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Shannon, J. L., Jr.

Publications and source records attributed to Shannon, J. L., Jr..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Closed form expressions for crack mouth displacements and stress intensity factors for chevron notched short bar and short rod specimens based on experimental compliance measurements

A set of equations are presented describing certain fracture mechanics parameters for chevron notch bar and rod specimens. They are developed by fitting compliance calibration data reported earlier. The equations present the various parameters in their most useful forms. The data encompass the entire range of the specimen geometries most commonly used. Their use will facilitate the testing and analysis of brittle metals, ceramics, and glasses.

Bubsey, R. T.↗
NASA NTRS1985

Fractured toughness of Si3N4 measured with short bar chevron-notched specimens

The short bar chevron-notched specimen is used to measure the plane strain fracture toughness of hot pressed Si3N4. Specimen proportions and chevron-notch angle are varied, thereby varying the amount of crack extension to maximum load (upon which K sub IC is based). The measured toughness (4.68 + or - 0.19 MNm to the 3/2 power) is independent of these variations, inferring that the material has a flat crack growth resistance curve.

Salem, J. A.↗
NASA NTRS1985

Analysis of an externally radially crack ring segment subject to three-point radial loading

The boundary collocation method was used to generate Mode I stress intensity and crack mouth opening displacement coefficients for externally radially (through-the-thickness) cracked ring segments subjected to three-point radial loading. Numerical results were obtained for ring segment outer-to-inner radius ratios (Ro/Ri) ranging from 1.10 to 2.50 and crack length to segment width ratios (a/W) ranging from 0.1 to 0.8. Stress intensity and crack mouth displacement coefficients were found to depend on the ratios Ro/Ri and a/W as well as the included angle between the directions of the reaction forces.

Gross, B.↗
NASA NTRS1983

Specimen size and geometry effects on fracture toughness of Al2O3 measured with short rod and short bar chevron-notch specimens

Plane strain fracture toughness measurements were made on Al2O3 using short rod and short bar chevron notch specimens previously calibrated by the authors for their dimensionless stress intensity factor coefficients. The measured toughness varied systematically with variations in specimen size, proportions, and chevron notch angle apparently due to their influence on the amount of crack extension to maximum load (the measurement point). The toughness variations are explained in terms of a suspected rising R curve for the material tested, along with a discussion of an unavoidable imprecision in the calculation of K sub Ic for materials with rising R curves when tested with chevron notch specimens.

Shannon, J. L., Jr.↗
NASA NTRS1983

Analysis of an Externally Radially Cracked Ring Segment Subject to Three-Point Radial Loading

The boundary collocation method was used to generate Mode 1 stress intensity and crack mouth opening displacement coefficients for externally radially cracked ring segments subjected to three point radial loading. Numerical results were obtained for ring segment outer-to-inner radius ratios (R sub o/R sub i) ranging from 1.10 to 2.50 and crack length to segment width ratios (a/W) ranging from 0.1 to 0.8. Stress intensity and crack mouth displacement coefficients were found to depend on the ratios R sub o/R sub i and a/W as well as the included angle between the directions of the reaction forces.

Gross, B.↗
NASA NTRS1983

Development of plane strain fracture toughness test for ceramics using Chevron notched specimens

Chevron-notched four-point-bend and short-bar specimens have been used to determine the fracture toughness of sintered aluminum oxide and hot-pressed silicon nitride ceramics. The fracture toughness for Si3N4 is found to be essentially independent of the specimen size and chevron notch configuration, with values ranging from 4.6 to 4.9 MNm exp -3/2. In contrast, significant specimen size and notch geometry effects have been observed for Al2O3, with the fracture toughness ranging from 3.1 to 4.7 MNm exp -3/2. These effects are attributed to a rising crack growth resistance curve for the Al2O3 tested.

Bubsey, R. T.↗
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 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

LVDT gage for fracture-toughness tests in liquid hydrogen

Linear-variable differential transformer replaces conventional resistance strain gages to measure crack-mouth-opening displacement. LVDT is superior in tests under liquid hydrogen, where boiling of hydrogen on resistive is suited to broad temperature range and hostile environments such as nuclear reactors.

Pierce, W. S.↗
NASA NTRS1980

Resonant-fatigue cracking apparatus

Apparatus produces controlled surface cracks in test specimens. It has been developed and is useful in production of surface cracks of controlled size and shape in fracture specimens. It consists of specially-designed stand-mounted clamping fixture, commercially available pneumatic actuator, and suitable sound-control mufflers.

Dorner, J. P.↗
NASA NTRS1980

Performance of Chevron-notch short bar specimen in determining the fracture toughness of silicon nitride and aluminum oxide

Ease of preparation and testing are advantages unique to the chevron-notch specimen used for the determination of the plane strain fracture toughness of extremely brittle materials. During testing, a crack develops at the notch tip and extends stably as the load is increased. For a given specimen and notch configuration, maximum load always occurs at the same relative crack length independent of the material. Fracture toughness is determined from the maximum load with no need for crack length measurement. Chevron notch acuity is relatively unimportant since a crack is produced during specimen loading. In this paper, the authors use their previously determined stress intensity factor relationship for the chevron-notch short bar specimen to examine the performance of that specimen in determining the plane strain fracture toughness of silicon nitride and aluminum oxide.

Munz, D.↗
NASA NTRS1980

Fracture toughness of brittle materials determined with chevron notch specimens

The use of chevron-notch specimens for determining the plane strain fracture toughness (K sub Ic) of brittle materials is discussed. Three chevron-notch specimens were investigated: short bar, short rod, and four-point-bend. The dimensionless stress intensity coefficient used in computing K sub Ic is derived for the short bar specimen from the superposition of ligament-dependent and ligament-independent solutions for the straight through crack, and also from experimental compliance calibrations. Coefficients for the four-point-bend specimen were developed by the same superposition procedure, and with additional refinement using the slice model of Bluhm. Short rod specimen stress intensity coefficients were determined only by experimental compliance calibration. Performance of the three chevron-notch specimens and their stress intensity factor relations were evaluated by tests on hot-pressed silicon nitride and sintered aluminum oxide. Results obtained with the short bar and the four-point-bend specimens on silicon nitride are in good agreement and relatively free of specimen geometry and size effects within the range investigated. Results on aluminum oxide were affected by specimen size and chevron-notch geometry, believed due to a rising crack growth resistance curve for the material. Only the results for the short bar specimen are presented in detail.

Shannon, J. L., Jr.↗
NASA NTRS1978

Surface-crack shape change in bending fatigue using an inexpensive resonant fatiguing apparatus

An inexpensive device for producing surface cracks of controlled size and shape is described along with results of its use which show how the shape of the surface crack changes as it grows in bending fatigue from a variety of crack starter shapes. The growth pattern for any crack is uniquely defined by the crack starter configuration and appears to be independent of alloy. Shape changes are substantial and all growth curves tend toward a common growth curve. Circular cracks tend to become elliptical with an associated increase in stress intensity factor. This increase accelerates the crack growth rate and proximity to the critical flaw size. Through-cracks produced by the extension of a surface crack in a bending stress field will have length ten times the section thickness. The results demonstrate the necessity of taking into account changing crack shape in the calculation of structural life when bending is a significant component of the stress field.

Pierce, W. S.↗
NASA NTRS1978

Investigation of the fracture mechanism of Ti-5Al-2.5Sn at cryogenic temperatures

Fractography and metallographic sectioning were used to investigate the influence of microstructure on the fracture mechanism and fracture toughness (KIC) of normal interstitial and extra low interstitial (ELI) Ti-5Al-25Sn at 20 K (-423 F) and 77 K (-320 F). Plates of each grade were mill annealed at 815 C followed by either air or furnace cooling. These variations in composition and cooling rate resulted in differences in the volume fraction and internal structure of the dispersed beta phase and in the ordering of the alpha matrix. The ELI alloys were tougher than the normal interstitial plates. KIC of the furnace-cooled ELI plate was 25% lower than that of the air-cooled ELI material. Variations in cooling rate had no influence of KIC of the normal interstitial alloys. Fractography showed that a large portion of the fracture surfaces were covered with elongated dimples. Metallographic sections of specimens deformed at 77 K showed that these features form at the intersections of slip bands or deformation twins with grain or twin boundaries. Ordering and higher interstitial levels increase the local strain in slip bands resulting in void nucleation at lower macroscopic strains and lower KIC values.

Van Stone, R. H.↗
NASA NTRS1977

Room temperature crack growth rates and -20 deg F fracture toughness of welded 1 1/4 inch A-285 steel plate

Data are presented which were developed in support of a structural assessment of NASA-LEWIS' 10-foot by 10-foot supersonic wind tunnel, critical portions of which are fabricated from rolled and welded 1 1/4 inch thick A-285 steel plate. Test material was flame cut from the tunnel wall and included longitudinal and circumferential weld joints. Parent metal, welds, and weld heat affected zone were tested. Tensile strength and fracture toughness were determined at -20 F, the estimated lowest tunnel operating temperature. Crack growth rates were measured at room temperature, where growth rates in service are expected to be highest.

Shannon, J. L., Jr.↗