Role of microstructures on the growth of long fatigue cracks

An analysis is presented to understand the role of microstructures on the two crack growth driving force parameters, and , without invoking the extrinsic crack closure concepts. Microstructural variables considered are: grain size, precipitates and stacking fault energy. It is shown that is strongly affected by the scale of the microstructure, such as grain size or precipitate spacing. For each case, the mode of slip deformation and environment affects the fatigue resistance as represented by . However, the microstructures seem to have a smaller effect on . Also, the enhanced planarity of slip from the reduction in stacking fault energy has a pronounced effect on when compared with the materials deforming under homogeneous slip.     

The orientation relationships and interfaces between δ-AuSn and Au

The orientation relationships and interfaces of δ-AuSn with the Au (001), (110) and (111) surfaces have been studied with transmission electron microscopy. Au was evaporated onto the NaCl (001), (011) and (111) surfaces to form epitaxial Au thin films and Sn was evaporated onto the Au films to form δ-AuSn. Two types of orientation relationships have been found: (1) _δ//(001)_Au, (0001)_δ//(110)_Au, and _δ//_Au, which was found on the _δ/(001)_Au interface and the _δ/_Au interfaces; and (2) _δ//_Au, (0001)_δ//(110)_Au, and _δ//_Au, which was found on the _δ/_Au interface. The interfaces are analyzed by the structures of the surfaces and the orientation relationships. The nucleation of δ-AuSn on these interfaces is also discussed.     

Physical meaning of ΔKRP and fatigue crack propagation in the residual stress distribution field

Previous papers have shown ΔK_RP to be a useful parameter describing fatigue crack propagation behavior, where ΔK_RP is an effective stress intensity factor range corresponding to the excess RPG load (re-tensile plastic zone's generated load) in which the retensile plastic zone appears under the loading process. In this paper, the relationship between ΔK_RP and the zone size () (which is smaller between the tensile plastic zone at maximum load and the compressive plastic zone at minimum load) was investigated using a crack opening/closing simulation model so as to consider a physical meaning of ΔK_RP. As a result, it becomes clear that ΔK_RP dominates the zone size where fatigue damage mostly occurs. This result supports the following crack propagation equation

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Very low temperature thermal conductivity of Kevlar 49

We measured the thermal conductivity of a Kevlar 49 in the temperature range. The data were fitted with a power-law: . Kevlar 49 is a candidate material for the supports of CUORE experiment.     

Analysis of a transiently propagating crack in functionally graded materials under mode I and II

Crack tip stress and displacement fields for a transiently propagating crack along gradient in functionally graded materials (FGMs) with a linear variation of shear modulus are developed. The higher order terms of the transient stress and displacement fields at crack tip were obtained by transforming the general partial differential equations of the dynamic equilibrium into Laplace’s equations whose solutions have harmonic functions. Thus, the fields can be expressed very simply. Using these stress components, isochromatics and the first invariant at crack tip are generated.The results show that the isochromatics (constant maximum shear stress) for mode I crack tilt backward around the crack tip with an increase of crack tip acceleration , and tilt forward around the crack tip with an increase of rate of change of dynamic mode I stress intensity factor . The isochromatics for mixed mode crack move to upper direction with an increases of and , and lower direction with an increase of . Contours of the first stress invariant for mode I crack enlarge around the crack tip with an increase of , and decrease around the crack tip with an increase of . As decreases at crack initiation, the predicted kinking angles increase. As increases, the predicted kinking angles also increase.     

Crack – fiber sensor interaction and characterization of the bridging tractions in mode I delamination

Fiber bridging is regularly encountered in mode I delamination tests of unidirectional fiber reinforced composites. However, characterization of the bridging tractions is rather difficult. One way to indirectly evaluate the bridging traction distribution is to embed a fiber Bragg grating (FBG) sensor close to the crack tip and to measure the distributed strain along this FBG. The strain measurements from the FBG sensor are used to characterize the fiber bridging tractions by an identification method. In this work, the sensor is embedded in a unidirectional carbon/epoxy composite. Firstly, it is treated as an inclusion near the crack plane and a numerical analysis is performed to study its effect on the measured strain field and energy release rate. The results demonstrate that the sensor, located at about two fiber diameters from the crack plane, has a negligible effect on the fracture process. Secondly, among the identified linear, bilinear, and exponential bridging traction distributions, the exponential one is found to be a suitable model. Characterization of the bridging tractions allows to calculate the energy release due to the bridging fibers which is similar to the difference between the initiation energy release and the propagation value . The results also agree with the bridging tractions evaluated from the conventional energy release rate – crack opening displacement method.     

Ion-molecule reactions in mixtures of methane with carbon tetrafluoride

Ion-molecule reactions have been measured for methane-carbon tetrafluoride mixtures of different composition using a quadrupole mass spectrometer with a high-pressure ion source. Concentration of methane in these mixtures ranged from 10% to 90% (at 10% increment). Primary ions , , F⁺, CF⁺ and were produced by electrons with energy of 300 eV. Secondary ions , , , , and were observed as the result of ion-molecule reactions. Relative current intensities for primary and secondary ions are presented as a function of both total mixture pressure and concentration of methane in the mixture. Potential of repeller electrode inside the ion source collision chamber was fixed at 5 V for all measurements. Total mixture pressure was changed from 0.7 to 33.3 Pa. Schemes of ion-molecule reactions were proposed.     

Analysis of high temperature fatigue crack growth behavior

High temperature fatigue crack growth has been examined in the light of the new concepts developed by the authors. We observe that the high temperature crack growth behavior can be explained using the two intrinsic parameters ΔK and K_max, without invoking crack closure concepts. The two-parameter requirement implies that two driving forces are required simultaneously to cause fatigue cracks to grow. This results in two thresholds that must be exceeded to initiate the growth. Of the two, the cyclic threshold part is related to the cyclic plasticity, while the static threshold is related to the breaking of the crack tip bonds. It is experimentally observed that the latter is relatively more sensitive to temperature, crack tip environment and slip mode. With increasing test temperature, the cycle-dependent damage process becomes more time-dependent, with the effect that crack growth is dominated by K_max. Thus, in all such fracture processes, whether it is an overload fracture or subcritical crack growth involving stress corrosion, sustained load, creep, fatigue or combinations thereof, K_max (or an equivalent non-linear parameter such as J_max) remains as one essential driving force contributing to the final material separation. Under fatigue conditions, cyclic amplitude ΔK (or an equivalent non-linear parameter like ΔJ) becomes the second necessary driving force needed to induce the characteristic cyclic damage for crack growth. Cyclic damage then reduces the role of K_max required for crack growth at the expense of ΔK.     

Electrohydrodynamic pulsatile flow in a channel bounded by porous layer of smart material

Electrohydrodynamic dispersion due to pulsatile flow in a channel bounded by porous layer of smart material is studied considering both steady and unsteady cases using both BJ and BJR-slip conditions. We found that in the case of steady flow, the dispersion coefficient, decreases with an increase in electric number W_e but increases with an increase in porous parameter σ_p in the case of BJ-slip condition. However this nature is different in the case of BJR-slip condition in the sense that the dispersion coefficient, increases for certain values of W_e and then decreases with an increase in W_e. In the case of unsteady flow, the dispersion coefficient, , decreases with an increase in W_e and σ_p for both BJ and BJR conditions. In particular, we found that the value of for steady flow in the case of BJ-slip condition is less than that of unsteady flow. The opposite is true for BJR condition. The findings are useful in the design of robust and efficient artificial organs in the human body.