Transition from small to large cracks in Ti-6Al-4V specimens

Light alloys allow us to have light components with interesting mechanical features. Ti-6Al-4V is a bimodal titanium alloy employed in aerospace, automotive, maritime and biomedical applications. This alloy has also a good corrosion strength which can be reduced by damages on the passivating surface layer. These damages can be due to an inadequate adhesion of the surface layer, variable loads and interactions with aggressive media [1,2]. For these reasons, quasi-static and fatigue tests on Ti-6Al-4V specimens in different inert, aggressive and very aggressive environments were carried out in the past. Air, air + beeswax coating, paraffin oil, 3.5wt.% NaCl-water solution and water-methanol solutions with several percentages were investigated in order to evaluate the chemical and mechanical forces of the corrosion fatigue phenomena [3]. Stress corrosion cracking (SCC) tests on a low strength steel [4] showed that the threshold of dJ/dt decreases with decreasing deformation rate and that the electrochemical energy contribution on the crack growth is independent from the displacement rate but dependent from the electrochemical conditions at the crack tip. As stated in [5] the crack size effect must be considered because small cracks have very high growth rate. Slow strain rate tensile and low-amplitude cyclic tests on micro-notched high strength low alloy steel specimens showed that crack growth strongly depends on the notch-tip plastic zone and hydrogen activity itself. High cycle fatigue tests with different notch shapes showed in [7] that the maximum stress and gradient increase with decreasing defect size. The stress state at the notch root is a function of the geometry. In [7] an elasto-plastic FE modelling with a multiaxial fatigue criterion and a correction for the stress gradient is also shown. Tanaka et al. [8] applied the fracture mechanics approach to fatigue crack initiation for also small notch-tip radius. In [9] micro-notched 316L steel specimens in a chloride medium gave the crack propagation rate in function of the global loading. A FE model was also developed in [9]. Micro-notched Ti-6Al-4V specimens were machined to get various notch lengths (up to about 100 ) & tested under static & cyclic loads to obtain fracture properties, incubation times. The notches were made using Electro Discharge Machining (EDM) in order to reach various values of stress concentration factor ( ) without notch tip plasticity. The tests were carried out on a testing machine previously designed by the Structural Mechanics Laboratory (SM-Lab) of the University of Bergamo and now modified so that bigger specimens can be used and easier setting can be reached. During the tests an axial load was applied with a fixed increment of it every fixed time. The machine has specific grips in order to avoid unwanted bending moment on the specimens. Test results are plotted on Kitagawa diagram to analyze the role of environment on static and cyclic applied loads. At the threshold i.e. at the endurance limit (horizontal line in Kitagawa diagram) we calculated: . [1] Brown B.F. 1972: 147–244, Naval Research Laboratory, Washington, D.C. [2] Lee E.U., Vasudevan A.K., Sadananda K. International Journal of Fatigue 2005; 27: 1597-1607. [3] Baragetti S., Villa F. Fracture and Structural Integrity 30 (2014), 84-94. [4] Sarioğlu, F. and Doruk, M. Conf.Proc., Advances in Fracture Research (ICF-7), Houston, Texas, USA, ed. K. Salama, 1989, 1, pp. 259–65. [5] Gangloff RP. Met Trans A 1985;16A (5):953-69. [6] Bosch, C., Delafosse, D., Longaygue, X. European Corrosion Congress 2010 (Eurocorr2010), Sep 2010, Moscou, Russia. 2, pp.1558-1570, 2010. [7] Franck Morel, Anne Morel, Yves Nadot. International Journal of Fatigue,Elsevier, 2009, 31 (2), pp.263-275. [8] Tanaka K., Nakai, Y., Kawashima, R. Engineering Fracture Mechanics 18, 5, pp.1011-1023,1983. Tinnes, J.P., Delafosse, D., Bosch, C., Raquet, O., Santarini, G. Euroorr 2004

study on a new mobile anti terror barrier

Abstract The vehicle-ramming terror attacks in Berlin, Barcelona, London and Nice highlighted our vulnerability: all of us could be wounded or killed during a walk in a crowded place, it is sufficient a car, a van or a truck. The authors of this paper designed a planter full of water and mainly made of steel and cast iron. For this reason, this device serves as both mobile anti-terror barrier and street furniture. This barrier can stop a 3500 kg vehicle running at 64 km/h and the system itself in less than five meters as demonstrated by the experimental crash test. Starting from these considerations, a simplified mathematical model of the impact was developed and a finite element model was calibrated. The first one points out the main features needed by the obstacle; the second one is a good base for further analyses

Effects of FOD on fatigue strength of 7075-T6 hourglass specimens

The term "Foreign Object Damage (FOD)" refers to the damage associated with the impact of particles on aircraft engine components that causes changes in fatigue strength as a result of induced stress concentrations, residual stresses and microstructural changes. This paper presents the Finite Element (FE) models created to study the stress field induced by the impacts of a steel sphere on a 7075-T6 hourglass specimen. The impacts occur at the minimum cross section, in the normal and oblique directions. The results reveal that in the event of an oblique impact the axial tensile stresses are greater and closer to the crater created by the impact. The superposition of the residual axial stresses to the bending stresses by fatigue test allows to identify the areas in which the maximum stresses are reached and therefore crack initiation is expected

Impact of DLC Coating Deposition on the Fatigue Strength of Al-7075-T6 Aluminum Alloy

Abstract: Al-7075 has interesting mechanical properties but is susceptible to corrosion. Physical Vapor Deposition (PVD) of coatings results in good corrosion resistance and compressive stresses of the order of 1 GPa on the surface of metallic components. However, the impact of PVD films on the strength of Al-7075-T6 is uncertain. This paper provides a summary of the findings of the Authors’ research group in recent years on the fatigue behavior of Al-7075-T6 with and without PVD Diamond-like Carbon (DLC) coating. The results indicated that DLC-coated specimens have lower fatigue strength than uncoated specimens for lives up to about 10000000 cycles. The failure mechanism was determined by observation of the fracture surfaces of the failed specimens. The stress analysis performed confirms the experimental observation, with crack nucleation expected below the surface of coated specimens, where the highest tensile stresses occur during fatigue loading

Quasi-static crack propagation in Ti-6Al-4V in inert and aggressive media

High strength-to-weight ratio Ti-6Al-4V alloy is used in many engineering applications. Its surface oxide film can protect the substrate from interaction with a lot of corrosive environments. Unfortunately, this surface layer can be damaged under mechanical and chemical actions with a consequent reduction in corrosion resistance. A characterization of the untreated alloy under quasi-static loading is here provided. Inert and aggressive environments have been investigated and the influence of the notch geometry of the alloy has also been analyzed

Rotating Bending Fatigue Tests on Uncoated and Dlc Coated 7075-T6 Aluminum Alloy

7075-T6 aluminum alloy is commonly used in high strength-to-mass ratio applications. However, its usage in corrosive environments, with fatigue loads, is still under investigation. Physical Vapor Deposition (PVD) coatings can improve the tribological properties of the surface. R=-1 rotating bending tests on uncoated and PVD Diamond Like Carbon (DLC) coated 7075-T6 specimens were carried out in air in order to evaluate the contribution of the coating deposition on the fatigue strength of the alloy

A new mobile anti-ramming system

The recent vehicle-ramming terror attacks in Berlin, Barcelona, London and Nice pointed out our vulnerability: all of us could be victims during a walk in our squares or in crowded places, it is sufficient a car, a van or a truck. The current solution to this problem is the application of concrete barriers in our cities. These devices are not goodlooking and give a sense of danger to the human beings. But are they effective? Starting from this question, the authors studied the effectiveness of these barriers and developed a new goodlooking mobile device. The study was carried out by means of mathematical and numerical models. In particular, a spring-mass system was used in order to macroscopically represent the problem and finite element analyses by explicit calculation in Abaqus were carried out. Also Smoothed Particle Hydrodynamics (SPH) method was used in the numerical models. The final shape of the barrier is a planter full of water which is urban furniture and anti-ramming system at the same time. The barrier is made of metal, without foundation and characterized by high friction coefficient between the ground and the barrier itself and a good mass distribution, studied to stop the vehicle in few meters. The high deformability and the ability of the water in the barrier to take away a lot of energy allow to stop a 3500 kg vehicle running at 64 km/h and the system itself in less than four meters as demonstrated by the crash test carried out. As consequence of this crash test, the anti-ramming system is IWA 14-1:2013 and PAS 68:2013 certificated

Fracture surfaces of 7075-T6 specimens tested in rotating bending fatigue tests and evaluation of the centrifugal force contribution

7075-T6 is a common high strength-to-mass ratio aluminum alloy. Its application in corrosive media under cyclic loading has to be more deeply investigated. Physical Vapor Deposition (PVD) coatings could improve the tribological properties of the surface. The here described R=-1 rotating bending tests on uncoated and PVD Diamond Like Carbon (DLC) coated specimens in air environment aim at evaluating the influence of the coating deposition on the fatigue strength of the alloy. Different fracture surfaces morphologies were detected and for this reason the stresses induced by the rotation of the specimens (without bending) were calculated by means of finite element analysis

Experimental tests and mathematical and numerical models for the impact dynamics: an application for the conception of an anti-terror barrier

The impact dynamics of a pendulum against several obstacles is investigated. The investigation is carried out by means of experimental tests and mathematical and numerical models. The study aims at identifying the best filler for an innovative mobile anti-ramming system