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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Time-dependent fracture strength of oriented polymers

A statistical consideration based upon a theory of microscopic molecular kinetics is developed to describe the time-dependent fracture process for oriented polymers. The interrelation between the macroscopic fracture strength and the microscopic behaviors is formulated with the aid of a double Fourier series expansion. The microscopic behavior in the vicinity of a point is expressed in the form of a spherical function which can be expanded into a series of symmetric tensorial terms. Subsequently, the fracture strength of polymers is evaluated in terms of both time and molecular orientation. By taking account of the individual values of the directional fraction of integrity, the time to fracture and the most probable direction of fracture within any volume element in a medium are predicted.

Mun, M. S.↗
NASA NTRS1986

Mechanism of strength degradation for hot corrosion of alpha-SiC

Sintered alpha SiC was corroded by thin films of Na2SO4 and Na2CO3 molten salts at 1000 percent. This hot corrosion attack reduced room temperature strengths by as much as 50 percent. Strength degradation was proportional to the degree and uniformity of corrosion pitting attack as controlled by the chemistry of the molten salt. Extensive fractography identified corrosion pits as the most prevalent source of failure. A fracture mechanics treatment of the strength/pit depth relationship produced an average K sub IC equal to 2.6 MPa sub m 1/2, which is cnsistent ith published values.

Smialek, James L.↗
NASA NTRS1987

Structure and tensile strength of LaS(1.4)

The tensile strength of LaS(1.4) has been estimated by diametral stress testing at room temperature, 800 and 1300 K. Brittle, tensile-type failures were obtained at all temperatures when the crosshead speed was 0.0021 mm/s; however, a 1300 K test at 0.00085 mm/s produced plastic flow. The microstructure of LaS(1.4) consisted of two phases with beta-La2S3 comprising about 15 vol percent of the structure and gamma-La2S3 the remainder. Because of the limited amount of material available for testing, no correlation between microstructure and mechanical strength could be drawn.

Whittenberger, J. Daniel↗
NASA NTRS1988

Strain intensity factor approach for predicting the strength of continuously reinforced metal matrix composites

A method was previously developed to predict the fracture toughness (stress intensity factor at failure) of composites in terms of the elastic constants and the tensile failing strain of the fibers. The method was applied to boron/aluminum composites made with various proportions of 0 to + or - 45 deg plies. Predicted values of fracture toughness were in gross error because widespread yielding of the aluminum matrix made the compliance very nonlinear. An alternate method was developed to predict the strain intensity factor at failure rather than the stress intensity factor because the singular strain field was not affected by yielding as much as the stress field. Strengths of specimens containing crack-like slits were calculated from predicted failing strains using uniaxial stress-strain curves. Predicted strengths were in good agreement with experimental values, even for the very nonlinear laminates that contained only + or - 45 deg plies. This approach should be valid for other metal matrix composites that have continuous fibers.

Poe, C. C., Jr.↗
NASA NTRS1986

Strength optimization of alpha-SiC by improved processing

Silicon carbide is of great interest for structural use in aircraft and automobile engines. This ceramic combines high thermal conductivity and low coefficient of thermal expansion, and consequently has good thermal shock resistance. However, like other ceramics, silicon carbide shows strength variability due to processing flaws such as large voids, shrinkage cracks, and inclusions. Agglomerates in the starting powder seem to be the predominant cause of such defects. Improved processing techniques such as slurry pressing and hot isostatic pressing (HIP) were employed to minimize these defects and to improve strength and reliability in the fabricated material. For this purpose 2-inch diameter disks were fabricated by various consolidation techniques. These include: dry pressing and sintering, slurry pressing and sintering, and slurry pressing and HIPing. The results are discussed.

Dutta, Sunil↗
NASA NTRS1987

A fractographic investigation of the influence of stacking sequence on the strength of notched laminated composites

The fracture behavior of T300/5208 CFRP laminate panels with 12 different combinations of ply orientation and stacking sequence is investigated experimentally, using optical microscopy, SEM, and X-ray radiography to characterize the notch-tip damage zones and fracture surfaces of center-cracked tension specimens subjected to tensile loading at constant crosshead displacement rate 20 micron/s. The results are presented graphically and analyzed in detail. Significant differences in notched strength are found for different ply fiber orientations and stacking sequences; the laminates with few major delaminations had a greater percentage of fracture due to broken fibers and also higher notched strength.

Harris, Charles E.↗
NASA NTRS1987

Strength characterization of yttria/alumina-doped sintered silicon nitride

The flexural strength of yttria/alumina-doped sintered silicon nitride (Ford Material-RM 20) was measured as a function of temperature (20 to 1400 deg C), applied stress and time. Flexural stress rupture testing at 800 and 1000 deg C indicated that the material can sustain 344 MPa and 276 MPa, respectively, without failure, for a limited time (less than or equal to 100 h). The RM 20 material was susceptible to both oxidation and early stages of creep deformation at temperatures above 1000 deg C and displayed extensive creep deformation and degradation in strength above 1300 deg C.

Govila, R. K.↗
NASA NTRS1988

Fibre-matrix bond strength studies of glass, ceramic, and metal matrix composites

An indentation test technique for compressively loading the ends of individual fibers to produce debonding has been applied to metal, glass, and glass-ceramic matrix composites; bond strength values at debond initiation are calculated using a finite-element model. Results are correlated with composite longitudinal and interlaminar shear behavior for carbon and Nicalon fiber-reinforced glasses and glass-ceramics including the effects of matrix modifications, processing conditions, and high-temperature oxidation embrittlement. The data indicate that significant bonding to improve off-axis and shear properties can be tolerated before the longitudinal behavior becomes brittle. Residual stress and other mechanical bonding effects are important, but improved analyses and multiaxial interfacial failure criteria are needed to adequately interpret bond strength data in terms of composite performance.

Grande, D. H.↗
NASA NTRS1988

Probabilistic constitutive relationships for cyclic material strength models

A methodology is developed that provides a probabilistic treatment for the lifetime of structural components of aerospace propulsion systems subjected to fatigue. Material strength degradation models, based on primitive variables, include both a fatigue strength reduction model and a fatigue crack growth model. Probabilistic analysis is based on simulation, and both maximum entropy and maximum penalized likelihood methods are used for the generation of probability density functions. The resulting constitutive relationships are included in several computer programs.

Boyce, L.↗
NASA NTRS1987

Mechanical strength and tribological behavior of ion-beam-deposited boron nitride films on non-metallic substrates

An investigation was conducted to examine the mechanical strength and tribological properties of boron nitride (BN) films ion-beam deposited on silicon (Si), fused silica (SiO2), gallium arsenide (GaAs), and indium phosphide (InP) substrates in sliding contact with a diamond pin under a load. The results of the investigation indicate that BN films on nonmetallic substrates, like metal films on metallic substrates, deform elastically and plastically in the interfacial region when in contact with a diamond pin. However, unlike metal films and substrates, BN films on nonmetallic substrates can fracture when they are critically loaded. Not only does the yield pressure (hardness) of Si and SiO2 substrates increase by a factor of 2 in the presence of a BN film, but the critical load needed to fracture increases as well. The presence of films on the brittle substrates can arrest crack formation. The BN film reduces adhesion and friction in the sliding contact. BN adheres to Si and SiO2 and forms a good quality film, while it adheres poorly to GaAs and InP. The interfacial adhesive strengths were 1 GPa for a BN film on Si and appreciably higher than 1 GPa for a BN film on SiO2.

Miyoshi, Kazuhisa↗
NASA NTRS1988

Autohesive strength development in polysulfone resin and graphite-polysulfone composites

The effects of bonding temperature and contact time on autohesive strength development in thermoplastic polysulfone resin and graphite-polysulfone composites were investigated. Two test methods were examined to measure autohesion in the neat resin samples. These included an interfacial tension test and a compact tension fracture toughness test. Autohesive strength development in fiber-reinforced composites was measured using a double cantilever beam interlaminar fracture toughness test. The results of the tests were compared with current diffusion theories explaining crack healing and welding of glassy polymers. Discrepancies between the results of the present investigation and the diffusion theories are discussed.

Howes, Jeremy C.↗
NASA NTRS1988

The strength of laminated composite materials under repeated impact loading

When low velocity and energy impact is exerted on a laminated composite material, in a perpendicular direction to the plane of the laminate, invisible damage may develop. It is shown analytically and experimentally that the invisible damage occurs during the first stage of contact between the impactor and the laminate and is a result of the contact stresses. However, the residual flexural strength changes only slightly, because it depends mainly on the outer layers, and these remain undamaged. Repeated impact intensifies the damage inside the laminate and causes larger bending under equivalent impact load. Finally, when the damage is most severe, even though it is still invisible, the laminate fails because of bending on the tension side. If the repeated impact is halted before final fracture occurs the residual strength and modulus would decrease by a certain amount.

Rotem, Assa↗
NASA NTRS1989

Fatigue strength reduction model: RANDOM3 and RANDOM4 user manual, appendix 2

The FORTRAN programs RANDOM3 and RANDOM4 are documented. They are based on fatigue strength reduction, using a probabilistic constitutive model. They predict the random lifetime of an engine component to reach a given fatigue strength. Included in this user manual are details regarding the theoretical backgrounds of RANDOM3 and RANDOM4. Appendix A gives information on the physical quantities, their symbols, FORTRAN names, and both SI and U.S. Customary units. Appendix B and C include photocopies of the actual computer printout corresponding to the sample problems. Appendices D and E detail the IMSL, Version 10(1), subroutines and functions called by RANDOM3 and RANDOM4 and SAS/GRAPH(2) programs that can be used to plot both the probability density functions (p.d.f.) and the cumulative distribution functions (c.d.f.).

Boyce, Lola↗
NASA NTRS1988

The solar origin of long-term variations of the interplanetary magnetic field strength

Using simple models for the coronal field structure, the spacecraft observations of the photospheric field during sunspot cycle 21 were extrapolated for the purpose of modeling quantitatively the long-term behavior of the IMF during the sunspot cycle 21. Results were compared with the measurements of the radial component of the IMF at earth. The results indicate that the solar source of the IMF can be represented to a first approximation by the dipole component of the photospheric field, whose axis is nearly perpendicular to the ecliptic plane around sunspot minimum, but tilts more strongly toward it around sunspot maximum. It was also found that the average radial IMF strength varies with heliographic latitude; around sunspot minimum, the radial IMF is expected to be roughly twice as strong above the sun's poles as near the ecliptic plane. The average strength of the photospheric field above latitude 55 deg is about 10 G around sunspot minimum.

Wang, Y.-M.↗
NASA NTRS1988

Measured line positions and strengths of methane between 5500 and 6180/cm

Calculations for the positions and strengths of over 2000 methane absorption lines stronger than 0.0001/sq cm per atm have been obtained in the 5500-6180/cm spectral region. Spectra were obtained at room temperature using the Fourier transform spectrometer at the Kitt Peak National Observatory solar telescope. The 2nu3 band strength of C-12H4 was shown to be 1.641/sq cm per atm. A Herman-Wallis-type correction of 1 + 0.0298 m + 0.000536 sq m was also found.

Margolis, Jack S.↗
NASA NTRS1988

Analytical and experimental investigation of the notched strength of thick laminates with surface notches

Experimental and analytical investigations of the fracture of a graphite/epoxy laminate with part-through semielliptic surface notches have been conducted. Experimental values of notched strength were determined for a variety of notch aspect ratios and notch depths. Analytical predictions of laminate failure were obtained from three different models. The three models were equally successful in predicting the fracture of laminates with deep notches. The experimental results of laminates with shallow notches indicated more notch sensitivity (that is greater strength reduction) than did the analytical models.

Harris, Charles E.↗
NASA NTRS1988

Fractal distributions of stress and strength and variations of b-value

A two-dimensional planar fault zone on which the difference between stress and strength follows a fractal distribution is simulated to study the variation of the frequency-magnitude b-value under different distributions of heterogeneities and ambient stress levels. It is suggested that earthquakes occur in regions where this difference exceeds a specified value and that the size of these regions is a measure of the magnitude of the associated earthquake. A systematic variation in b-value is observed. It is found that the b-value has a positive correlation with the fractal dimension of the distribution and is inversely related to the ambient stress level. Observational data are compared with the simulation, showing that the observed b-value variation before and during earthquake sequences is a result of changes in both the ambient stress level and in the fractal dimension of the stress-strength distribution.

Huang, J.↗