2013 Roma - IGF XXII

In this work the regimes of contact in misaligned crowned splined couplings have been analyzed. Experimental tests have been performed in order to identify if fretting damage on the components appears as fretting wear or fretting fatigue. A significant difference was identified on the surface of specimen by analyzing two different tests; the first test emphasized a fatigue damage and in the second test a wear phenomena has been achieved. Also a good correlation has been obtained by analyzing the fretting map obtained by using the Mindlin’s theory and experimental results

This paper analyzes hot torsion flow curves [1-5], microstructures [1,2,6-9], constitutive equations and extrusion finite element modeling (FEM) [10-13] of aluminum composites. Those results come mainly from previous studies of prof. H.McQueen et alii. Metal-matrix composites (MMC) of 6061, 7075, 2618 and A356 alloys with Al2O3 or SiC particles ( 15-30 µm) were produced by liquid metal mixing. Aluminum alloy matrices reinforced with particles of Al2O3 or SiC possess higher strength and stiffness as well as greater wear resistance and improved high temperature properties [14-17]. MMC produced by liquid metal mixing are secondarily fabricated by traditional mechanical forming (extrusion, forging or rolling) [18-21]. Materials were deformed over the temperature range 300 to 500°C and strain rates 0.1 to 4/5 s-1. At 400°C (and lower T) the strength of composites is higher than that of the alloys. With exception of 6061 and 2618, there is almost no difference in strength at 450°C while at 500°C composites appear to be softer than the alloys. 2618 MMC exhibit lower ductility then A356 and 6061 MMC that exhibit similar ductility. 7075 alloy and MMC decline from good ductility at 400° to very low at higher T because of GB precipitation [1-5]. The softening of the alloys with increasing T (and with decreasing strain rate) is due to improved DRV.

Some extreme working environments are characterised by corrosive conditions, able to develop hydrogen formation. The presence of atomic hydrogen localized in correspondence of plastic strains at the crack tip modifies the steel behaviour and its macroscopical mechanical properties. The phenomenon of hydrogen embrittlement is, indeed, one of the main responsible for the increase in fatigue crack growth rate and life reduction. For this reason, it is important to have validated numerical models able to estimate the mechanical behaviour of material in presence of hydrogen. Aim of this study is to develop a numerical model of two low-alloyed steels used in pipelines applications. Numerical simulations of C(T) specimens are carried out in different steps, considering hydrogen presence/absence combined with local plastic strains of the material. These two parameters are, indeed, responsible for a drop in material toughness, therefore for an increased crack growth rate. Two modelling techniques are used to simulate crack propagation: application of cohesive elements characterised by laws of degradation of mechanical properties, and the virtual crack closure technique (VCCT). Once model is validated by a comparison with experimental toughness measures, final considerations on the most valid simulation technique for the considered case are proposed.

Intersecting holes inside mechanical component, stressed with internal pressure, generate stress intensification; this kind of geometry detail is very common in the powertrain field. Triangular flaws has been taken into account at the intersection of two holes inside a specified specimen. Influence of bore hole D1/D2 and angle between their axes α are examined. Numerical analysis are performed to determine Stress Intensity factors (SIF) in many geometric configurations. Afterwards, fitting weight function’s parameters with FEM results, new SIF analytics expression are shown. Finally, the accuracy of weight functions in SIF predictions for different inner pressure has been checked.

For many years short fibre reinforced polymers have been commercially very attractive, mainly because of their superior mechanical properties over corresponding polymer matrices and their low-cost fabrication technique. Owing to their complex microstructure, especially in relation to fibre length and orientation distributions, an understanding of strength has not been well established. The aim of the present study is the investigation of the mechanical performance of injection moulded short glass fibre-reinforced polyamide composites. The glass fibre content of these composites was 35% by weight. Mono axial static tests were carried out. Specimens of the same material have been tested at different time starting form their production in order to analyse the natural ageing of the investigated composite material. The Digital Image Correlation (DIC) technique was applied during the tensile tests in order to obtain the stress-strain curves and to detect the failure zone at the early stages of the tests. From the experimental investigation, it results that there is a worsening of mechanical properties due natural aging of the test material.

Uniaxial static traction tests and fatigue tests, performed on smooth and notched specimens of C40 steel have confirmed that starting from the thermal analysis of the specimen under test it is possible to evaluate the fatigue limit of the material. Further, starting from post processing image analysis, the parameters that characterize the dynamic behavior of the material have been characterized; in particular in the case of notched specimen. Test results have shown that utilizing adequate sensors it is possible to make estimations very close to the real values of both the stress concentration factor and the fatigue-stress concentration factor. It is possible therefore to distinguish between the total elastic behavior and the partial elastic behavior that is the behavior characterized by some irreversible deformation of specific microarea of the material. The experimental results proposed in the present paper, have outlined the possibility to define some parameters related to the total deformation density energy. Those parameters can be determined also in the cases of local plasticization that usually does not allow to provide immediate and verified measurements.

For the first time, carbon nanotubes (CNTs) have been added to a grout. The samples were prepared with up to 0.8 wt% of CNTs, with respect to the binder, and were characterized by three-point bending and compressive tests. The results showed that these additions continuously increased the flexural and the compressive strength, as well as the fracture energy, for a CNTs content up to 0.12 wt% with respect to the binder.

Nickel-Titanium (NiTi) based shape memory alloys exhibit phase transition mechanisms and, consequently, common theories on mechanics of materials cannot be directly applied. Furthermore, local transformations near geometric discontinuities significantly affect crack formation and propagation mechanisms, under both static and cyclic loading conditions. A full field displacement measurement technique have been used in this investigation, which is based on the Digital Image Correlation (DIC) method, to analyze notched NiTi specimens. In particular, Single Edge Notch (SEN) specimens, obtained from a commercial pseudoelastic NiTi sheet, have been investigated. The localized stress-induced phase transformation mechanisms near the notch root have been analysed.

The use of Ni-Ti alloys in the practice of endodontic comes from their important properties such as: shape memory and superelasticity phenomena, good corrosion resistance and high compatibility with biological tissues. In the last twenty years a great variety of nickel-titanium rotary instruments, with various sections and taper, have been developed and marketed. Although they have many advantages and despite their increasing popularity, a major concern with the use of Ni-Ti rotary instruments is the possibility of unexpected failure in use due to several reasons: cyclic fatigue, novice operator handling, presence manufacturing defects, etc. Recently the use of an aqueous gel during experimental tests shown a longer duration of the instruments. The aim of the present work is to contribute to the study of the fracture behavior of these endodontic rotary instruments particularly assessing whether the use of the aqueous lubricant gel can extend the their operative life stating its reasons. A finite element model (FE) has been developed to support the experimental results. The results were quite contradictory, also because the Perspex (Poly-methyl methacrylate, PMMA) cannot simulate completely the dentin mechanical behavior; however the results highlight some interesting points which are discussed in the paper.

Advanced, high-strength steel sheets are increasingly used to make lighter and safer car-bodies. The main mechanical requirements of these steels in service are high fracture strengths and energy absorptions, relevant for crash tests, and fatigue endurances, relevant for ordinary car usage. The steel sheets are made by continuous casting, hot rolling, cold rolling and continuous annealing, or other continuous final heat treatments, and are then cold formed and welded to fabricate the car bodies. Two new high-strength car-body steels are examined here: a TWIP (TWinning Induced Plasticity) steel, which is already industrially available, but not yet widely used, and a Q&P (Quenching and Partitioning) steel, which was recently produced industrially as a prototype. The new steels are compared with a widely used, high-strength, DP (Dual-Phase) automotive steel grade. Automotive TWIP steels are high-Mn austenitic steels, with a medium-high C content, which exhibit a promising combination of strength and toughness, arising from the ductile austenitic structure, which is strengthened by C, and from the TWIP (TWinning Induced Plasticity) effect. The low-alloy Q&P steels are subjected to the Quenching and Partitioning (Q&P) final heat treatment, which consists of: 1) full or partial austenitizing; 2) quenching to the Tq temperature, comprised between Ms and Mf; 3) soaking at the Tp “partitioning” temperature, equal to or slightly higher than Tq, allowing carbon to diffuse from martensite to retained austenitized; 4) quenching to room temperature. The final microstructure consists primarily of low-carbon martensite, high carbon martensite, and carbon stabilized austenite. The fatigue behavior of these steels is examined both in the as-fabricated condition and after pre-straining and welding operations, which are representative of the cold forming and assembling operations performed to fabricate the car-bodies. Moreover, the microscopic fracture mechanisms are assessed by means of fractographic examinations.

Foams and porous materials with cellular structure have many interesting combinations of physical and mechanical properties coupled with low specific weight. By means of replication casting it is possible to manufacture foams from molten metal without direct foaming. A soluble salt is used as space holder, which is removed by leaching in water. This can be done successfully if the content of space holding fillers is so high that all the granules are interconnected. One of the main advantages of using the replication casting is a close control of the pore sizes which is given by the distribution of particle sizes of the filler material. This contrasts with the pore size distribution of the materials foamed by other processes where a wider statistical distribution of pores is found. On the other hand, the maximum porosities that can be achieved using space holders are limited to values below 60%, whereas the other methods allow for porosities up to 98%. Temperature of the mould and infiltration pressure are critical process parameters: a typical problem encountered is the premature solidification of the melt, especially due to the high heat capacity of the salt. In this work foam properties such as cell shape, distribution and anisotropy and defect presence are investigated by using digital image processing technique. For this purpose replicated AlSi7Mg0.3 alloy foams are produced by infiltrating preforms of NaCl particles, varying the metal infiltration pressure and the mould preheating temperature. An original procedure based on image analysis has been set up to determine size, morphology and distribution of cells. The paper demonstrates that this methodology, coupled with microstructural analysis is a useful tool for investigating the effects of process parameters on foam properties.

This paper describes the results of tensile tests and finite element (FE) calculations with representative volume elements (RVEs) of basalt fibre reinforced plastic with two different types of fabric reinforcements. As fabric reinforcements show repeating ondulations of warp and fill yarn, simple mixtures laws reach their limits. That is the reason why the mesoscopic dimension, lying between the microscopic and the macroscopic dimension, has to be taken into account when a mechanical characterization of fabric reinforced composites is carried out. The aim of this work is to determine the stiffness of a fabric reinforced composite in warp and fill direction with numerical investigations. The simulations are based on FE-calculation with two different RVEs. The tensile tests and the FE-calculations have been carried out for two different types of basalt fabrics, namely twill 2/2 and twill 1/3. The comparison between the experimental data and the results of the FE-calculations are provided in order to support the validity of the proposed model.

Pseudo-elastic (PE) materials are an important class of metallic alloy which exhibit unique features with respect to common engineering metals. Because of these unique properties, PEs are able to recover their original shape after high values of mechanical deformations, by removing the mechanical load (PE). From the microstructural point of view shape memory and pseudo-elastic effects are due to a reversible solid state microstructural diffusionless transitions from austenite to martensite, which can be activated by mechanical and/or thermal loads. Copper-based shape memory alloys are preferred for their good memory properties and low cost of production. This paper describes the main crack initiation and its propagation in an tensile test in order to evaluate crack path and its behaviour at low and at high values of deformation. Both grain boundary chemical properties and X-ray diffraction will be discussed in order to correlate structural transition involved in an Cu-Zn-Al alloy characterized by a PE behaviour.

The present paper is aimed at investigating the effect of shot peening on the very-high cycle fatigue resistance of the Al-7075-T651 alloy. Pulsating bending fatigue tests (R = 0.05) were carried out on smooth samples exploring fatigue lives comprised between 105 and 108 cycles. Three peening treatments with different intensity were considered to explore different initial residual stress profiles and surface microstructural conditions. An extensive analysis of the residual stress field was carried out by measuring with the X-ray diffraction (XRD) technique the residual stress profile before and at the end of the fatigue tests, so as to investigate the onset of a stabilized residual stress field. Fatigue crack initiation sites have been investigated through scanning electron microscopy (SEM) fractography. The surface morphology modifications induced by shot peening were evaluated using an optical profilometer. The influence of surface finishing on the fatigue resistance was quantified by eliminating the surface roughness in some peened specimens through a tribofinishing treatment.

A Light Utility Helicopter (LUH), in the course of a training flight, leaving the ground during the taxi to take off, went into an uncontrolled rolling to the right; consequently the helicopter gradually laid down on the right side. The impact with the runway destroyed the rotating blades up to the hubs rotor. The accident investigation focused on main rotor oscillatory plate servo actuators . These components, directly linked to the cloche movements, regulate main rotor blades plane tilt and pitch. Following the preliminary examination, only front servo actuator attachment was found to be broken in two parts. In detail, the present paper deals with the fracture analysis results. The servo actuator attachment material is a 2014 Aluminum alloy extrudate, undergone to T651 heat treatment. Fracture surfaces were examined by optical and electronic microscopy in order to determine the main morphological features and consequently to trace the origin of failure mechanism and causes. The accordance with the specification requirements about alloy composition was verified by quantitative elementary analysis through inductive coupled plasma spectroscopy (ICP); furthermore, semi-quantitative elementary analysis was locally verified by Energy dispersion spectroscopy X ray (EDS_RX). Finally, the hydrogen content of the material was evaluated by the total hydrogen analysis. Microstructural and technological alloy characteristics were verified as well by using metallographic microscopy and hardness testing of the material.

Alumina-mullite (AM) refractories are widely used as liners for the thermal insulation of the combustion chambers in gas turbines for power production. A complete thermomechanical characterization of a commercial AM refractory was performed according to the international standards for dense ceramic or ceramic composites. Four-point flexural test were carried out on standard specimens to determine the values of Modulus of Rupture (MOR) and Young’s modulus (E), at room temperature and up to 1500 °C: this temperature was chosen because the inlet temperature of the turbines for energy production was around 1400 °C. The most important property required to refractories for this application is the thermal shock resistance. In order to quantify it, four-point flexural tests at room temperature after quenching were carried out to calculate the residual MOR, according to the standard procedure for dense ceramics. The tests were performed with temperature differences up to 1000 °C, which is comparable to operating conditions during the turbine shutdown. New AM refractories were developed, by recycling a large amount (20 wt%) of industrial ceramic wastes coming from gas turbine investment casting process, and also adding zircon (ZrSiO4). Also for these materials, a thermomechanical characterization was performed, in order to compare the behaviour of the new materials to the commercial refractory’s one. Interesting results were found about the mechanical properties of the new materials. The refractories developed by recycling industrial ceramic wastes generally show better mechanical and thermal shock resistance than the commercial refractory taken into account.

In the present work, a computational strategy for the modeling of reinforced concrete beams with shape memory alloy (SMA) actuators for flexural cracks repair is developed. In particular, for the concrete, a nonlocal damage and plasticity model is adopted; the model is able to consider peculiar macroscopic behaviors which characterize the quasi-brittle materials, such as the tensile and compressive damaging, accumulation of irreversible strains and the unilateral phenomena. The development of the flexural cracks in concrete are modeled using the cohesive zone interface formulation, which accounts for the mode I, mode II and mixed mode of damage, the unilateral contact and the friction effects. The interface model considers even the coupling between the body damage and the interface damage ensuring that body damage and interface damage cannot evolve independently one from the other. A uniaxial SMA model able to reproduce both the pseudo-elastic behavior and the shape memory effect is adopted for the reinforcing SMA wires. Finally, finite element simulations are developed in order to reproduce the experimental behavior of smart concrete beams subjected to three-point bending tests.

The life of a mechanical component depends on the interaction between mechanical characteristics of the material of which it is made and to the stresses to which it is subjected. In order to determine the total stress acting in a mechanical component, in addition to the stresses caused by external loads imposed during the use, it is necessary to know the residual stress field resulting from manufacturing process and often associated with non-uniform plastic deformation. Typically, the residual stresses are not uniform throughout the deformed metallic material: these are detrimental because, very commonly, they reduce the elastic limit of the material and cause the tendency of the component to deform during subsequent processing. The particularly insidious aspect of residual stress is that its presence generally goes unrecognized until after malfunction or failure occurs. However, even if tensile residual stress fields reduce the mechanical performance of the material by causing the onset of brittle fracture and wear phenomena, the compressive residual stresses generally have a beneficial effect and cause a delay of the onset and the subsequent propagation of the fatigue crack.

In order to apply the “Leak Before Break” (LBB) Analysis is very important to know the cracks area. Particularly for pipes under bending the area depends from crack position related to the bending plane. Considering symmetrical cracks respect to the flexural plane cannot be a conservative assumption. In this work has been carried out an experimental investigation in order to evaluate COD distribution for off-axis cracks on pipes under bending. Have been tested pipes with different thickness and different cracks lengths and orientations. COD distribution has been monitored by digital image correlation (DIC). Experimental data have been used to validate an analytical model (HCM Hodograph Cone Method) previously proposed by the authors.

The cohesive zone model has been widely used for the description of quasi-static crack growth of interfaces and, recently, evolutions have been proposed in order to account for fatigue phenomena. In this work the cohesive zone model previously developed by the authors to simulate fatigue crack growth at interfaces in 2D geometries is extended to 3D cracks under mixed-mode I/II loading.