Papers by Author: Wen Bo Luo

Abstract: Mooney-Rivlin model and Ogden model are frequently used by engineers for finite element analysis of rubber material. Before simulation, simple, biaxial and planar extension tests are always done to get the model parameters. In this paper, we compare these two hyperelastic models with experimental data produced under simple, biaxial extension and planar extension loading conditions. The ability of the two models to reproduce different deformation modes is analyzed. Both material parameters and the stretch range of validity of each model are determined.

Abstract: A new 3D unit cell model is developed for homogenization calculation of composites containing randomly dispersed ellipsoid inclusions. The new unit cell is constructed using the Ansys Parameter Design Language (APDL), taking the inclusion volume fraction, inclusion orientation and spatial dispersion as variables. A series of unit cells containing multiple ellipsoids, showing random distributions in particle size and position, were constructed and used for finite element calculation at microscale, the effective modulus of the composites with periodic microstructures, which modeled by the unite cells, were then estimated by homogenization. The influences of particle volume fraction and the particle stiffness on the effective elastic modulus of the composites were examined. The estimated results were compared with different particle volume fractions were calculated, and the calculated data was compared with other classic models.

Abstract: This paper presented a parametric experimental study of electrokinetic instability phenomena in a cross-shaped configuration microfluidic device with varying channel depths and conductivity ratios. The flow instability is observed when applied electric field strength exceeds a certain critical value. The critical electric field strength is examined as a function of the conductivity ratio of two samples liquid, microchannel depth, and the treatment of microchannel wetted surface. It is found that the critical electric field strengths for the onset of electrokinetic instability are strongly dependent on the conductivity ratio of two samples liquid, and decrease as the channel depths increasing of microfluidic devices. In the present study, the surface inside microchannels is treated utilizing hydrophilic and hydrophobic organic-based SOG (spin-on-glass) nanofilms for glass-based microchips. The experimental results indicate that no significant difference for the critical electric fields for the onset of electrokinetic instability phenomena in both hydrophilic and hydrophobic SOG coating in the surface of microchannels. The critical electric fields for the onset of electrokinetic instability phenomena are slightly lower in both SOG coated cases in compare with that of the non-coated microchannel.

Abstract: Based on the observations that high temperature accelerates creep rate of polymer while physical ageing plays a reverse role, and that there is an analogy between the influences of stress and temperature on the intrinsic times of polymers, the time-ageing time superposition principle (TASP) and the time-temperature-stress superposition principle (TTSSP) are used to evaluate the long-term creep behavior of poly(methyl methacrylate) (PMMA). PMMA specimens were aged for 2 to 120 hours at identical temperature, their short-term creep strains with 2-hour test duration were measured under various stress levels ranging from 14 to 30 MPa at room temperature, and modeled by means of time-ageing time equivalence and time-stress equivalence. The results show that the creep rate increases with stress, but decreases with ageing time. The ageing time shift factors vary with the stresses at which the shifts are applied. The ageing shift rate is independent on imposed stress in linear viscoelastic region, while it decreases with increasing stress when the material behaves in a nonlinear viscoelastic manner. The master creep compliance curve up to about 1-month at reference ageing time 120 hours and stress 18 MPa, which is nearly 2.5 decades longer than the test duration, is constructed by shifting the creep curves horizontally along the logarithmic time axis. The result illustrates that TTSSP, combined with TASP, provides an effective accelerated test technique for long-term mechanical behaviors of polymers.

Abstract: The uniaxial tension under various strain rates, creep under various sustained loads, and equalamplitude- strain loading and unloading tests are carried out at room temperature with polyamide 6/nano- SiO2 composite specimens. According to the elasticity recovery correspondence principle, the recovered elastic stresses (strains) in the case of prescribed strain (stress) history are calculated, and the instantaneous elastic constitutive equations are deduced. The nonlinear viscoelastic constitutive relations in single integral form on the basis of the instantaneous elastic constitutive equations are constructed and applied to model the current stress (strain) responses of polyamide 6/nano-SiO2 composite. The theoretic results agree well with the experimental data, which demonstrates that the single integral constitutive relations used in this work can accurately simulate the physical nonlinear viscoelastic properties of polyamide 6/nano-SiO2 composite. Finally, the creep curve at higher stress level is horizontally shifted along logarithmic timescale using a stress shift factor in terms of the time-stress superposition principle and superposed on that at relative lower stress level to form a master creep compliance curve that spans a longer timescale interval than the short-term test curve does, which suggests that TSSP provides an accelerated characterization method for the long-term creep performance of polyamide 6/nano-SiO2 composite.

Abstract: In this work, the physical aging and its effect on nonlinear creep behavior of poly(methyl methacrylate) are presented. After annealing above Tg to release the previous thermal and stress history, the samples were quenched to 60oC, aged for various times, and were then tested at three different stress levels (22MPa, 26MPa and 30MPa) at room temperature of 27oC. At each stress level, the creep strain was converted to compliance and measured as a function of test time and aging time. The test results show that higher stress accelerates creep rate of the material while physical aging plays a reverse role. The time-aging time superposition is applicable to build a master creep compliance curve at each stress level, and it is demonstrated that the shift rate deceases with increasing stress. Moreover, based on the time-stress superposition principle, a unified master curve was constructed by further shifting the sub-master curves at 30MPa and 26 MPa to a reference stress level of 22MPa.

Abstract: Temperature induced change, and stress induced change as well, in intrinsic timescale were investigated by nonlinear creep tests on poly(methyl methacrylate). With four different experimental temperatures, from 14 to 26 degrees centigrade, time-dependent axial elongations of the specimen were measured at seven different stress levels, from 14 MPa to 30 MPa, and modeled according to the concept of time-temperature-stress equivalence. The test duration was only 4000 seconds. The corresponding temperature shift factors, stress shift factors and temperature-stress shift factors were obtained according to the time-temperature superposition principle (TTSP), the time-stress superposition principle (TSSP) and the time-temperature-stress superposition principle (TTSSP). The master creep compliance curve up to about two-year at a reference temperature 14 degrees centigrade and a reference stress 14 MPa was constructed by shifting the creep curves horizontally along the logarithmic time axis using shift factors. It is shown that TTSSP provides an effective accelerated test technique in the laboratory, the results obtained from a short-term creep test of PMMA specimen at high temperature and stress level can be used to construct the master creep compliance curve for prediction of the long-term mechanical properties at relatively lower temperature and stress level.

Abstract: The effect of geometric clearances on the stress and deformation distributions of a wedged-ring joint structure is studied, five cases of dimensional tolerance of each contact surface, i.e., the maximum, minimum, mean of maximum and minimum, mean of maximum and mean, and mean of minimum and mean are investigated, and the probable states that the wedged-rings attach with the upper cylinder or attach with the lower cylinder radially in the assembly process are considered as well. The results of finite element numerical simulations indicate that in the design tolerance zone, the effects of geometric clearances between contact surfaces on the displacement and stress distributions of the structural components are weak. Only when the wedged-rings attach tightly with the upper cylinder or with the lower cylinder, the displacement and stress distributions in the joint of the structure occur a dramatic variation, and the assembly processing measure making the wedged-rings attaching tightly with the lower cylinder can be adopted to effectively decrease the stresses and deformations in the joint of the structure.

Abstract: The uniaxial tensile creep of a commercial grade Poly(methyl methacrylate) was measured for 4000 seconds under various temperatures and stress levels ranging from 14 oC to 26 oC and 6 MPa to 32 MPa. The resultant creep compliance curves depart from each other for stresses beyond a critical value which varies with temperature, indicating nonlinear viscoelastic behavior. The time-temperature-stress superposition principle (TTSSP) was used to construct a smooth master compliance curve with a much longer time-scale interval from the short-term tests at higher stresses and temperatures. It is shown that the master curve covers a period of over 290 days, which is nearly 3.9 decades longer than the test duration. Moreover, it is verified that the time-temperature shift factors are dependent on stresses at which the shifts are applied, and that the time-stress shift factors are dependent on reference temperatures.

Abstract: Temperature field is formed due to heat dissipation when material is subjected to irreversible deformation. In this paper, the heat dissipation in the crack-tip plastic zone was considered. By considering the propagating crack-tip plastic zone as a running heat source and constructing a reasonable heat source density distribution function, the temperature field around a steady running crack was obtained. It is shown that temperature rise is dependent on the crack growth speed and the material parameters. The maximum temperature rise reaches to >50 oC in our example calculations for a steady running crack in PMMA.