Engineering topics

Salem, J. A.

Publications and source records attributed to Salem, J. A..

18 records

NASA NTRS2016

Additive Manufacturing and Characterization of Polylactic Acid (PLA) Composites Containing Metal Reinforcements

Additive manufacturing of polymeric systems using 3D printing has become quite popular recently due to rapid growth and availability of low cost and open source 3D printers. Two widely used 3D printing filaments are based on polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS) systems. PLA is much more environmentally friendly in comparison to ABS since it is made from renewable resources such as corn, sugarcane, and other starches as precursors. Recently, polylactic acid-based metal powder containing composite filaments have emerged which could be utilized for multifunctional applications. The composite filaments have higher density than pure PLA, and the majority of the materials volume is made up of polylactic acid. In order to utilize functionalities of composite filaments, printing behavior and properties of 3-D printed composites need to be characterized and compared with the pure PLA materials. In this study, pure PLA and composite specimens with different metallic reinforcements (Copper, Bronze, Tungsten, Iron, etc) were 3D printed at various layer heights and resulting microstructures and properties were characterized. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) behavior of filaments with different reinforcements were studied. The microscopy results show an increase in porosity between 3-D printed regular PLA and the metal composite PLA samples, which could produce weaker mechanical properties in the metal composite materials. Tensile strength and fracture toughness behavior of specimens as a function of print layer height will be presented.

brass↗

NASA NTRS2014

Burst Testing of Triaxial Braided Composite Tubes

Applications using triaxial braided composites are limited by the materials transverse strength which is determined by the delamination capacity of unconstrained, free-edge tows. However, structural applications such as cylindrical tubes can be designed to minimize free edge effects and thus the strength with and without edge stresses is relevant to the design process. The transverse strength of triaxial braided composites without edge effects was determined by internally pressurizing tubes. In the absence of edge effects, the axial and transverse strength were comparable. In addition, notched specimens, which minimize the effect of unconstrained tow ends, were tested in a variety of geometries. Although the commonly tested notch geometries exhibited similar axial and transverse net section failure strength, significant dependence on notch configuration was observed. In the absence of unconstrained tows, failure ensues as a result of bias tow rotation, splitting, and fracture at cross-over regions.

Triaxial Braided Composites↗

NASA NTRS2010

Test Method Variability in Slow Crack Growth Properties of Sealing Glasses

The crack growth properties of several sealing glasses were measured by using constant stress rate testing in ~2 and 95 percent RH (relative humidity). Crack growth parameters measured in high humidity are systematically smaller (n and B) than those measured in low humidity, and crack velocities for dry environments are ~100x lower than for wet environments. The crack velocity is very sensitive to small changes in RH at low RH. Biaxial and uniaxial stress states produced similar parameters. Confidence intervals on crack growth parameters that were estimated from propagation of errors solutions were comparable to those from Monte Carlo simulation. Use of scratch-like and indentation flaws produced similar crack growth parameters when residual stresses were considered.

Salem, J. A.↗

NASA NTRS2002

Mechanical Properties and Microstructure of Biomorphic Silicon Carbide Ceramics Fabricated from Wood Precursors

Silicon carbide based, environment friendly, biomorphic ceramics have been fabricated by the pyrolysis and infiltration of natural wood (maple and mahogany) precursors. This technology provides an eco-friendly route to advanced ceramic materials. These biomorphic silicon carbide ceramics have tailorable properties and behave like silicon carbide based materials manufactured by conventional approaches. The elastic moduli and fracture toughness of biomorphic ceramics strongly depend on the properties of starting wood preforms and the degree of molten silicon infiltration. Mechanical properties of silicon carbide ceramics fabricated from maple wood precursors indicate the flexural strengths of 3441+/-58 MPa at room temperature and 230136 MPa at 1350C. Room temperature fracture toughness of the maple based material is 2.6 +/- 0.2 MPa(square root of)m while the mahogany precursor derived ceramics show a fracture toughness of 2.0 +/- 0.2 Mpa(square root of)m. The fracture toughness and the strength increase as the density of final material increases. Fractographic characterization indicates the failure origins to be pores and chipped pockets of silicon.

Singh, Mrityunjay↗

NASA NTRS2001

Effects of Minor Alloying Additions on the Microstructure, Toughness, and Creep Strength of Directionally Solidified NiAl-31Cr-3Mo

A study of the effects of small (0.25 to 1.0 at%) fifth element additions to the structure and mechanical properties of directionally solidified (DS) NiAl-31Cr-3Mo has been undertaken. Essentially all the additions changed the as-DS'ed microstructure from lamellar eutectic grains to cells and, in some cases, introduced NiAl dendrites and/or third phases. In general the alloying additions did not improve strength or toughness over that possessed by the base composition; only Hf and, perhaps Ti, gave a minor increase in elevated temperature creep resistance. The lack of improvement in creep properties is probably due to inability to precipitation harden NiAl.

Whittenberger, J. D.↗

NASA NTRS2001

Development of Tough, Strong, and Pest-Resistant MoSi2-(Beta)Si3N4 Composites for High-Temperature Structural Applications

A new MoSi2-base composite was developed that contains in-situ reinforcement of whisker-type beta-Si3N4 grains in a MoSi2 matrix. The advantages of this in-situ reinforced MoSi2-Si3N4 are lower density, higher fracture toughness and better strength than typical MoSi2 alloys, combined with excellent environmental and pest resistance. The average fracture toughness of the in-situ reinforced material determined by one technique was 12.2 MPa.m(exp 1/2) compared to 4.9 to 5.5 MPa.m(exp 1/2) for similar materials with the exception that the beta-Si3N4 had a blocky morphology as opposed to the whisker-like morphology typical of the in-situ toughened material. This MoSi2-(beta)Si3N4 was also resistant to pesting at intermediate temperatures (400 to 600 C) even when precracked or under applied load; conditions that would quickly reduce typical MoSi2 alloys to oxidized powder.

Hebsur, M. G.↗

NASA NTRS2000

Microstructural Characterization of a Directionally-Solidified Ni-33 (at. %)Al-31Cr-3Mo Eutectic Alloy as a Function of Withdrawal Rate

The Ni-33 (at. %)Al-3lCr-3Mo eutectic alloy was directionally-solidified (DS) at different rates, V(sub I), varying between 2.5 to 508 mm/ h. Detailed qualitative and quantitative metallographic and chemical analyses were conducted on the directionally-solidified rods. The microstructures consisted of eutectic colonies with parallel lamellar NiAl/(Cr,Mo) plates for solidification rates at and below 12.7 mm/ h. Cellular eutectic microstructures were observed at higher solidification rates, where the plates exhibited a radial pattern. The microstructures were demonstrated to be fairly uniform throughout a 100 mm length of the DS zone by quantitative metallography. The average cell size, bar-d, decreased with increasing growth rate to a value of 125 microns at 508 mm/ h according to the relation bar-d (microns) approx. = 465 V(sup -0.22, sub I), where V(sub I) is in mm/ h. Both the average NiAl plate thickness, bar-Delta(sub NiAl), and the interlamellar spacing, bar-lambda, were observed to be constant for V(sub I) less than or = 50.8 mm/ h but decreased with increasing growth rate above this value as 0.93 bar-Delta(sub NiAl)(microns) = 61.2 V(sup -0.93, sub I) and bar-lambda (microns) = 47.7 V(sup -0.64, sub I), respectively. The present results are detailed on a microstructural map. Keywords Optical microscopy, microstructure, compounds intermetallic, directional solidification

Raj, S. V.↗

NASA NTRS1999

Effect of Growth Rate on Elevated Temperature Plastic Flow and Room Temperature Fracture Toughness of Directionally Solidified NiAl-31Cr-3Mo

The eutectic system Ni-33Al-31Cr-3Mo was directionally solidified at rates ranging from 7.6 to 508 mm/h. Samples were examined for microstructure and alloy chemistry, compression tested at 1200 and 1300 K, and subjected to room temperature fracture toughness measurements. Lamellar eutectic grains were formed at 12.7 mm/h; however cellular structures with a radial eutectic pattern developed at faster growth rates. Elevated temperature compression testing between 10(exp -4) to 10(exp -7)/s did not reveal an optimum growth condition, nor did any single growth condition result in a significant fracture toughness advantage. The mechanical behavior, taken together, suggests that Ni-33Al-31Cr-3Mo grown at rates from 25.4 to 254 mm/h will have nominally equivalent properties.

Whittenberger, J. Daniel↗

NASA NTRS1998

High-Temperature Slow Crack Growth of Silicon Carbide Determined by Constant-Stress-Rate and Constant-Stress Testing

High-temperature slow-crack-growth behaviour of hot-pressed silicon carbide was determined using both constant-stress-rate ("dynamic fatigue") and constant-stress ("static fatigue") testing in flexure at 1300 C in air. Slow crack growth was found to be a governing mechanism associated with failure of the material. Four estimation methods such as the individual data, the Weibull median, the arithmetic mean and the median deviation methods were used to determine the slow crack growth parameters. The four estimation methods were in good agreement for the constant-stress-rate testing with a small variation in the slow-crack-growth parameter, n, ranging from 28 to 36. By contrast, the variation in n between the four estimation methods was significant in the constant-stress testing with a somewhat wide range of n= 16 to 32.

Choi, Sung H.↗

NASA NTRS1993

Fracture Toughness of PMMA as Measured with Indentation Cracks

Fracture toughness of polymethyl methacrylate (PMMA) was evaluated with indentation cracks. The cracks were developed by indenting in a liquid acetone environment, as suggested by previous researchers. Due to the inconsistency in crack configurations with varying indentation loads and to the negligible residual stress from indentation, the application of the indentation strength method was limited in evaluating fracture toughness of PMMA. The semi-elliptical crack approximation, however, particularly at a low indentation load of 9.8 N, resulted in reasonable agreement with the value determined by three conventional fracture toughness testings using the compact tension (CT), double cantilever beam (DCB), and single edge notched beam (SENB) specimens. Measurements at other indentation load typically were in poor agreement with conventional methods due to poorly developed crack configurations.

Choi, S. R.↗

NASA NTRS1993

Interaction of Cracks Between Two Adjacent Indents in Glass

Experimental observations of the interaction behavior of cracks between two adjacent indents were made using an indentation technique in soda-lime glass. It was specifically demonstrated how one indent crack initiates and propagates in the vicinity of another indent crack. Several types of crack interactions were examined by changing the orientation and distance of one indent relative to the other. It was found that the residual stress field produced by elastic/plastic indentation has a significant influence on controlling the mode of crack interaction. The interaction of an indent crack with a free surface was also investigated for glass and ceramic specimens.

Choi, S. R.↗

NASA NTRS1993

Evaluation of the Fracture Toughness of Nb-40Al-8Cr-1W-1Y-0.05B Intermetallic Material by Indentation Techniques

The fracture toughness of an Nb-40Al-8Cr - 1W-1Y-0.05B intermetallic material was evaluated by indentation techniques at room temperature. Two widely used indentation methods, crack size measurement and indent strength, yielded excellent agreement with a conventional fracture toughness technique using straight-through precracked specimens, despite the occasional formation of poorly configured cracks. However, the modified indentation technique, using dummy indent flaws, resulted in a low fracture toughness compared to that evaluated by the other methods. The material did not exhibit rising R-curve behavior, as evaluated from the indentation strength data. These results indicate that indentation fracture principles are applicable to this brittle intermetallic material without modification of the residual contact stress term originally calibrated for ceramic materials.

Choi, S. R.↗

NASA NTRS1992

Mechanical behaviour and failure phenomenon of an in situ toughened silicon nitride

The Weibull modulus, fracture toughness and crack growth resistance of an in-situ toughened, silicon nitride material used to manufacture a turbine combustor were determined from room temperature to 1371 C. The material exhibited an elongated grain structure that resulted in improved fracture toughness, nonlinear crack growth resistance, and good elevated temperature strength. However, low temperature strength was limited by grains of excessive length (30 to 100 microns). These excessively long grains were surrounded by regions rich in sintering additives.

Salem, J. A.↗

NASA NTRS1992

Strength, toughness and R-curve behaviour of SiC whisker-reinforced composite Si3N4 with reference to monolithic Si3N4

The flexural strength and fracture toughness of 30 vol pct SiC whisker-reinforced Si3N4 material were determined as a function of temperature from 25 to 1400 C in an air environment. It was found that both strength and toughness of the composite material were almost the same as those of the monolithic counterpart. The room-temperature strength was retained up to 1100 C; however, appreciable strength degradation started at 1200 C and reached a maximum at 1400 C due to stable crack growth. In contrast, the fracture toughness of the two materials was independent of temperature with an average value of 5.66 MPa sq rt m. It was also observed that the composite material exhibited no rising R-curve behavior at room temperature, as was the case for the monolithic material. These results indicate that SiC whisker addition to the Si3N4 matrix did not provide any favorable effects on strength, toughness and R-curve behavior.

Choi, S. R.↗

NASA NTRS1990

Ultrasonic imaging of textured alumina

Ultrasonic images representing the bulk attenuation and velocity of a set of alumina samples were obtained by a pulse-echo contact scanning technique. The samples were taken from larger bodies that were chemically similar but were processed by extrusion or isostatic processing. The crack growth resistance and fracture toughness of the larger bodies were found to vary with processing method and test orientation. The results presented here demonstrate that differences in texture that contribute to variations in structural performance can be revealed by analytic ultrasonic techniques.

Stang, D. B.↗

NASA NTRS1990

Determination of the stress distributions in a ceramic, tensile specimen using numerical techniques

Finite element analyses (FEA) were used to determine the stress distributions in a ceramic, tensile specimen with two types of button-head gripping systems. The FEA revealed stress raisers at both the button-head and the transition from the gage section to the shank. However, the stress field within the bulk of the gage section is uniform and uniaxial. The stress ratio, kt, between the button-head and gage section stresses varied from 0.35 to 0.72 for the tapered collet or the straight collet systems, respectively. Previous empirical tests confirm these results whereby the tapered collet system, compared to the straight collet system, sustained over twice the average load before failure at the button-head.

Jenkins, M. E.↗

NASA NTRS1989

Thermal expansion and elastic anisotropy in single crystal Al2O3 and SiC whiskers

The crystalline thermal expansion and elastic anisotropies of SiC and Al2O3 whiskers are considered in the process of evaluation of the anisotropy of the alpha-E product, the thermoelastic stress index. It is demonstrated to exhibit considerable crystalline anisotropy and a substantial temperature dependence, both of which will influence the thermal and residual stresses in composites using these materials for reinforcement.

Salem, J. A.↗

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.↗