Zhufeng Yue

Evaluation of creep damage behavior of nickel-base directionally solidified superalloys with different crystallographic orientations

Abstract A crystallographic creep damage constitutive model is developed for nickel-base directionally solidified superalloys. The rates of material degradation and grain boundary void growth are considered. The governing parameters are determined from the creep test data of single crystals and directionally solidified superalloys with a crystallographic orientation. A finite element program is used to analyze the creep damage behavior of nickel-base directionally solidified superalloys for different crystallographic orientations. The results depend on the number of grains modelled and compare well with the experimental data.

Combining Analysis of Coupled Electrical-Thermal and BLOW-OFF Impulse Effects on Composite laminate Induced by Lightning Strike

A comprehensive simulation procedure combining electrical-thermal analysis and BLOW-OFF impulse (BOI) analysis was conducted to investigate lightning direct effects on damage behavior of composite. The nonlinear material model was elaborated combining the damage mechanism of composite laminate subjected to lightning strike. Results of electrical-thermal analysis indicated that temperature distribution of composite laminate is mainly affected by the electrical anisotropy because of Joule heating. By comparing results of BOI analysis with lightning test, it can be found that strain fields of analysis meet well with the damage pattern of lightning specimen. It could be concluded that the analysis procedure is suitable for modeling damage of composite due to lighting strike, and results of logarithmic strain field can be used to help estimate the zone which need to be repaired for composite.

Determination of creep parameters from indentation creep experiments: a parametric finite element study for single phase materials

Abstract This paper explores the possibilities of determining creep parameters for a simple Norton law material from indentation creep testing. Using creep finite element analysis the creep indentation test technique is analysed in terms of indentation rates at constant loads. Emphasis is placed on the evolving stress distribution in front of the indenter during indentation creep. Moreover the role of indenter geometry, size effects and of macroscopic constraints is explicitly considered. A simple procedure is proposed to translate indentation creep results into constitutive creep equations for cases where the dimensions of the tested material are significantly larger than the indenter. The influence of macroscopic constraints becomes important when the size of the indenter is of the same order of magnitude as the size of the testing material. As a striking example for size effects and for macroscopic constraints the indentation creep process in a thin film is analyzed. The results contribute to a better mechanical understanding of indentation creep testing.

Large-deformation and high-strength amorphous porous carbon nanospheres

Carbon is one of the most important materials extensively used in industry and our daily life. Crystalline carbon materials such as carbon nanotubes and graphene possess ultrahigh strength and toughness. In contrast, amorphous carbon is known to be very brittle and can sustain little compressive deformation. Inspired by biological shells and honeycomb-like cellular structures in nature, we introduce a class of hybrid structural designs and demonstrate that amorphous porous carbon nanospheres with a thin outer shell can simultaneously achieve high strength and sustain large deformation. The amorphous carbon nanospheres were synthesized via a low-cost, scalable and structure-controllable ultrasonic spray pyrolysis approach using energetic carbon precursors. In situ compression experiments on individual nanospheres show that the amorphous carbon nanospheres with an optimized structure can sustain beyond 50% compressive strain. Both experiments and finite element analyses reveal that the buckling deformation of the outer spherical shell dominates the improvement of strength while the collapse of inner nanoscale pores driven by twisting, rotation, buckling and bending of pore walls contributes to the large deformation.

Casting microporosity growth in single-crystal superalloys by a three-dimensional unit cell analysis

Finite element (FE) analysis is employed to investigate casting microporosity growth in nickel-base single-crystal superalloys DD3. Based on a finite deformation rate-dependent crystallographic constitutive equation, the simulation of casting microporosity in a three-dimensional unit cell model was carried out under variation in a range of parameters including triaxiality, Lode parameter and a type of different slip systems activated and loading orientation. The FE results show that the stress triaxiality has profound effects on growth behaviour, volume fraction of casting microporosity and shape transition; the Lode parameter is an important parameter for casting microporosity growth and shape transition as well as triaxiality. Large local cumulative shear strain around casting microporosity plays an important role in driving casting microporosity growth; the high casting microporosity volume fraction corresponds to big cumulative shear strain. The operative slip systems type and loading orientation has a remarkable effect on casting microporosity growth, which is associated with the Schmid Factor and the Youngs modulus of orientation.

A numerical study of damage development and creep life in circular notched specimens during creep

Abstract In this study, numerical calculations of creep damage development and life behaviour of circular notched specimens have been performed with the Kachanov–Rabotnov damage law. The emphasis was placed on the role of specimen geometry and material ductility. The creep deformation will relax the elastic stress concentrations. The redistribution of the stress concentration depends on the material properties and specimen geometry. ‘Brittle’ materials will redistribute faster than ‘ductile’ materials. The creep crack initiation place can be the centre of the specimen (large radius of notches e.g. R = 3 mm in this study) or the tips of the notches (small radius of notches). The characteristics of ‘notch strengthening’ and ‘notch weakening’ depend on the notch radius and notch type as well as material properties. For the same notch type, the creep lives decrease with the decreasing of notch radius and the depth of cracks. Generally, the ‘V-type’ notched specimens have longer lives than ‘U-type’ notched specimens for the same notch radius. For all specimens studied, the ‘ductile’ material specimens have longer creep lives than ‘brittle’ material specimens.

Experiment and Simulation Study on the Creep Behavior of PMMA at Different Temperatures

In the present study, creep experiments at different stress and different temperatures were carried out to study the creep behavior of PMMA (MDYB-3). The results show that the creep behavior of PMMA is significantly dependent on temperature in the temperature range of 20–75°C. It was indicated that the duration curves of creep could be divided into three phases. The Chen theory, Norton formula and exponential expression can be used to describe three phases of creep behaviors, respectively. Furthermore, the creep constitutive model based on the experiment results was introduced and implemented into a user subroutine UMAT of software ABAQUS. The finite element analysis (FEA) results prove that the constitutive model introduced in the paper can successfully simulate the creep behavior of structure made of PMMA.

Fracture behavior of a nickel-base single crystal superalloy as predicted by the strain energy density criterion

Abstract Results from compact tension specimens and scanning electronic microscopy are used to study the fracture characteristics of nickel-base single crystal superalloy DD3. Finite element analyses are also made to better understand the outcome. The experiments showed that the initiation and growth of cracks were dependent on the crystallographic orientation of specimens, and the fracture surfaces coincided with the slip planes. Three-dimensional stresses near the crack tip are obtained. The strain energy density criterion is applied to predict the fracture of the superalloy. Agreement with experiments is good.

The creep-damage constitutive and life predictive model for nickel-base single-crystal superalloys

A two-state-variable creep-damage constitutive and life predictive model that has been built is discussed in this article. The cavitation-controlled damage mechanism and microstructural deg-radation, i.e., material damage mechanism, are considered. The latter is derived mainly from the rafting and derafting of the precipitate γ′. The model has been verified by the creep ex-periments of nickel-base single-crystal DD3 at 760 °C and 850 °C. The steady creep and tertiary creep can be predicted satisfactorily. The active slip systems are confirmed as octahedral 〈112〉 {111} based on the lattice rotation. The parameter C reflecting material damage mechanism depends on the crystallographic orientation and can be assigned to the value C〈001〉α along 〈001〉 crystallographic orientation and C〈011〉α along 〈011〉 orientation partially. The life in different crystallographic orientations can be predicted satisfactorily.

Experimental and Constitutive Investigation on Tensile and Compressive Behavior of MDYB-3 at Different Temperatures and Loading Rates

In the present study, tensile tests and compressive tests at different loading rates and different temperatures were carried out to study the mechanical behaviors of MDYB-3 systematically. The experimental results show that the stress-strain behavior and mechanical performance of MDYB-3 material under tension differ significantly from it under compressive response. The tension failure strength, elastic modulus and yield strength increase basically with the rise of temperature, while the critical strain decreases with the rise of temperature. Furthermore, a tensile constitutive model was proposed for MDYB-3 material expressing in terms of elastic, viscous and plastic deformation.