Wenke Pan

Simultaneous determination of the Young's modulus and Poisson's ratio in micro/nano materials

Most mechanical properties of micro/nano materials cannot be determined using purely theoretical approaches or by extrapolation from bulk measurements. These properties are usually extracted experimentally by using micromechanical test structures such as cantilevers. This paper reports a novel cross-shaped structure used to simultaneously extract the Youngs modulus and Poissons ratio using the force–deflection principle. Equations for calculating the Youngs modulus and Poissons ratio are derived, and using these, an experimental demonstration of this method is presented. The average values of the Youngs modulus and Poissons ratio for the device layer commercially sourced silicon on a silicon-on-insulator (SOI) wafer are measured to be 116.5 ± 2.24 GPa and 0.32 ± 0.03, respectively.

A new distributed cooling method for mitigating residual stress in friction stir welding

An analytical and finite element investigation of the effect of different cryogenic cooling nozzle configurations on temperature and residual stress in a model friction stir weld is presented. A new configuration adopting a distributed cooling approach is proposed based on an analytical cooling model. Finite element models are implemented to verify the effect of distributed cooling on welding temperature and longitudinal residual stress. The results presented indicate that new active cooling methods can improve mitigation of welding-induced residual stress.

A sigmoidal model for superplastic deformation

Abstract A new phenomenological model, designed to capture the sigmoidal nature of stress dependency on strain rate for superplastic deformation, is presented. The model is developed for the Ti-6Al-2Sn-4Zr-2Mo alloy using data obtained under controlled strain-rate tensile tests spanning a range of strain rates and temperatures, from 930 to 980 °C. The sigmoidal model performance is compared with that of a more conventional double-power law, strain, and strain-rate hardening model using time-dependent finite element and theoretical analyses. The primary intended application of the sigmoidal model is for more accurate simulation of the effects of strain-rate variation within test specimens and sheet during superplastic deformation. Analysis of this variation within two designs of tensile test specimens is presented to illustrate this aspect.

Simplified Methods for Prediction of Elastic-Plastic Behaviour of Complex Structures with Repeated Structural Units

This paper reviews a range of simplified and approximate techniques developed and implemented by the authors in the context of elastic-plastic, elastic-creep and elastic-plastic-creep, monotonic loading of both simple components and complex structures. The primary motivation for the work is to provide more rapid methods for estimating failure variables for damage assessment of structures. The context of the work has included both aerospace structures and tubular frameworks, such as offshore jacket-type structures, both of which belong to a class of structures, which contain repeated units where localised plastic damage may be concentrated.

Identifying plastic properties of friction stir-welded material by small punch beam test

Average plastic properties of friction stir-welded AA2024-T3 are obtained by coupling novel small punch beam testing with a neural network algorithm. The small punch beam test utilizes a cylindrical punch head and miniature rectangular beam specimens. The specimens may be manufactured by material removed from in-service components with minimal effect on mechanical performances. Specimen preparation, material model, and identifying procedure are systematically presented. Predicted load–displacement results agree well with the experimental results and the identified strain–stress relationship demonstrates useful agreement with tensile test. Since the load–displacement curve is insensitive to base material properties, knowledge of these properties is not required in the proposed method.

Superplastic forming of Ti‐6Al‐2Sn‐4Zr‐2Mo: experiments and simulation

The paper investigates the tensile properties and formability of Ti‐6Al‐2Sn‐4Zr‐2Mo for superplastic forming (SPF) applications, using material testing, numerical modelling and forming tests. A series of constant strain rate tests were carried out, varying the strain rate and the temperature within the range 930°C to 980°C, to determine the effects of these variables on strain rate sensitivity and strain hardening. Power‐law constitutive relationships were adopted for both strain‐rate and strain hardening. The performance and finite element implementation of the latter relationships is assessed via comparisons between (i) a theoretical model of the tensile test, (ii) a finite element model of the tensile test and (iii) the tensile test data itself. Multiaxial validation of the constitutive model and associated formability window is investigated using superplastic forming trials and a two‐dimensional finite element simulation of the process.