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Theoretical study results are presented for the effects of strain on the cutoff frequencies of SiGe heterojunction bipolar transistors. The positive role played by the strain on the base resistance and transit time is shown to have a further effect on high-frequency performance.
Here, the effects of strain and strain rate on dynamic grain growth (DGG) and subgrain evolution are reported for an interstitial-free steel deformed at 850 ° C. Microstructures produced during tension tests at true-strain rates of 10 -4 and to 10 -3 s -1 true strains ranging from 0.02 to 0.2 were preserved following deformation. These were characterized using electron backscatter diffraction (EBSD), including the application of spherical harmonic transform indexing to produce high-angular-resolution EBSD (HR-EBSD) data. HR-EBSD data resolved the small misorientation angles of subgrain boundaries while imaging much larger data fields than possible with previously available techniques. The resulting data confirmed that steady-state flow stress is inversely proportional to the average subgrain size and that subgrain boundary misorientation angle increases with strain. The following new observations are reported. The rate of DGG increased with respect to time but decreased with respect to strain as strain rate increased. This behavior is rationalized through a simple model using separate rate parameters for the effects of time and strain. Subgrain size was not constant during steady-state deformation, but decreased slowly with increasing strain. Subgrain size distributions and subgrain boundary misorientation angle distributions were measured, and both remained approximately log-normal during steady-state deformation. Subgrain evolution demonstrated no dependence on parent grain size, crystallographic orientation, or Taylor factor. These new data suggest that steady-state flow stress is more likely controlled by the dislocation density internal to subgrains than by the spacing between subgrain boundaries.
High strain-rate (up to 6600 s –1 ) and quasi-static compression tests are conducted on a powder-sintered porous titanium with different pore sizes (mean: 30 μm and 120 μm). In situ X-ray imaging is implemented to characterize the pores-scale deformation dynamics. The yield strength as a function of strain rate exhibits two stages of rate sensitivity, and the transition occurs at 1600 s –1 . X-ray images show that pore compaction and strain localizations occur preferentially at pores oriented perpendicular to the loading direction under quasi-static loading, but become more random under high strain-rate loading as a result of higher driving force and plastic deformation nucleation rate. Here, the more homogeneous nucleation of plastic deformation contributes to the increased rate sensitivity beyond 1600 s –1 . At the same strain rate, the yield strength of porous Ti as well as strain field homogeneity decreases significantly with increasing pore size. The small pore spacing in fine-pored Ti reduces the degree of stress concentrations around pores. Therefore, the higher stress concentrations in coarse-pored Ti lead to an earlier yield of matrix around pores and thus a lower bulk yield strength.
Effective countermeasures to prevent loss of bone mineral during long duration space flight remain elusive. Despite an exercise program on MIR flights, the data from LeBlanc et al. (1996) indicated that there was still a mean rate of loss of bone mineral density in the proximal femur of 1.58% per month (n=18, flight duration 4 - 14.4 months). The specific mechanisms regulating bone mass are not known, but most investigators agree that bone maintenance is largely dependent upon mechanical demand and the resultant local bone strains. A plausible hypothesis is that bone loss during space flight, such as that reported by LeBlanc et al. (1996), may result from failure to effectively load the skeleton in order to generate localized bone strains of sufficient magnitude to prevent disuse osteoporosis. A variety of methods have been proposed to simulate locomotor exercise in reduced gravity. In such simulations, and in an actual microgravity environment, a gravity replacement load (GRL) must always be added to return the exercising subject to the support surface and the resulting skeletal load is critically dependent upon the magnitude of the GRL. To our knowledge, GRLs during orbital flight have only been measured once (on STS 81) and it is likely that most or all prior treadmill exercise in space has used GRLs that were less than one body weight. McCrory (1997) has shown that subjects walking and running in simulated zero-G can tolerate GRLs of 1 if an appropriate harness is used. Several investigators have attempted to measure in vivo strains and forces in the bones of humans, but have faced ethical and technical limitations. The anteromedial aspect of the tibial midshaft has been a common site for the placement of strain gauges; one reason to measure strains in the anterior tibia is that this region is surgically accessible. Aamodt et al. (1997) were able to measure strains on the lateral surface of the proximal femur only because their experimental subjects were already scheduled for hip surgery. Lu et al. (1997) used an instrumented massive proximal femoral prosthesis along with electromyographic measurements to demonstrate that femoral forces depend on muscular activity. These analyses of in vivo bone mechanics are valuable. The invasive nature of the procedures involved, however, limits both the number of subjects and the number of strain gauge locations. Further, the results of these studies may be confounded by the inclusion of subjects with pathological conditions. Gross et al. (1992) measured strain at three locations on the equine third metacarpal and used those data to construct a computer model of the internal strain environment of the bone. An analogous placement of multiple gauges in living humans would be difficult and potentially hazardous because of the depth of soft tissue overlying the tibia and femur.
An experimental investigation was conducted to determine the strain rate effects in fiber composites. Unidirectional composite specimens of boron/epoxy, graphite/epoxy, S-glass/epoxy and Kevlar/epoxy were tested to determine longitudinal, transverse and intralaminar (in-plane) shear properties. In the Longitudinal direction the Kevlar/epoxy shows a definite increase in both modulus and strength with strain rate. In the transverse direction, a general trend toward higher strength with strain rate is noticed. The intralaminar shear moduli and strengths of boron/epoxy and graphite/epoxy show a definite rise with strain rate.
Effect of strain rate and load cycling on tensile behavior and air permeability of coated fabric for inflatable decelerator system
Nanoscale ferroelectric 2D materials offer the opportunity to investigate curvature and strain effects on materials functionalities. Among these, CuInP 2 S 6 (CIPS) has attracted tremendous research interest in recent years due to combination of room temperature ferroelectricity, scalability to a few layers thickness, and ferrielectric properties due to coexistence of 2 polar sublattices. Here, we explore the local curvature and strain effect on polarization in CIPS via piezoresponse force microscopy and spectroscopy. To explain the observed behaviors and decouple the curvature and strain effects in 2D CIPS, we introduce the finite element Landau–Ginzburg–Devonshire model, revealing strong changes in hysteresis characteristics in regions subjected to tensile and compressive strain. The piezoresponse force microscopy (PFM) results show that bending induces ferrielectric domains in CIPS, and the polarization-voltage hysteresis loops differ in bending and nonbending regions. Furthermore, these studies offer insights into the fabrication of curvature-engineered nanoelectronic devices.
Specimens of wrought sintered commercially pure tungsten were made from 1/8-inch swaged rods. All the specimens were recrystallized at 4050 F for 1 hour prior to testing at temperatures from 2500 to 4000 F at various strain rates from 0.002 to 20 inches per inch per minute. Results showed that, at a constant temperature, increasing the strain rate increased the ultimate tensile strength significantly. The effects of both strain rate and temperature on the ultimate tensile strength of tungsten may be correlated by the linear parameter method of Manson and Haferd and may be used to predict the ultimate tensile strength at higher temperatures, 4500 and 5000 F. As previously reported, ductility, as measured by reduction of area in a tensile test, decreases with increasing temperature above about 3000 F. Increasing the strain rate at temperatures above 3000 F increases the ductility. Fractures are generally transgranular at the higher strain rates and intergranular at the lower strain rates.
The results of a theoretical study of the effects of the strain on the cutoff frequencies of SiGe HBTs are presented. The influence of the strain on the base resistance and transit time, and through them on high-frequency performance, was taken into account. The positive role played by the strain is demonstrated.
The objective of this paper is to evaluate various experimental techniques and analysis methods for the characterization of interlaminar fracture toughness, and to determine the effects of strain rate on that property for a graphite/epoxy composite. Mode I interlaminar fracture was investigated by means of a double-cantilever beam (DCB) specimen for AS-4/3501-6 graphite/epoxy. Hinged tabs were used to insure unrestrained rotation at the free ends. Specimens were loaded at quasi-static deflection rates of up to 8.5 mm/s corresponding to crack extension rates of over 51 mm/s. Crack extension was monitored by means of strain gages mounted on the surface of the specimen, or a conductive-paint circuit attached to the edge of the DCB specimen. Continuous records were obtained of load, deflection, and crack extension for determination of the strain energy release rate. The latter was expressed as a power law of the crack extension velocity. Results indicate that the strain energy release rate increases with crack velocity by up to 28 percent for the range of rates considered.
Na 3 Co 2 SbO 6 is a promising candidate to realize the Kitaev spin liquid phase since the large Kitaev spin exchange interaction is tunable via the change in electronic structure, such as the trigonal crystal field splitting (Δ TCF ). Here, in this study, we show that the uncorrelated electronic structure of Na 3 Co 2 SbO 6 is rather insensitive to the strain effect due to the low crystal symmetry accompanied by oxygen displacements and the presence of Sb s orbitals. Using density functional theory plus dynamical mean field theory, we find that the correlated electronic structure of Na 3 Co 2 SbO 6 is an orbital-selective Mott insulating state where the trigonal a 1g orbital is insulating due to the nearly full occupation, while other d orbitals behave as typical Mott insulators, resulting in the effective tunability of Δ TCF under the strain effect. The sign change of Δ TCF can occur as the in-plane tensile strain is applied, and the Kitaev spin liquid phase could possibly be realized due to the strongly suppressed Δ TCF under tensile strain. Our results show that the local Co-site symmetry and dynamical correlation effects will play an important role in engineering the novel magnetic phase in this and related materials.
Marine energy structures are typically made using composite materials and are repeatedly loaded by currents and waves. Submersion and repeated loading lead to two environmental effects: moisture intrusion and mechanical fatigue. To understand their combined effects, submerged fatigue testing can be used. Submerged fatigue testing often requires submerged instrumentation to validate component manufacturing methods and models. Strain measurements are critical for understanding marine energy component loads. One common method for measuring strain is by using foil strain gauges, but the durability of strain gauges in submerged fatigue conditions is not well-understood. To increase strain gauge durability and protection from contamination, delamination, and water intrusion, strain gauge coatings may be applied over strain gauges and wire connections. In this study, strain gauges were adhered to composite coupons, coated, and mechanically tested in a water tank. Cycles to composite failure, cycles to strain gauge failure (SGF), strains, and SGF modes were used to measure the effects of strain gauge coatings on composite fatigue life and strain gauge durability. The coatings did not have significant effects on composite fatigue life or strain gauge durability. The methods developed and measurements taken at the coupon scale in this study will be used to inform methods and designs for subsequent submerged subcomponent testing, full-scale testing, and standards development. The benefits of designing marine energy structures to informed standards and designs are decreased lifetime costs and increased reliability and energy production, ultimately leading to a sustainable and low-carbon energy system.
This paper presents the results of a program of analysis and tests to determine the dynamic properties of elastomers as a function of strain and ambient temperature. Measurements were also made to determine the temperature distribution in the elastomer samples during the tests. These measured properties were compared with analytical predictions based on a visco-elastic model designed to take into account the self-heating of the materials as a function of strain. The test method used was well-established Base Excitation Resonant Mass Technique. The specimens tested were two cylindrical button compression specimens and a shear specimen. Strain was shown to be an important parameter in determining the dynamic properties of the elastomers. In general, these properties were much more sensitive to strain than to frequency. The self-heating effect was found to account for a portion of the strain sensitivity of these properties.
Cellular materials are ubiquitous in our modern society. They may be stochastic gas-blown foams (e.g., polyurethane), foamed starches (e.g., cereals), or, in this case, 3D printed microlattices. Failure in these materials is often driven by surface or sub-surface defects, which may be nucleated at a surface roughness, an interior void, or inclusion interfaces that may not be typically observable. Obfuscating our understanding further, bulk materials are known to exhibit strain-rate-dependent mechanical response, making a subsurface understanding of damage even more critical. For the first time, an in situ uniaxial mechanical loading stage that simultaneously rotates specimens up to 18 Hz was fielded at a synchrotron for 3D tomographic imaging. This capability opens a plethora of materials science opportunities to explore strain rate effects in materials and examining deformation, fracture, and delamination’s (in composites) for a complete 3D picture (movie) of material response. We demonstrate the deformation of 3D printed polymer lattice structures, of three different material types, at 0.25, 1.1, and 2.2 s −1 strain rates. We successfully imaged the 3D deformation of these materials and can directly compare the same printed structure to the three material types at three strain rates, all in 3D. Material point method simulations were applied to one of the materials to better understand the role of voids on the 3D printed structure’s performance.
A general study is made of the effects of structural heating on calibrated-strain-gage load measurements on the wing of a supersonic airplane. The primary emphasis is on temperature-induced effects as they relate to slope changes and thermal shifts of the applied load/strain relationships. These effects are studied by using the YF-12A airplane, a structural computer model, and subsequent analyses. Such topics as the thermal environment of the structure, the variation of load paths at elevated temperature, the thermal response characteristics of load equations, elevated-temperature load-measurement approaches, the thermal calibration of wings, and the correlation of strains are discussed. Ways are suggested to measure loads with calibrated strain gages in the supersonic environment.
The combined effects of moisture (M), temperature (T), and strain rate (SR) on the properties of the unidirectional graphite/epoxy composite AS4/3501-6 are investigated experimentally. Longitudinal tensile, in-plane shear, and transverse tensile specimens with moisture content 0 or 1 percent were characterized using standard mechanical tests and the techniques described by Yaniv et al. (1987) at temperatures 23-128 C and strain rates from 5 x 10 to the -6th/sec to 5/sec. The results are presented in graphs and discussed in detail. The longitudinal properties of the composite were found to be generally unaffected by changes in the parameters, whereas transverse and interlaminar properties decreased with increasing T and M at constant SR and the transverse and in-plane shear moduli increased with SR. A time-T-M equivalence principle is used to derive master curves for all the matrix-dominated properties, expressed in terms of a two-variable shift function.
NASA has advanced NASA HR-1 as a solution for liquid rocket engine components that operate in hydrogen rich environments using various AM techniques. The NASA HR-1 for AM applications was specifically formulated for high ductility in high pressure gaseous hydrogen environment in addition to high strength and low cycle fatigue (LCF) resistance. NASA HR-1 meets materials requirements for liquid rocket engine components, including good hydrogen resistance, high conductivity, good LCF performance, and high elongation and strength for components in medium heat flux environments. NASA has completed fabrication of several subscale and full-scale channel wall nozzles in LP-DED NASA HR-1 and completed hot-fire testing. This includes refinement of the process to produce thin-walls and various channel geometries to meet the requirements for channel wall nozzle applications. The main objective of this investigation is to better understand the effects of tensile test strain rate and specimen surface finish on the amount of tensile ductility degradation in high pressure gaseous hydrogen environments. Performing smooth tensile tests in high pressure hydrogen and nitrogen environments can serve as a rapid and economical screening method to determine the relative susceptibility of materials to HEE. This paper presents the tensile test results for LP-DED NASA HR-1 in hydrogen and details the material evaluation on the effects of strain rate and surface finish condition on surface cracking behavior and fracture characteristics. The resistance to HEE for LP-DED NASA HR-1 and the wrought alloy is compared under the same testing conditions. Loss of tensile ductility is quantified in terms of changes in elongation at failure in hydrogen to compare HEE susceptibility. The most suitable strain rate and specimen surface finish condition for future HEE screening testing at MSFC are recommended.
A principal effect of turbulence on premixed flames in the flamelet regime is to wrinkle the flame fronts. For nonunity Lewis numbers, Le is not equal to 1, the local flame structure is altered in curved regions. This effect is examined using direct numerical simulations of 3D isotropic turbulence with constant density, single-step Arrhenius kinetics chemistry. Simulations of Lewis numbers 0.8, 1.0, and 1.2 are compared. At the local level, curvature effects dominated changes to the flame structure while strain effects were insignificant. A strong Lewis-number-dependent correlation was found between surface curvature and the local flame speed. The correlation was positive for Le less than 1 and negative for Le greater than 1. At the global level, strain-related effects were more significant than curvature effects. The turbulent flame speed changed significantly with Lewis number, increasing as Le decreased. This was found to be due to strain effects that have a nonzero mean over the flame surface, rather than to curvature effects that have a nearly zero mean. The mean product temperature was also found to vary with Lewis number, being higher for Le greater than 1 and lower for Le less than 1.