Xueling Fan

Dynamic mechanical behavior of 6061 al alloy at elevated temperatures and different strain rates

The compressive stress-strain relationships of 6061Al alloy over wide temperatures and strain rates are investigated. The dynamic impact experiments are performed using an improved high temperature split Hopkinson pressure bar apparatus. The experimental results are compared with those obtained by the modified Johnson-Cook constitutive model. It is found that the dynamic mechanical behavior depends sensitively on temperature under relatively low strain rates or on strain rate at relatively high temperatures. The good agreement indicates that it is valid to adopt the parameter identification method and the constitutive model to describe and predict the mechanical response of materials.

A MODIFIED DUCTILE FRACTURE MODEL INCORPORATING SYNERGISTIC EFFECTS OF PRESSURE AND LODE ANGLE

A modified ductile fracture model is proposed based on continuum damage mechanics and three-stress-invariant criteria. In the modified model, the effects of hydrostatic pressure and Lode angle are considered and combined in the developed fracture envelope. To predict the fracture behaviors of ductile materials, the model is implemented into the commercial finite element platform ABAQUS/Explicit through a user material subroutine VUMAT. The validity of this model is examined by comparing the numerical results with experimental data of round bar and compact tension tests. It is demonstrated that comparing with other three-stress-invariant models the modified ductile fracture model can predict values compare favorably with experimental data especially in the unloading stage. The model can be adopted to predict the fracture patterns and unloading process of ductile materials and thus to investigate the whole range of the elastic-plastic ductile fracture behaviors of materials.

TEMPERATURE AND STRAIN RATE SENSITIVITY OF ULTRAFINE-GRAINED COPPER UNDER UNIAXIAL COMPRESSION

Uniaxial compressive experiments of ultrafine-grained (UFG) copper fabricated by equal channel angular pressing method were performed at temperatures ranging from 77 K to 573 K under quasi-static and dynamic loading conditions. Based on the experimental results, the influence of temperature on flow stress, strain hardening rate and strain rate sensitivity (SRS) were investigated carefully. The results show that the flow stress of UFG copper displays much larger sensitivity to testing temperature than that of coarse grained copper. Meanwhile, both the strain hardening rate and its sensitivity to temperature of UFG copper are lower than those of its coarse counterpart. The SRS of UFG copper also shows apparent dependence on temperature. Although the estimated activation volume of UFG-Cu is on the order of ~10 b3, which is on the same order with that of grain boundary diffusion processes, these processes should be ruled out as dominant mechanisms for UFG-Cu at our experimental temperature and strain rate range. Instead, it is suggested that the dislocation-grain boundary interactions process might be the dominant thermally activated mechanism for UFG-Cu.

Origin of high strength, low modulus superelasticity in nanowire-shape memory alloy composites

An open question is the underlying mechanisms for a recent discovered nanocomposite, which composed of shape memory alloy (SMA) matrix with embedded metallic nanowires (NWs), demonstrating novel mechanical properties, such as large quasi-linear elastic strain, low Young’s modulus and high yield strength. We use finite element simulations to investigate the interplay between the superelasticity of SMA matrix and the elastic-plastic deformation of embedded NWs. Our results show that stress transfer plays a dominated role in determining the quasi-linear behavior of the nanocomposite. The corresponding microstructure evolution indicate that the transfer is due to the coupling between plastic deformation within the NWs and martensitic transformation in the matrix, i.e., the martensitic transformation of the SMA matrix promotes local plastic deformation nearby, and the high plastic strain region of NWs retains considerable martensite in the surrounding SMA matrix, thus facilitating continues martensitic transformation in subsequent loading. Based on these findings, we propose a general criterion for achieving quasi-linear elasticity.

Uniaxial compressive behavior of equal channel angular pressing Al at wide temperature and strain rate range

Uniaxial compressive experiments of ultrafine-grained Al fabricated by equal channel angular pressing (ECAP) method were performed at wide temperature and strain rate range. The influence of temperature on flow stress, strain hardening rate and strain rate sensitivity was investigated experimentally. The results show that both the effect of temperature on flow stress and its strain rate sensitivity of ECAPed Al is much larger than those of the coarse-grained Al. The temperature sensitivity of ultrafine-grained Al is comparatively weaker than that of the coarse-grained Al. Based on the experimental results, the apparent activation volume was estimated at different temperatures and strain rates. The forest dislocation interactions is the dominant thermally activated mechanism for ECAPed Al compressed at quasi-static strain rates, while the viscous drag plays an important role at high strain rates.

Design of Thermal Barrier Coatings Thickness for Gas Turbine Blade Based on Finite Element Analysis

Thermal barrier coatings (TBCs) are deposited on the turbine blade to reduce the temperature of underlying substrate, as well as providing protection against the oxidation and hot corrosion from high temperature gas. Optimal ceramic top-coat thickness distribution on the blade can improve the performance and efficiency of the coatings. Design of the coatings thickness is a multiobjective optimization problem due to the conflicts among objectives of high thermal insulation performance, long operation durability, and low fabrication cost. This work developed a procedure for designing the TBCs thickness distribution for the gas turbine blade. Three-dimensional finite element models were built and analyzed, and weighted-sum approach was employed to solve the multiobjective optimization problem herein. Suitable multiregion top-coat thickness distribution scheme was designed with the considerations of manufacturing accuracy, productivity, and fabrication cost.

Separation dynamics of large-scale fairing section: a fluid–structure interaction study

The separation dynamics of a large-scale fairing section in ground test is investigated numerically using a fluid–structure interaction method. The commercial finite element software MSC/Dytran is adopted to establish the dynamic fluid–structure coupling model of the fairing. Two coupling surfaces are constructed for the inner and outer surfaces of the fairing section. The coupling equations are solved using the sequenced-coupling method, in which the fluid and structural problems are examined by the finite volume method and the finite element method, respectively. A comparison between fluid–structure interaction and dynamical response analysis is performed under the conditions with and without atmosphere effect. Results shown that the consideration of atmosphere effect will attenuate the vibration frequency and slow down the center of mass velocity. The effect of aerodynamic interference on the displacement response indicates that a maximum of 13.3% relative displacement can be induced, which may cause collision between the lower trailing portion of fairing section and the core vehicle. Therefore, it can be concluded that the fluid–structure interaction analysis is essential for evaluating and validating the reliability of separation mechanisms in ground tests.

Experimental and Numerical Evaluation of the Ablation Process of Carbon/Carbon Composites Using High Velocity Oxygen Fuel System

The ablation process of carbon/carbon (C/C) composites was tested under hypersonic flowing propane flame. The microstructures of C/C composites were characterized and the numerical analysis was performed. Two typical ablation morphologies of the carbon fibers, which are drum-like and needle-like shapes, were observed depending on the alignments of fibers to the flame directions. Temperature fields in the composites were analyzed using finite element method, and the mechanisms that govern the formation of different ablation behaviors were elucidated. For paralleled fiber bundles, the highest temperature situates in the middle parts underlying the ablation pits, where the drum-like shape is formed. For perpendicular fiber bundles, the highest temperature appears at the turning point between the transverse section and the surface of fiber, which leads to the gradual ablation from the fiber surface toward the axis, and eventually the formation of the needle-like shape.

Experimental Investigation on the Tensile Strength of Composite Laminates Containing Open and Filled Holes

An experimental study is performed to evaluate the effects of clamping pressure, friction, and washer size on the static performance of composite laminates with open and bolt-filled holes. The static tensile strength and failure behavior of composite laminates with an open hole and a bolt-filled hole are analyzed and compared. Experimental results show that the static tensile strength of composite laminates is sensitive to pre-existing damage of both open- or filled-hole laminates. In contrast, a comparison between the experimental results of open- and filled-hole specimens proved that whether the hole is open or filled has a feeble influence on the tensile-tensile fatigue strength of studied composite laminates. In comparison, however, it is found that the inserted washer size, bolt clamping force, and friction force strongly affect the tensile strength of open- and filled-hole composite laminates. Moreover, application of thicker washers and hi-lock bolt will significantly increase the static strength and fatigue life of composite laminates with a bolt-filled hole.

Stress Distribution in the Vicinity of Thermally Grown Oxide of Thermal Barrier Coatings

Finite element simulation of stress distribution of thermal barrier coating system (TBCs) is presented. Two dimensional periodic unit cells are used to examine the stress development and critical sites with high potential of cracking during thermal cycling. During cooling, high tensile out-of-plane stresses in the peak of the thermally grown oxide (TGO) are formed, which lead to crack initiation in the vicinity of TGO and the interface. At the same time, high compressive stresses developed in the valley domain. The influence of crack within the top coat in the vicinity of the TGO is also investigated. The finite element analysis shows that crack seriously affects stress field development and the thermal-mechanical behavior of TBCs. Based on the finite element analysis results one can conclude that imperfections and its development should be always considered to be a crucial parameter for TBCs life.