim, Germany). The passivity of the frameworks was evaluated on the entire system, composed of a resin master cast, the implant analogues embedded in the cast and the frameworks. Strains were recorded by means of an electrical strain gauge connected to a control unit for strain gauge measurements. The experimental tests were carried out in the laboratories of the Department of INdustrial engineering at the University of Bologna. The results of the test campaigns, which compared three samples for each technological process, showed that no significant differences exist between the four framework types. In particular, the frameworks made by the resistance welding approach led to a mechanical response that is well comparable to that of the other tested frameworks.

A user-friendly computational algorithm for the structural analysis of wrapping machine rotating rings

Wrapping machines usually consist of a two- or a four-column frame, supporting a huge rotating ring, connected to a pre-stretch unit with film coil carrier. Stiffness is a key point of packaging machines, since it is strictly related to the accuracy of the wrapping task. It depends on the stiffness of the frame, which can be achieved by the four-column architecture, and on the ring constraint system. As a consequence, the ring structures are usually highly statically indeterminate. Nowadays, there is an increasing demand for higher rotational speeds and more reduced operation times. Therefore, an accurate structural analysis of the ring, considering its actual loading and constraints is more and more important. The structural analysis of the rotating ring is treated by many references; however, such a statically indeterminate constraining makes this problem highly complicated. The goal of this paper consists in the development of a general and original computational algorithm for the structural analysis of rotating rings. The results are collected in a user-friendly way in terms of normalized internal loads, so that they can be of a great help even for not expert users. This model has been experimentally validated and easily applied to case studies and optimization tasks.

Load Cell Training for the Students of Experimental Stress Analysis

Many numerical models are nowadays available for the structural analysis of complexly shaped structures. However, a critical problem consists of the estimation of the actual loads that a structure withstands. The most proper way to determine them under service conditions consists of executing in-field tests, where load cells have an important role. This article deals with a didactic project at the University of Bologna, focused on the development of strain gage load cells, from design to calibration and use. Details are provided on the course units involved and on the levels of teaching, and three case studies are presented. Two case studies deal with the design of decoupled load cells, using simple beam geometry. Students had the opportunity to become confident with the principles ofmechanics and to tackle the not trivial activities like calibration by decoupled load application and determination of the related matrices. The third one deals with the development of a load cell for impact loads, where the students had the chance to observe a practical application of the studied Wheatstone bridge connection, to participate in on-field tests, and to better understand the close relationship between experimentation and design.