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At least 19 records

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NASA NTRS1971

Basic failure mechanisms in advanced composites

Fundamental failure mechanisms in carbon-epoxy composites were studied for more reliable prediction of the performance of these materials. Single and multiple fiber specimens were tested under tensile loads, and the sequence of failure events was observed. Parameters such as resin crack sensitivity, fiber surface treatment and variations in fibers from batch to batch are being evaluated. The analysis of bulk composite fracture processes using acoustic emission techniques is being studied in order to correlate microscopic observations with bulk composite behavior. Control of the fracture process through matrix and interface modification is being attempted, and study of failure processes in composite/metal specimens is being conducted. Most of the studies involved DEN 438 epoxy novolac as the matrix, but some experiments are now underway using the higher temperature resin ERLA 4617.

Mullin, J. V.↗

NASA NTRS1978

The suppression of low-temperature intergranular brittleness in ferritic Fe-Mn alloys

A method for obtaining cryogenic toughness in an Fe-12Mn-0.2Ti alloy by a controlled cooling technique is presented. The intergranular fracture surface of the quenched alloy, using an AES, revealed no significant difference from that obtained by the bulk transgranular fracture surface, indicating the absence of large-scale grain boundary segregation during the austenitizing treatment. Attention is given to the step cooling method, using a furnace at temperatures of 100 to 120 C, followed by fast cooling to ambient temperature, which in turn produces a high-impact toughness without intergranular brittleness. It is concluded that the sources of brittle intergranular fracture in the alloy are both chemical, operating in martensitic structure, and nonchemical, active during quenching from the austenitizing treatment.

Hwang, S. K.↗

NASA NTRS1995

Mechanical properties of high-temperature superconducting wires

Bending strength, fracture toughness, and elastic modulus data were acquired for YBa2Cu3O(x), Bi2Sr2CaCu2O(x) (Bi,Pb)2Sr2Ca2Cu3O(x), and Tl2Ba2Ca2Cu3O(x) bars. These data and thermal expansion coefficients strongly suggest that the maximum possible tensile strain without fracture of bulk tapes or wires is approximately equals 0.2%. In Ag-clad conductors, residual stresses will be of limited benefit, but fractures produced by larger strains can be accommodated by shunting current through the Ag.

Goretta, K. C.↗

NASA NTRS1986

Interfaces and failure mechanisms in Al-SiC composites

Aluminum alloys reinforced with silicon carbide whiskers exhibit significantly higher strength and modulus than the unreinforced alloys. However, the composites also exhibit low ductility and poor fracture toughness for reasons which are not well-understood. In this study, high resolution and conventional TEM techniques are used to analyze interface microstructure and failure mechanisms in P/M 2124 and 6061 aluminum alloys reinforced with silicon carbide whiskers. Interfacial oxides are typical of both composite materials, often distributed in clusters or in a discrete layer 2-5 nm thick along the whisker-matrix interface. Highly deformed regions beneath bulk tensile fracture surfaces reveal possible fracture nucleation centers as well as sites of stress concentration where intense plastic strain has occurred. Observations of such highly deformed regions include (1) void initiation at whisker ends, (2) interface decohesion involving the thin oxide layer, (3) transverse cracks in whiskers, and (4) cracks in large constituent particles. TEM results are presented and discussed in relation to mechanisms of composite failure.

Nutt, S. R.↗

NASA NTRS1993

Experimentally reproduced relict enstatite in porphyritic chondrules of enstatite chondrite composition

Experiments are presented that test a model for the origin of porphyritic pyroxene (PP) chondrules in enstatite chondrites that contain phenocrysts of enstatite with blue cathodoluminescence (CL) set in a matrix of radial, dendritic enstatite with red CL. Established one-atmosphere, gas-mixing techniques were used. Relict enstatite phenocrysts with blue CL in a matrix of coarsely radial to dendritic enstatite with red CL were successfully produced. The relict crystals are preserved in runs with a melt time of 36 minutes or less at 1537 C. The relicts remain angular with smooth crystal/melt interfaces, and thus melting has occurred uniformly. Partial melting does occur along fractures produced when the blue CL enstatite was initially grown and cooled through the proto/ortho enstatite transition with the attendent volume change. There is either reaction with the melt and diffusion of Mn and Cr into the blue CL En, or there is an overgrowth of red CL En along the fractures. The bulk of the relicts remain blue. The melt enclosing the relicts crystalized to a coarsely radial to dendritic to micro porphyritic texture comprised of enstatite that has a bright red CL with decreasing melt time. The blue CL En has Mn and Cr contents at or below detection limits of the electron probe as described in earlier studies and in natural blue CL En. In the red CL En in this study, the Mn, Al2O3, and Cr are at previously observed levels and the levels change rapidly.

Lofgren, Gary E.↗

NASA NTRS2004

Progress Status of Skutterudite-Based Segmented Thermoelectric Technology Development

Young's modulus, bulk density, fracture strength, and fracture toughness of alpha silicon carbide manufactured by extrusion and by isopressing were measured at room and elevated temperature in order to determine material feasibility as augers for the Tile Overlay Repair attachment. The measured properties of extruded and isopressed materials differ significantly, with the extruded material exhibiting marked strength anisotropy and lower density. The measurements were made between July and September 2006 at Glenn Research Center (GRC), in order to obtain design data for use in structural and thermal mathematical models.

Seqmented Thermoelectric Technoloqy (STE)↗

NASA NTRS1981

An experimental investigation of silicon wafer surface roughness and its effect on the full strength of plated metals

Plated silicon wafers with surface roughness ranging from 0.4 to 130 microinches were subjected to tensile pull strength tests. Electroless Ni/electroless Cu/electroplated Cu and electroless Ni/electroplated Cu were the two types of plate contacts tested. It was found that smoother surfaces had higher pull strength than rougher, chemically etched surfaces. The presence of the electroless Cu layer was found to be important to adhesion. The mode of fracture of the contact as it left the silicon was studied, and it was found that in almost all cases separation was due to fracture of the bulk silicon phase. The correlation between surface roughness and mode of contact failure is presented and interpreted.

Spiers, G. D.↗

NASA NTRS2018

Microscale Fracture Testing of HfO2-Si Environmental Barrier Coatings

Environmental barrier coatings (EBCs) play a vital role in protecting advanced turbine components constructed from ceramic matrix composites. EBC performance is evaluated based on many factors such as mechanical and thermal properties and chemical stability. This study focuses on the mechanical properties of individual phases within a HfO2-Si EBC through the combined use of chevron-notched microcantilevers and finite element modeling. The Si- and HfO2-rich phases were found to have fracture toughness K(sub IC) values of 1.36+/-0.07 MPa·m1/2 and 1.99+/-0.36 MPa·m1/2, respectively. Despite irregular notch geometry, calculated K(sub IC) values agreed reasonably with literature values of monolithic samples for the respective phases. The relatively large scatter in the HfO2-rich samples is attributed to heterogeneity in fracture feature size and distribution of the HfO2 and HfSiO4 phases. Comparisons with bulk single-edge V-notched beams suggest bulk behavior is dictated by the fracture behavior of the Sirich phase with no contribution from the HfO2-rich phase.

Robertson, Andrew L.↗

NASA NTRS1984

Toughening mechanism in elastomer-modified epoxy resins, part 2

The role of matrix ductility on the toughenability and toughening mechanism of elastomer-modified DGEBRA epoxies was investigated. Matrix ductility was varied by using epoxide resins of varying epoxide monomer molecular weights. These epoxide resins were cured using 4,4' diaminodiphenyl sulfone (DDS) and, in some cases, modified with 10% HYCAR(r)CTBN 1300X8. Fracture roughness values for the neat epoxies were found to be almost independent on the monomer molecular weight of the epoxide resin used. However, it was found that the fracture toughness of the elastomer-modified epoxies was very dependent upon the epoxide monomer molecular weight. Tensile dilatometry indicated that the toughening mechanism, when present, is similar to the mechanisms found for the piperidine cured epoxies in Part 1. SEM and OM corroborate this finding. Dynamic mechanical studies were conducted to shed light on the toughenability of the epoxies. The time-dependent small strain behavior of these epoxies were separated into their bulk and shear components. The bulk component is related to brittle fracture, whereas the shear component is related to yielding. It can be shown that the rates of shear and bulk strain energy buildup for a given stress are uniquely determined by the values of Poisson's ratio, nu. It was found that nu increases as the monomer molecular weight of the epoxide resin used increases. This increase in nu can be associated with the low temperature beta relaxation. The effect of increasing cross-link density is to shift the beta relaxation to higher temperatures and to decrease the magnitude of the beta relaxation. Thus, increasing cross-link density decreases nu and increases the tendency towards brittle fracture.

Yee, A. F.↗

NASA NTRS2015

Mechanical Properties of Air Plasma Sprayed Environmental Barrier Coating (EBC) Materials

Development work in Environmental Barrier Coatings (EBCs) for Ceramic Matrix Composites (CMCs) has focused considerably on the identification of materials systems and coating architectures to meet application needs. The evolution of these systems has occurred so quickly that modeling efforts and requisite data for modeling lag considerably behind development. Materials property data exists for many systems in the bulk form, but the effects of deposition on the critical properties of strength and fracture behavior are not well studied. We have plasma sprayed bulk samples of baseline EBC materials (silicon, ytterbium disilicate) and tested the mechanical properties of these materials to elicit differences in strength and toughness. We have also endeavored to assess the mixed-mode fracture resistance, Gc, of silicon in a baseline EBC applied to SiCSiC CMC via four point bend test. These results are compared to previously determined properties of the comparable bulk material.

Ceramic Matrix Composites (CMC's)↗

NASA NTRS1971

Friction, wear, and lubrication in vacuum

A review of studies and observations on the friction, wear, and lubrication behavior of materials in a vacuum environment is presented. The factors that determine and influence friction and wear are discussed. They include topographical, physical, mechanical, and the chemical nature of the surface. The effects of bulk properties such as deformation characteristics, fracture behavior, and structure are included.

Buckley, D. H.↗

NASA NTRS1974

The use of the ion probe mass spectrometer in the measurement of hydrogen concentration gradients in Monel K 500

The ion probe mass spectrometer was used to measure hydrogen concentration gradients in cathodically charged Monel K 500. Initial work with the ion probe involved the calibration of the instrument and the establishment of a suitable experimental procedure for this application. Samples of Monel K 500 were cathodically charged in a weak sulfuric acid solution. By varying the current density, different levels of hydrogen were introduced into the samples. Hydrogen concentration gradients were taken by ion sputtering on the surface of these samples and monitoring the behavior of the hydrogen mass peak as a function of time. An attempt was made to determine the relative amounts of hydrogen in the bulk and grain boundaries by analyzing a fresh fracture surface with a higher proportion of grain boundary area. It was found that substantially more hydrogen was detected in the grain boundaries than in the bulk, confirming the predictions of previous workers. A sputter rate determination was made in order to establish the rate of erosion.

Truhan, J. J., Jr.↗

NASA NTRS1987

Elevated temperature crack growth

The objective of the Elevated Temperature Crack Growth Program is to evaluate proposed nonlinear fracture mechanics methods for application to hot section components of aircraft gas turbine engines. Progress during the past year included linear-elastic fracture mechanics data reduction on nonlinear crack growth rate data on Alloy 718. The bulk of the analytical work centered on thermal gradient problems and proposed fracture mechanics parameters. Good correlation of thermal gradient experimental displacement data and finite element prediction was obtained.

Malik, S. N.↗

NASA NTRS1979

Friction and wear characteristics of iron-chromium alloys in contact with themselves and silicon carbide

Sliding friction experiments were conducted with various iron-chromium alloys in contact with (1) themselves, (2) single crystal silicon carbide disks, and (3) single crystal abrasive grit of silicon carbide. Results indicate the coefficients of friction for the alloys sliding against themselves are between those for pure iron and pure chromium, and are only slightly different with 1, 5, 9, 14, and 19 weight percent chromium in iron. The wear is due, primarily, to shearing, or tearing fracture, of the cohesive bonds in the bulk metal and plowing of the bulk by lumps of wear debris. There are only slight differences in the coefficients of friction for the various alloys when sliding on silicon carbide. The coefficient of friction for the alloys are higher than those for pure iron and pure chromium. Alloy hardening observed in the alloys plays a dominant role in controlling the abrasive friction and wear behavior of the alloys.

Miyoshi, K.↗

NASA NTRS1990

Microstructure and mechanical properties of multiphase NiAl-based alloys

The effect of the gamma-prime phase on the deformation behavior and fracture resistance of melt-spun ribbons and consolidated bulk specimens of a series of Nial-based alloys with Co and Hf additions has been examined. The morphology, location, and volume fraction of the gamma-prime phase are significant factors in enhancing the fracture resistance of the normally brittle NiAl-based alloys. In particular, the results indicate that a continuous-grain-boundary film of gamma-prime can impart limited room-temperature ductility regardless of whether B2 or L10 NiAl is present. Guidelines for microstructure control in multiphase NiAl-based alloys are also presented.

Pank, D. R.↗

NASA NTRS1983

Compositional effects on Si3N4 fracture surfaces

Surface analysis techniques (X-ray, infrared reflection spectroscopy, Auger electron spectroscopy) applied to the same samples reveal that fracture surfaces of Si3N4 with Y2O3 densification aids possess a higher concentration of oxygen than the bulk. The oxide densification aids thus concentrate in the grain boundaries, and even low-temperature fracture is seen as occurring preferentially within the oxygen-enriched grain boundaries. It is found that increasing the concentrations of Y2O3 and Al2O3 increases the oxygen content of the fracture surface. A range of 13-15 percent Y2O3 + 6 percent Al2O3 gives an amorphous grain-boundary phase that is resistant to devitrification. Fracture occurs through the amorphous phase, and heat treatment at 1000 C has little effect on the amorphous phase.

Hench, L. L.↗

NASA NTRS1998

Microstructure and Mechanical Properties of Reaction-Formed Joints in Reaction Bonded Silicon Carbide Ceramics

A reaction-bonded silicon carbide (RB-SiC) ceramic material (Carborundum's Cerastar RB-SIC) has been joined using a reaction forming approach. Microstructure and mechanical properties of three types of reaction-formed joints (350 micron, 50-55 micron, and 20-25 micron thick) have been evaluated. Thick (approximately 350 micron) joints consist mainly of silicon with a small amount of silicon carbide. The flexural strength of thick joints is about 44 plus or minus 2 MPa, and fracture always occurs at the joints. The microscopic examination of fracture surfaces of specimens with thick joints tested at room temperature revealed the failure mode to be typically brittle. Thin joints (<50-55 micron) consist of silicon carbide and silicon phases. The room and high temperature flexural strengths of thin (<50-55 micron) reaction-formed joints have been found to be at least equal to that of the bulk Cerastar RB-SIC materials because the flexure bars fracture away from the joint regions. In this case, the fracture origins appear to be inhomogeneities inside the parent material. This was always found to be the case for thin joints tested at temperatures up to 1350C in air. This observation suggests that the strength of Cerastar RB-SIC material containing a thin joint is not limited by the joint strength but by the strength of the bulk (parent) materials.

Singh, M.↗

NASA NTRS1989

Elevated temperature crack growth

Alloy 718 crack growth experiments were conducted to assess the ability of the selected path-independent (P-I) integrals to describe the elevated temperature crack growth behavior. These tests were performed on single edge notch (SEN) specimens under displacement control with multiple extensometers to monitor the specimen and crack mouth opening displacement (CMOD). The displacements in these tests were sufficiently high to induce bulk cyclic inelastic deformation of the specimen. Under these conditions, the linear elastic fracture mechanics (LEFM) parameter K does not correlate the crack growth data. The experimentally measured displacement gradients at the end of specimen gage length were used as the boundary conditions in elastic-plastic finite element method (FEM) analyses. These analyses were performed with a node release approach using CYANIDE, a GEAE FEM code, which included a gap element which is capable of efficiently simulating crack closure. Excellent correlation was obtained between the experimentally measured and predicted variation of stress and CMOD with crack length and the stress-CMOD loops for Alloy 718 tests conducted at 538 C. This confirmed the accuracy of the FEM crack growth simulation approach. The experimentally measured crack growth rate data correlated well the selected P-I integrals. These investigations have produced significant progress in developing P-I integrals as non-linear fracture mechanics parameters. The results suggest that this methodology has the potential of accurately describing elevated temperature crack growth behavior under the combined influence of thermal cycling and bulk elastic-inelastic deformation states.

Kim, K. S.↗