Advanced Materials Research Vols. 891-892

Abstract: The propagation of long fatigue cracks in ultra fine-grained (UFG) copper obtained by equal channel angular pressing (ECAP) is investigated in the mid ΔK range and in the near threshold regime. The crack growth rates in UFG copper are substantially faster than in coarse-grained (CG) copper. A huge influence of environment is observed, with growth rates faster of more than two orders of magnitude in air compared to vacuum. The crack growth mechanisms are discussed on the basis of microfractographic observations and the deformation texture.

Abstract: Strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of an equiaxed crystal nicke-base superalloy were conducted at 850oC in order to investigate the effects of different dwell times and strain ratios on the fatigue behavior and life distribution. The cyclic damage accumulation (CDA) method and modified CDA method were employed to predict the fatigue life for the superalloy under complex loading condition, respectively. CDA method is employed to predict the fatigue life for the superalloy and the predicted fatigue life is within the scatter band of±6X. The fatigue life predicted by the modified CDA method agrees very well with the experimental fatigue life and the predicted fatigue life is well within the scatter band of±3X, which means that the modified CDA method is able to consider the influences of dwell time and strain ratio on the fatigue life for the superalloy.

Abstract: Aluminum alloys are the first chosen materials for aircraft structures since 1930 due to their high ratio between properties and density. Currently, aircraft industries use Al-Zn-Mg-Cu alloys, with T7451 temper, which it shows high strength, fatigue and corrosion resistance. The improvements properties are result from the η’ phase formation, MgZn₂. The aim the present work is to propose an alternative route for ageing heat treatment at one AA7050 aluminum alloy, the manner which the formation of η’ phase particles at lower temperatures and longer periods of treatment is obtained. According to the literature, ageing treatments at low temperatures and longer periods optimize the usage of the solution atoms, favoring the growth and nucleation particles. As a result, there is an increase in the density of the particle improving the alloy properties. These improvements show a decrease up to 13% in the crack propagation rate for moderate ΔK levels, keeping the same levels of Vickers hardness but with decay of 12% of ultimate tensile strength.

Abstract: Graphene, a flat monolayer of carbon atoms packed in a 2D honeycomb lattice, has outstanding mechanical properties and can be used as a reinforcement for developing composites, such as graphene/polymer composites. The interface properties between the reinforcing and the matrix phase influence significantly the performance of these new nanocomposites. Very limited experimental studies have been carried out to evaluate the interfacial characteristics of the nanocomposites due to difficulties in accessing individual interfaces. Evaluation of interfacial behaviour of the nanocomposites using numerical studies is available, but these studies mainly deal with separation in the shear (sliding) mode performed by pullout test. The purpose of this study is to develop a microscopic numerical model to simulate graphene/polymer peel test, where opening mode of fracture is dominated. A plane-strain model is developed using the finite element method (Abaqus). The interface bonding between the graphene and polymer matrix is described by using a cohesive zone model. The numerical results are compared with an experimental study published in literature.

Abstract: Combined methods to obtain ultra-fine grain (UFG) α-brass samples are proposed. Severe plastic deformation followed by recrystallization was conducted, where multiple rolling and equal channel angular pressing (ECAP) were employed. Recrystallization was accomplished by heat-treatment after the severe plastic deformation, and the grain size after the severe plastic deformation was decreased. By multiple rolling, plates with thickness of 0.1 mm and grain size of 1.0 μm were obtained. By ECAP process, square bar with cross-section of 6 mm × 6 mm and minimum grain size of 4.1 μm was obtained. The 0.2 % proof strength, ultimate tensile strength, and fatigue limit were increased with the value of inverse square root of grain size (Hall-Petch relationship). Then, the 0.2 % proof strength of UFG brass was tenfold, the ultimate tensile strength and the fatigue limit were two fold increases from the conventional α-brass. Because of the high strength, the scatter of fatigue strength of UFG brass was large, which reflects the sensitivity to defects in material.

Abstract: Ternary TiAl alloy with 8 at.% Nb and lamellar microstructure is subjected to low cycle fatigue tests at temperatures ranging from room temperature to 800 °C. The aim of the study is to find limit conditions when the microstructure is still stable and to study mechanisms of microstructural degradation when this limit is exceeded. Up to 750 °C, no cyclic softening or hardening is observed and cyclic stress-strain curve follows the tensile curve. Cyclic softening is characteristic for 800 °C. The TEM observation did not reveal any substantial changes in the microstructure due to the cycling up to 700°C. The lamellar structure is altered by cyclic straining at 750 °C and, to a higher extent, at 800°C. In specimens cycled to fracture at 800 °C, the domains without lamellar structure cover about 10% of volume and are almost dislocation free. The destruction of lamellar microstructure is the reason for the marked cyclic softening at 800 °C.

Abstract: An analysis of whether and how the occurrence of shallow (radial) thermal cracks promotes additional plastic deformation of a mechanically loaded wheel tread is carried out. The study employs numerical simulations of a 2D slice of an elastoplastic railway wheel tread containing thermal (radial) cracks. The cracked wheel material is subjected to repeated passes of a frictional rolling contact load. The effect of the existing thermal cracks on bulk deformation and subsequent rolling contact promoted growth is quantified. Results indicate that thermal cracks of a depth of 0.1 mm have a negligible effect, whereas 1.0 mm cracks significantly decrease the bulk resistance of the wheel material. Further, it is shown how the magnitude of stress, strain and deformation depends on the direction of applied traction.

Abstract: The ratcheting behaviour of a hypereutectoid high strength rail steel with carbon content of 0.85% was experimentally studied under both uniaxial and bi-axial cyclic loadings recently by the authors. To numerically simulate the multiaxial ratcheting behaviour of the rail steel, the Abaqus built-in Lemaitre-Chaboche model was applied first in current study. Following Abaqus documentation, the material data for the Lemaitre-Chaboche model were calibrated from the uniaxial loading test results. Comparing with experimental data, the Lemaitre-Chaboche model with the calibrated data provides overpredictions for the ratcheting responses of the rail steel under both uniaxial and bi-axial loadings. After that, a modified cyclic plasticity model with a coupling multiaxial parameter in the isotropic and kinematic hardening rules was applied for the material. The material data for this modified model were calibrated from both uniaxial and bi-axial loading tests. Comparison between the simulated results and the experimental data show that this modified cyclic plasticity model has the capacity to simulate both uniaxial and multiaxial ratcheting behaviour of the hypereutectoid rail steel with an acceptable accuracy.

Abstract: Recently, high-strength alloy steels have been developed and used for various products. It is well known that fretting fatigue does not show fatigue limit. In other words, fretting fatigue failure may occur in very high cycle regime more than 10⁷ cycles. However, it is difficult to investigate fretting fatigue property in very high cycle regime by using conventional fatigue testing machines because it is time-consuming. In this study, a fretting fatigue testing method for carburized alloy steels in very high cycle regime is explored by using an ultrasonic torsional fatigue testing machine. Carburized SCM420H was used for investigation. The experimental results showed that it is possible to conduct fretting fatigue testing of carburized alloy steels by using an ultrasonic torsional fatigue testing machine.

Abstract: To search for a single parameter to evaluate the stress state in rail head during wheel/rail rolling contact situations, the stress-based and the strain based phenomenological approaches for multiaxial fatigue analysis can be considered as the candidates. Following the stress-based approach, the maximum von Mises stress range can be applied as a single parameter to evaluate the stress state in the rail head. However, the von Mises stress range only relies on the stress field in the rail head for the fatigue analysis, which is not sufficient for assessing the fatigue resistance of the rail steel. The Smith-Watson-Topper (SWT) method, the strain-based phenomenological approach for multiaxial fatigue analysis which considers stress, elastic strain and plastic strain components, is then adopted to study rolling contact fatigue in the rail head. Combining with the three-dimensional finite element modelling of a steady-state wheel/rail rolling contact, the numerical procedure to calculate the SWT parameter in the rail head is presented. The capability of the SWT method to predict the initiation of fatigue cracks in the rail head is confirmed in a case study. Consequently, the maximum SWT parameter is proposed as a single parameter to effectively evaluate the stress state in the rail head.