Key Engineering Materials Vol. 627

Abstract: The present paper summarises the results from uniaxial-tension stress-controlled fatigue tests performed at different temperatures up to 650°C on Cu-Be specimens. Two geometries are considered: hourglass shaped and plates weakened by a central hole (Cu-Be alloy). The motivation of the present work is that, at the best of authors’ knowledge, only a limited number of papers on these alloys under high-temperature fatigue are available in the literature and no results deal with notched components.The Cu-Be specimens fatigue data are re-analyzed in terms of the mean value of the Strain Energy Density (SED) averaged over a control volume. Thanks to the SED approach it is possible to summarise in a single scatter-band all the fatigue data, independently of the specimen geometry.

Abstract: 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.

Abstract: Experimental tests investigating very-high-cycle fatigue (VHCF) properties of materials are commonly performed with ultrasonic testing machines, which allow for a significant reduction of testing time. In order to evaluate the effect of tested material volume (size-effect) on VHCF properties, the Authors recently proposed to adopt Gaussian specimens for VHCF tests. Investigation of size-effect with Gaussian specimen induces large mechanical power dissipation and temperature increment that must be taken into account. The present paper proposes an analytical model, which allows to approximately predict the dissipated mechanical power and the temperature increment in Gaussian specimens. The analytical model is also numerically verified through a Finite Element Analysis.

Abstract: The scientific community involved in the study of the gas storage processes faces with the problem of the gas hydrates. In fact when the natural gas goes in contact with liquid water the methane hydrates take place; such hydrates can be considered as a solid structure which could abrupt partially or totally a gas pipeline. In such cases, the differential pressure across the plug can put it in movement producing catastrophic scenarios in the whole plant. In this work a parametric analytical analysis was conducted in order to define the plausible kinematic conditions of the hydrates mass. Once selected an opportune set of boundary conditions, some of the consequences of the motion of hydrates were taken into account, such as the amplification of the displacements related to resonance phenomena, the inertial effects and impacts against the pipe internal walls.

Abstract: During the last few years various experimental destructive and non-destructive methods were developed to evaluate residual stresses. However it is impossible to obtain a full residual stress distribution in welded structures by means of experimental methods. This disadvantage can be solved by means of computational analysis which allows to determine the whole stress and strain fields in complex structures. In this paper the temperature distribution and residual stresses were determined in a single-pass butt joint welded by GMAW (Gas Metal Arc Welding) process by finite element model (FEM). A 3D finite parametric element model has been carried out to analyze temperature distribution in butt weld joints and thermo-mechanical analyses were performed to evaluate resulting residual stresses. Temperature fields have been investigated by varying an initial preheating treatment. Moreover the technique of “element birth and death” was adopted to simulate the process of filler metal addition The high stresses were evaluated, with particular regard to fusion zone and heat affected zone. The influence of preheating and post-heating treatment on residual stresses was investigated. The residual stresses decrease when preheating temperature increases. The maximum value of longitudinal residual stresses without pre-heating can be reduced about 12% and 38% by using the preheating and post-heating process respectively.

Abstract: This paper concerns the experimental characterization of static and fatigue strength of a flat panel stiffened by bonded pad made of aluminum alloy. The panels were full scale and tested under both static and fatigue loads, applied by means of an in house designed and built multi-axial static and fatigue machine. The fatigue crack propagation life of the stiffened panel has been compared to that of a simple flat panel and to that of a flat panel with chemical milling pad-up, getting that the fatigue crack growth life can be significantly improved.

Abstract: In the recent years, the study of the behaviour of damaged structures has been focused on cracked components in presence of an extensive material yielding at the crack tip; under this condition, linear elastic fracture mechanics theory (LEFM) is not able to describe the real plastic zone shape and size. Within this work, an extensive numerical analysis, based on elastic plastic fracture mechanics theory (EPFM), of the plastic zone size at the tip of a Mode I pre-crack at the notch edge in a plate is presented.

Abstract: The ductile damage behaviour of an aluminium laser welded joint is studied experimentally and numerically. The dimensions of the weld regions are fixed by hardness tests. Fracture toughness tests of Al6061 laser beam welded joints were performed with the compact tension (C(T)) specimens. The Rousselier model is used and the parameters: initial void volume fraction (f0) and average void distance (l_c) are identified by metallographic investigations, for the BM, the FZ and the HAZ. Numerical calibration of the Rousselier parameters is performed on notched round specimens. The same Rousselier parameters are used to predict force vs. Crack Opening Displacement (COD) of C(T) specimens.

Abstract: In this study, the effects of water absorption and freezing on flexural property and mode I interlaminar fracture toughness of Carbon Fiber Reinforced Plastics (CFRP) were investigated. Plain woven carbon fiber was used as reinforcement. Epoxy resin was used as matrix. Water absorption test was conducted at 90°C for 24, 100, 250 and 500 hrs. After water absorption, the specimens were kept under-15°C for 24 hrs. Static three point flexural and Double Cantilever Beam (DCB) tests were conducted after water absorption and freezing. As a result, flexural strength and mode I interlaminar fracture toughness of CFRP after water absorption and freezing decreased with an increase of immersion time. Flexural modulus of CFRP did not change after water absorption and freezing. And flexural strength and mode I interlaminar fracture toughness of frozen CFRP after water absorption were lower than those of water absorbed CFRP. From the observation using scanning electron microscope, it is understood that the water absorption and freezing affects the interfacial adhesion between fiber and matrix.

Abstract: Halloysite is one of clay minerals (a new family of crystalline aluminosilicates), and has a nano-tubular structure similar to carbon nanotubes. The typical sizes of the halloysite are 50 nm in outer diameter, 15 nm in inner diameter, and 500-1000 nm in length. This paper deals with mechanical properties of PVA/halloysite nanocomposite films which were fabricated by a casting method. Sulfuric acid surface treatments were performed on halloysite to improve its interfacial adhesion bonding between halloysite and PVA polymer matrix. The effect of acid surface treatment on the mechanical performance was also examined. In the case of PVA/untreated-halloysite composites, the tensile strength was decreased; on the other hand Young’s modulus was increased compared with those of neat PVA. It was found that the sulfuric acid treatment time played an important role in the mechanical performance of the PVA/halloysite nanocomposites.