Transient tests are used to demonstrate unusual or even paradoxical deformation behaviors in metallic alloys and polymers at ambient and elevated temperatures. Included are repeated relaxation tests and creep tests of short duration. It is shown that creep rate need not increase with a stress increase and that, at the same stress level, creep rate can be different on loading and unloading. Also, a stress magnitude increase can be found during relaxation. Once steady inelastic flow is reached, the relaxation rate is nearly independent of the stress and strain at which relaxation starts; it depends mainly on the preceding prior inelastic strain rate. These phenomena can be modeled using an overstress (effective stress) dependence of inelastic rate of deformation and a proper evolution law for a single state variable, the equilibrium stress (back stress) within the context of a “unified” state variable theory. The author and his students have developed the viscoplasticity theory based on overstress (VBO) that can model these unusual behaviors.
The changes in atomic structure around the crack-tip in FCC crystal under in-plane shear (Mode II) loading is analyzed by means of molecular dynamics (MD) simulation. The interatomic force interaction is assumed to be derived from the “N-body” potential proposed by Ackland et al. The crack is modeled as a vacancy sheet and fixed boundary condition is adopted. The ductile deformation with emission of edge dislocations from crack-tip is observed. The moment at which the crack-tip atom overcomes the potential barrier and an edge dislocation is generated is clearly defined as the time when the slope of the curve of the work vs. time suddenly falls down. From the result of MD-simulation in low temperature, the critical stress intensity factor KcrII for the dislocation nucleation which is estimated with this condition agrees well with Rice's theoretical prediction. At finite temperature T, the temperature dependence of the critical stress intensity factor is able to be explained from the properties of thermal activation process. Discrete jumps are recognized in KcrII-T relation in the cases of same reference velocity. The discrete jump seems to relate the interaction of different modes of atomic vibration under the effect of shear stress in slip plane and tension-compression stress wave.
Mode of debonding on the interface between Al matrix and SiC whisker aligned unidirectionally is simulated by using the finite element method, and the subsequent creep crack growth in the matrix at 300°C is treated in the framework of the continuum damage mechanics. In order to estimate damage and stress development in the vicinity of the whisker, a unit cell model which is based on a kind of periodical boundary condition is employed. Damage development associated with stress/strain for the unit cell model is calculated as the axisymmetric and also plane strain conditions, and the difference between both results are compared. The analyses of debonding and creep crack growth are carried out by using a local approach, and macroscopic creep deformation of MMC as well as microscopic damage and stress distribution is also evaluated, and the result is discussed in the comparison with the experimental data.
Overall thermomechanical behavior of the polycrystalline shape memory alloys is described from the micromechanical point of view by modeling the alloys as a matrix-inclusions material system. The martensite inclusions are assumed to be born in the austenite matrix at the moment when a transformation condition is satisfied there during thermomechanical loading. The macroscopic constitutive equation is derived in rate form by means of Eshelby's inhomogeneous theory. The Mori-Tanaka theory is employed to take into account the interaction of the martensite inclusions. The formula of the overall thermomechanical material parameters, which exhibits the effect of transformation, is derived explicitly.
Cavitation formation in a high strain rate superplastic Al-Mg-Si composite, reinforced with 20 vol.% Si3N4 particulates, was observed by a high resolution scanning electron microscope. The composites were pulled by the tension test under steady true stress at an optimum superplastic deformation condition of 8MPa and 833K. Many cavities were formed at/near the interfaces between the matrix and particulates. The number of cavities increased slowly and the growth was so sluggish with increasing strain. The size of most cavities, up to strain of 1.0 at least, were the order of the particulate sizes at nearly 0.5μm, whereas above strain of 1.4 a few large cavities almost equal to grain sizes appeared by growth and/or interlinkage of the cavities. The large cavities near the highly deformed tip of the failed specimens rapidly grew up and made alignments along the tensile axis.
In order to evaluate the relative quality of hard disks, two testing methods are introduced in this study: a continuous sliding test and an in-situ reversal pin-on-disk test. These two methods enable one to compare the relative quality of a head-disk interface in a short period of time. The process leading to failure of the hard disk is clearly fatigue. Above a certain critical load, damage occurs easily and quickly. The lubricant layer can have an adverse effect on fatigue life at low sliding speed. It also affects the stability of the frictional coefficient.
The technique of computerized image reconstruction of electrical resistance in earth has been newly developed by numerical analysis methods, such as, finite element, finite difference, resistance network, integral equation and these complex solution. In this study, a new sensitivity distribution technique is proposed to improve the reliability of resistivity interpretation, mainly, to improve the subsurface image reconstruction by not only apparent resistivity measurements but also differential electrical measurements, i.e., the extended differential sensitivity distribution methods. Special cases of CC (drillhole DH-1) PP (surface) hole-to-surface, C (hole DH-1) PP (hole DH-2) crosshole, and CC (tunnel TL-1) PP (surface) tunnel-to-surface configurations were discussed.
Monte Carlo simulation method was used to evaluate the theoretical distributions of the estimates of Weibull modulus and scale parameter for various sample sizes. It was found that the theoretical distributions of the estimates of Weibull modulus and scale parameter are well approximated by log-normal distributions, and the scatter and bias of each distribution decrease with the increase of sample size. The comparison of the distribution of Weibull modulus estimated by experiments with the theoretical distribution indicates that the scatter of the experimentally obtained Weibull modulus results from the statistical nature of the estimates depending on the sample size and the difference of test conditions is not a major factor for the scatter. And it was also revealed that the distribution of scale parameter obtained by experiments does not agree with the theoretical one obtained by Monte Carlo simulation. This means that the scale parameter or the intrinsic strength of sintered silicon nitride and sintered silicon carbide depends on the fabrication and testing techniques.
It is shown that in proof testing of ceramic components, a phenomenon that strength after proof testing, Sf, depends quite sensitively on initial strength, Si appears under certain conditions. For example, a slight difference of Si only by 0.005MPa results in a large difference of Sf by some 400MPa. The reason and conditions for this phenomenon to appear are investigated theoretically. It is shown that this phenomenon appears when the unloading stress rate is considerably low and the crack growth exponent n is large as in ceramics. A practical aspect of the phenomenon is also discussed.