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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Compliance calibration of the short rod chevron-notch specimen for fracture toughness testing of brittle materials

The short rod chevron-notch specimen has the advantages of (1) crack development at the chevron tip during the early stage of test loading, and (2) convenient calculation of plane-strain fracture toughness from the maximum test load and from a calibration factor which depends only on the specimen geometry and manner of loading. For generalized application, calibration of the specimen over a range of specimen proportions and chevron-notch configurations is necessary. Such was the objective of this investigation, wherein calibration of the short rod specimen was made by means of experimental compliance measurements converted into dimensionless stress intensity factor coefficients.

Bubsey, R. T.↗
NASA NTRS2005

Fabrication and Probabilistic Fracture Strength Prediction of High-Aspect-Ratio Single Crystal Silicon Carbide Microspecimens With Stress Concentration

Single crystal silicon carbide micro-sized tensile specimens were fabricated with deep reactive ion etching (DRIE) in order to investigate the effect of stress concentration on the room-temperature fracture strength. The fracture strength was defined as the level of stress at the highest stressed location in the structure at the instant of specimen rupture. Specimens with an elliptical hole, a circular hole, and without a hole (and hence with no stress concentration) were made. The average fracture strength of specimens with a higher stress concentration was larger than the average fracture strength of specimens with a lower stress concentration. Average strength of elliptical-hole, circular-hole, and without-hole specimens was 1.53, 1.26, and 0.66 GPa, respectively. Significant scatter in strength was observed with the Weibull modulus ranging between 2 and 6. No fractographic examination was performed but it was assumed that the strength controlling flaws originated from etching grooves along the specimen side-walls. The increase of observed fracture strength with increasing stress concentration was compared to predictions made with the Weibull stress-integral formulation by using the NASA CARES/Life code. In the analysis isotropic material and fracture behavior was assumed - hence it was not a completely rigorous analysis. However, even with these assumptions good correlation was achieved for the circular-hole specimen data when using the specimen data without stress concentration as a baseline. Strength was over predicted for the elliptical-hole specimen data. Significant specimen-to-specimen dimensional variation existed in the elliptical-hole specimens due to variations in the nickel mask used in the etching. To simulate the additional effect of the dimensional variability on the probabilistic strength response for the single crystal specimens the ANSYS Probabilistic Design System (PDS) was used with CARES/Life.

Nemeth, Noel N.↗
NASA NTRS1975

On fracture toughness evaluation for semi-brittle fracture

The existing methods of assessing the fracture toughness of materials exhibiting semi-brittle fracture are critically reviewed. The methods concern the Crack Growth Resistance (R-curve), the Crack Opening Displacement (COD), and the J-integral. An analysis of the shortcomings of the methods described makes it possible to formulate a new definition of fracture toughness appropriate to semi-brittle fracture. An improved simple experimental method for measuring fracture toughness for semi-brittle fracture is proposed which takes into account both crack growth and plastic nonlinear effects at crack front. The proposed method is shown to be free of the theoretical and experimental discrepancies encountered in the R-curve, COD, and J-integral methods.

Eftis, J.↗
NASA NTRS1994

Establishment of redox conditions during planetary collisions as an origin of chondrites

Collisions between a 'cometlike' body (mixtures of chondritic materials and ice) and a slightly differentiated body were proposed for shock origin of ordinary chondrites. In this model, chondrules were formed with shock melting, and matrices were formed both by fracturing of materials and by recondensation of evaporated materials. This model can explain different redox conditions of chondrite formations by ice evaporation. Although this model was originally proposed for ordinary chondrites, we assume here that the model can be extended to chondrite formation in general. In this paper, redox conditions during chondrite formation by collisions will be discussed in the light of phase diagrams for solid-vapor equilibria.

Tsuchiyama, A.↗
NASA NTRS1987

A parametric study of fracture toughness of fibrous composite materials

Impacts to fibrous composite laminates by objects with low velocities can break fibers giving crack-like damage. The damage may not extend completely through a thick laminate. The tension strength of these damage laminates is reduced much like that of cracked metals. The fracture toughness depends on fiber and matrix properties, fiber orientations, and stacking sequence. Accordingly, a parametric study was made to determine how fiber and matrix properties and fiber orientations affect fracture toughness and notch sensitivity. The values of fracture toughness were predicted from the elastic constants of the laminate and the failing strain of the fibers using a general fracture toughness parameter developed previously. For a variety of laminates, values of fracture toughness from tests of center-cracked specimens and values of residual strength from tests of thick laminates with surface cracks were compared to the predictions to give credibility to the study. In contrast to the usual behavior of metals, it is shown that both ultimate tensile strength and fracture toughness of composites can be increased without increasing notch sensitivity.

Poe, C. C., Jr.↗
NASA NTRS1987

A parametric study of fracture toughness of fibrous composite materials

Impacts to fibrous composite laminates by objects with low velocities can break fibers giving crack-like damage. The damage may not extend completely through a thick laminate. The tension strength of these damage laminates is reduced much like that of cracked metals. The fracture toughness depends on fiber and matrix properties, fiber orientations, and stacking sequence. Accordingly, a parametric study was made to determine how fiber and matrix properties and fiber orientations affect fracture toughness and notch sensitivity. The values of fracture toughness were predicted from the elastic constants of the laminate and the failing strain of the fibers using a general fracture toughness parameter developed previously. For a variety of laminates, values of fracture toughness from tests of center-cracked specimens and values of residual strength from tests of thick laminates with surface cracks were compared to the predictions to give credibility to the study. In contrast to the usual behavior of metals, it is shown that both ultimate tensile strength and fracture toughness of composites can be increased without increasing notch sensitivity.

Poe, C. C., Jr.↗
NASA NTRS2002

Evaluation of Fatigue Crack Growth and Fracture Properties of Cryogenic Model Materials

The criteria used to prevent failure of wind-tunnel models and support hardware were revised as part of a project to enhance the capabilities of cryogenic wind tunnel testing at NASA Langley Research Center. Specifically, damage-tolerance fatigue life prediction methods are now required for critical components, and material selection criteria are more general and based on laboratory test data. The suitability of two candidate model alloys (AerMet 100 and C-250 steel) was investigated by obtaining the fatigue crack growth and fracture data required for a damage-tolerance fatigue life analysis. Finally, an example is presented to illustrate the newly implemented damage tolerance analyses required of wind-tunnel model system components.

Newman, John A.↗
NASA NTRS1981

Microstructure and mechanical properties of bulk yttria-partially-stabilized zirconia

A commercially available bulk 4.5 mole percent yttria-Y2O3)-partially-stabilized zirconia (PSZ) was studied by light microscopy, X-ray analysis, microhardness measurement, and fracture toughness testing. The growth of the precipitates and the phase transformations were studied as a function of aging in air at 1500 C. Aging cuves were constructed for both the as-received and the solution-annealed-and-quenched materials; the curves showed hardness peaks at 1397 and 1517 kg/sq mm, respectively. A total of twelve different types of tetragonal precipitates were found. The rectangular plate-shaped tetragonal precipitates were found to have a (110) habit plane. Grinding of the Y2O3 PSZ into powder did not cause a significant amount of metastable tetragonal precipitates to transform into the monoclinic phase, thus indicating that transformation toughening is not a significant mechanism for the material. The fracture toughness of the aged and of the unaged solution-annealed-and-quenched PSZ was found to be between 2 and 3 MN/cu m/2.

Valentine, P. G.↗
NASA NTRS1986

The emission of atoms and molecules accompanying fracture of single-crystal MgO

The emission of particles due to deformation and fracture of materials has been investigated. The emission of electrons (exoelectron emission), ions, neutral species, photons (triboluminescence), as well as long wavelength electromagnetic radiation was observed; collectively these emissions are referred to as fractoemission. This paper describes measurements of the neutral emission accompanying the fracture of single-crystal MgO. Masses detected are tentatively assigned to the emission of H2, CH4, H2O, CO, O2, CO2, and atomic Mg. Other hydrocarbons are also observed. The time dependencies of some of these emissions relative to fracture are presented for two different loading conditions.

Dickinson, J. T.↗
NASA NTRS1996

Improved Damage Resistant Composite Materials Incorporating Shape Memory Alloys

Metallic shape memory alloys (SMA) such as nitinol have unique shape recovery behavior and mechanical properties associated with a material phase change that have been used in a variety of sensing and actuation applications. Recent studies have shown that integrating nitinol-SMA actuators into composite materials increases the composite material's functionality. Hybrid composites of conventional graphite/epoxy or glass/epoxy and nitinol-SMA elements can perform functions in applications where monolithic composites perform inadequately. One such application is the use of hybrid composites to function both in load bearing and armor capacities. While monolithic composites with high strength-to-weight ratios function efficiently as loadbearing structures, because of their brittle nature, impact loading can cause significant catastrophic damage. Initial composite failure modes such as delamination and matrix cracking dissipate some impact energy, but when stress exceeds the composite's ultimate strength, fiber fracture and material perforation become dominant. One of the few methods that has been developed to reduce material perforation is hybridizing polymer matrix composites with tough kevlar or high modulus polyethynylene plies. The tough fibers increase the impact resistance and the stiffer and stronger graphite fibers carry the majority of the load. Similarly, by adding nitinol-SMA elements that absorb impact energy through the stress-induced martensitic phase transformation, the composites' impact perforation resistance can be greatly enhanced. The results of drop-weight and high velocity gas-gun impact testing of various composite materials will be presented. The results demonstrate that hybridizing composites with nitinol-SMA elements significantly increases perforation resistance compared to other traditional toughening elements. Inspection of the composite specimens at various stages of perforation by optical microscope illustrates the mechanisms by which perforation is initiated. Results suggest that the out-of-plane transverse shear properties of the composite and nitinol elements have a significant effect on the perforation resistance. Applications that can utilize the hybrid composites effectively will also be presented with the experimental studies.

Paine, Jeffrey S. N.↗
NASA NTRS1975

Investigation of lightweight designs and materials for LO2 and LH2 propellant tanks for space vehicles, phase 1

Design, analysis, and fabrication studies were performed on nonintegral (suspended) tanks using a representative space tug design. The LH2 and LO2 tank concept selection was developed. Tank geometries and support relationships were investigated using tug design propellant inertias and ullage pressures, then compared based on total tug systems effects. The tank combinations which resulted in the maximum payload were selected. Tests were conducted on samples of membrane material which was processed in a manner simulating production tank fabrication operations to determine fabrication effects on the fracture toughness of the tank material. Fracture mechanics analyses were also performed to establish a preliminary set of allowables for initial defects.

Source record↗
NASA NTRS1979

Fracture modes and acoustic emission of composite materials

Unidirectional fibrous composite material laminae are the construction material of a laminate. Four failure modes can occur in this material, and the identification of these modes is as important as the identification of onset of fracture. Acoustic emission (AE) pulses, generated by the change in the stress field resulting from cracking in a material, have a unique pattern of amplitude distribution that depends only on the mode of fracture and the type of material. By using a tapered double-cantilever beam as grips for the unidirectional lamina, a crack with a constant stress intensity factor was induced, thus eliminating the factor of the stress level. Placing the lamina in different directions relative to the grips imposed a different failure mode. By plotting the log of the relative number of AE pulses above some level versus the log of the relative level, a single constant coefficient can be measured for each material and mode of fracture. The AE energy is proportional to the energy released by the cracking; this proportionality depends on the medium where the AE waves have to travel, and not on the cracking mode.

Rotem, A.↗
NASA NTRS1972

Lunar rock splitter/can sealer

A manually powered and operated force generating device fractures lunar material as received from the moon and seals cans containing lunar samples. The rock splitting and can sealing operations are carried out in a vacuum environment. Can sealing is a cold welding process that requires a pressure of 125,000 psi (generating 32,000 lb for the largest can used). The design and modification of the device and the operating experience are described.

Johnson, K. G.↗
NASA NTRS1974

Mechanical behavior of polycrystalline ceramics: Brittle fracture of SiC-Si3N4 materials

Various properties of ceramic materials were investigated. Magnesium oxide and the role of anion impurities were investigated together with the slow crack growth in silicon nitride-silicon carbide ceramics. The oxide program involved development of fabrication techniques for anion doped materials and evaluation of the role of these anions in the hot pressing response, grain boundary diffusion of nickel doped material, grain boundary microhardness, and grain growth. The carbide-nitride work employed commercial materials and the research involved evaluation of a recently reported technique for study of slow crack growth and the development of data for these commercial materials.

Leipold, M. H.↗
NASA NTRS1974

Mechanical behavior of polycrystalline ceramics: Brittle fracture of SiC-Si3N4 materials

The first study area involved magnesium oxide and the role of anion impurities, while the second area was directed toward slow crack growth in silicon nitride-silicon carbide ceramics. The oxide program involved development of fabrication techniques for anion doped materials and evaluation of the role of these anions in the hot pressing response, grain boundary diffusion of nickel doped material, grain boundary microhardness, and grain growth.

Leipold, M. H.↗
NASA NTRS1976

Advances in engineering science, volume 1

Proceedings from a conference on engineering advances are presented, including materials science, fracture mechanics, and impact and vibration testing. The tensile strength and moisture transport of laminates are also discussed.

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