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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Deformation mechanisms of NiAl cyclicly deformed near the brittle-to-ductile transition temperature

The intermetallic compound NiAl is one of many advanced materials which is being scrutinized for possible use in high temperature, structural applications. Stoichiometric NiAl has a high melting temperature, excellent oxidation resistance, and good thermal conductivity. Past research has concentrated on improving monotonic properties. The encouraging results obtained on binary and micro-alloyed NiAl over the past ten years have led to the broadening of NiAl experimental programs. The purpose of this research project was to determine the low cycle fatigue properties and dislocation mechanisms of stoichiometric NiAl at temperatures near the monotonic brittle-to-ductile transition. The fatigue properties were found to change only slightly in the temperature range of 600 to 700 K; a temperature range over which monotonic ductility and fracture strength increase markedly. The shape of the cyclic hardening curves coincided with the changes observed in the dislocation structures. The evolution of dislocation structures did not appear to change with temperature.

Cullers, Cheryl L.↗
NASA NTRS1992

Model of brittle matrix composite toughening based on discrete fiber reinforcement

An analytical approach for the analysis of the effectiveness of fiber reinforcement in brittle matrix composites is presented. The analytical method allows consideration of discrete fiber distribution and examination of the development of crack growth parameters on microscale. The problem associated with the bridging zone development is addressed here; therefore, the bridging zone is considered to be smaller than the main preexisting crack, and the small scale approach is used. The mechanics of the reinforcement is accurately accounted for in the process zone of a growing crack. Closed form solutions characterizing the initial failure process are presented for linear and nonlinear force - fiber pullout displacement relationships. The implicit exact solution for the extended bridging zone is presented as well.

Rubinstein, Asher A.↗
NASA NTRS1994

The effect of various metallurgical parameters on the flow and fracture behavior of polycrystalline NiAl near the brittle-to-ductile transition

An investigation of the effect of various metallurgical parameters such as interfaces, allowing additions, test temperature, and strain rate on the flow and fracture behavior of polycrystalline NiAl is summarized. From this study, a more complete understanding of the deformation and fracture behavior of polycrystalline NiAl near the brittle-to-ductile transition temperature has been developed. A mechanism for the BDTT is proposed that is based on the operation of localized dislocation climb processes that operate within the vicinity of the grain boundaries and provide the additional deformation mechanisms necessary for grain-to-grain compatibility during plastic deformation. Finally, methods for improving the low temperature mechanical behavior of NiAl were considered and reviewed within the context of the present knowledge of NiAl-based materials and the operative deformation and fracture mechanisms determined in this study. Special emphasis was placed on the use of second phases for improving low temperature properties.

Noebe, Ronald D.↗
NASA NTRS1995

Analysis of the progressive failure of brittle matrix composites

This report investigates two of the most common modes of localized failures, namely, periodic fiber-bridged matrix cracks and transverse matrix cracks. A modification of Daniels' bundle theory is combined with Weibull's weakest link theory to model the statistical distribution of the periodic matrix cracking strength for an individual layer. Results of the model predictions are compared with experimental data from the open literature. Extensions to the model are made to account for possible imperfections within the layer (i.e., nonuniform fiber lengths, irregular crack spacing, and degraded in-situ fiber properties), and the results of these studies are presented. A generalized shear-lag analysis is derived which is capable of modeling the development of transverse matrix cracks in material systems having a general multilayer configuration and under states of full in-plane load. A method for computing the effective elastic properties for the damaged layer at the global level is detailed based upon the solution for the effects of the damage at the local level. This methodology is general in nature and is therefore also applicable to (0(sub m)/90(sub n))(sub s) systems. The characteristic stress-strain response for more general cases is shown to be qualitatively correct (experimental data is not available for a quantitative evaluation), and the damage evolution is recorded in terms of the matrix crack density as a function of the applied strain. Probabilistic effects are introduced to account for the statistical nature of the material strengths, thus allowing cumulative distribution curves for the probability of failure to be generated for each of the example laminates. Additionally, Oh and Finney's classic work on fracture location in brittle materials is extended and combined with the shear-lag analysis. The result is an analytical form for predicting the probability density function for the location of the next transverse crack occurrence within a crack bounded region. The results of this study verified qualitatively the validity of assuming a uniform crack spacing (as was done in the shear-lag model).

Thomas, David J.↗
NASA NTRS1998

Aerogel: Tile Composites Toughen a Brittle Superinsulation

Pure aerogels, though familiar in the laboratory for decades as exotic lightweight insulators with unusual physical properties, have had limited industrial applications due to their low strength and high brittleness. Composites formed of aerogels and the ceramic fiber matrices like those used as space shuttle tiles bypass the fragility of pure aerogels and can enhance the performance of space shuttle tiles in their harsh operating environment. Using a layer of aerogel embedded in a tile may open up a wide range of applications where thermal insulation, gas convection control and mechanical strength matter.

White, Susan↗
NASA NTRS1998

Aerogel: Tile Composites Toughen a Brittle Superinsulation

Pure aerogels, though familiar in the laboratory for decades as exotic lightweight insulators with unusual physical properties, have had limited industrial applications due to their low strength and high brittleness. Composites formed of aerogels and the ceramic fiber matrices used as space shuttle tiles bypass the fragility of pure aerogels and can enhance the performance of space shuttle tiles in their harsh operating environment. Using a layer of aerogel embedded in a tile may open up a wide range of applications where thermal insulation, gas convection control and mechanical strength matter.

White, Susan↗
NASA NTRS2002

Reliability Analysis of Brittle Material Structures - Including MEMS(?) - With the CARES/Life Program

Brittle materials are being used, or considered, for a wide variety of high tech applications that operate in harsh environments, including static and rotating turbine parts. thermal protection systems, dental prosthetics, fuel cells, oxygen transport membranes, radomes, and MEMS. Designing components to sustain repeated load without fracturing while using the minimum amount of material requires the use of a probabilistic design methodology. The CARES/Life code provides a general-purpose analysis tool that predicts the probability of failure of a ceramic component as a function of its time in service. For this presentation an interview of the CARES/Life program will be provided. Emphasis will be placed on describing the latest enhancements to the code for reliability analysis with time varying loads and temperatures (fully transient reliability analysis). Also, early efforts in investigating the validity of using Weibull statistics, the basis of the CARES/Life program, to characterize the strength of MEMS structures will be described as as well as the version of CARES/Life for MEMS (CARES/MEMS) being prepared which incorporates single crystal and edge flaw reliability analysis capability. It is hoped this talk will open a dialog for potential collaboration in the area of MEMS testing and life prediction.

Nemeth, Noel N.↗
NASA NTRS2004

Ultrasonic Apparatus for Pulverizing Brittle Material

The figure depicts an apparatus that pulverizes brittle material by means of a combination of ultrasonic and sonic vibration, hammering, and abrasion. The basic design of the apparatus could be specialized to be a portable version for use by a geologist in collecting powdered rock samples for analysis in the field or in a laboratory. Alternatively, a larger benchtop version could be designed for milling and mixing of precursor powders for such purposes as synthesis of ceramic and other polycrystalline materials or preparing powder samples for x-ray diffraction or x-ray fluorescence measurements to determine crystalline structures and compositions. Among the most attractive characteristics of this apparatus are its light weight and the ability to function without need for a large preload or a large power supply: It has been estimated that a portable version could have a mass <0.5 kg, would consume less than 1 W h of energy in milling a 1-cm3 volume of rock, and could operate at a preload <10 N. The basic design and principle of operation of this apparatus are similar to those of other apparatuses described in a series of prior NASA Tech Briefs articles, the two most relevant being Ultrasonic/ Sonic Drill/Corers With Integrated Sensors (NPO-20856), Vol. 25, No. 1 (January 2001), page 38 and Ultrasonic/ Sonic Mechanisms for Deep Drilling and Coring (NPO-30291), Vol. 27, No. 9 (September 2003), page 65. As before, vibrations are excited by means of a piezoelectric actuator, an ultrasonic horn, and a mass that is free to move axially over a limited range. As before, the ultrasonic harmonic motion of the horn drives the free-mass in a combination of ultrasonic harmonic and lower-frequency hammering motion. In this case, the free-mass is confined within a hollow cylinder that serves as a crushing chamber, and the free-mass serves as a crushing or milling tool. The hammering of the free-mass against a material sample at the lower end of the chamber grinds the sample into powder in a relatively short time. The restriction of the free-mass to axial motion only makes the grinding very efficient. The free-mass can be fabricated to have teeth on its lower face to enhance the grinding effect. Optionally, there can be a hole at the bottom of the chamber covered with a sieve to tailor the size distribution of the powder leaving the crushing chamber.

Sherrit, Stewart↗
DOE OSTI2023

Large deformation and brittle failure calculated using the dual-domain material point method

The dual domain material point (DDMP) method is explored as a candidate to be implemented in a general purpose code to perform simulations of materials with complex geometry that undergo large history-dependent deformation and failure. To test its candidacy, we study its mesh convergence, its sensitivity to mesh orientation, and its ability to handle softening and failure of a material. Simulations of large deformation and simulations of mechanical failure are performed using both DDMP and the material point method (MPM). When cell-crossing of material points is not an issue and when there are a sufficient number of material points in each computation cell, the numerical error decreases with the square of the cell size as expected for both MPM and DDMP. DDMP has reduced error compared with MPM when there are many instances of material points crossing cell boundaries due to the continuous nature of the modified gradient of the shape functions. Simulations of a specimen under tension are also performed where the background mesh is aligned and misaligned with the tension direction. MPM displays a significant mesh-dependent stress field, DDMP shows negligible mesh dependency. Despite a mesh orientation-dependent stress field from MPM, the critical tension and failure mode from both MPM and DDMP calculations have negligible mesh dependency when using a non-local failure model. If only the failure mode is important (i.e., local stresses are unimportant), MPM with a non-local failure model is a suitable method for modeling failure with small deformations. However, if local stresses are also important or if there are large deformations with many cell-crossings before failure, DDMP should be the method that is used. A needed improvement for DDMP is identified from our numerical simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2020

An additive manufacturing design approach to achieving high strength and ductility in traditionally brittle alloys via laser powder bed fusion

Additive Manufacturing (AM) presents unprecedented opportunities to enable design freedom in parts that are unachievable via conventional manufacturing. However, AM-processed components generally lack the necessary performance metrics for widespread commercial adoption. We present a novel AM processing and design approach using removable heat sink artifacts to tailor the mechanical properties of traditionally low strength and low ductility alloys. The design approach is demonstrated with the Fe-50 at.% Co alloy, as a model material of interest for electromagnetic applications. AM-processed components exhibited unprecedented performance, with a 300 % increase in strength and an order-of-magnitude improvement in ductility relative to conventional wrought material. These results are discussed in the context of product performance, production yield, and manufacturing implications toward enabling the design and processing of high-performance, next-generation components, and alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Paul‐Mohr‐Coulomb failure surface of rock in the brittle regime

Abstract The Paul‐Mohr‐Coulomb failure criterion includes the intermediate principal stress σ II and friction angles at the limiting stress states of σ II = σ III and σ II = σ I , where σ I and σ III are major and minor principal stresses. Conventional triaxial compression ( σ II = σ III ), extension ( σ II = σ I ), and plane strain ( σ I ≠ σ II ≠ σ III ) experiments were performed on dry rock. The failure data were plotted in principal stress space, and material parameters were determined in the context of two internal friction angles and the theoretical uniform triaxial (all‐around equal) tensile strength. Assuming isotropy, the triaxial compression and extension results were used to construct a six‐sided pyramidal failure surface, and the extension friction angle was larger than the compression friction angle, a sufficient but not necessary condition of the intermediate stress effect. To capture the behavior of the rock in multiaxial loading, the Paul‐Mohr‐Coulomb criterion was extended to form a 12‐sided pyramid with best fit planes.

Makhnenko, Roman Y.↗
DOE OSTI2016

Direct observations of damage during unconfined brittle failure of Carrara marble

Abstract To observe and quantify the production of microfracturing from initial yield to failure, we deformed Carrara marble samples in uniaxial compression at 20, 105, and 180°C and continuously observed a region of about 1 mm 2 on an exposed face with a long‐working distance microscope. Using image processing and microscale strain‐mapping techniques, we measured local strains over a length scale of tens of micrometers. By treating the images with various filters, we identified linear damage features, as well as the magnitude of localized strain and the mode of deformation, i.e., shear versus normal deformation. In general, shear deformation is more prevalent after initial yielding, while tensile deformation dominates closer to peak stress. Independent measurements of both stress and microcrack density at different stages of each experiment provide a unique opportunity to explicitly compare the data with damage models. The model of Ashby and Sammis (1990) significantly underestimated the damage that the rock could sustain before peak stress, perhaps owing to the influence of weak grain boundaries on the damage production. In these samples, microcracks tended to form near boundaries before yield stress. During strain hardening, the damage parameters increased rapidly as longer microcracks grew along the boundaries and finally transected grains as loading neared peak stress. The microcrack density can be empirically related to the reduction of Young's modulus; stiffness ratios decay exponentially with increasing microcrack density for T ≤ 105°C.

Tal, Yuval↗
DOE OSTI2015

On the pore-scale mechanisms leading to brittle and ductile deformation behavior of crystalline rocks

Deformation mechanisms found at the pore scale are responsible for producing large strains in porous rocks. Furthermore, they include cataclastic flow, dislocation creep, dynamic recrystallization, diffusive mass transfer, and grain boundary sliding, among others. Here, we focus on two dominant pore-scale mechanisms resulting from purely mechanical, isothermal loading: crystal plasticity and micro-fracturing. We examine the contributions of each mechanism to the overall behavior at a scale larger than the grains but smaller than the specimen, which is commonly referred to as the mesoscale. Crystal plasticity is assumed to occur as dislocations along the many crystallographic slip planes, whereas micro-fracturing entails slip and frictional sliding on microcracks. It is observed that under combined shear and tensile loading, micro-fracturing generates a softer response compared to crystal plasticity alone, which is attributed to slip weakening where the shear stress drops to a residual level determined by the frictional strength. For compressive loading, however, micro-fracturing produces a stiffer response than crystal plasticity due to the presence of frictional resistance on the slip surface. Behaviors under tensile, compressive, and shear loading invariably show that porosity plays a critical role in the initiation of the deformation mechanisms. Both crystal plasticity and micro-fracturing are observed to initiate at the peripheries of the pores, consistent with results of experimental studies.

58 GEOSCIENCES↗