Key Engineering Materials Vols. 577-578

Abstract: The dynamic fracture behavior of 7075-T6 aluminum alloy was studied by finite element method to simulate a cracked three-point bending specimen loaded by stress wave loading. In order to determine the elastic-plastic dynamic fracture toughness using quasi-static fracture mechanics theory, the nominal load measured by Hopkinson pressure bar loaded fracture testing system was input into a finite element program to calculate the loading point displacement, and then this displacement was employed to obtain the load-displacement field in the vicinity of the crack tip without the inertia effect, the variation of J-integral as a function of time was established using the load-displacement parameters determined by finite element analysis. The critical J-integral corresponding to crack initiation time detected by a small strain gauge mounted on the three-point bending fracture specimen is determined as an elastic-plastic dynamic fracture toughness (JId). The comparison between the equivalent dynamic fracture toughness(KId) given by the aforementioned procedures and the value measured in previous studies was made to verify the validation of the proposed procedure.

Abstract: The paper contains results of a comprehensive research and development programme aimed at an evaluation of static strength of a GRP railway bogie of a new advanced design. The works were carried out as one of the essential phases of the implementation of a EUREKA project EUROBOGIE, which is aimed at development of advanced vehicle bogies and has a great potential to lead to more efficient movement of freight and passengers with lower environmental impact. The GRP bogie design consists of three main load carrying parts, namely two quite massive longitudinal side frames connected by a central transom plate which are manufactured in a single moulding operation using the resin transfer moulding (RTM) method. Since the component is unusually thick, all parameters of the RTM method have to be well balanced. One of the critical issues in the design and manufacture is the connection area of the transom with the side frames. Therefore, testing of this subcomponent was an important part of the bogie development. Results of the experimental programme provided necessary feedback for the optimization of the manufacture technology.

Abstract: Researched the effect of DNP on mechanical properties of materials with a coarse-grained and fine-grained initial structure of aluminum alloy 2024 - T3, D16 and nano-crystallite titanium VT1-0. It has been shown that self-organization of structures at dynamic non-equilibrium processes is a critical parameter for materials with a nano-structures, since it significantly reduces the strength at the subsequent loading.

Abstract: Corrosion processes are frequent reasons of failure of materials in many applications. Results of failure analysis of the crude-oil tank after more than 30 years of service are summarized in this work. The failure was caused by two different and independent corrosion processes – corrosion in crude oil inside the tank and corrosion from concrete base under the tank. Both corrosion processes usually occur equally over the whole surface. In this case, however, both the corrosion processes occurred with distinctly higher corrosion rate in basic material alongside of heat affected zone of weld joint as well. The crack with length about 420 mm was the final result of these processes. The effect of low-cycle fatigue from filling and draining of crude oil is usually significant and was discussed as well.

Abstract: It has been shown that the service lifetime of buried pipes can be reduced especially in relation to stress concentration caused by external point loads. If the pipe is loaded locally the stress is concentrated there and a crack can initiate at this position or the existing crack can be affected by corresponding stress redistribution. In the paper, the effect of a hard indenter, material properties, and the hoop stress level on the stress intensity factor of the axial inner crack are evaluated. A simple relation for estimation of the stress intensity factor value without FEM simulations is proposed.

Abstract: A fatigue testing setup based on electromagnetic excitation was built to apply cyclic stresses to the specimens near their resonance frequency. A test near the resonance frequency has the advantage that higher stresses can be applied to test specimens at a reduced input power. Stress amplitude up to 1000 MPa can be applied to the test specimens and up to four specimens can be tested simultaneously. The setup can test specimens at high fatigue cycle regime i.e. 100 million stress cycles can be achieved in 48 hours.The setup has been used to study the effects of internal and external microstructure on the fatigue strength of materials. Specimens especially stainless steel-304 was prepared by different techniques i.e. electric discharge machining EDM, etching and laser cutting. Specimens prepared by these techniques were tested and their fatigue strengths were compared. To probe the material endurance limit, tests were also performed on the above mentioned steel specimens in very high stress cycle regime i.e. > 10⁹ cycles. In order to investigate the effect of internal microstructure on fatigue strength of material, CuZn37 fabricated by etching was tested and the effect of different grain size on fatigue strength was compared. SN curves have been plotted for materials with no prior fatigue strength data. Stainless steel-1.4404 specimens prepared by Rapid Prototyping (RP) has been tested for fatigue analysis. The test results showed higher degree of scattering when compared to the traditionally manufactured steel. Fractography revealed the existence of inherent material flaws which was the main reason of higher degree of test point scattering. In addition to these data, the SN curve was plotted for Innolot which is an important soldering alloy and prompts to fatigue failure in electronic assemblies.

Abstract: Flanged joints arefundamental components of pipelines and they must be optimized to avoid leaks,bolts overstress and flanged joints damage. Overcoming standards indications asover-torqueing, and combination of different parameters, as difference in hardnessbetween flanges and gasket, can cause groove impressions. Aim of this study isto provide an efficient tool to optimize the choice of characteristicparameters of flanged joints, by numerical simulations. Geometries of a realtest case are simulated by FE models, considering different values of flangeand gasket hardness, as well as tightening torques, in order to cover possibleranges. From the analyses, it is possible to highlight that a particularcombination of hardness difference and over-torque is responsible for permanentflange impression.

Abstract: A viable solution to the ever-demanding weight-saving target in aerospace industry is the replacement of conventional engineering alloys with composite materials in primary structures. A major concern to the effective use of composite laminates is the substantial reduction in the compressive strength when the contact force has exceeded the delamination threshold load (DTL). This paper focuses on the study of the contact behavior of composite laminates under quasi-static indentation (QSI) forces. The effect of damage initiation and growth on contact behavior has been investigated via detailed assessment of the relation between the indentation force and the dent depth. Different phases corresponding to undamaged, local damage, global damage, and final failure of the laminate have been identified. A modification to the classical Hertz contact law has been proposed to account for the matrix material resistance to plastic deformation.

Abstract: In this paper, bolted joints were subjected to fatigue testing under load control with R-ratio equal to zero. Fatigue fracture surfaces show a crescent moon geometry for short cracks turning into quasi-straight fronts for long cracks. Fatigue cracking usually starts on the bolt’s first notch root inside the bolted joint, secondary cracks appearing sometimes with smaller size and a greater angle (in relation to the transversal section) in consecutive inner threads. The starting places, as well as the initial geometry of the cracks, are explained according to the distribution of the equivalent plastic strain in the bolt.

Abstract: Prestressing steels, obtained by cold drawing, are highly susceptible to hydrogen embrittlement (HE) phenomena. Stress and strain fields produced by cold drawing play an essential role in this process since they affect hydrogen diffusion. Therefore, variations of such fields due to changes in drawing conditions could modify life in-service of these structural components. In this work the effect on HE of a parameter of the wire drawing process, the bearing length, is analyzed by means of diverse numerical simulations by the finite element method (FEM). The results of this work allow the definition of a characteristic value of the die bearing length equal to the wire radius, and demonstrate that the effects of stress-strain fields produced by wire drawing on HE are reduced when the bearing length exceeds such a characteristic value, so that the optimum cold drawing process is that with a bearing length higher than the wire radius.