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This paper investigates the influence of plastic anisotropy on the tensile ductility of a high-strength aluminium alloy. To this end, finite element simulations of smooth and notched tension tests in different material directions are performed with an anisotropic plasticity model. The stress and strain histories from these simulations are then applied in localization analyses with the imperfection band approach, using the anisotropic plasticity model outside the band and an anisotropic version of the Gurson model inside the band. The imperfection within the band is represented by a volume fraction of void nucleating particles. The high-strength aluminium alloy AA7075-T651 is considered in this study. The results show that the directional dependency of the tensile ductility of the alloy found experimentally is predicted with good accuracy using the adopted approach. The numerical study indicates that plastic anisotropy plays an important role in determining the anisotropic tensile ductility of this high-strength aluminium alloy. [ABSTRACT FROM AUTHOR]

The paper presents an experimental and numerical investigation on the response of Kuru granite rock under confining compression and static indentation. Triaxial compression tests were executed at different levels of confining pressure. Indirect tensile tests were carried out with Brazilian disks, and quasi-static indentation tests with a spherical indenter were conducted. An elasto-plastic material model with piecewise yield function, plastic hardening/softening, coupled damage and non-associated plastic flow is presented and calibrated using the experimental test results. Finite element simulations of the triaxial compression tests and static indentation are performed. The influence of the material model features (e.g., the yield function and non-associativity) is discussed, and the effect of confining pressure during indentation is evaluated. It was found that, within the investigated pressure range, the dilatancy angle of the rock linearly decreased with pressure. With a saturating yield function, the simulated force-penetration curve is in good agreement with the experimental curve for the loading phase, but the simulated stiffness during unloading and the subsequent residual penetration are overestimated. The numerical simulation showed that the peak force of indentation increased approximately 11% with a confining pressure of 150 MPa compared to the unconfined case. The shape of the equivalent stress field near the free surface of the rock is affected by the confining pressure as well. [ABSTRACT FROM AUTHOR]

Age-hardened aluminium alloys have various degrees of precipitate-free zones (PFZs) along grain boundaries (GBs). The PFZs are weak zones and their existence promotes combined transgranular and intergranular fracture, thus reducing the ductility of the alloy. In this study, transmission electron microscopy (TEM) is used to display the geometrical properties and the chemical composition of the PFZs in the AA7075-T651 aluminium alloy. PFZs are found along grain and sub-GBs and their widths are about 40 and 20 nm, respectively. The PFZs are depleted of alloying elements compared with the nominal composition due to GB precipitation, but still they contain a certain amount of such elements in solid solution which will contribute to increase the yield strength and the work hardening compared to pure aluminium. Based on the results from the TEM study, a micromechanical finite element model of an idealized microstructure including grains and soft zones along the GBs is established. The Gurson model was used to represent the behaviour of the material in the grains and in the soft zones, using different initial void volume fractions to account for GB precipitation. Several loading conditions were applied to the micromechanical model in order to evaluate the localization of strains inside the soft zones and thus to get a better understanding of the role of the PFZs in ductile fracture of age-hardened aluminium alloys. It was found that the global failure strain varies non-monotonically with the global stress triaxiality due to the heterogeneity of the idealized microstructure. [ABSTRACT FROM PUBLISHER]

Abstract: The influence of stress state and plastic anisotropy on the fracture behaviour of a rolled AA7075-T651 aluminium plate under quasi-static loading conditions is studied both experimentally and numerically. Material tests in different directions of the plate were carried out using specimens with various shapes to provide a wide range of stress states. The strain to failure and the failure modes were found to vary strongly with the stress state, but also with the loading direction due to the complex microstructure of the alloy. Finite element simulations adopting an anisotropic plasticity model were used to obtain local values of stresses and strains and to define the strains at fracture for the various stress states. The numerical simulations show that due to the heterogeneous stress and strain fields in the specimens, it is very difficult to accurately locate the point where fracture initiates and thus to determine the local fracture strain as a function of stress state and loading direction. [Copyright &y& Elsevier]

Abstract: The influence of plastic anisotropy on the mechanical behaviour of a rolled aluminium plate under quasi-static loading conditions is studied experimentally and numerically. Material tests in different directions with respect to the rolling direction of the plate were carried out on various specimen shapes providing a wide range of stress states. The Yld2004-18p anisotropic yield function was identified through uniaxial tensile tests, shear tests and upsetting tests. This yield function was found to provide an adequate description of the significant anisotropic behaviour of a high-strength AA7075-T651 plate. Numerical simulations of all the material tests were then performed with an elasto-plastic material model using both the anisotropic and an isotropic version of the yield function. The numerical predictions of the mechanical response for notched tensile tests obtained with the isotropic version of the material model clearly over-estimated the experimental results. Similar results have been reported in the literature on other materials using isotropic constitutive relations. This over-estimation was significantly reduced when using the anisotropic version of the material model, and the reason why plastic anisotropy is so important for an accurate prediction of the notch-strengthening effect is explained. It was also established that the exponent of the yield function Yld2004-18p has a strong influence on the results for the notched tensile tests. When the exponent of the yield surface was assigned a sufficiently high value, an almost perfect fit between the numerical predictions and the experimental results was obtained. [Copyright &y& Elsevier]

This work considers a predictive numerical modelling approach for fracture-propagation control in CO 2 -transport pipelines, an area where current engineering tools do not work. Fluid–structure interaction model simulations are compared with three published medium-scale crack-arrest experiments with CO 2 -rich mixtures. The fluid flow is calculated by a one-dimensional homogeneous equilibrium model, and the thermodynamic properties of CO 2 are modelled using the Span–Wagner and the Peng–Robinson equation of state. The pipe material is represented by a finite-element model taking into account large deformations and fracture propagation. Material data commonly found in the literature for steel pipes in crack-arrest experiments is not sufficient to directly calibrate the material model used here. A novel three-step calibration procedure is proposed to fill the information gap in the material data. The resulting material model is based on J 2 plasticity and a phenomenological ductile fracture criterion. It is shown that the numerical model provides good predictions of the pressure along the pipe, the ductile fracture speed and a conservative estimate of the final crack length. An approximately plane-strain stress state ahead of crack tip implies that a fracture criterion accounting for a wide range of stress states is not necessary. [ABSTRACT FROM AUTHOR]

Understanding the influence of hydrostatic pressure and loading rate on the strength and fracture behavior of rocks is very important for the development of deep drilling technology. This paper presents a systematic study on the mechanical properties and behavior of Kuru Gray granite at confining pressures up to 225 MPa and at strain rates of 10 −6 s −1 and 600 s −1 . The low strain rate compression tests were carried out with a servo-controlled hydraulic testing machine with a radial confining chamber, and the dynamic tests with a special split Hopkinson pressure bar device with axial and radial confining pressure chambers. The results show that the rock strength increases significantly with strain rate and confining pressure. At confinements below 20 MPa, the strength of the material increases faster at the higher strain rate, but at confinements higher than this, the effect of confining pressure is stronger at the lower strain rate. The strain rate sensitivity increases when even a small confining pressure is applied. However, the rate sensitivity remains rather constant when the confining pressure is increased above 10 MPa. The parameters of the Hoek–Brown model and an alternative power-law model were calibrated for low and high rate data. Also, the fracture behavior of the rock was found to be strongly dependent on strain rate and confining pressure. At the low strain rate, the samples fail by axial splitting in the unconfined tests, whereas the dynamic unconfined tests result in a complete pulverization of the samples. At high confining pressures the fracture behavior is shear fracture for both studied strain rates. [ABSTRACT FROM AUTHOR]