The orientation dependence of fatigue-crack growth in 8090 Al-Li plate

A series of fatigue-crack growth rate (FCGR) tests was carried out on 8090 Al-Li plate to examine the effects of specimen orientation on fatigue-crack growth. The directionality of fatigue fracture behavior is found to be related to the strong {110}〈112〉 texture in this alloy. Based on a previously developed transgranular FCGR model using restricted slip reversibility (RSR) concepts, [1] a mechanistic model is developed for transgranular fatigue-crack growth in highly textured materials. The model takes the form of the Paris relationship with a power law exponent of 3, and the material texture is shown to strongly influence the proportional factor. The effect of texture on FCGR is related through a geometric factor cos² ϕ, where ϕ defines the angle between the load axis and the normal of the favorable slip plane. The effect of specimen orientation on FCGR in 8090 Al-Li alloy is shown to be related to a combination of its anisotropic mechanical properties and the variation of angleϕ with specimen orientation. The model further predicts that fatigue-crack growth rates will be slower in many textured materials than texturefree materials becauseϕ > 0 and cos₂ ϕ < 1.     

Assessment of service induced microstructural damage and its rejuvenation in turbine blades

Correlations between service induced microstructural degradation and creep properties in investment cast IN738LC turbine blades are discussed. Microstructural degradation in the form of γ’ coarsen-ing, MC carbide degeneration, formation of continuous networks of grain boundary M₂₃C₆ carbides, and the disappearance of serrated grain boundaries are considered in some detail. Their influence on primary (t _p ,ε _p ), secondary (t _s , ε _s ,ε _m ) and tertiary (t_t, ε_t) creep behavior is analyzed through rela-tionships of the form:

Creep crack growth in the absence of grain boundary precipitates in UDIMET 520

The effects of grain size and environment on creep crack growth (CCG) in Ni-base superalloy, UDIMET 520, were studied through experiments at 540 °C. Specially designed solution and aging treatments were used to produce γ′ strengthened microstructures with different grain sizes but without any M₂₃C₆ grain boundary precipitates. Five grain sizes, which fall into three groups (i.e., small, medium, and large), were employed. The creep crack growth rates (CCGRs) in specimens with small grain sizes were approximately 2.5 times lower than those with medium and large grain sizes, as a result of crack branching and the presence of some undissolved primary MC carbides at the grain boundaries. Otherwise, the CCGRs were insensitive to the grain size. Fractographic observations on the fracture surfaces and metallographic examinations on the cross sections of the interrupted CCG specimen revealed intergranular microcracks and a faceted intergranular mode of fracture in both air and argon environments. The test results suggest that the formation and propagation of intergranular cracks by grain boundary sliding (GBS) is the main micromechanism responsible for CCG in both air and argon environments at the relatively low test temperature employed. Grain boundary oxidation attack in the air environment simply accelerates the crack growth process. The present results are in agreement with the theoretical predictions of the GBS-controlled CCG model previously developed by the authors.     

Full-wave nonlinear analysis of nonradiative dielectric guide circuits including lumped elements

This paper describes the full-wave numerical analysis of complex nonradiative dielectric (NRD) guide structures including linear or nonlinear lumped elements. The numerical results described in this paper, validated through experimentation, show that the method is suitable for NRD guide and similar circuits. Nonlinear analysis of a single-ended NRD guide mixer is carried out, and the results are experimentally verified. Nonlinear harmonic-balance analysis of a novel leaky-wave balanced mixer circuit with very good tolerance of manufacturing inaccuracies in NRD guide configuration in Ka-band is then carried out. Due to curved segments, and orders of magnitude difference in the dimensions of different parts, the analysis of this component requires certain simplifying assumptions. How these can be implemented without sacrificing much accuracy is described. Until now, the design of NRD guide components, especially those using nonlinear effects, has been mostly trial-and-error-based. Here we demonstrate that a more systematic approach is possible.     

Comparison of graphic symbol learning in individuals with aphasia and right hemisphere brain damage

This study compared the differences in performance on recognition of graphic symbols across time by individuals with aphasia, individuals with right-hemisphere brain damage, and neurologically normal adults. The subjects, seen individually, learned 40 Blissymbols. The symbols were selected so that the effects of symbol translucency and complexity on the recognition of graphic symbols could be examined. A paired-associate learning paradigm was used to teach the symbol-referent pairs to subjects. The results indicated that individuals with aphasia and neurologically normal adults do not differ significantly in recognition of graphic symbols. However, individuals with right-hemisphere damage recognized fewer symbols compared to individuals with aphasia and normal adults, suggesting that they have difficulty in associative learning of graphic symbols. Additionally, translucency was found to be a potent factor in the recognition of Blissymbols by all groups. The finding that individuals with severe chronic aphasia can learn and retain graphic symbols has significant clinical implications for aphasia rehabilitation.     

Numerical and experimental analysis of nonradiative dielectric guide modulator and mixer at Ka band

This letter describes the numerical analysis of an on/off modulator and a single-ended mixer in nonradiative dielectric (NRD) guide configuration at Ka band. Currently, only experimental results are available for such structures. A shunt mounted diode switch in NRD guide configuration is analyzed using the finite element method (FEM) in conjunction with linear circuit simulation. A novel mixer is similarly analyzed using FEM and harmonic balance analysis. The results obtained from numerical analysis are compared with measured results. It is demonstrated that some counter-intuitive experimental results can be explained through numerical analysis.     

Reversibility with sodium bicarbonate of styrene maleic anhydride, an intravasal injectable contraceptive, in male rats

PURPOSE: Patients who use topical ophthalmic medications and wear soft contact lenses must remove their lenses before drop instillation to prevent absorption of the medication into the lenses. No previous study has examined how long such a patient should wait before reinserting their lenses. This study was designed to test the hypothesis that waiting 5 minutes before reinsertion of lenses would be sufficient to reduce absorption to a level below what is needed to produce a physiological response. METHODS: Naphcon-A was used as the test solution and pupillary dilation was the physiological response measured. The amount of benzalkonium chloride (BAC) extracted from the lenses was also measured. Twenty-three subjects, none of whom had any significant ocular or systemic abnormalities nor showed pupillary dilation to directly applied Naphcon-A, completed this 3-week study. The study used a 2-period crossover design with a 1-week screening phase. Results were analyzed with a repeated-measure analysis of variance. RESULTS: The pupils averaged 0.316 mm larger when subjects instilled Naphcon-A with lenses in place as compared to when dosing with lenses removed for 5 minutes (P = 0.0008). Nine of 23 subjects showed pupillary dilation greater than 0.5 mm when dosing with lenses in place as compared to none when lenses were removed for 5 minutes. Significantly (P < 0.01) more BAC was extracted from lenses that had been worn during dosing than from lenses removed for 5 minutes (0.9 microgram/lens as compared to no detectible amount). CONCLUSION: Removing soft contact lenses for 5 minutes was sufficient to prevent absorption of clinically significant amounts of Naphcon-A into the lenses.     

Serrated grain boundary formation potential of Ni-based superalloys and its implications

The serrated grain boundary formation potential of a large number of conventionally forged, powder processed, and investment cast Ni-based superalloys is reviewed. A mechanism of serrated grain boundary formation by which grain boundaryγ′ particles move and displace the local grain boundary segment is discussed and the prerequisite conditions for its occurrence are highlighted. The practical implications of the serrated grain boundary formation are also discussed. It is suggested that modifying the existing heat-treatment cycles in some investment cast and powder processed Ni-based superalloys would improve their properties. The possibility of minimizing weld cracking in superalloys by creating serrated grain boundaries in the base metal and the heat affected zone is also discussed.     

Pattern diversity of path loss compensated antennas for 5G base stations

In this article, a tapered slot antenna (TSA) operating from 27 to 29.8 GHz with an endfire gain of 9 dBi and high pattern integrity is presented. The gain specifications for path loss compensation on ground for a ceiling mounted millimeter wave base station is computed and the gain of the antenna elements with beam angled at ±45° was found to be 12 dBi. To enhance gain with minimal physical footprint, a combination of dielectric loading and electrical resonator metamaterial unit cells were integrated to the proposed TSA to achieve the expected gain enhancement of 3 dB across the band, operating in the same frequency band with aperture efficiency greater than 73% and a 1 dB gain bandwidth of 20.7%. A compact stacking topology for pattern diversity of all three antenna elements for path loss compensation is also investigated. The base station has a coverage of ±60° with uniform illumination and mutual coupling lesser than 35 dB. The detailed simulated and measured results are presented.     

Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.