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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

A single fracture toughness parameter for fibrous composite laminates

A general fracture toughness parameter Qc was previously derived and verified to be a material constant, independent of layup, for centrally cracked boron aluminum composite specimens. The specimens were made with various proportions of 0 and + or - 45 degree plies. A limited amount of data indicated that the ratio Qc/epsilon tuf' where epsilon tuf is the ultimate tensile strain of the fibers, might be a constant for all composite laminates, regardless of material and layup. In that case, a single value of Qc/epsilon tuf could be used to predict the fracture toughness of all fibrous composite laminates from only the elastic constants and epsilon tuf. Values of Qc/epsilon tuf were calculated for centrally cracked specimens made from graphite/polyimide, graphite/epoxy, E glass/epoxy, boron/epoxy, and S glass graphite/epoxy materials with numerous layups. Within ordinary scatter, the data indicate that Qc/epsilon tuf is a constant for all laminates that did not split extensively at the crack tips or have other deviate failure modes.

Poe, C. C., Jr.↗
NASA NTRS1981

Fundamental aspects in the quantitative ultrasonic determination of fracture toughness: General equations

The problem of establishing a theoretical groundwork for experimentally-found correlations between ultrasonic and fracture toughness factors in polycrystalline metals is discussed. It is noted that the link between these material properties and ultrasonic factors are the microstructural parameters that interact with stress wave propagation during deformation and fracture. The dynamic response of material inhomogeneities and the strains and displacements they undergo under incident stress waves are considered. Dynamic strains and displacements inside and outside scatterers are treated. The underlying approach, the formulation and governing equations for the eigenstrains, and the determination of the energy due to the presence of inhomogeneities are presented. The stress wave interaction problem is presented in terms of the dynamic eigenstrain concept.

Fu, L. S.↗
NASA NTRS1980

On the equivalence between semiempirical fracture analyses and R-curves

The relationship between the R-curves and semiempirical fracture analyses (SEFA) is investigated theoretically using a hypothetical material. Equivalent R-curves (ERC) are developed for real materials using data from the literature. It is shown that for each SEFA there is an ERC whose magnitude and shape are determined by the SEFA formulation and its empirical parameters. The ERC is equivalent in that it predicts exactly the same relationship between the fracture stress and the initial crack length (residual strength) as the SEFA. If the effective R-curve is unique, then the various empirical parameters cannot be constant, and vice versa. However, for one of the SEFA examined, Newman's SEFA, parameter variations are small enough to be within the range of normal data scatter for real materials.

Orange, T. W.↗
NASA NTRS1981

Fracture toughness of brittle materials determined with chevron notch specimens

Short bar, short rod, and four-point-bend chevron-notch specimens were used to determine the plane strain fracture toughness of hot-pressed silicon nitride and sintered aluminum oxide brittle ceramics. The unique advantages of this specimen type are: (1) the production of a sharp natural crack during the early stage of test loading, so that no precracking is required, and (2) the load passes through a maximum at a constant, material-independent crack length-to-width ratio for a specific geometry, so that no post-test crack measurement is required. The plane strain fracture toughness is proportional to the maximum test load and functions of the specimen geometry and elastic compliance. Although results obtained for silicon nitride are in good mutual agreement and relatively free of geometry and size effects, aluminum oxide results were affected in both these respects by the rising crack growth resistance curve of the material.

Shannon, J. L., Jr.↗
NASA NTRS1980

Thermal stress fracture in elastic-brittle materials

The reported investigation shows that the assessment of the possibility of the thermal fracture of brittle materials depends upon an accurate evaluation of the thermal stresses and the determination of the resulting stress intensity factors. The stress intensity factors can be calculated in a variety of ways ranging from the very precise to approximate, but only for a limited number of geometries. The main difficulty is related to the determination of the thermal stress field because of its unusual character and its dependence upon boundary conditions at points far from the region of thermal activity. Examination of a number of examples suggests that the best visualization of the thermal stresses and any associated fracture can be made by considering the problem to be the combination of thermal and isothermal problems or by considering that the prime effect of the temperature is in the generation of thermal strains and that the thermal stresses are simply the result of the region trying to accommodate these strains.

Emery, A. F.↗
NASA NTRS1981

Lunar floor-fractured craters - Evidence for viscous relaxation of crater topography

Viscous relaxation is evaluated quantitatively as a possible mechanism to account for the observed topographical modification of floor-fractured lunar craters. On the basis of a model of the moon as a viscous material of uniform Newtonian viscosity, the ultimate topographic profiles of fresh craters predicted taking into account viscous relaxation effects are compared with the observed profiles of floor-fractured craters of similar diameter. It is found that for a number of crater pairs, the agreement of the observations with the predictions of the simple viscous relaxation model is good, suggesting that isostatic adjustment of craters in areas of anomalously high temperatures at shallow depth may be a more important source of deformation than previously realized.

Hall, J. L.↗
NASA NTRS1982

Work of fracture in aluminum metal-matrix composites

Mechanical tests were conducted on B/Al composites and, for comparison, FP-Al2O3/Al composites in the as-fabricated condition and following high-temperature isothermal exposure or thermal cycling. In B/Al (1100), isothermal exposure at 773 K for 125 hr reduced toughness, measured by the work of fracture, from 76 to 10 kJ/sq m, and a similar reduction was observed after equivalent thermal cycling. In B/Al (6061), the same isothermal exposure reduced toughness from 44.5 to 8 kJ/sq m, but the effect of thermal cycling was less detrimental. In FP-Al2O3/Al, the work of fracture was insensitive to either type of treatment. Experimental results are interpreted in terms of matrix softening, interface properties, and fiber notch sensitivity.

Skinner, A.↗
NASA NTRS1982

Geoid height-age relation from Seasat altimeter profiles across the Mendocino Fracture Zone

Twenty-eight Seasat altimeter profiles crossing the Mendocino Fracture Zone are used together with seafloor ages determined from magnetic lineations to estimate the change in oceanic geoid height with age, between ages of 15 and 135 m.y. An unbiased estimate of the overall geoid offset along each profile is determined from a least-squares fit of the along-track derivative of the geoid to the geoid slope predicted from a simple two-layer gravitational edge effect model. Uncertainties based upon the statistical properties of each profile are also determined. A geoid slope-age relation is constructed by normalizing the geoid offsets and uncertainties by the age offsets. The results are in agreement with geoid slope-age relations determined from symmetrically spreading ridges (Sandwell and Schubert, 1980). However, the fracture zone estimates have smaller uncertainties and show less scatter. A comparison of these results with the geoid slope-age prediction of the boundary layer cooling model shows that the thermal structure begins to deviate from this model at an early age (20-40 m.y.). A plate cooling model with a thickness of 125 km is most compatible with the geoid slope-age estimates, although significant deviations occur; these may indicate that the lithospheric thermal structure is not entirely age dependent.

Sandwell, D. T.↗
NASA NTRS1982

Fracture of directionally solidified multicrystalline silicon

Fracture toughness data is given for multicrystalline silicon which has been prepared by directional solidification. Results indicated a plane-strain fracture toughness of 0.8 to 0.87 MN/m to the 3/2 power, which is consistent with data for single-crystal silicon.

Chen, C. P.↗
NASA NTRS1982

A simple test for the interlaminar fracture toughness of composites

A simple test method for the measurement of delamination resistance is assessed through its application to 11-ply graphite-epoxy laminates whose matrices employ the F185 and 205 resins. The F185 resin has the same base epoxy as the 205, to which liquid and solid elastomers have been added. The critical value of the mixed mode energy release rate, calculated from a closed form equation employing nominal strain measurements from the onset of edge delamination, show that addition of the elastomers to the base epoxy increases interlaminar fracture toughness. Comparison of these results with those of width-tapered double cantilever beam (WTDCB) test data for the two materials shows a similar strain energy release rate, establishing the accuracy of the novel method. Both tests are found to be needed, however, for the quantitative characterization of interlaminar fracture toughness.

Obrien, T. K.↗
NASA NTRS1982

Phenomenological and mechanics aspects of nondestructive evaluation and characterization by sound and ultrasound of material and fracture properties

Developments in fracture mechanics and elastic wave theory enhance the understanding of many physical phenomena in a mathematical context. Available literature in the material, and fracture characterization by NDT, and the related mathematical methods in mechanics that provide fundamental underlying principles for its interpretation and evaluation are reviewed. Information on the energy release mechanism of defects and the interaction of microstructures within the material is basic in the formulation of the mechanics problems that supply guidance for nondestructive evaluation (NDE).

Fu, L. S. W.↗
NASA NTRS1983

The effect of extreme temperatures on the elastic properties and fracture behavior of graphite/polyimide composites

The influence of elevated and cryogenic temperatures on the elastic moduli and fracture strengths of several C6000/PMR-15 and C6000/NR-15082 laminates was measured. Tests were conducted at -157 C, 24 C, and 316 C (-250 F, 75 F, and 600 F). Both notched and unnotched laminates were tested. The average stress failure criterion was used to predict the fracture strength of quasi-isotropic notched laminates.

Morris, D. H.↗
NASA NTRS1983

A nonlinear fracture mechanics approach to the growth of small cracks

An analytical model of crack closure is used to study the crack growth and closure behavior of small cracks in plates and at notches. The calculated crack opening stresses for small and large cracks, together with elastic and elastic plastic fracture mechanics analyses, are used to correlate crack growth rate data. At equivalent elastic stress intensity factor levels, calculations predict that small cracks in plates and at notches should grow faster than large cracks because the applied stress needed to open a small crack is less than that needed to open a large crack. These predictions agree with observed trends in test data. The calculations from the model also imply that many of the stress intensity factor thresholds that are developed in tests with large cracks and with load reduction schemes do not apply to the growth of small cracks. The current calculations are based upon continuum mechanics principles and, thus, some crack size and grain structure exist where the underlying fracture mechanics assumptions become invalid because of material inhomogeneity (grains, inclusions, etc.). Admittedly, much more effort is needed to develop the mechanics of a noncontinuum. Nevertheless, these results indicate the importance of crack closure in predicting the growth of small cracks from large crack data.

Newman, J. C., Jr.↗
NASA NTRS1982

The effects of weld-repair and hot isostatic pressing on the fracture properties of Ti-5Al-2.5Sn ELI castings

The Ti-5Al-2.5Sn extremely low interstitial alloy employed in the large castings which form the critical attachment fittings of the Space Shuttle External Tank was selected because of its high fracture resistance at cryogenic temperatures. Casting was selected over alternative fabrication methods because of its lower cost and adaptability to design changes, although it was found necessary to weld-repair surface and subsurface casting defects in order to reduce the scrap rate and maintain the inherent cost advantage of the castings. Hot Isostatic Pressing was experimentally found to heal the surface and internal defects of the castings, but did not improve tensile or fracture properties and was therefore rejected as a production technique. Production castings are instead weld-repaired, without any mechanical property degradation.

Misra, M. S.↗
NASA NTRS1983

On gravity from SST, geoid from SEASAT, and plate age and fracture zones in the Pacific

Data from an additional 50 satellite-to-satellite tracking (SST) passes were combined with earlier measurements of the high degree and order (n, m, 12) gravity in the central Pacific. A composite map was produced which shows good agreement with conventional GEM models. Data from the SEASAT altimeter was reduced and found to agree well with both the SST and the GEM fields. The maps are dominated especially in the east, by a pattern of roughly east-west anomalies with a transverse wavelength of about 2000 km. Further comparison with regional bathymetric data shows a remarkably close correlation with plate age. Each anomaly band is framed by those major fracture zones having large offsets. The regular spacing of these fractures seems to account for the fabric in the gravity fields. Other anomalies are accounted for by hot spots. The source of part of these anomalies is in the lithosphere itself. The possible plume size and ascent velocity necessary to supply deep mantle material to the upper mantel without complete thermal equilibration is considered.

Marsh, B. D.↗
NASA NTRS1984

Analysis of stress-strain, fracture and ductility behavior of aluminum matrix composites containing discontinuous silicon carbide reinforcement

Mechanical properties and stress-strain behavior for several types of commercially fabricated aluminum matrix composites, containing up to 40 vol % discontinuous SiC whisker, nodule, or particulate reinforcement were evaluated. It was found that the elastic modulus of the composites was isotropic, to be independent of type of reinforcement, and to be controlled solely by the volume percentage of SiC reinforcement present. The yield/tensile strengths and ductility were controlled primarily by the matrix alloy and temper condition. Ductility decreased with increasing reinforcement content, however, the fracture strains observed were higher than those reported in the literature for this type of composite. This increase in fracture strain is attributed to cleaner matrix powder and increased mechanical working during fabrication. Conventional aluminum and titanium structural alloys were compared and have shown that the properties of these low cost, lightweight composites have good potential for application to aerospace structures.

Mcdanels, D. L.↗
NASA NTRS1984

Interlaminar fracture of composites

Fracture mechanics has been found to be a useful tool for understanding composite delamination. Analyses for calculating strain energy release rates associated with delamination growth have been developed. These analyses successfully characterized delamination onset and growth for particular sources of delamination. Low velocity impact has been found to be the most severe source of composite delamination. A variety of test methods for measuring interlaminar fracture toughness are being developed to identify new composite materials with enhanced delamination resistance.

Obrien, T. K.↗
NASA NTRS1984

The statistical theory of the fracture of fragile bodies. Part 2: The integral equation method

It is demonstrated how with the aid of a bending test, the Weibull fracture risk function can be determined - without postulating its analytical form - by resolving an integral equation. The respective solutions for rectangular and circular section beams are given. In the first case the function is expressed as an algorithm and in the second, in the form of series. Taking into account that the cumulative fracture probability appearing in the solution to the integral equation must be continuous and monotonically increasing, any case of fabrication or selection of samples can be treated.

Kittl, P.↗